By Ken Barat, CLSO, Laser Safety Solutions
When thinking about how laser technology has changed our world, one pairing cannot be overlooked: laser diode and optical fiber. While the general public may associate this combination with telecommunication, those in the laser world know it is more far-ranging.
It is the delivery of laser energy through fibers that allows the multi-axis industrial laser revolution, allows the laser radiation source to be a distant from the actual work and support several robotic systems from one source. Further, the applications to which sensors have been applied has greatly increased due to the use of fiber optics — for example, the application of fiber gradings to the fiber.
Happily, laser accidents with fibers are almost exclusively related to what is termed non-beam hazards, and not the effect of direct laser radiation. The cases where laser radiation is involved overwhelmingly comprise procedural violations, rather than eye or skin injuries. In my experience, all incidents of an individual having been injured by laser radiation escaping a broken fiber are confined to the medical application of fibers.
Other common procedural incidents recorded by the Department of Energy over the years include fibers meant to be ‘off’ that were left ‘on’ and just lying on a table; fibers with the laser source in a separate room, once again thought to be ‘off,’ but found ‘on’; and end caps melted from a mixture of ‘on’ time duration and output.
Still, the most common injuries attributed to fiber systems in a non-medical setting are puncture wounds — all “sharps” related. Fiber optics work by the process of total internal reflection to contain the beam and, as long as the beam enters the fiber at the proper acceptance angle, there should not be any back reflection risk to users.
Once exiting the fiber, unless there is a micro lens at the end, divergence will be large. It should be noted that the days when being 8-10 cm away from the end of the fiber was a near-guarantee of establishing a safe distance have ended, a fibers now are regularly used to deliver 10, 60, or even 100+ Kw output.
Therefore, laser fiber users need to understand a number of factors relevant to fiber optic safety, and use that information to guide common sense approaches to safety:
Fiber cladding is the protective covering wrapped around bare optical fiber. Whether the cladding is metal armor or a plastic coating has no relationship with the beam power contained in the fiber — rather, the material is selected to protect the fiber from its work environment.
This is rather simple; you can only bend a fiber so far before it will break or cause laser radiation to escape.
Sharps containers deserve special attention, not so much for their function, but because of their frequent mislabeling. Most commercially available sharps containers are designated for medical applications and, as such, have a biohazard symbol printed or affixed on them. I have encountered many firms using biohazard sharps containers, and then disposing of the full containers in regular trash. Regardless of the fact they do not contain biologically hazardous materials, such containers, if found in a standard trash facility or dump, can lead to a significant amount of regulatory blowback and follow-up. Sharps containers can be found without the biohazard label; one just needs to look a little harder.
Scraps Hiding In Plain Sight
Fiber scraps on the typical optical table can appear at first glance to be scratches. This is why any area where fiber cutting and slicing is to be performed needs to be on a dark surface; black work mats are available for this purpose. Additionally, fibers can fly a distance many times their length, so the mat should not be too small.
Viewing The Fiber End
If it becomes necessary to view the end of a fiber, the best approach is to look at the end on a monitor screen, rather than a hand-held viewer. Note that not all viewers have a wavelength safety filter or are labeled as such. Still, viewing through a monitor will provide the best view, as well as the safest approach. Try hard to avoid Stephen King moments, where your arm takes on a mind of its own and tries to guide the fiber in your own eye.
Good Work Techniques
In addition to minding the safety factors listed above, laser fiber users will benefit from following these general safety tips:
- Do not eat in the same area where you work, and always wash your hands before eating. Particles of glass from an optical fiber are the same as glass splinters and can cause internal hemorrhaging;
- All fiber scraps shall be placed in a dedicated container (a “sharps” container) with a lid and disposed of per institutional policies. Sharps discarded in regular trash can pose a puncture hazard for environmental services staff.
- Because of glass particle hazards, always wear safety glasses when performing work directly on optical fibers, and do not touch your eyes while performing fiber connectorizing or splicing work. Do not touch contact lenses until you are sure your hands are clean; treat fiber optic splinters the same as you would glass splinters.
- The fiber strand ends are extremely sharp and can easily penetrate your skin or eyes. When broken off, they are very hard to find and remove. Thus, make sure fibers are terminated into a power meter or with suitable end caps.
