In-line fibre-optic laser doppler velocimeter using bragg grating interferometric filters as frequency to intensity transducers

Three dimensional complex flows particularly those of turbomachinery present challenges to current measurement technology in terms of restricted optical access, measurement accuracy for the on-axis velocity component, the need to resolve flow turbulence and measurement difficulty from close to surface or intra-channel measurements in rotating machinery. A novel non-intrusive in-line fibre-optic laser Doppler velocimeter is presented specifically for the measurement of the on-axis component of velocity. The measurement principle is based on a Doppler frequency to intensity transducer in the form of a fibre-optic Bragg grating based Fabry-Perot interferometric filter. The filters were fabricated at 514.5 nm but in principle any desired wavelength may be used thus permitting any laser wavelength source to be used. Filters with appropriate features were designed with the aid of the theoretical models based on the coupled mode theory and transfer matrix approach. The argon-ion laser emission wavelength was locked to a corresponding Doppler broadened absorption line of molecular iodine vapour while the Fabry-Perot interferometer phase was controlled in an independent feedback system using digital lock-in amplifiers. The optical frequency was stabilized to within 10 MHz for at least one hour while the phase was controlled to an equivalent of (within) ± 3 MHz in frequency. Both feedback loops utilized custom designed PID electronic circuit controllers. The bandwidth of the filter was tunable by up to 400 MHz, with a resolution of between 0.2 ms'1 and 1 ms"1, and a sensitivity range of between 0.5 [GHz]'1 and 1.7 [GHz]'1. In this technique the filter was tuned to the optical wavelength, rather than tuning the laser wavelength to match the filter. The finished instrument was applied to the measurement of the on-axis component of velocity, of a rotating disc, over an available range of up to ± 42 ms'1, limited only by the maximum velocity of the disc. The detection system was reconfigured for low velocity measurements at twice the sensitivity over a velocity range of ± 7 ms'1. This technique demonstrates a unique contribution to fluid dynamics for the measurement of the traditionally difficult in-line component of velocity.