Sources of loss in optical waveguides include free carrier absorption, band edge absorption, surface roughness, bending loss, and two photon absorption. Optical loss can be determined from the imaginary index of refraction.
Band edge absorption is a wavelength-dependent absorption based on material properties. For wavelengths above the bandgap wavelength (approx. 1 micron), the band edge absorption and free-carrier absorption of GaAs is greatly reduced. Free-carrier absorption caused by doping is still a concern for optical waveguide loss, however.
Free carrier absorption is loss in optical waveguides due to interaction of photons and charge carriers. The effects of free carrier absorption can be calculated using the free carrier coefficients of electrons and holes for the material and the doping concentration. Since doping is used to create a PIN structure, it is therefore wiser based on free carrier absorption to have the regions surrounding the intrinsic waveguide core to be lightly doped. The imaginary dielectric constant due to free carrier absorption, based on doping levels is calculated as follows. The doping concentration for electrons and holes are n and p respectively, the bulk refractive index is n0, the wavenumber is k, and FCN and FCP are the free carrier coefficients of electrons and holes respectively.