High-Beam-Quality, High-Average-Power Yb:YAG Lasers

Precision tracking of airborne targets, atmospheric sensing, and active imaging require high-average-power, high-beam-quality laser sources. We propose to address these requirements through the use of a side-pumped Yb:YAG laser where high beam quality isachieved by multi-passing the fundamental mode of the laser resonator several times through the gain region to effi-ciently extract the stored energy. The pump sources will be coupled to the Yb:YAG crystal using multimode fibers. This allows forflexibility in design and for the laser output power to be scaled to hundreds of watts by increasing the number of fibers pumping the crystal or the pump power in each fiber. In our proposed design, heat is removed from the laser crystal by direct contactto heat sinks; flowing liquid contacting the crystal is not required as it is in many high-average-power laser schemes, allowing the use of a variety of cooling schemes. Thermal lensing, which can cause aberration of the output beam, is kept low byspreading the heat load over a large area and minimizing the distance between the heat load and heat sinks. The use of Yb:YAG will also contribute to reduced thermal lensing due to its low quantum defect and high thermal conductivity. We anticipate thatthe lasers to be developed under this program would have wide applicability in laser machining and material processing. Specifically marking, drilling, cutting, welding, scribing, and ablation applications would benefit. Also, because of the high beamquality, harmonic conversion to other wavelengths for tailored laser machining applications would be very efficient. Military applications, where high beam quality illuminators are needed, would also be pursued.