1. Fiber, Cables, and Connectors 19
1.4 Multimode Fiber
While it is possible to use step-index multimode fiber, typical communica-
tions applications use graded-index fiber to reduce intermodal distortion.
The graded-index profile can be generated with the expressions
n
2
r =n
1
2
1 NA
2
r/a
q
for 0 <r<a
n
2
r =n
2
2
for r>a (1.13)
when q is called the profile exponent and r is the radial distance away
from the center of the fiber core, and n
1
is the index of refraction in the
center of the fiber and n
2
is the index of refraction of the cladding. The
optimum profile occurs for q slightly less than 2, which results in the
minimal dispersion.
The total number of modes that can propagate in a multimode fiber N
is given by the following expression, valid for large V numbers only:
N = 1/2 qV
2
/q +2 (1.14)
where q is the profile exponent and V is the normalized frequency. For
the special case of step-index fiber, q tends to infinity and N = V
2
/2. In
Table 1.4, we show typical specifications of multimode glass-optical
fiber.
Table 1.4 Typical specifications of multimode glass-optical fiber
Type of fiber Graded index with glass core and cladding
62.5/125 micron multimode fiber
Core diameter 625 ±30 m
Core noncircularity 6% maximum
Cladding diameter 125 ± 30 m
Cladding noncircularity 2% maximum
Core and cladding offset 3 m maximum
Numerical aperture 0275 ± 0015
Minimum modal bandwidth 500 MHz km at 2 km at 1300 nm or
160 MHz km at 850 nm
Attenuation 1.0 dB/km at 1300 nm or
4.0 dB/km at 850 nm

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