A feasibility study for an eye safe Raman Lidar transmissometer

The feasibility of using an eye safe Raman lidar transmissometer for solving the Slant Visual Range (SVR) problem in aircraft landing approaches was investigated. The SVR problem and the inadequacy of lidar techniques utilizing scattering from atmospheric dispersions was reviewed. An eye safety study for near ultraviolet (Uv) and near infrared (IR) lasers was conducted and the results were used as constraints on the lidar transmitter parameters.

Results of the investigation showed that the nonresonance Raman scattering cross sections of the major atmospheric gases considered, i.e., N2 and O2, are too low to achieve the desired backscattered return signals in A slight enhancement of lengths around 300 nm can be expected due to strong and 02 absorption bands at shorter wavelengths. Values on cm²/sr can be expected for the pure rotational Raman cross sections of N2 and O2 at 300 nm. With such cross sections it was determined that an eye safe Raman lidar transmissometer is physically feasible in transmittances as low as 10-3 in daylight (outside the at night or whenever the background noise is negligible with respect to the quantum fluctuation noise of the signal.

An investigation of the resonance Raman effect from major atmospheric gases was therefore conducted and an expression for the resonance Raman cross section in terms of the electronic oscillator strength, Franck-Condon factors and line strengths was derived. A search for absorption bands and lines of the major atmospheric gases in the near UV and near IR was conducted to determine the usefulness of the resonance Raman effect. It was found that oxygen has three weak bands in these regions but that these bands are too weak to give any noticeable resonance enhancement of the Raman cross section.