Patent ID: 12228710

ILLUSTRATION OF REFERENCE SIGNS

L1—first lens, P1—linear polarizer, P2—quarter-wave plate, STO—diaphragm, L2—second lens, L3—third lens, M—metasurface.

DETAILED DESCRIPTION OF EMBODIMENTS

In order to make the objectives, technical solutions and advantages of the present disclosure more clear and explicit, the present disclosure is further described in detail below in conjunction with embodiments with reference to drawings. Apparently, some but not all embodiments of the present disclosure are described. Based on the embodiments in the present disclosure, all of other embodiments obtained by those ordinarily skilled in the art without using any inventive efforts shall fall within the scope of protection of the present disclosure.

The terms used in the present disclosure are merely for the purpose of describing the embodiments, and are not intended to limit the present disclosure. The terms “include”, “contain” and the like used herein indicate existence of the feature, step, operation and/or component, but do not exclude existence or addition of one or more other features, steps, operations or components.

In the present disclosure, unless otherwise specified and defined explicitly, the terms such as “mount”, “join”, “connect”, and “fix” should be construed in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection, an electrical connection or communication with each other; it may be a direct connection, an indirect connection through an intermediary, or inner communication between two elements or interaction between two elements. For those ordinarily skilled in the art, specific meanings of the above terms in the present disclosure could be understood according to specific circumstances.

In the description of the present disclosure, it should be understood that orientation or positional relations indicated by terms such as “longitudinal”, “length”, “circumferential”, “front”, “rear”, “left”, “right”, “top”, “bottom”, “inner”, and “outer” are based on orientation or positional relations as shown in the drawings, merely for facilitating the description of the present disclosure and simplifying the description, rather than indicating or implying that related subsystem or element have to be in the specific orientation or be configured and operated in a specific orientation, and thus should not be construed as limitation to the present disclosure.

Throughout the drawings, like elements are denoted by like or similar reference signs. When an understanding of the present disclosure may be confused, a conventional structure or configuration will be omitted. Moreover, shapes, sizes, and positional relations of various components in the drawings do not reflect real sizes, scales or actual positional relations. In addition, in the claims, any reference signs located within parentheses shall not be construed as limitation to the claims.

Similarly, in order to simplify the present disclosure and aid in understanding one or more of various disclosed aspects, in the above description of exemplary embodiments of the present disclosure, various features of the present disclosure are sometimes grouped together in a single embodiment, drawing or description thereof. Description with reference to the terms such as “one embodiment”, “some embodiments”, “example”, “specific example” or “some examples” indicate that specific features, structures, materials or characteristics described in combination with this embodiment or example are included in at least one embodiment or example of the present disclosure. In the present description, exemplary expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples.

In addition, the terms “first” and “second” are used for descriptive purpose only, but should not be construed as indication or implication of importance in the relativity or implicit indication of the number of referred technical features. Thus, for a feature defined with the terms “first” and “second”, one or more such features may be explicitly or implicitly included. In the description of the present disclosure, the meaning of the term “multiple (a plurality of)” indicates at least two, e.g. two or three, unless otherwise expressly defined.

In the present embodiment, main design parameters of an imaging system are shown in the following table:

TABLE 1Design ParameterValueOperating Wave Band900-1700nmField of View178°System Focal Length F4.596mmImage Side F#4Total Track Length39.761mm

The operating wave band of the system can further be widened to 2000 nm and a wider spectrum through optimization on a metasurface structure. In the present embodiment, for ease of description, a main response wave band 900-1700 nm of a near-infrared detector is chosen for description.

In the present embodiment, when an operating bandwidth is the near-infrared wave band 900-1700 nm, an operating field of view may reach 178°, and in this case, the system satisfies 30 mm<TTL<40 mm, 1<φ×BFL<4, where TTL is a total track length of the system, Y is focal power of the system, and BFL is an optical back focal length of the system. It should be noted that for different wave band ranges, ranges of the TTL, φ and BFL of the system need to be redesigned. As shown inFIG.1, from an object side to an image side, a first lens L1having negative focal power, a linear polarizer P1, a quarter-wave plate P2, a diaphragm STO, a second lens L2having positive focal power, a third lens L3having positive focal power and a metasurface M are included in sequence.

