Research On The Key Technologies Of Low-coherence Fizeau Dynamic Interferometer

As a non-contact,high sensitivity and high accuracy testing equipment,interferometer can quickly and effectively obtain the surface information of optical elements,and is one of the most accurate and effective tools for testing optical elements.The traditional interferometer uses HeNe laser with high precision and narrow linewidth as light source and mechanical phase-shifting method for interferometry.It needs strict environmental situation to obtain high precision test results.In order to measure optical system or optical elements in complex environment,the cost-controllable dynamic interferometry system has become an alternatives program in engineering application.In this paper,the short-coherence Fizeau dynamic interferometer is taken as the research object.The space simultaneous phase-shifting method,the short-coherence light source,the registration technology of multi-channel interference fringe pattern,the reconstruction algorithm of interferometric surface and the error compensation method of interferometric system are deeply studied.The short-coherence Fizeau dynamic interferometer system is built for testing,and the overall test accuracy is relatively ideal.It provides a feasible program for dynamic interferometry.The space simultaneous phase-shifting method based on circularly polarized light interference is studied.The phase-shifting principle of two circularly polarized light beams with opposite rotation and fixed phase difference is analyzed.A space simultaneous phase-shifter composed of QWP,NPBS and two PBS is designed,which can simultaneous collect four interference fringe patterns with phase difference of 90 degrees in turn.Four 2048 *2048 high resolution CCD cameras are equipped to achieve the dynamic collection of high resolution patterns,and the structure of the system uses a common-light Fizeau structure,so it has the strong ability of anti-vibration and anti-environmental disturbanceThe longitudinal mode characteristics of semiconductor lasers are studied.The coherence length of light source is 0.504 mm measured by FWHM method and 0.514 mm measured by moving high-precision guideway.In the multi-beam received by CCD,the optical path difference between the measured light and the reference light is less than the coherence length,and the interference fringes are generated.The coherence length of the short coherent light source meets the requirements of the system.In order to obtain high contrast fringe pattern,Michelsion method and RMS method are used to calculate the image contrast under different working currents.When the working current is 43 mA,the image contrast is the highest.The registration technology of multi-channel interference fringe pattern is studied.Firstly,the hardware registration of multiple CCD cameras is accomplished by using the long focal length microscope and crosshair images to eliminate the errors in the direction of translation along Z axis,rotation around X axis and rotation around Y axis.Then,the relative positions of multiple corsshair images collected by CCDs are calculated and corrected by using software registration method to realize the image registration of translation along X axis,translation along Y axis and rotation around Z axis.It can achieve pixel-level registration of multiple interference fringe patterns.The algorithm of reconstructing interferometric surface is studied.In order to solve the difference of gray value of interferometric fringe pattern caused by the difference of optical element performance and CCD sensitivity,the image is normalized based on the difference of gray value of incoherent background light outside the interferometric region.The phase is unwrapped by the discrete cosine transform least square method with better singularity suppression effect,and the unwrapped phase is obtained.Continuously,Zernike polynomial is used to fit the phase surface.In order to eliminate the defocusing and tilt in unwrapping phase,the first four coefficients of Zernike polynomial are set to zero to complete the reconstructed mirror profile.Aiming at the problem of large error in measurement results,the error of optical components is analyzed through theoretical analysis and simulation experiments,an error compensation method is proposed,and an error compensation experimental platform is built.After compensation,the measurement accuracy of the system is improved,and the measurement results are close to ZYGO GPI XP/D interferometer,which verifies the effectiveness of the error compensation method in this paper.After 50 measurements and calculations,the repeatability of PV value and RMS value of the system is 2.66 and 0.48 nm respectively,which verifies the superior repeatability of the system.The dynamic interferometry system can be used for testing under the condition of uncontrollable environmental conditions,and can be miniaturized into portable testing instruments in future.