jing Li

Fiber-optic intrinsic distributed acoustic emission sensor for large structure health monitoring

A fiber-optic intrinsic distributed acoustic emission (AE) sensor is proposed. By measuring the time delay of two signals from two Mach-Zehnder interferometers, the location of AE can be deduced, and the corresponding sensor is experimentally verified to be feasible with a 206 m average location error in a 20 km sensing range, which shows that this proposed sensor is applicable for distributed AE sensing for large structure health monitoring, with the unique advantages of low cost, simple configuration, and long sensing range. The limitations of the proposed sensor are also discussed, and the future work is presented.

Influences of laser source on phase-sensitivity optical time-domain reflectometer-based distributed intrusion sensor

This paper investigates the influences of laser source on distributed intrusion sensor based on a phase-sensitivity optical time-domain reflectometer (φ-OTDR). A numerical simulation is performed to illustrate the relationships between trace-to-trace fluctuations and frequency drift rate as well as pulse width, and fluctuations ratio coefficient (FRC) is proposed to evaluate the level of trace-to-trace fluctuations. The simulation results show that the FRC grows with increasing frequency drift rate and pulse width, reaches, and maintains the peak value when the frequency drift rate and/or the pulse width are high enough. Furthermore, experiments are implemented using a φ-OTDR prototype with a low frequency drift laser (<5  MHz/min), of which the high frequency drift rate is simulated by frequency sweeping. The good agreement of experimental with simulated results in the region of high frequency drift rate validates the theoretical analysis, and the huge differences between them in the region of low frequency drift rate indicate the place of laser frequency drift among system noises. The conclusion is useful for choosing laser sources and improving the performance of φ-OTDR.

Design principle for sensing coil of fiber-optic current sensor based on geometric rotation effect

The design principle exploiting the geometric rotation effect for the sensing coil of the fiber-optic current sensor (FOCS) on the basis of the polarization-rotated reflection interferometer is investigated. The sensing coil is formed by winding the low birefringence single-mode optical fiber in a toroidal spiral. The effects of the linear birefringence on the scale factor of the sensor can be suppressed with the reciprocal circular birefringence by appropriately designing the geometric parameters of the sensing coil. When the rated current is 1200 A(rms), the designed sensing coil can ensure the scale factor error of the sensor to satisfy the requirements of the 0.2 S class specified in IEC60044-8 over a temperature range from -40 °C to 60 °C.

An Analysis on the Optimization of Closed-Loop Detection Method for Optical Voltage Sensor Based on Pockels Effect

In engineering practice, the closed-loop optical voltage sensor (OVS) based on Pockels effect cannot reach the required precision level mainly due to the various disturbance and noise of system, so the application of OVS for low voltage measurement is restricted. Considering the cross coupling of the main and second closed-loops, the model of disturbance and noise of system that adopts the four-state modulation method is analyzed in the main closed-loop of OVS. Based on the established noise-perturbed stochastic model of OVS, we design a closed-loop detection algorithm for the OVS system to guarantee the mean-square exponential stability with a prescribed H∞ performance in order to optimize the detection precision of OVS. The experimental results show that the detection precision of OVS is 0.144 V while the relative measurement error of the scale factor is within ± 0.15% in measuring low AC voltages from 140 to 500 V, which verifies the effectiveness of the proposed detection scheme.

Signal Detection for Optical AC and DC Voltage Sensors Based on Pockels Effect

A new signal detection technology is presented to improve the stability and robustness of the optical voltage sensors (OVSs) based on Pockels effect for the measurement of ac and dc voltages. The closed-loop error of the OVS is a weak and nonlinear signal vulnerable to unavoidable noise. Simultaneously, the nonlinearity and noise in physical components of OVSs are the major causes of performance deterioration of system in practical high-voltage applications. We design a signal detection hardware that can precisely extract nonlinear closed-loop error and be applicable for the measurement of ac and dc voltages. Based on the signal detection hardware, we analyze the dynamic model of closed-loop OVSs considering the effects of nonlinearity, noise, and time-delay. The control scheme of OVS is proved to obtain exponential stability with a desired attenuation level of noise. The experimental results show that the OVS has a wide bandwidth up to 24.5 kHz, the maximum step voltage 19.5 kV, the accuracy of ac and dc voltage within 0.2% and 0.5%, respectively. The experimental results validate the effectiveness and usefulness of our proposed detection method.

