Significance Optical gyro, as an indispensable part of the modern inertial navigation technology, has been widely used in aerospace, military equipment and even civil field. It is based on Sagnac effect, which induces the optical difference between two signals propagating in opposite directions within the optical path rotation. So far, optical gyro has developed from traditional laser gyro to fiber optic gyro and integrated optical gyro, which are smaller and consumes less power. In terms of working principle, optical gyro could be divided into two categories, active optical gyro and passive optical gyro. The former is essentially a laser, using the frequency beat of the opposite propagating light to derive the external rotation signal, while the latter is based on the phase or frequency difference generated by the outside laser to measure the rotation. However, both of them are necessary when considering the backscattering problems in gyro during the process of improving the working accuracy.
Progress In order to better understand the backscattering mechanism in optical gyro, it could start with self-consistent equation under semi-classical theory, followed by analyzing the coupling mode of opposite propagating light wave. And the practical research of backscattering mechanism in optical gyro mainly focuses on lock-in effect and noise characteristics under macro conditions, taking corresponding methods to suppress them. For laser gyros, the lock-in effect caused by backscattering mechanism is usually solved by frequency bias. With the development of optical fiber, fiber optic gyroscope is invented. According to different working principles, it can be divided into interference fiber optic gyroscope, resonant fiber optic gyroscope and Brillouin fiber optic gyroscope. Their manifestation of backscattering is also slightly different. For the fiber optic gyroscope, it is based on the phase difference generated by the interference of the opposite propagating light wave. Therefore, the noise of backscattering disturbs the actual phase measurement. The main solution is to use the wide spectrum light source or phase modulation. For resonator fiber optic gyroscope, the noise caused by backscattering can be divided into two aspects. One is the light intensity of the scattered light wave itself, and the other is the mutual interference between the scattered light and the signal. The former will lead to the nonlinearity of the gyroscope output, while the latter results in the bias noise of the frequency measurement. The common solution is to carry out frequency modulation or phase modulation on the system, which suppresses the effect of backscattering noise. For Brillouin fiber optic gyro, it is an active resonant gyro in essence, like laser gyro. Therefore, frequency bias and phase modulation are often used to overcome the influence of lock-in effect. At present, integrated optical gyro with more fine structure and smaller volume is also a valuable research direction. However, it is identical to fiber optic gyros except for the waveguide resonator or micro resonator. Therefore, there is no great difference with fiber optic gyros in analysis methods for backscattering problems. It is worth noting that the Brillouin gyros on chip, which benefit from the reduction of stimulated Brillouin scattering threshold, can spontaneously generate Stokes light with a large frequency difference, so as to fundamentally solve the problem caused by the lock-in effect.
Conclusions and Prospects Backscattering in optical gyro is a common phenomenon, which will bring in the inevitable noise or lock-in effect leading to the deterioration of the gyro performance. At present, the common research focuses on using different modulations to suppress the adverse effects of backscattering, with the purpose to improve the performance of gyro. However, there are still some unsolved problems. One is the precision measurement of backscattering in the optical gyro, and the other is to minimize the magnitude of backscattering in the optical gyro through reasonable design and processing. Both of them are beneficial to better understand the backscattering mechanism of optical gyro at the micro level, reduce the adverse effects of backscattering fundamentally, and improve the actual working efficiency of optical gyro. In addition, it is also worth thinking about how to cleverly use the characteristics of backscattering, remove its adverse aspects, and promote its application in precision measurement.
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