| Optically pumped semiconductor disk lasers (OPSDLs), also referred to as vertical-external-cavity surface-emitting lasers (VECSELs), have emerged recently as a new category of semiconductor lasers. As can be seen from the figure on the left, the overall layout of an OPSDL is similar to that of a classical optically pumped solid-state disk laser, with the solid-state gain medium being replaced by a semiconductor chip.
An OPSDL chip is composed of an epitaxially grown distributed Bragg reflector (DBR) of high reflectivity (R > 99%) acting as cavity mirror and an active region where the laser radiation is generated. The active region itself consists of a number of quantum wells that are embedded between thick barrier layers usually serving also as pump absorbing layers.
The OPSDL concept has been shown to be capable of multiple-Watt output powers and laser emission in circular, nearly diffraction-limited beams even at high power levels; the combination of these properties is in general not achievable using conventional edge-emitting diode lasers. Thus, an OPSDL combines the advantages of semiconductor lasers (efficiency, wavelength versatility) with those of a conventional solid-state laser (simultaneously high beam quality and output power). Using additional intra-cavity optical elements, further functionalities such as passive mode-locking, wavelength tunability, single-longitudinal mode operation or second harmonic generation have been demonstrated recently.
Compared to an electrically pumped VCSEL, the optically pumped OPSDL is capable of much higher output powers. This is due to the scalable approach of the thin disk lasers. Further on, the internal losses due to free carriers are reduced, as the OPSDL semiconductor structure is undoped. The latter is crucial for the realization of high power lasers at longer wavelengths, as the free carrier absorption is the dominant loss mechanism in this wavelength regime.
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