Page 213 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

Optical Fibers and Optical FIber Amplifiers 207

This is not discouraging for sending signals on different wavelengths
down the fiber. These signals may travel at different speeds, but they
can be easily distinguished from each other. A problem occurs because
a laser emits light over a finite range of wavelengths, typically about
0.1 nm. Although this is small, it is not zero. This leads to a spread in
the arrival time of a laser pulse that grows with the transmission dis-
tance. For a fiber of length L, the transmission time of the peak of a
light pulse is

L L dn
= = n – 0 (9.16)
v g c d 0
If the laser source has a linewidth of , then we can estimate the
range of the pulse spreading in time as
d L d n
2

2

= 0 = 0 (9.17)
c 2
d 0 d 0 0
The material dispersion is defined as
2
1 d n
M ps/nm-km (9.18)
2
c d 2
35
30
25
Material dispersion (ps/nm-km) 15 5
20

10

–5 0

–10
–15
1200 1300 1400 1500 1600 1700
Wavelength (nm)
Figure 9.10. The material dispersion of light by SiO 2 . The material dispersion becomes
negligible near 1280 nm.

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