Double-layer metasurface light beam scanning element for laser radar

Abstract

The invention discloses a double-layer metasurface light beam scanning element for a laser radar. The double-layer metasurface light beam scanning element comprises two parallel medium metasurfaces which are sequentially arranged on the same optical axis. Incident light passes through the two layers of medium metasurfaces in sequence, two times of different phase regulation and control are achieved, and then deflection of the incident light is achieved. And the two layers of medium metasurfaces are respectively rotated around the optical axis, and the emergent light beam regularly deflects in any direction within a certain field angle, so that the directional scanning of the light beam is realized. The element has the characteristic of polarization insensitivity, and when incident beams in different polarization states are regulated and controlled, scanning positions obtained on an object to be scanned are completely consistent. The double-layer metasurface light beam scanning element provided by the invention can realize deflection in any direction in a certain view field angle according to the rotation angle of the metasurface, can perform directional scanning in the view field range, and has great application potential in the field of laser radars. Meanwhile, due to the introduction of the metasurface, the problem that a traditional laser radar optical system is complex and huge is solved, and the requirements for miniaturization and integration of modern optical elements are met.

Description

Technical Field

[0001] The present invention belongs to the technical field of lidar scanning, and particularly relates to a double-layer metasurface beam scanning element for lidar. Background Art

[0002] Since the advent of lidar, its characteristics of high precision, high resolution, and fast response have quickly made it one of the core technologies in the fields of scanning and ranging. A lidar emits laser pulses and receives their reflected signals, and uses the time-of-flight principle to calculate the time difference between the emission and return of the light pulses, thereby accurately measuring the distance to the target object. This method can not only achieve single-point ranging, but also perform omnidirectional and high-density three-dimensional point cloud data acquisition of the surrounding environment through a scanning mechanism (such as mechanical rotation, MEMS mirrors, or optical phased arrays). Traditional lidar systems rely on bulky mechanical rotating components and complex optical elements to achieve beam scanning and target detection. These systems usually include multiple lenses, mirrors, and prisms for focusing and guiding the laser beam. This design results in a large volume and weight of its internal optical system, and it has extremely high requirements for mechanical alignment accuracy, which poses severe challenges to the cost and reliability of traditional lidar. In addition, the presence of mechanical moving parts limits the scanning speed and system life, while also increasing power consumption and maintenance requirements. With the rapid development of fields such as autonomous driving, robot navigation, and intelligent perception, the demand for miniaturization, integration, and high performance of lidar systems is becoming increasingly urgent.

[0003] Recently, the emergence of metasurfaces has provided new possibilities for the innovation of lidar systems. Compared with traditional optical elements, metalenses have the advantages of small size, light weight, and easy integration. Applying metalenses to lidar systems can significantly simplify the optical system structure, eliminate mechanical moving parts, and thus achieve all-solid-state beam scanning. This design not only improves the reliability and scanning speed of the system but also reduces power consumption and manufacturing costs. In 2020, He et al. (He M, Guo Y, Li C, et al. Metasurface-based wide-angle beam steering for optical trapping[J]. IEEE Access, 2020, 8: 37275-37280) achieved beam deflection by changing the structure of metasurface units. They adjusted two types of structural units of the metasurface to change the geometric phase to achieve the beam deflection function and realized scanning within a certain area. However, the adjustment of the structural units inevitably increases the complexity of the design. In the same year, Yin et al. (Yin S, He F, Kubo W, et al. Coherently tunable metalens tweezers for optofluidic particle routing[J]. Optics Express, 2020, 28(26): 38949-38959) achieved the screening of different beams by changing the incident conditions of the incident beam, ensuring that the phase of the output light only depends on the geometric shape of the nanocolumns and the illumination conditions. Compared with the former's work, the latter reduces the design complexity of the metasurface, but using a single metasurface makes it difficult to meet the requirements of various applications at this stage in terms of the scanning field of view and scanning accuracy of the beam. Summary of the Invention

[0004] The object of the present invention is to provide a double-layer metasurface beam scanning element for lidar in view of the problems of high complexity, small scanning field of view, and low scanning accuracy existing in the existing metasurface beam scanning elements.

