Investigation On High Power End-pumped TEM₀₀ Mode Solid State Lasers And Fiber Phase Conjugator For Beam Quality Improvement

This dissertation is composed of two parts:high power end-pumped TEMoo mode solid state lasers and large aperture tapered fiber conjugator and its application in high repetition rate, large energy MOPA laser system.In end-pumping configuration, serious thermal lens effect will be formed because of the pump light's convergent incidence. In Chapter 2, we solve the crystal's temperature field distribution using analytical method and obtain the crystal thermal lens focal length by calculating the OPD. The restriction of thermal lens on TEMoo mode output power and the advantage of composite crystal on alleviating the thermal effect are analyzed. The nonlinear change effect of thermal lens optric power is introduced.Pumping coupling system, which has important influence on the pump radius and waist position in crystal, is an indispensable part in end-pumping configuration. Therefore, we calculate the influence of the pumping coupling system's every parameter on the pump radius and waist position. It is meaningful for experiments.Energy transfer up-conversion (ETU) effect is a kind of spectral loss mechanism, which has important influence for strong pumped crystal. In Chapter 3, we make a comparative analysis on rate equation theory with and without considering the ETU effect and point out that ETU effect will result in upper state life's shortening, population number's reduction, laser threshold's advance and thermal conversion coefficient's increase. With both on ETU effect and the fact that TEM00 mode radius in crystal will change with the pump power's change considered, the nonlinear thermal lens effect is explained. For better laser performance and relaxing the ETU effect, laege pump radius and low doped crystal are preferred.As the link of pump source and gain medium, resonator has important influence on the TEM00 mode output power and efficiency. The first part of Chapter 4 describes the basic principle of thermal-lens resonator and dynamically stale resonator. Considering the experiment, we mainly analyze the functions of plane plane asymmetrical resonator's two arms, cavity-mirror's curvature and in-cavity compensation lens. The second part is focused on the forming and measuring of spherical aberration (SA) and its influence on the beam quality, eigenmode, maximum pump power and frequency degeneracy effect. Methods of SA's compensation and avoidance are summarized. By analyzing the influence of SA on resonator, we find that it will broaden the stable zone's back-edge of the plane-plane symmetrical resonator but separate different transverse mode's stable zone I for plane-plane asymmetrical resonator. Therefore, a new transverse mode differential mechanism based on "SA+ plane-plane asymmetrical resonator" is brought forward, which is also the theoretical precondition for our experiments.In experiments, we realized:①a 36 W double-end-pumped Nd:GdVO4 TEM00 mode laser;②a 30 W-class double-end-pumped composite Nd:YVO4 TEM00 mode laser, which could be modulated up to 200 kHz with AO Q-switch③a 50 W-class double-end-pumped composite Nd:YVO4 TEMoo mode laser oscillator, which could be modulated up to 600 kHz with AO Q-switch.As an application, we make a PPMgOLN based OPO pumped by abovementioned high quality pump source and obtained more than 3 W output power both at 2.7μm and 3.8μm band, and small power output at 4.3μm band.SBS based phase conjugator has the ability of compensating the aberration of laser amplifiers in real-time. We achieved a maximum 70% SBS reflectivity with a large aperture tapered fiber with high repetition rate (1000 Hz) and large energy (40 mJ). The influences of fiber end surface quality and pump pulse width on the SBS reflectivity are experimentally investigated. Fused silica rod Brillouin amplifier is also experimentally investigated and two new phenomena, gain guiding and pulse width broadening, are observed and explained. A high repetition rate, large energy MOPA laser system is prepared with the large aperture tapered fiber conjugator included in. Separated two cell structure based phase conjugator system is designed for further expanding its dynamic range and load-endured ability and verification experiment has been also completed.