A folded optical system with a large field of view

Abstract

The present invention discloses a folded optical system with a large field of view, and relates to the field of optical imaging technology. A folded optical system with a large field of view includes a four-fold folded optical system and a two-fold folded optical system; the four-fold folded optical system and the two-fold folded optical system include a first metasurface, a second metasurface, and a third metasurface; in the four-fold folded optical system, the first metasurface, the second metasurface, and the third metasurface are coaxially arranged with a substrate; in the two-fold folded optical system, the first metasurface and the third metasurface are coaxially arranged with the substrate, and the second metasurface is coaxially arranged in front of the third metasurface. The folded optical system with a large field of view described in the present invention has a field of view greater than or equal to 40°, an optical system thickness less than 2 mm, and can be flexibly zoomed, which reduces the thickness and volume on the basis of improving the lens performance, and expands the application range of the folded lens.

Description

Technical Field

[0001] The present invention relates to the field of optical imaging technology, and in particular to a folded optical system with a large field of view. Background Art

[0002] Traditional small optical systems basically use a combination of multiple refractive optical elements to achieve high-quality imaging. These optical systems have complex structures and large volumes, especially large axial dimensions, and can no longer meet the development needs of existing drone or mobile phone optical systems. Folding flat-plate lenses can maintain high resolution and light collection capabilities while significantly reducing volume and weight, and can be used in small drone imaging systems and mobile phone imaging systems. The folding flat-plate lenses currently used are highly folded reflective lenses created on the basis of traditional reflecting telescopes such as Schmidt-Cassegrain telescopes. The total optical path is consistent with the traditional lens by reflecting light back and forth between two parallel plates, and the thickness is greatly reduced. It has the advantages of miniaturization, lightweight, and good image quality. It has been widely used in the field of visible light imaging and can be used in drones, mobile phones and other imaging devices. However, multiple folding of flat-plate lenses will result in a smaller field of view. At present, the maximum field of view of 8-fold flat-plate lenses is about 5°, the maximum field of view of 6-fold flat-plate lenses is about 15°, and the maximum field of view of 4-fold flat-plate lenses does not exceed 20°. The field of view of the imaging system of a small drone is generally required to be greater than 30°, and the field of view of the imaging system of a mobile phone is generally required to be greater than 40°. Both require the imaging system to have a large field of view, so expanding the field of view of the folding flat lens is an urgent problem to be solved. Most folding flat lenses use planes, quadratic surfaces, high-order aspherical surfaces, etc. to reflect and converge light. Due to the limitation of the annular aperture, it is difficult to achieve the purpose of expanding the field of view on the basis of miniaturization of the lens using traditional refraction and diffraction methods. Therefore, it is of great significance to provide a folding optical system with a large field of view in the visible light band.

[0003] At present, folded optical systems with different structures have been proposed in domestic and foreign patents, each of which has its own advantages and disadvantages.

[0004] The Chinese invention patent with patent application number 201510539488.X proposes a micro-optical label system based on an annular aperture ultra-thin lens, including a transmitting end and a receiving end. The transmitting end includes a micro-label code and a label lens, and the micro-label code is located at the front focal plane of the label lens; the receiving end includes a four-reflection annular aperture ultra-thin lens and an image sensor, and the image sensor is located at the back focal plane of the four-reflection annular aperture ultra-thin lens. The light emitted by the micro-label code passes through the label lens and the four-reflection annular aperture ultra-thin lens in sequence and finally forms an image on the image sensor. The ultra-thin large-aperture long-focal-length lens is realized by annular folding, and this lens is used as the mobile phone lens of the receiving end of the micro-optical label system, which can increase the receiving distance and realize the long-distance reception of the micro-label code by the mobile phone. However, there is a certain distortion in the received image of the system, which may cause the recognition time of the QR code to be too long, and the field of view of the system is small. At the same time, the image receiver (such as CCD, etc.) located at the image plane position is embedded in the lens matrix. The image receiver cannot be moved and cannot be used as a compensator to compensate for manufacturing tolerances during assembly, and is susceptible to stray light interference.

[0005] The Chinese invention patent with patent application number 201810773750.0 proposes an annular aperture folded ultra-thin optical imaging system. The front surface is designed to be a plane that is easy to manufacture, and the rear surface is composed of multiple aspherical surfaces. The light enters the optical imaging system through the annular aperture, undergoes the first reflection at the first high-order aspherical reflector, i.e., the aperture position, and is reflected to the first plane reflector, i.e., the circular aperture on the front surface of the lens, for the second reflection, and is reflected to the second high-order aspherical reflector for the third reflection. The light reaches the second plane reflector for the fourth reflection, and the light reaches the high-order aspherical transmission surface. After refraction, all the light converges to the imaging plane. However, the folded optical system has a small field of view. Due to the structural limitations of the folded optical system, it has limitations in wide-angle imaging with a large field of view.

