>> Fiber Optic Connectors Are Fragile

All fiber optic connectors should be taken care of as a high quality lens of a good camera. Even a small fingerprint can lead to big trouble. The weak link in many fiber optic communication systems, and in optical instrument reliability, is the use and care of the fiber optic connector.

After so many years of improvement, current fiber connectors are so easy to use, that people tend to be vigilance in connector care and cleaning. However, remember this, it only takes ONE time of a missed cleaning for a piece of grit to permanently damage the glass and ruin the connector.

Fiber optic connectors are susceptible to damage that is not immediately obvious to the naked eye. This damage can have significant effects on measurements being made. Keep it in mind: any degradation of a ferrule or fiber endface, any stray particles or finger oil on the endface, can have a significant effect on connector performance.

>> The Effect of Improper Use of Fiber Optic Connector

Fiber optic connector maintenance is a big topic. The following figure shows a close-up of a clean connector endface.

In contrast, the following figure shows a connector end that was used with a cable end that avoided the alignment key. This may have been caused by a temporary ferrule without a key, or possibly a mismatched long ferrule put into a short connector assemble. Material is smeared and ground into the endface causing light scattering and poor reflection.

Even worse, the following figure shows that if the connector was used continuously this way, it will grind off the glass face and destroy the connector.

Repeated connections without removing the particles, or using improper tools can lead to physical damage of the glass fiber endface as shown in the following pictures. Where the damage is severe, the damage on one connector can be transferred to another good connector that comes in contact with it.

Picture <a>

Picture <b>

Picture <c>

Picture <d>

The cure is disciplined connector care. Soft swabs should be used – never use metal or sharp objects. Some connectors have a soft endface material that can be very easily scratched and damaged.

In the <a> picture above, for example, the results of using a pin on the connector surface is seen. In <b>, a close-up of this damaged connector, not only is there gouging in the endface around the fiber, but the glass fiber itself has been chipped off and broken. Broken or damaged glass can lead to a number of problems that are often ascribed to other causes. This connector will probably damage any other fiber that is mated to it.

In the <c> and <d> pictures above, another connector is seen with severe abuse. In this example, the soft ferrule core was pushed away from the glass and there is pitting in the fiber end. In addition, the glass fiber is sticking up almost 3um above the uneven surface (from severe wear of the soft metal that was once flush with the glass fiber).

>> Matching Gel and Oils Contamination

Another potential problem comes with matching gel and oils. While these often work well on first insertion, they are great dirt magnets. Damage is often caused as the oil or gel grabs and holds grit that is then ground into the fiber endface. Also, some early gels were designed for the FC, noncontacting connectors, using small glass spheres. When used in contacting connectors, these glass balls can scratch and pit the fiber.

Index matching oils is also discouraged. It runs and migrates into unwanted areas, attracting and holding dust. If an index matching gel or oil must be used, apply it to a freshly cleaned connector, make the measurement, and immediately clean the gel or oil off. Never use a gel for long-term connections and never use it to improve a damaged connector. The gel can mask the damaged end and continued use of a damaged fiber can transfer damage to another connector.

>> Not-Too-Tight Mating of Fiber Optic Connectors

Another tip for good fiber connector mating is: not too tight.Unlike electrical connections, tighter is not better.

The connector’s job is to bring the endfaces of two fibers together. Once they touch, tighter only causes a greater force to be applied to the delicate endface. In some connectors, the end can cock off-axis with a tight connection (due to the curved face) resulting in a worse return loss. Many measurements are actually improved by backing off the connector pressure. Also, if a piece of grit does happen to get by the cleaning procedure, a tighter connection is more likely to damage the glass. Tighten just until the two fibers touch.

>> Fiber Optic Cleaning Procedures

The basics of cleaning are simple. Use a pure grade of isopropyl alcohol on a clean cotton swab to wipe off the endface and ferrule. (While other devices and methods also work, this one is easy, problem free, and inexpensive.)

After the wet scrub, wipe off the wet endface with a dry swab, or blow it off with filtered, nonresidue compressed air. As the following figure shows, leaving the fluid to air dry will leave behind the dissolved oils in little puddles which will interfere with the correct physical contact. Then when reinserting the cable into the connector, insert it gently, in as straight as possible. Tipping, and inserting with an angle, can scrape off material from inside the connector or even break the inside sleeve of some connectors made with ceramic material.

It is pretty often that a cable or connector may require more than one cleaning. If the performance seems poor, clean again. Often the second cleaning will restore the proper performance where the first did not. The idea is that the first step is to gently remove any grit and oil using the alcohol as a solvent. Remove the solvent by blowing it off or using a gentle dry wipe. Then if there is a caked-on layer of material still present (such as can happen if the beryllium-copper sides of the ferrule retainer gets scraped and deposited on the endface during insertion of the cable), the second cleaning should be harder, with a scrubbing action. But if scrubbed first, grit can be caught in the swab and become a gouging element.

A system is only as good the weakest link. Don’t let the connector become a failure because of poor attention. Use fusion splicers on the more permanent critical nodes. Choose the best connector possible. Check and measure the connector’s return loss often, for degradation. Clean every connector, every time.

>> Use of Wipes and Cleaning Cassettes

Fabric and/or composite material wipes provide combined mechanical action and absorbency to remove contamination.   Wipes should be used with a resilient pad in order avoid potential scratching of the connector end-face.  This method is appropriate for cleaning connectors with exposed ferrules or termini but cannot be used to clean connector end-faces within alignment sleeves.  The wipe should be constructed of material that is lint free and non-debris producing during the cleaning process.  Please note that dry wipes have been shown to leave a static charge on the end-face of the connector which  can thereafter attract particulate contamination.   Therefore it is recommended that a static dissipative solvent be used with a dry wipe to eliminate this condition.

1. Technique for Cleaning with Wipes or Cleaning Cassette

As mentioned above it is recommended to use a lint-free, non-debris generating wipe with a static dissipative solvent.  The user should dampen a portion of the wipe with the solvent, place the connector end-face into the damp area of the wipe and draw the connector into the dry area of the wipe.  A physical wipe of 2 – 5 cm should be sufficient.  This may be repeated in a different area of the wipe if desired, however 1 or 2 strokes should be sufficient for most common contaminants.

Upon inspection, if the connector is not clean after the first cleaning, the process can be repeated perhaps with slightly more pressure on the connector to increase the mechanical action and perhaps making several stokes from the damp to dry sections of the cleaning material.

If the connector is still not clean and contamination has not moved it should be considered nonremovable contamination and depending on the location and size of the contamination the user should pass or reject use of the connector.

2. Swabs or Port Cleaning Device

Use of purpose built swabs or mechanical port cleaning devices provides mechanical action and absorbency to remove contamination.  However since the area within a port is very confined and limits mechanical action, it is recommended that a wet/dry cleaning process be utilized for cleaning connector end-faces within alignment sleeves.   A static dissipative solvent will add chemical action to the cleaning process as well eliminate any residual static charge that may be on connector.  The cleaning end of the swab or cleaning material used in the port cleaning device should be lint free, non-debris generating material.

Select a swab or port cleaning device that is manufactured for the size connector you are cleaning.  Do not touch or contaminate the cleaning end of the swab or the port cleaning device.  It is recommended that the user dampen the swab or port cleaning device with a static dissipative solvent.  The swab or port cleaning device should be damp and not wet.  (The process is analogous to wiping a smooth surface with a sponge.  A damp sponge will pick up contaminates, whereas a wet sponge will only spread them around.)  If a very fast drying solvent is used then a follow-up cleaning with a dry swab or an addition activation of the port cleaning device may not be necessary.

3. Swab Cleaning Detail

Place the dampened cleaning end of the swab into the port and rotate the swab while applying some pressure to the connector end-face.  Usually pushing so that the compression spring in the connector is slightly activated is ideal for 2.5mm connectors.  Rotating the swab 6 – 12 times is sufficient.  The swab should only be used once and then disposed.  If the user is cleaning angled polished connectors – APC (typically color coded with a green connector housing or bulkhead adaptor), then using a ¼ turn back and forth rotation may help the swab end-face conform to the 8 degree angle and thereby clean the entire surface of the connector end-face. 

If a very fast drying solvent was used, you are now ready to inspect.  If not, repeat the above process with a dry swab.

4. Port Device Cleaning Detail

Insert the solvent dampened device into the alignment sleeve and active the cleaner to perform the mechanical clean either by pushing the device or by pressing on a button on the device.  An additional activation of the device assures that any excess solvent has been removed. Upon inspection, if the connector is not clean after the first cleaning, the process can be repeated.

If the connector is still not clean and contamination has not moved it should be considered nonremovable contamination and depending on the location and size of the contamination the user should pass or reject use of the connector.

5. About Solvents

Solvents used to clean fiber optics should be static-dissipative and residue-free.  Many solvents are flammable and/or packaged so that transportation of the solvent is considered a hazardous material increasing cost of shipment and storage of the solvent.  However, there are solvents available that are non-flammable and non-hazardous and packaged so that shipping requires no additional fees or paperwork.

Historically, 99% pure isopropyl alcohol (IPA) has been used as a solvent.  However, IPA is easily contaminated.  The solvent is hygroscopic and therefore absorbs moisture from the atmosphere including any contaminates present contaminating the solvent and potentially leaving a haze on the connector end-face which can be a problem with higher power laser networks.  In addition, IPA is flammable and is considered a hazardous material.  Typically IPA must be repackaged for use and is therefore very subject to contamination.

Also, it is important to select a solvent that is compatible with the cleaning wipe, swab, or device.  You do not want a solvent will dissolve a binder or glue used in a wipe or swab and generate additional contamination.

>> Acceptable & Unacceptable Fiber Connector End-Face Finishes

For measurement of the performance criteria, Back-reflection and Insertion Loss, there are meters available which are generally familiar to polishers.  The geometrical criteria, Apex Offset, Radius of Curvature, and Fiber Undercut are confirmed by using an Interferometer.

Visual inspection will always play an important role in evaluating the polished surface (see diagram below), but the now increasingly used interferometer is needed to confirm geometry.  Interferometers are available from a number of sources, ranging from those which provide a monitor from which the user determines product acceptability, to computer aided programs that provide a printed readout which includes all performance and geometric characteristics of the connector.

>> A Brief Introduction of Rare-Earth Doped Fibers

Rare-earth doped fiber is an optical fiber in which ions of a rare-earth element, such as neodymium, Erbium or holmium, have been incorporated into the glass core matrix, yielding high absorption with low loss in the visible and near-infrared spectral regions.

Fiber lasers and fiber amplifiers are nearly always based on glass fibers which are doped with laser-active rare earth ions (normally only in the fiber core). These ions absorb pump light, typically at a shorter wavelength than the laser or amplifier wavelength (except in upconversion lasers), which excites them into some metastable levels. This allows for light amplification via stimulated emission. Such fibers are often called active fibers. They are gain media with a particularly high gain efficiency, resulting mainly from the strong optical confinement in the fiber’s waveguide structure.

1. Why Rare Earth Ions?

Rare earth ions are good candidate for active ions in laser materials because they show many absorption and fluorescence transitions in almost every region of the visible and the near-infrared range.

Rare earths have other important characteristics in comparison to other optically active ions as well: the wavelengths of their emission and absorption transitions are relatively insensitive to host materials, the lifetimes of metastable states are long and the quantum efficiency tends to be high. These properties lead to an excellent performance of rare earth ions in many optical applications.

2. Common Types of Rare-Earth Doped Fibers

The technologically most important rare-earth-doped fibers are erbium-doped fibers for erbium-doped fiber amplifiers (EDFA) and ytterbium-doped fibers for high-power fiber lasers and amplifiers.

The following table shows the most common laser-active ions and host glasses and also typical emission wavelength ranges of rare-earth-doped fibers:

3. Host Compositions

The glass host composition affects the solubility of the rare earth dopant which, in turn, may affect the fluorescence lifetime, absorption, emission, and excited state absorption cross sections of the dopant transitions. These quantities affect the ultimate ability of the active material to provide gain.

Devices of general interest span rare earth concentrations of tens to several thousand parts per million (ppm), resulting devices of one to tens of meters long. For some applications dopant levels of 1 ppm and less are advantageous, resulting in devices several kilometers long.

For all designs, the rare earth should ideally be confined as a delta function in the center of the core for maximum gain per unit pump power. Practically, there is a necessary tradeoff between the confinement and the rare earth concentration. The more confined structures require a higher rare earth concentration for an equivalent length, eventually running into the clustering limit for the particular host glass composition.  Clustering is to be avoided in that it induces fluorescence quenching and reduces the performance of the device.

>> Waveguide Designs

Let us consider the waveguide configurations used to bring the pump, signal, and active media together. The most widely used configuration consists of a rare earth doped fiber core, which allows the pump and signal to propagate together in a single mode fiber, there are a number of alternative fiber device geometries worthy of consideration too.

1. Core Confined

The most efficient conversion of pump to signal photons uses the design in which both pump and signal are confined in the fiber core. This configuration has been made especially attractive by the availability of commercial, low-insertion loss, low-reflectivity fiber couplers, which can be chosen to combine a variety of pump and signal wavelengths onto a common output fiber.

For this design, the launched pump threshold power P_th provides a reasonable figure of merit for the efficiency of a fiber laser or amplifier, a lower value being preferred. This quantity is proportional to

where

A_eff = effective core area

ε_p = fractional absorbed pump power

σ_e = stimulated emission cross-section

τ_f = pump fluorescence lifetime

The efficiency of this device, therefore, can be increased by diminishing the effective core area, increasing the pump absorption cross section, increasing the pump fluorescence lifetime, and increasing the stimulated emission cross section.

Furthermore, of these parameters, decreasing the mode field diameter (decreasing A_eff )  has the greatest effect on increasing the gain/pump power slope. Further improvement can be achieved by confining the rare earth to the central portion of the core, where the pump and signal intensities are generally highest.

The optimized waveguide design then requires consideration of both the device configuration and a number of material and waveguide properties determined by the fabrication methods used.

2. Double Clad

Another approach to achieving interaction of guided pump light with an active medium uses a single-mode guide or the signal surrounded by a multimode pump guide.

Pump light is launched from the fiber end into the undoped cladding, propagating in a zig-zag pattern through the doped core as it travels along the fiber. Configurations with the core offset in a circular cladding and a core centered in an elliptical cladding have been demonstrated.  A high brightness neodymium fiber laser based on the latter design provided an output greater than 100 mW for an 807 nm diode array laser pump power of 500 mW.

The guiding geometry of this configuration was designed to maximize the use of power available from laser diode arrays, thereby producing higher-output powers.

3. Evanescent Field

The wings or evanescent field of the optical signal guided by a single mode fiber may be used to interact with an active material outside of the core region. One approach to enhancing this effect is to locally taper the guide, thereby causing the optical power to increase outside of the glass material bounds of the fiber over a length of several millimeters.

This tapering method has been used to demonstrate a 20 dB gain amplifier, for a pump power below 1 W, with a dye solution circulating around the tapered fiber region. Both the signal at 750nm and the pump at 650 nm were copropagating in the core.

Similarly, the active media can be incorporated in the cladding glass, as has been demonstrated for erbium (Er) or neodymium (Nd). Reported gain for an erbium-doped cladding structure was 0.6 dB for a 1.55 um signal, with a 1.48 um pump power of 50 mW.

The evanescent field may also be accessed by polishing away a portion of the fiber cladding, thus creating a structure similar to a D-shaped fiber. Pulsed amplification of 22 dB for one such dye evanescent amplifier has been achieved.

The pump power required for these devices to obtain a sizable gain far exceeds that needed for schemes in which active media are contained in the core. However, by using this evanescent interaction, active media, such as dyes, which cannot be incorporated into a glass, can be explored.

>> Characterization of Rare-Earth-Doped Fibers

In addition to all the properties of a passive (undoped) optical fiber, such as the guiding properties (effective mode area, numerical aperture, cut-off wavelength, bend losses), nonlinearities, etc., active fibers can be characterized with respect to several other properties:

• One of the most important parameters is the rare-earth doping concentration, most often specified in ppm wt (parts per million by weight). A higher doping concentration allows for efficient pump absorption in a shorter length and thus also reduces the effect of nonlinearities in high peak power devices. However, it can also lead to concentration quenching.
• Wavelength-dependent effective absorption and emission cross sections (and possibly ESA cross sections) together with the upper-state lifetime (and possibly lifetimes of intermediate levels) are required for calculating the wavelength tuning behavior, power efficiency, etc.
• Parameters for quantifying the speed of energy transfer processes are important particularly for codoped fibers.

As an alternative method, so-called Giles parameters can be specified, which depend on the doping concentration, effective mode area and effective cross sections.

