Practical Tools for Laser Safety and Traps to Avoid

INTRODUCTION

A number of products and solutions exist in the marketplace to help people achieve laser safety. Some are for the laser safety officer's (LSO's) use, some are for the user, and some would be classified as engineering controls, while others fall into the administrative control camp. The goal of this section is to make you aware of these tools. Such tools include laser hazard evaluation software, warning signs and labels, beam alignment and viewing aids, power meters, laser eyewear samples, beam termination and laser radiation containment materials (curtains and barriers), digital cameras, and reference sources.

__2 THE LASER SAFETY "TOOL BOX" WISH LIST

As the authority with jurisdiction over the laser safety program, the LSO is responsible for ensuring "the knowledgeable evaluation and control of laser hazards." The LSO must do this within the confines of a limited safety budget.

The organization is rare that does not have to continually justify its expenditures.

This applies in particular to expenditures for safety programs, including laser safety. While effective safety departments save money by facilitating injury and illness prevention, they are seldom viewed as revenue generators.

__2.1 LASER HAZARD EVALUATION SOFTWARE

An important LSO responsibility is recommending or approving protective equipment, including laser-attenuating eyewear and windows. In order to properly recommend or select such eyewear or windows, the LSO must determine the required optical density (OD). To do this, the LSO has several options: rely on a vendor to determine the correct OD, use the guidance provided in the American National Standards Institute (ANSI) Z136.1 standard to manually calculate the OD, or use commercial software to perform the necessary calculations. Depending on the variety of laser wavelengths used at a facility, software may be the optimal choice. Using software has certain advantages, particularly if one is not routinely doing calculations or has many of them to do. By using default values for some of the OD calculation parameters ( For example, the correction factor and recommended exposure duration), the software simplifies the task of determining OD values.

The authors would be remiss if they did not note that all of the hazard evaluation software programs they have reviewed have some flaws or quirks. In order to confidently use these software programs, the LSO should have a feel for what the correct OD value should be given the particular laser output parameters and the potential exposure condition, For example, intrabeam versus diffuse reflection.

While all the calculations can be done manually, hazard evaluation software does them quickly and provides a detailed report that can be appended to a standard operating procedure (SOP) or other laser safety-related documentation.

Generally, vendors sell licenses to use hazard evaluation software rather than selling it outright. The purchase of a corporate license may be less expensive than purchasing separate licenses for multiple locations within one's organization.

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Above: Fgr. 1 () (a) Laser danger sign as of 2000; (b) notice sign: used for repair or alignment notification.

Laser Alignment in Progress

DO NOT ENTER EYE PROTECTION REQUIRED DANGER ! POSITION 1 BOLD BLACK LETTERING () POSITION 2 BOLD BLACK LETTERING () POSITION 3 BOLD BLACK LETTERING () (RED SYMBOL) (WHITE) (RED) (RED) (WHITE)

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__2.2 WARNING SIGNS AND LABELS

Approving the wording of signs and equipment labels is another LSO responsibility. To ensure that signs and labels are both accurate and properly posted, the LSO may wish to obtain and provide laser users with such signage and labels.

Typically, the LSO has three choices regarding sign and label acquisition: buy the signs and labels from a commercial source, use software developed specifically for making in-house signs or labels, or use a generic software program (Claris Draw, MacDraw, PowerPoint, Freelance Graphics, etc.) to generate in-house signs and labels.

If signs and labels are a limited need or rarely needed, purchasing commercial signs and labels may be the preferred option. When there is a routine requirement for signs and labels, the latter two choices are often quicker less expensive. Using these software options, one can easy customize both signs and labels; location-specific information, such as LSO contact number, user contact numbers, and OD requirements, can be easily incorporated. When deciding how to obtain laser safety-related signs and labels, the LSO should consider the different types of signs and labels that may be needed. Depending on the scope of the laser safety program, the LSO may need laser certification labels, manufacturer identification labels, aperture labels, warning logotype labels and signs, inventory labels, and protective housing warning labels. Making one's own signs and labels using software becomes more attractive when one also takes into account non-laser labeling and signage needs such as emergency shutdown button sign-age, lockout and tagout labels, high voltage warning labels, and collateral radiation warning signs.

