LASER SAFETY. 5 Dye lasers (3-6) to 1 mj/pulse 20 Hz or 1 khz up to 1

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1 LASER SAFETY Abstract - This is an in-house manual on safe laser practice. It is designed to supplement government and University regulations, but it is strictly subordinate to them. Introduction Our laboratory resides in rooms 1081 and 1087 of Smith Laboratory. With the exception of a small Helium-Neon laser, every laser in this laboratory is a "Class 4" laser. The Helium-Neon is Class 2. Class 4 is a federal classification meaning that a laser constitutes a hazard to eyes and skin, and can start fires (see the Appendix). Here is a list of the lasers in our lab: Laser Wavelength (nm) Pulse Energy Pulse rate Power (Watts) 1 Millenia 532 (not a pulsed laser) 5 2 Merlin mj/pulse 1 khz 15 3 GCR , 532, , 150, 50 mj/pulse 20 Hz 12, 1.8, Kerr Lens nj/pulse 87 MHz 0.5 Modelocker 5 Dye lasers (3-6) to 1 mj/pulse 20 Hz or 1 khz up to 1 All of the above lasers are pulsed, except for the Millenia, which runs continuously. The pulse width of the others varies from 10 ns to 30 fs, depending on the laser. The visible wavelength range is roughly 400 nm to 750 nm (blue to red). Nonetheless, the Kerr Lens Modelocker is visible because of its power. The GCR 150 wavelength list begins with 1060 nm followed by other values. This laser actually only produces light at 1060 nm, but nonlinear crystals are used to generate the other colors. This raises an important point: non-linear techniques can produce any color in the spectrum besides the many listed above. Finally, most of the lasers in our lab do not operate independently, but jointly in a system. For example, at any time our lab may have several dye lasers in use, and these are pumped either by the Merlin or the GCR 150. Currently, the Millenia, Merlin, and Kerr Lens Modelocker are combined to form a short pulse laser system with these specifications: Short Pulse Laser System Wavelength Pulse Energy Pulse Rate Pulse Width Power nm 1 mj 1 khz 30 fs 1 W The key point is this: There is a broad range of laser wavelengths and energies present in this lab. You will typically have to protect yourself from multiple beams at multiple wavelengths simultaneously. 1

2 Laser Hazards Electrical: More people have been killed by electrocution from the laser electronics than blinded from exposure to a laser beam. Lethal voltages are present in the power supplies of all of our lasers, as well as in the lasers themselves. If you are not experienced working with high voltages in general, and laser power supplies in particular, you have no business doing any work inside a power supply except the regular maintenance described in the relevant laser manual (invariably done with the power off and the power supply unplugged from its electrical outlet). Fire: The only lasers that could easily start a fire are the Millenia and the Merlin. If their beams hit anything flammable, you are almost guaranteed of ignition. Care must be taken with all the lasers, of course. Eye and Skin Damage: The greatest daily hazards are in this area. All of the Class 4 lasers in our lab can easily cause total blindness or painful sores. The type of damage inflicted depends on the wavelength, pulse energy, pulse width, power, and repetition rate. Damage from ultraviolet radiation tends to reside in the outer layers of the skin for body exposure or in the cornea if the eyes are exposed. Low energy/power beams can cause burns like sunburns. Higher energy/power can ablate skin. In general, long-term exposure to ultraviolet beams or even diffuse scatter may lead to cataracts, glaucoma, and skin cancer. Optical and infrared radiation penetrate more deeply into the body. Beams entering the eye are focused by the cornea onto the fovea. The intensity of the beam at the focus can be enormous, even though the total energy present in the beam may be small. Not only can the region of the fovea at the focus be damaged, but the laser can generate a shock wave damaging other parts of the eye. Eye and skin damage may be immediately painful, or not noticed for a time. It is difficult to place an upper limit on the energy a laser pulse can have and not cause blindness because of the many factors involved. Any beam should be considered hazardous. Even diffuse scatter can be dangerous, especially if the radiation is in the ultra-violet. There will in general be many more beams in the lab than the ones coming directly from the lasers. Every time a laser beam passes through any object, there will be reflections from the input and output surfaces. Suppose a laser passes through a lens. One or both of the surfaces will be curved, hence, the reflections that leave the lens will either converge or diverge. The converging beams can hurt nearby people or damage optics. Diverging beams are also dangerous because they can fill a large area with laser radiation. Unless everyone in the lab is careful, the lab will be filled with dangerous light. We work hard to contain all the beams in the lab. 2

