VECSEL basics

A Vertical-External-Cavity Surface-Emitting Laser (VECSEL) is a small semiconductor laser, where the laser light propagates perpendicular to the semiconductor wafer surface (surface-emitting). This is in contrast to conventional edge-emitting semiconductor lasers where the laser light propagates among the direction of the wafer surface and is usually emitted at a cleaved edge (edge-emitting). Another major difference to edge-emitters is the relatively short interaction length of the laser beam with the optical active material (often several QWs) of only a few 10 nm, compared to several 100 µm up to 1 mm or longer. The optical gain is therefore only in the region of a few percent and it is essential to minimize the losses in the cavity by using external page Bragg mirrors with a high reflectivity. Unlike a external page VCSEL, in which two high-reflective DBRs are incorporated into the semiconductor structure to form the optical cavity, in a VECSEL one of these mirror is placed outside semiconductor structure and forms an external cavity (see image below left). As mentioned above, the optically pumped gain region is typically very thin compared to the lateral dimensions of the beam. VECSELs are therefore also often referred to as Semiconductor Disk Lasers (SDLs) in analogy to the thin disk lasers.