__2.3 DIGITAL CAMERA

A digital camera can be a great asset to any laser program. It can be invaluable for documenting laser-related accidents or incidents. Investigative reports are enhanced, and the accident scene is more readily understood. Digital images can be used to clarify SOPs, audit reports, and control measure descriptions and aid in describing safety concerns. Such images can also be important adjuncts to any regulator reports, such as Center for Devices and Radiological Health (CDRH) reports.

As with other devices involving information storage technology, the cost of digital cameras continues to decrease. When new models are introduced with enhanced features, sales outlets for these cameras drop the prices on older inventory items. One need not have a camera with an inordinate amount of megapixels to obtain clear digital images for training as well as for incident prevention and investigation efforts.

__2.4 LASER EYEWEAR SAMPLES

Failure of laser eyewear to fit well or comfortably is a primary reason given for not wearing laser protective eyewear. By having samples of frames or complete laser eyewear available, the LSO can help ensure that the eyewear purchased will be used. Laser users can see whether or not the protective eyewear feels good on their faces and if its visible light transmission (VLT) is sufficiently high to allow the safe performance of critical tasks. Ordering unsuitable eyewear is avoided, money is saved, and the turnaround time for obtaining useful eyewear is reduced.

__2.5 RESOURCE MATERIAL

It’s important that the LSO keep abreast of new laser technology developments that may have a future impact on the laser use environment in which he or she has laser safety responsibilities. Despite the plethora of laser safety-related information on the World Wide Web, magazines and books are still valuable information sources for the LSO. While it’s important to have various written reference sources, whether in hard copy or on the Web, there is no substitute for talking with a consultant or peers when encountering an unfamiliar laser safety issue.

Information exchange and networking is seen by nearly all safety and health professionals as a value added activity. Participation in professional associations allows one to meet individuals with laser safety challenges similar to one's own.

__3 TRANSITION ZONE BETWEEN THE LSO AND LASER SYSTEM USERS

__3.1 BEAM ALIGNMENT AND VIEWING AIDS

As the ANSI Z136.1-2000 standard points out, "laser incident reports have repeatedly shown that an ocular hazard may exist during beam alignment procedures." Aids for beam alignment or viewing include low-power lasers (class 2 and visible class 3R), sensor cards or screens, and viewer or image converters. The use of lower-power laser beams' paths can simulate the beam paths of higher-power lasers while minimizing the potential exposure hazard. Converters and sensor cards can be invaluable to both the LSO and laser users in determining the adequacy of efforts to properly block beams and their stray reflections.

__3.2 VIEWERS

In general, infrared (IR)-wavelength alignment aids are generally more available than their ultraviolet (UV)-wavelength counterparts. IR viewers enable their users to locate invisible beams in the near-IR (NIR) portion of the spectrum, generally up to 1300 nm, although IR viewers can be found that extend the IR viewing range out to 1500 or 1700 nm ( Figure 5.2 and Figure 5.3). While less common than their IR viewer counterparts, UV-wavelength viewers are also available.

__3.2.1 How Do They Work?

IR and UV viewers are image converters rather than image intensifiers. The principle is based on a photocathode image converter. It translates the IR power distribution into an image visible through the eyepiece. An extremely low-level (picowatts of IR) is sufficient to produce an image.

Above: Fgr. 2 Assorted infrared viewers.

Above: Fgr. 3 IR viewer (size comparison).

Above: Fgr. 4 Well-used IR sensor card.

__3.3 SENSOR CARDS

Similar in purpose to viewers, sensor cards can also be used to locate invisible beams, either UV or IR ( Figure 5.4). These cards and converters permit invisible UV and IR beams and their reflections to be "seen." The phosphors in these devices are designed so that one can see the up-converted or down-converted invisible wavelengths. One should select sensor cards with care since they respond to different invisible wavelengths and minimum power density (irradiance) thresholds. Some sensor cards generate specular reflections, and appropriate laser eye protection should be worn as a precautionary measure. Sensor cards are available in a variety of sizes and shapes, and some of them require preactivation using UV or visible radiation.

