Research On The Measurement Method Of Air Refractive Index By Phase Modulated Laser Interferometry And Its Application

Laser interferometric displacement measurement technology based on laser wavelength is the mainstream technology in the field of displacement measurement.It is widely used in various high-end equipment fields such as ultra-precision machining,semiconductor integrated circuit chip manufacturing and so on.Because laser interferometric displacement measurement is mostly carried out in the air,and the wavelength of laser in the air is closely related to the air refractive index,the accuracy of air refractive index measurement is the key factor to improve the displacement measurement accuracy of laser interferometer.In this dissertation,the principle and method of phase modulated laser interferometric air refractive index measurement are studied,and the new methods of phase modulated laser interferometric air refractive index measurement based on variable length vacuum cavity is proposed.The high-precision air refractive index measurement is realized,and the related application experiments of large-scale precision displacement measurement are carried out.The main work and innovation of this dissertation are summarized as follows:1.The development status of laser interferometric displacement measurement technology is introduced,including the measurement range and accuracy of various measurement methods.The significance of air refractive index to laser interferometric displacement measurement is expounded.The research status of air refractive index measurement is analyzed,including Edlén formula method,pumping method,closed vacuum cavity method,beat frequency method and dual wavelength interferometry method.The advantages,disadvantages and limitations of each measurement method are summarized.The research objective and the content arrangement of each chapter are introduced.2.The method for measuring the refractive index of air by laser interferometry based on phase modulation and variable length true cavity is proposed.The principle of air refractive index measurement by laser interference based on variable length vacuum cavity is introduced.The air refractive index measurement methods used PZT（piezoelectric driver）triangle modulation differential Michelson interference and EOM（electro-optical modulator）sine modulation homodyne interference are studied.The influences of non-ideal optical elements and installation errors on the proposed methods are analyzed.A method to compensate the tilt of the vacuum cavity lens and the residual gas error is studied,and an EOM sinusoidal modulation laser interferometric air refractive index measurement and error compensation method based on a variable length vacuum cavity is designed.Finally,the scheme of the air refractive index measurement and displacement measurement compensation is designed.3.The design requirement of the variable length vacuum cavity device is analyzed and determined,and the mechanical structure of vacuum cavity with stainless steel welded bellows as the main body is built,and its air tightness is tested.Taking the variable length vacuum cavity as the sensing unit of air refractive index measurement,the experimental device of different laser interference air refractive index measurement methods with PZT triangle modulation,EOM sine modulation,EOM sine modulation and error compensation are built respectively,and the device of laser interference air refractive index measurement and displacement measurement compensation method is built.The influence of environmental factors on the measurement of air refractive index and the suppression method are analyzed.4.The PZT triangle modulation differential phase detection module is designed,and a simulation test with analog signal is carried out.The simulation results show that the phase resolution of the module can reach 0.05°,the measurement accuracy can reach±0.2°.The signal processing system of air refractive index measurement based on PZT triangle modulation laser interferometry is constructed.The phase detection module of EOM sinusoidal modulation is designed.The simulation test with analog signal shows that the phase resolution of the module can reach 0.02°,the measurement accuracy can reach±0.1°.The signal processing systems of different laser interference air refractive index measurement methods with EOM sine modulation,EOM sine modulation and error compensation,and the laser interference displacement measurement compensation systems with air refractive index measurement are constructed.5.The air refractive index measurement experiment of PZT triangular modulation differential Michelson interference is carried out,and the results show that the standard deviation between the measured results and the calculated results of the Edlén formula is 6.64×10^-8 and 3.58×10^-8 respectively when the tensile distance is 49mm and 98mm.The measurement experiment of air refractive index by the EOM sinusoidal modulation homodyne interferometry is carried out,the experimental results show that the standard deviation between the measured results and the calculated results of the Edlén formula is better than 2.93×10^-8 in 20 minutes and 4.74×10^-8 in 8.5 hours.These show that the two measurement methods are in good agreement with the Edlén formula,and can achieve high-precision measurement of air refractive index,but Measurement method of air refractive index by EOM sinusoidal modulation homodyne interferometry has higher measurement accuracy.The measurement experiment of air refractive index by the EOM sinusoidal modulation laser interference and error compensation is carried out.The experimental results show that the standard deviation between the measured results and the Edlén formula results is better than 2.89×10^-8 in short time and 2.34×10^-8 in long time.It shows that the proposed error compensation method can correct the influence of window tilt and residual gas in vacuum cavity on the measurement of air refractive index,and improve the measurement accuracy.Finally,the measurement experiment by the laser interference air refractive index measurement and displacement measurement compensation is carried out.Comparison experiment of long-distance displacement measurement of 50 mm and 100 mm are carried out respectively,and the standard deviation between the measured results and the measurement results of Renishaw laser interferometer is better than 20nm,the validity and practicability of the proposed methods in laser interferometric precision displacement measurement are verified.