Low-Finesse Snapshot Image-Plane Interferometric Imaging Spectrometer

Abstract

The present invention relates to the field of spectral imaging, and particularly to a low-finesse snapshot image-plane interferometric imaging spectrometer, which comprises a telescopic system, a microlens array, a Fabry-Perot array interferometric system, and a detection system. The telescopic system is used to collect the target light field and perform field-of-view limitation and collimation on the target light field; the microlens array is used to perform array imaging on the collimated target light field; the Fabry-Perot array interferometric system includes a parallel flat plate and a checkerboard stepped mirror, the step heights are distributed in a two-dimensional decreasing pattern in a checkerboard shape, the step height difference between two adjacent steps is the same, the highest step is closely attached to the parallel flat plate, and an air gap with a two-dimensional increasing thickness change is formed therebetween. The thickness of each region of the air gap corresponds to an optical path difference respectively; the detection system is used to image the modulated interference light field to obtain an interference image array. The present invention has the advantages of small volume, light weight, compact structure, static state, stability, reliability, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of spectral imaging, and particularly relates to a low-finesse snapshot image-plane interferometric imaging spectrometer. Background Art

[0002] Fourier transform spectroscopy is a widely used measurement method for obtaining the infrared spectrum of a target scene, with a very broad application range, which has extended from the laboratory to the remote sensing field. Most Fourier transform spectrometers use a time-modulated Michelson interferometer system to measure the time-modulated interference signal generated by the interaction between the incident spectrum and the Michelson interferometer system, and perform spectral demodulation through Fourier transform. In a Fourier transform infrared spectrometer based on the Michelson interferometer system, a moving mirror is used to perform time sampling on the interference pattern. The moving mirror generates a time-varying optical path difference between the two beams through the splitting of the input beam. Under the illumination of monochromatic light, the response of the detector to this time-varying optical path difference is a sine signal that varies with the optical path difference. By accurately measuring the change in the optical path difference, usually using a reference laser signal, the wavelength information of the incident light can be recovered from the sampled interference signal. The illumination of multiple wavelengths generates an additive composite interference pattern, and the intensity of each wavelength, that is, the spectral information, can be recovered using Fourier transform. Since the Michelson interferometer system uses a high-precision precision moving mirror scanning mechanism to finely scan the optical path difference, the Fourier transform spectrometer based on the Michelson interferometer system has a large volume, heavy weight, high integration difficulty, and is extremely vulnerable to the external environment, making it difficult to meet the application requirements of new scientific and technological fields such as unmanned aerial vehicle (UAV) airborne and micro-nano satellite on-board.

[0003] The Fabry-Perot interferometer system has the advantages of a compact structure, small volume, and light weight compared to the Michelson interferometer system, and is therefore particularly suitable for application requirements such as UAV airborne and micro-nano satellite on-board. The traditional Fabry-Perot interferometer system is usually used as a very narrow bandpass filter to achieve narrowband filtering by using a very high reflectivity coating. If a lower reflectivity coating is used on the surface of the Fabry-Perot cavity, the spectral transmission is essentially a sine curve, and the spectrum can be restored through Fourier transform. However, too low a coating reflectivity will result in a reduction in fringe contrast and cause interference failure. Summary of the Invention

[0004] In view of this, the present invention aims to provide a low-finesse snapshot image-plane interferometric imaging spectrometer to solve the technical problems of the existing Fourier transform spectrometer based on the Michelson interferometer system, such as large volume, heavy weight, high integration difficulty, and extreme vulnerability to the external environment.

[0005] To achieve the above object, the technical solution of the present invention is realized as follows:

[0006] A low-finesse snapshot image-plane interference imaging spectrometer, comprising a telescopic system, a microlens array, a Fabry-Perot array interference system, and a detection system; wherein,

[0007] The telescopic system is used to collect the target light field and perform field-of-view limitation and collimation on the target light field;

[0008] The microlens array is located in the collimation direction of the telescopic system. The microlens array adopts an image-space telecentric optical path structure and is composed of M×N microlens units. The microlens array is used to perform array imaging on the collimated target light field;

