**4. Conclusion**

detected electrical signals after passing a low-pass filter are sent into a digital oscilloscope for waveform display and signal processing. An experimental setup for evaluating sensor performances is shown in **Figure 15(c)**. In the experiments, the sensor was placed between two

In operating principle, when the sensor vibrates under the action of alternating electric field force, the Bragg wavelength of twin-grating-based fiber Fabry-Perot interferometer will be modulated by varying axial strains in the fiber, created by periodic bending of fibers, proportional to the vibrating amplitude, which in turn changes the reflected light power and generates a sinusoid fringe signal with an average amplitude proportional to the electric field strength.

A set of photos showing sensor prototype, experimental setup and sensor vibrating under the action of alternating electric field force is shown in **Figure 17**. The sensor shown in **Figure 17(a)** has a 40-mm long cantilever beam with a natural frequency at 50 Hz. The beam length depends on the power frequency and the hardness of coating material, varying from 39 to 40 mm. The frequency responses of the sensor can be investigated with a frequency-swept shaker. A photo in **Figure 17(a)** shows the sensor vibrating under a 50-Hz, 5-kV electric field environment

parallel electrodes which generated a uniform electric field in this space.

*3.3.2. Experimental results*

160 Optical Interferometry

**Figure 17(b)** is a photo of the experimental setup.

**Figure 17.** A set of photos on sensor (a) and experimental setup (b).

**Figure 18.** Test for investigating frequency response of sensor with a shaker (a) and results (b).

In this chapter, basic concepts of optic interferometry have been presented and clarified. Four main types of fiber interferometers with the help of simple mathematical descriptions also have been briefly introduced. By means of the wavelength of the light source as the length unit as well as the period of light wave as the time unit, optical interferometry is a precision metrology. Combined with the merits of optical interferometry and optical fiber, fiber-optic interferometers have many unique features capable of satisfying a variety of needs in industrial measurement fields, and can provide a much higher sensitivity than their bulk-optic versions, since the optical fiber can be rolled to form a coil which extremely extends the optical path in a limited space. The main applications of fiber-optic interferometers now are in sensor fields as fiber-optic sensors to detect various physical parameters with high accuracy and sensitivity.

Many properties of optical fiber, such as the random birefringence distributions along the fiber, temperature dependency, dispersion, and nonlinearity, however, sometimes become main factors to influence the ultimate performances of a fiber interferometer sensor. Therefore, when we design a fiber interferometer sensor or explore its new applications, these factors should be taken into account. Due to a limit of the number of pages, these properties of the optical fiber as well as their impact on sensor performances, however, have not been mentioned yet in this chapter.

Three novel fiber-optic sensor models, based on optical interferometry technologies and developed in our laboratory recently, have been proposed and demonstrated. Experimental results mainly used for describing basic sensing concepts and sensor structures, as well as for demonstrating sensor performances also were presented, although these are not all of our research achievements. These fiber interferometer sensors are intentionally used, in the future, in the electric power industry as one of most important application fields of fiber-optic interferometer sensing technologies, for various physical parameter measurements, which may be difficult for electric/electronic sensors.

Many research activities are in progress to characterize these proposed fiber sensors in respect of the sensitivity, directionality, frequency responses, and stability. Also how to pack these fiber sensors to make them useful in practical applications is still a challenging work.