For such characterization, a variety of measurement techniques are used. White-light absorption spectra can be used for finding absorption cross sections (for known doping concentrations). Emission cross sections are obtained from fluorescence spectra, with scaling e.g. via the reciprocity method or the metastable level lifetimes. Upper-state lifetimes are often obtained from fluorescence measurements with pulsed pumping, and ESA parameters can be obtained in experiments with a modulated pump power.

The resulting set of data can be used e.g. in laser and amplifier models based on rate equations. Such models allow one, e.g., to predict or check the performance of fiber laser or amplifier devices, the effect of possible modifications, etc.

Further characterization may be required for quantifying effects such as photodarkening, which can sometimes seriously degrade the efficiency of active fiber devices.

>> Rare-Earth-Doped Fiber Amplifiers

1. A Brief Review of Rare-Earth-Doped Fiber Amplifiers

An important advance in optical fiber technology occurred with the development of fibers that amplify light through stimulated emission. These led to dramatic increases in the channel capacities of fiber communication systems in addition to providing the key components in many new forms of optical sources and signal processing devices.

Such fibers are made by incorporating various rare-earth ion dopants into the core material, the most successful of which has been erbium. Erbium-doped fiber amplifiers (EDFAs) in their usual configuration provide gain that maximizes at 1.53 um when the fiber is pumped by additional light input at either 1.48 or 0.98 um wavelength. Lengths of amplifying fibers are used as repeater sections in communications systems, replacing the expensive and complicated electronic units that were commonly used.

The primary motivation in using a fiber amplifier repeater is that the transmitted signal remains in optical form throughout the link rather than being transformed into an electrical signal and back to optical whenever a repeater stage is encountered. This property offers additional advantages, which include the ability to change system data rates as needed or to simultaneously transmit multiple data rates without need to modify the transmission span.

A further advantage (also true for Raman amplifiers) is that a single EDFA can provide gain for multiple wavelengths simultaneously. Such a task would otherwise require a separate electronic repeater for each wavelength. It is this feature that contributed to the realization of dense wavelength division multiplexed (DWDM) systems. For example, 80 wavelength channels having 50 GHz spacing can be accommodated within the conventional 1.53 to 1.56 um EDFA gain bandwidth. More recent efforts have resulted in the extension of EDFA gain into the longer wavelength (L band) range between 1.56 and 1.63 um.

Aside from systems applications, numerous device applications for signal processing in addition to the construction of erbium-doped fiber-based lasers have been demonstrated.

Other rare-earth dopants or dopant combinations have been used to produce fiber amplifiers that poses gain at other wavelengths in the visible and near-infrared. Examples of these include praseodymium-doped fiber amplifiers (PDFAs), which provide gain at 1.3 um and are pumped at 1.02 um. Ytterbium-doped fibers (YDFAs) amplify from 975 to 1150 nm using pump wavelengths between 910 and 1064 nm; erbium-ytterbium codoped fibers (EYDFAs) enable use of pump light at 1.06 um while providing gain at 1.55 um. Thulium-doped fluoride fibers (TDFAs) have been constructed for amplification at 0.8 um and 1.48 um.

2. Basic Theory of Amplification by Stimulated Emission

The mechanism of amplification by stimulated emission can be demonstrated using a simple material model, as shown in the following figure.

The material consists of N_t identical atoms per unit volume. Each atom has four possible energy states associated with, for example, four possible electron configurations. upward transitions between energy levels in a single atom occur through the absorption of an incident photon. In the absence of additional light, downward transitions occur either by nonradiative relaxation or radiative relaxation through spontaneous emission (the random emission of a single photon in any direction). When an additional photon is incident on the atom, a downward transition can be stimulated, resulting in the emission by the atom of a second photon, which propagates with the incident one, As more photons are generated, these in turn stimulate downward transitions in adjacent atoms; this cascading effect can ultimately result in substantial power gain, provided a sufficient number of atoms can be initially excited to the higher energy states and provided the number of downward transitions per unit time can be made to exceed the upward transition rate.

In the four-level model, light inputs at two different frequencies are separately responsible for absorption and emission. The light that is absorbed, known as the pump, is at the higher frequency; its presence induces transitions from level 1 to level 4, whose energy difference is hω₂. The pump light is input at frequency ω₂ to coincide with this resonance. Fast nonradiative transitions occur from level 4 to level 3, allowing a substantial number of atoms to assume the level 3 energy state. This build-up of “population” in level 3 is assured if relaxation processes from level 3 to level 2 or level 1 are either slow or are not allowed.

The energy spacing between levels 3 and 2 is hω₁. The model assumes that relaxation of population from level 3 to level 2 can occur through stimulated or spontaneous emission; the latter occurs with characteristic relaxation time τ. The 3 –> 1 transition is assumed forbidden. From level 2, fast nonradiative decay again occurs to level 1. Because levels 4 and 2 both relax quickly, the populations of these levels are both essentially zero, meaning that the total atomic population N_t is divided in some proportion between levels 3 and 1, so that N_t = N₁ + N₃. This also means that the level 3 population can exceed that of level 2, resulting in a population inversion between these two levels. The result is that net gain can occur for light at frequency ω₁ because the rate of stimulated downward transitions between 3 and 2 exceeds the upward transition rate between these levels.

To determine the gain as a function of the various input and medium parameters, rate equations for the population densities of the important energy levels must be solved. Consider a fiber whose core is doped with erbium or another substance for which the excitation dynamics can be described by the four-level model. The rate equations that describe the populations of levels 1 and 3 are, respectively,

The pump and signal powers Pp and Ps, respectively, are expressed in terms of photon flux densities [photons (sec – m²)^-1]  by dividing both quantities by the energy per photon hω₂ or hω₁ and the fiber core area A_c (erbium is assumed to be present in the core only). σ_ap and σ_es are the absorption and emission cross-sections, respectively, expressed in m²; these, when multiplied by the appropriate photon flux densities, yield the probability of excitation or de-excitation of a single atom in a specified time period. Multiplying the cross sections by the associated number densities of the ground or excited states (N₁ or N₃) yields the exponential absorption absorption or gain coefficients for the pump and signal powers, respectively, that reside in the core (where absorption and gain exist).

The above two equations are most easily solved in steady state, in which all time derivatives are zero. The resulting expressions for N₁ and N₃ in terms of N_t are, respectively,

The saturation powers for the pump and signal are defined respectively as

We note that in the absence of signal power, P_sat^ap is the pump power required to equalize the two populations.

The signal and pump powers grow or attenuate with distance in the fiber according to the respective equations

The equations are coupled because N₁ and N₃ both depend on P_p and P_s. An analytic solution can be obtained for the special case in which P_p << P_sat^ap and P_s << P_sat^es. As a result, N₁ ≈ N_t and N₃ ≈ N_t(P_p/P_sat^ap). Under these conditions, the second equation above is readily solved to yield

The last equation can then be solved by assuming weak absorption for the pump, such that P_p(z) ≈ P_p(0). Then, using the foregoing approximation for N₃, we obtain

This result, although greatly simplified, demonstrates that at a given pump power level, the available gain is appreciable if (1) the absorption and emission cross sections are high and (2) if the lifetime of the metastable state (level 3) is long.

>> Rare-Earth Doped Fiber Fabrication Methods

1. Low-Loss Communication Fiber

The standard method of fabricating doped silica fiber fall into two basic categories, both based on the reaction of halides, such as SiCl₄, GeCl₄, POCl₃, SiF₄, and BCl₃, to form the desired mix of oxides.

Category 1 reacts the chlorides in a hydrogen flame and collects the resulting soot on a mandrel for subsequent sintering to a transparent glass. Processes based on this method are commonly referred to as vapor axial deposition (VAD) and outside vapor deposition (OVD).

Category 2 reacts the chlorides inside a substrate tube that becomes part of the cladding, simultaneously reacting, depositing, or sintering as a torch plasma fireball or microwave cavity traverses the tube. Processes based on this method are commonly referred to as modified chemical vapor deposition (MCVD), plasma chemical vapor deposition (PCVD), and intrinsic microwave chemical vapor deposition (IMCVD).

All these methods create a preform, or large-geometry equivalent, which is desired in the fiber. The preform is then drawn into an optical fiber by heating one end to the softening temperature and pulling it into a fiber at rates of 1-10 m/s.

Index-raising dopant ions, such as germanium, phosphorus, aluminum, and titanium, and index-lowering dopants such as boron and fluorine, are introduced into the reaction stream as halide vapors carried by oxygen at a temperature near 30°C. The halide compounds of rare earth ions are, however, generally less volatile than the commonly used chlorides and fluorides of the index-modifying dopants, thereby requiring volatilizing and delivering temperatures of a few hundred degrees. This requirement has stimulated the vapor and liquid phase handling methods to be discussed.

2. Rare Earth Vapor Phase

Methods to deliver rare earth vapor species to the reaction/deposition zone of a preform process have been devised for both MCVD and VAD or OVD techniques. The configurations employed for MCVD are shown in the following figure, for which rare earth dopants are delivered to an oxidation reaction region along with other index-controlling dopants. The low vapor pressure rare earth reactant is accommodated either by taking the vapor source close to the reaction zone and immediately diluting it with other reactants or by delivering the material as an aerosol or higher vapor pressure organic compound.

The heated frit source was made by soaking a region of porous soot previously deposited on the upstream inner wall of an MCVD tube with a rare earth chloride-ethanol solution. On heating to 900°C and being allowed to dry, the sponge became a vapor source. Two other source methods use the heated chloride directly as a source after dehydrating. The dehydration is necessary in that most rare earth chlorides are, in fact, hydrated. Dehydration may be accomplished by heating the material to nearly 900°C with a flow of Cl₂, SOCl₂, or SF₆. The attraction of the heated source injector method is that the rare earth reactant source is isolated from potentially unwanted reactions with the SiCl₄, GeCl₄, or POCl₃ index-raising reactants.

A variation of the heated chloride source method requires a two-step process referred to as transport-and-oxidation. Here, the rare earth chloride was first transported to the downstream inner wall by evaporation and condensation, followed by a separate oxidation step at higher temperatures. The resulting single-mode fiber structure consisted of a P₂O₅/SiO₂ cladding and a Yb₂O₃/SiO₂ core, one of the few reported uses of a rare earth dopant as an index-raising constituent. A 1-mol% Yb₂O₃/SiO₂ core provided the 0.29% increase in refractive index over the near silica index cladding.

The aerosol delivery method overcomes the need for heated source compounds by generating a vapor at the reaction site. A feature of this method is the ability to create an aerosol at a remote location and pipe the resulting suspension of liquid droplets of rear earth dopant into the reaction regions of the MCVD substrate tube with a carrier gas. The aerosols delivered this way were generated by a 1.5 MHz ultrasonic nebulizer commonly used in room humidifiers. Both aqueous and organic liquids have been delivered by this technique, allowing the incorporation of lead, sodium, and gallium, as well as rare earths. Given that most of the aerosol fluid materials contain hydrogen, dehydration after deposition is required for low OH content.

Vapor transport of rare earth dopants may also be achieved by using organic compounds that have higher vapor pressures than the chlorides, bromides, or iodides. These materials can be delivered in lines heated to 200°C, rather than the several-hundred degree requirements for chlorides. The application of this source to MCVD has been reported using three concentric input delivery lines. Multiple rare earth doping and high dopant levels are reported with this method, along with background losses of 10 dB/km and moderate OH levels of near 20 ppm.

Rare earth vapor, aerosol, and solution transport may also be used to dope preforms fabricated by the OVD or VAD hydrolysis processes. Such doping may be achieved either during the soot deposition (see the following figure) or after the soot boule has been created (two more pictures below). The introduction of low vapor pressure dopants to VAD was initially reported using a combination of aerosol and vapor delivery. The incorporation of cerium, neodymium, and erbium has been accomplished in the OVD method by introducing rare earth organic vapors into the reaction flame.

Cerium, for example, was introduced as an organic source, cerium β-diketonate. The high vapor pressure of this compound allowed delivery to the reaction flame by a more traditional bubbler carrier system with heated delivery lines. Another high vapor pressure organic compound used as the rare earth chelate. Here a 1.0-wt% Nd₂O₃ double-clad fiber was fabricated for high-output powers, with background losses of 10 dB/km. Concentration of Yb₂O₃ as high as 11 wt%, as required for the double-clad laser, were also achieved by this method.

3. Rare Earth Solutions

One of the first reported means for incorporating low-volatility halide ions into high-purity fiber preforms used a liquid phase “soot impregnation method”. A pure silica soot boule was first fabricated by flame hydrolysis, with a porosity of 60-90% (pore diameter of 0.001 – 10 um). The boule was immersed in a methanol solution of the dopant salt for 1 hour and then allowed to dry for 24 hours, after which the boule was sintered in a He/O₂/Cl₂ atmosphere to a bubble-free glass rod (see the previous figure). The dopant concentration was controlled by varying the ion concentration in the solution. This general technique, later referred to as molecular stuffing, has been used to incorporate Nd and Ca in silica.

A variation of this solution-doping technique combining MCVD and the solution doping has more recently been reported (see the figure below). Here, an unsintered (porous) layer of silica is first deposited inside a silica tube by the MCVD process. This layer is doped by filling the tube with an aqueous rare earth chloride solution; this solution is allowed to soak for nearly 1 hour, and then the solution is drained. The impregnated layer is dried at high temperatures in the presence of flowing chlorine/oxygen mixture. Index-raising dopants such as aluminum have also been incorporated by this method.

Although this process would seem to be inherently less pure, it has produced doped fibers with background losses of 0.3 dB/km. This general method has also been extended by replacing aqueous solutions with ethyl alcohol, ethyl ether, or acetone solvents for Al3⁺ and rare earth halides. Solubilities vary widely among the rear earth nitrates, bromides, and chlorides, and all are useful.

Fibers made with these nonaqueous solvents contained a relatively low OH impurity level as evidenced by the less than 10 dB/km absorption at 1.38 um. Aqueous solution methods may also produce low OH fibers with proper dehydration techniques.

As erbium-doped silica amplifiers were developed, it became clear that confinement of the dopant to the central region of the core was very important, as were uniformity and homogeneity of the deposit. To this end, another MCVD dopant method was developed, referred to as sol-gel dipcoating. The process coats the inside of an MCVD substrate tube with a rare earth-containing sol, which subsequently gels, leaving a thin dopant layer. (see the following picture). Both rare earth and index-raising dopants may be combined. The coating sol is formed by hydrolyzing a mixture of a soluble rare earth compound with Si(OC₂H₅)₄ (TEOS). The viscosity of the gel slowly increases with time as hydrolysis polymerizes the reactants. Deposition of the film then proceeds by filling the inside of the MCVD support tube with the gel, followed by draining. The gel layer thickness is controlled by the viscosity of the gel which, in turn, is determined by its age and the rate at which the gel liquid is drained. Film thickness of a fraction of a micrometer is typical, thereby allowing a well-confined dopant region. The coated tube is returned to the glass-working lathe for subsequent collapse.

4. Rod and Tube

The first optical fiber was made by drawing a preform assembly made of a core rod and cladding tube of the proper dimensions and indices. Recent adaptations of this method have been demonstrated for making compound glass core compositions. To retain the overall compatibility with communication-grade doped silica fiber, a small compound glass rod is inserted into a thick-walled silica tube. The combination is then drawn at the high temperatures required by the silica tube. As a result, a few of the less stable constituents of the compound glass are volatilized. In spite of this, lengths of fiber can be drawn that are long enough for practical use.

With interest in distributed Er-doped amplifiers, a need arose for a method to produce uniform and very low dopant levels. Although the solution doping and outside process methods were successful in controlling these low levels of dopant, a new, rod-and-tube like technique was also devised to meet this challenge. Here rare earth was introduced into an MCVD preform as the core of a fiber with a 150 um outside diameter and a 10 um core diameter (see the figure below). The “seed” fiber was inserted in the bore of an MCVD preform before the last collapse pass. During the final collapse pass, the fiber becomes a diffusion source for the dopant ions in the center of the preform core.

Fibers fabricated by this method have shown losses as low as 0.35 dB/km and well-controlled erbium levels of 0.01 ppm Er³⁺, corresponding to a ground-state absorption level of only 1 dB/km at 1.53 um. Ina sense this resembles a miniature rod-and-tube process except that the rod is effectively dissolved in the host core, as evidenced by the change in fluorescence spectrum from the seed composition to the core composition.

>> What is Power over Ethernet?

