Polarization Errors In Resonator Fiber Optic Gyroscopes

The gyroscope is an inertial sensor for the measurement of the rotation rate. The fiber optic gyro based on the Sagnac effect has the advantages of good resistance, high sensitivity and large dynamic range. The key element of the resonator fiber optic gyro (RFOG) is the fiber ring resonator (FRR), and its basic mechanism is the enhancement of the Sagnac effect due to the multiple transmissions of the light in the cavity. Compared with the interferometric fiber optic gyroscope (IFOG), the RFOG has the potential to achieve a higher detection precision with much shorter fiber, which gives obvious superiority in miniaturization and integration.For the suppression of the polarization fluctuation noise which has limited the long-term stability of the RFOG, this dissertation adopts the special fibers, including the single-polarization fiber (SPF) and the hollow-core photonic bandgap fiber (HC-PBF) to constitute the resonator. The analysis model of the polarization characteristics of the SPF hybrid resonator is established by using the Jones matrix method. To improve the suppression ability of the polarization fluctuation, the structure of the SPF hybrid resonator is analysised and optimized. In the polarization-maintaining HC-PBF hybrid resonator, the influence on the RFOG output fluctuation of the strong reflection characteristic of the two fiber end face in the cavity is mainly analyzed. The major innovations and achievements can be concluded as follows:(1) Since the sinusoidal phase modulation technique is widely applied in the RFOG, this dissertation optimizes the modulating frequency of the sinusoidal signal imposing on the phase modulator. The analytical expression of the demodulation slope of the linear region of the demodulation curve on account of both the reflection and transmission types of the FRRs are obtained by mathematic deduction. After drawing the relationship curve between the demodulation slope of the linear region and the modulating frequency, the optimal modulating frequency corresponding to the largest slope is given out. Compared with the original best modulating frequency calculated by Lorentz approximation, the optimized modulating frequency doubles the slope of the demodulation curve. The experimental RFOG system has been established and the experimental verification of the above simulation result has been carried out.(2) For the further suppression of the deterioration of the RFOG detection precision resulted from the polarization fluctuation in a larger temperature range, a hybrid resonator constituted by general PMF coupler and SPF is proposed. Based on the Jones matrix method, the suppression abilities against the polarization fluctuation of different hybrid FRR structures are compared and analyzed. The research proves that the reciprocal hybrid FRR based on the twice 90° polarization axis rotation splicing has the best suppression ability against the polarization fluctuation noise on the condition of the same instruments and parameters. In consideration of the finesse and the output amplitude of the hybrid resonator, the structure parameters of the resonator is designed and optimized. The tested finesse is 14.7, and with an input optical power of 1.3 mW, the theoretical sensitivity of the RFOG should be 0.32°/h. The simulation results shows that a temperature fluctuation of 100℃ brings about a polarization noise of about 0.29°/h, which has been decreased by 29 times compared with the resonator based on the twin 90° polarization axis rotation splicing with two in-line polarizers. The aforementioned reciprocal hybrid SPF resonator is applied to the practical RFOG system, and a minimum swing signal of -0.001°/shas been detected. In the test time of 1200 s, a random walk coefficient of 0.08°/(?). and a typical bias stability below 0.3°/h has been carried out. The temperature characteristic test shows that while the cavity temperature changes from 15℃ to 27℃ within 20 minutes, the output bias of the RFOG keeps stable.(3) In order to reduce the influence of the three main optical nosies from origin, a novel hybrid polarization-maintaining (PM) HC-PBF ring resonator is contructed by splicing a length of about 10 m of PM HC-PBF with a conventional solid-core PM fiber optical coupler. With consideration of both the finesse and the resonance depth, the HC-PBF hybrid resonator is optimized, and its tested finesse and resonance depth are 6.67 and 0.3246, respectively. By the transfer function method, the reflection characteristic of the end face between the fiber splicing points in the hybrid FRR is analyzed and measured. The research manifests that the influence of the odd components of the backscattering noise in the hybrid FRR is similar with the Rayleigh backscattering noise in a common PMF resonator, which is able to be reduced by the carrier suppression method. However, the even components of the reflection light are hard to be distinguished from the original optical signal, which deteriorates the short-term precision of the RFOG. The HC-PBF hybrid resonator is adopted in the RFOG system. During the test time of 1 hour, the output peak-to-peak value is 0.344°/s, with an integration time of 1 s, close to the 100 s short-term test result of 0.3°/s. Therefore, the detection precision of the RFOG with an HC-PBF hybrid resonator is limited mainly by the short-term noise. The Allan variance analysis towards the RFOG output result shows a bias stability of 0.007°/s.In conclusion, in lack of the developed low loss SPF and HC-PBF coupler, this dissertation applies common PMF coupler and the directly splicing technique of the SPF and the HC-PBF to the design of the resonator. The primary research verifies that the SPF hybrid resonator shows good suppression ability against the polarization fluctuation within a larger temperature fluctuation range. The bias stability of 0.007°/s obtained from the RFOG based on the HC-PBF resonator is the best result reported to date in the research of the HC-PBF resonator.