Development of an Extrinsic dual-cavity Fiber Fabry-Perot interferometer : Applications to periodic and non-periodic vibration measurements

Abstract

The work involved in this thesis principally concerns the development and characterization of a dual-cavity Extrinsic Fiber Fabry-Perot Interferometer (EFFPI), with the specific aims of analyzing both periodic and non-periodic vibrations. This thesis is divided into five chapters. In chapter I, we provide a brief overview of vibration measurements and their associated techniques, both optical and non-optical. A general description of the characteristics of fiber optic interferometers most suited for this application is next included. The emphasis on non-contact measurement, geometrical flexibility, accessibility to the mesurand in question and the ease of deployment orientates our choice towards the fiber Fabry-Perot device. Chapter II presents the operating principles of the EFFPI. The device contains a “virtual” pseudo-dual-cavity which is generated due to the introduction of polarization-controlling optics into the optical path of the sensing cavity. This configuration enables two sets of “quadrature phase-shifted” interference signals to be obtained, hence eliminating the problem of directional ambiguity. The general properties of the interferometer, such as its reflectance and fringe visibility, have been characterized. More importantly, the polarization states of the injected and output lightwaves have been studied to further understand polarization-induced signal attenuation with the aim of reducing this parasitic effect. A modified zero-crossing fringe demodulation technique is described in chapter III for processing the interference signals from the dual-cavity EFFPI sensor into useful displacement information. The resolution of the demodulation scheme is determined by the number of sub-levels into which the interference fringes can be divided. In this work, a λ/64 resolution is deemed sufficient for application in periodic vibrations with relatively large amplitudes. Various signal types, such as sinusoidal, square, and triangular excitations have been applied and experimentally verified. Possible errors due to temperature variation of the laser source as well as the target orientation during displacement measurements are also investigated. In chapter IV, a phase-tracking technique is described for demodulating the interference signals into the required/desired displacement of a target subjected to non-periodic vibration. The development of a simulation and demodulation program enables the analysis of out-of-quadrature phase errors, random noise effects, quantization noise, etc. The detected phase errors can subsequently be corrected by the demodulator while the noise can be reduced via an amplitude correction method. Experimental tests under squarewave excitation carried out with a PieZo-electric Transducer (PZT) incorporating a capacitive sensor demonstrated excellent agreement (difference of only a few nanometers). The EFFPI sensor is next employed for two specific applications. In seismometry, the possibility of our sensor for detecting both vibration amplitudes and velocities is aptly demonstrated. In addition, the fiber sensor is also shown to be relatively accurate in measuring liquid level variation in an optical inclinometry set-up based on two communicating short-base vases. The final chapter concludes the work carried out in this thesis and proposes perspectives for further enhancing the performance of the developed sensor