1. Introduction

Power-over-Ethernet (PoE) or "Active Ethernet" eliminates the need to run 110/220 VAC power to Wireless Access Points and other devices on a wired LAN. Using Power-over-Ethernet system, installers need to run only a single CAT5 Ethernet cable that carries both power and data to each device. This allows greater flexibility in the locating of AP’s and network devices and significantly decreasing installation costs in many cases.

Power over Ethernet (PoE) technology describes a system to pass electrical power safely, along with data, on Ethernet cabling. The IEEE standard for PoE requires category 5 cable or higher for high power levels, but can operate with category 3 cable for low power levels.

2. How is power supplied?

Power is supplied in common mode over two or more of the differential pairs of wires found in the Ethernet cables and comes from a power supply within a PoE-enabled networking device such as an Ethernet switch or can be injected into a cable run with a midspan power supply.

Power-over-Ethernet begins with a CAT5 "Injector" that inserts a DC Voltage onto the CAT5 cable. The Injector is typically installed in the "wiring closet" near the Ethernet switch or hub.

Some Wireless Access Points and other network accept the injected DC power directly from the CAT5 cable through their RJ45 jack. These devices are considered to be "PoE-Compatible" or "Active Ethernet Compatible".

Devices that are not "PoE Compatible" can be converted to Power-over-Ethernet by way of a DC "Picker" or "Tap". These are sometimes called Active Ethernet "Splitters". This device picks-off the DC Voltage that has been injected into the CAT5 cable by the Injector and makes it available to the equipment through the regular DC power jack.

Therefore in order to use Power-over-Ethernet (PoE) you need either:

1. (Injector) + (PoE compatible device)
2. (Injector) + (non-PoE compatible device) + (Picker)

The following figure shows a wireless LAN access point (non-PoE compatible) which is powered by a PoE splitter (picker).

Wireless LAN Access Point, Powered by a Power over Ethernet Splitter

>> Power over Ethernet Standard

The original IEEE 802.3af-2003 PoE standard provides up to 15.4W of DC power (minimum 44 V DC and 350 mA) to each device. Only 12.95W is assured to be available at the powered device as some power is dissipated in the cable.

The updated IEEE 802.at-2009 PoE standard also known as PoE+ or PoE Plus, provides up to 25.5W of power. Some vendors have announced products that claim to comply with the 802.3at standard and offer up to 51W of power over a single cable by utilizing all four pairs in the Cat5 cable.

Numerous non-standard schemes have been used prior to PoE standardization to provide power over Ethernet cabling. Some are still in active use.

>> Applications of Power over Ethernet (PoE)

Uses for Power over Ethernet include:

• Network routers
• A mini network switch installed in distant rooms, to support a small cluster of ports from one uplink cable. (These ports on the mini-switch do not themselves provide PoE.)
• Network webcams
• Network Intercom/Paging/Public address systems and hallway speaker amplifiers
• VoIP phones
• Wall clocks in rooms and hallways, with time set using Network Time Protocol
• Wireless access points
• Outdoor roof mounted radios with integrated antennas, 802.11 or 802.16 based wireless CPEs (customer premises equipment) used by wireless ISPs

>> Types of Picker / Taps

Two basic types of Pickers and Taps are available: Passive and Regulated.

A Passive Tap simply takes the voltage from the CAT5 cable and directs it to the equipment for direct connection. Therefore if 48 VDC is injected by the Injector then 48 VDC will be produced at the output of the Passive Tap.

A Regulated Tap takes the voltage on the CAT5 cable and converts it to another voltage. Several standard regulated voltages are available: 12VDC, 6 VDC, 5 VDC. This allows a wide variety of non-PoE equipment to be powered through the CAT5 cable.

>> Voltage and Pinout Standards

Although the IEEE has a PoE standard called IEEE 802.3af, different equipment vendors use different PoE voltages and CAT5 pin configurations to provide the DC power. Therefore it is important to select the appropriate PoE devices for each piece of equipment you plan to power through the CAT5 cable.

The IEEE has standardized on the use of 48 VDC as the Injected PoE voltage. The use of this higher voltage reduces the current flowing through the CAT5 cable and therefore increases the load and increases the CAT5 cable length limitations. Where the maximum cable length has not been a major consideration some vendors have chosen 24 VDC and even 12 VDC as their "injected" voltage.

>> Multi-Port Injectors

Several manufacturers offer Multi-Port Injectors including 6 and 12-port models. These models are less versatile since they are only used where many devices are to be powered through the CAT5 cable originating in a single wiring closet or from a single switch. They typically operate in exactly the same manner as their more popular single-port counterparts.

In the past, companies often had several cabling infrastructures because no single cabling system would support all of a company’s applications. Nowadays, a standardized cabling system is important not only for consumers but also for vendors and cabling installers. Vendors must clearly understand how to design and build products that will operate on a universal cabling system. Cable installers need to understand what products can be used, proper installation techniques and practices, and how to test installed systems.

In this tutorial, we will cover some of the important topics related to cabling standards.

1. Identify the key elements of the ANSI/TIA-568-C Commercial Building Telecommunications Cabling Standard
2. Identify other ANSI/TIA standards required to properly design the pathways and spaces and grounding of a cabling system
3. Identify key elements of the ISO/IEC 11801 Generic Cabling for Customer Premises Standard

>> Structured Cabling and Standardization

Typical business environments and requirements change quickly. Companies restructure and reorganize at alarming rates. In some companies, the average employee changes work locations once every two years. The data and voice cabling system had to support these reconfigurations quickly and easily.

Until the early 1990s, cabling systems were proprietary, vendor-specific, and lacking in flexibility. Cabling has changed a lot over the years. Cabling installations have evolved from proprietary systems to flexible, open solutions that can be used by many vendors and applications. This change is the result of the adaptation of standards-based, structured cabling systems. The driving force behind this acceptance is due not only to customers but also to the cooperation between many telecommunications vendors and international standards organizations.

A properly designed structured cabling system is based around components or wiring units. An example of a wiring unit is a story of an office building, as shown in the following figure. All the work locations on that floor are connected to a single wiring closet. All of the wiring units (stories of the office building) can be combined together using backbone cables as part of a larger system.

Note: A structured cabling system is not designed around any specific application but rather is designed to be generic. This permits many applications to take advantage of the cabling system.

The components used to design a structured cabling system should be based on a widely accepted specification and should allow many applications (analog voice, digital voice, 10Base-T, 100Base-TX, 16Mbps Token Ring, RS-232, etc.) to use the cabling system. The components should also adhere to certain performance specifications so that the installer or customer will know exactly what types of applications will be supported.

A number of documents are related to data cabling.

1. In the United States, the standard is ANSI/TIA-568-C, also known as the Commercial Building Telecommunications Cabling Standard. The ANSI/TIA-568-C standard is a specification adopted by ANSI (American National Standards Institute), but the ANSI portion of the document name is commonly left out.
2. In Europe, the predominant standard is the ISO/IEC 11801 Ed. 2 standard, also known as the International Standard on Information Technology Generic Cabling for Customer Premises.

These two documents are quite similar, although their terminology is different, and the ISO/IEC 11801 Ed. 2 standard permits an additional type of UTP cabling. Throughout much of the rest of the world, countries and specification organizations have adopted one of these standards as their own.

>> ANSI/TIA-568-C Cabling Standard

In the mid-1980s, consumers, contractors, vendors, and manufacturers became concerned about the lack of specifications relating to telecommunications cabling. Before then, all communications cabling was proprietary and often suited only to a single-purpose use. The Computer Communications Industry Association (CCIA) asked the EIA to develop a specification that would encourage structured, standardized cabling.

Under the guidance of the TIA TR-41 committee and associated subcommittees, the TIA and EIA in 1991 published the first version of the Commercial Building Telecommunications Cabling Standard, better known as ANSI/TIA/EIA-568 or sometimes simply as TIA/EIA-568.

> 1. ANSI/TIA-568-C Purpose and Scope

The ANSI/TIA/EIA-568 standard was developed and has evolved into its current form for several reasons:

• To establish a cabling specification that would support more than a single vendor application
• To provide direction of the design of telecommunications equipment and cabling products that are intended to serve commercial organizations
• To specify a cabling system generic enough to support both voice and data
• To establish technical and performance guidelines and provide guidelines for the planning and installation of structured cabling systems

The ANSI/TIA-568-C standard addresses the following:

• Subsystems of structured cabling
• Minimum requirements for telecommunications cabling
• Installation methods and practices
• Connector and pin assignments
• The life span of a telecommunications cabling system (which should exceed 10 years)
• Media types and performance specifications for horizontal and backbone cabling
• Connecting hardware performance specifications
• Recommended topology and distances
• The definitions of cabling elements (horizontal cable, cross-connects, telecommunication outlets, etc.)

The current configuration of ANSI/TIA-568-C subdivides the standard as follows:

• ANSI/TIA-568-C.0: Generic Telecommunications Cabling for Customer Premises
• ANSI/TIA-568-C.1: Commercial Building Telecommunications Cabling Standard
• ANSI/TIA-568-C.2: Balanced Twisted-Pair Telecommunications Cabling and Components Standard
• ANSI/TIA-568-C.3: Optical Fiber Cabling Components Standard

Note:

The ANSI/TIA-568-C standard contains two wiring patterns for use with UTP jacks and plugs. They indicate the order in which the wire conductors should be connected to the pins in modular jacks and plugs and are known as T568A and T568B. Do not confuse these with the documents ANSI/TIA/EIA-568-B and the previous version, ANSI/TIA/EIA-568-A. The wiring schemes are both covered in ANSI/TIA/EIA-568.

> 2. Subsystems of a Structured Cabling System

The ANSI/TIA-568-C.1 standard breaks structured cabling into six areas:

1. Horizontal cabling
2. Backbone cabling
3. Work area
4. Telecommunications rooms and enclosures
5. Equipment rooms
6. Entrance facility (building entrance)

1) Horizontal Cabling

Horizontal cabling, as specified by ANSI/TIA-568-C.1, is the cabling that extends from horizontal cross-connect, intermediate cross-connect, or main cross-connect to the work area and terminates in telecommunications outlets (information outlets or wall plates). Horizontal cabling includes the following:

• Cable from the patch panel to the work area
• Telecommunications outlets
• Cable terminations
• Cross-connections (where permitted)
• A maximum of one transition point
• Cross-connects in telecommunications rooms or enclosures

The following figure shows a typical horizontal-cabling infrastructure spanning out in a star topology from a telecommunications room. The horizontal cabling is typically connected into patch panels and switches/hubs in telecommunications rooms or enclosures. A telecommunications room is sometimes referred to as a telecommunications closet or wiring closet. A telecommunications enclosure is essentially a small assembly in the work area that contains the features found in a telecommunications room.

(A) Transition point

ANSI/TIA-568-C allows for one transition point in horizontal cabling.

The transition point is where one type of cable connects to another, such as where round cable connects to under-carpet cable. A transition point can also be a point where cabling is distributed out to modular furniture. Two types of transition points are recognized:

1. MUTOA – This acronym stands for multiuser telecommunications outlet assembly, which is an outlet that consolidates telecommunications jacks for many users into one area. Think of it as a patch panel located out in the office area instead of in a telecommunications room.
2. CP – CP stands for consolidation point, which is an intermediate interconnection scheme that allows horizontal cables that are part of the building pathways to extend to telecommunication outlets in open-office pathways such as those in modular furniture. The ISO/IEC 11801 refers to the CP as a transition point (TP).

If you plan to use modular furniture or movable partitions, check with the vendor of the furniture or partitions to see if it provides data-cabling pathways within its furniture. Then ask what type of interface it may provide or require for your existing cabling system. You will have to plan for connectivity to the furniture in your wiring scheme.

Application-specific components (baluns, repeaters) should not be installed as part of the horizontal-cabling system (inside the walls). These should be installed in the telecommunication rooms or work areas.

(B) Recognized Media

ANSI/TIA-568-C recognizes two types of media (cables) that can be used as horizontal cabling. More than one media type may be run to a single work-area telecommunications outlet; for example, a UTP cable can be used for voice, and a fiber-optic cable can be used for data. The maximum distance for horizontal cable from the telecommunications room to the telecommunications outlet is 90 meters (295’) regardless of the cable media used. Horizontal cables recognized by the ANSI/TIA-568-C standard are limited to the following:

1. Four-pair, 100 ohm, unshielded or shielded twisted-pair cabling: Category 5e, Category 6 or Category 6A (ANSI/TIA-568-C.2)
2. Two-fiber 62.5/125-micron or 50/125-micron optical fiber (or higher fiber count) multimode cabling (ANSI/TIA-568-C.3)
3. Two-fiber (or higher fiber count) optical fiber single-mode cabling (ANSI/TIA-568-C.3)

(C) Telecommunications Outlets

ANSI/TIA-568-C.1 specifies that each work area shall have a minimum of two information-outlet ports. Typically, one is used for voice and another for data.

The following figure shows a possible telecommunications outlet configuration. The outlets go by a number of names, including equipment outlets, information outlets, wall jacks, and wall plates. However, an information outlet is officially considered to be one jack on a telecommunications outlet; the telecommunications outlet is considered to be part of the horizontal-cabling system.

The information outlets wired for UTP should follow one of two conventions for wire-pair assignments or wiring patterns: T568A or T568B. They are nearly identical, except that pairs 2 and 3 are interchanged. Neither of the two is the correct choice, as long as the same convention is used at each end of a permanent link. It is best, of course, to always use the same convention throughout the cabling system. T568B used to be much more common in commercial installations, but T568A is now the recommended configuration. (T568A is the required configuration for residential installations, in accordance with ANSI/TIA-570-B.) The T568A configuration is partially compatible with an older wiring scheme called USOC, which was commonly used for voice systems.

Be consistent at both ends of the horizontal cable. When you purchase patch panels and jacks, you may be required to specify which pattern you are using, as the equipment may be color-coded to make installation of the wire pairs easier. However, most manufacturers now include options that allow either configuration to be punched down on the patch panel or jack.

The following figure shows the T568A and T568B pin-out assignments. The wire/pin assignments in this figure are designated by wire color. The standard wire colors are shown in the following table.

Although your application may not require all the pins in the information outlet, you should make sure that all wires are terminated to the appropriate pins if for no other reason than to ensure interoperability with future applications on the same media. The table below shows some common applications and the pins that they use, and clearly illustrates why all pairs should be terminated in order to make the structured-wiring installation application generic.

A good structured-wiring system will include documentation printed Tip and placed on each of the telecommunications outlets.

(D) Pair Numbers and Color Coding

The conductors in a UTP cable are twisted in pairs and color coded so that each pair of wires can be easily identified and quickly terminated to the appropriate pin on the connecting hardware (patch panels or telecommunication outlets). With four-pair UTP cables, each pair of wire is coded with two colors, the tip color and the ring color.

In a four-pair cable, the tip color of every pair is white. To keep the tip conductors associated with the correct ring conductors, often the tip conductor has bands in the color of the ring conductor. Such positive identification (PI) color coding is not necessary in some cases, such as with Category 5 and higher cables, because the intervals between twists in the pair are very close together, making separation unlikely.

You identify the conductors by their color codes, such as white-blue and blue. With premises (indoor) cables, it is common to read the tip color first (including its PI color), then the ring color. The table below lists the pair numbers, color codes, and pin assignments for T568A and T568B.

2) Backbone Cabling

The next subsystem of structured cabling is called backbone cabling. (Backbone cabling is also sometimes called vertical cabling, cross-connect cabling, riser cabling, or intercloset cabling.)

Backbone cabling is necessary to connect entrance facilities, equipment rooms, and telecommunications rooms and enclosures. Backbone cabling consists of not only the cables that connect the telecommunications rooms, equipment rooms, and building entrances but also the cross-connect cables, mechanical terminations, or patch cords used for backbone-to-backbone cross-connection.

cross-connect – A cross-connect is a facility or location within the cabling system that permits the termination of cable elements and their intereconnection or cross-connection by jumpers, termination blocks, and/or cables to another cabling element (another cable or patch panel).

