Xiaxiao Wang

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.

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.

Analysis and Design of Loop Gains to Optimize the Dynamic Performance of Optical Voltage Sensor Based on Pockels Effect

A design method of loop gains is proposed for improving the fast dynamic tracking performance of optical voltage sensor (OVS) based on Pockels effect. The distribution principle of loop gains is investigated in theory according to the characteristics of closed-loop error of OVS. Based on the obtained distribution principle of loop gains, the hardware circuit and the control parameters of the controller are designed to improve the signal to noise ratio (SNR) of closed-loop error and the dynamic performance of OVS, respectively. The experimental results demonstrate that the system can achieve the high dynamic performance under the high detection precision: OVS can accurately track 13th harmonic with 1.57% measurement error and 4.24° phase error, and the long term steady accuracy of power frequency voltage is within ±0.1%. The experimental results validate the effectiveness of our new design method of loop gains.

Influences of Imperfect Polarization Induced Effects to the Quasi-Reciprocal Reflective Optical Voltage Sensor

The influences of imperfect polarization induced effects are investigated in this paper for a novel optical voltage sensor (OVS), which consists of quasi-reciprocal reflective optical interferometer and digital closed-loop detection scheme. The numerical models are established to describe the relationship between the characteristic parameters of imperfect polarization and the performances of the OVS, which is also simulated to investigate quantitatively. The numerical models and the simulated results are experimentally verified with typical imperfect polarization induced errors. Accordingly, the primary characteristic parameters of imperfect polarization are illuminated. Whats more, the conclusions of this paper can provide useful references for the design and performance improvement of the novel Pockels effect based OVS.

Fiber optic gyro R and D at Beihang University

Beihang University has been carrying out systematic research from basic theory to industrialization of fiber optic gyroscope (FOG). Through the study on basic theory and establishment of physical and mathematical model, simulation platform is formed for the analysis of FOG system.. Production lines are established for integration, test and manufacture of components and FOG products. The test results of our typical products indicate that high performance FOG has a bias stability of 0.0008°h in laboratory, and three-axis FOG has a bias stability of better than 0.01°/h.

Error analysis and experiment for reflective sagnac interferometer-type fiberoptic current sensor

A reflective Sagnac interferometer-type fiber-optic current sensor (RS-FOCS) is presented with Y-branch integrated-optics phase modulator and polarization beam splitter. The digital closed-loop Fiber gyroscope technology is employed for signal detection and processing. The variation of linear birefringence and Verdet constant of sensing fiber and phase retardation of quarter-wave retarder with temperature is indicated as the primary scale factor error source. The effect of linear birefringence was suppressed by winding the sensing fiber spirally, and the ratio error caused by the temperature dependence of the Verdet constant is reduced by elliptical-core fiber quarter-wave retarder. The performance of RS-FOCS is tested, and the results show that: the error is less 0.1% for high current whereas more than 1% for low current; the maximal variation of scale factor is 0.066% in a month; the scale factor error is less than 0.2% at −40°C∼60°C.

Eigenfrequency detecting method with sawtooth wave modulation theory for navigation grade fiber optic gyroscopes

Abstract. Eigenfrequency is a key parameter for the fiber optic gyroscope (FOG). An eigenfrequency detecting method for FOGs, especially for high-grade FOGs, such as the navigation grade FOGs, is proposed. The eigenfrequency is detected with the sawtooth wave modulation theory. Adjusting the frequency of the sawtooth wave to an even integer of the eigenfrequency, the error signal caused by the sawtooth wave modulation will be zero, then the eigenfrequency can be calculated by the value of the sawtooth wave frequency exactly and the bias modulation frequency is at the eigenfrequency accurately. It is demonstrated experimentally with an FOG, the length of whose sensing coil is about 1200 m, that the accuracy of the eigenfrequency measurement is better than 1.2 ppm (0.1 Hz). With its high accuracy, not only can the frequency of the bias modulation be adjusted to the eigenfrequency precisely, but also this method can be used as an eigenfrequency detector for studying the characteristics of the sensing coil according to the eigenfrequency to study the mechanism of the errors generated in the FOGs.

Analysis and elimination of bias error in a fiber-optic current sensor

Bias error, along with scale factor, is a key factor that affects the measurement accuracy of the fiber-optic current sensor. Because of polarization crosstalk, the coherence of parasitic interference signals could be rebuilt and form an output independent of the current to be measured, i.e., the bias error. The bias error is a variable of the birefringence optical path difference. Hence, when the temperature changes, the bias error shows a quasi-periodical tendency whose envelope curve reflects the coherence function of light source. By identifying the key factors of bias error and setting the propagation directions of a super-luminescent diode, polarization-maintaining coupler and polarizer to fast axis, it is possible to eliminate the coherence of parasitic interference signals. Experiments show that the maximum bias error decreases by one order of magnitude at temperatures between -40°C to 60°C.

Influence of polarization-dependent crosstalk on scale factor in the in-line Sagnac interferometer current sensor

Abstract. The scale factor of the in-line Sagnac interferometer current sensor associated with the polarization-dependent crosstalk introduced by optical devices is theoretically investigated. The variations of output pigtail polarization crosstalk of the Ti-indiffused LiNbO3 phase modulator and polarization crosstalk of the polarization-maintaining (PM) delay optical fiber with the temperature can lead to the scale-factor error. The white-light interferometry is utilized to measure the pigtail polarization crosstalk of the phase modulator. The variation of the scale factor with the temperature has been tested over a range from −40°C to 60°C experimentally. The results confirm the influence of output pigtail polarization crosstalk of phase modulator and polarization crosstalk of the PM delay optical fiber on the scale factor. To ensure the scale factor error to be within ±0.2%, the output pigtail polarization crosstalk of phase modulator and polarization crosstalk of the PM delay optical fiber should not exceed −30  dB for the current sensor operating in the outdoor environment.