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 MIL-HDBK-1012/1
10.4.2.5 Components of a Fiber Optic System. A fiber optic system contains
three major components: the light source, which may be an Injection Laser
Diode (ILD) or a Light Emitting Diode (LED); the light detector, which may be
an Avalanche Photodiode Detector (APD) or a P-I-N diode; and the transmission
medium, the optical fiber cable.
a) Light source. Injection laser diodes provide greater power
output and a narrower spectral bandwidth of light than LEDs, enabling more
power to be coupled into the optical fiber and providing greater transmission
capacity or bandwidth. ILDs are not linear, which limits their capacity to
transmit analog channels. ILDs are generally used in digital systems. LEDs
become attractive when bandwidth requirements and the system length permit;
they are lower in cost, operate over a wider temperature range, have a longer
expected life, and offer greater long-term stability.
b) Light detectors. APDs improve receiver sensitivity by
approximately 10 dB or more over P-I-N detectors, since they provide internal
gain. APDs, however, require an auxiliary power supply, introduce more noise,
and are more costly than P-I-N detectors. The system requirements will
dictate the choice of detector.
c) Light transmission. In any material, light travels at a speed
characteristic of the material and lower than the speed of light in free
space. The ratio of the speed in the material to the speed in free space
defines the refractive index of the material. When light traveling in one
medium encounters another material of lower refractive index, the light is
bent towards the material of higher refractive index.
If the angle of incidence is increased sufficiently, the bent light
will travel along the interface of the materials. This angle is known as the
critical angle. At an angle greater than the critical angle, the light will
be totally reflected from the interface and follow the transmission path.
(See Figure 17.)
d) Transmission medium. An uncabled optical fiber consists of a
core, a cladding, and a protective coating. The core material has a higher
index of refraction than the cladding material, and therefore light tends to
be confined within it. The core material can be plastic or glass, although
glass provides lower attenuation and greater bandwidth performance. The
cladding material may be air (although this is not practical), plastic, or
glass; glass provides greater stability and compatibility with the core glass.
To confine the injected light to the core, the angles of reflection of
propagating ray must exceed the critical angle. This reflection angle is
unique to each mode and determines the length of the path associated with each
mode. The result is that each mode progates at a characteristic axial
velocity, so that dispersion limits the maximum rate at which information can
be transmitted.
Multi-mode fibers, where the change in refractive index from the
core to the cladding is a discrete step, are called step index fibers.
Typically, their bandwidth is limited to approximately 30 MHz per km. (See
Figure 17.) Since light travels faster in a material with a lower refractive
index, fibers with cores whose index of refraction is high at the center, but
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