- Optical fibers may be difficult to work with while wearing gloves, but consideration should be given to the use of protective gloves whenever possible. Note that gloves also serve to keep fiber free from (skin) oils and other contaminants. Also, to reduce the chance of contamination of personal clothing with fiber fragments, the use of a lab coat or disposable apron is recommended when cutting or splicing fibers.
- Keep all combustibles and flammable vapors and gases away from curing ovens and fusion splicers that are in-use. Fusion splicers use an electric arc and can ignite flammable vapors and gases. Splicing activities are prohibited in confined spaces or other locations where the buildup of gases is possible. Splicing should be performed in a clean, temperature-controlled location to ensure maximum splicing quality. Also, obviously, do not smoke in or near work areas.
- Always work with fiber optics as if they were active/live;
- Properly label all fibers in conduit and jacketed fibers (bare fibers may not accept labeling);
- Work areas should be thoroughly cleaned after working with fiber optics to remove all debris and sharp fragments. Because of this requirement, work areas should be well-ventilated areas — due to the potential use of curing ovens, chemical cleaners, and adhesives — and hazard mitigation should be addressed when using chemical cleaners and adhesives, especially during splicing and termination activities.
- The hazard distance from a fiber with a micro lens is similar to that of a collimated beam; the nominal hazard zone (NHZ). — the place where a decision must be made whether to don protective clothing/eyewear — from an unterminated fiber may extend quite far when the source laser is Class 3B or Class 4. Controls should be considered when operating higher-power fiber coupled systems.
Take Advantage Of Telecommunications Work
Several telecommunication standards contain significant safety efforts developed in that industry. Even if one’s work is not directly related to a telecommunications application, there is value in learning from that industry’s research and documentation — including guidance on such topics as fiber viewing, bend ratios, problems caused by dust at the ends, etc. Below are some of the major fiber optic telecommunication standards.
- ITU-T Series G39 supplement, Optical System Design and Engineering Considerations, presents best practices, including viewing fibers, signage, alarms, viewing equipment, terminations, and cleaning of connectors and optical amplifiers.
- ITU-T G.664, Optical Safety Procedures and Requirements for Optical Transport Systems, provides guidelines and requirements for techniques to enable optically safe working conditions on optical interfaces of the optical transport network. This standard includes Raman amplification techniques in restricted and controlled locations, as well as guidelines for automatic power reduction (APR), automatic laser shutdown (ALS), and automatic power shutdown (APSD) for systems employing high power.
- IEC/TR 61292-4, Optical Amplifiers Part 4: Maximum permissible optical power for damage free and safe use of optical amplifiers, describes fiber damage caused by high optical powers, maximum permissible exposure (MPE) for eyes and skin, and connector end-face damage induced by dust/contamination. It also describes optical power limits that cause thermal damage and fibre-coat burn/melt induced by macro bends, as well as addressesthe impact of long haul, high data rate (>40 Gb/s), DWDM, and optically amplified systems.
- ANSI/IEC Standards — When looking for user standard guidance, the first search for users in the US would be the ANSI Z136.2 standard, which applies only to telecommunication, though it is very similar to IEC 60825-2.
In terms of non-telecommunications fiber guidance, standard guidance can be found in two ANSI User Standards: ANSI Z136.1 Safe Use of Lasers, 2014 edition, which has guidance on cable disconnection; and ANSI Z136.8 Standard for Safe Use of Lasers in Research, Development and Testing, which contains a section on general fiber optic safety.
Fiber optics, utilizing an enclosed beam path, secure enclosure for nanowatt to megawatt laser radiation, automatic power reduction if a break occurs, and a common-sense approach to controls offer a very safe system. Long live the fiber.
About The Author
Ken Barat, CLSO, is the principal consultant at Laser Safety Solutions. He is the former Laser Safety Officer for Lawrence Berkeley National Laboratory and the National Ignition Facility. He is the author of several text books on laser safety. He has served as the laser safety adviser for Laser Interferometer Gravity Wave Observatory, ELI, and Allen Institute, among others. Laser Institute of America Fellow, Rockwell award winner and senior member IEEE & SPIE. Part of “Ask the expert” team for the Health Physics Society. The organizer and executive director of the first seven LSO Workshops.