Extended cavity surface-emitting semiconductor lasers

We review progress in the development of an unconventional type of semiconductor laser that has become the focus of much attention in recent years. The vertical-external-cavity surface-emitting laser is a diode-pumped solid-state laser with a semiconductor quantum well gain medium. It overcomes the limitation of conventional edge- and surface-emitting semiconductor lasers, which can offer either diffraction-limited beam quality, or high power, but not both in the same device. In this paper, we describe the physical principles of these lasers, emphasising those aspects that are unique to this hybrid. Optical characterisation of gain wafers is discussed, with particular attention to photoluminescence measurement; a powerful tool for the analysis of growth errors, the quantum efficiency of the active region, and the effect of multilayer interference effects on laser performance. Key achievements in the field to date, in high-power performance, ultrashort-pulse generation, and spectral coverage are summarised.