GaSb-based 1.9-2.4 µm quantum-well diode lasers with low-beam divergence

We present results on low beam divergence, low threshold current GaSb-based quantum-well diode lasers emitting in the 1.9 - 2.4 µm wavelength range. By carefully designing the active quantum-well region, low threshold current densities in the range of 148 to 190A/cm2 could be achieved in the entire wavelength range. A novel structure for the epitaxial waveguide was designed and realized experimentally, leading to a reduced beam divergence in the fast axis of 44° full width at half maximum (FWHM), compared to 67° FWHM of a conventional broadened waveguide design. This improvement was achieved without any sacrifice in the laser performance, i.e. the novel laser structure showed the same threshold current I(ind th) and differential quantum efficiency eta d as the standard one. Ridge-waveguide lasers employing the new waveguide design and emitting at 2.3 µm were operated in an external cavity configuration. Due to the improved coupling efficiency of the laser beam into the collimating optic, a wide tuning range of 130 nm could be achieved, limited only by the gain bandwidth of the active material.