[0009] The Fabry-Perot array interference system is located in the imaging optical path of the microlens array. The Fabry-Perot array interference system includes a parallel plate and a checkerboard stepped mirror. The parallel plate serves as the front interference plate of the Fabry-Perot array interference system. The checkerboard stepped mirror is located on the image-space focal plane of the microlens array and serves as the rear interference plate of the Fabry-Perot array interference system. The number of steps of the checkerboard stepped mirror is M×N, and the step heights are distributed in a two-dimensional decreasing checkerboard pattern. The step height difference between any two adjacent steps in one direction of the checkerboard stepped mirror is d, and the step height difference between any two adjacent steps in the other direction of the checkerboard stepped mirror is Md. Each step corresponds to a microlens unit. The highest step is in close contact with the parallel plate, so that an air gap with a two-dimensional increasing thickness is formed between the parallel plate and the checkerboard stepped mirror. The thickness of each region of the air gap corresponds to an optical path difference. The parallel plate, the air gap, and the checkerboard stepped mirror form an FP resonator array;

[0010] The detection system is located in the outgoing direction of the Fabry-Perot array interference system and is used to image the interference light field modulated by the Fabry-Perot array interference system to obtain an interference image array.

[0011] Further, the two surfaces of the parallel plate opposite to the checkerboard stepped mirror are respectively used as reflection interfaces. The two reflection interfaces and the air gap form an FP resonator, and the reflectivity of the two reflection interfaces is 30% - 40%.

[0012] Further, the parallel plate adopts a high-refractive-index plate. An antireflection film is deposited on the surface of the high-refractive-index plate facing away from the checkerboard stepped mirror, and the surface of the high-refractive-index plate facing the checkerboard stepped mirror is not coated. Or the parallel plate adopts a low-refractive-index plate. An antireflection film is deposited on the surface of the low-refractive-index plate facing away from the checkerboard stepped mirror, and a high-refractive-index dielectric film is deposited on the surface of the low-refractive-index plate facing the checkerboard stepped mirror and serves as the reflection interface of the FP resonator.

[0013] Further, the high-refractive-index plate adopts a silicon or germanium dielectric material, and the low-refractive-index plate adopts a quartz, calcium fluoride, magnesium fluoride, or sapphire dielectric material.

[0014] Further, the checkerboard stepped mirror includes a stepped substrate. The stepped substrate is made of a high refractive index medium. The upper surface of the stepped substrate of the high refractive index medium is not coated and serves as the reflection interface of the FP resonator, and an antireflection film is deposited on the lower surface of the stepped substrate of the high refractive index medium; alternatively, the stepped substrate is made of a low refractive index medium, a high refractive index medium film is deposited on the upper surface of the stepped substrate of the low refractive index medium and serves as the reflection interface of the FP resonator, and an antireflection film is deposited on the lower surface of the stepped substrate of the low refractive index medium.

[0015] Further, the low refractive index medium uses quartz, calcium fluoride, magnesium fluoride or sapphire dielectric materials; the high refractive index medium uses silicon or germanium dielectric materials; the high refractive index medium film uses a silicon film or a germanium film.

[0016] Further, let the step height difference between two adjacent steps be d. The step heights of the checkerboard stepped mirror are distributed in a checkerboard-like two-dimensional decreasing manner. The height decreases successively in steps of m in the d direction, and the height decreases successively in steps of n in the Md direction. The height of the ( m , n )-th step D ( m , n ) is:

[0017] D ( m , n ) = -[( n - 1) M + m - 1] d .

[0018] Further, let the step height difference between two adjacent steps be d. The thickness of the air gap is distributed in a checkerboard-like two-dimensional increasing manner. The thickness increases successively in steps of m in the d direction, and the thickness increases successively in steps of n in the Md direction. The thickness of the air gap of the FP resonator corresponding to the ( m , n )-th step h ( m , n ) is:

[0019] h ( m , n ) = [( n - 1) M + m - 1] d .

[0020] Furthermore, for the incident broadband spectrum, the step height difference d satisfies the following relationship:

[0021] ;

[0022] where λ min is the minimum wavelength of the broadband spectrum, θ max is the incident angle corresponding to the maximum aperture angle ray in the FP resonator, J is the spectral harmonic order, and n0 is the refractive index of the medium in the FP resonator;

[0023] For the incident narrowband spectrum, the step height difference d satisfies the following relationship:

[0024] ;

[0025] where λ S is the shortest wavelength of the narrowband spectrum, λ L is the longest wavelength of the narrowband spectrum, k is the spectral folding order, taking any integer less than or equal to .

