There is a number of national and international standards which regulate laser safety, but I have seen companies and laboratories which do not respect them. Some people are working on dangerous lasers without protection or with unsuitable protection. Also some people just call a few laser safety vendors and take the salesman advice.

But after having decided what protection level you need, how to choose a suitable manufacturer? For the same specifications, some are cheaper by a factor 3 or 5, but what justify this difference?

To help you decide, here is the first-ever on-line poll on laser safety manufacturers. If you are a laser user, please share your experience and let our reader know what you think.

We also strongly encourage you to develop your point of view on your laser goggles by posting a comment.

Wavefront deformation and poor focus or poor image.

Since light can be modelled as an electromagnetic wave, one can define a surface of constant phase, called wavefront. This is much like the crest of a wave in the water. The little animation below can help understand this concept quite easily.

Optical wavefront curved by a lens

Because of imperfections of the media in which the light is going through, the wavefronts are normally deformed and are not a perfectly curved (or “flat”) surface anymore. This in turn affects the propagation of the light rays and makes it impossible for them to focus in a single point (one can demonstrate that in any point the ray of light is perpendicular to the wavefront). The result is lower intensity at focus, blur and in general, aberrations. The picture below can give an example, it compares a perfect situation with a case where the wavefront is heavily deformed.

Deformation of the optical wavefront through the eye.

There is a number of very common reasons for this to happen. Heating up of the optical system, atmospheric turbulences, inhomogeneities of the media in which the light propagates, gradient of density in the air (mirage effect), etc…

Wavefront deformation and destructive interferences.

In addition of what we just mentioned above, aberrations in a beam of light will greatly reduce the intensity at focus due to destructive interferences. Once again, the images below will help understand why. First, keep in mind light is an electromagnetic wave. As it goes along, the electrical field varies from +E to -E

Light as an electromagnetic wave

In the ideal case, when there is no wavefront deformation, the light going through a media or an optical system will arrive at focus at the same time whatever the path it goes through. In this situation (picture below), the electrical fields add up at focus, and the intensity of the light is thus greatly increased.

Ideal lens

In reality, because of the wavefront is deformed, some of those electrical fields will arrive at the focus point at different “times” (phase). The electrical fields do not have the same values and their addition will be counter-productive, leading to reduced intensity in places.

Lens with aberrations

This leads to the intensity patterns at focus you can see below, and to a reduced Strehl ratio. It creates obvious problem when the aim is to get the highest possible intensity at focus or the best quality image.

Point Spread Functions

Practical consequences of poor wavefront quality.

As a direct result of what has been said above, a poor wavefront will:

• Reduce intensity at focus. In case of a welding/cutting laser this mean decreased efficiency. Any laser application that focuses the light down would be impacted by poor wavefront, such as welding, cutting, plasma generation, surgery, fluorescence or Raman excitation, etc… It is to be noted that a laser beam can potentially heat up the optics inside itself and create thermal lensing, which in turn will deform the wavefront.
• Create hotspots. This is particularly crucial in Chirp Pulse Amplification lasers. All the optical components of an amplification chain can induce phase aberrations responsible for spatial intensity modulations. These distortions can generate energy hot-spots and irreversible optical damages of the components, some of which are prohibitively expensive.
• Lower resolution. Aberrations are the plague of imaging systems, because they create blurry images and effectively reduce the imaging system resolution. This can be caused by the imaging system itself (poor quality lenses, for instance, or mis-alignment), of by the environment (the turbulence in the atmosphere create dynamic aberrations which lower the capabilities of non-adaptive optic telescopes)

What technology is currently available to measure wavefront aberrations?

Shack-Hartman

This is the most wide-spread type of wavefront sensor. A micro-lens array focuses the incident wavefront into a number of small spots on a CCD. Aberrations in the beam will make the spots move away from the place they would occupy in front of the centre of each micro-lens if the wavefront was perfectly flat. The deviation of each spot is directly proportional to the gradient of the wavefront, which can then be reconstructed.

The Shack-Hartman is the most versatile wavefront sensor available at the moment. It can measure wavefront aberrations 1,500 bigger than the wavelength at a precision of one-hundredth of a wavelength. It is the easier to align, the most documented and is already designed-in a number of turnkey solutions for industrial need.

Its main weakness is its poor spatial resolution. With a number of measurement points equal to the number of micro-lenses, it is typically of the order of 1000 to 5000 data points per wavefront.

This instrument is best suited for general measurements, when you need both a good dynamic range and good precision (resolution of the phase), but do not need a high spatial resolution (or transverse precision, helping with high spatial frequency aberrations).

Realistically, this includes most of the cases, since a wavefront reconstructed from 1000 points is able to include aberrations of well past beyond the 10th order.

