Thermal focal length determination of a laser crystal by modulating the pump source

A simple method for determining the thermal focal length of a laser crystal is developed only by modulating the pump source to the pulse operation mode. The maximum output energy of a laser pulse is initially measured with the modulated pump source at 1 Hz repetition rate, where the thermal effects are negligible. Therefore, the beam waist of the resonator mode is equal to that of the pump laser beam, and a relationship between the energy and cavity waist is deduced. Then, the output energies at different repetition rates of the pump source are measured and the corresponding cavity waist sizes are calculated using the above relationship. Eventually, the thermal focal length of the laser crystal is directly determined by using a formula for the cavity mode radius of the resonator with an internal lens. The present method paves the way to determine the thermal focal length for different laser crystals and hence appropriate laser resonator cavity can be designed for higher energy with good beam quality. (paper)