Laser safety can be somewhat of a mine field. Indeed, many lasers in industrial or research setting can cause serious permanent damages to the eye in a fraction of a second, and by choosing a safety eyewear you are trusting your precious eyesight to a pair of goggles.

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.

Characterizing a laser beam is a growing concern in the industry. A great number of instruments are available on the market, each with their specialities. When it comes to analysing the spatial behaviour of a laser beam, the most common solution is the beam profiler. However another solution starts to be affordable and user-friendly enough to be seriously considered by anyone who wishes to take laser beam characterisation to a whole new dimension. It extends its capabilities far beyond the laser applications, and is of very high interest for astronomy, microscopy, optics caracterisation and more.

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).

Because of the wide range of power and energy meter available on the market, and even more because they tend to be not totally versatile, you need to carefully examine your needs against the capabilities of the instrument you are planning to acquire. Here is a little check list to help you decide if a laser power meter or energy meter would fit your application.

• 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)…

The base of laser beam diagnostic is to know how much average power you got. Available off the shelves form different manufacturers are three main type of devices, based either on a photodiode, a thermopile or a pyroelectric detector.

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.

The level of laser exposure which is considered as the limit between safe and potentially harmful is called Maximum Permissible Exposure (or MPE). Maximum Permissible Exposures are set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and are also adopted by standardisation committees.

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.