For 10.6 μm laser radar systems which utilize a double sideband modulator as a local oscillator frequency shifter in the receiver section, it is necessary to remove the carrier signal and the unwanted sideband signal prior to routing the frequency-shifted radiation to the heterodyne detector. This paper discusses the use of a reflective-mode Fabry-Perot etalon in conjunction with an optical duplexer to remove the carrier signal. The duplexer is a thin-film polarizer together with a quarter-wave plate. Since the sideband-to-carrier power out of the modulator is typically a few percent, it is imperative that mirror surface error and finite coating reflectivity be properly accounted for in order to achieve an adequate filter design. A rejection ratio corresponding to 20 dB between the laser carrier and the sideband was measured for the present design.
R. T. Brown,
P. K. Cheo,
R. S. Eng,
"Transmission/Reflection Filter For Double Sideband IR Waveguide Modulators", Proc. SPIE 0999, Laser Radar III, (18 February 1989); doi: 10.1117/12.960224; https://doi.org/10.1117/12.960224