Phase demodulation scheme for long-distance distributed Mach-Zehnder interferometer

Abstract

A phase demodulation scheme of interferometic distributed optic fiber sensor (DOFS) for strain and vibrations measurement is proposed in this paper. We discuss the principles of interferometic DOFS based on long-length MZI, and present the preliminary results on the application of distributed optic fiber sensor for the measurements of distributed signals in long-distance region. And illustrate important points in regard to the phase demodulation. The system is all fiber and uses a 3×3 coupler for phase demodulation. Parameters such as strain sensitivity, transverse strain sensitivity, failure strain, and frequency response are discussed. In addition, this technique can yield a large dynamic range with phase amplitudes for its symmetry. The high speed digital processing (DSP) technology is used in the sensor system, which can carry out all operations in real time and promote the resolution of localization. The hardware design of phase demodulation scheme of the sensor is completed and the experimental results are obtained. The new scheme greatly reduces the complexity of practical fiber interferometers and an experimental setup has been implemented to demonstrate its expected merits. The interferometic DOFS has better sensitivity and resolution than the others. In theoretic, it allows fully distributed strain, vibration and acoustic wave measurement over a sensing length of tens of kilometer. At the time of writing, the system is only partially completed; therefore the content of this paper will focus on the principle of distributed optical fiber sensor. The results mainly are presented in laboratory.