Probing solid-state laser characteristics through polarization behavior

Abstract

It is difficult to accurately describe the polarization state of light emitted by lasers. Generally, laser polarization behavior is often probed by changing a single parameter, such as pump current, to see what happens. This kind of experiment can be used to formulate and validate theoretical models of the phenomenon. Detailed studies have found that many physical mechanisms contribute to the complex polarization behavior of semiconductor lasers. 3 However, similar studies have yet to be performed on optically-pumped solid-state lasers. Here, we describe our efforts to remedy this situation. Under some conditions, the polarization of solid-state lasers is known to depend on the pump-beam polarization and the stress-induced birefringence that determines the laser cavity polarization. While models have been developed that accurately describe the steady-state behavior, the dynamic evolution predicted by these models had not been compared to experiments. We have been found that abruptly changing the pump beam polarization switches the laser’s polarization state. However, contrary to the behavior seen in semiconductor lasers, the switching occurs after a significant delay. Further investigation has allowed us to accurately model the delay as due to three physical parameters. In our experiments, an electro-optic modulator adjusted the polarization of the pump beam, which was applied to a Nd:YAG laser. The laser’s polarization response to abruptly changing the pump beam’s polarization by 90◦, but keeping its power constant, appeared after a delay of several tens of microseconds (see Figure 1). The delay strongly increased as the pump amplitude approached the threshold pump strength. During this Figure 1. The response of a Nd:YAG laser’s linear polarization modes (black and blue lines) to a 90◦ change in pump polarization (vertical dashed line) is delayed. The pump strength is 116mW, or a factor of 1.26 higher than the threshold pump strength.