Atmospheric CW-FM-LD-RR ladar for trace-constituent detection: a concept development

We formulate a general framework for simultaneous remote ranging and spectral selection of trace constituents by an appropriate use of frequency modulation combined with mixing and filtering. A theoretical model of a spectroscopic continuous-wave, frequency-modulated, laser-diode-based range-resolving (CW-FM-LD-RR) ladar is developed for the expected echo signal modulated by the nonlinear absorption. Relationships between the heterodyne echo-signal amplitudes and frequencies and trace-gas concentration for each range are analyzed. Features of the echo-signal inversion and its calibration are studied. It is analytically shown that, by suitable processing, the range-dependent concentration profile of the absorbing gas can be extracted.