In-line laser Doppler velocimeter using fibre-optic Bragg grating interferometric filters

Abstract

An all fibre-based laser Doppler velocimetry system for the measurement of the predominantly on-axis component of velocity is presented. The technique employs Bragg grating-based Fabry–Perot interferometric filters custom-designed to suit the transduction of Doppler optical frequency shifts to intensity. The sensitivity of the in-fibre filters to strain is exploited to provide tuning of the filter relative to the centre wavelength of the laser and of the spectral shape of the filter, which in turn varies the velocity range, sensitivity and resolution. A phase-locking scheme incorporated to stabilize the filter is described together with a laser-wavelength stabilization system that locks to a Doppler-broadened absorption line of iodine vapour. An optical fibre-linked probe head is used to couple the laser beam at 514.5 nm to and from the measurement volume. The velocimeter is characterized by measuring the velocity of a rotating disc over ranges of ±42 and ±7 m s−1 with a resolution of 0.2 m s−1.