Scattered light which remains is an optical waveguide to be guided to the detector is an important limiting factor to the performance of integrated optical devices. In sputtered glass and niobium oxide waveguides the principal mechanisms for scattering are the scattering from refractive index variations in the waveguide and from surface roughness of the waveguide. Theories for scattering from surface roughness, in the regime expected for these waveguides, predict that the wavelength variation of the scattering should be proportional to the inverse square of the wavelength. Theory for scattering from refractive index variations predicts a wavelength dependence ranging from inverse fourth power of the wavelength to no dependence upon wavelength, when the range of possible scattering diameters is varied from small to large with respect to the optical wavelength. Experiments on a number of relatively lossy waveguides indicate that there is little relation between overall waveguide loss and the magnitude of the scattering in the near forward direction. Furthermore, overall loss and in-plane scattering have different variation with wavelength suggesting that the mechanisms involved in loss and in scattering are different. Waveguide surface preparation prior to sputtering the waveguide appears to be an important factor in determining overall quality but more as it influences the bulk properties of the waveguide film than as a direct scattering mechanism. Homogeneity of the waveguide film appears to be the controlling factor in production of low scatter waveguides.
Gerald B. Brandt,
"In-Plane Scattering In Glass And Niobium Oxide Waveguides", Proc. SPIE 0176, Guided Wave Optical Systems and Devices II, (26 July 1979); doi: 10.1117/12.957221; https://doi.org/10.1117/12.957221