Analysis On Output Signal Properties Of Prisms Laser Gyro With Mechanical Dither Bias

As a new type of optical inertial device which is used in attitude measurement,prisms laser gyro has drawn many attentions in recent years because of its excellentperformance under the adverse circumstance. Compared with the traditional mirror lasergyro such as reflector gyro and fiber optical gyro, prisms laser gyro has manyadvantages such as a quick starting, high accuracy, stable performance, high abilities ofpreventing interference and no need of coatings. With the increasing requirement ofhigh performance of inertial devices in the area of domestic navigation, the gyro withhigher reliability and stable output quality has become the study focus.To keep the gyroworking status out of lock-in, mechanical dither is usually used. However, it is foundthat the amplitude of output voltage signal of prisms laser gyro is always modulated bydither bias, which strongly influences the measurement accuracy of gyro. Currently, theperformance test is mainly made at the stage of the finished products. Any unqualifiedgyro needs to be disassembled to repair. That’s the mainly cause responsible for thelower rate of finished gyro products and higher production cost.This paper presents a theoretical analysis and experimental research on thegeometrical structure, prism’s material, a photodetector assembly situation, polarizationproperties of output light and the stress distribution in prisms of gyro. The mechanismof the output signal modulation has been presented by help of the simulation results oflaser gyro. Based on the polarized measurement results, a new method based on powervalue of output polarized light to estimate the output signal amplitude modulation oflaser gyros is proposed. And a new structure to improve the gyro performances has beenpresented and the numerical simulation is also made to examine the advantages of thenew structure laser gyro.1. Based on the fact that the amplitude of output voltage signal has been modulatedby dither bias, the effect of mechanical dither to the laser transmission trajectory andphotodetector is calculated and analyzed systematically. By use of the numericalsimulation and finite element analysis methods, it is found that the main factors to causemodulation depth are photodetector deviation and stress induced birefringence in prisms.The comparison between the numerical simulation and experiment shows that by themodel and the mechanism proposed can well describe the coupled light intensity.2. To test the polarization characteristics of the transmission laser in the prismsgyro, a specified test platform is established. Using the Brewster angle as the laser beam incident angle is a significant characteristic in prisms laser gyro. The analytical formulaof output light intensity in gyro and the relationship between polarized parameters andmodulation depth are given and discussed. For the first time to our knowledge, a newmethod to estimate the amplitude modulation of the gyro output signal is proposed byuse of the polarized power value of the output optical intensity. Experimental resultsshow that when the value of polarized power of output light is below25.5%of that inideal static situation which equals to-23.334dBm, the standard error is over0.0337dBm,and the displacement extent of prism is higher than the53%of radius of the beam waistin gyro cavity, the amplitude modulation extent of gyro output signal can be as high as16%, which can strongly influence measurement precision of laser gyro. According tothe results above, the production efficiency could be improved, and development cyclecould be shortened through testing the polarized power of output light before the prismsare assembled to gyro.3. Based on the analysis of the performance of an asymmetrical gyro, a new type ofgyro with symmetrical structure is proposed. This symmetry structure provides a newway for optimization design and reduction amplitude modulation for the prisms lasergyro. Compared with the results of finite element analysis between these two kinds ofgyros, it is shown that the stress value distribution is symmetry gyro and the value ismuch smaller than that in asymmetry one in prism. From the results of simulation, usingthe symmetry gyro structure could reduce the stress of the prism and off-axis range ofthe laser beam trajectory in the cavity, and improve the stability of output laser intensityand output signal quality of the laser gyro effectively.4. Based on the analysis results and the idea of the symmetry gyro structure, theinfluence of the refractive index of the prism and the vibration of photodetector on theoutput laser power of gyro is analysed.An optimization method has been presented. Theinitial position of the detector should be in the center of the output light spot area, andthe prism refractive index should be1.8857. In this case, analysis indicates that the gyrowith symmetric prisms of higher refractive index would improve the performance ofcoupling light and reduce the modulation depth by52.63%at least.The research results in this paper would provide an important reference forimproving the quality of the output signal and the reliability of prisms laser gyro.