[0026] Furthermore, the preparation process of the step substrate is as follows:

[0027] S1: Provide a planar substrate with a transverse width of a and a longitudinal width of b, and polish and clean the planar substrate;

[0028] S2: Vertically etch the planar substrate to form a longitudinal two-step structure, with an etching depth of NMd / 2 and an etching width of b / 2;

[0029] S3: Vertically etch the longitudinal two-step structure to form a longitudinal four-step structure, with an etching depth of NMd / 4 and an etching width of b / 4. Repeat step S3, ensuring that the etching depth of each time is 1 / 2 of the previous etching, and the etching width of each time is 1 / 2 of the previous etching, until the etching depth is Md and the etching width is b / N;

[0030] S4: Horizontally etch the longitudinal four-step structure to form a transverse two-step structure, with an etching depth of Md / 2 and an etching width of a / 2;

[0031] S5: Horizontally etch the transverse two-step structure to form a transverse four-step structure, with an etching depth of Md / 4 and an etching width of a / 4. Repeat step S5, ensuring that the etching depth of each time is 1 / 2 of the previous etching, and the etching width of each time is 1 / 2 of the previous etching, until the etching depth is d and the etching width is a / M.

[0032] Furthermore, the telescopic system includes a telescopic objective lens, a field stop, and a collimator; where

[0033] The telescopic objective lens is used to collect the target light field and image the target light field at the field stop;

[0034] The field stop is located on the image-side focal plane of the telescopic objective lens and the object-side focal plane of the collimator simultaneously. The field stop is used to limit the field of view of the imaging of the target light field;

[0035] The collimator is used to collimate the divergent light emitted by the target light field located at the field stop into parallel light.

[0036] Furthermore, the detection system includes a relay imaging lens, a cold shield aperture and a area array detector; wherein,

[0037] The relay imaging lens adopts an object-side telecentric optical path structure and is used to image the interference light field modulated by the Fabry-Perot array interference system onto the area array detector;

[0038] The cold shield aperture is located on the image-side focal plane of the relay imaging lens and is used to limit the object-side numerical aperture;

[0039] The area array detector is located at the image plane of the relay imaging lens and is used to perform photoelectric conversion on the interference light field to obtain an interference image array.

[0040] Furthermore, the area array detector is also used to segment the interference image array, segment the interference image array into interference image units corresponding to each FP resonator; and arrange the interference image units in the thickness order of each FP resonator to form an interference image data cube, and perform discrete Fourier transform operation on the interference image data cube with the optical path difference as the axis to obtain a spectral image data cube that varies with wavelength.

[0041] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0042] The present invention uses a checkerboard stepped mirror with medium and low reflectivity to construct a Fabry-Perot array interference system, replacing the beam splitter and the precision moving mirror scanning mechanism in the traditional Fourier transform imaging spectrometer, simplifying and lightening the opto-mechanical structure of the Fourier transform imaging spectrometer, improving the stability and robustness of the Fourier transform imaging spectrometer, and having the advantages of small volume, light weight, compact structure, static, stable and reliable. At the same time, the incident interface of the checkerboard stepped mirror uses medium and low reflectivity, and the spectrum can be restored through Fourier transform, reducing the difficulty of spectrum restoration of the traditional Fabry-Perot interference system. Through the coupling modulation transmission of the light field by the Fabry-Perot array interference system and the microlens array, the synchronous measurement of the interference image data cube can be realized, and then the snapshot effective detection of the target scene image information and spectrum information is realized, improving the real-time performance of multi-dimensional information detection. Description of the Drawings

[0043] The accompanying drawings that form a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and the descriptions thereof are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0044] Figure 1 is a schematic structural diagram of a low-finesse snapshot image-plane interference imaging spectrometer according to Embodiment 1 of the present invention.

[0045] Figure 2 is a schematic structural diagram of a checkerboard step mirror according to Embodiment 1 of the present invention.

[0046] Figure 3 is a schematic diagram of the spatial distribution of the air gap thickness between a parallel flat plate and a checkerboard step mirror according to Embodiment 1 of the present invention.

[0047] Figure 4 is a schematic diagram of the preparation process of a step substrate according to Embodiment 1 of the present invention.

[0048] Figure 5 is a schematic diagram of the process of image spectrum inversion according to Embodiment 1 of the present invention.

[0049] Figure 6 is a schematic structural diagram of a checkerboard step mirror according to Embodiment 2 of the present invention.

[0050] Explanation of the reference numerals in Embodiment 1: telescopic objective lens 10, field stop 20, collimating mirror 30, microlens array 40, parallel flat plate 50, checkerboard step mirror 60, step substrate 601, antireflection film 602, relay imaging mirror 70, cold screen stop 80, area array detector 90.

[0051] Explanation of the reference numerals in Embodiment 2: step substrate 603, antireflection film 604, high refractive index dielectric film 605. Detailed implementation manners

[0052] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not constitute a limitation to the present invention.

