Eye hazards may lurk in optical-fiber networks

FIGURE 1. Danger related to infrared-emitting lasers includes eye damage due to local heating, retinal damage from imaged light at 100-1400 nm, and corneal damage in the range between 1000 and 1400 nm.

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FIGURE 1. Danger related to infrared-emitting lasers includes eye damage due to local heating, retinal damage from imaged light at 100-1400 nm, and corneal damage in the range between 1000 and 1400 nm.

Until now, optical-fiber-communications systems (OFCS) have been relatively low on the list of workplace laser-related hazards. In 1996, for instance, there were just two recorded diode-laser injuries, both from 650-nm-output laser pointers. Reasons for concern are growing, though. Not only are fast systems being deployed closer to the end user, they are using higher-power lasers and optical amplifiers.

Infrared (IR) lasers could pose a particularly insidious threat (see Fig. 1). The light is invisible, and the retina of the eye, where damage is likely to occur, has no pain sensation. Someone receiving regular low-level damaging doses may not realize it until too late. It is, therefore, prudent for manufacturers of laser products for communications to actively promote effective and rational standards of laser safety (see "Need more information on laser safety?," and Laser Focus World, Aug. 98, p. 149).

Who is at risk?

Contrary to what one might expect, the exposure risks in OFCS are unlikely to lie with system users-they are more likely to tire of staring before being hurt. The larger risk lies with either the untrained or momentarily careless engineers.

Based on accessible-emission levels, receptacle laser devices are more challenging than the traditional fiber-pigtailed package. Pigtails basically block or attenuate most of the diode-laser light. The only accessible light is that coupled into the fiber core-about 10%-30% of total laser output.

The SC-duplex receptacle in many modern transceivers does not include the aperture feature of the pigtail. Instead, a lens system couples light from the laser facet onto the polished end-face of a fiber ferrule. With single-mode fiber, system-coupling efficiency is only about 10%-nine times the rated output of the transceiver emerges from the optical port with the fiber disconnected. For a given fiber-coupled power specification, the margin between actual accessible power and the Class 1 (eye-safe) limit is much smaller.

Component manufacturers sometimes deal with this problem by screening receptacle parts for accessible power. Normally, the associated cost burden would not be too high, were it not for the fact that most laser assemblies are now designed for precise alignment into single-mode fiber. To produce a nearly collimated beam that couples well, a typical compact, low-cost lensing system mounts a ball lens close to the laser facet. Even a small misalignment of the ball can produce large position variations in the beam exiting the port. US regulations require screening the accessible emission from the port by scanning the far-field pattern to locate the beam and then measuring the power passed by a 7-mm aperture located 20 cm from the source-bumping up the inspection costs per unit.

Another design mounts a single-mode fiber (stub) a few

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FIGURE 2. A laser with a fiber-stub aperture generates a far-field pattern where the white circle shows emission due to the core mode.

Protecting eyesight takes some insight

Perhaps the worst assumption any laser user could make is that protective eyewear guarantees complete safety from laser-inflicted eye damage. Recent reports indicate that it is not uncommon to find that exposure to damaging laser light resulted from carelessness-the person either failed to heed warning signs, did not wear recommended safety glasses, wore eyewear with the wrong optical density and wavelength for which protection is afforded, and so on.

In fact, laser standards have never suggested that protective eyewear should be the primary means of protection against laser radiation. In certain cases, engineering controls, such as beam-path enclosures, shields, or even locked doors may be more reliable safeguards. That`s not to downplay the importance of safety eyewear, though. According to the Guide for the Selection of Laser Eye Protection from the Laser Institute of America (LIA; Orlando, FL), the optimum approach toward ensuring safe operation of some laser systems may include eye protection, administrative controls to ensure it is worn, and efforts to control other hazards.

Design goals for eyewear

The following general design goals for laser eye protection are provided by the LIA:

Filters. Filters should not undergo reversible bleaching by pulsed laser radiation. Certain filter materials containing organic colorants in plastic substrates may temporarily lose their attenuation during a short-duration Q-switched pulse and recover their initial density following the exposure, making eyewear users susceptible to damage from extremely short pulses. Additionally, filters should not bleach during exposure to sunlight and UV radiation and should withstand damage from environmental contaminants such as chemicals, mildew, and high humidity.

Filters and frame should be flame resistant or self-extinguishing, and filter material should meet the standards of ANSI Z87.1 1989, when feasible, to provide impact resistance. The manufacturer of the eye protection should supply additional information related to the filter transmission curves or tables, scotopic and photopic (night and day) visual transmittance, and any data on filter or frame damage thresholds.

General eyewear. Except for a marginal zone of 5 mm around the edges of the filter plates, eyewear should not exceed a refractive error of ?0.06 diopters spherical error, 0.06 diopters astigmatic error, or 0.12 cm/m prismatic error (ISO Category 1). For high lens clarity, it is desirable that the incident light scatter from the lens should not exceed 1 cd/cm2/1X (1% haze). Except for a marginal zone of 5 mm around the edge of the filter eyepiece or eyepieces, there should be no defects, such as blisters, streaks, occlusions, and so on that would impair vision under conditions of normal use.

The 40-page eyewear-protection guide also provides detailed information about both the maximum permissible exposure limit of the eye and the protective optical density rating that all eyewear should be marked with. Understanding these ratings can be critical, especially for emerging laser technology. With tunable lasers, for instance, one pair of laser eyewear may not provide adequate protection from multiple or tunable wavelengths produced.

More information on the $15 publication ($10 for LIA members) is available on the Web at www.laserinstitute.org. For more on specific safety standards, see "Need more information on laser safety?" on p. 289.

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FIGURE 3. Accessible emissions limits vary dramatically according to the FDA (left) and the IEC (right) for a product with continuous-wave operation at 1300 nm.

BRIAN WHEELERis a field application engineering supervisor at Hewlett-Packard, Americas Marketing Center, 2850 Centerville Rd., Wilmington, DE 19808; e-mail: [email protected]

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