[0006] The Chinese invention patent with patent application number CN202010482151.0 provides an optical lens, a lens module including the optical lens, and an electronic device including the lens module, aiming to achieve good imaging effects while obtaining an optical lens with a telephoto effect and a small length, which is relatively simple to manufacture and has a low cost. The optical lens is divided into a first component and a second component, wherein the first component has a reversing lens with positive focal length, and the light is reversing twice in the reversing lens, thereby reducing the thickness of the lens in the optical lens while obtaining the desired light direction adjustment effect to achieve the long focal length of the optical lens, thereby reducing the length of the optical lens. The second component is a lens with negative focal length, and the second component includes at least two lenses, which cooperate with each other to achieve a better field of view correction effect, so that the optical lens can obtain higher quality imaging. In addition, the patent also proposes a lens module and an electronic device, wherein the lens module is used to achieve a better long-range shooting effect and has a shorter total optical length, so as to be better applied to thin electronic devices. The electronic device includes a housing and the above-mentioned lens module, which has the advantages of thinness and low cost. The optical lens proposed in the patent does not use a prism to achieve light path folding. Compared with the optical lens that uses a prism to fold the light path, the manufacturing accuracy and manufacturing cost of the optical lens are lower. In addition, compared with the periscope lens that uses a prism to fold the light path, the optical lens in the present application axially folds the light through a folding lens, so that the total optical length of the optical lens is shortened while occupying less space in other directions of the optical lens. The lens only uses a secondary folding lens component, and a quadruple folding lens can be used to further reduce the thickness of the first component. The second component of the lens is composed of at least two refractive lenses, so that the total length of the system cannot be compressed to a smaller thickness. The field of view of the optical lens is small, and imaging with a large field of view angle cannot be performed.

[0007] The U.S. invention patent with patent application number US201615048008 proposes a folded imaging system, which sets a lens array adjacent to an image sensor and has two operating modes: imaging mode and storage mode. The above-mentioned lens array has a plurality of lens elements. The imaging mode has a plurality of cooperative lens elements arranged around a common optical axis, and the storage mode has a plurality of cooperative lens elements arranged in a common thin plane volume. When the imaging system is folded, all lens elements of the imaging system deviate from the optical axis. When the imaging system is in the imaging mode, all lens elements form an image. The imaging system includes a plurality of lens elements distributed in two dimensions, and the focusing ability of the lens element depends on its distance from the optical axis. In contrast to the methods and devices of the prior art, the system first presented here does not collapse in an axial or telescopic mode, but collapses along an off-axis path. More specifically, these systems collapse from an imaging mode to a storage mode characterized as a thin plane volume. When considering these off-axis collapse schemes, the fundamental difference between the collapsible lens system of the present invention and the collapsible lens system of the prior art can be found. Further fundamental differences and distinguishing factors are easily discerned when considering the special means and strategies used to mitigate specific aberrations caused by imperfect lens placement. Specifically, the lens array processor is designed to operate on the incident wavefront to reduce image errors caused by known mechanical changes. The conversion between the above two modes results in imperfect placement of composite lens elements. Special provisions are included for correcting image errors to achieve very high resolution. Although the patent uses an off-axis approach to fold the optical system, this simple mechanical folding does not effectively optimize the internal optical path, resulting in some optical parameters that cannot be further improved and the system has certain limitations.

[0008] The Chinese invention patent with patent application number PCTCN2021095122 proposes a superlens and an optical system having the same. The patent combines a supersurface and a traditional optical lens to form a hybrid optical system to complete specific imaging applications. The supersurface structural units are arranged in an array, which is a regular hexagon or square. Each supersurface structural unit is provided with a nanostructure at the center and vertex positions. The arrangement of the nanostructure determines that the supersurface is insensitive to polarization, and its optical phase is related to the height of the nanocolumn structure, the shape of the cross section, and the material of the nanocolumn structure. The optical system described in the patent is composed of two superlenses, and the traditional lens part includes at least one of a refractive lens and a reflective lens. One structure is two superlenses and a refractive lens. The incident light passes through the first superlens and the second superlens in sequence into the refractive lens. The refractive lens is a convex lens, and all three lenses are coated with an anti-reflection film for the visible light band. Another structure is two metalenses and two reflecting lenses. The reflecting lenses include a main reflecting lens and a secondary reflecting lens. The main reflecting lens is arranged between the secondary reflecting lens and the third metalenses. The fourth metalenses is arranged on the side of the third metalenses away from the main reflecting mirror. The main reflecting mirror includes two sub-reflecting lenses arranged in a direction perpendicular to the third axis, which are concave mirrors, and the secondary reflecting mirror is a convex mirror. The incident light is reflected by the reflecting mirror and enters the secondary reflecting mirror. After reflection, it enters the two metalenses and is output. The hybrid optical system of this invention has the ability of wide-band achromatic imaging. Compared with the existing optical system, the optical system of this invention is small in size and light in weight. However, the metalenses and traditional lenses in this invention are relatively independent in terms of structural division, and there is no description of the method and application of the metasurface in expanding the field of view of the optical system.