[0053] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0054] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first", "second", etc. may explicitly or implicitly include one or more of such features. In the description of the present invention, unless otherwise stated, the meaning of "a plurality of" is two or more.

[0055] In the description of the present invention, it should be noted that unless otherwise clearly specified and limited, the terms "mounted", "connected", "coupled" shall be understood 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, or an electrical connection; it may be directly connected, or indirectly connected through an intermediate medium, and may be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood through specific circumstances.

[0056] The present invention will be described in detail below with reference to the drawings and in conjunction with the embodiments.

[0057] The present invention provides a low-finesse snapshot image-plane interference imaging spectrometer, which includes a telescopic system, a microlens array, a Fabry-Perot array interference system, and a detection system. Among them, the telescopic system is used to collect the target light field and perform field-of-view limitation and collimation on the target light field. The microlens array is located in the collimation direction of the telescopic system. The microlens array adopts an image-space telecentric optical path structure and is composed of M×N microlens units. The microlens array is used to perform array imaging on the collimated target light field. The Fabry-Perot array interference system is located in the imaging optical path of the microlens array. The Fabry-Perot array interference system includes a parallel plate and a checkerboard stepped mirror. The parallel plate serves as the front interference plate of the Fabry-Perot array interference system. The checkerboard stepped mirror is located on the image-space focal plane of the microlens array and serves as the rear interference plate of the Fabry-Perot array interference system. The number of steps of the checkerboard stepped mirror is M×N, and the step height is distributed in a two-dimensional decreasing checkerboard pattern. The step height difference between any two adjacent steps is the same. Each step corresponds to a microlens unit. The highest step is closely attached to the parallel plate, so that an air gap with a two-dimensional increasing thickness is formed between the parallel plate and the checkerboard stepped mirror. The thickness of each region of the air gap corresponds to an optical path difference. The detection system is located in the outgoing direction of the Fabry-Perot array interference system and is used to image the interference light field modulated by the Fabry-Perot array interference system to obtain an interference image array.

[0058] The telescopic system includes a telescopic objective lens, a field stop, and a collimating mirror. Among them, the telescopic objective lens is used to collect the target light field and image the target light field at the field stop. The field stop is simultaneously located on the image-space focal plane of the telescopic objective lens and the object-space focal plane of the collimating mirror. The field stop is used to perform field-of-view limitation on the imaging of the target light field. The collimating mirror is used to collimate the divergent light emitted by the target light field located at the field stop into parallel light.

[0059] The detection system includes a relay imaging mirror, a cold shield aperture, and a focal plane array detector. Among them, the relay imaging mirror adopts an object-space telecentric optical path structure and is used to image the interference light field modulated by the Fabry-Perot array interference system onto the focal plane array detector. The cold shield aperture is located on the image-space focal plane of the relay imaging mirror and is used to limit the object-space numerical aperture. The focal plane array detector is located at the image plane of the relay imaging mirror and is used to perform photoelectric conversion on the interference light field to obtain an interference image array.

[0060] In the present invention, the objective light field is imaged onto the field stop by a telescopic objective lens. After the field stop restricts the imaging field of view, it is collimated into parallel light by a collimating lens and then incident on a microlens array. The microlens array focuses the incident light field in an array manner, and multiplies and arrays the objective light field to image it into a Fabry - Perot array interference system. The Fabry - Perot array interference system consists of a parallel flat plate and a checkerboard stepped mirror. Each step of the checkerboard stepped mirror corresponds to an imaging channel, that is, to a microlens unit. By closely fitting the surface of the parallel flat plate with the highest step of the checkerboard stepped mirror, an air gap with a two - dimensional linear increasing change in thickness in a checkerboard pattern is formed between the parallel flat plate and the checkerboard stepped mirror. The parallel flat plate and the checkerboard stepped mirror are made of silicon or germanium materials with a high refractive index, so that the reflection interfaces on both sides of the air gap have medium - low reflectivity, forming a low - finesse Fabry - Perot interference array, and thus the spectral demodulation can be achieved through Fourier transform. The stepped structure of the checkerboard stepped mirror results in a two - dimensional discrete linear change in the thickness of the air gap, and different air gap thicknesses correspond to different optical path differences, thereby modulating the optical path difference of the objective light field incident thereon and generating a change in interference intensity. Therefore, after being modulated by the Fabry - Perot array interference system, the objective light field forms an interference light field array, and each interference light field unit corresponds to a specific optical path difference. The interference light field array is imaged onto a focal plane detector after being transmitted by a relay imaging lens, and an interference image array corresponding to the optical path difference array is obtained on the focal plane detector. By performing image segmentation and image registration on the interference image array and performing discrete Fourier transform along the optical path difference, the spectral information of each object point in the target scene can be restored, realizing the snapshot measurement of the image information and spectral information of the target scene. The present invention reduces the size of the Fourier transform infrared imaging spectrometer and has the advantages of being structurally compact, static, stable, and reliable.

