Investigation On Beam Quality Control In High Power Laser Oscillator And Amplifier

This disseratation is composed of four parts:the thermal effects in a diode-end-pumped solid-state laser; the beam quality evolution of the laser field oscillating inside an oscillator&the influence of spherical aberration on the laser resonator; investigation of beam quality improvement in a diode-end-pumped amplifier; side-pumped solid-state laser&nonlinear frequency convertion.Beam quality is a critical parameter for a laser. Control and improvement of laser beam quality are necessary. Laser beam quality is affected by the thermal effects and gain effects of the laser crystal. In this dissertation, the influence of thermal effects and gain effects on the laser beam quality is investigated, in order to improve laser beam quality.Firstly, the thermal effects in a diode-end-pumped solid-state laser are studied. The crystal’s temperature distribution is solved from the heat conduction equation using analytical method and finite difference method. The optical path difference (OPD) of the laser crystal is calculated, and the thermal lens effect and thermally induced aberrations are investigated. The influence of spherical aberration on the focal length of thermal lens is studied. A laser with different beam radius passing through a thermal lens has different focal length. With the decomposition of the OPD distribution over the slices in terms of the Zernike polynomials, it finds out that the main aberration is spherical aberration. The thermally induced aberrations can be represented by spherical aberration coefficient. A new method using the diffraction pattern of a probe laser to measure the focal length of a thermal lens is proposed, which can be used when the laser is on operation.Based on the study of thermal effects, a theoretical model of end-pumped laser oscillator is developed, considering both thermal effects and gain effects. The stable transverse mode oscillating in the resonator is simulated. The discrepancy of the beam quality at two cavity mirrors is discovered. The beam quality improvement in a laser oscillator is confirmed experimentally and theoretically. The beam quality improvement is attributed to the thermally induced aberrations. A laser with negative spherical aberration is compensated by the thermally induced aberrations of the laser cystal, when it passes through the laser crystal. The influence of spherical aberration on resonator’s stable zones is also studied. The spherical aberration induced stable zones separatioin of different order’s transverse modes is pointed out. Fundmental mode operation can be achieved by choosing proper working point. An experimental setup of a diodes dual-end pumped Nd:YVO4laser is built up.51.2W fundamental mode operation is achieved with a pump power of104W, and the beam quality factor M2=1.2.Laser amplifier is an effective approach for high power laser output. However, the beam quality of the amplified laser is always worse than that of the input laser because of thermally induced aberrations. The beam quality improvement in a laser amplifier is investigated using a theoretical model of end-pumped laser amplifier. The importance of the spherical aberrtion of the input laser on the beam quality of amplified laser is discovered. An input laser with negative spherical aberration is compensated by the thermally induced aberrations of the laser cystal when it is amplified, and the laser beam quality is improved. Both in the single-end pumped and dual-end pumped laser amplifier experiments, the beam quality improvements are confirmed. The beam quality facter M2is improved from2.2to1.4in a single-end pumped laser amplifier. The beam quality facter M2is improved from2.71to1.67in a dual-end pumped laser amplifier. The experimental and numerical simulation results are in good agreement.Finally, side-pumped solid-state laser and nonlinear frequency convertion are studied. The thermally induced birefringence is investigated. A two-rod fundamental mode dynamic stable laser resonator is built up, and35W fundamental mode operation is achieved. Using external cavity nonlinear frequency conversion,1.85W266nm and11W355nm ultraviolet laser output are obtained. The thermal effect of the BBO nonlinear optical crystal is also studied.