[0009] The above-mentioned folded optical systems have a small field of view in the visible light band and cannot achieve imaging with a large field of view, and no folded optical system that effectively expands the field of view has been proposed. To this end, the traditional folded flat lens is improved by introducing a metasurface, and a variety of subwavelength structures with broadband optical response in the visible light band are designed on the metasurface. The various subwavelength structures are arranged on the metasurface so that light with different incident angles is modulated by the metasurface to the corresponding exit angle, thereby expanding the field of view of the flat lens and achieving imaging with a large field of view in the visible light band. At the same time, the thickness and volume of the optical system are further reduced on the basis of improving the performance of the flat lens. It can be widely used in small drone imaging and mobile phone imaging optical systems, expanding the application range of folded flat lenses. Summary of the invention

[0010] The purpose of the present invention is to provide a folded optical system with a large field of view to solve the technical problems in the above-mentioned background technology.

[0011] To achieve the above-mentioned objectives, the present invention provides a folded optical system with a large field of view, including a four-fold reentrant folded optical system and a two-fold reentrant folded optical system; the four-fold reentrant folded optical system and the two-fold reentrant folded optical system include a first metasurface, a second metasurface, and a third metasurface; in the four-fold reentrant folded optical system, the first metasurface, the second metasurface, and the third metasurface are coaxially arranged with a substrate; in the two-fold reentrant folded optical system, the first metasurface and the third metasurface are coaxially arranged with the substrate, and the second metasurface is coaxially arranged on the front side of the third metasurface; array-distributed nanocolumn structures are arranged on the first metasurface, the second metasurface, and the third metasurface.

[0012] Furthermore, the four-fold folding optical system also includes a first plane mirror and a second plane mirror, the first plane mirror and the second plane mirror are coaxially arranged on the same substrate, and the first plane mirror and the second plane mirror are coaxially arranged on the front side of the first metasurface, the second metasurface, and the third metasurface.

[0013] Furthermore, in the four-fold folded optical system, the first metasurface and the second metasurface are arranged in a ring shape, the first metasurface is arranged at the periphery of the second metasurface, and the third metasurface is arranged at the center of the second metasurface.

[0014] Furthermore, in the secondary folding optical system, the first metasurface is arranged in a ring shape, and the third metasurface is arranged at the center of the first metasurface.

[0015] Furthermore, the nanocolumn structure includes a negative hollow nanocolumn structure, a square nanocolumn structure, a negative square nanocolumn structure, a hollow square nanocolumn structure, a negative hollow square nanocolumn structure, and a topological nanocolumn structure.

[0016] Furthermore, the nanopillar structure is optimized by combining the geometric phase method and the propagation phase method;

[0017] The phase distribution formula of the nanocolumn structure of the first metasurface is:

[0018]

[0019]

[0020] Where k0 is the free space wave number, f is the focal length, and n ext is the refractive index of the background medium, r is the distance from the preset position to the center of the surface circle;

[0021] The phase distribution formula of the nanopillar structure of the second metasurface is:

[0022]

[0023] in, is the theoretical phase, f is the focal length, λ is the incident wavelength, and r is the distance from the preset position to the center of the surface circle;

[0024] The third supersurface determines the structure and distribution of the nanocolumns according to the focal length adjustment range and in combination with the first supersurface and the second supersurface structures.

[0025] Furthermore, in the four-fold folded optical system, the light enters the optical system through the light plane of the annular light aperture, the light is incident on the first metasurface, reflected by the first metasurface to the first plane reflector, reflected by the first plane reflector to the second metasurface, reflected by the second metasurface to the second plane reflector, reflected by the second plane reflector to the third metasurface, and transmitted and zoomed by the third metasurface, the light converges onto the image plane.

