Fiber Optic Essentials

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 625 ±30 m

Core noncircularity 6% maximum

Cladding diameter 125 ± 30 m

Cladding noncircularity 2% maximum

Core and cladding offset 3 m maximum

Numerical aperture 0275 ± 0015

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