Cryogenic operation and room temperature application of an optically-pumped surface-emitting semiconductor laser

Abstract

This thesis reports how the performance of a Vertical-External-Cavity Surface-Emitting Laser (VECSEL) can be significantly improved by cooling the active region to cryogenic temperatures. Also presented is the development of a room temperature, stable, high power, wavelength-tuneable, mode locked VECSEL and its subsequent implementation as a pump laser in a system designed to generate single photons.

The VECSEL is a type of semiconductor laser capable of producing high output power in near diffraction-limited beams. The semiconductor gain region is highly temperature sensitive and the output power of a VECSEL is limited by non-radiative Auger recombination. Previous research has shown that by cooling the gain chip the gain per carrier is increased, the carrier density at the point of threshold lasing is reduced and the onset of Auger-recombination induced thermal rollover is delayed.

This thesis presents a VECSEL that uses a gain chip cooled to 83 K. The device exhibited a 53% x 10% reduction in the incident pump power required to reach laser threshold, a 4-fold increase in incident pump power tolerated prior to the onset of thermal rollover, and an increase in output power of more than an order of magnitude when its performance was compared to operation at 293 K. A mode locked VECSEL using a gain chip held at 143 K is also reported. Prior to this research the coldest temperature at which a VECSEL gain chip had been operated was 248 K. This work shows that cryogenic temperatures can significantly improve VECSEL performance and begins a new direction in VECSEL research.

The mode locked VECSEL has surpassed the performance of other types of mode locked semiconductor laser and is now approaching the performance of fibre and solidstate lasers. It has yet to be commercialised and so, to demonstrate that the mode locked VECSEL is now a mature, reliable technology, this thesis reports the development and implementation of a mode locked VECSEL as a pump laser in a single photon generation system. The system generates coincidental pairs of photons and, by detecting one photon in the pair, the presence of the companion will be heralded. The wavelength flexibility, excellent beam quality and high pulse repetition rate make the mode locked VECSEL ideal for both quantum state engineering and heralded single photon generation.

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