Importance of depolarized scattering contributions in filtered Rayleigh scattering measurement models

Abstract This paper investigates depolarization influences on detected filtered Rayleigh scattering (FRS) signals to understand, in general, when it is necessary to model both the polarized and depolarized portion of the signal, and when it is sufficient to just model the polarized portion. The effectiveness and robustness of spectroscopic FRS (i.e. the frequency scanning method) is driven by the ability to accurately model these signals across the full range of operating conditions for the instrument. Due to the molecular Rayleigh scattering process, the polarization of the scattered laser light will be slightly depolarized with respect to the incident light. The analysis in this work shows that for a subset of operating conditions (e.g. lower angles between laser propagation direction and observation direction, and near-ambient thermodynamic state), the molecular vapor filter for the FRS measurement suppresses most of the polarized signal, while allowing the depolarized signal to pass through. As such, there is a need to model this depolarized portion of the signal to obtain accurate measurements across the full FRS operating space. Applying the proposed signal model in this work to an extensive FRS database demonstrates vast improvement in residuals between measured and modeled FRS signals.