| Dynamic interferometry is widely used in the fields of astronomical optics,high power laser,laser fusion,and so on.At present,Most of the dynamic interferometers are based on the Twyman-Green type,where the polarization phase-shifting technique can be achieved easily.But there has a limitation in testing element with large aperture,and the system error is also increased,due to the non-common optical path structure of Twyman-Green interferometer.In this paper,a dynamic Fizeau interferometer(DFI)with a short coherent light source is studied,it is able to acquire four interferograms simultaneously,and realize high precision,real time measurement.According to the principle of polarization interferometry,the Jones vector and Jones matrix are employed to deduce the numerical model of the DFI.A short coherent orthogonal polarized light source module is properly designed,which contains a diode-pumped solid-state laser(DPSSL)and a special designed optical path delay device.The coherence of the DPSSL is modulated by the injection current.By adjusting the injection current and the optical path delay device,a pair of short coherent orthogonal polarized beams with a certain optical path difference(OPD)are acquired.Only when the OPD equals to twice the Fizeau interference cavity,the two short coherent orthogonal polarized beams will produce interference after reflected by the test flat and reference flat,respectively.In this way,the spurious interference is eliminated.Recording the interference intensity with the polarization phase-shifting module which contains a quarter wave plate,a pixelated micro-polarizer and a CCD,four interferograms with π/2 phase shift in a phase-shifting sequence will be extracted,and the phase distribution can be get through using four step phase-shifting algorithm.The system error is effectively suppressed by the Fizeau optical path system.Furthermore,it is no need to consider the spatial mismatch between interferograms owe to the polarization camera.As the coherence of the DPSSL changes with the injection current,it is necessary to study the variation regularity and select a working current to match the requirement for coherence length.Measuring the spectrum of the DPSSL by using a spectrometer,then the data of laser stability and drift of central wavelength will be got,and also the influence of temperature to the DPSSL is studied.All these results show that the DPSSL has a good stability and will be work well as the short coherence light source.By properly designing the optical path and optimizing the parameters of the cell modules in the DFI,an experimental apparatus is set up,which contains short coherent orthogonal polarized light source module,Fizeau interference module and polarization phase-shifting module.And then,the joint debugging of the whole opto-mechatronics systems is accomplished.To verify the reliability of the apparatus,an optical flat is measured by the apparatus,and also by Zygo GPI XP commercial interferometer,the difference between root-mean-square(RMS)value is 0.024λ,which meets the accuracy requirements.Furthermore,a sphere Fizeau interferometric measurement system is built,and realize optical path matching under different cavity length and surface shape recovery.Different error sources in the measurement system are analyzed,including polarization orientation error in pixelated micro-polarizer,retardation error of quarter wave plate,fast-axis orientation error of quarter wave plate.The analysis indicates that fast-axis orientation error of quarter wave plate will lead to intensity inconsistency between interferograms,and it shows up as a ripple error in the phase distribution,whose spatial frequency is the same as the fringe.To suppress this error,an intensity normalized algorithm is applied to normalize the background and modulation components in each interferogram,and then,demodulate the phase distribution.Both the simulation and experiment show the effectiveness of the intensity normalized algorithm. |
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