Phase locking of ring lasers can be accomplished by coupling via reverse waves. In one embodiment, a portion of the forward wave output from one laser is coupled into the reverse wave of another, with reverse wave suppressor mirrors used to couple each reverse wave output into the forward wave of the same laser. In an alternate approach, the reverse wave output of one laser is coupled into the forward wave of another, with a portion of each forward output being retro-reflected into the reverse wave of the same laser. A one dimensional model of the coupling schemes is presented, which predicts the result of mirror detuning. The coupling concept is demonstrated by a CO2 laser experiment with UR90 resonators. Predictions of the theory are verified, including the variation of piston phase difference with resonator length for the forward-to-reverse and reverse-forward schemes.
J. A. Benda,
G. E. Palma,
E. A. Sziklas,
"Theory Of Ring Lasers Coupled Using Reverse Waves", Proc. SPIE 1045, Modeling and Simulation of Laser Systems, (31 May 1989); doi: 10.1117/12.951318; https://doi.org/10.1117/12.951318