Hartman

Same as above with a holed mask in place of the micro-lenses array. This could be considered as obsolete technology only interesting when you cannot use lenses (for X-ray wavefront sensing for instance).

Curvature sensors

They measure the intensity profile of the beam in two different planes along the optical axis. By comparing the intensities, the software will compute the axial derivative of the intensity, and then calculate the second derivative of the wavefront using Poisson’s equation. This technique gives a very good spatial resolution because one pixel gives one phase data point. One of the main drawbacks of this technique is its limitation in terms of dynamic range, typically limited to a few microns (typically 3 µm). Just as critically, since it is working on the second derivative of the wavefront, it is by nature unable to measure tip-tilt aberrations. Finally, the light beam must be collimated and of reasonable intensity.

This has some uses to measure wavefront with high spatial frequencies of aberration, of low amplitude.

Multi-lateral shearing interferometer

A 2D diffraction grating replicates the incident beam into four beams which are propagated along slightly different directions. The interaction between the beams produces an interference pattern which is imaged on a CCD.

When aberrations are present on the beam, the interference pattern is distorted. The pattern deformations are directly connected to the phase gradients. A spectral analysis using Fourier transforms allows the phase gradient extraction in 2 orthogonal directions. The phase map is finally obtained by integration of these gradients.

Typically you can tune the position of the diffraction grating to change the behaviour of the sensor: either you get high precision measurement of the phase or you get high spatial resolution (to see high spatial frequencies). Also the overall dynamic range of the instrument is limited, so you can tune it either for high precision measurement of the phase or for measurement of a highly aberrated wavefront, but you cannot measure high level of aberration with a good precision. This would probably mean that you need to pay extra care to the alignment when making a precision measurement, otherwise the tip-tilt will bring the wavefront out of the dynamic range.

The wavelength range is the one of the CCD used (generally 350-1100nm), it is insensitive to vibrations. Finally, because the beam is split into 4, you need a reasonable intensity.

This type of instrument is suitable when the measurement you want to make do not tick all the boxes of high aberration amplitude, high precision and high spatial resolution at the same time.

In practice, what help can you expect from a wavefront sensor?

• Characterise optical aberrations and obtain their projection on Zernike polynomials. This is precious information to understand easily the imperfections of an optical system. Since wavefront sensors are relatively fast (tens of Hertz), they can as well characterise dynamic aberrations such as those induced by thermal effects. High end systems running at a kHz can even measure aberrations due to atmospheric turbulence.
• Characterise completely the light propagation. The characterisation of a beam of light in terms of intensity and wavefront allows a certain number of its fundamental parameters to be calculated by simply processing the initial measurement data. Using the Fresnel propagation equations, one can reconstruct the phase and intensity profile in any plane along the optical axis.


Laser true 3D profile

• Measure the intensity profile at focus of a laser (also known as Point Spread Function or PSF). Or, rather, reconstruct it. This is just a consequence of the point above. A number of people are experiencing issues while trying to measure the intensity of their laser at focus. A Wavefront sensor capable of measuring the intensity as well can reconstructs the PSF while being meters away from the focus, and can then become part of the solution.
• Characterise an optical system by measuring the Mode Transfer Function MTF. This is only the Fourier transform of the PSF.
• Measure a number of laser parameters such as M2, beam waist, optical intensity propagation along the optical axis.

Watch out for an exact software description before buying anything! Some of the above features involve advanced data processing and are not proposed by every wavefront sensor manufacturers.

A wavefront sensor will also:

• Help align the optics of your system. Some of those sensors make it very easy: the idea is to position the first optic of the system, and align it while using the wavefront sensor to find the position that minimizes the aberrations. After having found the right position you would then take a measurement as a reference, introduce a second optic, and subtract the reference to the new wavefront. What you then measure are the aberrations introduced by the latest optic alone, the positioning of which you can now optimise. And so on…
• Help improve the optical resolution of your system.
• Help increase the power at focus.
• Help produce tighter focal spots.

Those last three points all come from the same property. As shown in the point spread functions pictures above, aberrations reduce the quality of the response of an optical system, spreading its PSF (focal spot size) and reducing the intensity at its centre. This results in blurry images and effectively reduce the resolution. By characterising the aberrations introduced by an optical system, a wavefront sensor helps in taking the relevant actions to minimise them (through better alignment or adaptive optic for instance).

Signal to noise ratio

Also noted SNR or S/N. This is defined as the ratio between the signal power and the noise power. Hence:

Understanding the meaning of this is quite straightforward: the higher this ratio, the best signal you get. At equal input signal, the detector with the highest SNR is the less noisy one. If S/N<1, you cannot see anything, if S/N>>1, the signal is easy to pick up. As such, the signal-to-noise ratio is not a usable figure of merit of a detector. It is rather a measure of how strong your signal is compared to the “sensitivity” of your detector.

