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Attainment of the theoretical minimum input power for injection locking of an unstable-resonator KrF laser.

We report attaining a ratio of output to injected power as high as 10(8) in a KrF system, the limit being reached when injected power equals the noise per mode in the resonator. We find that, because of coherence effects, adjustment of the cavity length for correct value of the equivalent Fresnel number can change the threshold power for frequency locking by at least 2 orders of magnitude. The laser's locked bandwidth is <0.1 A, and the line can be tuned over a range exceeding 10 A. The beam divergence is near (i.e., 1.5x) the diffraction limit.