Multi-wavelength router, method for processing thereof, mid-wave infrared multispectral imaging system

Abstract

The application discloses a kind of multi-wavelength router, its processing method, mid-wave infrared multispectral imaging system.The processing method of multi-wavelength router includes the following steps: first, initialize the height surface of planar element;Then, set the objective function of different wavelength converging in different position point spread function, build optimization function based on objective function;Then, according to the target optical parameter of multi-wavelength router, the initial height surface of planar element, based on optimization function, after the iterative optimization of height surface, obtain the optimal height surface of planar element, according to the optimal height surface to planar element is processed and obtains multi-wavelength router.The application can realize multispectral imaging only by adding a piece of multi-wavelength router in front of the detector without changing the optical structure of mid-wave infrared imaging system, greatly realizing the miniaturization and light weight of spectral imaging system;It has higher spectral transmittance and light collection efficiency.There is no moving part in the system, the structure is stable, and it is beneficial to carry.

Description

Technical Field

[0001] This invention relates to a spectral imaging system in the field of spectral imaging technology, specifically to a multi-wavelength router, its processing method, and a mid-wave infrared multispectral imaging system. Background Technology

[0002] Spectral imaging technology is a novel technique that utilizes single or multiple spectral channels for spectral data acquisition and processing, image display, and analysis and interpretation. Its primary aim is to obtain nearly continuous spectral data for each pixel while acquiring a large number of narrow-band continuous spectral images of targets. Currently, spectral imaging technology is mainly applied in the fields of hyperspectral airborne and aerospace remote sensing.

[0003] Currently, spectral imaging systems mainly face the following technical challenges: First, pushbroom and swing-broom systems are too large and complex, failing to meet the miniaturization and lightweight requirements of the aerospace industry. Second, due to system limitations, both pushbroom and swing-broom systems require time, resulting in the inability to obtain spectral images at the same moment, lacking timeliness. Third, traditional filter-type imaging spectrometers use a series of narrowband filters for light dispersion; each filter only selects light of a specific wavelength, while other light is absorbed or reflected. This light does not reach the photodetector and therefore does not contribute to the detected signal, resulting in a significant waste of light energy.

[0004] A color router is a device that can directly and losslessly transmit all incident light to a photodetector of a predetermined color channel based on different parts of the visible spectrum. With the development of planar optics, emerging spectral routing schemes can achieve comprehensive control over optics in both the frequency and spatial domains and efficiently utilize the entire spectrum. In 2022, X Zou et al. achieved a color router by reverse engineering to obtain the optimal 0-1 encoding structure. However, the unit cells of metasurfaces are small, increasing the fabrication complexity for large pixels in the mid-infrared range. Furthermore, the 0-1 encoding structure limits the degrees of freedom. (2020) Philip et al. optimized and iterated polymer cubes (stacked with multiple two-dimensional layers) of three-dimensional nanostructures to achieve the desired functionality; in 2022, Li achieved the functionality of a Bayer array by optimizing a single-layer metasurface structure through reverse engineering. However, current color routers mostly use metasurface materials with wavelengths concentrated in the visible light range. No color routers utilizing diffraction elements to create mid-infrared bands have yet been developed. Furthermore, common metasurface materials have low transmittance in the mid-infrared range. Summary of the Invention

[0005] To address the technical problems existing in the background art, and considering the issues of complex structure, high cost, low timeliness of spectral information acquisition, and low energy utilization of filters in traditional mid-wave infrared spectral imaging systems, this invention employs a mid-wave infrared multispectral imaging system based on a multi-wavelength router. The purpose of this invention is to realize a snapshot-type mid-wave infrared multispectral imaging system using a multi-wavelength router based on diffraction elements. This imaging system utilizes a single diffraction element to cover the infrared detector, enabling direct and lossless routing of light of different wavelengths to pixels at different locations on the infrared detector, allowing for the simultaneous acquisition of spatial and spectral information of mid-wave infrared targets and scenes.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] I. A multi-wavelength router

[0008] Parallel light of different wavelengths converges at different locations after passing through the multi-wavelength router.

