Silica-based optical fibers are the most important transmission medium for long-distance and large-capacity optical communication systems. The most distinguished feature of optical fiber is its low loss characteristics. The lowest transmission loss ever achieved is 0.154 dB/km at ? = 1 .55- m wavelength [1]. This means that the signal intensity of light becomes half of the original strength after propagating 20 km along the optical fiber. Together with such low loss properties, low dispersion is also required for signal transmission. Signal distortion due to dispersion of the fiber is closely related to the guiding structure of optical fibers. In order to realize low-dispersion fibers, it is necessary to understand the transmission characteristics of fibers and to design and analyze the arbitrarily shaped guiding structures. In this chapter, first a rigorous analysis of step-index fiber is presented, to understand the basic properties of optical fibers. Then linearly polarized (LP) modes, which are quite important mode designations for the practical weakly guiding fibers, will be described. The derivation of dispersion equations is explained in detail in order to understand the dispersion characteristics of fibers. Signal transmission bandwidths of graded-index multimode fibers and single-mode fibers are discussed and compared in connection with their dispersion values. Finally, the principle of polarization-maintaining properties in birefringent fibers and their polarization characteristics are described.

3.1. BASIC EQUATIONS

The electromagnetic fields in optical fibers are expressed in cylindrical coordinates as

Substituting Eq. (3.1) into Maxwell s Eq. (1.31), we obtain two sets of wave equations...