3 Personal Safety The next section will cover controlling stray beams. Given that this has been done, an enormous step has already been taken toward protecting yourself. The main threat, then, lies to your hands and eyes. Typically, you will be continuously adjusting optical components, and so necessarily placing your hands close to a beam. Invariably, you will get hit once and awhile. Usually this will just sting a little. There are a few beams in the lab, however, that can give a nasty bruise. Before you modify or work on any system, you must know exactly where all the beams are and how powerful they are. Every intense infrared or ultraviolet beam must have its path clearly marked, or it must be shielded. When you are planning a new beam path, at any wavelength, try to place the optics so they may be easily reached without your hand crossing any beams. Your eyes are your greatest concern - they are the most easily damaged. We have laser goggles which provide some protection from the Millenia, Merlin, and GCR 150 lasers. The degree of protection will vary depending on the wavelength and pulse energy involved (see the Appendix). There is a problem with using goggles, however. Our goggles are nothing more than absorbing filters which try to block certain laser lines while permitting enough ambient room light to transmit so the user can see. Typically, if you tried to block all the wavelengths present in an experiment, you would be unable to see anything at all. Worse the lab is often darkened for one reason or another. Moreover, if you are trying to align a beam you need to be able to see it - just the thing our goggles try to prevent. There will be many times, then, when you will not want to wear goggles or wearing them will not be sufficient protection. Nonetheless, there is much you can do to protect yourself. Eye glasses offer extra protection against ultra-violet light without degrading overall vision. There is often diffuse UV scatter present in the lab and the hazards from long term exposure to such radiation have already been noted. The amount of protection offered varies with wavelength and lens type, but is easy to measure. Eye glasses also present a hazard. No beam should ever be at eye level, but if one were, a user's eye glasses could direct light into his or her eyes that would otherwise miss. The principle threat comes from surface reflections off the lenses from beams directed towards the side of the face. In effect, glasses make the user's eyes bigger targets. As described in the next section, by design beams produced by stray reflections and diffuse scatter travel mostly in a plane at waist height and parallel to the floor. You must never allow your open eyes to intersect this plane. For example, if you drop a tool on the floor, you put yourself at risk when you bend down to pick it up. You can protect yourself by simply keeping your eyes closed until you are below this plane - make this a habit. Often, you will want to look at some part of your apparatus that requires you to place your eyes in a dangerous position if the lasers are present. Do not depend on electronically controlled beam stops to protect your 3

4 eyes and do not use your hands as beam blocks. Use a beam block - a stable mounted shield, or a "beam eater" - a box designed to contain laser beams. There will be times when you wish to view your experiment in progress, and can do so only by placing your eyes near a beam. This is unacceptable. You must be willing to take the time to devise a safe means to view it. Alignment Procedures Most laser accidents occur while aligning an optical system. When you first place a mirror in a beam, you do not know where the reflected beam will go. If the vertical adjustment happens to be near the end of its range from the last time it was used, the beam could be sent careening upward - eventually to reach eye level. Fortunately, it is easy to align a laser system safely. The first rule is to align using the lowest beam power you can. There will usually be times when high power beams must be aligned, but the coarse alignment must always be done at low powers. If you want to direct a beam into an area where someone else is working, you must inform this person and agree amongst yourselves how to do this safely. It may be that one of you will have to wait until the other is done. (This is all obvious, of course. This document does not wish to suggest that safe practice requires much more than common sense. The goal of this document is to guarantee that everyone sees these ideas formally, at least once.) The greatest danger occurs when a mirror is first placed in the beam. This danger is completely nullified if a beam block is placed in the reflected path before the mirror is placed. Since you are doing this coarse alignment with low powers, almost anything will serve as a block. Once the mirror is in place, you can carefully move the block to the location of the next optical component, always keeping the beam on the block. You should follow the same procedure with any optical component. Lenses require a little extra care, however. Here you must also know where the reflections will go and how they are focused, before you place the lens. Also, note that a focusing reflection can easily damage any nearby optic. (If you notice a spark appearing just before or after a lens, that is probably a reflected beam breaking down the air.) The lasers that generate the beams are all at roughly waist height. You should strive to keep your beams at this level. This tends to keep spurious reflections in this plane where they are easy to block. Also, everyone is careful to keep their eyes well above or below this level. However, there will certainly be times when you have to send a beam from one level to another. Just be aware that this is when you are most likely to create a reflected beam moving upward at some random direction. We have a large laboratory, and if this beam is not blocked it will reach eye level. This is simply not acceptable. 4