[0026] Furthermore, in the described twice-return folded optical system, light enters the optical system through an annular aperture, the light is incident on the first metasurface to widen the field of view, the light is reflected by the first metasurface to the second metasurface, the light is reflected by the second metasurface and converges, and the light is transmitted and zoomed by the third metasurface and converges onto the image plane.

[0027] Furthermore, the field of view angle of the optical system is greater than or equal to 40°, and the thickness of the optical system is less than 2 mm.

[0028] The advantages and positive effects of the folded optical system with a large field of view described in the present invention are:

[0029] 1. The present invention combines metasurface technology to expand the field of view of the foldable flat lens and realize imaging of the optical system with a large field of view. In the visible light band of 400nm-750nm, by relaxing the restriction of diffraction-limited resolution, light with different incident angles can be modulated by the metasurface to the corresponding exit angles, enabling the metasurface to perform imaging with a large field of view.

[0030] 2. The field of view angle of the metasurface folded optical system in the present invention is greater than or equal to 40°, and the thickness of the optical system is less than 2 mm. At the same time, by introducing metasurfaces with different structures, flexible zoom and other functions are realized. On the basis of improving the performance of the flat-panel lens, the thickness and volume are reduced, thereby expanding the application scope of the folded flat-panel lens.

[0031] 3. For the optimization problem of hybrid optical systems, the present invention can be combined with corresponding efficient optimization methods to more quickly and accurately design the parameters required for large field of view imaging optical systems.

[0032] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1It is a schematic diagram of a four-fold folded optical system in the present invention;

[0034] Figure 2 Schematic diagram of a 4-fold folded optical system with variable focus in the present invention;

[0035] Figure 3 It is a schematic diagram of a double-folding folding optical system in the present invention;

[0036] Figure 4 Schematic diagram of the variable-focus double-reentrant folded optical system of the present invention.

[0037] Reference numerals

[0038] 11-12, first metasurface; 21-22, first plane reflector; 31-32, second metasurface; 41-42, second plane reflector; 5, image plane; 6, third metasurface. DETAILED DESCRIPTION

[0039] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0040] Unless otherwise defined, technical or scientific terms used in the present invention shall have the common meanings understood by one having ordinary skills in the field to which the present invention belongs.

[0041] Traditional folding flat-plate lenses are divided into calcium fluoride (CaF2) substrates, polymethyl methacrylate (PMMA) substrates and air substrates. From the perspective of processing technology, the folding flat-plate lens in the present invention uses an air substrate and introduces a metasurface in a four-fold flat-plate lens.

[0042] The metasurface substrate in the present invention is quartz glass, and the nanocolumn material is a dielectric material with high light propagation efficiency in the visible light band, such as single crystal silicon, silicon nitride, titanium dioxide, lithium niobate, silicon dioxide, calcium nitride, etc. The structure of the nanocolumn can be a negative hollow nanocolumn structure, a square nanocolumn structure, a negative square nanocolumn structure, a hollow square nanocolumn structure, a negative hollow square nanocolumn structure, and a topological nanocolumn structure. The present invention designs a hybrid structure optical system for the visible light band and realizes imaging with a large field of view, so the introduced metasurface uses the principle of combining the geometric phase method and the propagation phase method to optimize the design of the nanocolumn structure. The geometric phase regulation principle regulates, that is, modulates the rotation angle of the structural unit; the transmission phase regulation principle regulates, that is, changes the geometric parameters of the structural unit, wherein the geometric parameters include the number of nanocolumns in the structural unit, the length and width of the nanocolumns, and the gap between the nanocolumns.

[0043] The optical system of the present invention replaces the high-order aspheric structure in the traditional four-fold flat lens with a metasurface. The optical system is a circular aperture centrally symmetrical structure. The light enters the optical system through the circular aperture clear plane. The light passes through the clear plane and is reflected for the first time on the first metasurface 11-12. By setting the structure, size and arrangement of the metasurface nanocolumns, combined with the principle of joint regulation of geometric phase and propagation phase, the restriction of the diffraction limit resolution is relaxed in the visible light band of 400nm-750nm, so that light with different incident angles is modulated by the metasurface to the corresponding exit angle, so that the metasurface can perform large field of view imaging. The field of view of the metasurface folded optical system can be greater than 40°. At the same time, it can maintain a high resolution suitable for most applications. The hyperbolic phase distribution and quadratic phase distribution of the diffraction limit and wide field of view metasurfaces are:

[0044]

[0045]

[0046] Where: k0 and f are the free space wave number and focal length, n ext is the refractive index of the background medium, and r is the distance from the preset position to the center of the surface.

