Investigation Of Diode-Pumped Solid-State Laser With Power Amplifier

DPSSL (Diode-pumped solid-state laser) has become one of the most promising approaches in the high-power laser engineering. It employs diode laser as the pump source instead of conventional Helium or Xenon flashlamp, leading to a significant improvement of efficiency, reliability and beam quality, and thus called the revolution of the laser history. With the development of high power diode lasers, high power DPSSL has become one of the main objectives of current research and development activities within the laser community. Research of DPSSL has mainly concentrated on scaling of output power, while remaining of a high spatial beam quality. The main problem hindering scaling of diode-pumped solid-state lasers is heat deposition within the laser medium. A detailed investigation of the theory of scaling the output power while remaining the good beam quality of DPSSL, versatile Q-switching technologies and laser amplification technology is reported in this thesis, including the following contents.THEORETICAL INVESTIGATIONS(1) Investigations of thermal effects and strategies of eliminating of thermal effectsIn end-pumped solid-state lasers the requirement for small focused pump beam sizes leads to a very high pump deposition density and hence a high thermal loading density. This in turn leads to very strong thermal lensing and thermally induced birefringence, and even fracture of the crystal, which are seriously detrimental to laser performance. It is therefore natural that the study of heat generation in laser crystals, and its detrimental effects to laser performance, has been the focus of much research, with the view being to establish effective ways to scale laser output power without compromising beam quality. However, most research to date has been related to Nd:YAG, with limited work on other materials. In fact, Nd:YVO4 is currently one of the most popular laser crystals of DPSSL for its large stimulated emission cross-section at 1064 nm, especially in end-pumped configurations. It is evident, however, the scope of the research work done for Nd:YVO4 is not consistent with its acceptance as a widely used gain medium. A finite element model has been presented in this thesis to theoretically simulate the thermal effects in the crystal of diode end-pumped solid-state lasers, and also several strategies of eliminating the thermal effects. It was found that the thermal lensing from the end-face curvature is comparable with that from temperature-dependent index change in Nd:YVO4 crystal. The composite crystal not only effectively prevented the end-face from bulging, but also significantly reduced the temperature rise in the crystal. The high temperature and end bulging at the pumped face in the conventional crystal had a detrimental effect on the laser performance, and composite crystal eliminated these effects, resulting a high efficiency and good beam quality. It was also found that the saturation effects of pump light absorption by the crystal has a large effect on the thermal lensing and must be included in the prediction of the thermal lens strength. It has been concluded from the simulation that good beam quality can be achieved by designing a small ratio of laser beam size to pump beam size, and a 'top-hat' pump intensity profile and a longer laser rod with a lower active ion concentration are also preferred to scale the laser output with good beam quality.(2) Investigations of the effects of amplified spontaneous emission (ASE) on dynamic characteristics of passive Q-switchingUsually, the effects of ASE are insignificant and can be ignored. However, strong ASE is quite possible within longitudinally-pumped solid-state lasers because the focused pump beam can induce a high gain and excellent mode matching between the pump and lasing beams. The ASE is especially strong for Nd:YVO4 because of its short fluorescence lifetime and large stimulated emission cross-section. In the case where strong ASE is present, the conventional rate equations can no longer describe the behaviour of Q-switching a...