Coherent addition of laser beams in resonant passive optical cavities

An optical resonator, designed for frequency doubling of cw single-frequency radiation, is simultaneously injected by two phase-coherent laser beams with the same frequency. By using standard methods in laser-cavity stabilization, we are able to stabilize the cavity length on resonance with the laser, as well as the relative phase of the funda- mental beams, to fulfill the optimum coupling conditions simultaneously on the two input couplers. By using this method, we generate reliably more than 220 mW of single-frequency radiation at 399 nm using two 0:5 W semiconductor tapered amplifiers at 798 nm. This method can be generalized to a larger number of input couplers and holds promise for improving the performances of extreme-UV frequency combs. © 2010 Optical Society of America 140.3298, 140.4780, 140.7240, 190.2620, 300.6210, 020.3320.