Saturation pulse width effect for small signal gain measurement: theoretical model and experiment

Abstract

We report a new method to measure amplitude small signal gain of a chopper base Q-switched Nd:YAG laser using saturation pulse width effect of nanosecond relaxation oscillation (RO) spikes. Formation of such RO pulses is theoretically modeled and simulated by attributing a loss function to the internal Q-switch chopper and solving the standard rate equations using real practical values associated with a laboratory Q-switched cw Nd:YAG high-power laser. It is found that at a certain chopper frequency, pulse width of the simulated RO spikes is saturated and tends to a minimum value. The performance of the represented model is verified by experimental measurements through generating nanosecond RO spiked using a frequency-variable chopper inside the cavity of the laboratory Nd:YAG laser. We observed that at a certain input pump power of 180 and 290 W, when the frequency of Q-switch chopper is respectively raised up to 5.8 and 12 kHz, oscillation of single RO pulse is established. We also found that by further increasing of the chopper frequency up to 18 kHz, FWHM pulse width of the RO fundamental pulse is saturated toward \(\sim \)163 ns. Moreover, at this frequency of chopper and input pump powers of 270 and 180 W, saturated FWHM RO pulse widths of 170 and 330 ns are measured, respectively. Obtained results show good consistency between theoretical model and experimental measurements. This saturation effect, is used to obtain amplitude small signal gain of the utilized Q-switched Nd:YAG laser as 3 and 0.8 at the above input pumping levels, respectively. At the same time by using Findlay–Clay approach the small signal gain is measured to provide a quantitative comparison with the above results. From the final results we found excellent agreement between the presented method and Findlay–Clay approach.