A large field-of-view, high-precision solar sensor based on polarization-multiplexed metasurface array

Abstract

This invention discloses a large field-of-view, high-precision sun sensor based on a polarization-multiplexed metasurface array. It includes a CMOS image sensor and a metasurface array serving as a light introducer. The metasurface array achieves field-of-view multiplexing through multiple superimposed metalens units with different deflection phases. Simultaneously, using polarization multiplexing technology, each metalens unit forms a pair of spaced dual-spots on the CMOS image sensor as a line-of-view encoding. By calculating the spacing between these dual-spots, each sub-field of view can be unambiguously identified and the global field of view synthesized, thus resolving the inherent contradiction between large field of view and high precision in traditional digital sun sensors. This invention combines the advantages of a large field of view and high precision, and its lightweight design adapts to high-density integration requirements, providing technical support for spacecraft attitude measurement.

Description

Technical Field

[0001] This invention relates to solar sensors, and more specifically to a large field-of-view, high-precision solar sensor based on a polarization-multiplexed metasurface array. Background Technology

[0002] Sun sensors provide a spacecraft's orientation relative to the sun by detecting the direction of the sun's vector. Traditional sun sensors are mainly divided into two categories: analog and digital. Analog sun sensors (such as four-quadrant detectors) estimate the sun's angle by measuring the difference in photocurrent. However, due to sensor noise and operating principles, their accuracy is usually low. Digital sun sensors, on the other hand, form a solar spot on an image sensor using a light-introducing device (such as a pinhole or slit). The angle is calculated by determining the offset of the spot's center. This method offers higher accuracy than analog sun sensors and has become the mainstream solution.

[0003] However, the performance improvement of traditional digital sun sensors has always been limited by a core physical contradiction: a large field of view and high precision are difficult to achieve simultaneously. The working principle of a digital sun sensor is as follows: Figure 1 As shown, assume the distance between the plane of the light introducer 1 and the plane of the CMOS image sensor 3 is... When the angle of incidence of sunlight is At that time, the distance the sunspot has moved compared to when it was directly incident is ,according to An estimated value of the angle of incidence of the light can be calculated. For a pixel size of The size of the imaging area is CMOS image sensor 3, solar incident angle resolution The single-axis field of view of the sun sensor To achieve high precision, the distance between the light introducer 1 and the CMOS image sensor 3 needs to be increased. To improve angular resolution, but this directly leads to a narrowing of the field of view; conversely, if a larger field of view is obtained by reducing the distance between the light introducer 1 and the CMOS image sensor 3... If a fisheye lens is introduced, the measurement accuracy will be significantly degraded due to decreased resolution or severe aberrations.

[0004] In summary, traditional digital sun sensors are limited by optical principles and cannot achieve both a large field of view and high precision. This results in problems such as large size and low integration, making it difficult to meet the urgent needs of micro and nano satellite platforms for lightweight, high-performance sun sensors. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a large field of view, high precision, and miniaturized solar sensor based on a polarization multiplexing metasurface array.

[0006] Technical Solution: This invention provides a large field-of-view, high-precision solar sensor based on a polarization-multiplexed metasurface array, comprising a light introducer, a CMOS image sensor, and a signal processing unit. The light introducer employs a metasurface array, and the CMOS image sensor is positioned on the focal plane of the metasurface array. The metasurface array has multiple metalens units with superimposed deflection phases, each metalens unit corresponding to a different sub-field of view in the total field of view of the solar sensor. The metalens units are polarization-multiplexed metalens units, configured to produce different optical responses to any two orthogonal incident polarization states (e.g., left-handed circularly polarized light and right-handed circularly polarized light), such that for sunlight with a single incident direction (unpolarized or partially polarized sunlight), a pair of spaced double spots are formed on the CMOS image sensor. The spacing between the double spots serves as a line-of-view encoding and has a unique mapping relationship with the preset deflection phase of the metalens unit that generates the double spots, used to identify the metalens unit that generates the double spots and its corresponding sub-field of view. The signal processing unit processes the image acquired by the CMOS image sensor and calculates the angular information of the solar vector.

