Reverse wave-coupled ring lasers

Two approaches for phase locking of ring lasers by coupling via reverse waves are described. In one approach, coupling is accomplished by mutual injection of forward waves into reverse waves. Coupling of the forward and reverse waves in each laser is achieved with reverse-wave suppressors. In the alternate approach, the reverse-wave outputs are used for mutual coupling and partial suppressor mirrors are used to couple forward to reverse waves. The approaches to coupling are demonstrated with a CO₂ laser-coupling experiment with two UR90 resonators, giving a cavity-length locking range of one-sixth of a wavelength. A one-dimensional model of the coupling schemes is presented. Predictions of the theory are verified, including the variation of the piston-phase difference with resonator-length detuning for the forward-to-reverse and reverse to-forward approaches. Predictions for the performance of four coupled lasers are given