3.10. DISPERSION CONTROL IN SINGLE-MODE OPTICAL FIBERS

3.10.1. Dispersion Compensating Fibers

Single-mode optical fibers in which zero-dispersion wavelengths are shifted to 1 .55- m regions (Fig. 3.19(b)) are called dispersion-shifted fibers (DSFs). In order to distinguish standard single-mode fibers having zero-dispersion wavelengths around 1 .3 m regions (Fig. 3.19(a)) from DSFs, standard single-mode fibers are simply called single-mode fibers (SMFs). As described in Section 3.8, the maximum bit-rate B of fiber is given by B = 234 / (Gbit/s) where ? is chromatic dispersion in ps/km/nm and L is fiber length in km. Since chromatic dispersion ? of SMF is about 17 ps /km /nm at 1 .55 m region, the maximum bit-rate of SMF is about B = 5 .7 Gbit /s for L = 100 km. Then we cannot transmit 10 Gbit /s signal in such SMF fibers. Dispersion compensating fibers (DCFs) are quite attractive fiber-based components for upgrading the existing SMFs. Figure 3.36 shows chromatic dispersions of SMF and DCF, respectively. Refractive-index difference ? of DCF is 2.8% and core diameter 2a = 1 .6 m ( ?c = 0.74 m). Chromatic dispersion of DCF at 1.55 m is ?105 ps /km /nm. It is known that when we connect 16.2-km length of DCF to 100-km length of SMF the total chromatic dispersion of the transmission fiber becomes...