Novel semiconductor membrane external-cavity surface-emitting laser

Abstract

Optically pumped semiconductor vertical external-cavity surfaceemitting lasers (VECSELs) exhibit many desirable properties1, 2 and have therefore become an important stand-alone class of solid-state lasers over the last 20 years. For example, VECSELs can be used nowadays to reach 100W-level continuous wave output.3 However, a large quantum defect (resulting from the energy difference between pump and laser photons) means that heat is incorporated into the active region of VECSELs. This gives rise to a strongly temperature-dependent performance4 caused by the interplay of gain and cavity resonance and the limited charge-carrier confinement. The limited charge-carrier confinement is a particular challenge in the aluminum gallium indium phosphide (AlGaInP) material system, i.e., in which the thermal conductivity5, 6 is low and the laser structure is based on a thick distributed Bragg reflector (DBR). Indeed, the thermal conductivity of this type of DBR is an order of magnitude lower than well-conducting metals (i.e., which are often used as backside heatsinks) and two orders of magnitude worse than diamond (commonly used for the backside or as an intracavity heat spreader).7 In addition, the semiconductor structure itself—with a thickness of several micrometers (for the active region and the DBR)—and the substrate (with a typical thickness of 350 m) impede the heat flow out of the active region. To overcome the heat flow problems and to improve the performance of VECSELs, numerous thermal management strategies have been previously proposed. Such approaches include changes to the heat spreader arrangement,8 removing the substrate,1 flip-chip processes,9 or the insertion of compound mirrors.10 According to the natural progression of these Figure 1. Picture of the semiconductor membrane external-cavity surface-emitting laser (MECSEL) in operation. From left to right, the out-coupling/resonator mirror, diamond-sandwiched semiconductor gain membrane (integrated into a brass mount), birefringent filter, and pump optics with a 532nm pump laser beam (behind the birefringent filter), and a highly reflective resonator can be seen (as illustrated schematically in Figure 2).