Research On Nano Displacement Measurement Method Based On Differential Laser Interferometry With Phase Modulation

In recent years,with the rapid development of ultra precision machining,chip manufacturing and other modern technology fields,the requirements of nano measurement technology are also increasing.In nano measurement technology,laser interferometry has the advantages of high measurement accuracy and large measurement range,while sinusoidal phase modulation interferometry is the current research hotspot of laser interferometry.It has the advantages of simple optical structure,strong antiinterference ability and large dynamic range,and is widely used in various displacement,vibration,topography and other measurement fields.In order to further improve the accuracy of displacement measurement,a differential laser interferometric displacement measurement method based on phase modulation is proposed.Firstly,the optical structure of the whole system is designed,and the influence of non common mode disturbance between the measurement optical path and the reference optical path is eliminated by using the differential optical path structure.Secondly,an PGC demodulation method with calculation of phase delay and modulation depth is proposed to eliminate the nonlinear error caused by phase delay and modulation depth.Finally,a method of angle detection and Abbe error compensation based on three quadrant detector is proposed to eliminate the Abbe error in the process of displacement measurement.The improved PGC demodulation method uses the harmonic signals and differential signals to calculate the carrier phase delay and phase modulation depth,and uses the calculation results to compensate the influence of carrier phase delay and phase modulation depth in real time.The nonlinear error introduced in the signal processing process will be eliminated.Through theoretical analysis and simulation,the effectiveness of the improved PGC demodulation method is proved.The differential signal measured by three quadrant detector is used for real-time detection of angle and real-time compensation of Abbe error.On the basis of principle analysis,the limitation of measurement range of angle is discussed.In order to verify the effect of the improved PGC demodulation method in the actual experiment,the sinusoidal phase modulation interferometer was constructed,and the sinusoidal displacement measurement experiment and nanometer displacement measurement experiment were carried out.The experimental results shown that the improved PGC demodulation method eliminate the influence of phase delay and modulation depth,and the SINAD of demodulation results is all greater than 57 d B in the range of carrier phase delay from 0 ° to 30 ° and phase modulation depth from 2.23 rad to 3.03 rad;the nonlinear error of the sinusoidal phase modulation interference system using the improved PGC demodulation method is reduced to less than 0.15 nm.In order to verify the compensation effect of the non common mode disturbance between the measurement optical path and the reference optical path of the differential structure and the Abbe error compensation effect in long-distance measurement,a differential laser interferometric displacement measurement system based on phase modulation was constructed,and the stability experiment,angle measurement comparison experiment and displacement measurement comparison experiment were carried out.The experimental results shown that the differential optical structure designed in the system can effectively reduce the non common mode disturbance between the measurement optical path and the reference optical path,and the differential phase has no obvious drift within 30 minutes;the proposed method can achieve high-precision measurement of pitch angle and yaw angle,and the standard deviation of the difference between the measurement results of the proposed method and Renishaw interferometer is 0.572? and 0.425?,respectively.After compensating Abbe error,the maximum deviation between the measurement results of the proposed method and the reference position value is reduced to 1.686 ?m,and the standard deviation is reduced to 0.583 ?m.