Design And Analysis Of Achromatic Diffractive Lens

The lens is the core component of the imaging system.Currently,most imaging systems generally use the optical lens based on the refraction / reflection principle,which modulates the phase of light field through the accumulation of optical path,so such lenses often have a relatively large volume and weight.In addition,due to the dispersion of materials,it is usually necessary to combine multiple lenses to achieve achromatic imaging,which will further expand the volume and weight of the device,and cannot meet the requirements of miniaturization and integration for modern imaging system.On the other hand,the optical element based on diffraction principle is thin and light,which provides a possible solution for high integration imaging system.However,the traditional diffractive elements(such as Fresnel lens)often have serious chromatic aberration,which will limit their application in imaging system.In order to realize the "smaller,lighter and easier to integrate" imaging system,this thesis combines the diffraction theory with the optimization design methods,and designs a variety of achromatic diffractive lenses with different numerical apertures for practical application requirements.The main contents and innovations of this thesis are as follows:(1)An achromatic diffractive lens with low numerical aperture has been designed.A group of concentric rings with different heights are used as the diffractive lens structure,and the phase of the light field is controlled by the change of the ring height.On the basis of this,the diffraction field of the structure is calculated by scalar diffraction theory.Then,based on the target light field,the structure parameters of the diffractive lens are optimized by the direct binary search algorithm,and the corresponding simulation experiments are carried out by using the electromagnetic simulation software Lumerical FDTD Solutions and MATLAB mathematical software to verify the achromatic performance of the designed diffractive lens.Finally,the effects of the quantization levels and the maximum height of the ring on the achromatic performance are analyzed.(2)An achromatic diffractive lens with high numerical aperture has been designed.Due to the sub-wavelength size of diffraction lenses with high numerical aperture,the scalar diffraction theory will not be applicable.Aiming at this problem,a method of combining the finite difference time domain method(FDTD)with the direct binary search algorithm is proposed to design a achromatic diffractive lens with high numerical aperture.Using this method,an achromatic diffractive lens with numerical aperture of 0.75 is designed and simulated.Compared with the common diffractive lens,the designed achromatic diffractive lens can suppress the chromatic aberration effect well.(3)It often takes several hours to perform three-dimensional vector analysis on a diffractive lens with a diameter of one hundred microns using FDTD.In the process of comprehensive design,it usually requires hundreds to thousands of vector analysis,which will cost a lot of computing time and memory.Therefore,the computational power is the main factor to limit the aperture size of diffractive lens.Aiming at this problem,a body-of-revolution finite-difference time-domain(BOR-FDTD)method is proposed as a vector analysis method for diffractive lenses.In this way,the three-dimensional vector analysis problem is transformed into a two-dimensional problem,the calculation time is reduced by 20 to 50 times,and an achromatic diffraction lens with large diameter and high numerical aperture is designed.