Light-Guiding Fundamentals and Fiber Design

This chapter introduces the fundamental concepts of the guidance of light in optical fibers made from dielectric materials, such as silica, with an emphasis on optical transmission properties relevant to communications. Optical fibers are fabricated by first depositing high-purity silica soot, doped with germania to raise the index of refraction or fluorine to lower the index of refraction, to form a core rod of 1 cm or more in diameter and 1 m or more in length. The cutoff wavelength of a single-mode optical fiber is the wavelength above which only a single bound mode, the fundamental linearly polarized (LP) mode, propagates. For numerous reasons concerning transmission performance, it is desirable to operate fibers in the regime where only the fundamental mode propagates. An optical fiber's dispersion is the tendency for the fiber to either broaden or narrow a pulse as it travels along the fiber. In a single-mode fiber, chromatic dispersion of the fundamental mode is caused by the dispersive properties of the materials that the fiber is made from, referred to as material dispersion, as well as by the dispersive properties of the waveguide, referred to as waveguide dispersion.