Design of small beam divergence, low threshold, long wavelength GaSb lasers

GaSb-based quantum well lasers have been optimized for high coupling efficiency into an optical system. The use of a V-shaped Weaker Waveguide in the n-cladding layer is shown to dramatically reduce vertical beam divergence without any performance degradation compared to a conventional broad-waveguide laser structure. Starting from a broad waveguide laser structure design which gives low threshold current and a large vertical far-field (VFF), the structure was modified to decrease the VFF while maintaining a low threshold-current density. In a first step the combination of a narrow optical waveguide and reduced refractive index step between the waveguide and the cladding layers reduces the VFF from 67° to 42°. The threshold current density was kept low to a value of ∼190 A/cm² for 1000×100 μm² devices by careful adjustment of the doping profile in the p-type cladding layer. The insertion of a V-Shaped Weaker Waveguide in the n-cladding layer is shown to allow for further reduction of the VFF to a value as low as 35° for better light-coupling efficiency into an optical system without any degradation of the device performance.