When all the spectral components are separated from an optical signal, it is termed dispersion. It usually occurs when optical signals travel along optical fiber from transmitter to receiver in an optic–fiber communication link. Dispersion causes distortion in the transmitted optical signal (analog or digital transmission) along the optical fiber. As the optical pulses travel along the optical fiber channel, when digital modulation is used in transmitting optical signals, the dispersion phenomenon causes the broadening of optical pulses. There are different types of dispersion effects such as modal dispersion (in multimode fiber, transmitted optical pulse tends to spread due to time delay between lower- and higher-order propagation modes, causing bandwidth limitation), chromatic dispersion (combination of material dispersion as well as waveguide dispersion that results in spreading of transmitted optical pulse as they travel through the optical fiber), and polarization mode dispersion. Material dispersion happens because of variations in the fiber core refractive index with respect to operating wavelength, and waveguide dispersion happens due to nature of the physical structure of the optical fiber. Due to variations in the fiber core refractive index, different wavelengths of the light beam would travel at somewhat different velocities of light. As a result, an optical pulse gets broadened, causing dispersion. With the introduction of optical amplifiers (as discussed in the previous chapter) as in-line amplifiers in an optic–fiber link, the signal attenuation due to fiber is no more the major concern for achieving desired performance for optical fiber communication systems. However, they aggravate the dispersion problems. In order to achieve the lowest attenuation, there is a need of implementing efficient dispersion management techniques. There are different varieties of optical fibers available including dispersion compensating fibers. With an objective of controlling the spread of transmitted optical pulse in optical fiber communications systems, the dispersion management (also known as dispersion compensation) techniques must be applied.
This chapter focuses on dispersion management in optical fiber communications. The discussion begins with the need of dispersion management because dispersion-induced pulse broadening imposes the severe limitations on the performance of the system. This is followed by detailed discussions on different techniques of dispersion management as pre-compensation as well as post-compensation. The discussion is carried forward by describing various types of dispersion-compensating fibers including fiber Bragg gratings. Finally, different methods of fabrication of chirped fiber gratings are covered.