Wavelength-dependence reduction of the scale factor for tactical-grade fiber optic gyroscopes.

Fiber optic gyroscopes (FOGs) suffer from the scale-factor inaccuracy induced by the wavelength instability of the broadband source, which remains a bottleneck both in theory and in practical application. In this work, we propose a simple but effective technique for reducing the wavelength dependence of the scale factor by employing the size of the digital-ramp register as the actuator in the closed-loop scheme for nulling the ramp-reset-induced errors, instead of the conventionally-used feedback-chain gain. Experiments show that, for the tactical-grade FOG equipped with the super-luminescent diode (SLD) operating under temperatures from -40 °C to +60 °C, the proposed technique reduces the compensated scale-factor inaccuracy to 282 ppm, with respect to 2065ppm in the conventional case. This technique relaxes the stringent requirements on the wavelength stability of SLDs, which contributes to the large-scale production and application of tactical-grade FOGs.

Fiber-optic distributed acoustic sensor utilizing LiNbO3 straight through waveguide phase modulator.

A novel fiber-optic distributed acoustic sensor (DAS) utilizing a LiNbO3 straight through waveguide phase modulator as phase generation carrier (PGC) modulation module for the detection of acoustic signal is presented. The sensitive principle and the phase demodulation method of the system based on phase-sensitive optical time domain reflectometer (Φ-OTDR) are described. This scheme solves the problems of low modulation frequency and unstable performance of piezoelectric transducer (PZT) in the traditional homodyne detection system and depends only on the pulse repetition frequency. The efficacy of the new approach is demonstrated experimentally, showing that the weak acoustic signal can be demodulated accurately. The noise level of the system is < 4.2×10-3 rad/√Hz, the signal to noise ratio (SNR) is > 16 dB, and the spatial resolution is 10 m, as well as a detection frequency can theoretically achieve 25 kHz at 2 km sensing fiber. It provides a new research idea for DAS and is expected to replace PZT to achieve a high-frequency response, which has good potential in the applications of low cost, long distance and high frequency detection.

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.

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 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.