GaSb based diode laser arrays emitting at wavelengths around 2 μm have a significant potential for a variety of applications including material processing, such as welding of transparent plastic materials, and optical pumping of mid-infrared solid state lasers. Even though high output power broad area single emitters and laser arrays have already been demonstrated, they all suffer from a large fast axis beam divergence of typically 67° FWHM due to the broadened
waveguide design employed. Here we will present results on (AlGaIn) (AsSb) quantum-well diode laser single emitters and linear arrays consisting of 19 emitters on a 1 cm long bar emitting at around 1.9 μm. To improve on the poor fast axis beam divergence we abandoned the broadened waveguide concept and changed over to a novel waveguide design which features a rather narrow waveguide core. This results in a remarkable reduction in fast axis beam divergence to 44° FWHM for the new waveguide design. For single emitters a cw output power of more than 1.9 W have been observed. 16.9 W in continuous-wave mode at a heat sink temperature of 20 °C have been achieved for arrays. The maximum wall-plug efficiency amounts to 26% both for the single emitters and the laser arrays. These efficiencies are among the highest values reported so far for GaSb based diode lasers, and allow us to use passively cooled and thus less expensive heat sinking technologies.
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.
Semiconductor lasers with high beam quality and high optical output power are very attractive for a variety of applications such as molecular spectroscopy, fiber optic communication and frequency conversion. In the used power regime, devices based on tapered gain sections are the most promising candidates to reach these demands. However, two disadvantages of the tapered laser concept are the reduced output power provoked by their additional resonator losses and the astigmatism of these diode lasers. In case of high brightness diode lasers it is important to discuss the methods needed for an advanced output power also from the point of view of beam quality. The knowledge about astigmatism is essential for designing micro-optics. For the experimental results low modal gain, single quantum well InGaAs/AlGaAs devices emitting at 980 nm were grown by molecular beam epitaxy. The influence of the thermal resistance and of the tapered section length on the output power as well as on the beam quality has been investigated. In addition the impact of these parameters on the astigmatism of tapered diode lasers has been analysed. The experimental results have been correlated with simulations of the current-power curves and BPM simulations of the nearfield behaviour.
Trap centers and minority carrier lifetimes are investigated in InAs/(GaIn)Sb superlattices used for photodetectors in the far-infrared wavelength range. In our InAs/(GaIn)Sb superlattice photodiodes, trap centers located at an energy level of ~1/3 band gap below the effective conduction band edge could be identified by simulating the current-voltage characteristics of the diodes. The simulation includes diffusion currents, generation-recombination contributions, band-to-band coherent tunneling, and trap assisted tunneling. By including the contributions due to trap-assisted tunneling, excellent reproduction of the current voltage curves is possible for diodes with cut-off wavelength in the whole 8-32 μm spectral range at temperatures between 140 K and 25 K. The model is supported by the observation of defect-related optical transitions at ~2/3 of the band-to-band energy in the spectra of the low temperature electroluminescence of the devices. With the combination of Hall- and photoconductivity measurements, minority carrier lifetimes are extracted as a dependence of temperature and carrier density.