(A) Basic Requirements for Backbone Cabling

Backbone cabling includes:

1. Cabling between equipment rooms and building entrance facilities
2. In a campus environment, cabling between buildings’ entrance facilities
3. Vertical connections between floors

ANSI/TIA-568-C.1 specifies additional design requirements for backbone cabling, some of which carry certain stipulations, as follows:

• Grounding should meet the requirements as defined in J-STD-607-A, the Commercial Building Grounding and Bonding Requirements for Telecommunications.
• The pathways and spaces to support backbone cabling shall be designed and installed in accordance with the requirements of TIA-569-B. Care must be taken when running backbone cables to avoid sources of EMI or radio frequency interference.
• No more than two hierarchical levels of cross-connects are allowed, and the topology of backbone cable will be a hierarchical star topology. Each horizontal cross-connect should be connected directly to a main cross-connect or to an intermediate cross-connect that then connects to a main cross-connect. No more than one cross-connect can exist between a main cross-connect and a horizontal cross-connect.
• Centralized optical fiber cabling is designed as an alternative to the optical cross-connection located in the telecommunications room or telecommunications enclosure when deploying recognized optical fiber to the work area from a centralized cross-connect.
• The length of the cord used to connect telecommunications equipment directly to the main or intermediate cross-connect should not exceed 30 meters (98’).
• Unlike horizontal cabling, backbone cabling lengths are dependent on the application and on the specific media chosen. (See ANSI/TIA-568-C.0 Annex D.) For optical fiber, this can be as high as 10,000 meters depending on the application! However, distances of ≤ 550 meters are more likely inside a building. This distance is for uninterrupted lengths of cable between the main cross-connect and intermediate or horizontal cross-connect.
• Bridge taps or splices are not allowed.
• Cables with more than four pairs may be used as long as they meet additional performance requirements such as for power-sum crosstalk. These requirements are specified in the standard. Currently, only Category 5e cables are allowed to have more than four pairs.

(B) Recognized Backbone Media

ANSI/TIA-568-C recognizes several types of media (cable) for backbone cabling. These media types can be used in combination as required by the installation. The application and the area being served will determine the quantity and number of pairs required. The maximum distances permitted depend on the application standard and are available in ANSI/TIA-568-C.0 Annex D. In general, the higher the speed, the shorter the distance. Also, optical fiber maximums can range from 220 to 10,000 meters depending on the media and application, whereas UTP is limited to 100 meters.

The distances for recognized media are dependent on the application and are shown in ANSI/TIA-568-C.0 Annex D. (Note: distances are the total cable length allowed between the main cross-connect and the horizontal cross-connect, allowing for one intermediate cross-connect.)

Note: Coaxial cabling is not recognized by the ANSI/TIA-568-C version of the standard.

3) Work Area

The work area is where the horizontal cable terminates at the wall outlet, also called the telecommunications outlet. In the work area, the users and telecommunications equipment connect to the structured-cabling infrastructure. The work area begins at the telecommunications area and includes components such as the following:

• Patch cables, modular cords, fiber jumpers, and adapter cables
• Adapters such as baluns and other devices that modify the signal or impedance of the cable (these devices must be external to the information outlet)
• Station equipment such as computers, telephones, fax machines, data terminals, and modems

The work area wiring should be simple and easy to manipulate. In today’s business environments, it is frequently necessary to move, add, or remove equipment. Consequently, the cabling system needs to be easily adaptable to these changes.

4) Telecommunications Rooms and Telecommunications Enclosures

The telecommunications rooms (along with equipment rooms, often referred to as wiring closets) and telecommunications enclosures are the location within a building where cabling components such as cross-connects and patch panels are located. These rooms or enclosures are where the horizontal structured cabling originates.

Horizontal cabling is terminated in patch panels or termination blocks and then uses horizontal pathways to reach work areas. The telecommunications room or enclosure may also contain networking equipment such as LAN hubs, switches, routers, and repeaters. Backbone-cabling equipment rooms terminate in the telecommunications room or enclosure. The figure above illustrates the relationship of a telecommunications room to the backbone cabling and equipment rooms.

A telecommunications enclosure is intended to serve a smaller floor area than a telecommunications room.

TIA’s Fiber Optics LAN Section (www.fols.org) has compared the cost differences between network cabling systems using either telecommunications rooms or telecommunications enclosures on each floor of a commercial building and has found as much as 30 percent savings when using multiple telecommunications enclosures.

TIA-569-B discusses telecommunications room design and specifications. TIA-569-B recommends that telecommunications rooms be stacked vertically between one floor and another. ANSI/TIA-568-C further dictates the following specifications relating to telecommunications rooms:

• Care must be taken to avoid cable stress, tight bends, staples, cable wrapped too tightly, and excessive tension. You can avoid these pitfalls with good cable-management techniques.
• Use only connecting hardware that is in compliance with the specifications you want to achieve.
• Horizontal cabling should terminate directly not to an application-specific device but rather to a telecommunications outlet. Patch cables or equipment cords should be used to connect the device to the cabling. For example, horizontal cabling should never come directly out of the wall and plug in to a phone or network adapter.

5) Entrance Facility

The entrance facility (building entrance) as defined by ANSI/TIA-568-C.1 specifies the point in the building where cabling interfaces with the outside world. All external cabling (campus backbone, inter-building, antennae pathways, and telecommunications provider) should enter the building and terminate in a single point.

Telecommunications carriers are usually required to terminate within 50’ of a building entrance. The physical requirements of the interface equipment are defined in TIA-569-B, the Commercial Building Standard for Telecommunications Pathways and Spaces. The specification covers telecommunications room design and cable pathways.

TIA-569-B recommends a dedicated entrance facility for buildings with more than 20,000 usable square feet. If the building has more than 70,000 usable square feet, TIA-569-B requires a dedicated, locked room with plywood termination fields on two walls. The TIA-569-B standard also specifies recommendations for the amount of plywood termination fields, based on the building’s square footage.

Demarcation Point – The demarcation point (also called the demarc, pronounced dee-mark) is the point within a facility, property, or campus where a circuit provided by an outside vendor, such as the phone company, terminates. Past this point, the customer provides the equipment and cabling. Maintenance and operation of equipment past the demarc is the customer’s responsibility.

The entrance facility may share space with the equipment room, if necessary or possible. Telephone companies often refer to the entrance facility as the demarcation point. Some entrance facilities also house telephone or PBX (private branch exchange) equipment. The following figure shows an example of an entrance facility.

6) Equipment Room

The next subsystem of structured cabling defined by ANSI/TIA-568-C.1 is the equipment room, which is a centralized space specified to house more sophisticated equipment than the entrance facility or the telecommunications rooms. Often, telephone equipment or data networking equipment such as routers, switches, and hubs are located there. Computer equipment may possibly be stored there. Backbone cabling is specified to terminate in the equipment room.

In smaller organizations, it is desirable to have the equipment room located in the same area as the computer room, which houses network servers and possibly phone equipment. The following figure shows the equipment room. For information on the proper design of an equipment room, refer to TIA-569-B.

> 3. Media and Connecting Hardware Performance

ANSI/TIA-568-C specifies performance requirements for twisted-pair cabling and fiber-optic cabling. Further, specifications are laid out for length of cable and conductor types for horizontal, backbone, and patch cables.

1) 100 Ohm Unshielded Twisted-Pair Cabling

ANSI/TIA-568-C.2 recognizes four categories of UTP cable to be used with structured cabling systems. These UTP cables are specified to have a characteristic impedance of 100 ohms, plus or minus 15 percent, from 1MHz up to the maximum bandwidth supported by the cable. They are commonly referred to by their category number and are rated based on the maximum frequency bandwidth. The categories are found in the following table.

Ensuring a Specific Level of Cabling Performance

UTP cabling systems cannot be considered Category 3–, 5e–, 6–, or 6A–compliant (and consequently certified) unless all components of the cabling system satisfy the specific performance requirements of the particular category.

The components include the following:

• All backbone and horizontal cabling
• Telecommunications outlets
• Patch panels
• Cross-connect wires and cross-connect blocks

All patch panel terminations, wall-plate terminations, crimping, and cross-connect punch-downs also must follow the specific recommendations for the respective category. In other words, a network link will perform only as well as the lowest category-compliant component in the link.

Connecting Hardware: Performance Loss

Part of the ANSI/TIA-568-C.2 standard is intended to ensure that connecting hardware (crossconnects, patch panels, patch cables, telecommunications outlets, and connectors) does not have an adverse effect on attenuation and NEXT. To this end, the standard specifies requirements for connecting hardware to ensure compatibility with cables.

Patch Cables and Cross-Connect Jumpers

ANSI/TIA-568-C.1 also specifies requirements that apply to cables used for patch cables and cross-connect jumpers. The requirements include recommendations for maximum-distance limitations for patch cables and cross-connects, as shown here:

The total maximum distance of the channel should not exceed the maximum distance recommended for the application being used. For example, the channel distance for 100Base-TX Ethernet should not exceed 100 meters.

Note: Patch cables should use stranded conductors rather than solid conductors Tip so that the cable is more flexible. Solid conductor cables are easily damaged if they are bent too tightly or too often.

Patch cables usually have a slightly higher attenuation than horizontal cables because they are stranded rather than solid conductors. Though stranded conductors increase patch cable flexibility, they also increase attenuation.

Detailed requirements for copper cabling and connectivity components are found in ANSI/TIA 568-C.2. Fiber-optic cabling and connectivity components are contained in ANSI/TIA 568-C.3. We highly recommend that you familiarize yourself with cabling requirements if you need to specify performance to a cabling contractor. You should only have to reference the standard for purposes of the Request for Quotation, but your knowledge will help in your discussions with the contractor.

2) Fiber Optic Cabling

The ANSI/TIA-568-C standard permits both single-mode and multimode fiber-optic cables.

Two connectors were formerly widely used with fiber-optic cabling systems: the ST and SC connectors. Many installations have employed the ST connector type, but the standard now recognizes only the 568SC-type connector. This was changed so that the fiber-optic specifications in ANSI/TIA-568-C.3 could agree with the ISO 11801 standard used in Europe.

The ANSI/TIA-568-C.3 standard also recognizes small-form-factor connectors such as the MT-RJ and LC connectors as well as array connectors such as MPO connectors.

What Are Fiber Modes

Fiber-optic cable is referred to as either single-mode or multimode fiber. The term mode refers to the number of independent subcomponents of light that propagate through distinct areas of the fiber-optic cable core. Single-mode fiber-optic cable uses only a single mode of light to propagate through the fiber cable, whereas multimode fiber allows multiple modes of light to propagate.

Multimode Optical Fiber Cable

Multimode optical fiber is most often used as backbone cable inside a building and for horizontal cable. Multimode cable permits multiple modes of light to propagate through the cable and thus lowers cable distances and has a lower available bandwidth. Devices that use multimode fiber-optic cable typically use light-emitting diodes (LEDs) to generate the light that travels through the cable; however, higher-bandwidth network devices such as Gigabit Ethernet are now using lasers with multimode fiber-optic cable. ANSI/TIA-568-C.3 recognizes two types of multimode optical fiber cable:

1. Two-fiber (duplex) 62.5/125-micron (aka OM1 per ISO 11801)
2. 50/125-micron multimode fiber-optic cable

Within the 50/125-micron multimode fiber-optic classification, there are two options:

1. A standard 50-micron fiber (aka OM2 per ISO 11801)
2. A higher bandwidth option known as 850nm laser-optimized 50/125-micron (aka OM3)

ANSI/TIA-568-C.3 recommends the use of 850nm laser-optimized 50/125-micron (OM3) since it has much higher bandwidth and supports all Gigabit Ethernet applications to the longest distances.

The same connectors and transmission electronics are used on both 62.5/125-micron and 50/125-micron multimode fiber-optic cable. Since multimode fiber has a large core diameter, the connectors and transmitters do not need the same level of precision required with single-mode connectors and transmitters. As a result, they are less expensive than single-mode parts.

Single-Mode Optical-Fiber Cable

Single-mode optical-fiber cable is commonly used as backbone cabling outside the building and is also usually the cable type for long-distance phone systems. Light travels through single mode fiber-optic cable using only a single mode, meaning it travels straight down the fiber and does not “bounce” off the cable walls. Because only a single mode of light travels through the cable, single-mode fiber-optic cable supports higher bandwidth and longer distances than multimode fiber-optic cable.

Devices that use single-mode fiber-optic cable typically use lasers to generate the light that travels through the cable. Since the core size of single-mode cable is much smaller than multimode fiber, the connecting hardware and especially the lasers are much more expensive than those used for multimode fiber. As a result, single-mode based systems (cable plus electronics) are more costly than multimode systems.

ANSI/TIA-568-C.3 recognizes OSI and OS2 single-mode optical fiber cables.

Optical Fiber and Telecommunications Rooms

The ANSI/TIA-568-C standard specifies that certain features of telecommunications must be adhered to in order for the installation to be specifications-compliant:

• The telecommunications outlet(s) must have the ability to terminate a minimum of two fibers into 568SC couplings.
• To prevent damage to the fiber, the telecommunications outlet(s) must provide a means of securing fiber and maintaining a minimum bend radius of 30 millimeters.
• The telecommunications outlet(s) must be able to store at least one meter of two-fiber (duplex) cable.
• The telecommunications outlet(s) supporting fiber cable must be a surface-mount box that attaches on top of a standard 4” × 4” electrical box.

>> What is AWG?

A remarkable device that has been made using several planar-waveguide technologies and has found a variety of applications in WDM lightwave systems is the arrayed-waveguide grating, or AWG.

Arrayed-waveguide gratings (AWG) are based on the principles of diffractions. An AWG device is sometimes called an optical waveguide, a waveguide grating router, a phase array, or a phasar. An AWG device consists of an array of curved-channel waveguides with a fixed difference in the length of optical path between the adjacent channels.

An arrayed waveguide grating (AWG) is a generalization of the Mach-Zehnder interferometer. This device is illustrated in the following figure.

It combines two NxM star couplers through an array of M waveguides whose lengths are chosen in such a way that the length difference δl between any two neighboring waveguides is constant. As a result, the phase difference between two neighboring waveguides is also constant as an input signal propagates through it.

The Mach-Zehnder interferometer can be viewed as a device where two copies of the same signal, but shifted in phase by different amounts, are added together. The AWG is a device where several copies of the same signal, but shifted in phase by different amounts, are added together. It is this constant phase difference that creates the grating-like behavior.

When light enters the input cavity, it is diffracted and enters the waveguide array. There the optical path difference of each waveguide creates phase delays in the output cavity, where an array of fibers is coupled. The process results in different wavelengths having constructive interference at different locations, where the output ports are aligned.

>> How does AWG work?

The wavelength dependence of an AWG can be understood in simple physical terms as follows. Consider a WDM signal consisting of multiple channels at different wavelengths with a constant channel spacing Δν. When this signal is launched into one of the input waveguides, the first star coupler splits its power into many parts and directs them into the waveguides forming the grating. At the output end of the grating array, the wavefront is tilted because of linearly varying phase shifts in waveguides of different lengths. The tilt is wavelength-dependent and it forces each channel to focus on a different output waveguide of the second coupler. This behavior is similar to a bulk grating that also directs different wavelengths to different locations.

To fully understand the principles of operation. Let’s consider the AWG shown above. Let the number of inputs and outputs of the AWG be denoted by n. Let the couplers at the input and output be n × m and m × n in size, respectively. Thus the couplers are interconnected by m waveguides. We will call these waveguides arrayed waveguides to distinguish them from the input and output waveguides. The lengths of these waveguides are chosen such that the difference in length between consecutive waveguides is a constant denoted by L.

The MZI is a special case of the AWG, where n = m = 2. We will now determine which wavelengths will be transmitted from a given input to a given output. The first coupler splits the signal into m parts. The relative phases of these parts are determined by the distances traveled in the coupler from the input waveguides to the arrayed waveguides. Denote the differences in the distances traveled (relative to any one of the input waveguides and any one of the arrayed waveguides) between input waveguide i and arrayed waveguide k by d_ikⁱⁿ.

Assume that arrayed waveguide k has a path length larger than arrayed waveguide k −1 by ΔL. Similarly, denote the differences in the distances traveled (relative to any one of the arrayed waveguides and any one of the output waveguides) between arrayed waveguide k and output waveguide j by d_kj^out
. Then, the relative phases of the signals from input i to output j traversing the m different paths between them are given by

Here, n₁ is the refractive index in the input and output directional couplers, and n₂ is the refractive index in the arrayed waveguides. From input i, those wavelengths λ, for which φ_ijk , k = 1, . . . ,m, differ by a multiple of 2π will add in phase at output j . The question is, Are there any such wavelengths?

If the input and output couplers are designed such that and , then the above equation can be written as

Such a construction is possible and is called the Rowland circle construction. It is illustrated in the following figure. Thus wavelengths λ that are present at input i and that satisfy   for some integer p add in phase at output j.

The Rowland circle construction for the couplers used in the AWG. The arrayed waveguides are located on the arc of a circle, called the grating circle, whose center is at the end of the central input (output) waveguide.

Let the radius of this circle be denoted by R. The other input (output) waveguides are located on the arc of a circle whose diameter is equal to R; this circle is called the Rowland circle. The vertical spacing between the arrayed waveguides is chosen to be constant.

