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The CO2 laser DIAL concept has been applied successfully to the detection of common industrial byproducts and a variety of toxins and simulants (see Figures 1 and 2). Work to date has focused almost exclusively on detection of these compounds at low concentrations and at relatively short standoff ranges. The current techniques use large optics, multimode lasers and direct detection. The large optics partially offset the reduced sensitivity of the direct detection receiver. The standoff range can be enhanced by increasing the pulse energy but will still be severely limited by the inherently high noise equivalent power (NEP) oflong wavelength infrared (LWIR) detectors. It has long been recognized that range performance can be enhanced considerably in a photon counting sense by using a fully coherent system (see Figure 3 for state-of-the-art NEP estimates for direct and coherent detectors); good speckle averaging, however, should be retained in coherent sensors to achieve high-sensitivity and this will have a profound influence on the measurement technique. Standoff ranges of 100-200 km can be achieved with high altitude platforms (ER-2 or balloons) and coherent transmit/receive (shared) 30 cm aperture systems with pulse energies of< 1 .0 Joules/pulse (J/p) and repetition rates of 30-50 Hz. A key feature of the high sensitivity measurement approach is to use the coherent mode-locked pulse burst waveform as a wide bandwidth source to provide both speckle averaging (within each pulse) and coherent detection. There is also considerable synergism between the systems required for long-range imaging laser radar and remote sensing; both sensors will use coherent CO2 lasers and similar pulse formats. The laser radar/remote sensing functions can be combined into a single sensor suite. The key functional upgrade is to incorporate tunability in the mode-locked transmitter to produce, on demand, a selection of different CO2 laser wavelengths within the 9-1 1 pm atmosphere window and thus accommodate the remote sensing capability; a matched frequency agile local oscillator is also required for the shot noise limited coherent receiver, to provide 'photon counting' capabilities at all wavelengths of interest.
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