*3.3.1. System configuration and operation principles*

As schematically illustrated in **Figure 15(a)**, this fiber electric field sensor actually is a twingrating fiber Fabry-Perot interferometer-based vibration sensor [32]. In this sensor structure, a polyimide-coated, single-mode fiber including two identical gratings is clamped at a position close to the first grating by a fiber holder to form a fiber vibration sensor with the cantilever beam structure [33, 34]. The length of cantilever beam is chosen so that the cantilever beam has a natural frequency close to the power frequency of 50 Hz or 60 Hz. The polyimide coating adopted in this structure is first to protect the fiber gratings as well as to enhance the mechanical strength of fiber cantilever beam, and second to get an excitation power for vibration (see **Figure 15(b)**) from the applied alternating electric filed by means of the electrical polarization properties of polyimide film.

**Figure 15.** Schematics of electric field sensor structure (a), principle (b), and test setup (c).

The polyimide-coated fiber actually holds a trace of charged ions stored among polyimide crystalline layers, which make this fiber appear to have weak electrical polarization properties [31, 35]. As we assume, the charged ions may originate from the electrification occurred in the coating process, when the fiber was repeatedly painted with a brush filling with polyimide resin, or come from the electrostatic charging [36–38] under a high potential field environment. According to our observations and tests, the polyimide favors to trap the negative ions, so it is the polar dielectric [31], showing the electronegative characteristic. Since the polyimide is a dielectric with a relative permittivity = 3.4 at a frequency of 1 kHz at room temperature, and

also as one kind of high-quality electrical insulation material, widely used in the power industry, some of the trapped net negative ions probably can be stored permanently or semipermanently in polyimide crystalline layers after the polyimide resin is heat-cured.

When a fiber with polyimide coating in a loose state is placed into a power-frequency electric field, it starts to jitter under the action of alternating electric field force. However, this jittering is very small in amplitude and irregular, which can not bring us any useful information about surrounding electric field strength. A cantilever beam structure, as employed in many types of vibration sensors [32], can be utilized, in this case, to regularize the fiber's jittering and convert it into a regular mechanical vibration. Furthermore, the vibrating amplitude can be mechanically amplified over hundred times by adjusting the natural frequency of the cantilever beam and bringing it close to the power frequency.

Since the vibrating amplitude of the cantilever beam is proportional to the induced electric field force, in turn, the electric field strength, it is possible to evaluate the electric field strength by means of interferometric methods to measure the vibrating amplitude of cantilever beam. This can be realized with our sensor system to be introduced next by detecting the fringe signal intensity proportional to the vibrating amplitude of cantilever beam.

**Figure 16.** Schematic of electric field sensor system configuration.

Compared with other sensors proposed in [20, 21] with similar structures, this sensor does not need the diaphragm as a reflector, which resultantly make it simpler in fabrication and more

**3.3. A twin-grating fiber Fabry-Perot interferometer-based power-frequency electric field**

We will demonstrate an all-fiber power-frequency electric field sensor, based on a twin-grating fiber Fabry-Perot interferometer. This sensor technology intentionally is applied in the power industry for measuring of the power-frequency electric field strength near the high-voltage

Compared with other types of optical electric field sensing technologies based on several physical effects, such as Pockels, Kerr, piezoelectric, or electrostrictive effect [27–30], our electric filed sensing technology is based on the electrical polarization properties [28, 31] of the dielectric coating material with a variety of attractive features, such as small size, low cost, electrodeless, single-lead, high sensitivity, flexibility in a variety of environments and capa-

As schematically illustrated in **Figure 15(a)**, this fiber electric field sensor actually is a twingrating fiber Fabry-Perot interferometer-based vibration sensor [32]. In this sensor structure, a polyimide-coated, single-mode fiber including two identical gratings is clamped at a position close to the first grating by a fiber holder to form a fiber vibration sensor with the cantilever beam structure [33, 34]. The length of cantilever beam is chosen so that the cantilever beam has a natural frequency close to the power frequency of 50 Hz or 60 Hz. The polyimide coating adopted in this structure is first to protect the fiber gratings as well as to enhance the mechanical strength of fiber cantilever beam, and second to get an excitation power for vibration (see **Figure 15(b)**) from the applied alternating electric filed by means of the electrical polarization

**Figure 15.** Schematics of electric field sensor structure (a), principle (b), and test setup (c).

sensitive to ultrasonic waves propagating in the transformer oil.

**sensor**

158 Optical Interferometry

equipment.

bility of remote sensing.

properties of polyimide film.

*3.3.1. System configuration and operation principles*

The configuration of a sensor system is schematically shown in **Figure 16**, which consists of a CW DFB laser diode, an optical circulator, and a photodetector. The reflection spectrum of sensor is shown in **Figure 3**. The sensor operation is maintained at a quadrature point of the fringe signal by adjusting the laser wavelength with a feedback arrangement to get a quasilinear dependence of reflected light power on the phase shift produced by vibration. The 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 parallel electrodes which generated a uniform electric field in this space.

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.