[0007] In this invention, a metasurface array achieves field-of-view reuse through multiple superimposed metalens units with different deflection phases. These metalens units have different deflection phases, thus each metalens unit corresponds to a unique sub-field of view in the total field of view of the solar sensor, thereby achieving field-of-view reuse and expansion. Simultaneously, this invention uses the spacing between the two light spots as the line-of-view encoding, which is the fundamental mechanism for achieving unambiguous recognition of multiple sub-fields of view and global field-of-view synthesis.

[0008] Furthermore, the signal processing unit is configured to perform the following steps: extracting the centroid position of the two light spots imaged on the CMOS image sensor; calculating the distance between the centroids of the two light spots, identifying the target metalens unit that generates the two light spots and its preset deflection phase based on the distance; and calculating the angle information of the solar vector by combining the centroid offset of the two light spots relative to the center of the target metalens unit, the focal length of the target metalens unit, and its deflection phase.

[0009] Furthermore, the dual light spots are selected from the pair of dual light spots closest to the center of the CMOS image sensor.

[0010] Furthermore, the metasurface array and the CMOS image sensor are respectively positioned on the upper and lower sides of the 3D-printed support structure to achieve integrated construction.

[0011] Furthermore, the relative distance between the metasurface array and the CMOS image sensor can be finely adjusted for focal length calibration.

[0012] Furthermore, the CMOS image sensor is connected to the support structure by screws, which can convert the rotation into axial displacement of the support structure, thereby enabling continuous adjustment of the distance between the metasurface array and the CMOS image sensor.

[0013] Furthermore, the metasurface array is fabricated using micro-nano fabrication processes, and it is formed on a fused silica substrate along with a light-shielding mask.

[0014] Metasurface arrays are mass-produced using standard semiconductor micro / nano fabrication processes, demonstrating the potential for large-scale production. This approach, which integrates complex optical systems onto a single chip, significantly reduces manufacturing costs and provides an ideal, cost-effective solution for applications such as large-scale constellation networking.

[0015] Furthermore, the metasurface array is arranged in a square grid or a hexagonal grid.

[0016] Furthermore, the geometry of the meta-lens unit is an elliptical cylinder, a rectangular cylinder, or a polygonal cylinder.

[0017] Furthermore, the material of the metasurface array is monocrystalline silicon, polycrystalline silicon, amorphous silicon, or silicon nitride.

[0018] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0019] (1) This invention achieves field-of-view subdivision by superimposing multiple metalens units with different deflection phases in a metasurface array, and provides a unique line-of-view code for each sub-field of view through polarization multiplexing. This achieves a large field of view coverage while maintaining the high angular resolution of a single metalens unit through the synthesis of multiple metalens units. This invention achieves a synergistic improvement in field of view and accuracy.

[0020] (2) The present invention uses an ultra-thin, planar metasurface array to replace the complex optical components such as stereo lenses and slit masks in traditional sun sensors, which greatly reduces the volume, weight and structural complexity of sun sensors. It is suitable for platforms with strict limitations on load, size and weight, such as micro-nano satellites and high-altitude long-endurance UAVs. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the working principle of a traditional digital sun sensor.

[0022] Figure 2 This is a schematic diagram of a large field-of-view, high-precision solar sensor based on a polarization-multiplexed metasurface array, provided by an embodiment of the present invention.

[0023] Figure 3 This is a schematic diagram of line-of-sight encoding based on a metasurface array in an embodiment of the present invention;

[0024] Figure 4 This is a schematic diagram of the nano-unit structure of the metasurface array in an embodiment of the present invention;

[0025] Figure 5 This is a schematic diagram of the fabrication process of the metasurface array in an embodiment of the present invention. Detailed Implementation

[0026] The invention will now be further described with reference to the accompanying drawings.

[0027] Appendix Figures 1 to 5 The accompanying figure labels are as follows:

[0028] 1. Light introducer; 2. Support structure; 3. CMOS image sensor; 4. Signal processing unit; 5. Light-shielding mask; 6. Fused silica substrate.

[0029] like Figure 2 As shown, this embodiment of the invention provides a large field-of-view, high-precision solar sensor based on a polarization-multiplexed metasurface array, including a light introducer 1, a support structure 2, a CMOS image sensor 3, and a signal processing unit 4. The light introducer 1, employing a metasurface array, is mounted on the upper side of the support structure 2 and is used to introduce and modulate sunlight. The CMOS image sensor 3 is disposed on the focal plane of the metasurface array by the support structure 2 and is used to acquire images formed by the modulated sunlight. The signal processing unit 4 is electrically connected to the CMOS image sensor 3 and is used to process the images acquired by the CMOS image sensor 3 to calculate the angular information of the solar vector.