In the present embodiment, Nd1>1.5, Nd2>1.8, Nd3>1.8, Vd1-Vd2>17, and Vd1-Vd3>17, where Nd1 is a refractive index of the first lens, Nd2 is a refractive index of the second lens, Nd3 is a refractive index of the third lens, Vd1 is a dispersion coefficient of the first lens, Vd2 is a dispersion coefficient of the second lens, and Vd3 is a dispersion coefficient of the third lens.

The first plane-concave lens L1has the negative focal power, the second plane-convex lens L2has the positive focal power, and the third biconvex lens L3has the positive focal power. Object side surfaces and image side surfaces of the first lens, the second lens and the third lens are planar or spherical surfaces, only the three spherical lenses are involved in the whole system, and others are all planar optical elements, which facilitates batch processing and has a low assembling and integration difficulty. By combining the refractive lenses having the focal power and the metasurface, a refractive/meta hybrid system is formed, and broadband ultra-wide-angle imaging is realized. In the present embodiment, for convenience of illustration in the drawings, the first lens L1is a plane-concave lens, the second lens L2is a plane-convex lens, and the third lens L3is a biconvex lens. The three spherical lenses are not limited in specific shape, and those skilled in the art could make a choice as required. In addition, in the present embodiment, in order to improve light transmittance, all of the three spherical lenses in the system achieve 98% transmittance by double-sided coating, and all of the linear polarizer, the quarter-wave plate and the metasurface structure are improved in transmittance by coating and reduced in reflectivity. The above components may also be of a non-coating type, which could be selected by those skilled in the art as required and is not limited herein.

In the present embodiment, parameters of system elements are as follows. Numbers marked in Surface are gradually increased in an order from the object side to the image side (from S1 to S13):

TABLE 2RadiusSpacingRefrac-of Cur-or Thick-tiveAbbeSemi-vaturenessIndexNumberdiameterSurfaceName(mm)(mm)NdVd(mm)S1First lensInfinite3.81.51764.19913.2S2L16.37210.9185.974S3LinearInfinite2.51.45867.8215polarizerP1S4quarter-Infinite0.451.76872.2375S5wave plateInfinite1.41.89P2S6DiaphragmInfinite2.81.229STOS7SecondInfinite3.61.81646.5497.5S8lens−13.61417.5L2S9Third lens193.31.81646.5497.5S10L3−45.2391.57.5S11Meta-Infinite0.4751.76872.2377.5S12surfaceInfinite8.0187.5MS13Image——4.589surface

It should be noted that for different of wave band ranges, the above element parameters may be specifically designed using target values such as system focal length and F #, without being bound to the above specific values. The linear polarizer P1and the quarter-wave plate P2may be separated or integrated with each other, which is not limited herein. In order to save space, in the present embodiment, the linear polarizer P1and the quarter-wave plate P2are integrated together. The linear polarizer P1and the quarter-wave plate P2are used for realizing polarization imaging, that is, the linear polarizer and the quarter-wave plate in the system are coaxial, and can rotate at the same speed about an optical axis, so as to convert different linearly polarized light in the light emitted, reflected or radiated by an imaged target into circularly polarized light, and by making use of the characteristic of the metasurface M being sensitive to a polarization state of incident light, and realize polarization imaging of light in different polarization states, thus acquiring polarization information on the target, and realizing the polarization imaging. In the present embodiment, polarization imaging of light in different directions may be obtained by remotely controlling rotation of the linear polarizer and the quarter-wave plate by a remote-control miniature motor, so that it may be directly used for long-distance imaging at a distance of up to 1 km, instead of being merely restricted to laboratory use, thus greatly improving the use convenience and practicality thereof. In the present embodiment, the miniature motor is chosen to regulate and control the rotation of the linear polarizer and the quarter-wave plate, but those skilled in the art could also select, according to actual use, devices that drive the linear polarizer and the quarter-wave plate to rotate, which is not limited herein.

