The amount of light that is reflected back can cause undesirable system and network effects, thus it must be minimized. With today’s 10Gbit/s, 40Gbit/s and even 100Gbit/s high date rates, video over fiber links with lots of connections, connection back-reflection has become very critical.

In those undesirable effects, increased noise if one of them. This noise is analogous to multipath interference noise of radio waves in the atmosphere. Both noise sources are generated by reflection propagating back and interfering.

There is also a secondary effect of increased noise caused by the back-reflection also being reflected and then traveling down the fiber in the original direction but with a time delay as compared with the original signal.

The time-proven method to reduce back-reflection on fiber optic connectors is to ensure that the fibers are in physical contact (PC) (touching). One way is to grind a radius on the ferrule, and another is to make a angle on the ferrule.

So four types of fiber connector physical contact shape are introduced in the industry. They are called PC (physical contact, <-30dB back reflection), SPC (super physical contact, <-40dB back reflection), UPC (ultra physical contact, <-50dB back reflection) and APC (angled physical contact, <-60dB back reflection).

You can find SC/PC, SC/SPC, SC/UPC and SC/APC connectors. But not all four types are available on all fiber connectors.

Fiber optic cable installers have always been trying to get the maximum number of fibers into a duct. For example, a fiber cable with diameter of 1 inch fills 64 percent of a 1.25 inch duct. The rule of thumb is that you can add a fiber cable to a duct if the cable does not exceed 70% of the area of the duct.

Let’s still take 1.25 inch duct as a example, the requirement of the cable diameter not only has to be smaller than 1.25 inch, but it also has to be small enough so it can accommodate the bends and length of the conduit route.

Most fiber cable manufacturers produce fiber cables containing less than 432 fibers in order to meet the 1 inch diameter size requirement for 1.25 inch innerduct. So for a 4 inch conduit, the maximum fiber counts is 1296 fibers.

But compared to copper cables, fiber cables still have the advantage of higher fiber counts with smaller diameters. The common practice in fiber optic cable installation then, is to place multiple 1.25 inch innerducts in the common 3.5 inch or 4 inch conduit structures, this practice basically doubles or triples the duct capacity.

For OSP (outside plant) fiber cable installations, the challenge is different than indoor applications. In this case, you have to choose the correct inside diameter of the innerduct.

The most common conduit is 100mm in diameter and 150mm diameter is becoming more popular. Conduit is laid in the ground, and innerducts are drawn through the conduit. Fiber optic cables are then deployed and pulled through the innerducts.

The most critical failure factor is the massive tension applied to these innerducts. The eventual performance of the installed system is very critical since a failed innerduct or fiber inside can be very expensive to repair.

The materials of fiber innerduct is commonly HDPE. It must have excellent mechanical properties and also must have good resistance to other environmental factors such as temperature, humidity and moisture, UR radiation, or even ozone.

OK. Here is the point: fiber optic cables should be tested after shipping and handling. This is one of the most common mistakes made by fiber optic cable installers and contractors.

Damage to cabling can occur during shipping or installation. Failing to test fiber cables after it is delivered is a common mistake made by installers. This failure makes damaged cable detection difficult and returns awkward.

An OTDR could be used in this case to shoot an optical profile on each fiber after the cable is received and still on the shipping reel. A permanent record will then be available for future use.

Failing to perform testing, verification, and documentation prior to the installation of the fiber end-termination equipment is a problem. If the fiber is not tested after installation, it cannot be determined whether it was installed correctly; serious equipment performance problems can occur.

Furthermore, failing to document in the cable plant could make trouble-shooting difficult later; as well as voiding warranty conditions of the installed network.

When testing short runs of fiber, there is typically not much information about the fiber available except for length and attenuation. Connectors and splices are generally not present, needed, or used for short lengths; new short runs would be reinstalled.

The sampling rate of an OTDR will determine how much resolution the instrument has when capturing trace information. While it is important to maximize resolution for short distances, it is not mandatory for longer distances.

Since it takes more time to take more sampling or data points, longer stretches of fiber can use a lower sampling rate, whereas medium lengths can use a medium sampling rate. This kind of incremental improvement in time helps when testing hundreds of fibers.

Buy Fiber Optic Cleaving Tool (Fiber Cleaver) Here

A fiber cleave is initiated by lightly scratching the surface of the fiber. When the fiber is thereafter pulled or bent, a crack will originate at the scratch and propagate radially across teh width of the fiber. This produces a nearly flat cleave of an optical fiber.

Fiber Cleaving Tool

The stress field within the fiber created by tension or bending determines the speed at which sound will propagate. If the crack exceeds this speed, the crack will suddenly change direction by almost 90°. This results in an excess of glass on one fiber and a shortage on the other fiber (called a hackle).

