Page 199 - Introduction to Information Optics

184 3. Communication with Optics

1.2 1.3 1.4 1.5 1.6 1.7
Wavelength (Mm)
Fig. 3.10. Dispersion of silica fiber as a function of operating wavelength for different types of
single mode fibers.

si on. Since the waveguide dispersion contribution D w depends on fiber par-
ameters, such as the core radius and the index difference A, it is possible to
design the fiber such that the zero dispersion point is shifted to the vicinity of
1.55/mi, as shown in Fig. 3.10. Such fibers are called dispersion-shifted fibers.
It is also possible to tailor the waveguide contribution such that the total
dispersion is relatively small over a wide wavelength range (e.g., 1.3 to 1.6/mi).
Such fibers are called dispersion flattened fibers, as shown in Fig. 3.10.

3.3. CRITICAL COMPONENTS

3.3.1. OPTICAL TRANSMITTERS FOR FIBER-OPTIC
COMMUNICATIONS -SEMICONDUCTOR LASERS

We would like to find out what kind of light sources are required for
fiber-optic communications. The light sources to be used for fiber-optic
communications must have the following unique properties:

• Proper output wavelengths — centered at 1 = 1.3 jwm range or A = 1.5 /im
so that lower attenuations can be achieved.
• Narrow bandwidth AA so that low intramodal dispersion can be achieved.
• High energy efficiency and compact size.
Based on the above requirements, semiconductor lasers or laser diodes are
the best candidates because they have output spectral range from visible to
infrared, narrow bandwidth (< 1 nm), compact size, and high energy efficiency