TABLE OF CONTENTS Microwave frequencies are propagated on coaxial cables, inside metallic waveguides, or through-the-air. Optical frequencies are propagated on dielectric (non-metallic) fibers, waveguides embedded in substrates, and also through-the-air. This Chapter analyzes cylindrical glass optical fibers. Rectangular optical waveguides, used in planar lightwave integrated circuits, are discussed in Chapter 8.
Section 2.2 analyzes the propagation of light in cylindrical optical fibers. Ray optics, also referred to as geometrical optics, is used to initially describe propagation in and around the core of multimode fibers. Rays are an acceptable substitute for solving the complex equations required to model multimode fiber propagation. Single-mode fiber analysis can use some of the results of geometrical optics, but requires the use of EM wave theory in addition. The step index fiber is used as an initial model because it provides a simple platform for analysis. In a step index fiber the index of refraction change at the core/cladding interface is abrupt. Section 2.2 also develops the concepts of polarization, total internal reflection, reflection and reflectance, numerical aperture, and wave velocities using the stepindex fiber model.
Section 2.3 builds on the velocity concepts to explain core propagating modes and cutoff frequencies. The two basic types of fibers used for telecommunications, single-mode (SM) and graded index (GMM) multimode, are described. Different SM core indices of refraction, beyond the step-index, are considered. Single-mode fibers do not have the problem of modal dispersion. As a result, their core/cladding interfaces can be optimized to control total...