For use as a demultiplexer, all the wavelengths are present at the same input, say, input i. Therefore, if the wavelengths, λ₁, λ₂, . . . ,λ_n in the WDM system satisfy for some integer p, we infer from the previous equation that these wavelengths are demultiplexed by the AWG.

Note that though δ_iⁱⁿ and L are necessary to define the precise set of wavelengths that are demultiplexed, the (minimum) spacing between them is independent of δ_iⁱⁿ and L, and determined primarily by δ_j^out .

>> AWG Applications

For most arrayed-waveguide gratings, the diffraction orders are very large. This is an advantage of arrayed-waveguide gratings over conventional gratings that typically operate with low diffraction orders. The wavelength resolution of AWG varies inversely with mN. Since arrayed-waveguide gratings can resolve small wavelength differences, they are used extensively in WDM communications.

The above figure shows schematically the used of 4 × 4 AWG devices as multiplexers, demultiplexers, drop/add multiplexers, and full interconnections.

In Figure (a), signals having four different wavelengths and impinging upon the four input ports are combined and “multiplexed” in an output port. In a demultiplexer, Figure (b), an input signal containing four wavelengths λ₁, λ₂, λ₃ and λ₄ is sorted and routed to ports 1, 2, 3, and 4, respectively. In a drop–add multiplexer [Figure (c)], information contained in a light beam of wavelength λ₂, for example, is dropped and replaced by new and different data before the beam exiting from the output port. In a full interconnect [Figure (d)], a signal arriving at input port 1 with different spectral components is distributed to the output ports according to the signal wavelengths. A signal of wavelength λ₁ goes to output port 1, wavelength λ₂ to output port 2, and so forth. For signals impinging upon input port 2 with wavelengths λ₁, λ₂, λ₃, and λ₄ going to output ports 2, 3, 4, and 1, respectively. In short, arrayed-waveguide gratings can perform many functions and are capable of resolving fine wavelength differences. As a result, they find applications in many WDM communications.

Since the path lengths of different grating elements are different, and the difference are defined and determined lithographically. Arrayed-waveguide gratings are also useful in generating and shaping femtosecond pulses.

>> More Discussion

The AWG has several uses. It can be used as an n × 1 wavelength multiplexer. In this capacity, it is an n-input, 1-output device where the n inputs are signals at different wavelengths that are combined onto the single output. The inverse of this function, namely, 1 × n wavelength demultiplexing, can also be performed using an AWG. Although these wavelength multiplexers and demultiplexers can also be built using MZIs interconnected in a suitable fashion, it is preferable to use an AWG. Relative to an MZI chain, an AWG has lower loss and flatter passband, and is easier to realize on an integrated-optic substrate. The input and output waveguides, the multiport couplers, and the arrayed waveguides are all fabricated on a single substrate. The substrate material is usually silicon, and the waveguides are silica, Ge-doped silica, or SiO2-Ta2O5. Thirty-two–channel AWGs are commercially available, and smaller AWGs are being used in WDM transmission systems. Their temperature coefficient (0.01 nm/◦C) is not as low as those of some other competing technologies such as fiber gratings and multilayer thin-film filters. So we will need to use active temperature control for these devices.

Another way to understand the working of the AWG as a demultiplexer is to think of the multiport couplers as lenses and the array of waveguides as a prism. The input coupler collimates the light from an input waveguide to the array of waveguides. The array of waveguides acts like a prism, providing a wavelength-dependent phase shift, and the output coupler focuses different wavelengths on different output waveguides.

The AWG can also be used as a static wavelength crossconnect. However, this wavelength crossconnect is not capable of achieving an arbitrary routing pattern. Although several interconnection patterns can be achieved by a suitable choice of the wavelengths and the FSR of the device, the most useful one is illustrated in the following figure. This figure shows a 4 × 4 static wavelength crossconnect using four wavelengths with one wavelength routed from each of the inputs to each of the outputs.

In order to achieve this interconnection pattern, the operating wavelengths and the FSR(Free Spectral Range) of the AWG must be chosen suitably.

>> Background History of Optical Circulator

An optical circulator is a multi-port (minimum three ports) nonreciprocal passive component.

The function of an optical circulator is similar to that of a microwave circulator—to transmit a lightwave from one port to the next sequential port with a maximum intensity, but at the same time to block any light transmission from one port to the previous port. Optical circulators are based on the nonreciprocal polarization rotation of the Faraday effect.

Starting from the 1990s optical circulators has become one of the indispensable elements in advanced optical communication systems, especially WDM systems. The applications of the optical circulator expanded within the telecommunications industry (together with erbium-doped fiber amplifiers and fiber Bragg gratings), but also expanded into the medical and imaging fields.

>> Background Technology

Since optical circulators are based on several components, including Faraday rotator, birefringent crystal, waveplate, and beam displacer, we will have to explain these technologies before jumping into the detail of circulator.

1. Faraday Effect

The Faraday effect is a magneto-optic effect discovered by Michael Faraday in 1845. It is a phenomenon in which the polarization plane of an electromagnetic (light) wave is rotated in a material under a magnetic field applied parallel to the propagation direction of the lightwave. A unique feature of the Faraday effect is that the direction of the rotation is independent of the propagation direction of the light, that is, the rotation is nonreciprocal. The angle of the rotation θ is a function of the type of Faraday material, the magnetic field strength, and the length of the Faraday material, and can be expressed as

θ = VBL

where V is the Verdet constant of a Faraday material, B the magnetic field strength parallel to the propagation direction of the lightwave, and L the length of the Faraday material.

The Verdet constant is a measure of the strength of the Faraday effect in a particular material, and a large Verdet constant indicates that the material has a strong Faraday effect. The Verdet constant normally varies with wavelength and temperature. Therefore, an optical circulator is typically only functional within a specific wavelength band and its performance typically varies with temperature. Depending on the operating wavelength range, different Faraday materials are used in the optical circulator.

Rare-earth-doped glasses and garnet crystals are the common Faraday materials used in optical circulators for optical communication applications due to their large Verdet constant at 1310 nm and 1550 nm wavelength windows. Yttrium Iron Garnet and Bismuth-substituted Iron Garnets are the most common materials.

The Verdet constant of the BIG is typically more than 5 times larger the YIG, so a compact device can be made using the BIG crystals. All these materials usually need an external magnet to be functional as a Faraday rotator. Recently, however, a pre-magnetized garnet (also call latching garnet) crystal has been developed that eliminates the use of an external magnet, providing further potential benefit in reducing overall size.

Faraday rotators in optical circulators are mostly used under a saturated magnetic field, and the rotation angle increases almost linearly with the thickness of the rotator in a given wavelength (typically 40 nm) range. The temperature and wavelength dependence of the Faraday rotation angle of the typical BIG crystals at wavelength of 1550 nm is 0.04-0.07 deg/°C and 0.04-0.06 deg/nm, respectively.

2. Light Propagation in Birefringent Crystals

Another common material used in the construction of optical circulators is the birefringent crystal. Birefringent crystals used in optical circulators are typically anisotropic uniaxial crystals (having two refractive indices with one optical axis). In an anisotropic medium, the phase velocity of the light depends on the direction of the propagation in the medium and the polarization state of the light. Therefore, depending on the polarization state of the light beam and the relative orientation of the crystal, the polarization of the beam can be changed or the beam can be split into two beams with orthogonal polarization states.

The refractive index ellipsoid for a uniaxial crystal is shown in the above figure. When the direction of the propagation is along the z-axis (optic axis), the intersection of the plane through the origin and normal to the propagation direction S_o is a circle; therefore, the refractive index is a constant and independent of the polarization of the light. When the direction of the propagation S forms an angle θ with the optic axis, the intersection of the plane through the origin and normal to S becomes an ellipse. In this case, for the light with the polarization direction perpendicular to the plane defined by the optic axis and S, the refractive index, is called the ordinary refractive index n_o, is given by the radius r_o and independent of the angle θ. This light is called ordinary ray and it propagates in the birefringent material as if in an isotropic medium and follows the Snell’s law at the boundary.

On the other hand, for light with the polarization direction along the plane defined by the optic axis and S, the refractive index is determined by the radius r_e and varies with the angle θ. This light is called the extraordinary ray and the corresponding refractive index is called the extraordinary refractive index n_e. In this case n_e is a function of θ and can be expressed as

The n_e varies from n_o to n_e depending on the direction of propagation. A birefringent crystal with n_o < n_e is called a positive crystal, and one with n_o > n_e is called a negative crystal.

Therefore, the function of a birefringent crystal depends on its optic axis orientation (crystal cutting) and the direction of the propagation of a light. Birefringent crystals commonly used in optical circulators are quartz, rutile, calcite, and YVO4.

3. Waveplates

One of the applications of the birefringent crystal is the waveplate (also called retardation plate). A waveplate can be made by cutting a birefringent crystal to a particular orientation such that the optic axis of the crystal is in the incident plane and is parallel to the crystal boundary (zx-plane in the second figure). When a plane wave is perpendicularly incident onto the incident plane (zx-plane), the refractive index for the polarization component parallel to the x-axis equals n_o and that parallel to the z-axis equals n_e.

Therefore, when a linearly polarized light with the polarization direction parallel to the z- or x-axis is incident to the waveplate, the light beam experiences no effect of the waveplate except for the propagation time delay due to the refractive index. However, when the polarization direction of the incident light is at an angle to the optic axis, the components parallel to the x- and z-axes travel at difference velocities due to the refractive index difference. Therefore, after passing through the waveplate, a phase difference exists between these two components, and the resulting polarization of the output beam depends on the phase difference. The phase difference can be expressed as

where δ is the wavelength of the light, Δn the refractive index difference between the ordinary and extraordinary refractive indices, and t the thickness of the crystal.

When the thickness of the crystal is selected such that the phase difference equals to m • (π/2) (quarter of the wave), the waveplate is called a quarter-waveplate, and similarly the phase difference in a half-waveplate is m • π (where m is called the order of the waveplate, and is an integer and odd number).

The quarter-waveplate is best known for converting a linearly polarized light into a circularly polarized light or vice versa, when a light beam is passed through the quarter-waveplate with the polarization direction at 45° to the optic axis (see the following figure). The half-waveplate is used most frequently to rotate the polarization direction of a linearly polarized light.

When a linearly polarized light beam is launched into a half-waveplate with an angle θ against the optic axis of the waveplate, the polarization direction of the output beam is rotated and the rotation angle equals to 2θ (see the above figure (b)). Crystal quartz is widely used for making waveplates, due to its small birefringence.

4. Beam Displacer

Another commonly used form of the birefringent crystal is the beam displacer, which is used to split an incoming beam into two beams with orthogonal polarization states, the intensity of each beam dependent on the polarization direction of the incoming beam.

The birefringent crystal-based beam displacer is made by cutting a birefringent crystal in a specific orientation such that the optic axis of the crystal is in a plane parallel to the propagation direction and having an angle α to the propagation direction (see the above figure). The separation d between the two output beams depends on the thickness of the crystal and the angle between the optic axis and the propagation direction, and can be expressed as

where t is the thickness of the crystal.

The optic axis angle to yield a maximum separation is given as

Rutile, calcite, and YVO4 are common birefringent materials for the beam displacer due to their large birefringence (Δn of more than 0.2 at 1550 nm wavelength). For rutile crystal, the n_e and n_o at a wavelength of 1550 nm are 2.709 and 2.453, respectively, resulting in the α_max of 47.8°. For YV04 crystal, the n_e and n_o at a wavelength of 1550nm are 2.149 and 1.945, respectively. Calcite is rarely used in optical circulators due to its softness and instability in a damp heat environment.

>> How Optical Circulator Works

Optical circulators can be divided into two categories.

1. polarization-dependent optical circulator, which is only functional for a light with a particular polarization state. The polarization-dependent circulators are only used in limited applications such as free-space communications between satellites, and optical sensing.
2. polarization-independent optical circulator, which is functional independent of the polarization state of a light. It is known that the state of polarization of a light is not maintained and varies during the propagation in a standard optical fiber due to the birefringence caused by the imperfection of the fiber. Therefore, the majority of optical circulators used in fiber optic communication systems are designed for polarization-independent operation.

Optical circulators can be divided into two groups based on their functionality.

1. Full circulator, in which light passes through all ports in a complete circle (i.e., light from the last port is transmitted back to the first port). In the case of a full three-port circulator, light passes through from port 1 to port 2, port 2 to port 3, and port 3 back to port 1.
2. Quasi-circulator, in which light passes through all ports sequentially but light from the last port is lost and cannot be transmitted back to the first port. In a quasi-three-port circulator, light passes through from port 1 to port 2 and port 2 to port 3, but any light from port 3 is lost and cannot be propagated back to port 1. In most applications only a quasi-circulator is required.

The operation of optical circulators is based on two main principles.

1. Polarization splitting and recombining together with nonreciprocal polarization rotation.
2. Asymmetric field conversion with nonreciprocal phase shift.

We will explain both designs in detail.

1. Nonreciprocal Polarization Rotation-based Circulators

A. Early Development

Dielectric coatings-based polarization beam splitters were used to construct optical circulators in the early stage of circulator development. A schematic diagram of a 4-port circulator is shown in the figure above, where two dielectric coating-based polarization beam splitter cubes were used to split the incoming beam into two beams with orthogonal polarization.

In operation, a light beam launched into port 1 is split into two beams by the polarization beam splitter that transmits the light with horizontal polarization (along the y-axis) and reflects the light with vertical polarization (along the x-axis). The two beams are then passed through a half-waveplate and a Faraday rotator. The optic axis of the half-waveplate is arranged at 22.5° to the x-axis so that the vertically polarized light is rotated by +45°. The thickness of the Faraday rotator is selected for providing 45°-polarization rotation and the rotation direction is selected to be counter-clockwise when light propagates along the z-axis direction.

Therefore, the polarization of the two beams is unchanged after passing through the half-waveplate and Faraday rotator because the polarization rotation introduced by the half-waveplate (+45°) is cancelled by that of the Faraday rotator (-45°). The two beams are recombined by the second polarization splitter  and coupled into port 2.

Similarly, when a light beam is launched into port 2, it is split into two beams with orthogonal polarization by the second polarization beam splitter. Due to the non-reciprocal rotation of the Faraday rotator, in this direction the polarization rotations introduced by both the half-waveplate and Faraday rotator are in the same direction, resulting in a total rotation of 90°. Therefore, the two beams are combined by the first polarization splitter in a direction orthogonal to port 1 and coupled into port 3. The operation from port 3 to port 4 is the same as that from port 1 to port 2.

B. Current Development

However, the isolation of this type of optical circulator was relatively low due to limited extinction ratio (around 20 dB) of the polarization beam splitters. Various designs using birefringent crystals have been proposed to increase the isolation by utilizing the high extinction ratio property of the crystal.

One of the designs is shown in the figure below (a), where birefringent beam displacers are used for splitting and combining of the orthogonally polarized light beams. As shown in the figure blow (b), where each circle indicates the beam position and the arrow inside the circle indicates the polarization direction of the beam.

A light beam launched into port 1 is split into two beams with orthogonal polarization states along the y-axis. Two half-waveplates, one (upper) with its optic axis oriented at 22.5° and the other (lower) at -22.5°, are used to rotate the two beams so that their polarization direction becomes the same. The Faraday rotator rotates the polarization of both beams 45° counter-clockwise, and the two beams are vertically polarized (along the y-axis) and passed through the second birefringent crystal without any spatial position change because the polarization directions of the two beams match the ordinary ray direction of the crystal. After passing through another Faraday rotator and half-waveplate set, the two beams are recombined by the third birefringent crystal, which is identical to the first one.

Similarly, a light beam launched into port 2 is split into two beams and passed through the half-waveplate set and the Faraday rotator. Due to the nonreciprocal rotation of the Faraday rotator, the two beams become horizontally polarized (along the x-axis), and are therefore spatially shifted along the x-axis by the second birefringent crystal because they match the extraordinary ray direction of the crystal. The two beams are recombined by the first crystal at a location different from port 1 after passing through the Faraday rotator and half-waveplates. The distance between port 1 and port 3 is determined by the length of the second birefringent crystal.

The use of the birefringent crystals generally results in an increase in size and cost of the circulator due to the cost of crystal fabrication. Extensive development efforts have been concentrated on improvement of various designs. Due to the performance advantages, currently all commercially deployed optical circulators are based on the use of birefringent crystals.

2. Asymmetric Field Conversion-based Circulators

An optical circulator can be constructed using two-beam interference with nonreciprocal phase shifting without the need for polarization beam splitting. One example of this kind of optical circulator is shown in the figure below, where a four-port circulator is constructed using two power splitters and nonreciprocal phase shifters.