__3.3.1 How Do They Work?

The cards contain a special sensitive area coated with a chemical phosphor layer, which emits clearly visible light when illuminated by NIR, IR, and some UV sources. IR sensor cards work on a principle known as electron trapping, where phosphor-based compounds are employed to absorb and "trap" incoming light energy from a short wavelength and release that stored light in the form of visible light upon stimulation from a longer IR wavelength. The visible result is a localized glow, which is relative in intensity to the amount of stored light and IR power levels exciting the active area. Many cards have the sensor area encapsulated between durable clear plastic layers, allowing for easy handling.

__3.4 OPTICAL GLASS SENSORS

While not as common as IR sensor cards, optical glass sensors do have applications. They are constructed of optically clear glass with specific phosphors adhered to one side of the glass substrate. IR radiation emitting from a laser or LED source will be visualized from either side of the sensor screen.

__3.5 BEAM BLOCKS AND DUMPS

Beam blocks, backstops, traps, dumps, and stops provide a means to terminate unneeded laser energy and keep beams confined to a particular area, such as an optical table. These can be commercially obtained or manufactured in-house.

An example of an in-house beam block would be L-shaped 1/8 to 1/4 inch thick, 8 to 14 inches high, and 2 to 12 inches wide, made of anodized aluminum. The anodization process should be of a beaded pattern, rather than a flat shiny black surface. The diffuse surface reduces the chance of specular reflections. The base of the L would have, depending on the width, holes drilled into it, allowing it to be secured to an optical table. The beam blocks can be located behind the optics or turning mirrors. Beam dumps are generally used as part of a system to capture a split-off beam

__3.6 BEAM CONTAINMENT ENCLOSURES

Enclosures around laser setups or systems are a great aid to laser safety. These can be composed of interlocked or noninterlocked panels. Once again, commercial firms manufacturer enclosures to user specifications or they can be build in-house. Usually, they are a metal frame with opaque panels.

__3.7 CURTAINS

Laser curtain barriers are either used in a fixed mode capacity or a temporary set-up. The LSO at one time or another may face the issues surrounding the need for a temporary control area barrier. While such a barrier is usually needed during equipment service and maintenance, it can also be useful during laser alignment in an open area or an area not designed for open beam hazards. A fixed mode of use would be when one is trying to separate the laser use area from a larger space. These curtains are similar in appearance to many welding curtains; they are designed to withstand either direct or, more commonly, diffusely scattered laser beams. Barriers must be opaque to the laser wavelengths in use, cannot be combustible, and must be designed to withstand the intensity of the laser beam that may strike them. Barriers can be passive or active. Most are passive. Commercial laser curtains can be a rather expensive; at times more cost-effective alternatives are available. Commercial laser curtains are certified by the manufacturer to withstand laser irradiation in general at the level of 100 to 300 W/cm2 for beam sizes of 5 mm or less when the exposure time is 100 sec or less. Higher-irradiance models are also on the market. Laser curtains can be purchased on portable frames to provide a temporary containment when it’s needed.

__3.8 ACTIVE BARRIERS

The active guard or enclosure incorporates a dedicated sensor into the passive guard to detect the presence of an errant beam on the enclosure wall. The guard has only to maintain its integrity for a time sufficient for the sensor to detect the beam and feed a signal back to shut down the laser. The sensor may be intrinsic or extrinsic to the guard. Active guarding systems were developed for use in conjunction with CO2 lasers and Nd:YAG laser fiber optic delivery systems.