[0030] Here is a specific example.

[0031] (a) Metasurface arrays

[0032] like Figure 3 As shown, a silicon nitride metasurface array and a metallic light-shielding mask 5 are formed on a fused silica substrate 6. The metasurface array consists of 7×7 metalens units, each 200μm×200μm in size, with the designed focal length of each metalens unit... The diameter is 20mm and the working wavelength is 550nm.

[0033] No. Line number List Phase distribution function of the metalens unit under left-handed circular polarization Phase distribution function under right-hand circular polarization for:

[0034]

[0035] in, , It is the operating wavelength; and The coordinates are two-dimensional rectangular coordinates on the metasurface plane; and Axis coding for determining the distance between two light spots.

[0036] for , The directions and incident angles are respectively , The incident light, after the first Line number Behind the array of metalens units, the wavefront phase of its left-hand circularly polarized and right-hand circularly polarized components. for:

[0037]

[0038] in, Is with , An irrelevant constant that does not affect the phase distribution.

[0039] Therefore, the generated dual light spots are located on the CMOS image sensor 3. and At that location, the two light spots are , Spacing in direction , They are respectively:

[0040]

[0041] set up , Then it can be calculated to ensure the first Line number The incident light from the dual light spots of the array of meta-lens units falls on the 6.44mm × 4.62mm imaging area of ​​the CMOS image sensor (Aptina MT9J003). Direction deflection angle satisfy:

[0042]

[0043] in, The shorter side of the CMOS image sensor 3 is 4.62 mm.

[0044] No. Line number Sub-field angle of a super-lens unit It can be represented as:

[0045]

[0046] Overall field of view of metasurface arrays It can be represented as:

[0047]

[0048] Since for any satisfy:

[0049]

[0050] Therefore, it can be guaranteed that within the overall field of view... Within any field of view, at least one complete pattern will be imaged on the CMOS image sensor 3. During the transition phase, before the image formed by one pattern leaves the CMOS image sensor 3, another pattern will be imaged on the CMOS image sensor 3, thus avoiding blind spots during the field of view transition.

[0051] To achieve the multiplexing of left-handed circular polarization (LCP) and right-handed circular polarization (RCP), a target Jones matrix is ​​constructed from nanoscale unit structures forming a metasurface array. Must meet:

[0052]

[0053] in, Represents the imaginary unit. This represents finding the inverse of a matrix.

[0054] Figure 4 In this diagram, P represents the periodicity of the nanounit structure arrangement, H represents the height of the rectangular prism, W represents the width of the rectangular prism, and L represents the length of the rectangular prism. The rotation angle is for the rectangular prism. This LCP and RCP multiplexed metasurface array consists of nanounit structures arranged with a period of 360 nm. These nanounit structures are 900 nm high silicon nitride rectangular prisms on a quartz substrate, and their actual Jones matrix... It can be represented as:

[0055]

[0056] in, The complex electric field of the dipole in the TE mode for this nanounit structure at a wavelength of 550 nm. The complex electric field of the dipole in the TM mode for this nanounit structure at a wavelength of 550 nm. It is the rotation angle of a rectangular prism. The corresponding rotation matrix satisfies .

[0057] To achieve polarization multiplexing, the metasurface array is located at... The actual Jones matrix of the nanounit structure at the location Should meet:

[0058]

[0059] For the target Jones matrix The target rotation angle of the metalens unit can be calculated by performing eigenvalue decomposition. With the goal and A database of the relationship between parameters such as the size or shape of nanoscale structures and their electromagnetic response can be constructed using the finite-difference time-domain (FDTD) method. Appropriate structural parameters can then be selected from this database to meet specific requirements. and The requirements are as follows. Based on the designed target phase response, the metalens units (including length, width, and rotation angle) in the metasurface array are determined.

