Planar Waveguides with Continuous Dielectric Constant Variation

Abstract

In the last chapter, we discussed electromagnetic wave propagation in planar slab waveguides consisting of a slab of high refractive index material sandwiched between two layers. From the point of view of geometrical optics a light beam is guided by repeated total internal reflections at the interfaces. One of the disadvantages of such waveguides is the rapid increase in the transmission loss with increasing irregularities of the interfaces; this places rigid tolerance limits for low-loss waveguides. Recently, much interest has been aroused in waveguides in which the guidance is the result of continuous variation of refractive index. The fabrication of such waveguides is discussed in Appendix A. The waveguides act as a cylindrical lens with periodic focusing of the beam; most of the energy of the beam is concentrated around the central portion (where the refractive index is maximum) and hence the effect of irregularities (at the peripheries) on the beam is negligible.