Study On Off-axis Digital Holographic Microscopy Technology Based On Divided Apertures

With the rapid development of microelectronics,micro optics and high-end manufacturing industry,the components with micro-nano-structure have been more and more refined in volume,diversified in function and widely used,resulting in a requirements for the measurement accuracy and speed of three-dimensional surface topography of microstructure.Optical microscopy system with the characteristics of non-contact,high precision and high efficiency,has been widely used in the measurement of three-dimensional surface topography of microstructure.The reflection characteristics of the rough surface of the measured object will cause light scattering phenomenon in the high-precision reflective measurement of optical microscope,which affects the measurement accuracy seriously.Therefore,how to measure the 3D topography of reflective microstructure in full field of view,high resolution and real-time under the condition of surface scattering light has become an important topic in the field of modern precise measurement.The subject "Study of off-axis digital holographic microscopy with divided apertures technology" is to combine the off-axis digital holographic microscopy(DHM)and divided apertures(DA).A three-dimensional microscopic measurement system with strong anti scattering ability,high measurement accuracy and good real-time performance is studied by introducing carrier technology,polarization phase shift technology and bipolar adaptive filtering technology,which provides a new method for parameter extraction and performance evaluation of reflective samples.The main contents of this paper are as follows:Firstly,a separate path off-axis DHM with DA in reflective mode is proposed to reject the scattered light of reflective sample,which combines the separate path off-axis DHM with the DA technology.The imaging principle of DA-DHM is studied based on the analysis of the antiscattering characteristics of the DA structure and its influence on the system resolution.The incident light and the reflected light olny overlop in the focused region with the DA,which reduces the influence of scattered light on the imaging optical path and improve the anti scattering ability and measurement accuracy of holographic microscopic imaging system.A slightly off-axis DHM with DA is proposed by introducing the polarization phase-shifting technology and double DA.With polarizing element and polarizing prism,two phase-shift offaxis interferograms can be otained in one shot.The zero order term can be eliminated by using subtraction betweent the two interferograms,which can improve the measurement resolution and optimize camera bandwidth and field of view utilization.Secondly,a common path off-axis DHM with DA is proposed to improve the antiinterference ability,which combines the DA with Fizeau interferometer,based on studying the anti-interference principle of the common path DHM system.A grating is used to diffract the object light and reference in to different orders to generate the off-axis interferograms,which can improve the anti-interference ability of the DHM system while suppressing the scattered light.Finally,a digital holographic phase retrieval technology based on bipolar adaptive filter is proposed to meet the requirement of off-axis DA-DHM for speed and resolution,which takes the balance of the loss of effective information and noise in the actual filtering process into account.This technology deduces the relationship between the original phase,the interferogram and the recovered phase based on the adaptive filtering principle of the least mean square error.And then,we design the bipolar filter structure to extract more sample information under the condition of minimizing noise and other distractors which can be applied to the off-axis digital holographic system with different carrier by modifying the parameters of the bipolar filter.Experimental results show that the resolution of imaging results is higher,the imaging speed is faster,and the performance of the system is improved effectively when the phase retrieval technology is applied to the reflective off-axis and slightly off-axis DA-DHM.