Crystal stack for coherent optical beam combination
 A stack of optically contacted birefringent crystals is configured to carry out coherent beam combination (multiplexing) of narrow-bandwidth phase-locked beams from multiple lasers or laser amplifiers (e.g., multimode Yb-doped fiber amplifiers). A stack of N crystals can multiplex the output of 2.sup.N laser amplifiers into a single diffraction-limited beam. Phase control of the beams is maintained by an electronic servo which monitors the optical power emitted into certain undesired beams and minimizes this power by means of phase adjusters (e.g., piezoelectric fiber stretchers) on each amplifier. A configuration is described where a front-end laser master oscillator (FMO) (e.g., a Nd:YAG laser) is demultiplexed by the crystal stack, passes through multiple laser amplifiers, is reflected back through the amplifiers by phase-conjugating mirrors (e.g., passive multimode fibers generating stimulated Brillouin scattering), and is multiplexed on the return trip through the crystal stack. Phase conjugation removes beam distortion and depolarization produced on a single pass through the amplifiers, and the reflected beams are generated with the spatial profiles and alignment needed for beam recombination. An additional back-end laser master oscillator (BMO) (e.g., a second Nd:YAG laser) seeds the phase-conjugate field to phase-lock the different amplifiers. 
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Gerald T.
Moore