Research On Long- Pulse YAG Laser Amplifier

A theoretical and experimental research on Nd: YAG solid- state laser amplifier in kilowatt- level and long- pulse operation have been conducted in this thesis.First, the laser oscillator and amplifier are analyzed and simulated in both time and space domain. Due to the analysis results of output coupling mirror transmittance, output power, extraction efficiency, Gaussian beam transmission characteristic, gain characteristic, etc, the laser structure are designed. Gaussian beam matching between each component is ensured, so that the incident beam can full fill the end surface of laser rod and make full use of effective modulus volume. Mode selection of self- aperture can inhibit the decrease of M2 factor caused by thermal induced spherical aberration. Also obtained that the amplification stages will not change the beam parameters product(BPP) of incident beam by achieved the matching state of stage spacing and the thermal focal length.Second, based on the nonlinear heat conduction model, thermal effect, include the thermal induced birefringence and characteristics of polarization in [111]- cut Nd: YAG crystal, is analyzed in strong pumping and high power operation mode, and the distribution of the thermal induced birefringence ellipse is obtained. According to the nonlinear model fitting analysis of optical path difference(OPD), it is concluded that the quadratic term related to the radius of rod makes contribution to the thermal focal length, and fourth power item determines the thermal induced spherical aberration. Thermal induced spherical aberration is nonnegligible in high power operation and will influence the beam quality. The thermal focal length expression is also derived based on the nonlinear model, experimental results fit the theoretical analysis. The results show that nonlinear model provides more realistic and more general description for a Nd: YAG crystal in [111]- cut.Last, MOPA structure with single stage of oscillation and double stage of amplification is adopted in experimentation, in repetition frequency of 90 Hz and duration time of 1 ms condition, average output power of 1024 W, maximum single pulse energy of 11.4J, peak power of 11.4 k W, electro-optical efficiency of 3.41 % and BPP of 20.9 mm×mrad are achieved. Among them, the BPP of oscillation stage, which is 19.8 mm×mrad, is basically unchanged through the double stage of amplification within the measurement error ranges and complied with the expected design target. Compared with the same level of kilowatt laser structure of two or three rods resonator, the results show that the MOPA structure laser has the smaller BPP, higher electro-optical efficiency, and the optical damage of intracavity laser medium and optical element can be avoided, also the complexity level of system assembly, commissioning and maintenance can be decreased.