LASER DIODE PUMPED ND: YAG SOLID STATE LASER FOR APPLICATIONS IN SECOND HARMONIC GENERATION

Abstract

In this thesis, a new pumping approach for the YAG laser pumped by laser diode is proposed to achieve high energy/power performances. In this approach, the central part of the pump beam from the laser diode is absorbed directly by the YAG rod through its convex end surface, which is very much like the conventional end pump approach, while the outer part of the pump beam is absorbed by the YAG rod through its side surface after multiple diffused reflections in a specially designed pump module, which is very much like the conventional side pump approach. We call this the end-side(ES) pump configuration. The laser mode properties are analyzed by using the ABCD law. A stable convex-concave laser cavity is adopted to achieve the largest possible laser mode volume. The ES-pump approach is analyzed for two different cases-uniform distribution and Gaussian distribution of the pump laser beam. The calculated pump efficiency of the pump module is 60% and 81 % for the uniform distribution and Gaussian distribution respectively. The Q-switched YAG laser properties, including the laser threshold, the output energy and the optical efficiency, are analyzed by using the Q-switched laser theory. Based on the theoretical design and consideration, an ES-pumped YAG laser is constructed and its properties are studied experimentally. The experimental value of the threshold is 81.2mJ. The maximum output energy and optical efficiency of the ES-pumped Y AG laser in free lasing mode is 16.5mJ and 11.5% respectively. In the Q-switched mode, the maximum output energy is 4.4mJ and the optical efficiency is about 3.1 %. The low optical efficiency is mainly due to the low pumping efficiency of the ES-pump module, which is estimated to be only 17% from the experimental data. This pumping efficiency can be enhanced by improving the material used as well as the fabrication process. The application of the ES-pumped YAG laser in second harmonic generation(SHG) is investigated by using a KTP crystal. For Type II SHG of KTP, the phase matching angles and the relative entrance angle of polarized direction of the Q-switch YAG laser are calculated as follows: ?=90°, ?=23.5° and ?1=45°. The length of KTP crystal used is 6mm. The maximum SHG energy obtained is 1.9mJ and the conversion efficiency from 1.06µm to 0.53µm is 44% in our work.