[0009] II. A fabrication method for a multi-wavelength router

[0010] 1) Based on the target optical parameters of the multi-wavelength router, set the objective function for the diffusion function of different wavelengths converging at different locations, and construct the optimization function FOM based on the objective function;

[0011] 2) Initialize the height plane of the planar element;

[0012] 3) Based on the initial height surface of the planar element, the height surface is iteratively optimized using the optimization function FOM to obtain the optimal height surface of the planar element. After processing the planar element based on the optimal height surface, a multi-wavelength router is obtained.

[0013] The formula for the optimization function FOM is as follows:

[0014]

[0015]

[0016]

[0017] Where N represents the total number of sampling wavelengths, ω i Here, μ is the weighting factor, a is the influence factor, and μ is the weighting factor. i Indicates energy efficiency, ∈ i I represents the crosstalk caused by other wavelengths at the current sampling wavelength. i (x′) is the intensity distribution on the image plane after optimization of the i-th sampling wavelength, I j (x′) is the intensity distribution of the crosstalk signal after optimization for the j-th sampling wavelength; T i(x′) is the objective function value at the i-th sampling wavelength, x′ min and x′ max These are the minimum and maximum values ​​of the x-coordinate x′ in the objective function, respectively.

[0018] The target optical parameters include the operating band, number of channels c, focal length f, and the pixel size of the detector in the operating band.

[0019] The formula for calculating the focal length f is as follows:

[0020] f = p × 2p / λ

[0021] Where p is the side length of the detector cell in the working band, and λ is the average wavelength of the working band.

[0022] III. A Mid-Wave Infrared Multispectral Imaging System

[0023] The mid-wave infrared multispectral imaging system includes the multi-wavelength router, which is placed between the optical system and the detector along the optical axis.

[0024] The beneficial effects of this invention are as follows:

[0025] The method of this invention only requires adding a multi-wavelength router in front of the image of a mid-wave infrared camera, which greatly realizes the miniaturization and weight reduction of the spectral imaging system.

[0026] The method of this invention acquires image and spectral information within a single exposure time, thus possessing timeliness.

[0027] This invention utilizes a wavelength-splitting focusing method with diffraction elements to achieve spectral imaging, resulting in higher spectral transmittance and light collection efficiency.

[0028] The system of this invention has no moving parts and a stable structure. At the same time, the technology is relatively mature and the cost is low, which is conducive to the miniaturization of the mid-wave infrared spectral imaging system and makes it easy to be flexibly mounted. Attached Figure Description

[0029] Figure 1 This is a schematic diagram illustrating the specific process of the present invention.

[0030] Figure 2 The four spectral transmittance curves for the optimized 2×2 pixel.

[0031] Figure 3 This is a point spread function image of a 2×2 pixel at different wavelengths.

[0032] Figure 4 This is the height diagram of the optimized diffraction element.

[0033] Figure 5 A flowchart for optimizing diffraction elements.

[0034] Figure 6 This is a diffraction imaging model that converges at a non-central location.

[0035] Figure 7 This is a schematic diagram of a mid-wave infrared multispectral router.

[0036] Figure 8 This is an example image of spectral imaging.

[0037] Figure 9 This is a schematic diagram of the multispectral imaging system that has been built.

[0038] In the diagram: 1. Multi-wavelength router. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings.

[0040] To address the technical problems existing in the background art, and considering that traditional mid-wave infrared spectral imaging systems are complex in structure, expensive, have low timeliness in acquiring spectral information, and have low energy utilization of filters, this invention adopts a diffraction-based mid-wave infrared multi-wavelength router imaging system.

[0041] The purpose of this invention is to realize a snapshot-type mid-wave infrared multispectral imaging system using a multi-wavelength router based on diffraction elements. This imaging system uses a single diffraction element to cover an infrared detector, enabling light of different wavelengths to converge onto pixels at different positions on the infrared detector, thus simultaneously acquiring spatial and spectral information of mid-wave infrared targets and scenes.

