APPENDICES
APPENDIX 1: USING AN OPTICAL FIBER
Single mode optical fiber can provide low loss and low distortion if
attention is paid to a few important details. First, if more than one
mode is allowed to propagate in a step-index fiber, the bandwidth
will be degraded to approximately ƒ
3-dB
= [
cn
] / [2
L
(
NA
)
2
], where c is
the speed of light in free space, n is the index of the core, L is the
length of the fiber, and NA is the numerical aperture of the fiber.
Modal distortion can be eliminated by using a fiber with a core
small enough that only a single mode will propagate. In this case,
the bandwidth of the fiber will be limited by material dispersion
which is a property of the glass used in the fiber core. In this limit,
the bandwidth is approximately* ƒ
3-dB
= 1 / [2
LM
∆λ
], where L is the
fiber length in kilometers, M is the material dispersion in ps/(nm
×
km), and
∆λ
is the line width of the optical source in nm. This
bandwidth limitation can be ignored for glass fibers less than 10
meters in length, but can be serious for longer fibers and spectrally
broad sources.
10
*Palais, C.J., Fiber Optic Communications, Prentice-Hall, Inc., Englewood Cliffs, NY., 1984
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