5 You will often have high power beams present that you no longer need. For example, you could send a 1064 nm beam through a nonlinear, doubling crystal to generate 532 nm light, after which you may no longer need the 1064 nm light. We have high power "beam eaters" that can swallow any (non-focused) beam generated in the laboratory. If none are available, they are easy to construct. Ask someone. Special Hazards This is a description of some components that pose special hazards. Dielectric Mirrors: If a mirror is a dielectric mirror, than it can actually transmit if it is placed at the wrong angle initially. Even if it is at the correct angle, there can be leakage light through it. Usually this can be blocked with black tape. Glan-Thompson Polarizers: These polarizers work by transmitting light of one polarization and reflecting light of the orthogonal polarization. They are often placed in mounts that allow them to be rotated. As one is rotated, the reflected beam traces out a cone which can easily intersect your eye or anyone else's in the laboratory. The output port for the reflected light must be blocked unless you intend to use this light. (Note: if the output port is blocked, you can damage the polarizer if you send a high power beam through it.) Gratings: Here, you just need to be aware that a grating will typically have multiple beams leaving it from the different diffraction orders. Each one must be blocked if it is not used. Dye lasers: These tend to produce a large number of extra beams as well as copious diffuse scatter. This light should be well contained, but do not assume it is. This is difficult to arrange during alignment. Infra-red Viewing Cards: These cards glow red when exposed to infra-red light. They are essential when working with weak infra-red beams. The problem is they are often laminated and the lamination acts as a good reflector. You must always hold one of these cards such that a specular reflection would be directed downward. Actually, this is a good idea for anything you place in the beam path. These cards must not be used in high power beams. Infra-red Viewer: This is a device that converts IR light to electrons which then create an image on a screen. It looks a little like a camera or monocular, however it should not be treated as such. A beam that enters the device can reach your eye. This is for surveying a scene only. Machined Hardware: You will probably have to machine your own mounting hardware on occasion. Generally, this presents no difficulty. Still, you must avoid creating shiny surfaces. These can cause dangerous reflections. If you use aluminum, the hardware can be anodized after the machining is complete if there is a dangerous surface present. Anodization is a 5

6 chemical process that gives aluminum a flat black outer layer. Most of our optical hardware is made from anodized aluminum. Watches and Jewelry: The face of a wrist watch is reflective, as are rings, metal wristbands, and jewelry. If you are wearing a wrist watch and, as you reach towards an optical component, the face intersects a laser beam, you will send a dangerous reflection in an uncontrolled direction. All such apparel should be removed before working with laser radiation. Conclusion This document has described how to work with lasers safely in our lab. The guidelines given should be followed not only for your protection, but to keep you from endangering others. Safe laser practice is mostly common sense - it is easy to be safe. You should keep in mind that most laser accidents have not involved novices, but experienced workers. In part, this may be because experienced workers have had more time to make a mistake. Most likely, however, the problem is lack of caution. A beginning user is especially aware of the dangers involved, and perhaps even a little intimidated. As time goes by, this initial caution is slowly forgotten. There is no reason to be intimidated, but your safety depends on your continued caution. 6

7 APPENDIX Laser Safety Classification Scheme Laser safety standards, accepted world wide, group all lasers into four general hazard classes described in the table. LASER CLASSIFICATIONS Class Description Maximum Power (assuming a cw Argon ion laser) 1 Eye Safe 0.4 µw 2 Theoretical eye hazard 1 mw 3a Marginal eye hazard 5 mw 3b Significant eye hazard 500 mw 4 Serious eye hazard >500 mw Theoretical eye hazard, refers to a case where a person stares into a visible laser source. The maximum power for a given class depends on the wavelength and pulse width. The values given are for an Argon ion laser, which lases at a number of green, blue, and ultraviolet wavelengths, running continuously. Optical Density and Laser Goggles The filtering capability of our lab s laser goggles is listed here (and on the goggles themselves). The transmission as a function of wavelength is specified in terms of "optical density" (OD). If the optical density at some wavelength is d, then the transmission, T, at that wavelength is given by: T = 10 -d. Thus, an OD of 3 would attenuate the intensity by a factor of Note that these goggles will not give much protection from the Short Pulse Laser System. Glendale LGB Laser Goggles Wavelength (nm) Comment OD Ultraviolet to blue > Green, primary Argon laser line > Green > Green, 532 nm = doubled Nd:YAG fundamental > Near infra-red (IR) > ,070 Near IR to IR > ,000 IR > 7 7