Research On Holographic Projection System Based On Diffraction Axilens

In a traditional holographic projection system,the Spatial Light Modulator(SLM)is used to modulate the incident Light wave,and a clear reconstructed image is formed only in the focal plane of the Fourier lens.Liquid Crystal on Silicon(LCoS)-as a SLM commonly used by researchers,has the advantages of small size with high pixel filling rate and high diffraction efficiency.It should be pointed out that in the above holographic projection system,when the reconstructed plane is far away from the focal plane,the holographic reconstructed image will become more and more fuzzy.In order to expand the application range of holographic projection systems,many researchers have carried out research work on multi-plane holographic projection,which can use Fresnel phase holograms with different propagation distances or multi-planes with Fresnel phase lens.However,for holographic projection in different planes along the propagation direction of the light field,the above methods need to separately calculate the corresponding Fresnel phase hologram or adjust the focal length of the Fresnel phase lens,which affects the scalability of the system.In order to solve the above problems,this paper tries to fully explore the programmable phase modulation characteristics of SLM itself based on pixels,flexibly codes the diffraction optical elements with large focal depth characteristics,and co-modulates the incident light field by combining with the phase hologram.The main research work and innovation points of the paper are as follows:(1)The large focal depth characteristic of the programmable axilens is applied to the holographic projection,and relatively clear holographic images are formed within a range along the propagation direction of the light field.First,the iterative Fourier algorithm is employed to generate a phase hologram.A new phase distribution is formed by superimposing the axilens phase on the phase hologram,and loading onto the phase-only LCoS.With the programmability of the LCoS,the different focal-depth parameters of the axilens are tuned.With the laser after collimation as the light source incident LCoS,holographic images at different positions of the focal plane are observed along the propagation direction of the light field.The experimental results show that compared with the holographic projection system based on Fresnel lens,the holographic reconstructed images in good focus can be observed simultaneously on a multi-plane in a range by dynamically changing the focal depth of the axilens.(2)LCoS can be used to dynamically load the phase-only hologram.However,due to the limitation of the space bandwidth product,it is difficult to design the diffraction element with small pixel characteristic size.So in this paper,we use laser direct write lithography system to fabricate the desired diffraction phase element.Firstly,the phase distribution of the axilens is compressed by computer to obtain the diffractive axilens.In the simulation experiment,the generated phase hologram and the phase distribution of the diffractive axilens are superposed to form a new phase distribution and then observe the relative intensity distribution of the Fresnel diffraction re-phenomenon along the optical axis propagation direction.Then,laser direct writing lithography is used to expose the required diffraction phase elements on the photoresist substrate,and a limited quantization phase is realized in the photoresist(number of steps = 2,= 4).By contrast experiments,the reconstructed images of the phase hologram superimposed with the phase of the diffractive axilens and the phase of the diffractive Fresnel lens are observed.The experimental results show that the near clear hologram can be observed on a continuous plane in a certain range by using the diffractive phase element in the holographic projection system.Because the laser direct writing system can control the exposure to produce the required lithographic structure on the photoresist surface,and the system is simple in structure,stable in operation,and can achieve a higher lithographic resolution,further simplifying the holographic projection system.