單模Nd:YVO4/Cr4+:YAG雷射動力學之研究

Abstract

The main purpose of the thesis study is to investigate the single mode operation of a Nd:YVO4/Cr4+:YAG laser; especially the changes which occur during the transition between continuous wave (cw) and pulsed modes of operation. Theoretically, the standing wave effect in laser cavity is considered in evaluating the overlap efficiency so that a quantitative explanation to the slope efficiency can be made. In the experimental part, we used diode laser as our pump light source. A folded laser cavity was formed with a Nd:YVO4 laser crystal (serving as a terminal mirror), a thin-film polarizer, a KTP crystal and a output coupler. The KTP crystal and the polarizer function as a mode-selector. We found that stable single-mode operation can be easily achieved even when the cavity length is 18-cm long. At a pump power of 570 mw, an output power of 94 mw was obtained, and the vertical to horizontal polarization ratio is about 200:1. As far as the threshold pump power and slope efficiency of the laser are concerned, our experiments show very good agreement with theory. As the length of the folded cavity was changed to 8 centimeters, a Cr4+:YAG crystal was used as a saturable absorber. By changing the pump power, we measured the spectrum of the laser output which underwent a transition from cw to pulsed operation modes. We found that in the course of the transition, the laser does not change in its wavelength, and its output power still keeps the same linear relationship with the pump power. Thus, the mechanism of the transition may be simply attributed to the cross modulation between the intra-cavity laser intensity and the concentration of the absorbing centers within the Cr4+:YAG crystal.