Diode laser pumping sources for cryogenically cooled solid-state lasers

One of the recent advances in solid-state laser (SSL) defense technology is the 100W level Er-doped "eye-safe" laser with low quantum defect pumping at 1.53 mu m [1]. Major technical challenges in achieving high-wattage devices include increasing the system power conversion efficiency and arranging the removal of heat from both the crystal and the pumps. It is known that performance of the crystal can be improved dramatically by cryogenic cooling [2]. Hence, it is desirable to have cryo-cooled pumps to realize ergonomic and efficient diode-pumped SSL with unified cryogenic cooling. In this paper we report on the development of LN2-cooled InP-based lambda similar to 1.5-1.6 mu m diode pumps. The broad area lasers demonstrated 11W in continuous-wave (CW) regime at an operating current of 20A. Despite the highest CW power measured to date from an InP-based emitter, we did not observe catastrophic optical mirror damage. The spectral width of the radiation from a cooled device decreased 1.5-2 from its room-temperature value, which will significantly improve pumping efficiency. We show that laser diode design has to be optimized for performance at cryogenic temperatures. Reviewing the data on LN2 cooled lasers emitting in the wavelength range of 1.13 - 1.8 mu m, we discuss the route to increase the power conversion of the LN2 cooled InP-based pumps to greater than 60% and further narrow and stabilize the laser emission spectrum.