Research On Extended Depth Of Field With0/π Phase Mask

Lens imaging system is based on point-to-point imaging, and it has an obviousdisadvantage that when the object and image planes are not in conjugate position, theresultant image is rapidly blurred. An optical imaging system with extended depth of fieldmeans it can include more information of the object，and it can be widely used in mostfields such as: industry manufacture and scientific research. Therefore, it has been anactive research topic to extend the depth-of-field of optical imaging systems. The aim ofthis thesis is to present a0/πbinary phase mask and prove it can extend the depth offield of imaging systems.The main research works are as follows:The method of calculating the OTF (Optical Transfer Function) of a general defocusdiffraction-limited imaging system is analyzed in terms of frequency spectrum. Therelationship between the OTF and the defocus value is given with some curves. With theincreases of the defocus value, there will appear the phase inversion and amplitudezero-crossing phenomenon in the pass-band, which cut off the high-frequency detail partsof the image. Therefore a binary phase mask is introduced in the pass-band, toenhance the cut-off frequency and keep no zero-crossings within the pass band.A micro-imaging system with a large depth of field is designed through adding a0, binary phase mask into the infinite tube length microscope. The PSF (Point SpreadFunction) is expanded with the normalize pupil function. A searching algorithm, combinedby global rough search and local careful search, is used to seek an optimal pupil mask thatprovides the largest spatial cut-off frequency band in certain desired contrast values5%and10%. The effect for MTF curves by the positive and negative manufacture errors ofphase is analyzed.Using the optical design software ZEMAX, an infinite tube length micro imaging system with the binary phase mask is arranged and optimal parameters are acquired afterseveral optimizations. The MTFs (modulation transfer function) curves and the normalizePSFs of the imaging system corresponding to the different defocus values (i.e.differentobject distance) are analyzed. Imaging results for the objects of letters, resolution and chipsare compared in the cases of without and with the mask for the same defocus position.A liquid crystal spatial light modulator is proposed to simulate the0/πbinaryphase mask, and it is feasible method affirmed by the theoretic analyzing. An interferenceoptical system is designed for measuring phase shift, and a method is presented to calculatethe phase shift in terms of the peak-to-peak values of the interference fringes. Twoexperimental results show the relationship between the image gray value and the phasedepth, and the π phase shift can be obtained in the case of angles polarizer φ1=40°,φ2=160°and gray value190.According the theoretic calculation and ZEMAX simulated results, the SLM,simulating the optimal binary phase mask, insert the4f optical system which instead of theinfinite tube length microscope. Imaging experiments are performed with the objects ofcircle, dual slit,2-D grating words and letters.ZEMAX simulation results and imaging experiments show that binary phasemask can effectively extend the depth of field of the imaging system, which has a highresolution in a long frequency band and can obtain clear images without anypost-processing.