Signal Detection Technique For Double Closed-loop Resonator Micro Optic Gyro

An optic gyro is a high-precision inertial angular rotation sensor based on the Sagnac effect. It plays an important role in the sensor technology. An resonator micro optic gyro (RMOG) uses a waveguide ring resonator as the core sensitive element. Compared with the well-developed interferometic fiber optic gyroscope (IFOG), the RMOG has the unique advantages in system miniaturization and integration. In a single closed-loop RMOG, the loop delay limits the maximum loop gain. A high loop gain is preferred to reduce the laser frequent noise. To further reduce the laser frequency noise, a narrow bandwidth of the output for the open-loop is used. It limits the final response bandwidth of the RMOG and deteriorates the tracking performance. The double closed-loop RMOG can make better use of its reciprocity and get the resonance frequency difference directly. It has the advantages of high linearity of the scale factor and wide dynamic range over the single closed-loop RMOG.In this article, a double closed-loop RMOG with a silica waveguide ring resonator is proposed and demonstrated. A thoroughly reciprocal in the optical elements is setup between the clockwise (CW) and counterclockwise (CCW) lightwaves. The double closed-loop operation is realized through adding an acoustic optic frequency shifter (AOFS) into the CW and CCW lightwaves, respectively. Both of the CW and CCW lightwaves are locked to their respective resonance frequency with the double closed-loop operation. The resonance frequencies difference between the CW and CCW lightwaves shows the gyro rotation rate directly. Compared to the single closed-loop RMOG, the double closed-loop RMOG can improve the performance of the laser frequency noise suppression without affecting the system bandwidth.The double closed-loop RMOG is setup based on the sinusoidal phase modulation technique and two AOFSs. A proportional integral (PI) controller is adopted in the laser frequency lock-in loop. A simulation model is setup to compare the bandwidth characteristics and the laser frequency noise suppression characteristics between the double closed-loop RMOG and the single closed-loop one. The loop parameters are optimized according to the nonreciprocal parameters difference between the CW and CCW lightwaves in the practical RMOG. The system bandwidth has been maximized only not to deteriorate the laser frequency noise suppression. A proper group of modulation parameters are obtained according to noise reduction and signal detection requirements. Digital signal generators and synchronous demodulations are all realized on FPGA using the coordinate rotation digital computer (CORDIC) algorithm. Frequency lock-in loops based on the PI controller are all realized on LabVIEW program with a National Instruction (NI) data acquisition (DAQ) board.Based on the aforementioned works, a bias stability of about 2.52°/s in one hour is successfully demonstrated in the double closed-loop RMOG. The theoretical bandwidth is about 5 Hz.