[0042] like Figure 1 The present invention includes the following steps:

[0043] 1) Based on the target optical parameters of the multi-wavelength router, set the objective function for the diffusion function of different wavelengths converging at different locations, and construct the optimization function FOM based on the objective function;

[0044] Target optical parameters include the operating wavelength [λ] min , λ max The parameters are: number of channels (c), focal length (f, distance from the image plane), and pixel size of the detector in the operating band. The number of channels (c) is the total number of channels in the multispectral system, satisfying c = M × N, where M is the number of vertical channels and N is the number of horizontal channels. The M × N number of channels corresponds to the minimum unit of a multi-wavelength router needing to cover M × N pixels on the image plane, and this minimum unit is repeated until the entire image plane is covered.

[0045] The formula for calculating focal length f is as follows:

[0046] f = p × 2p / λ

[0047] Where p is the side length of a pixel in the working band detector, the size of a pixel on the image plane, i.e., the smallest unit, satisfying p × pμm. 2 λ is the average wavelength of the operating band.

[0048] In this embodiment, the router is designed to cover c = 2 × 2 pixels, similar to a color router. Therefore, the unit element size of the multi-wavelength router is 2p × 2pμm. 2 (It should be noted that the 2×2 pixel distribution is the case in this embodiment; other M×N coverage methods can also be chosen.) The material chosen for the diffraction element is Si, with a refractive index of approximately 3.42, and a processing precision of 1μm. Considering a maximum diffraction element height of 5μm, 100 steps, and each step height of 50nm, the selected wavelength range is 3.7μm-4.8μm, and the pixel size is 16μm×16μm.

[0049] In practice, the objective function of the diffusion function, which allows different wavelengths to converge at different locations, can be approximated as a Gaussian function, with its center located at (x′). min +x′ max ) / 2, half-height and full width are W i The formula for the objective function of the diffusion function, which allows different wavelengths to converge at different locations, is set as follows:

[0050]

[0051]

[0052]

[0053] Among them, T i (x′) represents the objective function value at the i-th sampling wavelength, where x′ is the abscissa of the objective function. min and x′ max These are the minimum and maximum values ​​of the x-axis, respectively. W i Let λ be the full width at half maximum (FWHM) of the i-th sampling wavelength. i λ is the i-th sampling wavelength. L is the length of the diffraction element, f is the distance between the diffraction element and the image plane, and NA represents the numerical aperture.

[0054] The formula for the optimization function FOM is as follows:

[0055]

[0056]

[0057]

[0058] Where N represents the total number of sampling wavelengths, the first term of FOM represents the weighted average efficiency of the N sampling wavelengths, the second term represents the weighted average crosstalk of the N sampling wavelengths, and ω i is a weighting factor used to balance the contributions of different sampling wavelengths. 'a' is an influence factor, where a∈(0,1) is used to balance the two terms. Specifically, μ... i and ∈ i In the sampled wavelength sequence, subscript i represents the current sampling wavelength number, and subscript j represents the number of other wavelengths. i Indicates energy efficiency, ∈ i I represents the crosstalk caused by other wavelengths at the current sampling wavelength. i (x′) is the intensity distribution on the image plane after optimization of the i-th sampling wavelength, I j (x′) is the intensity distribution of the crosstalk signal after optimization for the j-th sampling wavelength; T i (x′) is the objective function value at the i-th sampling wavelength, x′ min and x′ max These are the minimum and maximum values ​​of the x-coordinate x′ in the objective function, respectively.

[0059] 2) Randomly initialize the height plane of the planar element (i.e., the diffraction element);

[0060] 3) Based on the initial height surface of the planar element and the machining accuracy, the initial value of FOM is obtained. Then, based on the optimization function FOM, the height surface is iteratively optimized using the gradient descent-assisted DBS (Direct Binary Search) algorithm. This integrates the phases of multiple wavelengths into a single phase surface, obtaining the optimal height surface of the planar element. The specific optimization process is as follows: Figure 5 As shown, the final optimized diffraction element height map is as follows. Figure 4 As shown, a multi-wavelength router is obtained by processing planar elements according to the optimal height. Parallel light of different wavelengths converges at different positions after passing through the multi-wavelength router, that is, by using diffraction elements to converge wavelengths separately, such as... Figure 7 As shown.

[0061] The mid-wave infrared multispectral imaging system based on a multi-wavelength router includes a multi-wavelength router 1, which is placed along the optical axis between the optical system and the detector. The detector is positioned at the focal length of the multi-wavelength router. The system is constructed as follows: Figure 9 As shown.

