An interferometric device consisting of two metallic mirrors and a dielectric beam splitter of low reflectivity is used to reduce the spectral emission of a CO2 TEA laser to a single longitudinal mode (SLM). The finesse of the interferometer can be varied continuously by turning the polarization of the laser beam or in steps by using different beam splitters. The resonant reflectivity of the device was found to vary from 20 to 90% depending on transverse mode matching and alignment. With a low reflectivity beam splitter, the reliability of producing SLM pulses reached 100%. As the beam splitter reflectivity was increased, the interferometer finesse decreased resulting in a low reliability of producing SLM pulses. The maximum free spectral range which gave SLM reliably was 5 GHz. We also replaced one of the mirrors of the interferometer by a grating, and we observed SLM output on forty lines of the CO2 spectrum with a reproducibility of 80% or better. We discuss how the device could be used for intracavity pulse selection.
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