Imaging Three-Frequency CO 2 Laser Radar

Abstract

This paper describes an experimental imaging CO2 laser radar breadboard that is capable of producing simultaneous intensity, Doppler, and frequency modulated range images. The uniqueness of this system is that three-frequency homodyne detection, combined with FM chirp pulse compression, allows for wide frequency shifts in the intermediate frequency signal while retaining the signal-to-noise advantages of FM pulse compression. Three-frequency detection gives simultaneous Doppler-independent range and Doppler information. Higher signal-to-noise ratio gives increased system sensitivity and/or range capability. The technique for generating the two transmitted beams necessary for implementing the three-frequency detection is discussed. These beams must be parallel with spatial overlapping wavefronts. The methods for processing the return signal for obtaining intensity, Doppler, and range information are also described. The technique for obtaining Doppler-independent FM range information is discussed. Since the Doppler shift is eliminated from the signal for range processing, only one side of the FM chirp is needed to process the range and thus only one surface acoustic wave (SAW) device is needed. Moreover, the ambiguity range can be varied by fixed increments without changing any hardware and hence without changing the range resolution. Finally, some resulting images are presented to illustrate the system capability.