Birefringence elimination of bismuth germanate crystal in quasi-reciprocal reflective optical voltage sensor.

Bismuth germanate (Bi(4)Ge(3)O(12), BGO) has been widely utilized for the application of Pockels effect-based voltage and electric field sensors, because it possesses no unwanted effects ideally. However, there are multiple birefringences in BGO crystal induced by natural imperfections, temperature-dependent strain, and external pressure (or stress), which influences the demodulation of the Pockels effect induced by the voltage to be measured. For a Pockels effect-based quasi-reciprocal reflective optical voltage sensor, the influences of the multiple birefringences in BGO crystal are investigated and an elimination scheme is also proposed in this paper. The feasibility of the proposed elimination scheme is simulated and experimentally verified.

Design of Closed-Loop Detection System for Optical Voltage Sensors Based on Pockels Effect

A closed-loop detection system is established to make optical voltage sensors (OVSs) based on Pockels effect particularly ideal for high-voltage applications. For the closed-loop OVSs, the signal intensity fluctuation due to temperature variation contributes to the gain drift of the forward channel and finally influences the dynamic performance of the OVSs. Meanwhile, the closed-loop error of the OVS is also a weak signal that is hard to be accurately extracted. In this work, we propose a signal detection circuit based on weak signal detection theory to reduce noise levels for improving detection accuracy of OVS. In consideration of the influence of signal intensity fluctuation, we design a control algorithm for the closed-loop detection system to guarantee that the OVS is exponentially stable in order to have a fast dynamic response. The experimental results show that the OVS based on the proposed closed-loop system has accuracy within 0.2%, a fast rise time less than 25.2 μs and a wide bandwidth up to 24.5 kHz.

Review on Development and Applications of Fiber-Optic Sensors

Fiber-optic sensors (FOSs) have experienced a tremendous growth since the 1970s and a number of important transitions into commercial sector have been achieved. Due to the specific advantages, FOSs have been considered as not only the substitutes of conventional sensors but also the unique solution in fields of industry, engineering and scientific research itself. This paper provides a review on the concepts, principles, methodology and applications of FOSs. Firstly, the current state of the art of FOSs is reviewed. As significant cases of developments in FOSs,the interferometric sensors, fiber grating sensors, photo crystal fiber sensors and scattering based sensors are outlined, respectively. Furthermore, several potential areas, where FOS is believed to be the one and only effective solution, are also discussed. Finally, trends in the development of FOSs are briefly outlined by combine with development of photonics.

Signal-to-noise ratio enhancement of phase-sensitive optical time-domain reflectometry based on power spectrum analysis

Abstract. A location technique based on power spectrum analysis for the phase-sensitive optical time-domain reflectometry is proposed. The frequency characteristics of the backscattered signal at a time interval over the sensing fiber are provided to discriminate the disturbance region from other regions. Compared with conventional location techniques, the proposed method significantly enhances the signal-to-noise ratio (SNR). Therefore, it can provide a high-performance and cost-effective solution avoiding the use of the laser with super low-frequency drift. Although the frequency drift of the laser utilized in the experiment is 230  MHz/min, the average SNR is improved to be 20.3 dB and the maximum location error is 100 m over the monitored length of 9 km during 20 experiments.

Perimeter security system based on fiber optic disturbance sensor

The design and field test of a perimeter security system based on fiber optic disturbance sensor was described. The system consisted of fiber optic disturbance sensor and control computer. The fiber optic disturbance sensor was in Mach-Zehnder interferometer configuration using single mode fiber cable, which made the system relatively low cost. A digital Phase Generated Carrier (PGC) demodulation technique was used to eliminate phase drifts in the interferometric sensor. The demodulator was based on Field Programmable Gate Array(FPGA) and Digital Signal Processor(DSP). A prototype system with 1Km sensing cable was constructed and tested. The sensing cable was bound on the fence to detect disturbance generated by intruder. The test result verified that this sensor was sensitive to intrusion behavior. Typical disturbance signal was recorded when intrusion behavior was going on. There was obvious characteristic in the recorded signal. Intrusion behavior generated non-stable random signal, which had many amplitude peaks. And the frequency was below 500Hz. This was helpful for recognition algorithms development. Wind in the test field also generated low frequency background noise and increased recognizing difficulties.