In situ temperature mapping of high energy, high average power cryogenic amplifiers.*

Heat generation is a key obstacle to scale high energy solid state laser amplifiers to the multi-kilowatt average powers required for several applications. A leading approach to overcome this limitation is cryogenic cooling of the gain media. Accurate measurements of the temperature profiles within the laser active medium are vital for engineering new amplifier geometries and improving cooling techniques. However, methods to map the temperature of high average power amplifiers operating at cryogenic temperatures have not been available. In this work, by using a neural network to analyze the 1015 nm to 1029 nm fluorescence generated by a 940 nm diode laser, we demonstrate an accurate, in situ, noninvasive optical technique to generate two dimensional (2-D) maps of the temperature profile within cryogenic Yb-doped amplifiers operating at high average power. The effect on the temperature profile under different pump powers and cooling interface conditions is also presented.