Eye Safety and Wireless Optical Networks (WONs) - OED

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The AEL refers to the power level at a given wavelength that signifies a laser as belonging to a particular class. Generally, a standard includes tables, graphs, and equations of AELs that allow a person to classify a given laser based on wavelength, power, and pulse duration. The AEL may be related by some safety factor to the MPE values. The MPE values are based on actual biophysical research of laser-tissue interactions spanning wavelength, power, and pulse duration for a specific tissue. They are defined as the level of laser radiation to which, under normal circumstances, persons may be exposed without suffering adverse effect. The MPE are related to the wavelength, pulse duration, and in the case of visible and infrared, the size of the retinal image. There is a considerable body of research supporting the MPE. As lasers reach new benchmarks of performance particularly in pulsewidth and power, research is performed to estimate the hazard presented. It should be noted that the MPE values are generally derated by a safety factor themselves, adding further margin to MPE-based AELs. In summary, it should be understood that class limits in the form of AELs are not tissue-damage thresholds, but rather are significantly reduced below such limits in the interest of safety. CERTIFICATION A Class 1-certified laser is considered eye-safe with no special precautions required for persons who may be exposed to the system. Being Class 1 requires that a system comply with all Class 1 requirements in the applicable standard(s). Most generally this means that the system must limit access to radiation below the Class 1 AEL under any reasonably foreseeable single-fault condition. As previously indicated, AELs for a particular class definition are wavelength-dependent. The CDRH CFR 1040 and the IEC 60825-1 are self-regulated standards in the sense that the manufacturer has the responsibility to correctly interpret and apply the standards to products. There is no mandatory U.S. Federal or international compliance testing. The perils of not fully understanding and complying with applicable standards are manifold, and include product liability and, most importantly, potential hazard to humans. There are a number of nationally and internationally recognized, independent laboratories that perform laser product safety testing. Examples include Underwriters Laboratories (UL) and TÜV (Technischer Überwachungs Verein, English translation: Technical Surveillance Organization). The use of an independent test laboratory is good safety practice to ensure that an unbiased party comes to the same conclusion as the people who designed and plan to sell the product. This is particularly important in light of the relative complexity of the laser standards. Most individuals, upon first interpreting the standards, find them difficult to navigate. Employing a laser safety consultant often will save time and money in the development of new laser products, as fundamental design decisions can determine the ultimate laser class of the product and thus the market that the product can address. EYE SAFETY AND WIRELESS OPTICAL NETWORKS (WONS) With the rapid increase in demand for broadband access and the relative expense and delay in deploying fiber, wireless optical networks are being viewed as an affordable, reliable, and fast method of providing fiber-like bandwidth. Currently, carrier-grade equipment is being built to provide reliable, redundant access networks. Wireless optical technology proposes to break the ‘lastmile’ barrier -- the relatively short distance between a fiber optic network and a user. In the case of AirFiber’s OptiMesh wireless optical network, for example, this is accomplished by Class 1 eye-safe optical transceivers strategically mounted in a mesh configuration on the rooftops of commercial buildings in dense urban areas. Eye Safety and Wireless Optical Networks 802-0004-000 6
EYE-SAFE SYSTEM ARCHITECTURE As laser beams become prevalent in public spaces, new challenges arise for manufacturers and users. In response, the IEC is currently drafting a standard under the IEC 60825-1 base standard that specifically addresses open-air communication systems. Like many other enabling technologies, potential hazards posed by lasers have been mitigated by the application of safety standards. Lasers are now safely operated in open-air environments as exemplified by the widespread use of laser pointers, laser range-finders, and light shows. Safe operation requires that human access be limited to Class 1 radiation. Evaluating such access for wireless optical networks includes the laser system class, the deployment environment, and beampath. For example, WONs deployed in a space where casual passersby could potentially intercept a beam requires that the system be Class 1 and incapable of delivering radiation in excess of Class 1 (i.e., under single-fault conditions). This may be an unlikely deployment scenario since a WON subject to frequent beam interrupts would not be reliable; however, consider beams being aimed into receivers behind office building windows. The space between the terminals may be virtually inaccessible, but mispointing or beam-spread into adjacent windows must be addressed with reliable systems engineering that meets the standard of safety. Similarly, consider beams being sent and received between towermounted rooftop terminals. Although the terminals and space between them may be inaccessible, effective beam-stopping and mispointing again must be addressed and solved with engineering solutions. There is great diversity in the WON system architectures being developed today. Effecting safe public deployment of these systems requires top-level design choices to limit the access to harmful laser radiation. For example, design features such as the number and size of transmitted beams, divergence, power, wavelength, pulse duration, physical size of terminals, and installation location (i.e., network planning) all must be considered in determining laser system classification and human access. Furthermore, active safety systems that control the laser in response to a potential access event are often implemented. An example is a Location Monitor (LM) system or an Automatic Power Reduction (APR) system. ON OFF T APR Event begins (t = 0) Tdelay T restart FIGURE 2. Example Timing Diagram for an APR Response An example timing diagram for such an APR is shown in Figure 2, and important time scales are labeled. The constraints on the timing are determined by all the factors discussed above unique to the architecture of WON equipment. The shutdown time of the APR (T APR ) refers to the time from the instant an exposure event begins to the time the laser is turned off. This response time is determined T ping Eye Safety and Wireless Optical Networks 802-0004-000 7
Page 1 and 2: Eye Safety and Wireless Optical Net
Page 3 and 4: Absorbed Retinal Irradiance (W/cm 2
Page 5: Table 1 provides a summary of laser
Page 9: The draft standards at present natu

Eye Safety and Wireless Optical Networks (WONs) - OED

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