[0061] Embodiment 1

[0062] As Figures 1 - 5 shown, the low - finesse snapshot image - plane interference imaging spectrometer provided by Embodiment 1 of the present invention includes a telescopic objective lens 10, a field stop 20, a collimating lens 30, a microlens array 40, a parallel flat plate 50, a checkerboard stepped mirror 60, a relay imaging lens 70, a cold - screen diaphragm 80, and a focal plane detector 90 (using devices such as HgCdTe or InSb).

[0063] The target light field is first imaged onto the field stop 20 by the telescopic objective lens 10. The field stop 20 restricts the imaging field of the target light field, thereby controlling the spatial sampling on the area array detector 90 and the size of the imaging field, and suppressing the crosstalk of the imaging fields between adjacent interference channels. Then it is collimated into parallel light by the collimating lens 30 and incident into the microlens array 40. The microlens array 40 is composed of M×N microlens units, which performs aperture splitting and array imaging on the incident parallel light field. The microlens array 40 adopts an image-space telecentric optical path structure to make the chief ray of the array imaging beam parallel to the optical axis. The microlens array 40 divides the incident light field into M×N sub-apertures and arrays them onto the Fabry-Perot array interference system, where interference is formed in the FP resonator of the Fabry-Perot array interference system. The interference plate of the Fabry-Perot array interference system consists of a parallel plate 50 and a checkerboard stepped mirror 60. The parallel plate 50 serves as the front interference plate of the Fabry-Perot array interference system, and the checkerboard stepped mirror 60 serves as the rear interference plate of the Fabry-Perot array interference system. The checkerboard stepped mirror 60 includes a stepped substrate 601, and the multiple steps of the stepped substrate 601 are arranged in a two-dimensional checkerboard shape. Each step has a specific thickness, thus corresponding to an FP resonator. Each FP resonator corresponds to a specific microlens unit, and interference modulation is performed on each image field unit imaged by the microlens array. The two reflection interfaces of the Fabry-Perot array interference system form a low-finesse interference cavity with medium reflectivity, approaching two-beam interference, so that spectral restoration can be performed through Fourier transform. The parallel plate 50 is closely attached to the highest step of the checkerboard stepped mirror 60, so that an air gap with a two-dimensionally increasing thickness is formed between the two, thus forming an optical path difference that varies with the spatial position, and zero optical path difference is achieved at the optical contact positions of the two opposite surfaces of the parallel plate 50 and the checkerboard stepped mirror 60.

[0064] The two surfaces of the checkerboard stepped mirror 60 opposite to the parallel plate 50 serve as reflection interfaces and form a distributed FP resonator with the air gap. Each FP resonator has a specific thickness, thus having a fixed optical path difference. The light incident on the two reflection interfaces on both sides of the FP resonator will be reflected multiple times within the FP resonator, and interference will occur between the light beams passing through the FP resonator different numbers of times. The intensity of the interference signal of the light beam exiting through the FP resonator depends on the thickness of the air gap between the two reflection interfaces, the reflectivity of the reflection interfaces, the incident angle of the light, and the refractive index of the medium in the FP resonator between the two reflection interfaces. The medium in the FP resonator can be air or vacuum, or a low-refractive-index medium. In this invention, the air embodiment is used for illustration. The reflection interfaces of the traditional Fabry-Perot interference system usually adopt very high reflectivity to generate a narrow transmission band, while in this invention, the reflection interfaces of the Fabry-Perot array interference system adopt medium reflectivity to generate a quasi-cosine interference signal that can be demodulated by Fourier transform.

[0065] The interference image signal formed by the FP resonator corresponding to the (m,n)-th step in the checkerboard stepped mirror 60 of the Fabry-Perot array interference system can be expressed as:

[0066] ;

[0067] where R1 and R2 are the reflectivities of the two reflection interfaces of the FP resonator; ν is the wave number, ν = 1 / λ, λ is the wavelength, and δ(m,n) is the optical path difference of the FP resonator corresponding to the (m,n)-th step, and its expression is:

[0068] ;

[0069] where n0 is the refractive index of the medium in the FP resonator. When the medium in the FP resonator is air, n0 = 1; θ is the incident angle of the light in the FP resonator, corresponding to different field angles of view in the target scene; h(m,n) is the gap thickness of the FP resonator, which varies with different spatial positions due to the checkerboard stepped structure.