[0060] like Figure 5 As shown, the metasurface array needs to be fabricated using micro-nano fabrication processes. The specific process includes: 1) Cleaning the fused silica substrate and depositing a silicon nitride dielectric layer on the substrate surface using plasma-enhanced chemical vapor deposition (PECVD). The thickness of this layer is the same as the height of the target columnar structure of the metalens unit, which is 900 nm; 2) Spin-coating photoresist and electron beam conductive adhesive; 3) Using an electron beam lithography system, exposing the patterned areas of the light-shielding mask and the overlay markings; 4) Removing the electron beam photoresist from the unexposed areas through development and fixing processes; 5) Depositing a Cr and Au metal thin film as a light-shielding layer on the entire substrate surface using a thermal evaporation process; 6) Using a resist stripper to perform a stripping process to remove the light-shielding mask. 7) Spin-coating photoresist and electron beam conductive adhesive; 8) Using an electron beam lithography system, the meta-lens unit structure pattern is exposed and the electron beam photoresist in the exposed area is removed through development and fixing processes, thereby forming a negative pattern corresponding to the meta-nanostructure on the photoresist layer; 9) A chromium layer is deposited using a thermal evaporation process, and the mask pattern is reversed through a lift-off process to form a high-precision hard mask layer; 10) Using a chromium metal hard mask as a barrier layer, an inductively coupled plasma (ICP) dry etching process is used to etch the exposed silicon nitride dielectric layer to form the required nanopillar array; 11) The etched mask is removed, and the processed metasurface array is thoroughly cleaned.

[0061] It should be noted that this solar sensor operates in any visible light wavelength band. The material of the metasurface array can be monocrystalline silicon, polycrystalline silicon, amorphous silicon, silicon nitride, etc. The geometry of the metalens unit can be any polarization-sensitive pattern, such as elliptical cylinders, rectangular cylinders, polygonal cylinders, etc. The periodic arrangement of the metalens units can be a square grid or a hexagonal arrangement. The polarization multiplexing can select any two orthogonal polarization modes, not limited to the left-handed circular polarization and right-handed circular polarization introduced in the example.

[0062] (ii) CMOS image sensor

[0063] The CMOS image sensor selected is the Aptina MT9J003, with an imaging area size of 6.44mm × 4.62mm and a single pixel size of... The value is 1.67 μm. The estimated angle of incidence is... ,in The distance between the centroid coordinates of the light spot and the centroid offset of the target metalens unit is given by the solar incidence angle resolution. ≈ .

[0064] (III) Supporting Structure

[0065] Support structure 2 is fabricated from PLA material using precision 3D printing technology (using a Bamboo Lab X1 Carbon printer with a precision of 0.08 mm). This support structure 2 is used to fix the metasurface array and the CMOS image sensor 3. The relative distance between the metasurface array and the CMOS image sensor 3 is designed to be 20 mm (consistent with the focal length). The relative distance between the metasurface array and the CMOS image sensor 3 can be finely adjusted for focal length calibration. Specifically, metric M2 threaded holes are machined on the backplate of the CMOS image sensor 3 and the support structure 2, and the two are connected by M2 screws. By tightening the M2 screws, the rotation of the screws can be precisely converted into the axial displacement of the support structure 2, thereby achieving micron-level continuous adjustment of the distance between the metasurface array and the CMOS image sensor 3.

[0066] (iv) Signal Processing Unit

[0067] The process by which signal processing unit 4 processes the image and calculates the angle information of the solar vector is as follows:

[0068] 1) Image Acquisition and Preprocessing: Several metalens units in the metasurface array, with their field of view corresponding to the current solar direction, converge sunlight and form several pairs of dual-spot images on the CMOS image sensor 3. The signal processing unit 4 reads the image and performs noise reduction processing using adaptive Wiener filtering to improve the image signal-to-noise ratio.

[0069] 2) Dual-spot recognition and centroid localization: Select a pair of dual-spot images that are closest to the center of the CMOS image sensor 3 and use the Sobel operator for edge detection. Then, use a two-dimensional Gaussian surface fitting method based on least squares to perform sub-pixel-level centroid localization for each spot and obtain its precise coordinates.

[0070] 3) Field of view recognition: Calculate the centroid of the pair of light spots in... , Spacing in the direction. Map this spacing to a pre-defined "spacing-cell index" database. , By comparing the two lenses, the metalens unit that produces the dual light spots can be uniquely identified. Thus, the preset deflection phase of the metalens unit can be determined. .

