**1. Introduction**

Optical interferometry, as a precision metrology widely used in the optical system for extremely accurate measurements of a variety of physical quantities in laboratories as well as in industry fields [1], has been developed for over a hundred years. The optical interferometry technology is based on the interference of light beams, two beams or multiple beams, launched from same light source, laser source, or other monochromatic source, propagating through space or dielectric mediums such as glass waveguides with different optical paths, and arriving simultaneously at a point in space or on the surface of an object. As a result, the light intensity will vary periodically with the optical path difference, that is the optical phase difference, between the beams [2]. This is the well-known phenomenon of light

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

interference. The periodic change in the intensity of interference light is usually referred to as the interference pattern or the interference fringe [2]. A very small change in the optical path difference in the wavelength scale of light source can induce an obvious and measurable change in the intensity of the interference light. Therefore, by measuring the changes of interference light intensity, one can get the information about the changes of optical paths in an optical measurement system. Based on this mechanism, the optical interferometer is built as an instrument and is widely used for accurate measurements of many physical quantities, such as the distance, displacement, and velocity as well as for tests of optical systems.

With quick developments of laser and fiber optic technologies, the optical interferometry technology also had great progress and evolved from the classical bulk optics to the fiber optics [3]. Based on fiber-optic technologies, applications of the optical interferometers have been expanded to such areas as underwater acoustic detections, voltage and current measurements inside electric power systems [3–7], and biomedical pressure monitoring in living bodies [8]. As one of the most important applications, the optical interferometer is used as the optical interferometer sensor for detections of unknown and uncontrolled physical parameters [3]. The fiber-optic-based interferometer sensor uses optical fibers as light carriers and obtains the detection information from fiber-connected transducers or directly from fibers themselves [3, 8]. Compared with the classical, bulk-optic interferometer sensors, the fiberoptic interferometer sensors can achieve remote sensing and have a number of attractive features, such as excellent sensitivity and large dynamic range, small size with rugged packages, potential for low cost, and high reliability. In general, optical fibers and fiber-optic transducers/sensors are made with totally dielectric materials that are chemically inert and completely immune to electromagnetic interference (EMI), and can also withstand relatively high temperatures [4, 5, 8]. These unique properties make them very favorable to be used in harsh environments [9, 10], such as inside an electric power system in which the strong EMIs often make conventional electronic sensors work unstable and result in the increase of the fault rate.

The main aim of this chapter is to give a brief introduction of optical interferometers based on fiber-optic technology and their practical applications in the electric power industry for monitoring of the power system's running states, as well as for measuring of some crucial physical parameters. In Section 2, we will roughly classify the most commonly used fiber interferometers, according to their architectures, operation principles, and application areas. As a key fiber-optic component, the fiber Bragg grating plays a very important role in the constitution of an in-line fiber-optic interferometer [9, 11, 12], so the principles of the fiber Bragg grating as well as twin-grating-based fiber interferometer also will be introduced briefly. In Section 3, three prototypes of fiber interferometer sensors, developed in our laboratory recently, and intentionally used in the electric power industry for partial discharge (PD) sensing and the measurements of power-frequency electric field strength, are presented. Some preliminary experimental results for demonstrating the performances of these sensors also are presented. In the final section, a conclusion for summarizing our work is given to close this chapter.