[0062] In practice, a broadband light source, combined with a narrowband filter, emits parallel light that enters the multispectral imaging system, resulting in a point spread function image on the image plane. Then, the narrowband filter is switched sequentially (each time, the center wavelength of the filter increases by 100 nm while the bandwidth remains constant), obtaining point spread function images of different wavelength bands on the image plane. The results are as follows: Figure 3As shown. The transmittance of each region is obtained by dividing the intensity of each region by the intensity of the entire image plane, within that wavelength band. This invention continuously changes the wavelength band, ultimately yielding the mid-wave infrared spectral response curves for four regions, as shown. Figure 2 As shown, the spectral resolution of the imaging system is thus obtained. Finally, the constructed multispectral imaging system is used to acquire the spectral and spatial information of the target scene. The decrease in spatial resolution of the imaging result is exchanged for an increase in the number of spectral channels, and the two are consistent. Since the 2×2 arranged pixels are divided into 4 regions in this example, the spatial resolution of the imaging result is reduced by a factor of 4. At the same time, the obtained 2×2=4 channel spectral response is shown in the figure. Figure 8 As shown.

[0063] Analyzing the multi-wavelength router's splitting process, the principle of converging light to a non-central position on the image plane is as follows: Figure 6 As described above, parallel light rays are incident and converge to a point in three-dimensional space after passing through a planar element (flat optics). The phase that the planar element needs to provide can be expressed as:

[0064]

[0065] Where, λ d For the design wavelength. and Let f represent the focal length and the distance between the arbitrary position B of the planar element and the focal point F, respectively. The specific phase can be solved as follows:

[0066]

[0067]

[0068]

[0069] Among them, (r p ,φ p Let φ be the polar coordinate representation of point B. p Let θ be the polar angle of point F. f Let be the deflection angle of point F relative to the optical axis; the rest is consistent with the diffraction imaging model converging to the center position. Since the multi-wavelength router is placed very close to the detector (on the order of micrometers), this invention uses angular spectrum theory to describe the light emission portion. It should also be noted that the above equations only apply to a single wavelength λ. d .

[0070] The diffraction principle described above proves the feasibility of implementing a multi-wavelength router.

[0071] Finally, it should be noted that the above embodiments and descriptions are only used to illustrate the technical solutions of the present invention and not to limit it. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the disclosure of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the protection scope of the claims of the present invention.

Claims

1. A method of processing a multi-wavelength router, characterized by, Includes the following steps: 1) Based on the target optical parameters of the multi-wavelength router, set the objective function for the diffusion function of different wavelengths converging at different locations, and construct the optimization function FOM based on the objective function; The formula for the optimization function FOM is as follows: in, Indicates the total number of sampling wavelengths. As a weighting factor, As the impact factor, Indicates energy efficiency. This indicates the crosstalk caused by other wavelengths at the current sampling wavelength. It is the first The intensity distribution on the image plane after optimization of each sampling wavelength. It is the first The intensity distribution of crosstalk signals after optimization of each sampling wavelength; For the first Objective function value at each sampling wavelength These are the x-coordinates in the objective function. The minimum and maximum values; 2) Initialize the height plane of the planar element; 3) Based on the initial height surface of the planar element, the height surface is iteratively optimized using the optimization function FOM to obtain the optimal height surface of the planar element. After processing the planar element based on the optimal height surface, a multi-wavelength router is obtained. Parallel light of different wavelengths converges at different positions after passing through the multi-wavelength router.

2. The processing method of a multi-wavelength router according to claim 1, characterized in that, The target optical parameters include the operating wavelength and the number of channels. ,focal length And the pixel size of the detector in the working band.

3. The processing method of a multi-wavelength router according to claim 2, characterized in that, The focal length The calculation formula is as follows: in, The side length of the detector cell in the working band. This represents the average wavelength of the operating band.

4. A mid-wave infrared multispectral imaging system, characterized in that, The mid-wave infrared multispectral imaging system includes a multi-wavelength router fabricated by the processing method described in claim 1, which is placed between the optical system and the detector along the optical axis.

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