High Power, Long Wavelength Semiconductor Disk Lasers for Continuous and Mode-Locked Operation

Doktorsavhandling, 2006

The semiconductor disk laser has recently emerged as an attractive alternative to bulky and power consuming gas and solid state lasers. It offers compact, powerful and efficient devices with W level output power and beam properties earlier not achieved with semiconductor lasers. The concept of using an external laser cavity together with a semiconductor gain chip also opens up possibilities to integrate additional elements in the cavity for improved functionality, for instance wavelength conversion to accomplish emission at otherwise hard to reach visible wavelengths, or to stabilize the emission wavelength of the device. Another technique that is currently subject to intense research is that of passive mode-locking with a saturable absorber to force the laser to emit a train of short pulses instead of continuous emission. The repetition frequency of such a pulse source is set by the round trip time in the laser cavity and is in the GHz range. The concept therefore enables high repetition rate optical pulse sources without the use of any high speed electronics. Although continuous and mode-locked devices with emission wavelengths around 1 µm have successfully been demonstrated, the long wavelength (1.5 µm) devices have suffered from the poor thermal properties of the semiconductor materials used. This thesis describes the design and performance of such long wavelength devices with emphasis on improvements in thermal management schemes to achieve a high output power. We also show how the technique to reduce the thermal impedance simultaneously can improve the spectral properties of the device and stabilize the emission wavelength and as much as 470 mW of single frequency output power was achieved. In the last part of the work a passively mode-locked device emitting near chirp free pulses at a repetition frequency of 3 GHz and an average output power of 120 mW is presented along with the characterization of the semiconductor saturable absorber mirror used to sustain the mode-locking.