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 resonator's resonant frequencies with manipulating the serrodyne waveform.
KEYWORDS: Resonators, Digital modulation, Fiber optic gyroscopes, Phase modulation, Backscatter, Modulation, Signal detection, Digital signal processing, Amplifiers
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 resonator's
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.
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.
Experiment on bias stability measurement of resonator fiber optic gyro with a newly developed digital feedback scheme
is performed and a sensitivity of 5.6×10-4 rad/s is demonstrated. Under the digital control scheme, the short term and
long term laser central frequency drift are reduced by the output of a fast loop determined by proportional calculation to
the order of several least significant bits (LSBs) and the output of a slow loop decided by an up/down counter,
respectively. Resettling pulse noise due to imperfect 2π voltage of the hybrid digital serrodyne phase modulation
waveform is also reduced by automatic proportional and integration (PI) control. Besides, polarization noise is
suppressed by exciting single eigen state of polarization (ESOP) of the resonator with twice 90 degree polarization-axis
rotated splicing.
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.
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