General laser information
Optical fiber telecommunication systems, their associated test sets, and
similar operating systems use semiconductor laser transmitters that
emit infrared (IR) light at wavelengths between approximately 800
nanometers (nm) and 1600 nm. The emitted light is above the red end
of the visible spectrum, which is normally not visible to the human
eye. Although radiant energy at near-IR wavelengths is officially
designated invisible, some people can see the shorter wavelength
energy even at power levels several orders of magnitude below any
that have been shown to cause injury to the eye.
Conventional lasers can produce an intense beam of monochromatic
light. Monochromatic light is a single wavelength output of pure color
that may be visible or invisible to the eye. A conventional laser
produces a small-size beam of light, and because the beam size is
small the power density (also called irradiance) is very high.
Consequently, lasers and laser products are subject to federal and
applicable state regulations as well as international standards for their
safe operation.
A conventional laser beam expands very little over distance, or is said
to be very well collimated. Thus, conventional laser irradiance
remains relatively constant over distance. However, lasers used in
lightwave systems have a large beam divergence, typically 10 to 20
degrees. Here, irradiance obeys the inverse square law (doubling the
distance reduces the irradiance by a factor of 4) and rapidly decreases
over distance.
Lasers and eye damage
The optical energy emitted by laser and high-radiance LEDs in the
400-1400 nm range may cause eye damage if absorbed by the retina.
When a beam of light enters the eye, the eye magnifies and focuses
the energy on the retina magnifying the irradiance. The irradiance of
the energy that reaches the retina is approximately 105 or 100,000
times more than at the cornea and, if sufficiently intense, may cause a
retinal burn.
The damage mechanism at the wavelengths used in an optical fiber
telecommunications is thermal in origin, for example, damage caused
by heating. Therefore, a specific amount of energy is required for a
definite time to heat an area of retinal tissue. Damage to the retina
occurs only when one looks at the light sufficiently long that the
product of the retinal irradiance and the viewing time exceeds the
damage threshold. Optical energies above 1400 nm cause corneal and
skin burns, but do not affect the retina. The thresholds for injury at
Laser safety
Safety
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Lucent Technologies
365-372-333 R2.1
Issue 4, June 2005
