Xijing Wang

Reduction of polarization-fluctuation induced drift in resonator fiber optic gyro by a resonator with twin 90° polarization-axis rotated splices

A method to suppress polarization-fluctuation induced drift in resonator fiber optic gyro (R-FOG) is demonstrated by a polarization-maintaining fiber (PMF) resonator with twin 90 degrees polarization-axis rotated splices. By setting the length difference of the fiber segments between two 90 degrees polarization-axis rotated splicing points to a half of the beat-length of the fiber, a single eigen-state of polarization (ESOP) is excited with incident lightwave linearly polarized along the polarization-axis of the fiber. Compared to the previously reported resonator employing single 90 degrees polarization-axis rotated splice [1], in which two ESOPs are excited, our new scheme avoids the effect from the unwanted ESOP and thus suppresses the polarization-fluctuation induced drift in R-FOG output significantly.

Automated Suppression of Polarization Fluctuation in Resonator Fiber Optic Gyro With Twin 90

A method for automatically suppressing the polarization fluctuation in a resonator fiber optic gyro by adopting a resonator with twin 90° polarization-axis rotated splices is proposed and demonstrated experimentally. The scheme ensures that the input electric field excites only the desired eigenstate of polarization in the resonator by automatically setting the length difference of the fiber segments between the two 90° polarization-axis rotated splicing points at one half of the beat length of the polarization maintaining fiber. The feasibility of the proposed scheme is shown by simulation using a mathematical model based on the Jones transfer matrix. It is also demonstrated experimentally that the bias stability is enhanced when the feedback scheme is adopted.

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To get rid of the complex analogue control loops that are traditionally used in resonator fiber optic gyroscope (R-FOG) and to meet the requirements of high accuracy as well as small-size and light-weight in Inertial Navigation Systems (INSs), a digital controller is developed and demonstrated experimentally to control the R-FOG with the digital serrodyne modulation. The digital controller is designed to implement the function of tracking the laser diode frequency drift and to compensate for the imperfect 2π modulation voltage of the phase modulator for the serrodyne modulation. To acquire the resonant frequency, the digital serrodyne modulation with symmetric frequency shift is adopted as a counter-measure for Rayleigh backscattering noise. The optimal serrodyne modulation generated frequency shift is decided to be in the range of 100~120 kHz according to the numerical calculation. The successful demonstration of the open loop operation with the digital controller is deemed as the basis for the digitalized closed-loop experiment in future.

Polarization-Axis Rotated Splices

A field-programmable gate array-based digital processor is proposed and demonstrated experimentally for a resonator fiber optic gyro (R-FOG) with a bipolar digital serrodyne phase modulation scheme, which we previously proposed especially for R-FOG signal processing and its noise reduction. The processor has multi functions. First, it suppresses both the fast- and slow-drift components in the difference between the laser frequency and the resonators resonant frequency. The fast-drift with a small amplitude is compensated for by a proportional controller with an oversampling function to reduce the quantization error, while the slow-drift with a large amplitude is tracked using an up/down counter. Second, it automatically adjusts the amplitude of the waveform for bipolar digital serrodyne phase modulation for waves travelling both in the resonator clockwise and counterclockwise. Bipolar laser frequency alternation required to track the resonators resonant frequency is ideally realized by adjusting the phase modulation amplitude. This automatic adjustment also realizes an additional function for reducing the gyro drift caused by backscattering in the fiber resonator, which was originally implemented in the shape of the waveform for bipolar digital serrodyne phase modulation. Third, the FPGA generates a gyro output with open-loop operation. The R-FOG performance is demonstrated to be improved by applying these three functions with the FPGA.

Resonator fiber optic gyroscope with digital serrodyne scheme using a digital controller

A closed-loop resonator fiber optic gyro (R-FOG) with a precisely controlled bipolar digital serrodyne phase modulation scheme is experimentally demonstrated. The serrodyne modulation scheme serves multiple functions in the R-FOG, from reducing the backscattering induced performance degradation to achieving the closed-loop operation. To improve the backscattering suppression efficiency, a precise amplitude adjustment method using a gain variable amplifier and the oversampling technique is utilized to control the waveform of the bipolar digital serrodyne phase modulation. Compared with the amplitude control via adjusting the digital gain of the function generator, improved bias stability is realized. Moreover, the closed-loop operation is needed for the R-FOG to achieve high linearity in a wide dynamic range as the frequencies of both the clockwise (CW) and the counter clockwise (CCW) lightwaves are maintained at the resonators resonant frequency. To balance the resonant frequency difference, an additional bipolar digital serrodyne waveform, with the slope proportional to the rotation speed to compensate for the resonant frequency difference, is superimposed on the original bipolar digital serrodyne waveform. Measurement results of different rotation speeds show good linearity thanks to the adoption of closed-loop operation.

