High power, diode pumped, single frequency lasers
This thesis presents experimental and theoretical work on the development of high power (i.e. multi-Watt) single frequency lasers, with the main emphasis on using intra-cavity second harmonic generation to produce high power visible output. With such a laser, pumped by a beam shaped 20W diode bar, we have obtained over 3W of TEM00 single frequency output at 532nm. Because of the strong non-linear effects introduced into the resonator cavity by the frequency doubler, the behaviour of the laser can be significantly altered. In particular, there is the previously unreported phenomenon of mode-hopping suppression, where the non lasing modes are suppressed by the nonlinear loss (due to sum-frequency generation with the lasing mode). This allows the lasing mode to be smoothly tuned over many mode spacings simply by scanning the cavity length. Tuning ranges of up to 80GHz have been measured, and are in good agreement with theoretical calculations. There are also some less desirable consequences of the high nonlinear loss, such as parasitic lasing, self misalignment, and bidirectional lasing, that have had to be overcome in order to achieve efficient operation. Ring lasers can provide, arguably, the most robust and stable single frequency operation. However care must be taken in their design to minimise spatial hole burning. Even a small amount of residual spatial hole burning can cause multi-frequency operation in ring lasers. This thesis contains experimental measurements of spatial hole burning and compares these with a numerically calculated theory. Also, the technique of pump beam displacement to increase the single frequency performance of end-pumped ring lasers, suffering from residual spatial hole burning, is described. Thermal effects, such as thermal lensing and thermally induced birefringence often prove to be a major limiting factor in efficient operation of high power lasers. However these effects are dependent on several factors, such as the resonator design, and the heat sinking of the laser rod. Described in this thesis are the steps we have taken to minimize these effects in the designs of the high power lasers we have constructed.