is channel spacing accommodates 16 channels from 1310 to 1625 nm and supported by ITU-T recommendation G.652C. A schematic of the spectral loss and the CWDM channel scheme is shown in Figure 5.19. Special Fibers The discussion SO far has been limited to fibers designed for transmission. However, there are a large number of fibers designed with the aim of addressing special requirements. By virtue of necessity we will outline briefly the fibers designed for special needs in the field of optical communications. Erbium-doped silica fiber. This fiber is used as the gain medium in optical fiber amplifiers for the C-band of the spectrum (1530-156 nm). Polarization-preserving fiber. This fiber is used in situations where the polarization of the optical radiation inhibits the use of nonpolarization preserving fibers. Bearing in mind that the normal transmission fiber does not preserve polarization, polarization- preserving fibers are used in laser fiber tails, external modulators, polarization mode dispersion compensators, and in general devices (active and passive) that require control of the polarization of the radiation. Photosensitive fiber. This fiber is used in the fabrication of Bragg gratings, which are used in dispersion compensators employing the Bragg effect and other optical fiber components. Bend-insensitive fiber. The aim of this fiber is to minimize the losses due to the tight bending of fibers and finds applications in the packaging of optical devices. The parameters of the fibers previously enumerated vary from manufacturer to manu- facturer, and the reader is encouraged to look at the Web sites of fiber manufacturers. Chapter 5 Optical Fibers Linear Propagating Regime Nonlinear Propagating Regime Fig. 5.20 Typical nonlinear characteristic of optical fiber. Input Power Nonlinear Optical Effects in Fibers The discussion of fiber performance was based implicitly on a linear model. That is, when the input is doubled the output is also doubled. However, the optical fiber is essentially a waveguide loaded wit