[0047] By converting the linear phase of the secondary phase distribution of the above-mentioned metasurface into spatial displacement, the wide-field imaging of the metasurface can be realized, thereby obtaining the nanocolumn arrangement structure of the first metasurface 11-12. The function of the first metasurface 11-12 is to expand the field of view of the optical system. After the light is reflected for the first time, it is emitted for the second time at the first plane reflector 21-22 on the front surface of the optical system, and after the second reflection, it is reflected for the third time at the second metasurface 31-32. By setting the structure, size and arrangement of the nanocolumns on the metasurface, combined with the principle of joint regulation of the geometric phase and the propagation phase, the light reflected by the first plane reflector 21-22 is converged. At this time, the phase distribution of the metasurface satisfies the formula:

[0048]

[0049] Where: is the theoretical phase, f is the focal length, λ is the incident wavelength, and r is the distance from the preset position to the center of the surface.

[0050] The second metasurface 31-32 is used to converge and image the light of the optical system in the visible light band. After the light is reflected for the third time, it is reflected for the fourth time by the second plane reflector 41-42 on the front surface of the optical system, and all the reflected light is converged onto the imaging plane.

[0051] For the above-mentioned large field-of-view imaging metasurface folded optical system, an efficient inverse optimization algorithm can be used to optimize the hybrid optical system structure to obtain a wide field-of-view optical structure. Finally, the metalens is made using electron beam lithography, and the plane mirror is made using single-point diamond turning technology.

[0052] An image sensor is placed at the image plane of the above-mentioned metasurface folded hybrid optical system. The hybrid optical system receives visible light rays incident on the first metasurface 11-12 through the annular aperture. The light rays are reflected by the first metasurface 11-12 to the first plane reflector 21-22. The light rays passing through the first plane reflector 21-22 are reflected by the second metasurface 31-32. The reflected light rays are then reflected by the second plane reflector 41-42 and converged, and finally imaged on the image sensor at the image plane 5. Through the corresponding optical system design and optimization methods, the parameters of the metasurface and the entire optical system are optimized. The virtual aperture method proposed in the paper can also be used to expand the field of view of the metalens, and a virtual diffraction aperture of any shape and size is introduced into a specific axial position between the metasurface and the target plane, which is used as an optimized modulator. It can be mapped with various patterned fields to reversely infer the required field distribution on the metasurface under a given illumination source. It converts the optimization of the complex complex amplitude distribution on the metasurface into the optimization of the shape and position of the virtual aperture. The field of view of the metasurface can be expanded to more than 90°. Ultimately, the optical system can achieve imaging with a large field of view on the image sensor.

[0053] An image sensor is placed at the image plane 5 of the above-mentioned metasurface folded hybrid optical system. The hybrid optical system receives visible light rays incident on the first metasurface 11-12 through the annular aperture. The light rays are reflected by the first metasurface 11-12 to the first plane reflector 21-22. The light rays reflected by the first plane reflector 21-22 are reflected by the second metasurface 31-32. The reflected light rays are incident on the second plane reflector 41-42 for reflection. The light rays reflected by the second plane reflector 41-42 are incident on a third metasurface 6 in the same plane as the first metasurface 11-12 and the second metasurface 31-32. The metasurface has a special arrangement of nanocolumns. The light rays are transmitted through the third metasurface 6 and finally imaged by the image sensor at the image plane 5. The image sensor of this design can be flexibly moved along the optical axis. By rotating the third metalens, the focal length of the optical system can be adjusted to achieve the function of flexible zooming. The design can also optimize the parameters of the metasurface and the entire optical system through the corresponding optical system design and optimization method.

[0054] The above-mentioned metasurface folded hybrid optical system is a 4-fold type. According to the properties of the metasurface and the structural principle of the Cassegrain telescope, a 2-fold metasurface optical system can also be provided to further reduce the outer diameter of the optical system and keep it ultra-thin and small in size. The optical system is composed of two metasurfaces. An image sensor is placed at the image plane of the metasurface optical system. The optical system receives visible light rays incident on the first metasurface 11-12 through the annular aperture. The first metasurface 11-12 realizes the function of widening the field of view. The light rays are reflected from the first metasurface 11-12 to the second metasurface 31-32. The second metasurface 31-32 realizes the function of converging the light rays. The light rays are reflected from the second metasurface 31-32 and converged. Finally, the image is formed at the image sensor at the image plane 5, which can also realize imaging with a large field of view.

[0055] The technical solution of the present invention is further illustrated by the following examples.