[0070] It can be seen from the interference image signal formula that when the reflectivity of the reflection interface is very low, the interference signal is a cosine function, but the modulation degree of the interference fringes is also very low; when the reflectivity of the reflection interface is very high, the interference fringes have a very high modulation degree, but the interference signal is an extremely narrow comb function. The traditional Fabry - Perot interferometer uses a reflection interface with a very high reflectivity to generate very narrow interference characteristics through resonance. In the Fourier transform spectrum of these very narrow comb-shaped interference signals, sidelobes will be generated. The sidelobes will take away signals from the main frequency, causing distortion of the restored spectrum. In order to suppress the spectral distortion caused by harmonic signals and reduce the interference caused by the sidelobe signal spreading to the main frequency signal, the maximum wave number of the incident spectrum should be less than or equal to twice the minimum wave number, that is, the maximum wavelength should be less than or equal to twice the minimum wavelength, which means that the wavelength bandwidth of the incident spectrum should be less than or equal to the minimum wavelength, or less than or equal to half of the maximum wavelength. For short-wave infrared 1μm - 2μm, mid-wave infrared 3μm - 5μm, and long-wave infrared 8μm - 14μm, the application requirements can basically be met.

[0071] Therefore, the present invention uses a reflection interface with a medium reflectivity to generate a quasi-cosine interference signal. At this time, the spectral finesse is relatively low, and the checkerboard stepped mirror 60 is close to double-beam interference. The interference pattern function can be approximately expressed as:

[0072] ;

[0073] Therefore, the spectral information of the incident light signal can be restored from the interference signal through Fourier transform.

[0074] Although the lower the reflectivity, the closer the interference signal is to the cosine function, and thus the higher the spectral restoration accuracy, too low a reflectivity will cause the modulation depth of the interference fringes to also decrease. Therefore, in order to achieve effective demodulation of the spectrum through Fourier transform, it is necessary to balance the modulation depth of the fringes and the suppression of spectral side lobes. By balancing the modulation depth of the interferogram and the cosine degree of the interference signal, the reflectivity of the reflection interface is determined, minimizing the spectral side lobes and maximizing the modulation depth of the interferogram. That is to say, by optimizing the reflectivity of the reflection interface, the best balance between the modulation depth of the interference fringes and the cosine characteristics is achieved. In the present invention, the reflectivities of the two reflection interfaces of the FP resonator are set between 30% and 40%, which can achieve the best balance between the modulation depth of the interferogram and the cosine characteristics. Therefore, each FP resonator in the present invention is a Fabry-Perot cavity with low finesse, modulating the interference intensity of the interference image at a given optical path difference.

[0075] For infrared materials, silicon and germanium materials have relatively high refractive indices and thus relatively moderate reflectivities. For silicon materials, its refractive index is 3.4 and the surface reflectivity is 30%; for germanium materials, its refractive index is 4.0 and the surface reflectivity is 36%. Therefore, the parallel plate 50 is made of a high refractive index medium such as silicon or germanium. An anti-reflection film is deposited on the surface of the parallel plate 50 facing away from the checkerboard step mirror 60, and no film is deposited on the surface of the parallel plate 50 facing the checkerboard step mirror 60. The step substrate 601 can be made of a high refractive index medium material such as silicon or germanium. No film is deposited on the surface of the step substrate 601 facing the parallel plate 50, so that the reflection interface has a medium-low reflectivity to achieve low finesse. An anti-reflection film 602 is deposited on the surface of the step substrate 601 facing away from the parallel plate 50 to ensure high transmittance. The highest step of the step substrate 601 is in contact with the uncoated side of the parallel plate 50, and a zero optical path difference is achieved at the optical contact surface.

[0076] An air gap with an increasing thickness is formed between the checkerboard step mirror 60 and the parallel plate 50, thus constituting a spatial distributed modulation of the optical path difference. The height of each step is complementary to the thickness of the air gap of the FP resonator. Let the step height difference of the checkerboard step mirror 60 be d and the number of step levels be M×N. Then, in order to form continuous optical path difference sampling, the thickness of the air gap of the FP resonator corresponding to the (m,n)th step is:

[0077] 。

[0078] In order to achieve effective spectral restoration, the step height difference d should satisfy the Nyquist sampling theorem. In order to suppress the spectral aliasing caused by the Fourier transform of the high-order side lobe signals, the sampling signal cut-off frequency is extended to J sub-harmonics (in general, JTaking 3 can meet the requirements). For the incident broadband spectrum, the step height difference d should satisfy the following relationship:

[0079] ;

[0080] where λ min is the minimum wavelength of the broadband spectrum, θ max is the incident angle corresponding to the maximum aperture angle ray in the FP resonator, J is the spectral harmonic order, and n0 is the refractive index of the medium in the FP resonator.

