Toward a Fully Fiber-Integrated Filtered Rayleigh Scattering System

This paper presents an experimental investigation of the feasibility of a fully fiber-integrated filtered Rayleigh scattering (FRS) system that utilizes both fiber optic laser delivery and imaging bundle collection. A fully fiber-based delivery architecture offers a promising path towards moving away from bulky, constrained optical systems that will allow for measurements in increasingly difficult and confined spaces. A series of controlled measurements were performed using multi-mode fibers, combined with simultaneous direct and fiber-bundle imaging across multiple scattering geometries, to better understand the impact of fibers on the measured FRS signal. The experiments were designed to assess preservation of the filtered Rayleigh line shape, wavelengthdependent transmission behavior, and the introduction and potential removal of fiber-generated background signal. Across all the tested light delivery fibers, no observable distortion or broadening relative to the free-space laser was measured, and the iodine transmission curve was preserved in both the direct and fiber-based imaging. The system was then evaluated through staged flow-on measurements, progressing from a controlled benchtop jet to an in situ Virginia Tech High-Speed Wind Tunnel configuration. These measurements demonstrated recovery of the primary velocity and Mach number trends, with final wind-tunnel results showing qualitative agreement between FRS-derived and five-hole-probe flow-field measurements. These results establish a foundation for applying fully fiber-integrated FRS to confined internal flows and future inlet distortion measurements.