An example of an active guarding system for use with high-power CO2 lasers is the hot spot detector (HSD) developed by the Safety and Reliability Directorate (Culcheth), Atomic Energy Authority (Culham), and SIRA. The HSD is an errant beam detector; its function is to detect potential hot spots on the walls on the enclosure and to feed back a signal to shut down the laser system. It was considered that the detector could make it possible to relax constraints on enclosures to resist penetration and use more lightweight enclosures. After several trials of the system to determine response time compared to burn-through time, detection ranges, and so on, the following enclosure strategies were proposed for use in conjunction with the HSD.

For powers up to 1 kW (continuous wave [CW]), 1.5- to 2.0-mm painted steel panels with 6-mm thick polycarbonate windows provide a safety factor of 1:10 in terms of detection to burn-through time for beams of 10 mm; for powers >10 kW, 1.6-mm painted steel with 6-mm polycarbonate windows provides only a safety factor of 1:4 (1:5 deemed acceptable). Double skin panels filled with sand or water would improve the safety factor.

Because of the variation of burn-through time with beam polarization, it was suggested that 100-mm thick thermal building bricks would provide the best solution. Clearly, the use of such fortress style enclosures defeats the initial object of the HSD in offering an inflexible solution. Furthermore, this active approach is highly reactive, its entire operation being based on the occurrence of the hazardous event.

Above: Fgr. 6 Illuminated warning sign (can flash or be steady light).

__3.9 ROOM ACCESS INTERLOCKS

In some circumstances access to a laser use area may require an interlock system.

The purpose of these systems is to keep unauthorized personnel out. The majority of laser use facilities build their own interlock access systems. A limited number of commercial laser access interlock systems are on the market. These systems shutter or deactivate laser systems in use. At times just room access control will achieve a similar level of safety. This includes key card access. Card access, while not affecting the laser, has many advantages. It can restrict access to only those authorized and even monitor training compliance.

__3.10 ILLUMINATED WARNING SIGNS

Particularly when a class 4 laser is in use, an indicator of that use is required.

The most common means of indicating that use is through an illuminated sign or visual indicator (light) immediately outside the laser use area ( Figure 5.6).

These devices can be obtained commercially. A word of advice: Such illuminated signs will serve you better if the illumination source is LED based. The extended lifetime of LEDs versus standard light bulbs or torches can justify not incorporating a failsafe mechanism into the warning sign. With a fail-safe mechanism, if the light burns out, the laser won’t be able to fire or a shutter will drop in front of the laser beam.

Illuminated signs fall into different styles:

1. Illuminated warning sign that looks like a laser warning sign

2. Illuminated sign with message levels, that is, laser off, laser powered, beam on

3. Illuminated signal with no indication of purpose (red light over door)

4. Illuminated programmable sign (with custom information, such as wavelength information) The light over the door, in the author's opinion, needs to have an accompanying sign indicating what it means, For example, "light on equals laser on."

__3.11 POWER METERS

The LSO can use laser power meters to determine or confirm the classifications of laser products or systems. For example, the LSO can use a power meter to determine if the output of a helium neon laser labeled class 3b has degraded enough to allow its reclassification as class 3a. Such reclassification allows the relaxation of laser safety-related control measures for that laser's use. The LSO may also use a power meter to self-certify laser barriers and attenuating windows.

Loaning power meters to laser users may also help develop good rapport between those users and the LSO. The majority of power meter use is designed for user applications.

All power meters contain the four basic elements. Light striking the detector results in a current flow through the detector. The amplitude of the current is proportional to the total optical power striking the detector for power within the detector design range. This current is amplified and used to drive the meter read-out device. This may be an analog or a digital display. It’s the detector's role to convert the incoming optical signal into an electrical signal, which is then amplified and recorded. Most power meters are modular. They typically consist of two parts: a main control unit that features a read-out display, signal-processing electronics, and an interface for transferring data to a computer, and a detector head that is placed in the path of the light beam to be measured. Many manufacturers sell a wide variety of detector heads designed for different power levels.

These simply plug into the main unit and can greatly enhance the flexibility of a power meter.