[0005] The technical solution to achieve the object of the present invention is: A double-layer metasurface beam scanning element for lidar, the double-layer metasurface beam scanning element includes two parallel dielectric metasurfaces arranged coaxially and successively.

[0006] An incident light beam passes through the two dielectric metasurfaces successively, realizing two different phase modulations, and further realizing the deflection of the incident light beam; by rotating the two dielectric metasurfaces around the optical axis respectively, the outgoing light beam deflects regularly in any direction within a certain field of view, and further realizes the directional scanning of the light beam.

[0007] Furthermore, the dielectric metasurface includes a plurality of metasurface structural units arranged in an array, and the entire dielectric metasurface has the same phase gradient from left to right and from top to bottom.

[0008] Furthermore, the metasurface structural unit is a cylindrical nano-unit structure with symmetry.

[0009] Furthermore, the phase change of the incident light by the metasurface structural unit satisfies:

[0010]

[0011] where φ WG represents the phase change amount of the incident light by the metasurface structural unit, λ d represents the wavelength of the incident light, h is the height of the dielectric cylinder, n eff is the effective refractive index, β is the propagation function, and k 0 is the free space wavenumber.

[0012] Furthermore, the cylindrical nano-unit structure with symmetry includes a glass substrate and a high-refractive-index titanium dioxide cylinder disposed on the glass substrate.

[0013] Furthermore, the radius of the high-refractive-index titanium dioxide cylinder is adjustable, ranging from 30 nm to 120 nm, to cover a phase change of 0 to 2π.

[0014] Furthermore, the set parameters and theoretical working performance of the double-layer metasurface beam scanning element need to satisfy:

[0015]

[0016] where (X, Y) are the coordinates where the beam is projected onto the object to be scanned, D 1 is the distance between the two dielectric metasurfaces, D 2 is the distance between the second dielectric metasurface and the object to be scanned, θ 1 is the rotation angle of the first dielectric metasurface around the optical axis, θ 2 is the rotation angle of the second dielectric metasurface around the optical axis, ψ is the acute angle between the path of the beam emerging from the second dielectric metasurface and the z-axis, and φ is the acute angle between the perpendicular to the optical axis at the final beam landing position and the x-axis; G 1 is the phase gradient of the first dielectric metasurface, and k 0 is the free space wavenumber; the z-axis is along the direction of the optical axis, and the x-axis is the horizontal direction perpendicular to the z-axis in the plane of the dielectric metasurface.

[0017] Furthermore, the double-layer metasurface beam scanning element has polarization-insensitive characteristics. When modulating incident light beams with different polarization states, the scanning positions obtained on the object to be scanned are exactly the same.

[0018] Compared with the prior art, the remarkable advantages of the present invention are as follows:

[0019] (1) The optical metasurface in the present invention can replace the combined functions of traditional optical elements such as spatial light modulators and diffractive optical elements in traditional radar systems. Its core function depends on the micro-nano arrangement structure of titanium dioxide nanocylinders, and different optical modulation effects are achieved by adjusting the arrangement mode. This design can flexibly modulate incident light while meeting the requirements of miniaturization and integration of optical systems.

[0020] (2) Compared with the existing single-layer metasurface scanning element, after designing the two-layer dielectric metasurface in the present invention, only by adjusting its rotation angle can beam scanning within a certain range be achieved, which is efficient, simple and easy to implement.

[0021] (3) The double-layer metalens beam scanning element used to replace the beam deflection element of lidar in the present invention has advantages such as a larger scanning angle and higher accuracy compared with the traditional double-layer optical wedge system and galvanometer.

[0022] The present invention will be further described in detail below with reference to the accompanying drawings. Description of the Drawings

[0023] Figure 1 It is a schematic diagram of the composition of a double-layer metasurface beam scanning element for lidar in an embodiment.

