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 MIL-HDBK-1012/1
decreases in a parabolic profile to the value of the cladding index at the
core/cladding interface, are generally available. This type of fiber is
called graded index fiber and provides band-widths greater than 1 GHz. (See
Figure 18.)
e) The parameters affecting the transmission characteristics of
optical cables are attenuation, pulse dispersion, and numerical aperture.
Attenuation is a measure of the light loss (signal loss) per unit length of
transmission medium and is designated in decibels of optical power lost per
kilometer (dB/km). The losses result from absorption (due to impurities in
the glass), scattering (due to density fluctuations in the glass), and
radiation losses (due to microbending of the glass). Attenuation is inversely
proportional to temperature change.
Pulse dispersion (also called pulse spreading) is the measure
of the spreading or broadening of the light signal (pulse) and is designated
in nanoseconds per kilometer (ns/km). The spreading results from material
dispersion (due to the spectral width of the light source) and modal
dispersion (due to the slightly different velocities for each optical mode
present). Consequently, pulse spreading is a function of the path length.
The bandwidth can be calculated from the pulse dispersion and is given in MHz-
km. Numerical Aperture (NA) is a measure of the light-collecting capability
of an optical fiber and is a function of the difference in the refractive
indices of the fiber core and cladding. A large NA permits more light to
enter the fiber.
f) Cable installation criteria. Fiber optic cables are similar to
conventional cables in terms of the handling and installation procedures
required. The limiting tensile load varies from manufacturer to manufacturer
and among different cable types of a given manufacturer. The tensile load
typically found is in excess of 300 pounds; therefore, many conventional
installation tools and procedures can be adapted to optical cable. A few
essential differences must be recognized, however, to ensure a successful
installation. Glass, although an intrinsically strong material, is relatively
brittle. As a consequence, it is important to monitor and control the cable-
pulling force during installation. Unlike copper conductors, which can
stretch up to 20 percent before breaking, glass fibers Can only be strained 1
to 2 percent before breaking.
Another important factor is that fiber optic cables have a minimal
bend radius, generally 10 times larger than the cable diameter. This is not
unlike metal-base cables of the same bandwidth capability. Considerable
attention should be given to determining the correct lengths of cable to order
and install. The objective should be to determine the cable segment lengths
required for the installation without exceeding maximum recommended tensile
load and-to permit cable splicings and connectorization as required.
g) System design. Design of an optical fiber link occurs at two
levels. First, the individual components must be designed or specified, and
then their interaction must be considered in a system-level design. The
design goal of a communication link is to transfer information from one point
to another without degradation. For analog systems, this means designing for
a desired Signal-to-Noise Ratio (SNR), bandwidth, and distortion level. In
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