[0056] Example 1

[0057] Figure 1 Schematic diagram of the 4-fold folded optical system in the present invention. Light enters the optical system through the light plane of the annular light aperture, firstly, the light is incident on the first metasurface 11-12, and the light is regulated by the nanocolumn array on the first metasurface 11-12, so that light incident at different field angles can be emitted along the corresponding optical path, reflected by the first metasurface 11-12 to the first plane reflector 21-22, and reflected by the first plane reflector 21-22 to the second metasurface 31-32, and the light is regulated by the second metasurface 31-32 and reflected to the second plane reflector 41-42, and finally the light is reflected by the second plane reflector 41-42, and all the light is converged on the image plane 5.

[0058] First, the object reflects visible light, and three wavelengths of 486nm, 588nm and 656nm are selected as reference wavelengths in the visible light band of 400nm-750nm. The light is incident on the 4-folding metasurface folded hybrid optical system at different angles. The aperture stop of the optical system is set at the first metasurface 11-12 after the light is incident. The metasurface substrate in this folded hybrid optical system is silicon dioxide, and the nanocolumns can be made of single crystal silicon or other materials with high light propagation efficiency in the visible light band. The structure of the nanocolumns can be a negative hollow nanocolumn structure and other structures. By changing the geometric parameters of the nanocolumn structural unit and the arrangement of the nanocolumns on the substrate, the incident light of different wavelengths and angles has corresponding The output angle is set to 100°, and the metasurface is made polarization-insensitive. After that, the light is incident on the second metasurface 31-32 through the plane reflector 21-22. The light is also converged by changing the geometric parameters of the nanocolumn structural unit and the arrangement of the nanocolumns on the substrate. The geometric parameters include the number of nanocolumns in the structural unit, the length and width of the nanocolumns, the rotation angle of the nanocolumns, and the gap between the nanocolumns. The nanocolumn structure is optimized and designed by combining the principle of the geometric phase method and the propagation phase method. Through the above optimization of the optical system, a metasurface structure capable of imaging with a large field of view angle and a metasurface structure capable of efficiently focusing light are obtained. A CMOS image sensor is added to the rear end of the optical system, and the light is propagated through the optical system to achieve high-quality imaging with a field of view angle greater than 40°. The optimized optical system needs to fix the plane reflector and the metasurface by designing opaque connectors. The plane reflector is processed using single-point diamond turning technology, and the metasurface is processed using electron beam lithography.

[0059] Example 2

[0060] Figure 2 Schematic diagram of the 4-fold folded optical system with variable focus in the present invention. First, light is incident on the first metasurface 11-12, and the light is regulated by the nanocolumn array on the first metasurface 11-12, so that light incident at different field angles can be emitted along the corresponding optical path, and reflected by the first metasurface 11-12 to the first plane reflector 21-22, and the light is reflected by the first plane reflector 21-22 to the second metasurface 31-32, and the light is regulated by the second metasurface 31-32 and reflected to the second plane reflector 41-42, and the light is incident on the third metasurface 6 after being reflected by the second plane reflector 41-42, and the nanocolumn array of the third metasurface 6 is designed, and the light is transmitted through the third metasurface 6, and by rotating the third metasurface 6, the flexible zoom of the optical system can be realized, and finally an image is formed on the image plane 5.

[0061] First, the object reflects visible light, and three wavelengths of 486nm, 588nm and 656nm are selected as reference wavelengths in the visible light band of 400nm-750nm. The light is incident on a variable-focus 4-times-return metasurface folded hybrid optical system at different angles. The aperture stop of the optical system is set at the first metasurface 11-12 after the light is incident. The metasurface substrate in this folded hybrid optical system is silicon dioxide, and the nanocolumns can be made of single-crystal silicon or other materials with high light propagation efficiency in the visible light band. The structure of the nanocolumns can be a negative hollow nanocolumn structure and other structures. By changing the geometric parameters of the nanocolumn structural units and the arrangement of the nanocolumns on the substrate, incident light of different wavelengths and angles has corresponding exit angles after being reflected by the first metasurface 11-12, and the metasurface has polarization-insensitive characteristics. Then the light is incident on the second metasurface 31-32 through a plane reflector. , similarly, by changing the geometric parameters of the nanocolumn structural unit and the arrangement of the nanocolumns on the substrate, the light is converged, and finally the light is incident on the third super surface 6 through the plane reflector and emitted, and by changing the geometric parameters of the nanocolumn structural unit and the arrangement of the nanocolumns on the substrate, the third super surface 6 is rotated to realize flexible zooming of the optical system, wherein the geometric parameters include the number of nanocolumns in the structural unit, the length and width of the nanocolumns, the rotation angle of the nanocolumns, and the gap between the nanocolumns, etc., and the nanocolumn structure is optimized and designed using the principle of combining geometric phase and propagation phase. Through the above optimization of the optical system, a super surface structure capable of imaging with a large field of view angle, a super surface structure that efficiently focuses light, and a super surface structure that flexibly zooms the optical system by rotation are obtained, and a CMOS image sensor is added to the rear end of the optical system. The light propagates through the optical system to realize high-quality imaging with a field of view angle greater than 40° and a variable focal length. The optimized optical system needs to fix the plane reflector and the super surface by designing a light-proof connector, the plane reflector is processed using single-point diamond turning technology, and the super surface is processed using electron beam lithography.

