HIGH FREQUENCY ACOUSTO-OPTIC FREQUENCY SHIFTER FOR CO_2 LASER RADAR APPLICATION

摘要

A high-frequency acousto-optic frequency shifter is needed as the laser local oscillator(LO) in a coherent CO_2 laser radar receiver for earth-orbiting satellite tracking. To avoid frequency fold-over problem, we require either a 1-GHz frequency shifter or a shifter with a +/-500 MHz shift capability. Previous efforts in developing a 1-GHz frequency shifter was abandoned in favor of the +/-500 MHz shifter configuration because of low conversion efficiency achieved for the former. A combination of a recent reduction in the CO_2 laser LO power requirement and an improved fabrication technique on high-frequency shifters has enabled us to generate enough laser LO power for efficient shot-noise-limited heterodyne detection operation. Detailed frequency shifter fabrication and optical conversion measurements using a frequency stable CO_2 waveguide laser as the optical source for driving the AO frequency shifter will be discussed. Since the spectral purity of the frequency shifter output depends on that of the CO_2 waveguide laser, the frequency stability of the latter under simulated airborne environments has been measured and will be reported. Finally, the spectral purity of the AO laser frequency shifter output will be presented.