Ultraviolet (UV) Laser Implementation and Measurement Sensitivities in Filtered Rayleigh Scattering for Aerodynamic Flows

Abstract

Filtered Rayleigh scattering (FRS) is a non-intrusive, seedless, optical measurement technique that provides time-averaged, planar measurements of three-component velocity, static temperature, and static density of aerodynamic flows. Previous work in FRS development has commonly employed 532 nm Nd:YAG lasers and iodine vapor cells due to the ready availability of optics, achievable laser power levels, and well-documented transmission spectrum of iodine in this wavelength range. However, significant improvements in the strength of the Rayleigh scattered signal can be attained by utilizing shorter laser wavelengths due to the dependence of the Rayleigh scattering signal on the inverse of the wavelength to the fourth power. This study assesses the implementation and theoretical feasibility of an FRS system nominally at 387 nm with the use of cesium vapor as the molecular filter. Cesium vapor exhibits two deep absorption lines corresponding to the 62S(1/2) → 82P(3/2) atomic transitions around 387 nm. These absorption lineshapes are considered along with camera specifications to simulate an ultraviolet filtered Rayleigh scattering (UV FRS) measurement of aerodynamic flows. A signal model is developed using numerical functions for the cesium vapor cell transmission, camera specifications, signal dependent shot noise, and signal independent electronic detector read noise. Using this noise-inclusive model, velocity, static temperature, and static density measurement sensitivities for this proposed configuration are analyzed by evaluating and deriving the Cramér-Rao lower bound (CRLB) for each quantity. The effects of different flow conditions, Mie and geometric scattering levels, cesium vapor cell temperatures, and spectral resolutions are demonstrated and compared to the evaluation of the CRLB for a 532 nm system, representing current and traditional measurement capabilities, with a 2 torr iodine cell as the filter over the same set of simulated conditions. It is shown that the best possible theoretical measurement results are obtained for high-speed wind tunnel condition flows and at high spectral resolution.