Power meters can be divided into two types, each of which detects power by a different method. The first type uses thermal (thermopile) detectors, while the second type relies on semiconductor photodiodes. A special kind of pyroelectric thermal detector is also available for measuring pulse energies rather than power.

If you are looking for a general-purpose power meter for measuring CW laser powers of more than a few milliwatts, a thermopile-based solution is probably the most appropriate. Thermopile detectors offer a very wide, flat spectral response with a high damage threshold. They are a popular choice for those working with more powerful lasers such as Nd:YAG, Ti:sapphire, carbon dioxide, excimer, and argon ion. Detector heads for taking measurements from around 0.5 mW up to 10 kW are commercially available with a spectral range from 200 nm to 11 µm.

For very sensitive power measurements at a wavelength of less than 2 µm, a semiconductor photodiode is the best choice. These can measure powers as small as a fraction of a picowatt and are often available with heads that are specially designed for accepting optical fiber connectors for those working in the telecommunications field. Semiconductor-based meters are also available for making pulse measurements such as pulse energy and peak power.

If you need to measure light from a strongly diverging source such as a laser diode or the end of a bare optical fiber, consider an integrating sphere. If you are looking for a low-cost, convenient solution for quick but less accurate measurements, battery-powered hand-held semiconductors and thermopile probes are now available. However, these cannot transfer data to a computer and are not as precise as more sophisticated models.

When selecting a power meter as a consumer purchase, it’s important to know what you want and to find a product that meets your particular applications.

In doing this you will find yourself with the correct product, not one with features you will never use or need. So, determine what you need by asking yourself a number of questions:

Do I need to measure the power of CW beams or the peak power or energy of individual pulses? Most meters are designed for one task, although some are compatible with heads that allow both. Thermopile based systems are often useful for measuring the power of CW lasers or the average power of pulsed lasers above a few milliwatts. Photodiode based systems can also make both pulsed-power and CW measurements but are often limited to low powers. Both photodiodes and thermopiles can calculate or infer the energy per pulse, while pyroelectric detectors can directly measure pulse energy.

What is the maximum and minimum detectable power I need, and is a probe with the appropriate performance available? If you need to measure very low power levels (below 1 mW), then a photodiode-based solution is often best. Sensitive, low-noise photodiode heads that can measure subpicowatt power levels are now available. By contrast, if you are checking power levels in the watt or kilowatt regime, a thermopile solution is best.

What response time do I need? Thermopile probes often need to be left in the beam for a few seconds. Semiconductor detectors, however, are designed to have a fast response.

Can the meter connect to a wide range of probes in case requirements change? Probes can usually be disconnected from the meter and exchanged with a different model to suit different power ranges or wavelengths. See what is on offer and how much the individual probes cost.

Is the meter's calibration traceable to internationally recognized standards? If the results from the meter are to be trusted, it’s vital that it has been properly calibrated. For complete confidence, check to see if the meter is traceable to a standards body such as the U.S. National Institute of Science and Technology (NIST). Also find out at what wavelength the calibration was performed; ideally, you want it to be as close to your operating conditions as possible. A properly calibrated meter will be able to make a measurement with an uncertainty of less than 2 to 3%. Be sure to recalibrate the meter regularly; otherwise it may start to lose its accuracy. Most firms recommend annual recalibration. Also find out what calibration costs are from the manufacturer; this is a hidden cost that may affect how often you can afford to calibrate your equipment.

How large is the meter's detector entrance aperture? Make sure that the diameter of your light beam is not larger than the detector. If you need to measure a strongly diverging beam, then consider using your detector with an integrating sphere, which is guaranteed to collect all the light.

__3.12 DETECTORS

Detectors used in laser measurements can generally be characterized as photon detectors and thermal detectors. The photon detector responds to the number of individual photons incident onto the active surface of the detector, whereas the thermal detector responds to total optical power. In the thermal detector this input radiation is absorbed by the detector, producing a temperature increase that then results in a change in some other parameter, such as resistance, contact potential, or polarization. Thermopile detectors, also known as thermal detectors, measure optical power by sensing the heat that is released into an absorber when it’s irradiated by a light beam. The detector head is made of two parts: an absorbing front surface and a cooled heat sink. A thermocouple built into the head is used to generate an electrical signal that relates to the difference in temperature between the absorber and the heat sink. This signal is passed to the control unit for conversion into an optical power reading.