However, comparing the optical power needed to get a SNR of 1 is a step in the right direction to compare detector noise. According to the optical noise models explained earlier,

Obviously, the noise depends completely on the bandwidth of the detector. This is understandable: to differentiate a true experimental result from random experimental error, you need to repeat the experiment. Translated in detector terminology, to get a better signal you need to increase the integration time of the experiment (= decrease the bandwidth).

To define a good figure of merit, it needs to show the minimum detectable optical power and not to depend on the integration time. Enters the noise equivalent power.

Noise equivalent power

Also noted NEP. This is a slightly confused definition. The initial concept is to define the noise equivalent power as the optical power which will yield a signal to noise ratio of 1. This is then the limit of what can be detected. But with this definition the noise equivalent power can only be given at a specific bandwidth (Δν enters in the expression of S/N).

Since not two detectors have the same integration time, manufacturers tend to call Noise Equivalent Power the minimum detectable power per square root of bandwidth. We will note this noise equivalent power per unit of bandwidth to avoid confusion. In this situation we have then:

this normalised only depends on the detector itself (and sometime on the ambient temperature!) and is measured in . The smallest the NEP, the better is the detector.

Getting back to the ambient temperature issue, the fluctuations of the ambient temperature are generally too small in comparison of the absolute temperature to introduce a bias in the comparison. However, it is true that the higher the temperature, the more noisy a detector is. For that reason some high quality detector are cooled (generally thermoelectrically but sometime with cryogenic cooling).

Detectivity and Specific detectivity

The detectivity D is defined as the reciprocal of the NEP: . Since all of parameters we defined depend on the area of the detector, in some cases this introduces a bias in the detector comparison. Thus sometime is specified a specific detectivity D* (D star), defined as:

In fairness, I have very rarely encountered people using this specific detectivity in optical detectors.

Quantum effect and noise: the shot noise.

Fundamental physics tells us the light is made of particles (photons), which are emitted by the source at random. For that reason, the amount of photons emitted by the source (sun, bulb, laser, etc.) is not constant, but exhibits detectable statistical fluctuations. And this is in a nutshell what shot noise is. Because of its nature, it does not depends on the quality of the detector and is unavoidable. However the shot noise becomes a real issue only when the optical intensity is fairly low: in this case quantum fluctuations become much more noticeable.

The random process of light emission can generally be modelled using a Poisson distribution, the properties of which are very well known. If we note p(n) the probability that n photons arrive on the detector:

where is the standard deviation. What this means is that for 100 photons arriving on the detector, the uncertainty about the number of photon is of ±10 (±10%). If the number of photon is somewhat closer to common levels, e.g. , the uncertainty becomes , which is ±0.000,01%. It then becomes obvious that the shot noise is an issue only at low light level.

Let’s find out now what would be the fluctuations of the signal current due to the shot noise. Since each photon induce a free electron with an efficiency , during the time the number of electron produced is then . Every electron contributes to the signal current for a charge , the average value of the signal current is then:

Fluctuation in the number of photons create a fluctuation in the signal current, and those fluctuations are characterised by the standard deviation:

If we note the bandwidth of the detector, we can find the useful formula below:

And this standard deviation characterise the shot noise current.

Dark current

The dark current is the constant response exhibited by a detector during periods when it is not actively being exposed to light. It is sometime classified as another type of shot noise. It is also referred to as reverse bias leakage current in non optical devices and is present in all diodes. Physically, dark current is due to the random generation of electrons and holes within the depletion region of the device that are then swept by the electric field applied to the diode.

Similarly to the photon noise, it can be modelled by a Poisson distribution, with:

Let’s note that depends from many parameters, but generally speaking:

• Si photodiodes : ranges from 1 to 10 nA
• Ge photodiodes: ranges from 50 to 500 nA
• InGaAs photodiodes: ranges from 1 to 20 nA

Thermal noise

Thermal noise, also called Johnson noise or Nyquist noise is the electronic noise generated by the thermal agitation of the electrons inside an electrical conductor at equilibrium, which happens regardless of any applied voltage. It was discovered by Johnson in 1927 and explained by Nyquist.

