The semiconductor waveguide facet reflectivity problem

The problem of the facet reflectivity of a semiconductor slab waveguide is reexamined as an extension of Ikegami´s original approach that includes radiation-like modes. Using a guide-within-a-guide geometry, the radiation-like modes are included as modes bound to a thick air-cladding guide that contains the core profile of interest. In this model, the coupling from the fundamental model to radiation modes can be analyzed using a relatively simple analysis. The cross-coupling to the radiation modes for the simple double heterostructure waveguide is considered in detail and is shown to be important only for a large core-cladding index differences and for strong model confinement that results in a true facet loss. The conditions for this are the same as for low threshold lasers so that the loss sets a maximum limit on the equivalent internal quantum efficiency. A separate 1D finite-element numerical mode matching program, which treats evanescent and propagating radiation modes, is used as a comparison. The two methods of accounting for radiation modes are shown to be in good agreement. Modern graded core cases are treated as general examples. Specific quantum well laser structures taken from the literature, including II-VI and III-V structures spanning wavelengths from 0.5 μm to 10.0 μm, are also considered