Silicon Photonics: Fundamentals and Devices

10.7 Loss Mechanisms in Waveguides

Attenuation or loss suffered by a lightwave, as it travels the guide, is an important parameter that should be minimized for a good device.

There are three contributions to the loss: from scattering, absorption, and radiation. Scattering loss is usually important in glass or dielectric waveguides, while absorption loss is most important in semiconductor guides. Radiation loss becomes important when the guides are bent through a curve. We shall discuss briefly the various loss mechanisms at play within a guide.

10.7.1 Scattering Loss

There are two types of scattering loss in optical waveguides: volume scattering and surface scattering.

10.7.1.1 Volume Scattering

Volume scattering is caused by imperfections present in the guide material, such as impurities, defects, voids, and so on. The loss per unit length is proportional to the number of imperfections per unit volume of the medium. Also the size of the imperfections relative to the wavelength of radiation is another factor determining the loss. In a well-prepared guide material, the density of imperfections should be minimized leading to a negligible loss due to volume imperfections.

There are situations in which volume imperfections may play a dominant role. For example, an ion implantation process may introduce a sufficient number of defects in the waveguide material. Also, the size of the defects with respect to the wavelength of propagation, and the correlation length along the guide, are ...