Fiber Cleave Hackle

So in order to produce a nearly flat cleave of an optical fiber, the crack speed must propagate slower than the speed of sound in the fiber.

This rule should hold regardless of fiber material. Just be certain that you know the speed of sound in the fiber. Keep in mind that most bad cleaves are due to the initial scratch being too deep. Torsion will not change the speed of sound within the fiber, but it will produce non-perpendicular endfaces.

It is easy to cleave an 80um and 125um dia. fibers, but usually difficult to cleave >200um fibers. To some extent, the difficulty in cleaving these fibers results from the fact that the material of the fiber is not crystalline. Again, torsion will produce a non-perpendicular endface. In fact, most commercially available angle cleavers rely on torsion. The endface angle is proportional to the amount of torsion.

In BPON, an upstream frame consists of 53 timeslots, where each timeslot is comprised of one ATM cell and 3 bytes of overhead. When two consecutive timeslots are given to different ONUs, these 3 bytes or approximately 154 ns of the overhead should be sufficient to shut down the laser in the first ONU, turn it on in the second ONU, and perform gain adjustment and clock synchronization at the OLT.

Similarly, very tight timing is specified for GPON. For example, in GPON with a 1.244 Gbps line rate, only 16-bit times (less than 13 ns) are allocated for the laser-on and laser-off times. Such short intervals require more expensive, higher-speed laser drivers at the ONU.

A very tight bound of 44-bit times (less than 36 ns) is allotted for the gain control and clock recovery. In many cases, the dynamic range of the signal arrived from different ONUs will require a longer AGC time than the allotted overhead (guard interval). To reduce the range of necessary gain adjustment, BPON and GPON perform a power-leveling operation, in which the OLT instructs individual ONUs to adjust their transmitting power, so that the levels of signals received at the OLT from different ONUs are approximately equal.

The IEEE 802 work group has traditionally focused on enterprise data communication technologies. In EPON, the main emphasis was placed on preserving the architectural model of Ethernet. No explicit framing structure exists in EPON; the Ethernet frames are transmitted in bursts with a standard interframe spacing. The burst sizes and physical layer overhead are large in EPON. For example, the maximum AGC interval is set to 400 ns, which provides enough time to the OLT to adjust gain without ONUs performing the power-leveling operation. As a result, ONUs do not need any protocol and circuitry to adjust the
laser power. Also, the laser-on and laser-off times are capped at 512 ns, a significantly higher bound than that of GPON. The relaxed physical overhead values are just a few of many cost-cutting steps taken by EPON.

Another cost-cutting step of EPON is the preservation of the Ethernet framing format, which carries variable-length packets without fragmentation. In contrast, both BPON and GPON break the packets into multiple fragments. BPON uses AAL5, discussed above, to break a packet into cells at the transmitting end and to reassemble multiplecell payloads into a complete packet at the receiving end. GPON employs the GPON encapsulation method (GEM) to enable packet
fragmentation. This method uses a complicated algorithm to delineate variable-size GEM segments and reconstruct the packets at the receiving device.

Several operators have deployed BPON systems; however, the foretold mass deployment and corresponding equipment cost reduction have never materialized. At the time of this writing, there are no announced GPON field trials, let alone commercially deployed systems. Given the level of complexity of the GPON or tight specification for various physical-layer parameters, it is very doubtful that the cost of GPON equipment can match that of an EPON.

The cable sheath is said to contain the cable core and may vary in complexity from a single extruded plastic jacket to a multilayer structure comprising two or more jackets with intermediate armoring.

However, the plastic sheath material tends to give very limited protection against the penetration of water into the cable. Hence an additional water barrier is usually incorporated. This may take the form of an axially laid aluminum foil/polyethylene laminated film immediately inside the sheath as used by British Telecom.

Alternatively the ingress of water may be prevented by filling the spaces in the cable with moisture-resistant compounds. Specially formulated silicone rubber or petroleum-based compounds are often used which do not cause difficulties in identification and handling of individual optical fibers within the cable form.

These filling compounds are also easily removed from the cable and provide protection from corrosion for any metallic strength members within the fiber. Also the filling compounds must not cause degradation of the other materials within the cable and must remain stable under pressure and temperature variation.

Water Blocking Filling Compounds

Filling compounds are used in fiber optic cables to prevent the ingress of water into the cables. Moisture around the fiber can cause existing microcracks to propagate which can cause degradation or even failure of the system over time.

Filling materials are generally used in two different places in the cable. The first location is in the loose tubes or fiber enveloping areas. The second is in the interstices of the cable which includes all the areas not in direct contact with the fiber.

