Research On Extended Depth Of Field With0/πPhase Mask

Lens imaging system is based on point-to-point imaging, and it has an obvious disadvantage that when the object and image planes are not in conjugate position, the resultant image is rapidly blurred. An optical imaging system with extended depth of field means it can include more information of the object, and it can be widely used in most fields such as: industry manufacture and scientific research. Therefore, it has been an active research topic to extend the depth-of-field of optical imaging systems. The aim of this thesis is to present a "0/π" binary phase mask and prove it can extend the depth of field of imaging systems.The main research works are as follows:The method of calculating the OTF (Optical Transfer Function) of a general defocus diffraction-limited imaging system is analyzed in terms of frequency spectrum. The relationship between the OTF and the defocus value is given with some curves. With the increases of the defocus value, there will appear the phase inversion and amplitude zero-crossing phenomenon in the pass-band, which cut off the high-frequency detail parts of the image. Therefore a "0/π" binary phase mask is introduced in the pass-band, to enhance 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 a "0,π" binary phase mask into the infinite tube length microscope. The PSF (Point Spread Function) is expanded with the normalize pupil function. A searching algorithm, combined by global rough search and local careful search, is used to seek an optimal pupil mask that provides 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 of "0/π" phase 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 after several optimizations. The MTFs (modulation transfer function) curves and the normalize PSFs of the imaging system corresponding to the different defocus values (i.e.different object distance) are analyzed. Imaging results for the objects of letters, resolution and chips are 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 the "0/π" binary phase mask, and it is feasible method affirmed by the theoretic analyzing. An interference optical system is designed for measuring phase shift, and a method is presented to calculate the phase shift in terms of the peak-to-peak values of the interference fringes. Two experimental results show the relationship between the image gray value and the phase depth, and the π phase shift can be obtained in the case of angles polarizer ψ1=40°, ψ2=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 the infinite tube length microscope. Imaging experiments are performed with the objects of circle, dual slit,2-D grating words and letters.ZEMAX simulation results and imaging experiments show that "0/π" binary phase mask can effectively extend the depth of field of the imaging system, which has a high resolution in a long frequency band and can obtain clear images without any post-processing.