A solid-state laser system for the next generation of gravitational wave detectors with an output power of 220 W at the wavelength of 1064 nm is presented. Singlefrequency operation of the laser was achieved by injectionlocking of a high-power ring oscillator to an amplified nonplanar ring oscillator (NPRO) following the Pound-DreverHall scheme. The high-power stage which features four longitudinally pumped Nd:YAG laser crystals as active media in a ring resonator configuration was designed for reliable long term operation. Using a non-confocal ring cavity to filter the output beam, a pure TEM 00 mode with 168 W output power was obtained.

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