Absolute Distance Measurement Using Sinusoidal Phase Modulating Interferometer Calibrated By An Optical Frequency Comb

As a high-precision length measurement method,laser interferometry ranging technology has been widely used in ultra-precision machining,high-end equipment assembly and national defense construction.Incremental laser interferometers require a continuous movement of the target,so it is difficult to apply in the engineering fields where the guide rail cannot be laid.Therefore,the absolute distance measurement technology without the guide rail emerges as the times require.The development of high-end equipment manufacturing industry puts forward higher requirements on the measurement range and accuracy,and it is urgent to meet the absolute distance measurement technology with both large-scale and high-precision at the same time.In this dissertation,an absolute distance measurement method using sinusoidal phase modulating interferometer calibrated by an optical frequency comb is proposed.It combines the frequency-sweeping and multi-wavelength interferometry to perform large-scale and high-precision absolute distance measurement,and traces the distance measurements directly to atomic clocks.The main work and innovations of the dissertation are summarized as follows:1.The research status of absolute distance measurement technology based on the time-of-flight method and laser interferometry is analyzed in detail.The advantages and disadvantages of each measurement method and its limitations in application are summed up.In order to take into account both the measurement range and the measurement accuracy,the absolute distance measurement method combined with frequency-scanning interferometry and multi-wavelength interferometry is adopted,and the problems need to be solved urgently in this method are proposed.2.In order to monitor and compensate the distance drift in the absolute distance measurement method combining frequency-sweeping and multi-wavelength interferometry,a sinusoidal phase modulation interferometry with a monitor interferometer is proposed to realize absolute distance measurement,and the corresponding optical path structure is designed.The laser source of the measurement interferometer is an ECDL in of⁷80 nm,and the laser source of the monitor interferometer is a He-Ne laser.The interference signals of the two interferometers are separated according to the spectral band with a dichroic mirror and detected independently.The principles of absolute distance coarse measurement with frequency-sweeping interferometry and fine measurement with multi-wavelength interferometry are theoretically analyzed,respectively,and the validity of the measurement theory are verified by numerical simulation.3.In order to improve the phase demodulation precision of the interference signal in absolute distance measurement,a real-time compensation algorithm for carrier phase delay is proposed.The phase delay is calculated by finding the maximum value of the quadrature signal,and compensated to the carrier signal to make the carrier synchronize with the carrier component of the interference signal.Then the problem caused by the carrier phase delay which is induced by optical path propagation,photodetection,and circuit transmission is solved.Methods for nonlinear error correction based on real-time normalization of the quadrature signals and for real-time evaluation of the nonlinear error based on fixed-phase-difference method are proposed to solve the problems of the nonlinear error correction and evaluation in PGC-Arctan demodulation.The validity of the proposed carrier phase delay compensation algorithm and the nonlinear error correction and evaluation methods are verified by simulation experiments.4.A sinusoidal phase modulation interferometric system based on optical frequency comb for absolute distance measurement is designed,including an interferometry module,a light source module,a signal acquisition and processing module,and an air refractive index measurement and compensation module.The structure and function of each module are introduced,and test experiments are conducted to verify the performance of each module.For the interferometric measurement module,combined with the real-time correction method for nonlinear errors in PGC-Arctan demodulation,an EOM-based sinusoidal phase modulation differential interferometer is proposed to improve the interferometric optical path structure.For the light source module,a slow feedback module is designed to achieve long-term locking of the ECDL to the femtosecond optical frequency comb and the 100-minute long-term locking experiment indicates a frequency stability of 5.4×10^-12 for the locked ECDL.For the signal acquisition and processing module,a signal acquisition and processing system based on Lab VIEW is designed.For the air refractive index measurement and compensation module,the measurement uncertainty of the air refractive index is 2.04×10^-8 according to the Ciddor empirical formula and the measurement accuracy of the environmental parameter sensors.Combining the structure and function of each module,the overall design of the absolute distance measurement system is done to realize interconnection of each module,and a corresponding software system is designed.5.Experiments were carried out to verify the validity of the proposed real-time compensation algorithm for carrier phase delay,and real-time correction and evaluation methods for nonlinear errors in PGC-Arctan demodulation.The experimental results show that the compensation accuracy of the designed phase delay compensator is better than 0.1?,and the maximum non-linear error after correction is 0.25?.6.A sinusoidal phase modulation interferometric absolute distance measurement system based on femtosecond frequency comb was constructed.By comparison with the incremental laser interferometer,experiments for the measurement stability,linear displacement measurement capability,micrometer step measurement and distance measurement with large range were respectively carried out to verify the performance of the absolute distance measurement system.The experimental results show that:the relative stability of the absolute distance measurement can reach 6.4×10^-88 at 4.5 m;the linearity of the fitted line are 0.999987 and 0.999999,respectively,for the linear displacement of 300 mm at the near-end?1 m?and the far-end?8 m?;micron-scale resolution can be achieved according to the 3?m step measurements at the near-end and far-end;the residual standard deviation is 0.42?m compared with the incremental laser interferometer for absolute distance measurement in 8 m with an increment of 0.5 m,which is verified that the absolute distance measurement system can achieve sub-micron-level measurement accuracy within 8 m.Finally,the uncertainty of the absolute distance measurement system is analyzed,and the analysis results are in good agreement with the experimental results.