A device (a photodiode for instance) thermal noise can be modelled as a voltage source in series with an ideal resistor R. has a Gaussian distribution with a mean value of zero. In this case,

Where is the standard deviation of the voltage, is the Boltzmann’s constant in joules per kelvin and T is the resistor’s absolute temperature in kelvins. It can also be modelled a current source in parallel with R:

• Is the meter’s calibration traceable to internationally recognized standards such as NIST?
• Is your laser wavelength within the wavelength range of the power meter?
• What is the power range you expect to measure (highest and lowest limit)? Does it fall within the range the power meter can measure?
• What is the diameter of your beam at measurement point? Do you have any control on this (using a lens for instance)? Is the power meter aperture big enough?
• What is your power density (W/cm²) and energy density (J/cm²)? Is it below the damage threshold of the power meter?
• Is your laser a pulsed femtosecond? If yes you will need a flat spectral response across the laser bandwidth. This may also be the case if your laser is widely tunable and you can’t adjust the wavelength setting manually, or simply if you don’t know your wavelength.
• Is your laser pulsed and do you need to measure each pulse’s energy or an average power is sufficient? If the average power is enough or if you want to measure a single pulse energy, a thermopile is better. Otherwise you will have to go for a pyroelectric sensor or a specialised photodiode
• Are there a lot of vibrations in your environment? If so this would rule out a pyroelectric detector.
• Most power meters are sold nowadays in a set of two separate items: a display and a sensor. Make sure you order both and that they are compatible with each other
• Assess what type of display you need: do you need computer connectivity, LabView compatibility, is it to go “in the field”, do you need it wireless (yes some manufacturer do that now)…

Of course, many factors will influence the quality of a power meter, the most important being its calibration. One should go for power meters which calibration is traceable to a recognised standard (such as NIST).

Photodiodes: precision for low power lasers.

When a photon source, such as a laser, is directed at a photodiode detector, a current proportional to the light intensity and dependent on the wavelength is created. A photodiode sensor has a high degree of linearity over a large range of light power levels - from fractions of a nanowatt to about 2 mW (this higher limit depends a bit on the photodiode). Above that light level, corresponding to a current of about 1mA, the electron density in the photodiode becomes too great and its efficiency is reduced causing saturation and a lower reading. Most manufacturers offer a removable ND filter to allow extending somewhat the dynamic range of the power meter, generally up to about a watt maximum.

Photodiodes are generally made of silicon, thus their response is typically 350-1100 nm, and can be extended to 200-1100 nm. Occasionally one can find an off the shelf calibrated germanium or InGaAs photodiode which will allow precise measurement on the 800-1600 nm range. As you can see on the picture below, the typical response curve of a silicon photodiode is highly wavelength-dependent.

Silicon reponse curve

This importance of the wavelength dependence leads to two main drawbacks: you need to have a clear idea of the wavelength of your laser, since the power meter will ask you for it and the result will depend on the answer. Plus photodiode power meters are inappropriate for broadband light sources power measurements (for instance it is not the way forward when using femtosecond lasers).

On the positive side, photodiodes are relatively insensitive to temperature fluctuations, have a very small form factor, are fast (from a fraction of a second to some tens of microsecond response time, limited by the electronic) and are insensitive to vibrations. But their main and unique advantage lies in their ability to measure very small optical power.

Some manufacturers even offer a background light cancellation feature, which uses a second photodiode placed outside of the laser beam path but close enough to the measuring photodiode. The light measured by this second photodiode is considered as the background noise and subtracted to the reading of the first one.

Thermopiles: stability for medium and high powers

Using a thermopile sensor is a very robust and well established way to measure laser energy. The underlying principle is quite simple: it uses some thermocouples to measure the temperature gradient between the point where the laser beam hit the thermopile and the periphery where the heat is dissipated using a heatsink. It is then easy to calculate the incident laser power.

Thermopiles tend to be more accurate than photodiodes, but their sensitivity is lower. This means the error is lower in percentage, but they are unable to measure low power lasers. Typically their power range can go as low as a few hundreds of microwatt while some high power thermopile sensors can measure up to nearly 10 kW. Usable wavelength range commonly span 200-20,000 nm for a single broadband sensor.

On the down side, they are slow, at generally a couple of second response time despite software acceleration. Plus, since the measurement is based on heat exchange, a quick fluctuation of housing temperature will decrease the accuracy of the result. This is an issue for instance if the beam hits the housing or if you hold a low power thermopile by hand. Keep in mind that part of the beam energy is distributed outside the defined beam diameter, and this energy can hit the housing if your beam is too large.

Due to their slow response time, they are only really capable of measuring average power. They generally have an energy mode which allow them to measure the energy of a single pulse. Interestingly, the pulse width does not really matter: however short, the energy of the pulse will produce a heat increase and thus the meter will deliver a reading. However some thermopiles are better equipped to measure short pulses with high energy: in this situation the energy needs to be absorbed in the volume of the absorber and not only on its surface, otherwise there is a possibility to damage the sensor.

Because the measurement relies on thermal exchanges, thermopile technology is quite diverse. One can find sensor specialised on short pulses, some on long pulses, some give better results at specific wavelength, some have a spectrally flat response over hundreds of nanometer allowing broadband light measurement, and some have a slightly different technology, based on a Peltier device, which allows sub-second response time.

