Periodic Nonlinear Error Suppression And Correction Technology Of Homodyne Lasre Interference Technique With Unstable Signal Strength

The homodyne laser interferometry technology is an indispensable important technology in the field of precision displacement measurement due to its traceability,high precision,and no contact.Periodic nonlinear error is one of the important systematic errors affecting the measurement accuracy of the homodyne interferometer in sub nanometer precision measurement and micrometer displacement measurement.In recent years,the requirements for displacement measurement accuracy in the advanced manufacturing and scientific research fields have been continuously improved,and the diversity of the motion characteristics of measurement objects has led to more frequent changes of the interference signal strength.Therefore,the influence of nonlinear error on measurement accuracy becomes more and more prominent,which becomes a key problem that restricts the measurement accuracy of homodyne laser interferometry.This thesis aims to reduce the influence of nonlinear error on the measurement accuracy of homodyne laser interferometry,especially with non-stationary intensity interference signals.Based on the in-depth analysis of the influencing factors of nonlinear error,the global model of nonlinearity of homodyne laser interferometer is established and the characteristics and influencing factors of the three-difference of nonlinear error are clarified.Based on these,in order to reduce the nonlinear error of the homodyne laser interferometer,a nonlinear error suppression method based on phase compensation and parameter matching and a nonlinear error correction method based on active optical phase shifts are proposed.The specific research contents are as follows:The current model in the nonlinear error analysis is incomplete and does not take the influencing factors of the interference signal intensity variation into consideration.Aiming at this problem,this thesis considered the global optical path propagation process from the light source to the detector and with the equivalent reflectivity to express the variation of intensity of interference signal,and established a nonlinear error model of the homodyne interferometer based on the Jones matrix analysis.The model analysis shows that the influence of the parameters of each optical component to the three errors is universal and highly coupled.The relationship between the equivalent reflectivity and the three errors can be described as follows: DC offset changes linearly with equivalent reflectivity;the AC amplitude of the interference signal is proportional to the equivalent reflection coefficient,but the AC amplitude ratio remains unchanged;the non-orthogonal error is related to the phase delay of the NPBS and the coefficient of the different phase delay components,and there is no relationship between the equivalent reflectivity and non-orthogonal error.In this way,the generation and characteristics of nonlinear error and the influence of interference signal intensity to three errors are clarified,which provides a theoretical basis for the suppression and correction of nonlinear errors.For the problem that the adjustment process is complicated in the suppression of the non-orthogonal error and the DC offset error varies with the interference signal strength.Firstly,the generation mechanism of non-orthogonal error is deeply analyzed,and the essence of the non-orthogonal error is revealed as the nonideal of the discrepancy of the phase difference between the reference beam and the measuring beam introduced by the optical path structure in two interference signals.Based on this,a non-orthogonal error suppression method based on phase compensation is proposed.The method compensates for the nonideal of the discrepancy of the phase difference by using an adjustable phase compensation component to achieve the suppression of non-orthogonal error.Theoretical analysis shows that this method can suppress the non-orthogonal error to tens of picometers.Secondly,by analyzing mechanism of DC offset error changing with equivalent reflectivity,the influence of DC offset parameters on DC offset error is revealed.Based on this,a DC offset error suppression method based on parameter matching is proposed.The method achieves the mutual matching between the DC offset parameters by adjusting the angle of rotation of the Wollaston prism around the optical axis,thereby achieving a good suppression of the change of DC offset error with the equivalent reflectance,and the DC offset is independent of the interference signal strength.Aiming at the problem that the nonlinear error correction parameters(DC offset,AC amplitude ratio,and non-orthogonal angle)need a certain amount of displacement and are susceptible to the interference signal strength,a correction method based on active optics phase shifting is proposed.Firstly,the method realizes the independence between the DC offset correction parameter and the interference signal strength based on the aforementioned nonlinear error suppression method.Secondly,the liquid crystal phase retarder is used to introduce a controllable optical path change in the measurement optical path to replace the displacement of the object and achieve highly precise obtaining of the nonlinear error correction parameters without changing the equivalent reflectivity.Finally,the measurement results are corrected by using the above correction parameters,and the correction of the nonlinear error of the homodyne interferometer with unstable intensity signal is realized.Based on the above research,a homodyne laser interferometry system is designed and built,and the nonlinear error global model and nonlinear error suppression and correction methods are experimentally verified.In the verification experiment of the nonlinear model of homodyne interferometer nonlinear error with unsteady intensity signals,the error model is proved by the experiment of the variation of the three errors with equivalent reflectance and the experiment of phase delay of the non-polarizing beam splitter on the non-orthogonal error.The verification experiment of the non-orthogonal error suppression method based on phase compensation is carried out.The experimental results show that the proposed method can reduce the nonlinear error caused by non-orthogonal error by about three orders of magnitude.The verification experiment of DC offset error suppression method based on parameter matching is carried out.The experimental results show that the proposed method can reduce the change rate of DC offset with equivalent reflectivity by about two orders of magnitude.The verification experiment of nonlinear error correction method based on active optical phase shift is carried out.The experimental results show that under the condition of stable signal strength,the residual nonlinear error of the system is 33 pm,under the condition of unstable signal strength,when the interference signal strength changes from 100% to 50%,the residual nonlinear error of the system is reduced from several nanometers of traditional method to 90 pm,which effectively reduces the nonlinear error of the homodyne laser interference system under the condition of unstable signal strength.