The Modernization of a Long-Focal Length Fringe-Type Laser Velocimeter

Abstract A long-focal length laser velocimeter constructed in the early 1980's was upgraded using current technology to improve usability, reliability and future serviceability. The original, free-space optics were replaced with a state-of-the-art fiber-optic subsystem which allowed most of the optics, including the laser, to be remote from the harsh tunnel environment. General purpose high-speed digitizers were incorporated in a standard modular data acquisition system, along with custom signal processing software executed on a desktop computer, served as the replacement for the signal processors. The resulting system increased optical sensitivity with real-time signal/data processing that produced measurement precisions exceeding those of the original system. Monte Carlo simulations, along with laboratory and wind tunnel investigations were used to determine system characteristics and measurement precision. 1. Introduction In the early 1980s a two-component, fringe-type laser velocimeter operating in co-axial backscatter was developed and constructed to investigate flow fields about fixed wing and rotorcraft models in the NASA Langley 14- by 22-Foot Subsonic Tunnel (Hoad