Optical Design Of Ultra-Thin Lens Based On Annular Multi-Reflective Structure

The focal length of an optical system is closely related to the volume of the system.The longer the focal length,the larger the volume of the system.Currently,optical detection platforms are mostly small platforms such as micro unmanned aerial vehicles and small satellites.Therefore,miniaturized long focal length optical lenses have become an urgent need at present.The annular multi reflection optical lens evolved from the structure of the Gregory telescope.This paper analyzes its structural characteristics,derives the relationship between the initial structure of the lens and the occlusion ratio,and then presents a design method for the annular multi reflection ultra-thin lens.In order to reduce the cost of machining and testing,the surface shape of an aspherical surface is studied.Based on the standard quadric surface,the expression of Q-bfs type aspherical surfaces was derived,and the method of controlling the surface shape of Q-bfs type aspherical surfaces was discussed.Based on the basic theory of annular multi reflection ultra-thin lenses,a design and optimization method for annular multi reflection ultra-thin lenses is presented.First of all,a monolithic annular fourth and sixth reflection visible/shortwave infrared ultra-thin lens based on calcium fluoride（Ca F₂）was designed.The two structures have a field of view angle of 6.2°,a focal length of 75mm,an outer diameter of 50mm,and a working wavelength of 400nm～1700nm.The design results show that the MTF of both structures is less than 0.1 at 100lp/mm,and the magnification color difference is greater than 30μm.The imaging quality cannot meet the requirements.In order to further improve the imaging quality of the lens and reduce chromatic aberration,a dual panel multi reflection structure was proposed.Based on the design parameters of the single panel structure,a 1.3 megapixel dual panel annular fourth reflection and sixth reflection ultra-thin visible/short wave infrared lens was designed,respectively.The thickness to focal length ratio of the dual panel fourth reflection structure was 0.33,and the full field MTF was greater than 0.35 at 100lp/mm;The thickness to focal length ratio of the sixth reflection structure is 0.3,and the full field MTF is greater than 0.22 at 100lp/mm.The results show that the edge field magnification chromatic aberration of the two-piece four and six reflection lenses is smaller than the diffraction limit,effectively reducing the lens chromatic aberration.In order to verify the feasibility of a dual panel annular fourth reflection visible/shortwave infrared ultra-thin lens,a related process analysis was conducted.First,a thermal analysis was performed.After image plane displacement compensation,the lens had a temperature variation range of-20℃～40℃,and a full field MTF of greater than 0.3 at 100lp/mm;After tolerance analysis,the machining tolerance requirements for this lens are obtained.Combining existing machining methods and accuracy,a single point diamond turning technology is proposed for machining this lens;Finally,according to the structure of the system,the existing alignment methods are analyzed,and the centering adjustment alignment method is finally selected to align the lens,and the overall structure of the lens is designed.Taking into account factors such as processing and assembly errors,in order to ensure that the full field MTF of the lens is greater than 0.2 at 100lp/mm,a dual chip circular quadric reflector lens was ultimately selected to achieve the design of a high-resolution,long focal length visible/shortwave infrared integrated ultra-thin imaging lens.Compared to traditional refractive lenses,the dual panel annular multi reflective visible/shortwave infrared ultra-thin lens designed in this article has small size,long focal length,and is easy to carry.With the development of processing technology,it can achieve mass production,meeting the market demand for miniaturized long focal length lenses.