[0024] Figure 2 It is a result diagram of the scanning range of a small-size double-layer metalens system obtained theoretically in an embodiment.

[0025] Figure 3(a) is a three-dimensional schematic diagram of a metasurface structural unit in an embodiment, and Figure 3(b) is a schematic diagram of the cross-section of the metasurface structural unit in the xoy plane.

[0026] Figure 4 It is a partially enlarged schematic diagram of the metasurface in an embodiment.

[0027] Figure 5(a) is a result diagram of the phase distribution in the xoz plane modulated by a double-layer metasurface with rotation angles of (45°, 45°) respectively in an embodiment, and Figure 5(b) is a result diagram of the beam deflection under this condition.

[0028] Figure 6 It is a simulation result diagram of the scanning range of simultaneously rotating the double-layer metasurface beam scanning element in an embodiment. Detailed Embodiment

[0029] To make the objectives, technical solutions, and advantages of this application more clearly understood, the following further elaborates on this application in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for explaining this application and are not intended to limit this application.

[0030] It should be noted that if there are directional indications (such as up, down, left, right, front, back...) involved in the embodiments of the present invention, then such directional indications are only used to explain the relative positional relationship, movement conditions, etc. between components in a specific posture (as shown in the accompanying drawings). If this specific posture changes, then the directional indications will also change accordingly.

[0031] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, then such descriptions of "first", "second", etc. are only for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one such feature. Additionally, the technical solutions between various embodiments can be combined with each other, but it must be based on the ability of those of ordinary skill in the art to implement. When the combination of technical solutions results in contradictions or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0032] In one embodiment, in combination with Figure 1 , a double-layer metasurface beam scanning element for lidar is provided. The double-layer metasurface beam scanning element includes two parallel dielectric metasurfaces arranged coaxially in sequence.

[0033] The incident light passes through the two dielectric metasurfaces in sequence, achieving two different phase modulations, and thus realizing the deflection of the incident light. By rotating the two dielectric metasurfaces around the optical axis respectively, the outgoing beam deflects regularly in any direction within a certain field of view, thereby realizing the directional scanning of the beam.

[0034] The double-layer metasurface beam scanning element has polarization-insensitive characteristics. When modulating incident beams with different polarization states, the scanning positions obtained on the object to be scanned are exactly the same.

[0035] Furthermore, in one of the embodiments, each of the dielectric metasurfaces includes a plurality of metasurface structural units arranged in an array, and the entire dielectric metasurface has the same phase gradient from left to right and from top to bottom.

[0036] Preferably, in some embodiments, the metasurface structural unit is a cylindrical nano-unit structure with symmetry, and its phase change of the incident light satisfies:

[0037]

[0038] Among them, φ WG represents the phase change amount of the incident light by the metasurface structure unit, λ d represents the incident light wavelength, h is the height of the dielectric cylinder, n eff is the effective refractive index, β is the propagation function, k 0 is the free plane wave number.

[0039] Further preferably, the cylindrical nano-unit structure with symmetry includes a glass substrate and a high-refractive-index titanium dioxide cylinder disposed on the glass substrate, as shown in FIGS. 3(a) and 3(b).

[0040] Preferably, the size of each layer of the dielectric metasurface is 6μm * 6μm and includes 24 * 24 structure units.

[0041] Preferably, the entire dielectric metasurface has the same phase gradient of 3141.5 rad / mm from left to right and from top to bottom.

[0042] Preferably, the lattice constant of the metasurface structure unit is 250 nm.

[0043] Preferably, the height of the high-refractive-index titanium dioxide cylinder is 600 nm, and the radius is adjustable, ranging from 30 nm to 120 nm, so as to cover the phase change of 0 to 2π.