[0062] Example 3

[0063] Figure 3 Schematic diagram of the double-return folded optical system in the present invention. The optical system receives visible light rays incident on the first metasurface 11-12 through the annular aperture, and the first metasurface 11-12 realizes the function of widening the field of view. The light rays are reflected by the first metasurface 11-12 to the second metasurface 31-32, and the second metasurface 31-32 realizes the function of converging the light rays. The light rays are reflected by the second metasurface 31-32 and converge, and finally form an image at the image plane 5.

[0064] First, the object reflects visible light, and three wavelengths of 486nm, 588nm and 656nm are selected as reference wavelengths in the visible light band of 400nm-750nm. The light is incident on the twice-returning metasurface folded optical system at different angles. The aperture stop of the optical system is set at the first metasurface 11-12 after the light is incident. The metasurface substrate in this folded hybrid optical system is silicon dioxide, and the nanocolumns can be made of single crystal silicon or other materials with high light propagation efficiency in the visible light band. The structure of the nanocolumns can be a negative hollow nanocolumn structure and other structures. By changing the geometric parameters of the nanocolumn structural unit and the arrangement of the nanocolumns on the substrate, the incident light of different wavelengths and angles has corresponding exit angles after being reflected by the first metasurface 11-12. And the metasurface has polarization insensitivity. After that, the light is incident on the second metasurface 31-32. The light is also converged by changing the geometric parameters of the nanocolumn structural unit and the arrangement of the nanocolumns on the substrate. The geometric parameters include the number of nanocolumns in the structural unit, the length and width of the nanocolumns, the rotation angle of the nanocolumns, and the gap between the nanocolumns. The nanocolumn structure is optimized and designed using the principle of combining geometric phase and propagation phase. Through the above optimization of the optical system, a metasurface structure capable of imaging with a large field of view angle and a metasurface structure capable of efficiently focusing light are obtained. The second folding can reduce the outer diameter of the optical system and maintain a small thickness. A CMOS image sensor is added to the rear end of the optical system. The light propagates through the optical system to achieve high-quality imaging with a field of view angle greater than 40°. The optimized optical system needs to fix the metasurface by designing opaque connectors, and the metasurface is processed using electron beam lithography.

[0065] Example 4

[0066] Figure 4 Schematic diagram of the 2-fold folded optical system with variable focus in the present invention. The optical system receives visible light rays incident on the first metasurface 11-12 through the annular aperture, and the first metasurface 11-12 realizes the function of widening the field of view. The light rays are reflected from the first metasurface 11-12 to the second metasurface 31-32, and the second metasurface 31-32 realizes the function of converging the light rays. After the light rays are reflected from the second metasurface 31-32, they are transmitted through the third metasurface 6, zoomed, and converged onto the image plane 5.

[0067] First, the object reflects visible light, and three wavelengths of 486nm, 588nm and 656nm are selected as reference wavelengths in the visible light band of 400nm-750nm. The light is incident on the variable-focus two-times reentrant metasurface folded optical system at different angles, and the aperture stop of the optical system is set at the first metasurface 11-12 after the light is incident. The metasurface substrate in this folded hybrid optical system is silicon dioxide, and the nanocolumns can be made of single crystal silicon or other materials with high light propagation efficiency in the visible light band. The structure of the nanocolumns can be a negative hollow nanocolumn structure and other structures. By changing the geometric parameters of the nanocolumn structural units and the arrangement of the nanocolumns on the substrate, the incident light of different wavelengths and angles has corresponding exit angles after being reflected by the first metasurface 11-12, and the metasurface has polarization-insensitive characteristics. Then the light is incident on the second metasurface 31-32, and the nanocolumn structure is also changed. The geometric parameters of the structural unit and the arrangement of the nanocolumns on the substrate converge the light, and finally the light is emitted through the third metasurface 6. By changing the geometric parameters of the nanocolumn structural unit and the arrangement of the nanocolumns on the substrate, the third metasurface 6 is rotated to realize flexible zooming of the optical system, wherein the geometric parameters include the number of nanocolumns in the structural unit, the length and width of the nanocolumns, the rotation angle of the nanocolumns, and the gap between the nanocolumns, etc. The nanocolumn structure is optimized and designed using the principle of combining geometric phase and propagation phase. Through the above optimization of the optical system, a metasurface structure capable of imaging with a large field of view angle, a metasurface structure that efficiently focuses light, and a metasurface structure that flexibly zooms the optical system by rotation are obtained. The second folding can reduce the outer diameter of the optical system and maintain a small thickness. A CMOS image sensor is added to the rear end of the optical system. The light propagates through the optical system to achieve a field of view angle greater than 40° and high-quality imaging with a variable focal length. The optimized optical system needs to fix the metasurface by designing opaque connectors, and the metasurface is processed using electron beam lithography.

