Gain spiking and mode-beating control by signal injection in CO2 lasers

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Abstract
Gain spiking and mode‐beating effects in a high‐power pulsed CO2 laser have been experimentally shown to be controllable by the injection of a low‐power signal. The laser was pumped by a 1‐atm 10–100‐ kW/cm3 discharge controlled by a 175‐kV e‐beam at 0.4 A/cm2. A line‐selectable low‐power CO2 laser beam was injected on axis into the relatively large Fresnel number square unstable resonator cavity of the pulsed laser through a centered hole in the primary mirror. Analysis of the pulse output without signal injection showed as many as eight longitudinal modes were present which produced beats. On rare occasions the beat pattern envelope was seen to rise above the gain spike. Injecting a cw plane‐polarized 0.3‐W P‐20 signal into the CO2 oscillator cavity caused the pulsed laser to operate plane polarized at a single frequency for several hundred nsec. Increasing the injected signal power to 5 W caused a substantial suppression of the gain spike. The observed spike suppression was significantly more than that predicted by the one‐dimensional model analysis.