Conditions for stable coherent operation of two CO2 lasers with unstable resonators

The influence of a phase shift in a channel coupling two unstable-resonator CO₂ lasers on the band width of stable phase locking of the laser fields was studied experimentally for the first time. The fraction of the radiation effecting the optical coupling was determined experimentally and found to vary in the range 10 ^– 3–10 ^– 2 for different coupling apertures. The maximum mismatch of the resonator lengths which did not disrupt the coherence amounted to ΔL ≈ λ / 20. A method developed for calculating the diffraction made it possible to determine numerically the stable phase locking range of the output radiation from two lasers with unstable resonators. The calculated and experimental parametric dependences of the phase-locking band width were in qualitative agreement.