Diode-end-pumped solid-state lasers
Thesis (MSc (Physics. Institute for Laser Research))--University of Stellenbosch, 2005.
This thesis consists of two parts: a discussion on diode-pumped solid-state lasers and a detailed description of the development of a diode-end-pumped Nd:YLF laser. A background motivation, which places this research area in perspective, is also given. Part One introduces solid-state lasers and their physics. The focus is on the Nd3+ active ion and describes its energy level structure as a typical four-level solid-state laser. An overview of optical pump sources for solid-state lasers is given, focussing on the construction, operation and advantages of diode lasers. It is motivated that diode-end-pumping solid-state lasers produce laser systems with the highest efficiency and diffraction limited beam quality. It is, however, emphasised that power scaling of diode-end-pumped solid-state lasers is problematic due to localised heat generation in the solid-state laser medium. The adverse effect of heat generation on the laser performance is also described. In the design of diode-endpumped solid-state lasers, the management of thermal effects is suggested as the approach to scale the output power of these lasers. Part Two of the thesis describes the design and the results of a novel high-power diode-end-pumped solid-state laser developed at the Laser Research Institute. The description of the design is split into three components: the laser material, the pump source and the laser resonator. The choice of laser material is motivated in detail, focussing on Nd:YLF’s advantage of having a very weak thermal lens when operated on the σ-polarization at 1053 nm. Its disadvantage of having a low fracture limit is also highlighted, but the approach to power scale it to the multi-10-watt level, with the use of low doping concentration, a low absorption pump wavelength, and a large pump beam, is described. It is further shown that this approach led to the development of a novel laser resonator, which allows a large fundamental mode in the laser material to match the large pump beam, and it can compensate for the astigmatic thermal lens in Nd:YLF. The experimental results of the high-power diode-end-pumped Nd:YLF laser are presented, showing the influence of doping concentration, output coupling efficiency and resonator adjustments on the continuous wave and Q-switched laser performance. It is shown that the optimum laser parameters were determined, resulting in the Nd:YLF laser producing more than 26 W of continuous wave output power with a close to diffraction limited beam quality (M2 < 1.4), and more than 3 mJ of energy per pulse at a repetition rate of 6 kHz when Q-switched. It is concluded that the powerscaling concept proved to be efficient and that further power scaling would be possible with this scheme.