In operation, a light beam launched into port 1 is split into two beams with equal intensity by the first power splitter. The two beams are then passed through two sets of phase-shifting elements (half-waveplate and Faraday rotator) that are selected such that they provide no phase shift between the two beams in one direction, but in the reverse direction a phase shift of π is introduced between the two beams. Therefore, from port 1 to port 2 the two beams are in phase and will be constructively recombined by the second splitter and coupled into port 2.

Similarly, a light beam launched into port 2 is split into two beams by the second splitter, and passed through the phase shifter set. Because a phase shift of π is introduced this time, the two beams are out of phase and no longer will be coupled into port 1 but will be coupled into port 3 due to the out-of-phase relation between port 1 and port 3.

The structure of this type of circulator is very simple and potentially could lead to lower cost. However, because phase information is used for the circulator function, control of the phase in each element and control of path length difference between beams are very critical for the performance.

Currently, circulators based on this principle have only been investigated in waveguide devices and no commercial products are available due to manufacturing challenges and performance disadvantages.

>> Newer Optical Circulator Designs to Reduce the Use of Materials and Size

Cost and stability have been the main limiting factors in expanding the applications of optical circulators. Recently, several designs have been developed in an effort to reduce the cost and realize high reliability. In the design shown in the second figure above, the circulator is used in a collimated beam and each port is collimated using a lens; therefore, relatively large size elements have to be used in order to construct the design due to the beam size. In recent designs, efforts have been concentrated on reducing the use of materials and size.

1. Circulator Design Using Diverging Beam

A compact low-cost circulator design has been proposed, placing optical elements in a diverging beam instead of in a collimating beam to reduce the overall use of expensive materials.

As shown in the figure below, in this design, all optical elements are placed in a diverging beam between the input/output ports and lenses. Two identical groups of elements are placed near the focal point of the lens, resulting in reduced size and manufacturing complexity. Each group of elements consists of two birefringent crystals, one Faraday rotator with 45° rotation angle, and two half-waveplates with their optic axes oriented in opposite directions (22.5° and -22.5°).

In operation, a light beam from port 1 is split into two orthogonally polarized beams in the y-axis by the first birefringent crystal. The two half-waveplates and the Faraday rotator are arranged such that after passing through the rotators the polarization directions of the two beams are the same and match the ordinary ray direction of the second birefringent crystal. Therefore, the two beams pass through the second birefringent crystal without any displacement. Two lenses are used for providing a one-to-one imaging system. Because the second group of the element is the same as the first one, the two beams are recombined and launched into port 2.

Similarly, a light beam launched into port 2 is split and passed through to the rotators. Due to the nonreciprocal rotation of the Faraday rotator, the polarization directions of the two beams are rotated matching the extraordinary ray direction of the second birefringent crystal. Therefore, the two beams are shifted a certain amount along the x-axis and shifted again the same amount by the second birefringent crystal in the other group. If the sum of beam shifting by the two birefringent crystals is designed such that it is the same as the distance between the first and third ports, the two beams will be recombined and coupled into port 3.

Because port 1 and port 3 share a single lens and the beam shifting is done at the diverging beam, the required beam shifting in this case is very small and typically equal to the fiber diameter of 125 μm. On the other hand, the required beam shifting in the design shown previously is determined by the diameter of a lens due to the use of collimated beams and is typically in the order of millimeters.

To further reduce the required thickness of the birefringent crystal, mode-field diameter of the input and output fiber is expanded to reduce the divergence angle of the beam. With this compact design, a circulator with a size of 5.5 mm in diameter and less than 60 mm in length has been developed, as shown in the figure below, compared to a typical size of over 25 mm in cross-section and over 90 mm in length for the design shown in previous figure.

2. Circulator Design Using Beam Deflection

A compact circulator using collimated beam deflection is also proposed and demonstrated. In the design, polarization-dependent angle deflection is used instead of the polarization-dependent position shift. As shown in the figure below, a single lens is used to collimate the light for both port 1 and 3 and all elements of the circulator are positioned in the collimated beam. The main difference is that a Wollaston prism is used in place of a birefringent beam displacer and a single lens is used for collimating two beams.

In operation, a light beam launched into port 1 is collimated and split into two beams with orthogonal polarization by the first birefringent crystal. The polarization directions of the two beams are rotated by the half-waveplates and Faraday rotator so that they become the same. Because port 1 is off-axis of the lens, the resulting collimated beam from the lens forms an angle θ to the propagation axis. This angle is corrected by the Wollaston prism and the two beams are propagated straight to the second Faraday rotator (solid lines). After passing through the half-waveplates and being recombined by the third birefringent crystal, the combined beam is focused by the second lens into port 2.

Similarly, light launched into port 2 is collimated and split into two beams with their polarization direction rotated. Due to the nonreciprocal rotation of the Faraday rotator, the two beams from port 2 are deflected to a direction opposite to the angle θ by the Wollaston prism (dotted lines). Therefore, after passing through the polarization rotators and the first birefringent crystal, the combined beam is focused by the first lens to a position different from that of port 1. The required deflecting angle of the Wollaston prism can be determined by the position distance between port 1 and port 3 and the focal length of the lens.

This design reduces the size of materials considerably. However, because the beam splitting and recombining is still performed in the collimated beam, it still requires relatively long crystals.

3. Reflective Optical Circulators

As shown in design examples described in this section, most optical circulators have a symmetric structure in terms of element materials and their relative positions. Therefore, a proposed design concept using imaging folding to redirect the light beam and reuse the common elements has advantages in reducing the overall device size and cost. A schematic diagram of one of the compact reflective circulator designs is shown in the figure below, where a single lens and a mirror are used to couple lights between all ports that are at the same side of the circulator. In this design, all elements are passed through twice to reduce the element account to half while maintaining the same performance as a conventional circulator.

In operation, a light beam launched into port 1 is split into two beams by the first birefringent crystal, and passed through the second crystal without any lateral position change, because the rotation angles of the polarization rotators (+45° or -45° rotation) are designed such that the polarization directions of the two beams match the ordinary ray direction of the second birefringent crystal. After being collimated by the lens and reflected by the mirror, the two beams are passed through the same elements again except for half-waveplates and recombined into port 2.

Similarly, a light beam launched into port 2 is split into two beams with orthogonal polarization directions. After passing through the polarization rotators, the polarization directions of both beams are aligned with the extraordinary ray direction of the second birefringent crystal due to the nonreciprocal rotation of the Faraday rotator, and the physical locations of the two beams are shifted after passing through the crystal. The two beams receive the location shift again after being reflected by the mirror and passed through the crystal. Therefore, after the proper polarization rotation the two beams are recombined at a location different from port 1 and will be coupled into port 3 if the distance between port 1 and port 3 matches two times the beam shift introduced by the second birefringent crystal. Multi-port circulators can be made by adding more ports into the design.

With the reflective design, the size and required optical elements can be significantly reduced, resulting in overall cost savings.

There are many variations in the circulator design, however, all nonreciprocal polarization rotation-based designs share a common structure with a minimum of three functional elements; polarization splitting and recombining elements, nonreciprocal polarization rotation elements, and polarization-dependent beam steering (angular or positional) elements.

>> Applications of Optical Circulators

Optical circulators were originally used in telecommunication systems for increasing transmission capacity of existing networks. By using optical circulators in a bi-directional transmission system, the transmission capacity of the network can be easily doubled without the need for deploying additional fibers, which has become increasingly expensive.

However, with the rapid advancement in optical communication technologies and the readily availability of low-cost and high-performance circulators, the applications of optical circulators have drastically expanded into not only the telecommunication industries but also the sensing and imaging fields. Optical circulators have become an especially important element in advanced optical networks such as DWDM networks.

In the traditional bi-directional optical communication system, a 50/50 (3 dB) coupler, which splits a light beam into two beams with equal intensity, was used to couple the transmitters and receivers as shown in the following figure (a). However, there are two main problems with this kind of structure. One is the need for an optical isolator in the transmitters to prevent light crosstalk between the transmitters, and the other is the high insertion loss associated with the use of the 50/50 coupler, because two couplers have to be used and each has a minimum loss of 3 dB, which results in a minimum 6 dB reduction of the link budget from the system.

The use of an optical circulator can solve both of the problems by providing the isolation function as well as a loss of less than 3 dB as shown in the following figure (b).

Optical circulators are powerful devices for extracting optical signals from a reflective device. Therefore, optical circulators are often used in conjunction with the fiber Bragg gratings that are typically reflective devices. Together with fiber Bragg gratings, optical circulators have become one of the indispensable elements in advanced DWDM optical networks. Circulators are used as MUX/DEMUX devices, but are also used with the fiber Bragg grating in dispersion compensation, tunable optical Add/Drop, and other applications.

Another application of the circulators is use with a mirror for double passing an optical element to increase efficiency. One example is the reflective erbium-doped fiber amplifier shown in the figure below. In operation, signal light is launched into port 1 of a circulator and passed through port 2 with minimum loss. The signal is combined with the pump light from a pump laser by a WDM coupler, and both lights are launched into an erbium-doped fiber. The amplified signal and residual pump lights are reflected by the mirror and passed through the erbium-doped fiber again so that the signal is amplified twice by the erbium-doped fiber, reducing the required length of the fiber, and the residual pump power is also re-used to increase the pump efficiency.

The idea has been adapted into different devices, such as replacing the coupler and erbium-doped fiber with a dispersion compensation fiber to reduce the required fiber length and adding a Faraday rotator between the mirror and fiber to reduce the polarization-induced effects. Bi-directional fiber amplifiers are also proposed for taking full advantage of the circulator.

With the development of advanced optical networks, applications of optical circulators are expanding rapidly and new functionality and applications are emerging quickly. For example, recently it has been reported that by adding wavelength-selective functions into circulators, a bi-directional wavelength-dependent circulator can be configured, which opens a new dimension of applications in advanced DWDM optical networks.

>> A Brief Introduction to Optical Amplifiers

Because fiber attenuation limits the reach of a nonamplified fiber span to approximately 200 km for bit rates in the gigabit-per-second range, wide area purely optical networks cannot exist without optical amplifiers.

Optical amplifiers are typically used in three different places in a fiber transmission link.

1. Power Amplifiers
   Power amplifiers serve to boost the power of the signal before it is launched on the line, extending the transmission distance before additional amplification is required.
2. Line Amplifiers
   Line amplifiers are located at strategic points along a long transmission link to restore a signal to its initial power level., thereby compensating for fiber attenuation.
3. Preamplifiers
   Preamplifiers raises the signal level at the input of an optical receiver, which serves to improve signal detection performance (i.e., the receiver sensitivity).

In each of the three cases, the desired properties are different. For power amplifiers, the important feature is high gain; preamplifiers require a low noise figure, and line amplifiers require both.

Optical amplifiers are also employed at various other points in a network (for example, within an optical switching node to compensate for losses in the switch fabric).

Semiconductor Optical Amplifiers (SOA) were developed in the 1980s but they never had a serious impact on long-distance transmission because of a number of negative features. In the case of fiber amplifiers, especially the EDFA (Erbium Doped Fiber Amplifier) and the RA (Raman Amplifier), however, the situation was quite different.

The first papers on EDFAs appeared in 1987. Within a few years of that time, 9000 km unrepeatered transmission was demonstrated. Shortly thereafter, soliton experiments showed that transmission distances could be extended almost indefinitely. All of these experiments used EDFAs. It is not an exaggeration to say that these devices have revolutionized optical communications.

Although the EDFA played a fundamental role in extending the reach of optical transmission systems it still had some drawbacks, including operation confined to a limited band of the optical spectrum and a nonflat gain profile. In contrast, Raman Amplifiers (RA), which were first demonstrated well before the EDFA and then virtually ignored for three decades, have more recently attracted renewed interest. This stems mainly from their ability to increase both the reach and the aggregate bit rate carried on a fiber; that is, the usable fiber bandwidth.

>> Erbium-Doped Fiber Amplifiers (EDFAs)

The EDFA belongs to a family of rare-earth-doped fiber amplifiers, the class of other possible dopants, including praseodymium (used for amplification in the 1300-nm range), neodymium (originally used for very high-power lasers), ytterbium (which has been used as a codopant with erbium), and thulium (amplifying in the S band). The important place of the EDFA in optical communications is due primarily to the fact that the properties of erbium produce amplification in a fairly wide band (approximately 35 nm) within the 1550 nm low-attenuation window in fibers. Furthermore, the EDFA has many other desirable features.

1. EDFA Module Structures

Three different EDFA structures are shown in the following figure.

In each of the above cases, the amplifier is of the travelling wave type, consisting of a strand of single-mode fiber, typically on the order of tens of meters long, doped with erbium. (The points S in the figure represent fiber splices.)

The EDFA is an optically pumped device, so energy is supplied by an optical source (Laser Diode), which injects power into the doped fiber at a wavelength matched to the characteristics of erbium (980 or 1480 nm). Pumping can be forward, backward, or bidirectional. The pump is typically coupled into the transmission fiber via a wavelength-selective coupler (WSC). Amplifications occurs by transfer of power from the pump wave to the signal wave as it propagates down the doped fiber.

Note that EDFA modules used in the field typically include other components, such as optical isolators to eliminate reflected power, and various devices for signal power monitoring, stabilization, and control.

2. EDFA Three-Energy Level System

Like many other forms of amplifiers of electromagnetic radiation, the EDFA operates via a three-energy level system. The model representing this process is shown in the following figure.

Levels E₁, E₂, and E₃ are the ground, metastable, and pump levels, respectively. The populations (fractional densities) of erbium ions in the three energy levels are denoted N₁, N₂, and N₃, where N₁ > N₂ > N₃ when the system is in thermal equilibrium (no pump or signal present). When pump and signals are present, these populations change as ions move back and forth between levels, accompanied by the emission or absorption of photons at frequencies determined by the energy-level difference.

The wavelengths associated with the dominant transitions are indicated in the above figure. The wavelength λ for each transition is given by the quantum relation λ = hc /ΔE, where h is the Planck’s constant and ΔE is the difference in energy levels. In actuality, the three levels in the simplified diagram are narrow bands, so each transition is actually associated with a band of wavelengths rather than a single line.

Two pump wavelengths are typically used for EDFAs: 980 and 1480 nm. As shown in the above figure, by absorbing energy from a 980 nm pump, Er³⁺ ions in the ground state are raised to state E₃. The rate at which these transitions occur is proportional to N₁P_p, where P_p is the pump power. These excited ions decay spontaneously to the metastable state E₂, and this transition occurs at a rate much faster than the rate from level E₁ to level E₃. This means that in equilibrium under the action of the pump, the ion population in the ground state is reduced and accumulates largely in state E₂. This process is referred to as population inversion because we now have N₂ > N₁, the reverse of the situation in thermal equilibrium.

The transition rate from level E₂ to level E₁ is very slow compared with the other transitions, so that the lifetime τ, in the state E₂ (the reciprocal of its transition rate to E₁) is very long (approximately 10 ms). Similar pumping action can occur at 1480 nm, in which case the ions are raised directly to the upper edge of the E₂ band. reliable semiconductor laser pump sources have been developed for EDFAs at both the 980 and 1480 nm pump wavelengths.

The wavelength band for transitions from state E₂ to the ground state is in the 1530 nm range, making it ideal for amplification in the lowest attenuation window of fibers. The dominant transitions from E₂ to E₁ are radiative, which means that they are of two types: spontaneous emission and stimulated emission. 

In the case of spontaneous emission , an ion drops spontaneously to the ground state, resulting in the emission of a photon in the 1530 nm band, and this appears as additive noise. Spontaneous emission noise is an unavoidable by-product of the amplification process, predicted by quantum theory. Its phase, direction, and polarization are independent of the signal.

In the case of stimulated emission, an incident photon in the 1530 nm range stimulates the emission of another photon at the same wavelength in a coherent fashion (with the same direction, phase, and polarization). If the incident photon is from a signal, this produces the desired amplification of the optical field. However, the incident photon could also have originated as a spontaneous emission “upstream” on the fiber, in which case this is called amplified spontaneous emission (ASE), which represents the major source of noise in amplified fiber transmission systems.

3. Gain Profile of EDFA

The fairly large amplification bandwidth of the EDFA is due to the finite width of the energy bands. The width of the energy bands is caused by a number of physical phenomena, including the Stark effect, which splits the main energy levels in to many sublevels. Because the population is not distributed uniformly within the E₂ band, the gain is not flat.

A typical plot of gain as a function of wavelength is shown in the following figure.

The uneven gain profile, with a peak at approximately 1530 nm, produces significant problems in a multiwavelength system when many amplifiers are cascaded over a long transmission span. Not only does uneven gain amplify different wavelengths unequally, but it also causes a large accumulation of ASE at the peak of the gain profile, which can eventually saturate the amplifier.