The linear polarizer is used for screening out polarized light in an specific polarization direction, and the quarter-wave plate is used for converting the linearly polarized light into the circularly polarized light. They both may be of deep subwavelength structures or conventional components, which is not limited herein. In order to reduce a system length, the linear polarizer may also be the deep subwavelength structures, wherein the unit structure thereof has a period value range of 0.08λ0˜0˜0.6λ0, a width value range of 0.04λ0˜0.08λ0, and a height value range of 0.08λ0˜0˜0.5λ0. Meanwhile, the quarter-wave plate may be deep subwavelength structures, wherein a period is p and 0.1λ0<p<0.5λ0; a unit structure width is w and 0.05λ0<w<0.2λ0; a unit structure height is h and 0.1λ0<h<λ0, and a base thickness is not more than 0.5 mm. In the above, λ0is center wavelength. In the present embodiment, the quarter-wave plate P2is of deep subwavelength structures. As shown inFIG.2, the unit structure thereof is a strip-shaped subwavelength silicon nanorod, which has a width w of 156 nm, a period p of 264 nm, and a height h of 300 nm, and a substrate of sapphire with a thickness of 0.45 mm, and the quarter-wave plate achieves average efficiency of 98.698%. The linear polarizer P1is of deep subwavelength structures, and a unit structure thereof is a strip-shaped subwavelength silicon nanorod, which has a width of 64 nm, a period of 153 nm, a height h of is 120 nm, and linear polarizer has the same substrate as the quarter-wave plate. Due to the use of the deep subwavelength structures, the system is small in volume and light in weight, and thus is easy to be mounted on a flight platform to realize the ultra-wide-angle broadband polarization imaging.

Meanwhile, most of the existing wide-angle imaging systems use lenses of an aspherical surface type and conventional diffractive optical elements, and are greatly restricted in efficiency, bandwidth and functional diversity. Discrete steps structure or nanorods structure of a different effective refractive index used in such system has different phase responses to different wavelengths, and thus the operating bandwidth is limited. Moreover, it is insensitive to polarization, and cannot realize polarization imaging detection. However, the metasurface M used in the present embodiment is based on a geometric phase principle: (Φ=±2θ, that is, a phase generated by the metasurface is two times a rotation angle of the unit structure thereof, and only by rotating the unit structures, phase distribution required by the system under a wide spectrum can be realized. As geometric phase only depends on the rotation angle of the unit structure and is independent of the wavelength, the phase responses of the metasurface to different wavelengths are kept consistent, and thus can operate under an ultra-wide band, without being restricted by the wavelength or bandwidth. Since the geometric phase principle is used, the metasurface in the system is polarization-sensitive, polarization information on the imaged target can be acquired using this characteristic, to perform the polarization imaging detection, thus further expanding richness of information acquired by the imaging system, and being of great significance to fields such as anti-counterfeiting surveillance.

In the present embodiment, the phase distribution of a surface type of a light incident surface S11(see Table 2) of the metasurface M satisfies the following equation:
Φ=MorderΣi=1NAiρ2iwhere Φ represents phase, Morderis order of diffraction, N is the maximal degree of polinomial in series, ρ is normalized radial aperture coordinate, Aiis coefficient of the 2i-th term of ρ. Here, Morderis the first order, N is 4, and a value of ρ is equal to a ratio of r to R, where r is a radius value corresponding to different positions in a plane of the metasurface, a maximum value of r is actual radius of the metasurface M, R is normalized radius, and R is 1 mm herein. It should be noted that diagram (c) inFIG.3is only an intuitive schematic diagram, but is not a diagram representing actual structure distribution. Actual phase distribution in the present embodiment needs to meet the above formula.

In the present embodiment, the metasurface M is a binary plane2, whose polinomial coefficient Aiare shown in the following table.

TABLE 3QuadraticQuarticSix-PowerEight-PowerTermTermTermTermSurfaceCoefficientCoefficientCoefficientCoefficientTypeA1A2A3A4Binary−15.2251640.0714807−0.00298365967.5031731e−5Plane 2

That is, the phase distribution of the metasurface satisfies the following equation:
Φ=−15.225164ρ2+0.0714807ρ4−0.0029836596ρ6+7.5031731e−5ρ8

