Design And Process Study Of Direct Laser Writing Diffractive Optical Element

Two-photon polymerization is a technique widely applied for manufacturing threedimensional micro/nano structures with subdiffraction-limited precision.By tightly focusing femtosecond laser pulses into a transparent photoresist,two-photon polymerization is triggered only in a tiny area around the focal point,which can achieve feature sizes in the sub-micro order.In addition,with the help of mechanical equipments such as vibration mirrors,piezo nanopositioning stages,and linear stages,it is possible to write high-resolution threedimensional microstructures freely.Therefore,two-photon polymerization is widely used in many fields,such as micro-optics,communications,biomedicine,microfluidic devices,and metamaterials.Diffractive optical element(DOE)is an optical device that is specially designed for manipulating the spatial distribution of light.It has find a vast number of applications in holographic displays,medical imaging,augmented reality/virtual reality,spectroscopy,beam shaping and etc.With the inherent capability of 3D processing,direct laser writing based on two-photon polymerization is suitable for writing DOEs.Compared to traditional manufacturing methods such as photolithography,direct processing,and replication,direct laser writing is maskless,low-cost,and high-precision.This paper investigates the design of DOEs and the two-photon direct laser writing process for fabricating binary diffractive optical elements.A desktop-level three-dimensional direct laser writing system based on two-photon polymerization technology was built,and DOE processing technology research was carried out based on this system.The computer-generated hologram(CGH)for generating the desired far-field diffraction pattern is designed based on the Error-Tracking-Control-Reduction algorithm.For the design of image reproduction diffraction optical elements,the algorithm outputs a binary graph of 1000×1000 pixels.To prevent the optical field pattern from overlapping with zero-level light and conjugate images,the input image is adjusted and the desired target image is placed in the lower-left corner of the input image.Then,the binary diffractive optical elements are manufactured using a home-made femtosecond direct laser writing system with a DOE size of 1 mm×1 mm.Each pixel features a square cross-section of 1 μm,and a height about 500 nm.Line sweep filling is used for path filling.Due to the direct laser writing system’s field of vision restriction,the original DOE is processed by splicing.The structure height of the diffractive optical element is adjusted by varying the height difference between the piezoelectric objective ring and the actual work interface.The processing parameters are optimized,and the effects of different laser power,scanning speed,line spacing,and other parameters on the direct laser writing diffractive optical elements are investigated experimentally.The designed target light field pattern is successfully reproduced using a parallel laser beam of 532 nm with a diameter of 1 mm lighting the diffractive optical element.Then,we design the DOE image reproduction of the dynamic pattern by decomposing a dynamic pattern into 36 frames and making the diffractive optical element corresponding to each frame of the pattern separately.By moving the DOEs in serial,we can realize the dynamic playback effect of the diffractive pattern in the diffraction plane.In this case,the pixel size of the diffractive optical element for each frame is 0.5 μm,with a total 2000×2000 pixels,and the DOEs of the frames are stitched together.In this work,we demonstrate the fabrication of binary DOEs by the two-photon laser direct writing technique.Multiple binary DOEs are designed,written and stitched together to generate the targeted dynamic holographic movie.The approach presented herein illustrates the great potential of the two-photon polymerization technique for the preparation of micro and nanooptical devices.