An Accurate Modeling, Simulation, and Analysis Tool for Predicting and Estimating Raman LIDAR System Performance

摘要

BAE Systems presents the results of a program to model the performance of Raman LIDAR systems for the remote detection of atmospheric gases, air polluting hydrocarbons, chemical and biological weapons, and other molecular species of interest. Our model, which integrates remote Raman spectroscopy, 2D and 3D LADAR, and USAF atmospheric propagation codes permits accurate determination of the performance of a Raman LIDAR system. The very high predictive performance accuracy of our model is due to the very accurate calculation of the differential scattering cross section for the specie of interest at user selected wavelengths. We show excellent correlation of our calculated cross section data, used in our model, with experimental data obtained from both laboratory measurements and the published literature. In addition, the use of standard USAF atmospheric models provides very accurate determination of the atmospheric extinction at both the excitation and Raman shifted wavelengths. Raman LIDAR is a useful and powerful tool for remote atmospheric probing where one can accurately determine the identification and concentration of a molecular specie of interest. Raman scattering is considered the generation of Rayleigh scattering sidebands caused by a modulation of the electric dipole of the molecule at a characteristic internal vibrational or rotational frequency. These sidebands appear at optical frequencies shifted from the incident frequency by plus or minus the values of the internal molecular frequencies. Hence, this process works will with laser excitation. While Raman scattering fundamentals in spectroscopy are well understood both theoretically and experimentally, few models exist to predict the performance of Raman LIDAR systems. Raman specrtoscopy is a valuable tool for the study of molecular structure, and Raman LIDAR is a simple, reliable, and accurate tool for remote sensing of gases, liquids, solids, and other molecular species of interest, and, is used with great effect as a standoff detector with an operational range from several to hundreds of kilometers. BAE Systems Raman LIDAR model can be used to accurately predict the Raman LIDAR system performance, specie concentration and identification, and the feasibility for making Raman measurements. When inverted, this model can be used to isolate and identify a specific molecular species. The use of Raman LIDAR in remote sensing is motivated both by the limitations of conventional in-situ probes and limitations of remote sensing techniques such as Mie, DIAL, LIBS, and by the many desirable characteristics of the Raman process. These include: 1) Specific Wavelength Shift - spectral shift of the Raman scattered light uniquely identified with a specific type of-molecule and its associated excitation level; 2) Well Determined and Independent Response -Raman line intensity is directly proportional to the number density of the Raman scattered specie and independent of the density of other molecules; 3) Three Dimensional Resolution -instantaneous response of the Raman process enables dimensional LIDAR techniques to be employed, and; 4) Accessibility of Temperature Information - Raman spectrum for gases in thermal equilibrium is a function of both specie concentration and temperature.