Pyroelectric: energy and power

Some applications absolutely need a pulse-to-pulse measurement of the energy. In those situation where an average reading of the power is not enough, a pyroelectric energy meter is the way forward.

Pyroelectricity is the ability of certain materials (generally a polar crystal or a ferroelectric) to generate an electrical potential when they are heated or cooled. When a pulse of light hits the detector, it heats it up and create that electric potential. The electrical voltage read by the measuring instrument is then proportional to the energy. Average power can be calculated by the electronic.

Pyroelectric energy meters are very fast (up to tens of kHz) and very broadband (typically 200-20,000 nm). These energy detectors will also make accurate measurements in spite of changing temperature in the environment or heating of the detector.

Unfortunately they are less durable and less accurate than thermopiles or photodiodes. They are also sensitive to vibrations and can’t measure continuous light (CW lasers) nor long pulses (it typically has to be less than 10 ms, but this varies a lot from detector to detector). It also has a maximum repetition rate. Therefore they should only be used when the measure of each pulse energy is necessary.

As Maximum Permissible Exposure evaluation and the determination of hazard areas (NHZ: Nominal Hazard Zone) are quite involved, a laser safety classification scheme has been designed by international standardisation committees to help users to decide if their laser is a potential hazard. Below is a summary of the different laser classes with their description.

Class 1

• Meaning: safe
• Type of laser: very low power lasers or enclosed lasers.
• Maximum Permissible Exposure: is never exceeded, even for very long exposure (hours), or with the use of optical instruments.
• Nominal Hazard Zone: none.
• Typical Accessible Emission Limit*: 40 µW for blue.

Class 1M

• Meaning: safe for the naked eye only, but potentially hazardous when optical instruments** are used.
• Type of laser: medium power lasers either collimated with a large beam or highly divergent.
• Maximum Permissible Exposure: can be exceeded when using optical instruments**.
• Nominal Hazard Zone: none for the naked eye.
• Typical Accessible Emission Limit*: a laser can be classified as Class 1M if the total output power is below class 3B (0.5 W for continuous in the visible) but the power that can pass through the pupil of the eye is within Class 1.

Class 2

• Meaning: safe for unintended exposure, (less than 0.25 s) but hazardous when looking at for more than 0.25 s.
• Type of laser: visible (400–700 nm) low power lasers.
• Maximum Permissible Exposure: are not exceeded provided the viewings are accidental only. MPE calculation assumes the blink reflex will stop the light after 0.25 s
• Nominal Hazard Zone: none for accidental exposure.
• Typical Accessible Emission Limit*: 1 mW for continuous lasers.

Class 2M

• Meaning: safe for the naked eye when the exposure is unintended, (less than 0.25 s) but hazardous when looking at for more than 0.25 s or when optical instruments** are used.
• Type of laser: visible (400–700 nm) medium power lasers either collimated with a large beam or highly divergent.
• Maximum Permissible Exposure: are not exceeded provided the viewings are accidental only and only with naked eyes. MPE calculation assumes the blink reflex will stop the light after 0.25 s. Using optical instruments** might bring the exposure above the MPE as well.
• Nominal Hazard Zone: none for accidental exposure to the naked eye.
• Typical Accessible Emission Limit*: a laser can be classified as Class 2M if the total output power is below class 3B (0.5 W for continuous in the visible) but the power that can pass through the pupil of the eye is within Class 2.

Class 3R

• Meaning: unsafe, except when handled carefully by experienced users. Accidental short exposure is considered as a small hazard.
• Type of laser: low power lasers.
• Maximum Permissible Exposure: can be exceeded up to 5 times.
• Nominal Hazard Zone: hazard area for the eye, none for the skin.
• Typical Accessible Emission Limit*: typically 5 mW in the visible.

Class 3B

• Meaning: unsafe without exception, Personal Protective Equipment (laser safety goggle) must be worn within the nominal hazard zone. Focused lasers of this class are a potential fire hazard.
• Type of laser: medium power lasers.
• Maximum Permissible Exposure: is exceeded more than 5 times. Skin MPE is not generally exceeded, except at focus.
• Nominal Hazard Zone: hazard area for the eye, none for the skin.
• Typical Accessible Emission Limit*: 500 mW.

Class 4

• Meaning: dangerous, Personal Protective Equipment for eyes and skin must be worn within the nominal hazard zone. Class 4 lasers are fire hazards as well. Diffuse reflections may be hazardous. Those lasers are commonly used for cutting or welding. This can create hazardous fumes. Cutting lasers generally create a small plasma which in turn emits UV light. UV light is another hazard to consider on a manufacturing floor.
• Type of laser: high power lasers.
• Maximum Permissible Exposure: ocular and skin MPE are exceeded. Diffuse reflections exceed the Minimal Permissible Exposure.
• Nominal Hazard Zone: hazard area for the eye and for the skin.
• Typical Accessible Emission Limit*: no limit.

