Though less compact than vertical-cavity surface-emitting lasers (VCSELs) or edge-emitting lasers, vertical-external-cavity surface-emitting lasers (VECSELs) offer diffraction-limited beams, power scalable to multiwatt levels, and the ability to introduce filters and nonlinear elements into the external cavity to achieve narrow linewidth, tunability, short-pulse generation, or efficient frequency conversion.
In addition to demonstrated operation at 1, 1.31, and 1.55 µm, high-power VECSEL operation at 2 µm has been achieved by researchers at Tampere University of Technology (Tampere, Finland), Universität Würzburg (Würzburg, Germany), and nanoplus Nanosystems and Technologies (Gerbrunn, Germany), a manufacturer of distributed-feedback (DFB) laser diodes in the 0.7 to 2.8 µm wavelength range.1 The realized VECSEL device operates at a wavelength particularly useful for applications in gas spectroscopy and environmental monitoring.
This long-wavelength operation is achieved by using antimonide (Sb)-based compound semiconductors for the gain medium. A unique fabrication process for the gain medium allows the research team to achieve 1 W output power.
In a single epitaxial step, the VECSEL structure was grown on a gallium antimonide (GaSb) substrate using molecular-beam epitaxy. The VECSEL structure comprised a distributed Bragg reflector (DBR) and a quantum-well (QW) gain section. The DBR comprised 18 pairs of quarter-wave-thick aluminum arsenide antimonide (AlAsSb) and GaSb layers, while the active region consisted of five groups of three gallium indium antimonide (Ga0.78In0.22Sb) quantum wells. Each QW group was placed at an antinode of the optical field in the three-wavelength GaSb Fabry-Perot cavity defined by the DBR and the semiconductor/air interface. An additional AlAsSb layer was grown on top of the gain section to confine the photocarriers generated within the GaSb layer by optical pumping and to avoid nonradiative recombination on the surface, and a 30-nm-thick GaSb cap layer was applied to protect against oxidation of the AlAsSb layer.
To fabricate the gain medium, a 2.5 × 2.5 mm2 chip was scribed off the 2 in. wafer and capillary bonded with water to a type-IIa natural-diamond heat spreader with larger dimensions of 3 × 3 mm2. The gain medium was then attached to a water-cooled copper heat sink and placed inside the external-cavity configuration (see figure). The gain region was optically pumped by a 790 nm diode laser; emission from the gain medium was collected by a mirror and transmitted through an optical coupler.
The output from the laser was tested with optical couplers having either 1% or 2% transmission at 2 µm. Near-room-temperature operation at 15°C yielded output power levels of 500 and 700 mW for the 1% and 2% couplers, respectively. The M2 quality factor of the Gaussian-shape beam profile was less than 1.45 based on a knife-edge test. When the copper-mount temperature was cooled to 5°C, the researchers were able to achieve 1 W output power using the 2% coupler.
The VECSEL has fringed spectral-output characteristics because of the etalon effect created by reflection from the diamond surface.
Researcher Antti Härkönen says that research will continue toward improving the operation efficiency, scaling the power to multiwatt levels, and increasing the operation wavelength for the VECSEL.
REFERENCE
1. A. Härkönen et al., Optics Express 14(14) 6479 (July, 10, 2006).
