Modal properties of depressed cladding semiconductor waveguides and lasers

The TE modal properties of three symmetric core (uniform, parabolic, and linear) depressed cladding slab waveguide structures are investigated for situations appropriate for use as lasers and routing waveguides. The results show that if the confinement factor, or mode size, is the criterion for comparison, there is little difference between the standard and depressed cladding cases for guides which are near the cutoff of the higher order modes. The primary difference is in the fraction of the mode which overlaps the cladding regions, which is always smaller for the depressed cladding cases. We show that there are numerous combinations of conditions leading to spot sizes comparable to the vacuum wavelength of light. Experimental data are presented for three parabolic core depressed cladding Al_xGa_1-xAs heterostructures doped to form lasers. In particular, the measured Gaussian equivalent spot size deduced from far-field measurements agree well with model situations for the largest core sizes. The largest Gaussian equivalent near-field spot size was ~1.2 μm, and resulted in a divergence angle of ~30° (full width half power) and a threshold current density only 50% larger than a standard small optical cavity laser structure optimized for maximum confinement factor. The scaling laws of amplifiers and lasers with confinement factor are discussed. Experimental routing tests were performed using rib delineated raised cosine S bends. The symmetric depressed cladding laser structure was found to have a much lower bend loss than the standard structure, leading to tighter routing geometries with an etch depth which did not penetrate the quantum well