High-Performance Photodetector Circuit for Doppler Lidar

A photodetector circuit has been built for use in the receiver portion of a continuous-wave infrared Doppler lidar system for atmospheric research. As in nearly all Doppler lidar systems, the detection of the return signal in this system involves heterodyning, in a photodetector, with a signal at the same frequency as that of the transmitted laser beam. Thus, the Doppler frequencies manifest themselves as beat frequencies (typically of the order of 40 MHz in this system) that appear in the photodetector output. The reason for developing the present photodetector circuit instead of using one of the many commercially available photodetector circuits is that a unique combination of high gain, high bandwidth, and low noise is needed for detection of the faint (power as low as 10-15 W) return signal in this system.