Comparison of radiometric and chemical detection sensitivities for heterodyne and direct detection DIAL

A high repetition rate, wavelength agile CO 2 laser has been developed at the Air Force Research Laboratory for use as a local oscillator in a heterodyne detection receiver. Rapid wavelength selection is required for measurements of airborne chemical vapors using the differential absorption lidar (DIAL) technique. Acousto-optic modulators are used in the local oscillator to tune between different wavelengths at high speeds (greater than 100 Hz) without the need for moving mechanical parts. Other advantages obtained by the use of acousto-optic modulators are laser output power control per wavelength and rugged packaging for field applications. A series of experiments to simultaneously characterize the radiometric and chemical detection sensitivities of heterodyne and direct detection DIAL systems is being performed at Kirtland AFB, NM, and will be described. The wavelength agile local oscillator (WALO) has been incorporated into a heterodyne receiver, with the Laser Airborne Remote Sensing (LARS) system providing the laser transmitter and direct detection receiver. The experiment series is studying radiometric issues, spread spectrum operation, the effects of target-induced speckle, and the influence of atmospheric turbulence for both detection mechanisms. Measurements are being performed over a horizontal path at standoff ranges of 4 to 15 km, using both natural and man-made targets. Comparisons of the heterodyne and direct detection radiometric and chemometric results will be presented, and contrasted with predictions from simulations and models. The results will also be discussed in terms of the implications for fielding operational DIAL systems.