Résumé
In this chapter, we tried to give a precise picture of the current state of the art of VCSEL technology in the 2–3 µm spectral range. We described classical microcavities but also structures with external-cavity geometry which allows one to fabricate devices with improved output beam properties (single transverse mode TEM00 operation) at high power. The electrically-pumped microcavities with highest performance in the 2–3 µm range are obtained from the AlGaInAs/InP materials system. But λ = 2.1 µm seems to represent the high wavelength limit of such devices. Concerning optically-pumped devices, Sb-based structures seem to have the best properties. Circular TEM00 output beam and single frequency laser operation was obtained above room temperature from both microcavities and external cavity devices with Type-I GaInAsSb/AlGaAsSb MQW active regions.
These mid-infrared emitting VCSELs were developed essentially for spectroscopic applications. The single mode operation, together with the broad continuous tuning range (without mode-hops) and the high repetition rate of VCSELs, are particularly interesting and highly suitable for gas detection or environmental monitoring. However, the recent development of vertical-cavity structures combining high power and high output beam quality opens supplementary fields of applications, for example in medicine, fiber amplifier pumping, free-space communications or military infrared countermeasures.