Notes

Accessible Emission Limit (AEL): an AEL is the maximum value of accessible laser radiation to which an individual could be exposed during the operation of a laser and is dependent on the laser class. The AEL above are given as an indication for continuous lasers, but may change for pulsed lasers or infrared lasers.

Optical instruments: two types of optical instruments increase the hazard of M lasers:

• instruments which will reduce the diameter of a collimated beam (telescopes, beam reducers, binoculars). This is dangerous when using lasers with large beams (>7mm) since it is likely to increase the amount of light entering the pupil of the eye.
• Converging optics such as lenses, loupes, prescription eyewear… this is an increased hazard when using highly divergent beams since it will make it less divergent for the eye, allowing a greater amount of light to enter the eye.

As a small comparison, the energy level passing through the pupil of the eye when looking directly at a 2 mW HeNe laser is of similar level to the one when looking directly at the midday sun under the tropics by cloudless weather. Except that the image of the sun inside the eye is more widespread. The laser light on the other hand is collimated out of the laser and will image as a very small spot on the retina (10-20µm). In those conditions, it is easy to reach power densities as high as a few thousands of W/cm². Keep in mind that generally speaking, fire hazard ^[1] starts at 10 W/cm². What happens inside one’s eye when looking directly at a laser is exactly the same as when using a magnifying glass to focus the sunlight over some delicate paper.

Here is another example: consider a 2 W, 532 nm pulsed DPSS laser with a pulse width of 1 ns and repetition rate of 20 Hz. Nothing fancy, this level of power is quite common in the industry. This laser’s diffuse reflection (on a chair frame or optical mount for instance) can blind someone 10 km away. And that is only due to indirect viewing.

Accidents can be avoided by a few simple policies:

• Hire a professional to assess the safety of your laser lab or factory.
• NEVER, EVER LOOK DIRECTLY INTO A LASER BEAM, even if you think it’s safe and even if you wear laser protection goggles.
• Work in a windowless room or seal off windows with certified laser barriers or curtains.
• No one should be allowed in the laser room without wearing proper protective eyewear, certified and rated against your laser specifications.
• Keep track of your beam path and cut off all beams with beam dumps where appropriate.
• Establish a protocol for entering or exiting the laser room. This will include a laser hazard warning sign, ideally a doorbell, an interlock, emergency shut-off button and a warning light.
• Reduce the power of your laser when aligning it.
• Remove chairs and stools from the surrounding of your laser table, to prevent people sitting around it (which would place their eyes at beam level).
• Remember that even diffuse reflectance (or reflectance of reflected light) from a class IV laser can be dangerous. This include reflectance on optical mounts, the table itself or even the floor

This is by no mean an exhaustive list. Common sense must rule, be aware of the dangers at all time and never underestimate them.

Reference
1. Section 7.2.3 of the ANSI Z136.1-2007

Why is it important?

Well, an optic with a bad rating on surface flatness will introduce some wavefront distortions, which are responsible for aberrations and bad quality focus. Aberrated wavefront leads to poor Strehl ratio (ratio of the observed peak intensity at the image plane compared to the theoretical maximum peak intensity of a perfect optical system), so poor optics makes one loose valuable optical power at focus. Plus scratches or digs on an optic create diffraction and stray light, which no one wants either.

Surface flatness

This is the measurement of the difference between the actual surface of the optic and the surface it would have if it was defect-free. There are two main way to measure it, the most common is called “peak-to-valley” (P-V). This is the difference between the “highest” and “lowest” parts on the surface of the optic, those “top” and “bottom” being defined as the local difference between the actual optic and the ideal one. Of course this ignores the curvature of the optics, which is not a defect. We consider this method of measuring defects on optic as inaccurate and misleading: it is a maximum measurement, and it does not say how many peaks and valley there are on the whole surface. Consequently it is difficult to predict how an optic will perform with this sort of measurement. An optical system having a large P-V error may actually perform better than a system having a small P-V error. Unfortunately it is by far the most widespread flatness quality control in the industry.

A much better measurement is the RMS (Root Mean Square) value of the flatness. This technique involves measuring a substantial amount of the optic’s surface at many points and then calculating the standard deviation of the surface from the ideal form. This measurement has direct mathematical implications: for instance it is possible to calculate the Strehl ratio from it.

Once again, in short the Strehl ratio is a very good indication on how much power you get at the image plane of the optical system versus what power you will get from an ideal aberration-free system. Once the Strehl ratio has been calculated, the quality of the optical system may be ascertained using the Maréchal criterion. The Maréchal criterion states that a system is regarded as well corrected if the Strehl ratio is greater than or equal to 0.8, which corresponds to an rms wavefront error λ/14. For instance an optical system introducing a λ/3 RMS deformation will have his actual power at focus reduced to approximately 3% of its theoretical power. The reason for this drop in power at the focus is that some interferences are created in the focus, with different rays arriving with a different phase.

