Step-Index Circular Waveguides

Abstract

The circular waveguide has found extensive use in optical communications systems, especially long distance communication links. The circular waveguide has no intrinsic advantage over rectangular waveguides except in one critical area: cost. Manufacturing circular waveguides from glass is a well established technology. Industry can produce hundreds of thousands of kilometers of circular dielectric waveguide each year. The same cannot be said about planar or rectangular waveguides. In this chapter, we will develop a description of wave propagation along a circular waveguide. This chapter deals with the “step-index” fiber (Fig. 4.1). Light is guided by a high-index circular core of uniform index, surrounded by a lower-index cladding layer. The cladding layer is usually covered with a plastic coating to protect the fiber from environmental hazards and abrasion. To find the modes of the circular step-index fiber, we must solve the wave equation in cylindrical coordinates. The modes of the cylindrical structure are more abstract than those of the planar structure. Not only are they circular in symmetry which will require a more complicated solution to the wave equation, but they are two dimensional, so there will be two mode numbers. We will see a similar effect when we discus rectangular waveguides in the next chapter.