[0044] Here, according to the propagation phase calculation formula, there is a certain relationship between the phase delay amount increased by the nano-phase column to the light beam and the radius of the phase column. Therefore, the phase delay applied by the phase column to the light beam within the range of 0 - 2π can be changed by changing the radius of the nano-phase column. The relationship between the phase columns with different radii within one period and the phase delay values applied by the phase columns to the light beam can be obtained through simulation measurement. In order to obtain this specific relationship, parameter scanning needs to be performed in the simulation software. In this embodiment, the Lumerical FDTD Solutionos simulation software is used for simulation.

[0045] In the database of the radius of the circular nano-column and the phase delay, 8 kinds of high-refractive-index dielectric cylinders within the radius range of 30 nm to 120 nm are discretely selected. The 8 selected basic structure units basically cover the phase change of 0 - 2π, and the radii are 37.8 nm, 54.2 nm, 64.2 nm, 72.8 nm, 80.8 nm, 88.4 nm, 96.2 nm, and 104.2 nm respectively.

[0046] Furthermore, in one of the embodiments, the set parameters and the theoretical working performance of the double-layer metasurface beam scanning element need to satisfy:

[0047]

[0048] where (X, Y) are the coordinates where the light beam is projected onto the object to be scanned, D 1 is the distance between the two layers of the dielectric metasurface, D 2 is the distance between the second layer of the dielectric metasurface and the object to be scanned, θ 1 is the rotation angle of the first layer of the dielectric metasurface around the optical axis, θ 2 is the rotation angle of the second layer of the dielectric metasurface around the optical axis, ψ is the acute angle between the path of the light beam emerging from the second layer of the dielectric metasurface and the z-axis, and φ is the acute angle between the perpendicular to the optical axis passing through the final landing position of the light beam and the x-axis; G 1 is the phase gradient of the first layer of the dielectric metasurface, k 0 is the free-space plane wave number; the z-axis is along the direction of the optical axis, and the x-axis is the horizontal direction perpendicular to the z-axis in the plane of the dielectric metasurface.

[0049] As a specific example, in some embodiments, the present invention is further verified.

[0050] In this embodiment, the object to be scanned is a light screen, D 1 is set to 4 μm, D 2 is set to 3 m, and other parameters adopt the above-mentioned preferred parameters. The double-layer metasurface light beam scanning element of the present invention can control the deflection of the light beam in different directions by controlling the different rotation angles of the two layers of metasurfaces, so that the final landing points of the light beam after being modulated by the double-layer metasurface system are different. Finally, all the landing points are displayed on the light screen, and it can be seen that the landing points basically cover an annular region, as Figure 4 shown. This region is the theoretical scanning range of the double-layer superlens system.

[0051] In the superlens simulation system, two far-field monitors are respectively placed between the first layer of the dielectric metasurface and the second layer of the dielectric metasurface, and at the light screen, for observing the far-field results of the light beam passing through the first superlens and the double-layer superlens system respectively.

[0052] Fig. 5(a) is a diagram of the phase distribution result in the x-z plane regulated by the phase of the double-layer dielectric metasurface with rotation angles of (45°, 45°) respectively. It can be seen that after the incident light is regulated by the phase of the double-layer dielectric metasurface with rotation angles of (45°, 45°) respectively, the phase direction is significantly at a certain angle with the z-axis direction, indicating that the incident light is deflected after being modulated by the nanorods. Fig. 5(b) is a diagram of the light beam deflection result under this condition, proving that the present invention can achieve light beam deflection as required. Lens1 and lens2 in the figure respectively represent the first layer of the dielectric metasurface and the second layer of the dielectric metasurface.

[0053] Next, the scanning range and scanning accuracy of the present invention are verified by modifying the rotation angle of the metasurface. Under the condition of ensuring that the structural parameters of the metasurface and the system settings remain unchanged, the rotation angle of the first-layer dielectric metasurface of the metasurface is fixed at 0°, and the second-layer dielectric metasurface structure rotates with a certain step. The landing point of the light beam on the light screen after being deflected by the double-layer metasurface light beam scanning element can be approximately regarded as a water droplet shape. On this basis, the first-layer dielectric metasurface is also rotated with a certain step, while keeping the second-layer dielectric metasurface rotating one week unchanged. In this way, the final scanning result can be regarded as rotating the landing point result of the deflected light beam obtained by only rotating the second-layer dielectric metasurface one week around the optical axis of the incident light. The circular ring area swept by the water droplet finally is the range that the deflected light beam can reach, and it is also the scanning result after the light beam is deflected and emitted in different directions, as Figure 6 shown, it can be seen that the scanning coverage in a certain area is basically achieved, and at the same time, extremely high scanning accuracy is ensured.