Since most manufacturers are specifying their optics flatness in peak-to-valley, here is a short comparison of what one should expect. This is without guarantee: as explained above, peak-to-valley is imprecise and misleading.

Scratch-dig

This is yet another very subjective quality measurement. Scratch-dig, sometimes called surface quality relates to the number and apparent size of visible defects, typically scratches and pits (called digs), on the part surface. While this may seem straightforward, probably no optical specification causes greater confusion. The problem arises because the assessment of scratches and digs is performed using a purely visual, non-quantitative comparison to a set of standards which conform to the US military specification MIL-O-13830. This situation arose because the specification was developed many years before the advent of the laser, when surface quality was primarily a cosmetic consideration without performance information. Scratch-dig is specified by two numbers, such as 40-20. The first number is the maximum width allowance for a scratch measured in microns, and the second is the maximum diameter for a dig in hundredths of a milimetre. So 40-20 would permit a scratch width of 0.04mm and a dig diameter of 0.2mm

This measurement is obviously badly limited: not only does it entirely rely on a visual inspection, but there is no measure of irregularities depth and scratches length, nor of their number, nor of their position (centre being worse). The problem is that this measurement has the potential to give important information on the optic. Small size defects are responsible light scattering, loss of contrast and stray light which can damage sensitive components in high power applications.

A much better measurement would be the Fourier transform of the surface of the optic, if this were available from manufacturers. Once again to help people getting an idea of what they are getting, here is a comparison of the average scratch-dig quality from quality manufacturers. Just keep in mind how imprecise this measurement is, though.

The simplest of all

The simplest coatings are made of thin layer of metal, such as aluminium, sliver or gold. They are used for broadband mirrors. The best reflectivity comes at higher price: aluminium (Al) is the cheapest (R=88%-92%), followed by silver (Ag, R=95%-99%) and then gold (Au, 98%-99%). But in the blue-violet part of the spectrum, only aluminium is suitable.

Dielectric coatings

These coatings use different thin layers of material (various metal oxides, calcium or magnesium fluorides) to create the desired effect. There are three major techniques used for dielectric coating: electron-beam deposition (E-beam), ion-assisted electron-beam (IAD) and ion beam sputtering (IBS). All of these process are quite similar in their principle. They consist in evaporating some coating material on the substrate. The difference lies in the deposition energy.

Because of the low energies involved when using electron-beam deposition, the thin film material contains bubbles and micropores, like a sponge. These will eventually fill with water, which will change the refractive index of the coating and thus the properties of the optics. (This is known as “environmental shifting”). The presence of water also lowers the damage threshold of the optics: when submitted to an intense light, the water will tend to vaporise and scrap off bits of the coating. Finally, even in the absence of water, the inhomogeneities of the coating layers lower the theoretical damage threshold. The positive points about this technology is that it is cheap, widespread and very versatile. The coating itself is also slightly flexible, which makes the optic more resistant to mechanical stress. Some of the major optics manufacturer only have access to that type of coating at the moment and outsource IBS-coated optics.

Ion-assisted electron-beam is an intermediate technique, between ion-beam sputtering and e-beam. So are its results.

Ion beam sputtering involves energies 100 times higher than e-beams. As a result the molecules of the coating layers form covalent bound when deposited. The result is free from bubbles or pores, more homogenous, more durable, have higher damage threshold and is more repeatable and controllable. They also show lower scattering and absorption properties, and overall higher specifications (more broadband, steeper transitions when needed, better spectral stability…). This is high precision coating, and the surface roughness can be controlled at better than 1 Å RMS (!), that is <λ/5000. Of course, this comes at a higher cost (atom-by-atom removal is very slow), and even worse, it is limited in the types of coatings it can handle: most of the UV coatings for instance involve fluorides which dissociate when sputtered. In this case, e-beam is the only option.

Coating types comparison

We will provide here a brief description of the choices one has to make when choosing its coating type.

Anti-reflective coatings

• Broadband vs damage threshold: since broadband generally means multi-layer coatings, the more broadband a coating is, the lower its damage threshold and the higher its price. Broadband coatings also show the property to be less sensitive to angle of incidence than other anti-reflection coatings, which makes them valuable when the optics has to be tilted over the course of an experiment for instance. Broadband coating also show lower reflectivity than single layer or V-types coatings.
• Single-layer vs V-types anti-reflection: Single-layer is more durable than V-types (which are multi-layers), and are enough to lower the reflectivity of BK7 from 4% to about 1.3%. On high refractive index materials (sapphire, Nd:YAG, ruby), single layer coating can go as low as 0.25% reflectivity at normal incidence. V-type coatings, AKA narrowband anti-reflection coatings, are best suited for laser application, since they can show reflectivities lower than 0.25% on common lens substrates.

