The use of optoelectronic devices, fiber-optic
connectors and fiber-optic cables in military system designs is increasing. Fiber-optic systems are relatively immune to
an Electromagnetic Pulse (EMP) and do not produce electromagnetic radiation
which, along with their intrinsic high speed response, makes them ideal for low
signal level and fast response time applications. Tiny optic-fibers comprise the basis for
photo-emitters, photo-detectors, and optical cables in these fiber-optic
systems. A variety of discrete
components, such as PIN diodes, light emitting diodes (LEDs) optical couplers,
etc., are employed in these systems. Recommended de-rating values and other
reliability and application information are discussed in this section.
CONTACTS & COMMENTS
The selection of an LED or laser diode light source depends upon the required
information bandwidth, transmission distance, fiber attenuation, coupling
efficiency, and pulse broadening considerations. Diode power and radiance are determined by
fiber attenuation and coupling efficiency.
Assuming that fiber dispersion is the limiting factor, pulse broadening
dictates the broadest acceptable spectral bandwidth. In the case of fiber optic communications for
long distance applications, the laser diode would likely be the preferred source
due to its narrow spectral bandwidth and high radiance, while the LED would be
preferred for short distance data-rate applications because of its reduced
temperature sensitivity and simple construction.
It is important to select a cable that will
adequately protect the optical fibers.
MIL-PRF-85045 provides guidelines, which can assist the user in selecting
the appropriate cable for a given application.
Optical fibers, although inherently reliable, are susceptible to failure
from physical abuse. Selecting a cable
best suited for the environment that the optical fiber will experience needs to
consider the following for specific
optical fiber applications:
attenuation to maximize repeater spacing or eliminate
of coupling to inexpensive large area
tolerances to allow for inexpensive
of the fiber.
of transmission bandwidth or speed.
Tolerance of extreme temperatures or other environmental
The slow response of phototransistor detectors limits their use to systems
below the megahertz range.
Phototransistors are ideal for short-distance, low speed
transmission. Faster phototransistors
operational characteristics are possible with heterojunction devices (e.g. GaAs).
The speed and sensitivity of photodiodes are more than adequate for many
fiber optic applications; however, they are not as well suited to the needs of
low-cost, low-speed fiber systems.
It is recommended that manufacturers of high
reliability, military level optoelectronic components be given first
consideration for part selection. In
addition to the recommendations already provided, caution is urged in the
consideration of new and untried designs, commercial quality product, and
product from new, unfamiliar sources. Also risky is continued procurement
without periodic re-assessment, availability of a special/unique design from a
single source, and dealing with a supplier that is having financial