The filling material in the loose tubes can be either a gel or powder compound. The filling material is used to block water from entering and to prevent wicking of the water along the fiber. The filling material used in the interstices of the cable is a water blocking material which is a very thick gel.

Depending on the application of the cable, filling material may be used in both the buffer tubes and the interstices or only in one of the locations.

For direct buried, aerial or duct installations, cables are generally manufactured with filling materials in both locations. More recently, most cable designs have employed filling materials in both locations.

The chemicals that the filling materials are made of are very important as they may affect the optical parameters. There have been numerous studies in the past few years on hydrogen migration into the core of the fiber which causes an increase in attenuation. Silicon has proven to be one material which has experienced problems with hydrogen migration.

Electroabsorption Modulator (EAM)

EAM is small and can be integrated with the laser on the same substrate. An EAM combined with a CW laser source is known as an electroabsorption modulated laser.

An EML consist of a CW DFB laser followed by an EAM, as shown above. Both devices can be integrated monolithically on the same InP substrate, leading to a compact design and low coupling losses between the two devices.

The EAM consists of an active semiconductor region sandwiched in between a p- and n-doped layer, forming a p-n junction. The EAM works on the principle known as Franz-Keldysh effect, according to which the effective bandgap of a semiconductor decreases with increasing electric field.

Without bias voltage across the p-n junction, the bandgap of the active region is just wide enough to be transparent at the wavelength of the laser light. However, when a sufficiently large reverse bias is applied across the p-n junction, the effective bandgap is reduced to the point where the active region begins to absorb the laser light and thus becomes opaque.

In practical EAMs, the active region usually is structured as an MQW, providing a stronger field-dependent absorption effect (known as the quantum-confined Stark effect).

The relationship between the optical output power, Pout, and the applied reverse voltage, Vm, of an EAM is described by the so-called switching curve. The following figure illustrates such a curve together with the achievable ER for a given switching voltage, Vsw.

The voltage for switching the modulator from the on state to the off state, the switching voltage Vsw, typically is in the range of 1.5 to 4 V, and the dynamic ER usually is in the range of 11 to 13 dB.

Because the electric field in the active region not only modulates the absorption characteristics, but also the refractive index, the EAM produces some chirp. However, this chirp usually is much less than that of a directly modulated laser. A small on-state (bias) voltage around 0 to 1 V often is applied to minimize the modulator chirp.

Lithium Niobate Mach-Zehnder Modulator (MZ Modulator)

Lithium Niobate Mach-Zehnder modulators are suited for use in metro, long-haul (LH) and ultra long-haul (ULH) optical transport applications.

The incoming optical signal is split equally and is sent down two different optical paths. After a few centimeters, the two paths recombine, causing the optical waves to interfere with each other. Such an arrangement is known as an interferometer.

If the phase shift between the two waves is 0°, then the interference is constructive and the light intensity at the output is high (on state); if the phase shift is 180°, then the interference is destructive and the light intensity is zero (off state).

The phase shift, and thus the output intensity, is controlled by changing the delay through one or both of the optical paths by means of the electro-optic effect. This effect occurs in some materials such as lithium niobate (LiNbO3), some semiconductors, as well as some polymers and causes the refractive index to change in the presence of an electric field.

The guided-wave LiNbO3 interferometers used to modulate laser beams was fabricated as early as 1980. LiNbO3 has been the material of choice for electro-optic MZ modulator because it combines the desirable qualities of high electro-optic coefficient and high optical transparency in the near-infrared wavelength used for telecommunications.

LiNbO3 MZ modulator can operate satisfactorily over a wavelength range of 1300 – 1550nm. It has been widely used in today’s high-speed digital fiber communication.

LiNbO3 MZ modulators with stable operation over a wide temperature range, very low bias-voltage drift rates, and bias-free operation are commercially available.

High-speed, low-chirp modulators are needed to take advantage of the wide bandwidth of optical fibers. Modulators have became a critical component both in the high-speed time-domain-multiplexing (TDM) and wavelength-division-multiplexing systems (WDM).

Modulators have been traditionally used to modulate a continuous wave (CW) laser to generate the digital signal to be transmitted through a fiber. High-speed modulator with >40GHz bandwidth has been fabricated. Low drive-voltage operation is the key to brining such modulators into practical use because this eliminates the need for high–power electrical amplifiers.

There is general a tradeoff between the speed and the drive voltage. The modulator chirp must also be taken into consideration in the link design. The design of the modulator and the associated chirp can be used as a degree of freedom to extend link distance.

In the last several years, FTTH installation, led by Verizon, has become really hot. But traditionally, there is one big issue where fiber lags behind copper cable: macro bending loss.

Traditionally, fiber cables must be handled and installed very carefully to avoid small bends along the fiber path, which can cause signal loss. This is especially true for indoor installations where contract installers encounter many sharp turns.