[0054] In summary, the double-layer metasurface light beam scanning element provided by the present invention can achieve deflection in any direction within a certain field of view according to the rotation angle of the metasurface, and can perform directional scanning within this field of view, having great application potential in the field of lidar. At the same time, the introduction of the metasurface solves the problem of the complex and large traditional lidar optical system, meeting the requirements of miniaturization and integration of modern optical elements.

[0055] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A double-layer metasurface beam scanning element for laser radar, characterized in that: The double-layer metasurface beam scanning element comprises two parallel dielectric metasurfaces arranged in sequence with the same optical axis; The incident light passes through two layers of dielectric metasurfaces in sequence, achieving two different phase controls, thereby realizing the deflection of the incident light; the two layers of dielectric metasurfaces are rotated around the optical axis respectively, and the outgoing light beam is regularly deflected in any direction within a certain field of view, thereby realizing directional scanning of the light beam.

2. The double-layer metasurface beam scanning element for laser radar according to claim 1, characterized in that: The dielectric metasurface includes a plurality of metasurface structural units arranged in an array, and the entire dielectric metasurface has the same phase gradient from left to right and from top to bottom.

3. The double-layer metasurface beam scanning element for laser radar according to claim 2, characterized in that: The super surface structural unit is a cylindrical nano unit structure with symmetry.

4. The double-layer metasurface beam scanning element for laser radar according to claim 3, characterized in that: The phase change of the incident light by the metasurface structure unit satisfies: Among them, φ WG The phase variable of the incident light to the metasurface structure unit, λ d represents the wavelength of the incident light, h is the height of the dielectric cylinder, and n eff is the effective refractive index, β is the propagation function, and k0 is the free plane wave number.

5. The double-layer metasurface beam scanning element for laser radar according to claim 3, characterized in that: The symmetrical cylindrical nanometer unit structure comprises a glass substrate and a high-refractive-index titanium dioxide cylinder arranged on the glass substrate.

6. The double-layer metasurface beam scanning element for laser radar according to claim 5, characterized in that: The radius of the high refractive index titanium dioxide cylinder is adjustable from 30nm to 120nm to cover the phase change from 0 to 2π.

7. The double-layer metasurface beam scanning element for laser radar according to claim 1, characterized in that: The setting parameters and theoretical working performance of the double-layer metasurface beam scanning element must meet the following requirements: Wherein, (X, Y) is the coordinate of the light beam projected onto the object to be scanned, D1 is the distance between the two layers of dielectric metasurfaces, D2 is the distance between the second layer of dielectric metasurface and the object to be scanned, θ1 is the rotation angle of the first layer of dielectric metasurface around the optical axis, θ2 is the rotation angle of the second layer of dielectric metasurface around the optical axis, ψ is the acute angle between the path of the light beam after passing through the second layer of dielectric metasurface and the z-axis, and φ is the acute angle between the perpendicular line of the optical axis passing through the final beam landing point and the x-axis; G1 is the phase gradient of the first layer of the dielectric metasurface, k0 is the free plane wave number; the z-axis is the direction along the optical axis, and the x-axis is the horizontal direction perpendicular to the z-axis in the plane where the dielectric metasurface is located.

8. The double-layer metasurface beam scanning element for laser radar according to claim 1, characterized in that: The double-layer metasurface beam scanning element has a polarization-insensitive characteristic. When the incident beams with different polarization states are regulated, the scanning positions obtained on the object to be scanned are completely consistent.

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