Through optimization on the subwavelength unit structures forming the metasurface, the efficiency may be greatly improved, and high-efficiency imaging with average efficiency of 97.869% can be achieved, as shown inFIG.3. In the present embodiment, the phase distribution achieved by this surface type is achieved by the metasurface. The metasurface is composed of the deep subwavelength structures. As shown inFIG.3, the unit structure thereof is a subwavelength silicon nanorod, of which a width wMis 166 nm, a period pyin y direction is 277 nm, a spacing between structures in x direction is 100 nm, a height hMis 600 nm, and a substrate is sapphire with a thickness of 0.475 mm, and the metasurface achieves average efficiency of 97.869%. The implementation of the phase is according to the geometric phase principle, Φ=±2θ, where Φ is phase of the unit structure, θ is rotation angle of the unit structure, which is positive or negative depending on a polarization state of circularly polarized light. It can be seen therefrom that the geometric phase-based metasurface can achieve arbitrary phase distribution only by rotating the unit structures, the phase thereof is only related to the rotation angle and the polarization state, and is independent of the wavelength, and thus it can operate under an ultra-wide band, without being restricted by the wavelength or bandwidth, and can also realize the polarization imaging.

The use of the metasurface, on one hand, can well correct the chromatic aberration, and on the other hand, can reduce the complexity of the system, reduce the use of conventional lenses, improve the integration degree of the whole system, reduce the weight and volume of the system, and reduce the assembly difficulty. Meanwhile, the novel function of the metasurface can also further expand the functions of the system, such as the polarization imaging function in the embodiments.

In the present embodiment, the optical transfer function graph of the system is shown inFIG.4. It can be seen from the graph that an optical transfer function curve of the system is quite close to a diffraction limit curve, and at 50 1p/mm, an optical transfer function (OTF) modulus is still above 0.5. In optical imaging design evaluation, if the OTF modulus at a cutoff frequency is higher than 0.5, the system can be considered as having good imaging quality. In this system, if a pixel size of the detector at the spectral wave band of the operating wave band 900-1700 nm is 15 μm, the cutoff frequency of the detector is 33.3 1p/mm, and in this case, the OTF modulus of the system at this frequency is higher than 0.6. Therefore, the imaging performance of this system is good.

In the present embodiment, the optical spot diagrams of the system are shown inFIG.5. The optical spot diagram is another important index for evaluating the optical imaging quality, which reflects a size of defocused spot, and when a dimension of the defocused spot is closer to Airy disk, the imaging quality of the system is higher. It can be seen from the diagrams that under various fields of view, RMS radii of defocused spots of the system are all smaller than Airy disk radius; even at the maximum half field-of-view angle of 89° C., the RMS radius is still smaller than the Airy disk radius; and GEO radii of the defocused spots are also close to the Airy disk radius; therefore, the imaging quality of the system is high.

In the present embodiment, graphs of axial chromatic aberration and lateral chromatic aberration of the system are shown inFIG.6. The axial chromatic aberration graph shows the axial chromatic aberration of different wavelengths as a function of entrance pupil height, where ordinate represents pupil height and abscissa represents the axial chromatic aberration. It can be seen from the graph that axial chromatic aberration of various wavelengths of the system at various entrance pupil heights is less than 0.1 mm, which is less than a product (0.12 mm) of the pixel of the common detector in this spectrum and twice the F number; therefore, the axial chromatic aberration of the system is small. The lateral chromatic aberration graph shows differences of distances between various wavelengths and an intersection point between a dominant wavelength and an image plane, where ordinate represents field of view and abscissa represents the lateral chromatic aberration. It can be seen from the graph that the lateral chromatic aberration of various wavelengths of the system in various fields of view is less than 8 m, which is less than the pixel (15 μm) of the common detector in this spectrum; therefore, the lateral chromatic aberration of this system graph is small, and the performance of the system is good. In conclusion, the chromatic aberration of the system in the whole operating wave band and in the whole wide-angle field of view can be well corrected.

The above imaging system may be mounted on a detection apparatus, for realizing multi-scene imaging.

It can be seen therefrom that the metasurface-based ultra-wide-angle broadband polarization imaging system provided in the present disclosure has good imaging performance.

The above design process, embodiments and simulation results well verify the present disclosure.

Therefore, the principle and embodiments of the present disclosure are described in the above in conjunction with the drawings, while the present disclosure is not limited to the above embodiments, and the above embodiments are merely illustrative rather than restrictive. Under the guidance of the present disclosure, various improvements and variations made on this basis, including surface types of lenses, replacement, increase and decrease of materials, wave band changes, etc., all fall within the scope of protection of the present disclosure. The present disclosure does not elaborate the well-known technologies to those skilled in the art.