Because amplifier cascading on long links accentuates these effects seriously, gain flattening is an important consideration in EDFAs. Several solutions to this problem are currently in use. One approach is to modify the design of the amplifier itself by using different materials such a fluoride glass. Other approaches use gain equalization via controllable attenuators or inverse filtering.

4. Gain Saturation

A. Small-Signal Gain

The gain of an EDFA is approximately independent of the signal power as long as the pump power is made high enough so that the pumping rate is much larger than the stimulated emission rate. This is called the unsaturated gain or small-signal regime. The small-signal gain under these conditions is an increasing function of pump power.

For a given fiber structure and doping, and a given pump power, these is an optimal fiber length that maximizes gain. For lengths smaller than the optimum, the pump power is not maximally utilized, and for larger lengths, pump power is exhausted somewhere along the fiber, and attenuation takes over. Typical optimal lengths are in the range of tens of meters. Maximum small-signal gains for EDFAs are typically 30 to 40 dB.

B. Gain Saturation

All amplifiers eventually exhibit gain saturation as the signal power is increases. In the saturated case, the signal extracts so much power from the pump as it propagates down the fiber that the stimulated emission rate becomes comparable with the pumping rate. The larger the input signal, and the higher the unsaturated gain, the sooner saturation is reached.

As saturation increases, the gain decreases. The saturation output power P_sat^out is defined as the output power at which the gain is compressed by 3 dB. The values of P_sat^out for typical EDFAs are in the hundreds of milliwatts. It should be noted that ASE also contributes to saturation in an EDFA . When input signals are very small, it is the ASE that saturates the amplifier first. This is known as amplifier self-saturation.

Because saturation is a nonlinear effect, it produces a number of complications when multiple signals are being amplified. One problem is that the saturated gain for any one signal depends on the aggregate power of the other signals as well as its own power. Thus signals (as well as accumulated ASE) tend to “steal” power each other. An advantageous effect of saturation is that a small amount of it in each amplifier in a cascade of several amplifiers tends to produce a self-regulating effect.

Several other nonlinear effects are a consequence of this power-stealing phenomenon but on a shorter time scale. The amplifier gain at any instant in time is a function of the excited state population N₂, which is depleted momentarily by stimulated emission when a signal is present. One manifestation of this occurs when an intensity-modulated digital signal changes from a 0 to a 1. The resultant fluctuation in N₂ causes corresponding gain fluctuations, which are most pronounced in the saturated regime and in the presence of large signals. Another manifestation occurs when beats from two signals spaced closely in optical frequency cause gain fluctuations at the beat (difference) frequency.

The gain fluctuations affect all signals being amplified and thus can potentially produce undesirable cross-talk, with one signal’s intensity fluctuations changing the gain for the others. These effects are significant only when the gain dynamics are such that gain can vary on a time scale as fast as that of the signal fluctuations. A simplified interpretation of gain dynamics in an EDFA is based on the assumption that the maximum speed for gain fluctuations is on the order of the reciprocal of the lifetime in the excited state, which is approximately 10 ms. However, actual gain transients in EDFAs can occur on time scales of hundreds of microseconds, which cannot be predicted using the lifetime alone.

In any case, these numbers indicate that signals fluctuating on time scales more rapid than, say, 100 us will cause no significant cross-talk in EDFAs. This corresponds to a minimum bit rate of approximately 10 Kbps to avoid cross-talk (or a WDM signal separation of approximately 10 KHz to avoid beat frequency effects). The lack of this cross-talk effect for bit rates higher than 10 Kbps is one of the important advantages of the EDFA over the SOA.

5. Noise and Noise Figure

The ASE noise generated in an EDFA can be the limiting performance factor in an optical transmission link. It is therefore important to quantify this effect.

For an amplifier with gain G, the ASE noise power spectral density at the output at optical frequency ν (in each polarization state ) is

where nsp, the spontaneous emission factor, is a function of the state population and approaches its minimum value of 1 with full population inversion. The ASE noise spectrum for an EDFA is roughly the same shape as the gain profile.

The significance of the ASE noise is most clearly expressed in terms of SNRs and the amplifier noise figure F_n. These quantities are defined in terms of electrically detected signals in an ideal system, as shown in the following figure.

The noise figure is defined as

where SNR_in is the electrical SNR seen when a signal of power Pin is converted to a photocurrent at the output of an ideal photodetector (PD). The noise in this case is shot noise due to the fact that the ideal detector is counting photons, which arrive randomly at the detector. (The detection process must be an integral part of any noise calculation, reflecting the quantum limits of lightwave transmission.) The numerator in the above equation is given by

where (RP_in)² is the square of the average photocurrent, σ² = 2qRP_inΔf is the shot noise power (the variance of the photocurrent), R=q/hν is the responsivity of an ideal detector, q is the electron charge, and Δf is the bandwidth of the electrical detector.

The quantity SNR_out is the electrical SNR seen with the amplifier inserted before the photodetector. To find SNR_out, we compute the variance of the photocurrent after amplification with gain G. Because the detector acts as a square-law device, the photocurrent variance contains terms due to shot noise and ASE noise by themselves, as well as signal-spontaneous emission beat noise because of the mixing between the signal and the ASE in the photodetector. It turns out that that latter is the dominant term, provided that G >> 1, and most of the ASE noise is filtered out at the input of the detector. This can be done by making Δf small enough to exclude extraneous noise but include the desired signal. Then we have

so

Using the above equations, we get

which corresponds to at least a 3-dB SNR degradation in the high-gain case. (In real systems, F_n is typically at least 4 dB.)

6. Amplifier Chains

Over a long transmission link, it is necessary to use several EDFAs interconnected by fiber sections to compensate for fiber attenuation. The gain of each amplifier is normally adjusted so that it compensates for the attenuation on one section of fiber. The question of optimal amplifier spacing then arises. It turns out that this is a fairly complex issue that depends, among other things, on the way in which the amplifiers are pumped, effects of fiber nonlinearities, and practical issues such as amplifier accessibility, cost, and so forth.

We examine a fairly simple model here, in which a fiber of length L is divided into N sections of spacing s = L/N. An amplifier is placed after each section, with a saturated gain that just compensates for the fiber attenuation on one section: G = e^αs. The total accumulated noise power spectral density at the end of this chain (taking into account both polarization states) is then

Note from the above equation that for a fixed amplifier spacing the effect of accumulated noise in the cascade grows linearly with the length of the link but decreases as the amplifier spacing decreases (i.e., as the number of amplifiers increases). Thus, the optimal strategy in this case is to place a very large number of low-gain amplifiers very close together, with the limiting case being one long, distributed amplifier. Cost, however, dictates the opposite strategy! In current practice, a compromise is reached, with spacings ranging from 20 to 100 km, typically giving an SNR at the receiver of at least 15 dB. The spacings are based on constraints such as maximum permissible power on a fiber, effects of fiber nonlinearities, and receiver sensitivity.

>> Raman Amplifiers (RA)

1. How Raman Amplifier Works

The discussion of the EDFA provides a useful framework for describing the Raman amplifier: They are both fiber amplifiers, with important similarities as well as differences, so they can often complement each other in applications.

Stimulated Raman scattering (SRS) can cause transmission impairments in fibers, but it can also be used for amplification. When SRS is used for amplification, pump power is introduced into a fiber carrying an optical signal, with the pump operating at a frequency higher than the signal frequency, just as in the EDFA (and other rare-earth-doped fiber amplifiers).

The pump photons interact with the material in the fiber through inelastic collisions, producing scattered photons at lower energy (and frequency) than the pump photons, with the remaining energy imparted to the fiber medium in the form of vibrational waves, called optical phonons. If the frequency of the scattered photon is the same as that of a signal photon propagating in the fiber, it can stimulate the emission of a second signal photon, thereby amplifying the signal, a process identical to that which occurs in the EDFA.

The performance of the RA can be expressed in terms of a Raman gain coefficient (RGC). An illustration of the form of a normalized RGC as a function of frequency shift between the pump and signal appears in the following figure.

As the figure shows, RA gain is polarization dependent. The gain coefficient for copolarized pump and signal waves is an order of magnitude higher than in the orthogonally polarized case. Polarization dependence is mitigated by the averaging effect of the polarization mode dispersion in the fiber medium and can be circumvented by using  either polarization diversity pumping or a single depolarized pump.

An important difference between the RA and the EDFA is that the energy levels of Er³⁺, which determine the gain profile of the EDFA are fixed, thereby fixing the position of the amplification band of the device, as well as the possible pump frequencies. The amplification band for the EDFA is fixed in the vicinity of 1530 nm – the middle of C-band – which is a primary reason for its importance in optical communication but which limits its flexibility in exploiting other transmission bands in optical fibers.

In contrast, for the RA it is only the pump/signal frequency difference (a band centered around 13 THz) that is fixed by the physics of the process, and any pump frequency can be used. Changing the pump frequency automatically shifts the waveband where amplification occurs.  Thus the amplification band of an RA can be centered at any desired frequency in the optical fiber transmission window by adjusting the pump frequency appropriately. Furthermore, for a single pump the amplification bandwidth is large (about 6 THz), and this band can be extended by superimposing several pumps at different frequencies. This makes the Ra an excellent tool for widening the usable bandwidth of long-haul WDM transmission systems beyond C- and L-bands into the S- and U-bands and beyond.

2. Raman Amplifier Configurations

The RA can be configured either as a distributed or discrete (lumped) amplifier. A typical distributed RA (DRA) consists of a long transmission fiber into which a counterpropagating (backward) Raman pump is injected. (Backward pumping reduces the effect of pump noise, as explained below.) The distributed amplification results in reducing the perceived loss along the span, which effectively improves the reach of the span and/or increases its capacity.

In a discrete RA, the amplifier consists of a coil of fiber together with pump(s) and ancillary equipment for monitoring, control, and perhaps other purposes such as dispersion compensation, gain flattening, or adding and dropping channels. Isolators are used to keep the pump power from escaping into the line. The fiber medium used in the discrete case is shorter than in the DRA, but it still is typically of the order of kilometers – two orders of magnitude longer than the EDFA. A significant advantage of the discrete RA is that the amplifying fiber can be chosen at will to suit a number of criteria. For example, a dispersion compensating fiber can be used to provide dispersion compensation for the transmission fiber, with the additional benefit of improving the Raman gain coefficient. The primary purpose of discrete RAs is generally to expand the usable bandwidth of a transmission link, whereas the primary purpose of a DRA is to improve the reach of a fiber span.

When several amplified spans are placed in tandem, with lumped line amplifiers placed at the junction points, the result is a hybrid arrangement as shown in the following figure.

The advantage of this arrangement is illustrated by a comparison of signal powers along the line with and without the DRAs (see the following figure).

Without the DRAs the signal level drops linearly along each span. Due to the overall span loss a high signal power must be launched into each span, which tends to produce nonlinear impairments at the beginning of the span. But at the far end of the span, the attenuation drops the signal into the noise level. Clearly the span is too long for discrete amplification alone. However, by adding distributed amplification throughout the span, the signal initially attenuates at almost the same rate as without RA but then increases in power toward the far end of the span as it encounters stronger pump power. The DRA pump signal power is also shown, decreasing from right to left in the figure, which is why distributed gain is highest toward the far end of the span. The net effect of the distributed amplification in the spans is to improve the overall system performance by reducing noise as well as nonlinear effects.

Distributed amplification keeps the signal above the noise level at the far end of each span, so the optical SNR at the input of each line amplifier is improved. Furthermore, distributed amplification makes it possible to launch the signal into each span at a lower power level, thereby reducing nonlinear impairments due to high signal levels.

3. The Good and Bad of Raman Amplifiers

There are a number of additional considerations that work for and against the RA. On the positive side, it operates in ordinary silica fibers, requiring no special materials or dopants. This makes it ideal as a means of adding distributed amplification to existing long transmission links. Furthermore, it has better ASE noise properties than the EDFA. RA acts like an EDFA with full population inversion.

However, there are additional sources of noise in RAs that can be more serious than ASE: in particular, multipath effects caused by reflections and double Rayleigh scattering. Rayleigh scattering causes forward propagating signals (or noise) to be scattered backward, but when a signal encounters this phenomenon twice, the doubly scattered signal propagates in the forward direction, recombining with the original signal after a multipath delay. Discrete double reflections due to imperfections, splices, and connectors in the fibers cause similar multipath effects.

Because the Raman effect is weak, long fibers are required in RAs, which tend to increase the multipath effects. Unintended reflections and Rayleigh scattering are present in all fiber systems, but they are attenuated in a passive fiber. However, when the fiber is pumped the Raman gain magnifies these effects to the point where the multipath interference places a limit on the usable gain in an RA.

Another drawback of the RA is that it has a very fast response to pump fluctuations. This can lead to coupling of pump noise into the amplified signals. These effects can be mitigated by using a counter-propagating pump, in which case the effects of the pump fluctuations are averaged out over the length of the pumped fiber. Otherwise, they require the use of “quiet” pumps; i.e., pumps with very low relative intensity noise.

In deploying Raman amplifiers as discrete amplifiers, there are some other practical concerns due to the high pump powers employed. Connectors should be minimized in favor of splices to reduce reflections and attenuation, and when connectors are required they must be designed to survive the high pump powers. Also, to protect personnel, automatic laser shutdown systems must be employed.

4. The Efficiency of Raman Amplifiers

RAs are normally less efficient than EDFAs in converting pump power to output signal power. However, their efficiency improves, exceeding that of the EDFA, at the large aggregate signal powers that occur in long-haul WDM systems with high channel counts. Furthermore the gain in the fiber medium depends strongly on the type of fiber being used. Because gain is proportional to pump intensity, it increases when a given amount of pump power is confined to a small fiber core. Thus, fibers with smaller cores such as Dispersion Shifted Fibers (DCFs) produce significantly higher Raman gain. This is a particular advantage in discrete Raman Amplifiers, where there is some choice in the type of fiber being used.

>> Semiconductor Optical Amplifiers (SOA)

1. How Semiconductor Optical Amplifier (SOA) Works

The structure of an SOA is similar to that of a semiconductor laser. It consists of an active medium (a p-n junction) in the form of a waveguide, with a structure much like the stripe geometry laser. The mobile carriers (holes and electrons) now play the role of the Er³⁺ ions in the EDFA.

The energy levels of the electrons in a semiconductor are confined to two bands: the conduction band, containing those electrons acting as mobile carriers, and the valence band, containing the nonmobile electrons. A hole, representing the absence of of an electron in the valence band, also acts as a mobile carrier. Mobile electrons and holes are abundant (i.e., are majority carriers) in n-type and p-type material, respectively.

The two energy bands in semiconductors play a role analogous to band E₂ and E₁ in the EDFA, but they are much broader than the EDFA bands. A band gap, E_g, separates the lower edge of the conduction band from the upper edge of the valence band so that the energy change involved in moving from one band to the other is at least E_g. Transfer of an electron from the valence band to the conduction band (with the absorption of energy) results in the creation of an electron-hole pair. One way in with this occurs is through the absorption of a photon, as in a photodetector. The reverse phenomenon, electron-hole recombination (with release of energy), occurs either nonradiatively (by transferring energy to the crystal lattice) or radiatively, with the emission of a photon.

The radiative case is of interest to us here for applications in light sources as well as amplifiers. Radiative electron-hole recombination occurs either spontaneously or through stimulated emission involving interaction with an identical photon. These two processes are analogous, respectively, to the spontaneous and stimulated emission processes in an EDFA. By proper choice of the semiconductor materials (e.g., InGaAs or InGaAsP), bandgaps that yield emission and/or absorption wavelengths in the ranges desired for optical communications (e.g., 1300 or 1550 nm) can be produced.

For photon emission to occur by electron-hole recombination at an optical frequency, ν, an electron-hole pair must be present with energy levels separated by an amount ΔE = hν. Furthermore, if the recombination is by stimulated emission, a photon of the same frequency must be present to interact with the electron-hole pair. The conditions for these effects to occur depend on the various carrier concentrations and the photon flux in the active region (the layer around the p-n junction).

In an unbiased p-n junction, a “depletion layer” exists around the junction caused by diffusion of majority carriers across the junction and subsequent recombination on the other side. This creates a net charge on each side of the junction and hence a retarding electric field, preventing further diffusion and draining carriers from the layer around the junction. The depletion layer can be broadened by reverse-biasing the junction, thereby augmenting the retarding field. This is the condition for operation of the p-n junction as a photodetector.

