Optical phase lock loop with a photorefractive optical beam combiner

Two narrow linewidth Nd:YAG unidirectional nonplanar ring oscillator lasers were electronically phase locked using photorefractive two-wave mixing in an iron-doped indium phosphide (InP:Fe) crystal as an optical phase detector. The phase lock loop phase error signal was derived from a low-amplitude sinusoidal phase modulation signal impressed on the strong local oscillator beam at a frequency large compared to the inverse of the photorefractive-grating formation time. A 60-mW pump laser was phase locked to a 10-nW signal laser beam with an rms phase error less than 0.1 rad. No spatial mode matching, phase conjugate mirror configuration or optical coupling between the two laser cavities were required. >