Characteristic Analysis And Fabrication Of The Multilayer Harmonic Diffractive Optical Elements

Since the 1980 s,the development of diffractive optics has injected new vitality into the research of traditional optical systems.The diffractive optical element has a precise three-dimensional relief structure,which can modulate the wave front of the incident light freely.It has special dispersion property which is different from the traditional refraction and reflection elements,and its volume is light and thin,so it is easy to copy in batches.In view of its excellent characteristics,scientists creatively combined it with the traditional refractive and reflective optical elements to form a refractive diffractive hybrid optical system,which significantly improved the imaging quality of the optical system.At the same time,it simplifies the internal structure of the system,reduces its volume,and promotes the further development of the optical system towards miniaturization.The refraction-diffraction hybrid system has been widely used in both visible light and infrared light bands.As an imaging optical element,the imaging chromatic aberration and wide spectrum diffraction efficiency of the diffractive optical element directly affects its feasibility and application prospect.Traditional single-layer diffractive optical element has large chromatic aberration,and the diffraction efficiency is greatly reduced when the incident wavelength deviates from the main wavelength,which has a serious wavelength dependence.The harmonic diffractive optical element can effectively eliminate chromatic aberration,but also can not achieve high diffraction efficiency in wide band.The multilayer diffractive optical element can achieve high diffraction efficiency in a wide spectral range,but its large chromatic aberration makes it impossible to image independently and needs to be used in conjunction with traditional refractive lenses.Therefore,the development of the new diffractive optical element with high diffraction efficiency and small chromatic aberration will greatly promote the application of diffractive optical elements in the field of optical imaging.Based on the imaging characteristics of the existing diffractive elements,this paper designs and proposes a new diffractive optical element—multilayer harmonic diffractive optical element,and further explores its processing and preparation work.Harmonic diffractive optical elements and multilayer diffractive optical elements have achromatic ability and wide spectrum imaging characteristics respectively.Based on this,a multilayer harmonic diffractive optical element with both excellent properties is proposed.First of all,the high-order design of harmonic diffraction is used to make the three-color resonant wave sources(625nm,469 nm,375nm)in the imaging band achieve the same optical focus and eliminate part of the chromatic aberration.However,the diffractive element at this time is not suitable for wide-band imaging and is easily affected by inter-order stray light.Through the material matching method,the appropriate liquid material is selected for filling,and the partial chromatic aberration is eliminated,while the overall diffraction efficiency reaches more than 90%,and the multilayer harmonic diffractive optical element with high diffraction efficiency and wide-band imaging is obtained.Using femtosecond laser direct writing technology to process diffractive optical elements,aiming at the problem of focusing accuracy of femtosecond laser direct writing systems,an image focusing technology is designed that temporarily places focusing light sources and objects in the optical path.By adjusting the position of the object,its imaging plane is consistent with the laser focusing plane,so that the laser focusing state can be indirectly reflected by the clear and distinguishable imaging state.The feasibility of this method is verified by the simulation analysis of zemax software.The imaging error is mainly caused by the focal depth of the imaging lens(3.9?m).It has been verified by multiple experiments that the height error of the processed components is within the range of 1.5?m(close to 1/2 focal depth),which is consistent with the theoretical analysis and meets the requirements of machining accuracy.Single layer and harmonic diffraction optic elements are fabricated and their imaging properties are tested.Further exploring the fabrication of the multilayer harmonic diffraction optical elements.