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 190 A/cm<sup>2</sup> 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 and differential quantum effciency 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 effciency of the laser beam into the collimating optic, a wide tuning range of 130nm could be achieved, limited only by the gain bandwidth of the active material.
Type-I diode lasers based on the (AlGaIn)(AsSb) material family are ideally suited to cover the 2-3 μm wavelength range. In this paper recent progress in terms of output power, beam quality and wavelength tunability is reported, achieved for broad-area and tapered single emitters as well as for linear broad-area laser arrays. Special attention has been paid to the reduction of the fast axis far-field beam divergence, employing improved vertical waveguide laser designs. Furthermore, tapered diode lasers have been developed in order to increase the slow axis beam quality at high output powers. An improved beam quality is of particular importance as many applications, including coupling the laser output into an optical fiber or into an external resonator, require diode lasers with a low beam divergence and a high brightness rather than sheer output power.