Cryogenic Diode Laser Array Pump for High Power YAG Lasers

ABSTRACT: This Phase II SBIR proposal presents Spire’s plan to build, test, and deliver ultra-high power, cryogenically cooled diode laser array modules designed to optically pump the AFRL cryogenic Yb:YAG thin disk laser. Spire-designed epitaxial wafers will be grown and fabricated into diode laser bars for optimal 940 nm output power at liquid nitrogen temperature operation. These bars will be incorporated into >2 kW CW sub-modules, to be then combined into >6 kW optical pump modules for the existing AFRL disk laser. Spire will also produce additional 940 nm cryogenic diode laser array modules from purchased diode laser bars to obtain comparative performance data. Spire’s cryogenic diode laser arrays will be based on relatively low-cost CEO impingement coolers, which were successfully demonstrated during Phase I on a complementary MDA-funded program. The >2 kW diode laser pump arrays will consist of twenty-two bars fitted with cylindrical lens arrays to produce collimated laser beams for uniform disk illumination. Both copper and diamond heat spreaders directly under each laser bar will be investigated and compared for high power operation. Spire will work closely with AFRL personnel for delivery and use of several diode laser array pump modules. BENEFIT: The performance of commercially-available high-power solid state lasers such as the Nd:YAG and Yb:YAG has reached upper limits of output power in the low-kW range. At these power levels, the thermal conductivity of the solid-state lasers is not high enough to dissipate the thermal loading resulting from the optical pumps, and detrimental effects such as thermal lensing, thermal stress, and overheating prevent further significant increases in optical output power. By operating such solid-state lasers at liquid nitrogen (LN) temperatures, these detrimental thermal effects are greatly reduced because of the large improvement in thermal conductivity of almost all materials at or near LN temperatures. Diode laser arrays used to optically pump these solid-state lasers are also expected to benefit from such cryogenic cooling for the same reasons and, are expected to have significantly better reliability and optical efficiency. For example, the Phase I work at Spire demonstrated that a 60 W room temperature (RT) diode laser bar emits 150 W at LN temperatures. The combined benefits of LN operation of both the solid-state laser materials and diode laser pump array materials are expected to result in improved high power solid-state laser performance, making reproducible multi-kW output power levels a commercial reality. Commercial applications of such high power solid-state lasers include metal welding, soldering, cutting, precision shaping and annealing for automotive, aircraft, ship-building, and industrial heavy construction. Higher laser power levels are expected to result in increased manufacturing speed as well as improved laser performance and reliability, resulting in reduced operating cost.