Several approaches have been designed to reduce the bend performance of optical fibers.

1. Reducing the mode field diameter

2. Depressing the cladding

3. Adding a low index trench

However, these enhancing options are incremental modifications of existing approaches to process and design, the achievable macro-bend improvements are limited and only incremental as well.

There is another completely different approach which is called hole-assisted fiber (HAF). This is shown below. It has a very different waveguide profile compared to the profiles of legacy fibers that leverage the technology of chemical-doped waveguide fabrication.

4. Adding a ring of symmetric holes within the cladding

While hole-assisted fiber (HAF) produces very bend-insensitive fibers, it is very costly to make in large quantities and long lengths, difficult to connect and not backward compatible with the industry standards. It even is not compatible with existing termination and field procedures.

>> Corning ClearCurve Bend-Insensitive Single Mode and Multimode Fibers

In 2007, Corning announced their new fiber design technology, called nonoStructures. This is a really breakthrough that shows superior bend performance that meet the FTTH requirements and are compatible with large-scale manufacturing and field installation procedures.

This design consists of a germania-doped core and a nanoStructres ring within the cladding. This fiber design consists of engineered features in the range of a few nanometers to several hundred nanometers.

Many industry experts have confirmed that the products is, as Corning claims, a major advance in fiber technology. It has better bending properties and is more forgiving of construction mistakes.

Download Corning ClearCurve Bend-Insensitive Single Mode Fiber Spec Sheet

Download Corning ClearCurve Bend-Insensitive Multimode Fiber Spec Sheet

As most fiber optic cable installers know, the labor and costs associated with accessing and grounding a traditional armored fiber optic cable is pretty high. Because the armor is usually made of aluminum foil or steel, so grounding is a must for all installations.

On April 6, 2009 Corning Cable Systems introduced its MIC DX serials all-dielectric armored fiber cable. This completely removes the need to ground the cable. As described by Corning, the cable core is protected by a flexible all-dielectric armor, that offers more than 4 times the crush protection compared to regular unarmored fiber cables. The cable also has a smaller outside diameter.

This all-dielectric armored fiber cable is designed to eliminate the need to ground the cable, offer greater tensile strength than interlocking armored cables. Absolutely no metallic components are used in the cable, this also simplifies the access to the fibers inside.

This all-dielectric armored fiber cable is offered in single mode, 62.5um multimode, 50um multimode and even hybrid (single mode and multimode mixed) fiber configurations. They can be used for riser (OFNR and FT-4), plenum (OFNP and FT-6), general purpose for intrabuilding backbone and horizontal cabling installations.

These cables are available with Gigabit Ethernet and 10-Gigabit Ethernet performance.

Download Corning MIC DX All-Dielectric Armored Fiber Optic Cable OFNR Riser spec here

Download Corning MIC DX All-Dielectric Armored Fiber Optic Cable OFNP Plenum spec here

What is VCSEL?

VCSEL stands for Vertical-Cavity Surface-Emitting Laser. It is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers which emit from surfaces formed by cleaving the individual chip out of a wafer.

Advantages of VCSEL laser 

A typical VCSEL consists of two oppositely-doped distributed Bragg reflectors (DBR) with a cavity layer between. In the center of the cavity layer resides an active region, consisting of multiple quantum wells. Current is injected into the active region via a current guiding structure either provided by an oxide aperture or proton-implanted surroundings. As the entire cavity can be grown with one-step epitaxy, these lasers can be manufactured and test on a wafer scale. This presents a significant manufacturing advantage.

The VCSEL cavity is very short, 100-1000 times shorter than that of a typical edge-emitting laser. There is typically only one Fabry-Perot(FP) wavelength within the gain spectrum; hence the FP wavelength(and not the gain peak) determines the lasing wavelength. The optical thickness variation of the layers in a VCSEL changes the lasing wavelength. The position of the layers with thickness variation to the center of the cavity is crucial for the resulting wavelength variation; the closer they are to the cavity center, the larger the wavelength change. This property lends to simple designs of wavelength-tunable VCSEL and multiple wavelength VCSEL arrays.

The VCSEL is, today, an established light source for data transmission in short-distance links, interconnects, and local networks (LANs, SANS, etc.). In these applications, the VCSEL is on-off modulated for the transmission of digital signals. Recent work on analog modulation of VCSELs indicates that VCSELs are suitable light sources also for the transmission of RF and microwave signals in, e.g., radio-over-fiber (RoF) networks used in antenna remoting in cellular systems for mobile communication.

Common to all these applications is that they put certain requirements on the high-speed modulation performance. With higher data rates and modulation frequencies, the requirements become more demanding and it becomes more difficult to identify VCSEL designs that fulfill the requirements.