Research On Signal Processing Method Of Two-color Sinusoidal Phase Modulation Interferometer

With the development of science,technology and the progress of equipment manufacturing industry,higher requirements for the measurement accuracy of displacement measurement technology has been put forward.The laser interferometry technology uses the laser wavelength as the"measuring scale"to measure the displacement,and has the advantages of non-contact,high precision,high sensitivity and traceable to the definition of"meter".Sinusoidal phase modulation interferometry is widely used in precision displacement measurement because of its advantages such as simple optical path structure,large dynamic range and strong anti-interference ability.As the laser"measuring scale"is related to the refractive index of medium,with the increase of the displacement measurement range,the correction error of refractive index of air gradually becomes the main factor affecting the precision of displacement measurement.In order to achieve a wide range and high precision displacement measurement,a two-color sinusoidal phase modulation interferometer based on electro-optical phase modulator（EOM）is proposed in this thesis.On the other hand,two near-wavelength lights are used to reduce the adverse effects of laser intensity and interference signal fluctuation caused by optical drift in the measured environment.Use two-color sinusoidal phase modulation interferometer for high precision displacement measurement is the key problem of phase demodulation precision of the interference signal,PGC-DCDM demodulation algorithm is proposed and the PGC-Arctan algorithm based on real-time correction of ellipse fitting is introduced in this thesis,through the orthogonal signal in PGC demodulation differential-cross-dividing-and-multiplying operation,and eliminate the influence of phase modulation depth（z）and carrier phase delay（θ）,so as to avoid the linear and nonlinear distortion introduced by them.Through theory of validation,simulation analysis and experimental verification,the feasibility and effectiveness of the proposed algorithm are proved,in which the experimental verification is implemented by signal processing board based on FPGA and ARM,and the result display and data storage are carried out by upper computer based on Labview.The experimental results show that when the modulation depth changes from 1.5 rad to 3.5 rad or the phase delay changes from-180°to 180°,the total harmonic distortion（THD）corresponding to the demodulation results is less than 0.6%and the signal-noise distortion ratio（SINAD）is higher than 44 d B,which proves that the algorithm is insensitive to z andθ,and avoids the demodulation distortion introduced by them.In order to verify the two-color sinusoidal phase modulation interferometry（two-color method）in a wide range of air refractive correction effect of displacement measurement,this thesis constructs the EOM two-color sinusoidal phase modulation interferometer,and made contrast measurements with monochromatic sinusoidal phase modulation interferometry（monochromatic method）,which uses empirical formulae for air refractive index correction.In the experiment,the measuring mirror was placed at a distance of about 4 m,and the stability measurement experiment of the two-color method and the monochromatic method was carried out simultaneously under stable and unstable experimental conditions.The experimental results show that the relative stability of the displacement measurement results of the monochromatic method and the two-color method is 2.4×10^-9 and 3.3×10^-8 respectively when the environment is stable,which verifies that the dispersion coefficient of the two-color method has an amplification effect on the measurement error when the environmental parameters such as temperature and pressure are relatively stable.In the unstable environment with rapid environmental temperature change and large air flow,the displacement measured by the monochromatic method within 160 s drifted 6μm,with a relative stability of 4.04×10^-7,while the displacement measured by the two-color method did not drift significantly,with a relative stability of 1.05×10^-7.The experiment shows that compared with the monochromatic method,the two-color method is not sensitive to the change of environmental parameters,and is more suitable for the application where the measurement range is large and the environmental parameters are difficult to be controlled stably.