Thermopiles are detectors that measure beam power by measuring tempera ture differences within the detector that are produced by the heating effect of the laser beam. They consist of metal disks connected to heat sinks at their edges.

This heat sink may be either air cooled or water cooled. Thermocouples located at the center of the disk and the outer edge are connected in series to produce a voltage that is proportional to the temperature difference from the center of the disk to the edge. During CW power measurements, the thermopile operates in a steady state with a constant heat flow and constant temperature difference. When the power changes, new thermal equilibrium conditions must be reached before the reading will be accurate. Thus, the response time of a typical thermopile is longer than 2 sec. Thermopiles are useful for all wavelengths at power levels above 1W. Air-cooled detectors are used for powers up to 300W, and water cooled models may measure powers of a few kilowatts.

__3.12.1 Pyroelectric Detectors

This type of thermal detector uses a ferroelectric crystal instead of a thermocouple to sense the temperature difference. Incident photons heat the crystal, causing an electric current to flow. This kind of detector is used for directly measuring the energy of an optical pulse. It cannot be used for measurements of optical power.

Pyroelectric materials are nonconductors in which the electrical polarization is a function of the temperature of the material. It consists of a slab of pyroelectric material with electrodes deposited on the surface where the polarization appears.

The charge on the electrodes corresponds to the polarization of the material and thus to its temperature. When the temperature changes, current flows through the load resistor. Thus, pyroelectric detectors respond to the change in detector temperature and cannot be used for direct measurement of CW laser power. For CW radiometers, the input beam to the pyroelectric detector is converted to a pulsed input by a light chopper. The system is calibrated to indicate the true CW power of the beam. Pyroelectric radiometers are useful from the blue end of the visible spectrum through the far IR for low-power beams. They are most important in IR regions where other low-light-level detectors cannot be used.

__3.12.2 Semiconductor Photodiode Detectors

Photodiodes are usually made from silicon, germanium, and indium arsenide and directly convert incoming photons into an electrical signal. This kind of detector offers a fast and sensitive response, but they are highly wavelength dependent.

__3.12.3 Integrating Sphere Detectors

These are spherical domes that are used to collect and homogenize the light before it enters a detector. An integrating sphere detector ensures that all the power is collected from a strongly divergent source and is a good way to obtain very accurate, reproducible results.

The most common photodiode detector is the PIN photodiode. In this device there is a region of intrinsic semiconductor material between the p and n regions.

This intrinsic region increases the width of the junction, which in turn provides for more efficient conversion of photons to charge carriers. These devices can be operated in either a photoconductive or photovoltaic mode. Silicon PIN photo diode detectors are probably the most widely used laser detectors in the visible and NIR regions of the spectrum.

__4 LASER SAFETY TRAPS

A number of traps or obstacles to laser safety have developed with the growth of laser technology. This section highlight those traps, in the hope that an alert laser user will avoid them. These traps have been the cause of or a contributing factor to many a preventable laser accident.

__4.1 800-NM ICEBERG

To paraphrase Rod Sterling, " You are now entering the iceberg zone." It’s well known that only 12 to 20% of an iceberg is above the waterline, meaning it’s extremely hard to determine its actual size by visual perception alone. The same is true of light past 700 nms. Our eye detects less than 1% of the available photons at such wavelengths, thus yielding a faint image. Mentally, we equate that faint visualization with a weak or low power source.

Many a researcher has been injured while attempting to view, For example, 751-nm or 810-nm beams they perceived only faintly. In almost all these cases, the user knew the actual beam output, but that fact was masked by the faint equal weak theorem, or iceberg effect. Needless to say, they were not wearing any protective eyewear to look at such a weak beam. NIR laser diodes, Ti:shappire, and Alexandrite lasers are some of the lasers that have been the source of such wavelengths in many of these incidents.