[0081] For the incident narrowband spectrum, especially when used for gas detection, in order to achieve high spectral resolution, the step height difference d should satisfy the following relationship:

[0082] ;

[0083] where λ S is the shortest wavelength of the narrowband spectrum, λ L is the longest wavelength of the narrowband spectrum, k is the spectral folding order, and any integer less than or equal to is taken.

[0084] The step substrate 601 is composed of M×N steps. The step height difference d between any two adjacent steps in one direction of the step substrate 601 is constant, and the step height difference Md between any two adjacent steps in the other direction of the step substrate 601 is constant. It is fabricated by an etching method, and the fabrication process is as follows:

[0085] S1: Provide a planar substrate with a transverse width of a and a longitudinal width of b, and polish and clean the planar substrate.

[0086] S2: Vertically etch the planar substrate to form a longitudinal two-step structure, with an etching depth of NMd / 2 and an etching width of b / 2.

[0087] S3: Vertically etch the longitudinal two-step structure to form a longitudinal four-step structure, with an etching depth of NMd / 4 and an etching width of b / 4. Repeat step S3, ensuring that the etching depth each time is 1 / 2 of the previous etching, and the etching width each time is 1 / 2 of the previous etching, until the etching depth is Md and the etching width is b / N.

[0088] S4: Horizontally etch the longitudinal four-step structure to form a transverse two-step structure, with an etching depth of Md / 2 and an etching width of a / 2.

[0089] S5: Transversely etch the horizontally two-step structure to form a horizontally four-step structure, with an etching depth of Md / 4 and an etching width of a / 4. Repeat step S5, ensuring that the etching depth each time is 1 / 2 of the previous etching, and the etching width each time is 1 / 2 of the previous etching, until the etching depth is d and the etching width is a / M.

[0090] After the imaging light field is modulated by the Fabry-Perot array interference system, it is imaged onto the area array detector 90 with a certain reduction ratio through the relay imaging lens 70. The relay imaging lens 70 samples the object-side telecentric optical path structure, and its image-side focal plane is matched with the cold shield aperture 80 of the area array detector 90. That is to say, the cold shield aperture 80 of the area array detector 90 is located on the image-side focal plane of the relay imaging lens 70. The Fabry-Perot array interference system and the area array detector 90 satisfy the object-image conjugate relationship with respect to the relay imaging lens 70. The relay imaging lens 70 relays and images the interference light field modulated by the Fabry-Perot array interference system onto the area array detector 90 to obtain a set of interference image arrays. Each interference image unit in the interference image array corresponds to an FP resonator with a specific medium thickness in the Fabry-Perot array interference system, and thus corresponds to an inherent optical path difference δ. In the present invention, each interference image unit in the interference image array is a replication of the same field of view and is intensity-modulated by the corresponding FP resonator. The area array detector 90 performs image segmentation on the interference image array, divides the interference image array into interference image units corresponding to each specific FP resonator, arranges the separated interference image units in the order of the thickness of the FP resonator into an interference image data cube. The thickness of each FP resonator corresponds to a specific optical path difference δ. After image registration, the interference pattern of each target point can be extracted. Each spatial pixel in the two-dimensional image plane of the interference image data cube is extracted, and the interference light intensities corresponding to the respective optical path differences of this spatial pixel are extracted along the optical path difference δ axis, so as to obtain the interference pattern sequence corresponding to each spatial pixel. Perform discrete Fourier transform operations on the interference pattern sequence of each spatial pixel, and the spectral information corresponding to each spatial pixel can be obtained, so as to obtain a spectral image data cube that changes with the wavelength λ, and thus demodulate the spectral information of each point in the target scene, realizing the snapshot measurement of the image information and spectral information of the target scene.

