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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 2 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 th and differential quantum efficiency η 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.

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We report on GaSb-based quantum-well diode lasers in a micro-machined external cavity setup using the Littrowconfiguration. An electrostatically actuated Si -grating, optimized to achieve a wide tuning range in the 2.3 µm wavelength range, is used as a tuning element.

There is an increasing interest in room-temperature diode lasers emitting in the infrared wavelength region between 1.9 and 2.4 gm for applications like spectroscopic gas-sensing, material processing, medical diagnostics and therapy.Beside a low threshold current and a high power efficiency of the laser diode, a high coupling efficiency of the laser beam into an optical system or fiber of given aperture is essential, in order to achieve a high net power efficiency of the entire system. So far, the highest CW-output powers for GaSb-based diode lasers at room temperature has been achieved with a broadened waveguide design. This way, low optical losses cx, combined with a low threshold current can be achieved, enabling output powers exceeding 1.7 W at 2.0 gm wavelength [1,2]. The drawback of this vertical laser design is the large beam-divergence in the fast axis with a full width at half maximum (FWHM) of typically 63°-670, limiting the optical coupling efficiency.Here we will present a new design for the vertical waveguide structure of (AlGaIn)(AsSb)-based diode lasers leading to a reduced beam divergence in the fast axis of only 440 full width at half maximum (FWHM), compared to 67°FWHM of a standard broad waveguide design (see Fig. 1). This improvement was achieved without sacrificing the laser performance, i.e. the novel laser structure showed the same threshold current Ith and differential quantum efficiency Ild as the standard one (see Fig.2). Uncoated ridge-waveguide diode lasers emitting at 2.3 gm with a 1000x64 jm2 geometry showed a threshold current density of 180 A/cm2 (or 60 A/cm2 per QW), which is among the lowest values, reported for GaSb-based diode lasers.Diode lasers with the new vertical waveguide design and a 1000 x 6 jm2 narrow ridge-waveguide geometry were operated in an extemal cavity setup in Littrow configuration. As can be seen in Fig.3, a broad tuning range of 130 nm was achieved, due to the high coupling efficiency of the laser beam into the collimating optics. This tuning range is limited by the gain bandwidth of the active layer material, which is indicated by the electroluminesence spectra in Fig. 3.~~~X 25 *(BX),S