The best way to avoid this trap is awareness, making sure users have been informed of this problem and reminded of it. In addition, alternative means of viewing such as CCD cameras, IR sensor cards, and, as always, laser protective eyewear are good options.

__4.1.1 Beam Visualization Chorus

If Pink Floyd's The Wall were a laser safety song, the most famous line would be, "How do you align if you can't see the beam?" This refrain has been a chorus from laser users since the dawn of laser technology. Of course, it has a true side, but rather than being the mantra that justifies laser alignment without protective eyewear, it should be the signal to call the LSO for input.

The purpose of alignment eyewear is to allow the user to visualize the beam while lowering the intensity of any beam that is transmitted through the user's eyewear to a class 2 level. To address this issue there is a European norm that recommends the OD for alignment eyewear versus the output of lasers used.

This author has often met laser users who proudly show off their laser protective eyewear for visible lasers, but when asked how they align with them on, the answer is silence or a lifting up of the eyewear. So, how do you align safely? One option to consider is the use of laser-alignment eyewear, which provides partial protection and allows visibility. A second is remote viewing by a CCD camera, which, especially with the newer small cameras, can be positioned to view a target, mirror, or other optics. With the addition of motorized mounts, alignment can be made without the users being at risk. The cost of CCD cameras and video monitors has dropped sharply in the past few years, making this a real option. As a third option is the old standby of lowering the laser's output power during alignment by neutral density filters or control of power supply current.

Very simply, full power is rarely needed to align a beam path. A fourth option would be using a low-power coaxial beam to show the beam path, generally a class 3A (or 3R). Yet another simple option is fine-tuning the alignment by the use of an iris shutter or a series of irises and holes in a senor card. A simple but under utilized technique is the Hartman plate for telescope alignment. Last, we have fluorescent plates, paper, and sensor cards; even some cross-hair setups can aid in alignment procedures.

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TABLE 5.1 European Norm 208 Optical Density recommendations for Alignment Eyewear

Scale # OD Max instantaneous power | Continuous Wave laser (W) | Maximum energy for pulsed lasers (J)

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__4.2 REFLECTIONS: VISIBLE OR INVISIBLE

A number of laser incidents can be traced to stray reflections from optical setups.

These incidents fall into three categories:

1. A reflection that was present but unknown and not looked for

2. A reflection that was known but thought not to be a problem

3. An instant reflection generated by some action (e.g., moving the power meter into live beam, reflection off a tool) 5.4.2.1 Present but Unknown Reflections

During the set up and alignment of a laser system, it’s essential to stop several times to check for reflections leaving the plane of the table. Using an IR or UV viewer or lowering the light level are acceptable approaches.

Consider this example from a September 2004 Department of Energy incident report: While aligning the diagnostics for an ultrafast Ti:Sapphire class 4 laser (800 nm), an experimenter raised his laser safety eyewear to rub his eye to alleviate an irritation due to an existing eye infection. He felt a bright flash and afterwards a light cloudiness in his left eye. Repairs on the laser were completed earlier in the day. In his eagerness to get his experiment underway, the experimenter introduced beam onto the table while he aligned the optics. He rotated one of the polarizing beam splitters. The secondary beam was not considered or accounted for, therefore not blocked or contained. By doing so, an unwanted/undetected beam left the plane of the optical table at an upward 45 degree angle, which subsequently struck his eye.

__4.2.2 Known Reflections

One of the biggest mistakes one can make is to know of a reflection that leaves the table and not block or contain it because one does not think it’s worth the effort or presents a hazard. An experimental setup had an invisible reflection (3000 nm) leaving the table at such a steep angle that it struck a spot 8 feet up on an adjacent wall. Because the reflection left the table at such a steep angle and no one was over 8 feet tall, the decision was made to just disregard it - until someone was being shown how to place the offending optic in place. During the placement of the optic, the reflection traveled up and down the wall and struck the person being instructed, who was standing directly opposite the optic to get a better view of the procedure. Another fact in this incident is that a coaxial HeNe (633 nm) beam ran with the 3000-nm beam. The injured person could have worn eyewear that would have allowed 633-nm visualization but blocked the 3000-nm beam, but no eyewear was worn.

