Optical fibre Bragg grating based Fabry-Perot interferometer for in-line laser Doppler velocimetry

There is increasing demand to accurately measure all three components of fluid velocity as complete 3D characterisation of complex fluid flow phenomena is only available from advanced CFD models whose reliability relies on the availability of experimental data. This article presents an all fibre-based laser Doppler velocimetry system (LDV) for the on-axis component of velocity that features Bragg grating based Fabry-Perot interferometric filters custom designed to suit Doppler frequency transduction to intensity. The sensitivity of the in-fibre filters to strain is exploited to provide tuning to the centre wavelength and the spectral shape of the filter, which in turn varies the velocity range, sensitivity and resolution. This feature offers the potential to adjust the instrument sensitivity (to velocity) to match any given flow regime. A phase-locking scheme incorporated to stabilise the filter is described together with an optional laser-wavelength stabilisation system that locks to a Doppler broadened absorption line of iodine vapour. The discrimination of direction of velocity is automatic, as is the case with the iodine cell but not so with LDV, where a Bragg cell is normally used. However, the reference beam implementation of LDV for measuring the on-axis velocity is restricted to velocities of a few tens of metres per second due to the close fringe spacing in the measurement volume coupled with bandwidth restriction on the available electronic processors and the use of an iodine cell fixes the laser wavelength and limits the low velocity range and resolution. The measurement principle of the new technique is simple and offers adjustable velocity range.