Mirrors and partial reflectors

• Broadband vs reflectivity: generally speaking, the more broadband, the lower reflectivity and the lower damage threshold. For very broadband mirrors requirements with more than 1 µm bandwidth, the only solutions available are the metallic coatings. Bare aluminium for instance has >86% reflectivity from 200 nm to well over 20 µm
• Working in transmission requires a partial reflector. This is used for instance to sample 1% of a beam for diagnostic purposes. Mirrors are not suitable because their back face is generally uncoated with non laser polishing. Beamsplitters can also do that job.
• Incident polarisation plays an important role in how well a mirror performs. Although mirror are tuned for a specific polarisation and angle of incidence, some configurations are physically limited. Best results are achieved when used at an angle with S polarisation or at normal incidence. At equivalent coatings, a mirror tuned for P polarisation at 45° incidence can hardly achieve less than 2% loss in reflectivity compared to its S equivalent. Sometime the loss is even much higher

Filters

• Filters for fluorescence: the main issue with fluorescence is to separate the excitation light from the usable signal. Although both are generally far from each other in terms of wavelength and a sensible set-up would use the fluorescent light on a different optical axis than that of the excitation light, fluorescence is generally a low-light measurement. As such it is necessary to have the best achievable transmission at the wavelength of interest. We would generally advise a minimum blocking OD of 3 and a minimum transmission of 80%. Bandwidth and cut-off steepness are of lesser importance provided the excitation wavelength is in the blocking range.
• Raman spectroscopy can be considered as a demanding application, because of the weakness of the signal and specially because of its spectral closeness to the excitation wavelength (typically ~10-50nm, sometime as far as 100nm). Therefore we would generally advise to go for high quality filters, such as those made using ion-beam sputtering. This can produce filters with transitions of ~3-8nm. There are four basic types of filters to choose from: Long-Wave-Pass (LWP) Edge Filters, Short-Wave-Pass (SWP) Edge Filters, Notch Filters, and a Laser-Line Filters. Laser-Line Filters will transmit only the excitation light (less noise), and Notch Filters are an obvious choice for blocking the excitation line. In systems using these two filter types, both Stokes and Anti-Stokes Raman scattering can be measured simultaneously. However, in many cases Edge Filters provide a superior alternative for both Transmitting and Blocking filters. Edge filters offer better transmission, higher laser-line blocking, and the steepest edge performance to see Raman signals extremely close to the laser line.
• Broadband attenuation is easily achieved using a ND filter (Neutral Density). Those filter have an uniform spectral response (typically over 400-1200nm) but are limited in the amount of power they can handle (typically less than a few watts). Higher powers require a partial reflector. They are extremely useful for photography and to measure some light with a low damage threshold instrument. They are normally defined by their OD (optical density) value: their transmission on a scale from 0 to 1 is equal to 10^-OD. For instance a ND filter with an OD of 3 will transmit 10^-3=0.001, which equals 0.1%.

Beamsplitters

• Wavelength separation / mixing: a beamsplitter can be used to separate or mix beams at different wavelengths. In this case they act as a mirror at one wavelength and as a window at the other wavelength. When building a system that involves wavelength separation, one must pay particular attention to its own design choices: physical laws limit what is achievable with a beamsplitter. When spectral purity at a wavelength is most important, it is better to transmit this wavelength and reflect the others. If getting as much light as possible at a wavelength is the main target rather than cleaning the spectrum, then it is better to reflect that wavelength and transmit the others. Finally, control of the polarisation is important when working at 45°. Best results are achieved when reflecting S polarised light and transmitting P polarised light. Good result are achieved when reflecting and transmitting P polarised light. Other combinations must be avoided if possible, the worst one being reflecting P and transmitting S.

A step towards the future?

Last year, a group of researcher^[1] at the Rensselaer Polytechnic Institute in Troy, N.Y., reported achieving broadband virtually reflection-free anti-reflection coating using oblique-angle deposition. This coating results in a thin-film of nanorods, which grow by themselves. Self-organisation of the nanorods is obtained by tilting the substrate during the coating process. This technique is also susceptible to increase highly reflective coating to near 100% reflectivity. If this finds its way to production, the market is likely to be huge.

Reference
1. Xi, J.-Q., Martin F. Schubert, J. K. Kim, E. Fred Schubert, Minfeng Chen, Shawn-Yu Lin, Wayne Liu, and Joe A. Smart “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection” Nature Photonics 1, 176 (March 2007)

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