__4.2.3 Instant Reflections

The technique of moving a power detector head into an active beam is poor practice, no matter how one defends it. This practice is not as uncommon as we in laser safety would like to think. LSOs start examining power meter detector heads for burn marks and burn-off coatings. Jewelry and ID badges are ready made reflective sources if not removed (in the future all such valuables can be sent to this author for safekeeping). In such cases injury could be prevented if all staff in the laser use area wear laser protective eyewear. Poor housekeeping on the optical table is a set-up for a reach into the beam path or bringing reflective tools, optics, and so on into the beam path.

You may ask, "How can I see visible reflections if I am wearing my eyewear?" This is a valid question because even with alignment eyewear, some visible diffuse reflections may be hard to see. A possible solution is to view the room through a digital or video camera; this way you might be able to image the source area.

Other options are to look from a known safe vantage point or view the area with an IR viewer - IR may be leaking through with the visible light.

__4.3 HOUSEKEEPING

Any investment counselor will tell you real estate is a premium asset. The same is true for laser labs. If only our labs were like the TARDIS from the BBC Dr.

Who TV series, which is larger on the inside than the outside. For those of us who have not solved the space-time dimensional problem, space is a real issue. Even as lasers become smaller, we still find objects to fill all our space. Therefore, many laser labs looks like people's garages. Unfortunately, this clutter is not confined to the space around the optical tables, but is also on optical tables themselves (even vertical optical table set-ups are not immune to clutter). Spare optical mounts, tools, lenses, mirrors, plastic bags, and plastic and cardboard boxes all tend to find a home on optical tables. Rather than being in a staging area separate from beam paths, they are under and next to active beam paths, acting as lures to attract hands into live beams or a reflection source when lifted through the beam.

Some solutions are arranging a staging area on the optical table outside the active beam line; constructing a second, upper, level on the optical table; and organizing cabinets, removing unused items that have been taking up space since 1980. Secondary storage for such items is an option all facilities should consider as well as a housekeeping day once a quarter. Even if you clean up a lab today, things will creep back in over time. Setting designated housekeeping days, just like preventative maintenance days, frees one from the pressure of stopping while project deadlines loom.

__4.4 FIBER OPTICS

Say "fiber optics," and the response is like the story of the blind men describing an elephant; the answer all depends on what part they are feeling. Most people respond with a telecommunication response, but a laser user might give a completely safe system response. While both responses are accurate, they don’t capture the whole of fiber optics in laser technology today. Laser radiation delivered through fiber optics today (CW to femtosecond pulses) extends to telecommunications, medicine, robotics, manufacturing (welding), and scientific applications.

Safety folks like fiber optics because it contains the laser beam; the bare fiber can be jacketed to provide additional protection. So, how can this frail fiber be a safety trap? The rule of thumb for years was that the divergence of the beam from the end of a fiber is so broad that it was not an eye hazard beyond about 10 cm (4 inches for the metric challenged). Here is where the trap starts. Many fiber applications today include a micro lens at the end of the fiber, creating a collimated beam rather than a quickly diverging one, so the hazard zone can be meters long. Second, the amount of energy being transmitted through fibers has steadily increased, as demonstrated by the development of diodes and diode arrays. Third, most wavelengths applied to fiber work are invisible, and the fiber end is far from the source. This leads to the issue of whether the fiber is active or not when it’s found disconnected (warning labels are a good way to solve this problem). Fourth, handling and cutting or splicing fibers presents a sharps and UV hazard. One can see that fiber lasers, while a great asset to laser technology, are not free from problems. Then there is the issue of broken fibers (a number of operating room fires have been started by laser beams escaping from broken fibers).