On the other hand, by forward-biasing, the retarding field is reduced, allowing more majority carriers to cross the junction, becoming minority carriers on the other side. This creates a condition favorable to recombination in the active region because once the mobile electrons from the n side cross over to the p side (at which point they become minority carriers), they encounter a large concentration of holes with which to recombine. A similar situation occurs for the mobile holes moving in the opposite direction. This effect, which increases the population of minority carriers in the active region on each side of the junction, is called minority carrier injection.

The current flow through the forward-biased junction acts as an electrical pump, supplying the energy necessary to produce an inversion of the carrier population in the active region. This is analogous to the Er³⁺ ion population inversion in the EDFA produced by optical pumping. The light-emitting diode (LED) is a simple application of radiative recombination. It is a forward-biased p-n junction producing its radiation by spontaneous emission. This effect is called injection electroluminesence.

Now suppose an optical signal is introduced into a waveguide embedded in a forward-biased p-n junction, which we now want to use as an amplifier. By applying sufficient injection current, conditions can be established in which stimulated emission dominates spontaneous emission and absorption in the guide. At this point, optical gain is produced, and the device becomes a semiconductor amplifier. Because the energy bands are broad in a semiconductor, the SOA amplifiers cover a much wider band than an EDFA.

2. The Good and Bad of Semiconductor Amplifiers (SOA)

Although its broadband gain characteristic is a positive feature, the SOA has a number of negative features.

First, the carrier lifetime in the high-energy state is very short (on the order of nanoseconds). As indicated earlier, this means that signal fluctuations at gigabit-per-second rates cause gain fluctuations at those rates, producing cross-talk effects between simultaneously amplified signals. These effects do not occur in EDFAs until the bit rate drops into the 10 Kbps range.

Second, because of its asymmetrical geometry, the SOA is polarization dependent. The EDFA, with its cylindrical geometry, is not.

Third, the coupling losses between the fibers and the semiconductor chip reduce substantially the usable gain and output power.

Fourth, the noise figure of a typical SOA is slightly higher than that of a typical EDFA due to fiber-chip coupling losses, although advances in packaging technology have improved that.

Because of recent improvements in broadband SOAs, polarization-dependent gain (PDG) and noise figure (rather than gain flatness and saturation-induced cross-talk) are becoming the predominant limiting performance factors. The best commercial SOAs can be specified having PDG as low as 0.5dB over the C-band (30 nm bandwidth). However, CWDM-capable SOAs typically exhibit PDGs of 1 dB or more over a 70 nm band.

>> The Basics of Quantum Wells Lasers

Regular double heterostructure (DH) semiconductor lasers have an active region of 0.1 to 0.2um thick. Since the 1980s, lasers with very thin active regions, quantum well lasers, were being developed in many research laboratories.

A quantum well laser is a laser diode in which the active region of the device is so narrow that quantum confinement occurs. The wavelength of the light emitted by a quantum well laser is determined by the thickness of the active region rather than just the bandgap of the material from which it is constructed. This means that much shorter wavelengths can be obtained from quantum well lasers than from conventional laser diodes using a particular semiconductor material. The efficiency of a quantum well laser is also greater than a conventional laser diode due to the stepwise form of its density of states function.

Quantum well lasers have active regions of about 100 Å thick, which restricts the motion of the carriers (electrons and holes) in a direction normal to the well. This results in a set of discrete energy levels and the density of states is modified to a two-dimensional-like density of states. This modification of the density states results in several improvements in lasers characteristics such as lower threshold current, higher efficiency, and higher modulation bandwidth and lower CW and dynamic spectral width. All of these improvements were first predicted theoretically and then demonstrated experimentally.

Quantum well lasers require fewer electrons and holes to reach threshold than conventional double heterostructure lasers. A well-designed quantum well laser can have an exceedingly low threshold current.

Moreover, since quantum efficiency (photons-out per electrons-in) is largely limited by optical absorption by the electrons and holes, very high quantum efficiencies can be achieved with the quantum well laser.

To compensate for the reduction in active layer thickness, a small number of identical quantum wells are often used. This is called a multi-quantum well laser.

The development of InGaAsP quantum well lasers was made possible by the development of MOCVD and GSMBE growth techniques. The transmission electron micrograph (TEM) of a multiple Quantum Well laser structure is shown in the following figure. It shows five InGaAs quantum wells grown over n-InP substrate. The well thickness is 70 Å, and the wells are separated by barrier layers of InGaAsP λ = 1.1 um. Multiquantum well lasers with threshold current densities of 600 A/cm² have been fabricated.

The schematic of a Multi-Quantum-Well Buried Heterostructure laser is shown in the following figure. The laser has a Multi-Quantum-Well (MQW) active region and it utilizes Fe doped InP semi-insulating layers for  current confinement and optical confinement.

The light versus current characteristics of a MQW BH laser are shown in the following figure. The laser emits near 1.55 um.

The MQW lasers have lower threshold currents than regular Double Heterostructure (DH) lasers. Also the two-dimensional-like density of states of the QW lasers makes the transparency current density of these lasers significantly lower than that for regular DH lasers. This allows the fabrication of very low-threshold (I_th 1 mA) lasers using high-reflectivity coatings.

The optical gain (g) of a laser at a current density J is given by

where a is the gain constant and J₀ is the transparency current density. The cavity loss α is given by

where α_c is the free carrier loss, L is the length of the optical cavity and R₁ and R₂ are the reflectivity of the two facets.

At threshold, gain equals loss; hence, it follows from the two equations that the threshold current density (J_th) is given by

Thus, for a laser with high-reflectivity facet coatings (R₁ R₂ 1) and with low loss (α_c ~ 0), J_th J₀. For a QW laser, J₀ ~ 50 A/cm² and for a DH laser, J₀ ~ 700 A/cm²; hence, it is possible to get much lower threshold current using QW laser as the active region.

The light versus current characteristics of a QW lasers with high-reflectivity coatings on both facets are shown in the following figure. The threshold current at room temperature is ~ 1.1 mA. The laser is 170 um long and has 90% and 70% reflective coating at the facets. Such low-threshold lasers are important for array applications.

Recently, QW lasers were fabricated that have higher modulation bandwidth than regular DH lasers. The current confinement and optical confinement in this laser are carried out using MOCVD grown Fe doped InP lasers. The laser structure is then further modified by using a small contact pad and etching channels around the active region mesa. These modifications are designed to reduce the capacitance of the laser structure. A 3-dB bandwidth of 25 GHz is obtained.

>> What is Transparency Current Density?

The transparency current density represents a fundamental limit to achieving the lowest lasing threshold for semiconductor lasers in general.

The current needed for lasing is composed of two parts: the first part being the current needed for maintaining the electron density at the optical transparency level, and beyond that a second part to attain the necessary gain to overcome all the losses in the laser cavity. It can be argued (and can actually be demonstrated experimentally) that a laser cavity can be designed such that the losses are minimal, but this can only reduce the second part of the threshold current while the first part, that responsible for optical transparency, is unaffected.

The key to building an ultralow threshold laser is thus to design a laser cavity with a very low loss, with a material that has the lowest transparency current density. A single quantum well structure is one that possesses both of these qualities and, when combined with high reflectivity coatings to minimize mirror loss, results in some of the lowest lasing threshold currents achieved to date.

>> Strained Quantum-Well Lasers

Quantum well lasers have also been fabricated using an active layer whose lattice constant differs slightly from that of the substrate and cladding layers. Such lasers are known as strained quantum-well lasers.

Over the last few years, strained quantum well lasers have been extensively investigated all over the world. They show many desirable properties such as

1. A very low-threshold current density
2. A lower linewidth than regular Multi-Quantum-Well (MQW) lasers both under continuous wave (CW) operation and under modulation

The origin of the improved device performance lies in the band-structure changes induced by the mismatch-induced strain. Strain splits the heavy-hole and the light-hole valence bands at the Τ point of the Brillouin zone where the bands gap is minimum in direct band-gap semiconductors.

Two material systems have been widely used for strained quantum well lasers

1. InGaAs grown over InP by the MOCVD or the CBE growth technique
2. InGaAs grown over GaAs by the MOCVD or the MBE growth technique

The first material system is of importance for low-chirp semiconductor laser for lightwave system applications. The second material system has been used to fabricate high-power lasers emitting near 0.98um, a wavelength of interest for pumping erbium-doped fiber amplifiers (EDFA).

>> The Background

In recent years there has been a lot of work on dispersion-compensating fibers (DCFs), which are being used extensively for upgrading the installed 1310nm optimized optical fiber links for operation at 1550nm. In the following two sections, we will discuss the basic principle behind dispersion compensation, and the characteristics of dispersion compensating fibers (DCFs).

>> What is Dispersion Compensation

Let’s look at a pulse (with spectral width of Δλ₀) which is propagating through a fiber characterized by the propagation constant β. The spectral width Δλ₀ could be due to either the finite spectral width of the laser source itself or the finite duration of a Fourier transform-limited pulse. We consider the propagation of such a pulse with the group velocity given by:

For a conventional single mode fiber with zero dispersion around 1300nm, a typical variation of ν_g with wavelength is shown by the solid curve in the following figure.

As we can see from the above figure,  ν_g has a maximum value at the zero dispersion wavelength and on either side it monotonically decreases with wavelength. So, if the central wavelength of the pulse is around 1.55 μm, then the longer wavelengths will travel slower than the smaller wavelengths of the pulse. Because of this (chromatic dispersion) the pulse will get broadened. The leading edge of the output pulse is blue shifted and the trailing edge is red shifted.

Now, after propagating through such a fiber for a certain length L₁, we allow the pulse to propagate through another fiber where the group velocity varies, as shown by the dashed cure in the above figure. The longer wavelengths will now travel faster than the shorter wavelengths and the pulse will tend to reshape itself into its original form. This is the basic principle behind dispersion compensation.

Now the total dispersion of a single mode fiber is given by:

Thus, d²β/dω² < 0 implies operation at λ₀ > λ_z (λ_z is the zero dispersion wavelength) and conversely.

Let (D_t)₁ and (D_t)₂ be the dispersion coefficient of the first and second fiber, respectively. Thus, if the lengths of the two fibers (L₁ and L₂) are such that

then the pulse emanating from the second fiber will be identical to the pulse entering the first fiber.

In order to fully understand this, let’s look at the following figure.

In the above figure (a), we can see the broadening of an unchirped pulse as it propagates through a fiber characterized by (D_t)₁ > 0  (λ₀ > λ_z). Thus, because of the physics discussed above, the pulse gets broadened and chirps, the front end of the pulse gets blue shifted, and the trailing edge of the pulse gets red shifted. The pulse is said to be negatively chirped. If such a negatively chirped pulse is now propagated through another fiber of length L₂ characterized by (D_t)₂ < 0, then the chirped pulse will get compressed (see (b) of the above figure), and, if the length satisfies the previous equation, then the pulse dispersion will be exactly compensated.

>> Dispersion Compensating Fiber

Conventional single mode fibers are characterized by large (~ 5-6 μm) core radii and zero dispersion occurs around 1300 nm. Operation around λ0 at 1300nm thus leads to very low pulse broadening, but the attenuation is higher than at 1550 nm. Thus, to exploit the low-loss window around 1550nm, new fiber designs were developed that had zero dispersion around 1550nm wavelength region. These fibers are called Dispersion Shifted Fibers (DSF) and have typically a triangular refractive index profiled core. using DSFs operating at 1550nm, one can achieve zero dispersion as well as minimum loss in silica-based fibers.

Now, in many countries, tens of millions of kilometers of conventional single mode fibers (CSFs) already exist n the underground ducts operating at 1300nm. One could increase the transmission capacity by operating these fibers at 1550nm and using WDM techniques and optical amplifiers. But, then there will be significant residual (positive) dispersion. On the other hand, replacing these fibers by DSFs would involve huge costs. As such, in recent years, there has been considerable work in upgrading the installed 1310nm optimized optical fiber links for operating at 1550nm. This is achieved by developing fibers with very large negative dispersion coefficients, a few hundred meters to a kilometer, which can be used to compensate for dispersion over tens of kilometers of the fiber in the link.

Compensation of dispersion at a wavelength around 1550nm in a 1310nm optimized single mode fiber can be achieved by specially designed fibers whose dispersion coefficient (D) is negative and large at 1550nm. These types of fibers are know as Dispersion Compensating Fibers (DCFs).

Since the DCF has to be added to an existing fiber optic limit, it would increase the total loss of the system and, hence, would pose problems in detection at the end. The length of the DCF required for compensation can be reduced by having fibers with very large negative dispersion coefficients. Thus, there has been considerable research effort to achieve DCFs with very large (negative) dispersion coefficients.

As an example, if we consider propagation in a 50 km length fiber (i.e., L = 50km) with D = + 16 ps/km*nm, then to compensate the dispersion by a 2 km long fiber we must have D’ = –400 ps/km*nm.

The higher the dispersion coefficient of the compensating fiber, the smaller will be required length of the compensating fiber. The next figure shows the waveforms at the input to a 50km conventional single mode fiber, the output without the dispersion compensator, and the output with a DCF with D = -548 ps/km*nm and of length 1.44 km. Note that without the compensating fiber, no information can be retrieved wile the DCF fully restores the pulses.

To achieve a very high negative value of D, the core of the compensating fiber has to be doped with relatively high GeO₂ compared with the conventional fibers. Unfortunately, the total fiber loss (α) increases because of this doping. Hence, for DCFs a measure of the dispersion compensation efficiency is given by the figure of merit (FOM), which is defined as the ratio of the dispersion coefficient to the total loss and has a unit of ps/(dB-nm)

                                                                                           FOM(ps/(dB*nm)) = |D|/α

A typical refractive index profile of DCF is shown in the following figure which has D around – 300 ps/(km*nm) and FOM around  – 400 ps/(dB*nm).

>> The Birefringence of Single Mode Fiber

Circular core fibers whose axes are straight are not birefringent – that is, the two orthogonally polarized LP₀₁ mode have the same effective indices. Bending such a fiber introduces stresses in the fiber and makes the fiber linearly birefringent with the fast and slow axes in the plane and perpendicular to the plane of the loop, respectively. The bending-induced birefringence of a single mode silica fiber is given by:

where n_ex and n_ey represent the effective indices of the LP₀₁ modes polarized in the plane and perpendicular to the plane of the bend, respectively, b is the outer radius of the fiber, R is the radius of the loop, and C is a constant that depends on the fiber material and the elastooptic properties of the fiber. For silica fibers, C is about 0.133 at 633nm.

The above equation tells us that the smaller the loop radius R, the larger is the birefringence. Note that any bending will also introduce attenuation and, hence, very small bend radii are not very practical.

Let’s look at some examples.

> Example A

Let’s consider a silica fiber of outer radius b = 62.5 um bent into a circular loop of radius 30 mm. The birefringence of the fiber at 633nm is then

which is indeed very small compared with the core-cladding indices difference.

Although the induced birefringence is very small, by having the two polarizations propagate over a long fiber length, one can obtain large phase shifts. Thus, if the fiber is coiled around N loops of radius R, then the bend-induced phase difference between the two polarization is

Substituting for Δn_eff from the previous equation, we obtain

where we have disregarded an unimportant negative sign. For achieving phase differences of π (corresponding to a half-wave plate) or π/2 (corresponding to a quarter-wave plate), we must have

> Example B

For simulating a quarter-wave plate at λ = 633 nm, using bend-induced birefringence, if we have a single loop (N=1), then

Using the same loop radius of 2.1 cm, we can simulate a half-wave retardation plate using two loops (N=2). Bend-induced linear birefringence can be used to build in-line polarization controllers as shown in the following section.

>> In-line Fiber Polarization Controllers

The following figure shows an in-line fiber optic polarization controller that utilizes bend-induced birefringence. It consists of three fiber birefringence components; the first and the last are quarter-wave retarders and the central one is a half-wave retarder. The bent fibers are fixed at points marked A, B, C, and D. The three fiber loops are free to rotate as shown.

A rotation of each of the loops will rotate the principle axes of the birefringent fiber sections with respect to the input polarization state. This is analogous to rotation of a classical bulk half-wave or a quarter-wave plate with respect to the incident light. Thus, rotation of the three loops is equivalent to the rotations of a combination of a λ/4, λ/2, and λ/4 plate. One can show that with this combination, any input polarization state can be transformed to any other output polarization state.

The polarization controller described above is used in many applications such as in fiber optic sensors where control of the state of polarization of a the light propagating through the fiber is required.

Polarization controllers operating over a wavelength range of 1250 – 1600 nm with optical insertion loss variations of less than 0.004 dB over the band are commercially available. Such polarization controllers are extremely important components in the measurement of polarization dependence of optical devices such as optical isolators, EDFAs, etc..