| With the rapid development of technical fields such as integrated circuit manufacturing and ultra-precision machining,higher requirements are placed on the accuracy and range of displacement measurement.As a typical high-precision displacement measuring instrument,heterodyne laser interferometer has been widely used due to its advantages of high measurement accuracy,wide measurement range and strong anti-interference ability.In this thesis,the compensation of air refractive index is taken as the starting point of the application,and the signal processing technology of heterodyne laser interferometry is studied for the high-precision measurement requirements of the interference signal phase in the two-color heterodyne laser interferometer.Firstly,the thesis introduces the research status and principles of two-color method for air refractive index compensation and heterodyne laser interferometer,and proposes the overall design scheme of two-color heterodyne laser interferometer.Then,the principle of the quadrature phase detection algorithm based on lock-in amplification and the key aspects of the algorithm implementation on FPGA are introduced in detail.In order to improve the phase measurement accuracy,the measurement error caused by sampling signal crosstalk when multi-channel signals are simultaneously collected is studied.By analyzing the mathematical model of signal crosstalk error in detail,a pre-compensation method based on spectrum analysis is proposed and verified by experiments.The experimental results of phase measurement show that the phase static measurement fluctuation is about 0.015°within two hours,the corresponding displacement fluctuation is about 20 pm.The phase measurement error after signal crosstalk error compensation decreases from the maximum 0.34°to 0.01°,which can meet the requirements of high-accuracy phase measurements in two-color heterodyne laser interferometer.On the basis of the above researches,heterodyne interferometry optical paths were built based on two-color Nd:YAG lasers at 532 nm and 1064 nm to further verify the practical application performance of the signal processing system,and the two-color method was used to calculate the refractive index of air.The specific experimental contents include:（1）The linewidth and frequency stability of the laser are determined by the beat frequency experiment with the iodine frequency stabilization device,in order to ensure the reasonable value of the light wavelength in the heterodyne interferometry.In this experiment,the measured value of the laser linewidth was less than 2 k Hz,and the maximum drift of the laser frequency was less than 25 MHz within two hours.When the averaging time is 819.2 s,the relative Allan variance is 1.59×10-8.（2）Based on the realized heterodyne interference signal processing system,a 1064nm monochromatic heterodyne interferometer was built for the step-by-step experiment of displacement comparison.The step interval of the experiment is 100 nm,the displacement range is 2μm.In this experiment,the maximum measurement error is less than 10 nm,and the measurement standard deviation is 5.19 nm,which proves that the signal processing system can meet the application requirements of heterodyne interferometry.（3）In order to verify the measurement capability of the signal processing system in the complex two-color heterodyne laser interferometer,the step-by-step experiment of the two-color laser displacement comparison was carried out,and calculated of air refractive index by two-color method.In the step experiment with the displacement range of 10μm and the step interval of 1μm,the maximum error of the two-color laser interferometry is less than 20 nm.At the distance of 200 mm,the 1-minute mean value of the air refractive index calculation results by the two-color method deviates from the empirical formula method by 10-7 orders of magnitude. |
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