Resonator Fiber Optic Gyro Based On Digital Triangle Wave Phase Modulation

Fiber optic gyro is a high accuracy inertial rotation sensor based on Sagnac effect, and it plays a decisive role in navigation and guidance system. The interference fiber optic gyro has been well developed, but it has some difficulty in minimization. Compared with interference fiber optic gyro, resonator fiber optic gyro(R-FOG) would use less fiber length to achieve the same accuracy, which shows advantages in the high accuracy and the minimization, so it is meaningful to study R-FOG The signal detection method for R-FOG can be concluded as ditigal and analog signal detection methods. Compared with analog circuit, digital circuit has advantages in stability, anti-interference, signal processing speed and bulk. Therefore, in order to achieve the high accuracy and minimization, digitalizing the R-FOG system must be the trend. In Japan, Hotate in the University of Tokyo has put forward an R-FOG based on digital serrodyne phase modulation technology. However, the error caused by reset pulse will emerge when using this modulation technology. In China, the researches on R-FOG last for many years, but the theoretical and experimental research on digital R-FOG is not thoroughly done. This dissertation conducts researches on the R-FOG based on digital triangle wave phase modulaton technology. This system uses LiNbO₃ phase modulator to realize triangle wave phase modulation, and employs the digital circuit based on FPGA to realize the detection of Sagnac resonance frequency deviation. Because there is no reset for triangle wave, this modulation technology can void the error caused by the reset pulse. The general system scheme is proposed and the system parameters are optimized in the dissertation. The R-FOG experimental system is setup, and the response for the rotation rate is tested. What's more, previous analysis of the fiber ring resonator most concerned the frequency domain in the steady-state condition, so this dissertation illustrates the dynamic response of the fiber ring resonator, the core sensing element in the R-FOG, in time domain when the frequency of the input laser is swept. Based on this, further analysis of the dynamic characteristics of the R-FOG is made. The detailed work and achievement of this dissertation can be concluded as follows: 1. The open-loop system of the R-FOG based on digital triangle wave phase modulation technology is setup. The system uses triangle wave phase modulation to realize double frequency modulation. According to the signal characteristics when using double frequency modulation, the signal detection method of the R-FOG is illustrated. In the R-FOG system, feedback of the demodulation signal in one loop is needed to the laser's frequency control port to realize the resonance frequency track for laser's frequency. Therefore, the frequency track technology will directly affect the locking accuracy of the system. With the resonance frequency deviation as the research object, the frequency track algorithm is optimized. And according to the simulation results, it is proved that the algorithm can achieve rapid locking for the resonance frequency. Based on the above theory, the digital detection circuit based on FPGA is put forward.2. Through the demodulation curve, the resonance frequency deviation detection is realized in the system. Thus, by using the phase modulation spectroscopy and optical field overlapping method, the expression for the demodulation curve is deduced. Together with the carrier component and demodulation curve characteristics as the research object, the effects on the system from the parameters of the triangle wave are analyzed thoroughly. In the R-FOG system, the backscattering noise is one of the main optical noises that affect the detection accuracy, and suppressing the carrier component would reduce the backscattering noise. Based on this, the optimized phase modulation index that can reduce the carrier component to 0 is obtained. Through the comparision of triangle and sine modulation, it is found that when the differences between the practical phase modulation index and the optimized value for triangle and sine modulation are identical, the carrier component increment for sine modulation is greater. This means that triangle modulation is advantageous in carrier component suppression. Increaseing the slope of the demodulation curve at the resonance point can raise the sensitivity of the system, thus, the effect on the resonance point's slope from the triangle wave's phase modulation index and modulation frequency is analyzed, and the optimized modulation frequency is obtained to realize the maximum for the slope of the demodulation curve at the resonance point. Apart from the sensitivity, dynamic range is also an important factor for gyro. The effect on the gyro's dynamic range from the modulation frequency of the triangle wave is analyzed. Through simulation, it is proved that there are different optimized modulation frequencies corresponding to different scale factor nonlinearities that would maximize the system's dynamic range. Because the demodulation curve's zero point error will directly affect the gyro's accuacy, the characteristics of the demodulation curve's zero point is studied by the comparision of digital triangle and serrodyne phase modulation technologies. It is proved that triangle modulation can eliminate the zero point error caused by the stair period of digital wave, compared with digital serrodyne phase modulation technology.3. Fiber ring resonator is the core sensing element in the R-FOG, so it is meaningful to conduct research on its characteristics. In general, previous analysis of the fiber ring resonator most concerned the frequency response in the steady-state condition, and the response in the dynamic condition has not been well studied. Considering the resonance curve's ringing phenomenon observed in the frequency sweeping experiment, this dessertion uses the optical field overlapping method to analyze the phase characteristics of the optical fields in the resonator, and it is proved that ringing would happen when the optimal destructive interference condition is destroyed. Through theoretical and experimental research, the condition when ringing occurs is obtained, and the relationship between characteristic parameters of the resonance curve and the sweep rate is analyzed. It is proved that the resonance curve will be changed in the dynamic condition that the frequency is swept, which would definitely affect the performance of the R-FOG. Through analyzing the zero point characteristics of the demodulation curve, it is proved that dynamic condition will cause zero point error for the R-FOG, and the zero point error increases as the product of the system's dynamic range and bandwidth increases. These results will provide theoretical basis for dynamic system parameter optimization.4. The R-FOG experimental system is set up. The resonance curve is tested, and the characteristic parameters are obtained. The practical effects of the triangle wave phase modulation are tested, and it is proved that this modulation technology can realize the double frequency modulation. The demodulation curve and the locking time of the system are tested, and it is proved that the closed-loop system can realize the tracking for the resonance frequency. The gyro signal and the bias stability are tested. The R-FOG response for 1°/s is obtained, and the peak-to-peak value of the bias stability is0.277s.