Nd:YVO4/Cr4+:YAG雷射的連續及脈衝式運作之研究

Abstract

The continuous-wave (CW) and pulsed modes of operation of a Nd:YVO4/Cr4+:YAG laser and the transition between the two modes have been investigated. The laser is end-pumped by a diode laser, and it has a linear cavity which is close to a hemispherical one; thus, the beam size within the cavity varies significantly. In experiment, a Cr4+:YAG crystal with an initial transmittance of 87.8% at 1064 nm was used as a saturable absorber, and the second threshold condition can be tested by varying the position of the saturable absorber. It was found that because of the overlap of the intracavity standing wave and the gain the threshold pump power is nearly twice that of the theoretical expectation when the standing wave feature is ignored. Likewise, in CW operation, the slope efficiency of the laser was found to be lower than theory. The laser may generate pulsed output when the second threshold condition is satisfied; however, pulsed operation may also be possible when the condition is not met, and there are two ways to generate pulsed output: one is achieved by increasing the pump power, the other by tapping the saturable while the laser is under a critical condition of CW operation. For comparison, a LiF: crystal with an initial transmittance of 95.7% at 1064nm was used as saturable absorber instead. We found it was harder for the LiF: crystal than the former to satisfy the second threshold condition. We use rate equation approximation to simulate the experiment, and our calculations found good agreement with the experiment. However, maybe the model employed is oversimplified, and the parameters used in our simulation are not accurate, our simulation cannot account for the experiment in every respect. The model and the physical parameters may need further refinement for more accurate results.