Zigzag slabs for solid-state laser amplifiers: batch fabrication and parasitic oscillation suppression

Applications such as gravitational-wave detectionrequire high-average-power laser sources with ahigh degree of spectral and spatial coherence. Forexample, the proposed Advanced Laser InterferometerGravitational Wave Observatory (LIGO)requires a 200 W, single-longitudinal-mode, singletransverse-mode Nd:YAG laser as compared with the10 Wlaser used continually on site since 1997.1,2 Severalapproaches to meet this requirement are beinginvestigated. Traditional rod-based injection-lockedlasers operate at 114 W.3 Thermal lensing and stressinducedbirefringence present challenges to theirpower scaling. Large-core double-clad fiber amplifierbasedsources have reached 264 W of output power.4However, their phase noise, pointing stability, andlong-term reliability have to be characterized withrespect to the demanding LIGO requirements. Wehave investigated a master oscillator power amplifier(MOPA) approach based on end-pumped zigzag slabamplifiers to take advantage of the well-knownpower scaling of slab lasers and the reliability andcoherence-preserving properties of power amplification.In addition to scientific applications, commercialapplications also motivate several approaches forscaling solid-state lasers to high average powers.For example, active mirror slab lasers also knownas thin-disk lasers, first invented by Martin andChernoch5 and extensively developed by Giesen andcolleagues,6,7 have reached the 1 kW class in multipletransverse modes. Power scaling is difficult becauseof their one-sided cooling and the practicalaspects to operate at a thickness below 100 m. Thezigzag, rectilinear geometry, slab-based5 MOPA systemcan scale to higher average powers while maintaininga high beam quality. The slab lasers arecooled symmetrically on both sides and their averagepower output scales with the cooled area. The nearlyone-dimensional thermal gradients and the zigzagoptical path of slab laser gain media significantlyreduce thermal lensing and stress-induced birefringencecompared with traditional rod-based designs.8,9Early zigzag slab laser designs had low efficienciesdue to flashlamp pumping. Residual phase distortionsand a complex direct water-cooled laser headadded to the engineering challenges.10 Most of theseengineering problems have now been solved by laserdiode pumping through the end11 and edge12 ofconduction-cooled zigzag slabs.13 Nd:YAG zigzag slablasers now operate at multikilowatt output powerswith good beam quality.