DC to 12-GHz Photoreceivers
Appendix I: Optical-Fiber Performance • 15
Appendix I:
Optical-Fiber Performance
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
f
3-dB
=
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*
f
3-dB
=
where
L
is the fiber length in kilometers,
M
is the
material dispersion in ps/(nm x km), and
∆λ
is the
linewidth 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.
c
n
2L NA
(
)
2
----------------------
1
2LM
∆λ
-------------------
*
Palais, C. J.,
Fiber Optic Communications,
Prentice-Hall, Inc.,
Englewood Cliffs, NJ, 1984.
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