[0071] 4) Solar vector calculation: Based on the centroid coordinates of the spot corresponding to left-handed circular polarization in the dual-spot configuration. Calculate its relative to the target metalens unit center coordinates offset Combined with the known focal length =20mm and the deflection phase of the identified metalens unit, the angle of incidence of the solar vector is calculated according to the following geometric model. and :

[0072]

[0073] The solar sensor in this embodiment achieved the following technical specifications on the prototype: theoretical field of view coverage of -38.0° to 35.8°, and angular resolution better than [previous specification] throughout the entire field of view. The core optical head can be controlled to a volume in the cubic centimeter range and weighs only tens of grams. This embodiment fully verifies the outstanding effects of the invention in achieving a large field of view, high precision, and lightweight design.

Claims

1. A large field of view high precision sun sensor based on polarization multiplexing metasurface array, comprising a light ray introducer (1), a CMOS image sensor (3) and a signal processing unit (4), characterized in that, The light introducer (1) adopts a metasurface array, and the CMOS image sensor (3) is set on the focal plane of the metasurface array. The metasurface array has multiple superimposed metalens units with different deflection phases. Each metalens unit corresponds to a different subfield in the total field of view of the solar sensor. The meta-lens unit is a polarization multiplexing meta-lens unit, which is configured to produce different optical responses to any two orthogonal incident polarization states, so that for sunlight with a single incident direction, a pair of spaced double spots are formed on the CMOS image sensor (3). The spacing between the double spots is used as the line-of-sight encoding and has a unique mapping relationship with the preset deflection phase of the meta-lens unit that generates the double spots, which is used to identify the meta-lens unit that generates the double spots and its corresponding sub-field of view; the signal processing unit (4) is used to process the image acquired by the CMOS image sensor (3) and calculate the angle information of the solar vector; The steps that the signal processing unit (4) is configured to perform include: extracting the centroid position of the dual spot imaged on the CMOS image sensor (3); The distance between the centroids of the two light spots is calculated, and the target meta-lens unit that generates the two light spots and its preset deflection phase are identified based on the distance. The angle information of the solar vector is calculated by combining the centroid offset of the two light spots relative to the center of the target meta-lens unit, the focal length of the target meta-lens unit and its deflection phase.

2. The large field-of-view, high-precision solar sensor based on a polarization-multiplexed metasurface array according to claim 1, characterized in that, The dual light spots are selected from the pair of dual light spots closest to the center of the CMOS image sensor (3).

3. The large field-of-view, high-precision solar sensor based on a polarization-multiplexed metasurface array according to claim 1, characterized in that, The metasurface array and CMOS image sensor (3) are respectively set on the upper and lower sides of the 3D printed support structure (2) to achieve integrated construction.

4. The large field-of-view, high-precision solar sensor based on a polarization-multiplexed metasurface array according to claim 3, characterized in that, The relative distance between the metasurface array and the CMOS image sensor (3) can be finely adjusted for focal length calibration.

5. The large field-of-view, high-precision solar sensor based on a polarization-multiplexed metasurface array according to claim 4, characterized in that, The CMOS image sensor (3) is connected to the support structure (2) by screws. The screws can convert the rotation amount into the axial displacement of the support structure (2), thereby realizing the continuous adjustment of the distance between the metasurface array and the CMOS image sensor (3).

6. The large field-of-view, high-precision solar sensor based on a polarization-multiplexed metasurface array according to claim 1, characterized in that, The metasurface array is fabricated using micro-nano fabrication processes and is formed on a fused silica substrate along with a light-shielding mask.

7. The large field-of-view, high-precision solar sensor based on a polarization-multiplexed metasurface array according to claim 1, characterized in that, The metasurface array is arranged in a square grid or a hexagonal grid.

8. The large field-of-view, high-precision solar sensor based on a polarization-multiplexed metasurface array according to claim 1, characterized in that, The geometry of the meta-lens unit is an elliptical cylinder, a rectangular cylinder, or a polygonal cylinder.

9. The large field-of-view, high-precision solar sensor based on a polarization-multiplexed metasurface array according to claim 1, characterized in that, The material of the metasurface array is monocrystalline silicon, polycrystalline silicon, amorphous silicon, or silicon nitride.

Images