| Fresnel zone plate(FZP)is a critical diffraction optical element,which has been widely used in aircraft carrier optical landing aid,space telescope flexible lens,computed tomography,solar photovoltaic and other aspects.It is the core of strengthening national strategic scientific and technological forces and seizing the commanding heights of science and technology.With the development of the integration of optical,mechanical and electrical systems,the size of FZPs has reached the level of micron and nanometer.Traditional micro-optical components are fabricated by mask lithography,which has many processing links and complicated mask fabrication.Femtosecond laser direct writing processing technology does not need mask,and has outstanding advantages such as a wide range of processing materials,high processing precision and non-hot melt processing,so it has become an important method for making micro FZPs.In the study of femtosecond laser processing and diffraction performance of FZPs,there are several outstanding problems to be solved:Femtosecond laser machining has low efficiency,limited precision and other defects,so it is necessary to optimize the processing technology.The diffraction properties of FZPs are required to be higher and higher in optical systems,and the diffraction rate,resolution and focal depth need to be further improved.With the expansion of the application field,the traditional circular FZP structure can no longer meet the application requirements of non-circular symmetric beam shaping,and it is urgent to develop new structures.Because of the above problems,this paper carried out the following research:(1)The surface quality and machining efficiency of femtosecond laser machining are improved based on wet etching and spatial light modulator(SLM).Through the wet corrosion process,the micro-groove’s protrusions on the wall and bottom are corroded away,and the roughness decreases by 75.68%.Based on the anisotropic corrosion characteristics of the material,the femtosecond laser irradiation of Ga As lower than the ablation threshold can enlarge the crystal cell and change the crystal direction of the material in the irradiated area.The corrosion efficiency can be increased nearly 10 times.SLM is used to modulate the phase of the femtosecond laser and eliminate the effect of zero-order light,the Gaussian distribution of the laser is regulated to annular distribution.After modulation,the processing process is optimized from single-focus serial processing to patterned processing,which can realize the processing of a single zone with a single exposure,reducing the processing time of a single FZP from 90 minutes to 90 seconds and significantly improving the processing efficiency.(2)The diffraction properties of circular FZP are optimized,and the diffraction rate,focal depth and resolution are improved.Based on the physical optics model,the FZP is modeled,and the optimization law of diffraction performance is studied by simulation and experimental processing.By increasing the height consistency and aspect ratio of zones and eliminating modified area,the diffraction rate of the FZP is increased from 6.96% to 8.4%.The fractal FZP is designed by traditional segmental FZP,and the focal depth is increased from 6 mm to 10 mm without a negative impact on the resolution,which solves the restriction problem of the transverse and longitudinal resolution of the microscope.Combined with the outer zone,the extended FZP is designed,and the focal radius is reduced from 12.6 μm to 8 μm,which not only breaks the minimum line width limit but also helps improve the imaging resolution.In addition,a circular FZP with a radius of 52.5 μm is fabricated on the end face of the multi-mode fiber.The outgoing light divergence angle is reduced by 19%,and the optical coupling efficiency can be improved.The resulting focusing ring has a radius of 82.5 μm and a width of 11.82 μm,which can be used for high-precision optical tweezers particle operation.(3)The FZP structure is innovated,and new diffraction performances are achieved.The FZP with an annular structure is designed and fabricated,and the single-ring laser beam and double-ring laser beam without a bright spot in the center are obtained after focusing,which makes up for the defects of the existing annular beam.The structure parameters can be adjusted flexibly,and the annular beam with different radii and focal lengths can be realized.A variety of square FZPs are designed and prepared.The similarities and differences of diffraction properties of linear FZP,square FZP,orthogonal FZP and circular FZP are compared,and the linear focusing and cross focusing are realized.The double cross-focusing is realized by phase translation,and the rectangular diffraction pattern with an arbitrary aspect ratio is adjusted.The non-traditional shape laser beam has a broad application prospect in beam shaping,optical vortex,array generator and other fields. |
