Optical Fibre Interferometric System for Doppler Effect Measurement

The optical fibre laser Doppler velocimetry (LDV) is a technique used for the velocity measurement of fluid flows. The two-dimensional directional LDV using a special technique based on generation of heterodyne signals of differential carrier frequencies has been described in literature [1,2]. The capability of simultaneous measurement of three orthogonal velocity components has also been presented in 3D LDV fibre optic system using wavelength and time division multiplexing techniques [3–5]. Recently, the presented system, based on optical fibre low-coherence interferometer, gives a possibility of measurement of the velocity distribution along depth, directly from a width of the Fourier spectrum of the interference signals [6]. Development of the above technique is based on a velocity vector component measurement directly from a peak position of correlation signal between two backscattering lights [7]. Other LDV applications are vibration measurements, by an unbalanced Michelson interferometer [8], self-mixing interference effect [9], or electronic speckle pattern interferometer [10]. In all the above situations, a compact and a simple probe is feasible by introducing optical fibres into the system. However, the use of optical fibres introduces the problems with obtaining efficient and stable system. If fibre LDV is used for a detection of moving surface [11], the use of interferometric Michelson configuration with polarisation maintaining fibre [12] or special interferometric processing [13] is needed for environmental stability and elimination of high frequency detection, respectively. Recently, fibre optic sensor using a frequency modulated laser to simultaneously measurement of both the position and velocity of a refracting target has been presented [14]. However, this system needs an expensive FFT spectrum analyser operating for sufficiently high resolution. In this paper, we present theoretical and experimental results of a new LDV system using single-reflection Fabry-Perot (SRFP) fibre interferometer. The special design of a compact optical head provides polarisation stability, as well as good environmental shielding. The unique numerical processing of interferometric output signals by the technique based on FFT gives an easy possibility of rotation element testing – especially ‘face run-out’ of spinning disc or precession parameters of the whirling mass. The experimental data obtained for the commercially available magnetic hard disc and the dynamically tuned electromechanical (DTEM) gyro are presented as examples of the system application.