Attenuation and Nonlinear Effects in Waveguides

Abstract

An optical signal will be degraded by attenuation and dispersion as it propagates through a material. Dispersion can sometimes be compensated or eliminated through clever design, but attenuation simply leads to a loss of signal. Eventually the energy in the signal becomes so weak that it cannot be distinguished with sufficient reliability from the noise always present in the system. Attenuation therefore determines the maximum distance that optical links can be operated without amplification. Attenuation arises from several different physical effects. In an optical waveguide, one must consider i) intrinsic material absorptions, ii) absorptions due to impurities, iii) Rayleigh scattering, iv) surface scattering, v) bending and waveguide scattering losses, and vi) microbending loss. In terms of priority, intrinsic material absorption and Rayleigh scattering are the most serious cause of power loss for long distance optical fiber systems. Surface scattering dominates integrated waveguide losses. Impurity absorption has become less of a problem as improved material processing techniques have been developed over the years. In this chapter, we will establish the fundamental limits of attenuation, and provide a basic understanding of the attenuation processes that can be applied directly to materials such as glass or semiconductor.