Fiber Optics

Abstract

An optical fiber is a cylindrical dielectric waveguide made of low-loss materials such as silica glass. It has a central core in which the light is guided, embedded in an outer cladding of slightly lower refractive index. Light rays incident on the core–cladding boundary at angles greater than the critical angle undergo total internal reflection and are guided through the core without refraction. Rays of greater inclination to the fiber axis lose part of their power into the cladding at each reflection and are not guided. Optical fibers are replacing copper coaxial cables as the preferred transmission medium for electromagnetic waves, thereby revolutionizing terrestrial communications. Applications range from long-distance telephone and data communications to computer communications in a local area network. In this chapter we introduce the principles of light transmission in optical fibers. These principles are essentially the same as those that apply in planar dielectric waveguides, except for the cylindrical geometry. In both types of waveguide light propagates in the form of modes. Each mode travels along the axis of the waveguide with a distinct propagation constant and group velocity, maintaining its transverse spatial distribution and its polarization. In planar waveguides, we found that each mode was the sum of the multiple reflections of a TEM wave bouncing within the slab in the direction of an optical ray at a certain bounce angle. This approach is approximately applicable to cylindrical waveguides as well. When the core diameter is small, only a single mode is permitted and the fiber is said to be a single-mode fiber. Fibers with large core diameters are multimode fibers. One of the difficulties associated with light propagation in multimode fibers arises from the differences among the group velocities of the modes. Modal dispersion can be reduced by grading the refractive index of the fiber core from a maximum value at its center to a minimum value at the core–cladding boundary. The fiber is then called a graded-index fiber, whereas conventional fibers with constant refractive indices in the core and the cladding are called step-index fibers. In a graded-index fiber the velocity increases with distance from the core axis (since the refractive index decreases). Although rays of greater inclination to the fiber axis must travel farther, they travel faster, so that the travel times of the different rays are equalized. Optical fibers are classified as step-index or graded-index, and multimode or single-mode. This chapter emphasizes the nature of optical modes and their group velocities in step-index and graded-index fibers. These topics are presented. The optical properties of the fiber material (which is usually fused silica), including its attenuation and the effects of material, modal, and waveguide dispersion on the transmission of light pulses, are discussed. Optical fibers are revisited in a later chapter, which is devoted to their use in lightwave communication systems.