Yuchao Yan

Reducing polarization-fluctuation induced drift in resonant fiber optic gyro by using single-polarization fiber

A novel hybrid single-polarization (SP) fiber ring resonator is demonstrated by using a polarization-maintaining coupler formed by splicing a section of SP fiber into the resonator. The SP fiber selectively eliminates the unwanted resonance by introducing high loss for the unwanted eigenstates of polarization in the resonator. The calculated result shows that this hybrid SP resonator is a good candidate for a tactical-grade performance gyro with a high environmental temperature stability. The experiment shows that the desired resonance in the resonator can keep an excellent stability in a wide temperature range, thus the temperature-dependent polarization-fluctuation drift in the resonant fiber optic gyro is sufficiently suppressed. As a result, a random walk coefficient of 0.08°/√h and a typical bias stability below 0.3°/h for an integration time of 300 s have been carried out.

Improving Long-Term Stability of a Resonant Micro-Optic Gyro by Reducing Polarization Fluctuation

Resonant micro-optic gyros (RMOGs) with waveguide-type ring resonators are promising candidates for applications demanding small, light, and robust gyros. However, the nonzero bias at the gyro output fluctuates in terms of both the bias and the bias stability, owing to the temperature variations of the waveguide-type resonator. In particular, the temperature sensitivity of the bias stability varies greatly depending on the temperature. This paper intends to reveal that these observed fluctuations arise from the temperature-dependent polarization fluctuation. We will investigate how such polarization fluctuation affects the bias of the gyro-output through the state of polarization of the input beam, the polarization characteristics of the resonator coupler, and the temperature variation of the resonator. Both our numerical simulation and experimental verification are carried out, which, for the first time to our best knowledge, demonstrate that the long-term stability of the RMOG can be significantly improved by optimizing the temperature of the waveguide resonator.

Effect of Fresnel reflections in a hybrid air-core photonic-bandgap fiber ring-resonator gyro

A novel hybrid polarization-maintaining (PM) air-core photonic bandgap fiber (PBF) ring resonator is firstly demonstrated by using a conventional solid-core PM fiber optical coupler formed by splicing a section of PM air-core PBF into the resonator. Due to Fresnel reflections exist at the two junctions between the air-core PBF and the solid-core fiber, the forward output signal of this hybrid ring resonator is the normal resonant curve with the superposition of the lightwaves that experienced even numbers of Fresnel reflections and the backward output signal is composed of lightwaves that experienced odd numbers of Fresnel reflections. Rigorous derivations of the forward and backward output signals are given out. The biggest resonant depth and finesse of the hybrid air-core PBF ring resonator predicted are 0.352 and 6.3 respectively by assuming a splice loss of 1.8 dB per junction. These predictions are finally confirmed by testing both the forward and backward output signals of the hybrid ring resonator. With the countermeasures against the influences of the odd numbers of Fresnel reflections, a bias stability of 0.007°/s is successfully demonstrated in a hybrid PM air-core PBF ring-resonator gyro.

Laser frequency noise induced error in resonant fiber optic gyro due to an intermodulation effect.

For the first time, a significant noise source in the resonant fiber optic gyroscope (RFOG) called intermodulation induced error is proposed and deeply analyzed in this paper. The intermodulation error is produced by the laser frequency noise at even multiples of the modulation frequency due to an intermodulation effect, which will seriously limit the random noise performance of the RFOG. Experiments are designed and conducted to verify and measure the intermodulation induced error in the RFOG. The experimental results confirm the existence of intermodulation error, and fit well with the theory. As for the design of the RFOG, light sources with a narrow intrinsic linewidth and a high modulation frequency are preferable to achieve a high rotation-rate sensitivity.

Resonant fiber optic gyro based on a sinusoidal wave modulation and square wave demodulation technique

New developments are made in the resonant fiber optic gyro (RFOG), which is an optical sensor for the measurement of rotation rate. The digital signal processing system based on the phase modulation technique is capable of detecting the weak frequency difference induced by the Sagnac effect and suppressing the reciprocal noise in the circuit, which determines the detection sensitivity of the RFOG. A new technique based on the sinusoidal wave modulation and square wave demodulation is implemented, and the demodulation curve of the system is simulated and measured. Compared with the past technique using sinusoidal modulation and demodulation, it increases the slope of the demodulation curve by a factor of 1.56, improves the spectrum efficiency of the modulated signal, and reduces the occupancy of the field-programmable gate array resource. On the basis of this new phase modulation technique, the loop is successfully locked and achieves a short-term bias stability of 1.08°/h, which is improved by a factor of 1.47.

Eliminating bias drift error in lock-in frequency for a resonant fiber optic gyro

The accuracy of the resonant frequency servo loop is a major concern for high-performance operation of a resonant fiber optic gyro. This is usually resolved by adopting the central frequency of the laser source to track the resonance of the optical fiber ring resonator in one direction. However, the drift of the resonant frequency arising from resonator temperature fluctuation must be eliminated to maintain this accuracy. The traditional proportional integral (PI) frequency servo loop cannot address this issue very well. For instance, a bias error as large as tens or even hundreds of degrees/hour has been observed at the demodulated output of a resonant frequency servo loop. In this paper, we propose a method to eliminate this bias error by adding a double integral term in the traditional PI-based resonant frequency servo loop. We demonstrate that the double integral term can precisely track the linear resonant frequency drift, evidenced by our experimentally achieved close-to-zero bias error of -0.0009  deg/h at the demodulated output of the resonant frequency servo loop.

Hybrid air-core photonic bandgap fiber ring resonator and implications for resonant fiber optic gyro

A novel hybrid polarization-maintaining (PM) air-core photonic bandgap fiber (PBF) ring resonator is demonstrated by using a conventional PM fiber coupler formed by splicing a section of air-core PBF into the resonator. The coupling loss between the PM air-core PBF and the conventional solid-core PM fiber is reduced down to ∼1.8 dB per junction. With the countermeasures proposed to reduce the backscattering induced noise, a bias stability of approximately 0.007 °/s was observed over a 1 hour timeframe, which is the best result reported to date, to the best of our knowledge, for RFOGs equipped with a hybrid air-core PBF ring resonator.

Research progress of the resonant fiber optic gyroscope technology

The resonant fiber optic gyro (RFOG) is a high accuracy inertial rotation sensor based on the Sagnac effect. The existence of various noises, including the nonreciprocal noises such as the polarization noise and the Kerr noise as well as the reciprocal circuit noise, limits the performance improvement of the RFOG. An improved scheme by inserting two in-line polarizers in the polarization maintaining fiber transmission-type resonator has been proposed to suppress the polarization-fluctuation induced drift. Furthermore, the adoption of the air-core photonic bandgap fibers (PBFs) offers a novel solution to reduce the optical Kerr effect. In addition, A digital signal processor is designed to reduce the reciprocal noises and detect the rotation information. A minimum actual rotation of 0.001°/s is achieved. The dynamic range is improved by a factor of 7 and the scale factor nonlinearity is decreased by a factor of 60.

Hybrid single-polarization fiber ring resonator and implications for resonant fiber optic gyro

A novel hybrid single-polarization fiber ring resonator is demonstrated for reducing the polarization error in a resonant fiber optic gyro. A bias stability below 0.7°/h with an integration time of 100 s has been carried out.

Digital Signal Processing for a Closed-Loop Resonant Fiber Optic Gyro

A digital signal processor for a closed-loop resonant fiber optic gyro (RFOG) is implemented on an FPGA. The detection limit is 9.95nV, which can detect an equivalent Sagnac effect of 1 × 10⁻⁴°/s in an RFOG.