The Fourth Harmonic Generation In BBO Crystal

In this dissertation, BBO crystals and KTP crystals were utilized in the Fourth Harmonic Generation (FHG) both for flash-pumped Electro-Optic Q switched laser and Laser-Diode (LD) pumped Acousto-Optic Q switched laser. The mathematics models of the both FHG mechanisms were established. Based on these models, it is found that the UV conversion efficiency and the beam quality of UV laser depended on the critical acceptance angle of BBO crystal in the e ray vibration plane, the walk-off effect of e ray and the harmonic gain saturation.Firstly, the nonlinear optical characteristics of BBO crystal were studied. The acceptance angles in both e ray vibration plane and o ray vibration plane were found as:Δθx=0.276mrad*cm andΔθy=46.25mrad*cm, respectively. We also divided the walk-off effect on the UV conversion efficiency in three regions by comparing the crystal length L and the aperture length La; then made the improved "Heuristic theory" available for strong focusing condition.In the FHG experiments, for flash-pumped Electro-Optic Q switched laser, pulses train of 2.26mJ/pulse was achieved. The IR～UV energy conversion efficiency was 17.8% at most, the repetition frequency was 1Hz～5Hz; for LD pumped Acousto-Optic Q switched laser, pulses train of 215mW average power was achieved by using normal convex lens focusing. The IR～UV energy conversion efficiency was 9.8% at most, the repetition frequency was 15KHz. Based on these results, for 0.9W input green laser power, we used a column lens focusing instand of the the normal convex lens, and found UV conversion efficiency from 14.1% to 18.1%.Two different theoretical models, were presented on the optimization of the FHG by green laser beam in BBO crystal, for Electro-Optic Q switched laser and Acousto-Optic Q switched laser, respectively. For the former one, the Gaussian distribution of laser beam, phase-mismatch as well as gain saturation in the BBO crystal was considered in the model; for the latter, the Gaussian distribution of laser beam, phase-mismatch as well as walk-off effect was considered in the model. The results of models matched the experimental results well.At last, the mechanism of both self-Q-switched microchip laser and gain-switched microchip laser were investigated theoretically and experimentally. The time and spectrum characteristics curves were achieved. Based on these curves, we made a feasibility analysis of FHG on self-Q-switched microchip pulses train but found the UV conversion efficiency was below 1%.