Resonator Fiber Optic Gyro with Bipolar Digital Serrodyne Scheme Using a Field-Programmable Gate Array-Based Digital Processor

We present the theoretical analysis on the effectiveness of the polarization-fluctuation suppression feedback control in resonator fiber optic gyro (R-FOG) with twin 90° polarization-axis rotated splices. Previously reported experimental results have shown that this feedback scheme is effective in improving the long-term bias stability of R-FOG by keeping the length difference of the fiber segments between two 90° polarization-axis rotated splicing points (Δl) to a half of the beat-length of the polarization maintaining fiber (B/2). In this paper, the effectiveness of the feedback loop is verified theoretically using Jones transfer matrix. Simulation results indicate that: (1) the error signal of the feedback loop (the yaxis polarized component in the output of the resonator) changes linearly with Δl; (2) the error signal diminishes as Δl is adjusted to the ideal condition of B/2, which are in good accordance with our experimental results.

Closed loop resonator fiber optic gyro with precisely controlled bipolar digital serrodyne modulation

We numerically analyze the polarization-fluctuation induced bias error in a resonator fiber optic gyro (R-FOG) with twin 90° polarization-axis rotated splices in the polarization maintaining fiber (PMF) resonator, especially when polarization dependent loss (PDL) exists inside the resonator. Simulation results indicate that the twin 90° polarization-axis rotated splicing method can effectively suppress the polarization-fluctuation induced bias error in an R-FOG, when the length difference of the fiber segments between the two splicing points is set around a half of the beat-length of the PMF. It is also shown that such an R-FOG is still tolerant to PDL if the polarization crosstalk in the resonator is low, although PDL degrades the performance of the R-FOG.

Automated Suppression of Polarization-Fluctuation in Resonator Fiber Optic Gyro by a Resonator with Twin 90° Polarization-Axis Rotated Splices- Theoretical Analysis

5-order enhancement in system sensitivity of resonator fiber optic gyroscope by polarization-noise suppression using twice 90o polarization-axis rotated splicing is numerically demonstrated. The optimal condition to suppress polarization-noise is demonstrated experimentally by temperature control.

Analysis of Polarization-Fluctuation Induced Bias Error in Resonator Fiber Optic Gyro with Twin 90° Polarization-Axis Rotated Splices

We have proposed and studied a resonator fiber optic gyro (R-FOG) with bipolar digital serrodyne phase modulation scheme. The serrodyne modulation serves multiple functions from reducing noises caused by fiber characteristics, such as backscattering and optical Kerr effect, to achieving gyro signal processing including closed-loop operation. Because the sensing fiber length in R-FOG is much shorter than that in interferometer FOG (I-FOG), the Shupe effect, which is caused by temporally variant temperature distribution along the fiber, can effectively be reduced in R-FOG. In this paper, a resonator made of a polarization-maintaining optical fiber (PMF) with twin 90o polarization-axis rotated splices has been proposed to suppress the polarization-fluctuation induced drift. An automated control to optimize the suppression has been proposed and demonstrated in experiments. To suppress the backscattering induced noise effectively, a precise adjustment of amplitude of the bipolar digital serrodyne waveform has also been introduced. Additionally, a closed-loop operation has been demonstrated by locking both the frequencies of clockwise (CW) and counter clockwise (CCW) travelling lightwaves to the resonators resonant frequencies with manipulating the serrodyne waveform.

Polarization-noise suppression by twice 90° polarization-axis rotated splicing in resonator fiber optic gyroscope

Automated suppression of polarization-fluctuation in a fiber optic gyro made of a polarization-maintaining fiber (PMF) resonator with twin 90° polarization-axis rotated splices is experimentally demonstrated. The y-axis polarized component in the output of the resonator is detected to generate the feedback signal to adjust the length difference of the two fiber segments between the two 90° polarization-axis rotated splicing points. When the length difference becomes a half of the beat-length of the PMF, only one eigen state of polarization is excited linearly polarized in x-axis direction, ensuring the suppression of polarization-fluctuation induced bias drift.