Single-mode operation of Doppler-broadened lasers by injection locking

A multimode He-Ne laser can be forced to oscillate in a single mode by injecting a narrow-band signal into the resonator. We show experimentally that the minimum injected intensity required for single-mode operation occurs at an injection frequency detuned from line center. This is in agreement with the results from a semiclassical theory of an inhomogeneously broadened injection-locked laser.