Laser Doppler velocimetry by modulating a CO_2 laser with backscattered light

A portion of the Doppler shifted light backscattered from a moving diffuse target is allowed to reenter the CO2 source laser. The laser power, monitored by a photodiode through a partially transmitting mirror on the rear of the laser, is modulated at the Doppler frequency. Two regimes of operation are observed. For the laser operating well above threshold, the Doppler signal level is relatively insensitive to the laser operating point. A narrow region of enhanced sensitivity is observed, however, for laser operation at a point near threshold.