Research On High Brightness And High Gain All-Solid-State Picosecond Laser Amplifier Technology

Picosecond pulsed lasers at 1064 nm with peak power up to several megawatts and repetition rates below 1 MHz are in great demand in various industrial and scientific fields, such as laser micromachining processes and nonlinear optics research. A master oscillator power amplifier （MOPA） is an option for meeting the demand because it provides considerable pulse energy and peak power for the future applications. Multistage fiber amplifiers can offer high gain. They are affected, however, by unwanted nonlinear effects and optical damage due to the high peak power during pulse amplification. Regenerative amplifiers with diode pumped bulk crystals can be an alternative solution to fiber amplifiers. However, these systems are complex and expensive. Direct amplification of a low power picosecond seed with a high gain bulk crystal is currently widely employed. In this dissertation, all-solid-state bulk amplifier with Nd:YVO₄ medium is used to amplify a SESAM-based passively passively Q-switched （PQS） microchip seed laser. The investigations offer a robust and cost-effective all-solid-state picosecond laser amplifier with both high gain and high brightness.This dissertation is composed of two parts:investigation of a high gain and high brightness all-solid-state picosecond laser amplifier with the end-pumped Nd:YVO₄ rod configuration; investigation of a high gain and high brightness all-solid-state picosecond laser amplifier with the Nd:YVO₄ bounce configuration.The intensively end-pumped Nd:YVO₄ rod configuration leads to a severely heat load. The thermally induced spherical aberration effect of the thermal lens degrades the output beam quality. Numerical calculations show that （the gain saturation effect is neglected）:the spherical aberration coefficient of the aberrated beam wavefront from the amplifier can change the sign in free space propagation; the wavefront can thus be compensated by another amplifier, which has nearly identical pumping parameters with the first amplifier. As the gain in the end-pumped Nd:YVO₄ rod amplifier is Gaussian or super-Gaussian distribution, we can neglect the beam quality degradation induced by intensity distributions and improve the beam quality based on spherical-aberration compensation more easily. A three-stage end-pumped Nd:YVO₄ rod amplifier with spherical-aberration compensation is delicately designed to amplify a 10 mW Nd:YVO₄ microchip seed laser with a pulse duration of ～95 ps at a repetition rate of 100 kHz. The pre-amplifier delivers an average power of 12 W, corresponding to an optical-optical extraction efficiency of 20%. The output power from the MOPA system is 65 W, corresponding to a pulse peak power of ～7 MW and pulse energy of 650μJ. The final beam quality factor M² is better than 1.3.A combination of high inversion density and a large stimulated emission cross section results in an ultrahigh single-pass gain of the side-pumped bounce Nd:YVO₄ slab amplifier. Numerical calculations of the thermally induced aberration effect of the thermal lens show that: the variation of pump size in the vertical direction （Gaussian distribution） influences little of the wavefront aberrations; as the bounce angle increases, the wavefront aberrations in the horizontal direction first increase and then reduce （a small grazing incidence angle results in both high gain and high brightness）. The relationship between the bounce angle and the extraction efficiency is analyzed theoretically. A liquid pure metal grease is used to replace the indium foil as the thermal contact material in experiments for better heat load transfer of the Nd:YVO₄ slab. A 10 mW Nd:YVO₄ microchip seed laser with a pulse duration of-95 ps at a repetition rate of 100 kHz is amplified by two single-pass Nd:YVO₄ bounce amplifiers. The first Nd:YVO₄ bounce amplifier delivers an average power of 12 W with pump power of 55 W, corresponding to an optical-optical extraction efficiency of 22%. The output power from the second amplifier is 32 W, corresponding to a pulse peak power of-3.4 MW and pulse energy of 320μJ. The final beam quality factor M² is 1.25 in the horizontal direction and 1.20 in the vertical direction.