Study On Multiple Wavelengths Interferometry For Surface Topography With Optical Phase Shift

At present,for the surface measurement of precision instrument,the better technology is: multi-wavelength phase-shift interference.Compared with the single wavelength measurement range of less than half a wavelength,the multi-wavelength interference technology has better measurement effect and a larger measurement range.Using traditional measuring devices will cause some errors,which are caused by the mechanical motion of piezoelectric ceramics.In view of this,a method combining optical phase shift and multi-wavelength interference technology is proposed by using liquid crystal spatial light modulator.The main research contents of this paper are as follows:Firstly,the working principle of the optical phase shift surface topography multiwavelength interferometry system is described.Based on the analysis of the phase modulation principle of the pure phase liquid crystal spatial light modulator and the feasibility of using it in the multiwavelength interferometry system.According to the selected device type,a suitable test optical path was built,and the phase modulation characteristics of the selected wavelength were measured after the performance of the device was tested.This method avoids the mechanical error of PZT and other driving devices and the repeated positioning error caused by the hysteresis of PZT when switching different wavelengths.Then,aiming at the phenomenon that the light intensity of white light is inconsistent after passing through different filters,a method of controlling the light intensity consistency of multi-wavelength interference is proposed.This study presents a method for controlling the intensity consistency of multi-wavelength micro-interferometry.A white LED with tunable intensity is used as the multi-wavalength source.A detector is placed in the micro-interferometry optical path to detect intensity of light emitted from different filters,and signal is fedback to an STM32 microcontroller in real time.Light intensities from different optical filters are quickly adjusted to be consistent gray distribution are obtained.The experimental results show that the presented method can quickly adjust the illumination intensity at different wavelengths,accurately stabilize at the set value,and reduce interferogram contrast error from 18% to 2%.Finally,in terms of phase extraction and recognition accuracy,an improved iterative phase shift extraction algorithm based on least square is discussed,and the effectiveness of this method is verified by simulation and experiments.This algorithm is used to optimize and improve the four-step phase shift method,and the minimum interference pattern is selected to quickly identify the phase when the measurement variance is minimum.In terms of the anti-interference performance of the system,the GUI camera acquisition interface written by MATLAB was used to repeatedly test its anti-interference performance,and the impact of environmental vibration on phase shiftwas observed after processing by the program.For a single wavelength,the deviation of phase shift extraction accuracy was between-0.079 and 0.078 rad.Finally,the measured object was measured.