1070 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 9, MAY 1, 2006 Optically Pumped GaSb-Based VECSEL Emitting 0.6 W at 2.3 m N. Schulz, M. Rattunde, C. Manz, K. Köhler, C. Wild, J. Wagner, S.-S. Beyertt, U. Brauch, T. Kübler, and A. Giesen Abstract—We report on the operation and beam profile anal- ysis of an optically pumped GaSb-based vertical-external-cavity surface-emitting laser at 2.33 m. To compensate for the low thermal conductivity of the laser chip, an intracavity heat spreader made from polycrystalline chemical vapor deposition diamond was bonded to the top surface of the chip. In this configuration, at 18 C, a maximum continuous-wave output power of 0.6 W in a multitransverse mode beam was achieved, limited by the available pump power. Optimizing the resonator for TEM laser emission , an output power exceeding 0.4 W was observed at the same temperature. Index Terms—GaInAsSb, infrared, single mode, vertical- external-cavity surface-emitting laser (VECSEL). T HE vertical-external-cavity surface-emitting laser (VECSEL) combines in itself the edge-emitting diode laser’s capability of high output power with the excellent beam quality of a conventional vertical-cavity surface-emitting laser; the latter emits typically at powers in the several milli- watts range in single-transverse mode operation. Compared to classical solid state and solid state disk lasers, the VECSEL offers the advantage of being a semiconductor laser with the associated efficiency and wavelength versatility. These unique properties open up a wide range of possible applications, which stimulated considerable current research interest into this class of lasers. In the well-explored GaInAs–AlGaAs–GaAs materials system, highly efficient VECSELs reaching output powers of several watts, combined with a good beam quality have been demonstrated at wavelengths around 1.0 m [1]–[4]. Further- more, nearly watt-level output powers have been achieved at 1.55 m for an InGaAsP-based VECSEL [5]. At wavelengths exceeding 2 m, research has been focused so far on low-power single-frequency tunable GaInAsSb–GaSb-based VECSELs intended for intracavity gas absorption spectroscopy [6], [7]. Emitting a nearly diffraction-limited beam, the output power of these long-wavelength devices is restricted now up to several milliwatts. There are a variety of applications for which compact laser sources emitting between 2 and 3 m with simultane- ously a high output power and a good quality are required. Examples are LIDAR applications, remote sensing, as well as free-space optical communication and gas absorption Manuscript received February 3, 2006; revised February 20, 2006. N. Schulz, M. Rattunde, C. Manz, K. Köhler, C. Wild, and J. Wagner are with the Fraunhofer Institut für Angewandte Festkörperphysik (IAF), 79108 Freiburg, Germany (e-mail: schulz@iaf.fraunhofer.de). S.-S. Beyertt, U. Brauch, T. Kübler, and A. Giesen are with the Institut für Strahlwerkzeuge (IFSW), Universität Stuttgart, 70569 Stuttgart, Germany. Digital Object Identifier 10.1109/LPT.2006.873360 spectroscopy. For these purposes, the power capability of GaInAs–AlGaAs–GaAs-based near-infrared VECSELs has to be transferred to GaSb-based VECSELs covering wavelengths beyond 2 m. In this letter, we report on the output power and beam profile characteristics of an (AlGaIn)(AsSb)–GaSb VECSEL emitting at 2.33 m. The VECSEL’s epitaxial layer structure was grown by molecular-beam epitaxy on a 2-in GaSb substrate in a single epitaxy run. It consists of a 7- m-thick distributed Bragg reflector (DBR) composed of 21.5 GaSb–AlAs Sb layer pairs with individual optical layer thicknesses of about one quarter-wavelength related to the laser emission wavelength, an active region containing nine 10-nm-thick Ga In As Sb quantum wells embedded between Al Ga As Sb barrier and absorber layers, and finally an Al Ga As Sb top window layer [8]. The active region is designed to absorb approximately 90% of the incident Nd : YAG pump laser emitting at 1.064 m. Due to the large quantum deficit between the pump photons and those emitted by the VECSEL, about 60% of the net pump power (in this context, the fraction of the incident pump power eventually entering the semiconductor chip is referred to as “net pump power”) is converted into heat, mostly within the active region and to a lesser extent in the DBR. Therefore, an appropriate thermal management is of prime importance. For substrate-side-down mounting, the large thickness of the DBR layer stack combined with the poor thermal conductivity of the quaternary materials used result in an effective thermal barrier even when the substrate has been thinned [9]. Neverthe- less, mounting an unprocessed piece of the present VECSEL structure substrate-side-down (substrate thickness: 500 m), continuous-wave (CW) operation could be achieved at room temperature with a threshold pump power density as low as 730 W/cm (i.e., 81 W/cm per quantum well). But the max- imum CW output power was restricted to 2 mW at 100-mW pump power, limited by thermal rollover due to the very inefficient heat-sinking [8]. To bypass the poor heat transfer through DBR and substrate, an intracavity heat spreader was bonded to the semiconductor chip’s top surface using the liquid capillarity bonding technique [10]. Due to its high optical transparency and outstanding thermal conductivity, we chose polycrystalline chemical vapor deposition (CVD) diamond as the heat spreader material [11]. A 6 6 mm VECSEL chip was bonded epi-side-down to an uncoated 475- m-thick polished CVD diamond disk (diameter: 15 mm) fabricated in-house, providing efficient heat extraction from the active region via the epitaxial layer surface to the diamond heat spreader. In the present experiment, the laser cavity is defined by the semiconductor DBR and an external concave output coupling 1041-1135/$20.00 © 2006 IEEE