Bends in Optical Dielectric Guides

A dielectric rod, embedded in one or more dielectrics of lower refractive index, is the basic ingredient of three types of optical waveguide which differ only in their relative dimensions and consequently in their guiding properties. The first is a small cross section guide which supports only the fundamental mode; most of the power travels in a lower loss external medium. Thus, the attenuation of the mode is smaller than if all the power flowed through the higher loss internal medium. Tiny rods, thin ribbons, or films made of glass or other substances embedded in either air or low loss liquids are typical examples. 1-3 The second is a medium size guide capable of supporting only a few 2103 2126 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 196!) modes; most of the power travels in the internal medium. Such a guide, (Fig. 1 of Ref. 10) has been proposed as the building block of passive and active components for integrated optical circuitry. 4-6 Lasers, modulators, directional couplers, and filters are some of the many devices which could be built in a single substrate utilizing the high precision techniques available from integrated circuitry; consequently they would be compact, mechanically stable, and reproducible. The third, a large size guide (clad fiber) which can support many modes, is used typically in fiber optics. 7 These basic guides, having round or rectangular cross section and straight axis, have been studied both analytically and through computer calculations. 8-13 Also the directional coupler (Fig.