| Laser interferometry is increasingly used in the field of nanometer displacement measurement and because of its advantages of high precision,high resolution,and large measurement range.And it is a key technology to promote the development of high-end equipment manufacturing industries such as microelectronic manufacturing and ultra-precision machining.Therefore,the study on high-precision nano-displacement measurement technology has important research significance and application value.In this thesis,the nanometer displacement measurement with laser synthetic wavelength interferometer?LSWI?based on the improvement of laser wavelength stability is proposed to further improve the measurement accuracy of LSWI.The research status at home and abroad of the nano-displacement measurement technology and the laser frequency stabilization method is illustrated.The measurement principle of LSWI is described,the influence of the single wavelength stability and the synthetic wavelength stability on the measurement uncertainty of LSWI is analyzed in detail and then it is concluded reasonably that increasing the stability of laser source by using the frequency stabilization technique is quite necessary in LSWI.Therefore,a synthetic wavelength stabilization scheme using offset-frequency locking for LSWI is proposed.By locking an external cavity diode laser to the+3rd sideband of an I2 frequency-stabilized He–Ne laser modulated with an electro-optic modulator,a stabilized synthetic wavelength with frequency difference of 815 MHz is obtained for LSWI.And the optical configuration and principle of offset-frequency locking are described in detail.A novel method of fraction displacement detection with the direct digital synthesize is proposed,and it addresses the influence of the DC-offset drift and AC-amplitude variation of the interference signal on the phase difference detection.The coarse integer fringe number is obtained by using arc tangent operation based on CORDIC algorithm.A new method of combining the integer and fraction fringes by using the fraction displacement to correct the coarse displacement is proposed and this method eliminates the integer fringe counting error when the measured displacement is around integral multiple of the interference fringe.The interference signal processing system using Zynq-7010 all programmable system-on-chip and the host computer system software using LabVIEW are developed.And the entire control system is able to implement the synthetic wavelength measurement and displacement measurement.In order to verify the feasibility and effectiveness of the proposed nanometer displacement measurement with LSWI based on offset-frequency locking,an experimental setup was constructed and the experiments of frequency stability,synthetic wavelength stability and displacement measurement were carried out.The frequency stability experimental results show that the relative stability of the single wavelength is improved from 2×10-8 to 1.7×10-11 at 1 s averaging time,the relative stability of the synthetic wavelength is improved from 1.2×10-2 to1.4×10-5 at 1 s averaging time,and the measurement uncertainty in the millimeter range is reduced from nanometer scale to picometer scale.These results verified the effectiveness of the offset-frequency locking system.The synthetic wavelength measurement results show that the dead path error caused by the non-common optical path of the optical configuration and the nonlinear error resulting from the polarizing leakage induced by the imperfection or misalignment of the polarization beam splitter are the main factors that lead to the synthetic wavelength measurement error.And the maximum value of the displacement measurement uncertainty in the millimeter range,introduced by the synthetic wavelength measurement error and the laser source constructed by this offset-frequency locking system,is about 0.4 nm.Stepping measurement results with nanometer displacement and repeat measurement results with micrometer displacement show that the average error and standard deviation are both within±1nm,which indicate the measurement uncertainty with subnanometer scale has been realized in LSWI after the stabilization of the synthetic wavelength using offset-frequency locking.These results comprehensively verify that the proposed nanometer displacement measurement with LSWI using offset-frequency locking is able to evidently improve the displacement measurement accuracy of LSWI. |
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