30 September 1994 Fourier transform Raman lidar for trace gas detection and quantification
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The Raman technique, while a valuable tool in chemical and combustion research, is limited in many remote sensing applications because of the low Raman scattering cross-section, which may be three to five orders of magnitude below the Rayleigh (elastic) values. Two concepts for increasing the signal level are discussed. First, use a range-gated Fourier transform spectrometer to increase the system throughput and allow multiplexing advantages. The spectrum is obtained by performing a FFT on the resulting interferogram. Second, since the cross section goes as the fourth power of the optical frequency, use ultra-violet laser illumination, and separate the resulting florescence radiation by placing a known dispersion on the transmitted waveform. The techniques for achieving this function, and the mathematical formulation for the phase-modulated auto-correlation which result, are not evaluated in this paper. However, the approach does not appreciably lower the available resolution because the limits are imposed by the sampling function inherent to the finite-duration Michelson mirror scan. A conceptual design using a long-pulse, flashlamp-pumped dye laser is shown, and typical performance equations in the detection of Freon 12, CCl2F2, are presented. For a one joule laser and a thirty (30) cm aperture operating in darkness, a concentration of 1023 molecules/m3 can be detected in a 60 km visibility at a range of 3.4 km.
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James C. Sentell, James C. Sentell, "Fourier transform Raman lidar for trace gas detection and quantification", Proc. SPIE 2266, Optical Spectroscopic Techniques and Instrumentation for Atmospheric and Space Research, (30 September 1994); doi: 10.1117/12.187592; https://doi.org/10.1117/12.187592

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