[0068] In summary, the present invention can be applied to small drone imaging systems and mobile phone and tablet imaging to achieve the purpose of expanding the field of view angle imaging.

[0069] Therefore, the present invention adopts the above-mentioned folded optical system with a large field of view angle, the field of view angle is greater than or equal to 40°, the thickness of the optical system is less than 2 mm, and it can also flexibly zoom, thereby improving the lens performance while reducing the thickness and volume, thereby expanding the application scope of the folded lens.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A folded optical system with a large field of view, characterized in that: It comprises a four-fold reentrant folded optical system or a two-fold reentrant folded optical system; the four-fold reentrant folded optical system or the two-fold reentrant folded optical system both comprise a first metasurface, a second metasurface, and a third metasurface; in the four-fold reentrant folded optical system, the first metasurface, the second metasurface, and the third metasurface are coaxially arranged with a substrate; in the two-fold reentrant folded optical system, the first metasurface and the third metasurface are coaxially arranged with a substrate, and the second metasurface is coaxially arranged at the front side of the third metasurface; array-distributed nanocolumn structures are arranged on the first metasurface, the second metasurface, and the third metasurface; The nanocolumn structure is optimized by combining the geometric phase method and the propagation phase method; the phase distribution formula of the nanocolumn structure of the first metasurface is: Where k0 is the free space wave number, f is the focal length, and n ext is the refractive index of the background medium, r is the distance from the preset position to the center of the surface circle; The phase distribution formula of the nanopillar structure of the second metasurface is: in, is the theoretical phase, f is the focal length, λ is the incident wavelength, and r is the distance from the preset position to the center of the surface circle; The third supersurface determines the structure and distribution of the nanocolumns according to the focal length adjustment range and in combination with the first supersurface and the second supersurface structures.

2. The folded optical system with a large field of view according to claim 1, characterized in that: The four-fold folding optical system also includes a first plane mirror and a second plane mirror. The first plane mirror and the second plane mirror are coaxially arranged on the same substrate. The first plane mirror and the second plane mirror are coaxially arranged on the front side of the first metasurface, the second metasurface, and the third metasurface.

3. The folded optical system with a large field of view according to claim 1, characterized in that: In the four-fold folding optical system, the first metasurface and the second metasurface are arranged in a ring shape, the first metasurface is arranged at the periphery of the second metasurface, and the third metasurface is arranged at the center of the second metasurface.

4. The folded optical system with a large field of view according to claim 1, characterized in that: In the described secondary folding optical system, the first metasurface is arranged in a ring shape, and the third metasurface is arranged at the center of the first metasurface.

5. The folded optical system with a large field of view according to claim 1, characterized in that: The nanocolumn structure includes a negative hollow nanocolumn structure, a square nanocolumn structure, and a topological nanocolumn structure.

6. The folded optical system with a large field of view according to claim 1, characterized in that: In the four-fold folding optical system, light enters the optical system through the light plane of the annular light aperture, is incident on the first metasurface, is reflected by the first metasurface to the first plane reflector, is reflected by the first plane reflector to the second metasurface, is reflected by the second metasurface to the second plane reflector, is reflected by the second plane reflector to the third metasurface, is transmitted and zoomed by the third metasurface, and converges onto the image plane.

7. The folded optical system with a large field of view according to claim 1, characterized in that: In the twice-return folded optical system, light enters the optical system through an annular aperture, is incident on the first metasurface to widen the field of view, is reflected by the first metasurface to the second metasurface, is reflected by the second metasurface and converges, is transmitted by the third metasurface, is zoomed, and converges onto the image plane.

8. The folded optical system with a large field of view according to claim 1, characterized in that: The field of view angle of the optical system is greater than or equal to 40°, and the thickness of the optical system is less than 2 mm.

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