[0091] Embodiment 2

[0092] The difference between Embodiment 2 and Embodiment 1 is only that the structure of the checkerboard step mirror is different, as Figure 6As shown, the stepped substrate 603 of the checkerboard stepped mirror in Embodiment 2 is made of low-refractive-index transparent dielectric materials such as quartz, calcium fluoride, magnesium fluoride, and sapphire to ensure a high transmittance at the interface. A high-refractive-index dielectric film 605 such as silicon or germanium is evaporated on the stepped surface of the stepped substrate 603, so that the reflection interface has a medium-low reflectivity to achieve low fineness. An antireflection film 604 is evaporated on the lower surface (i.e., the bottom plane) of the stepped substrate 603 to ensure a high transmittance.

[0093] It should be understood that the various forms of the processes shown above can be used, with steps reordered, added, or deleted. For example, the steps recited in the disclosure of the present invention can be executed in parallel, sequentially, or in a different order, as long as the desired results of the technical solution disclosed in the present invention can be achieved, and no limitation is imposed herein.

[0094] The above specific embodiments do not constitute a limitation on the protection scope of the present invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A low-finesse snapshot image-plane interferometric imaging spectrometer, characterized in that, It includes a telescopic system, a microlens array, a Fabry-Perot array interference system, and a detection system. Among them, the telescopic system is used to collect the target light field and perform field-of-view limitation and collimation on the target light field; the microlens array is located in the collimation direction of the telescopic system. The microlens array adopts an image-space telecentric optical path structure and is composed of M×N microlens units. The microlens array is used to perform array imaging on the collimated target light field; the Fabry-Perot array interference system is located in the imaging optical path of the microlens array. The Fabry-Perot array interference system includes a parallel plate and a checkerboard stepped mirror. The parallel plate serves as the front interference plate of the Fabry-Perot array interference system. The checkerboard stepped mirror is located on the image-space focal plane of the microlens array and serves as the rear interference plate of the Fabry-Perot array interference system. The number of steps of the checkerboard stepped mirror is M×N, and the step heights are distributed in a two-dimensional decreasing checkerboard pattern. The step height difference between any two adjacent steps in one direction of the checkerboard stepped mirror is d, and the step height difference between any two adjacent steps in the other direction of the checkerboard stepped mirror is Md. Each step corresponds to a microlens unit. The highest step is closely attached to the parallel plate, so that an air gap with a two-dimensional increasing thickness is formed between the parallel plate and the checkerboard stepped mirror. The thickness of each region of the air gap corresponds to an optical path difference. The parallel plate, the air gap, and the checkerboard stepped mirror form an FP resonator array; The two surfaces of the parallel plate opposite to the checkerboard stepped mirror are used as reflection interfaces respectively. The two reflection interfaces and the air gap form an FP resonator, and the reflectivity of the two reflection interfaces is 30% - 40%; The parallel plate adopts a high-refractive-index plate. An antireflection film is deposited on the surface of the high-refractive-index plate facing away from the checkerboard stepped mirror, and the surface of the high-refractive-index plate facing the checkerboard stepped mirror is not coated. Or the parallel plate adopts a low-refractive-index plate. An antireflection film is deposited on the surface of the low-refractive-index plate facing away from the checkerboard stepped mirror, and a high-refractive-index dielectric film is deposited on the surface of the low-refractive-index plate facing the checkerboard stepped mirror and serves as the reflection interface of the FP resonator. The high-refractive-index plate adopts a silicon or germanium dielectric material, and the low-refractive-index plate adopts a quartz, calcium fluoride, magnesium fluoride, or sapphire dielectric material; The checkerboard stepped mirror includes a stepped substrate. The stepped substrate adopts a high-refractive-index dielectric. The upper surface of the stepped substrate of the high-refractive-index dielectric is not coated and serves as the reflection interface of the FP resonator, and an antireflection film is deposited on the lower surface of the stepped substrate of the high-refractive-index dielectric. Or the stepped substrate adopts a low-refractive-index dielectric. A high-refractive-index dielectric film is deposited on the upper surface of the stepped substrate of the low-refractive-index dielectric and serves as the reflection interface of the FP resonator, and an antireflection film is deposited on the lower surface of the stepped substrate of the low-refractive-index dielectric. The low-refractive-index dielectric adopts a quartz, calcium fluoride, magnesium fluoride, or sapphire dielectric material; the high-refractive-index dielectric adopts a silicon or germanium dielectric material; the high-refractive-index dielectric film adopts a silicon film or a germanium film; For the incident broadband spectrum, the step height difference d satisfies the following relationship: ； where λ min is the minimum wavelength of the broadband spectrum, θ max is the incident angle corresponding to the light ray with the maximum aperture angle in the FP resonator, J is the spectral harmonic order, and n0 is the refractive index of the medium in the FP resonator; For the incident narrowband spectrum, the step height difference d satisfies the following relationship: ； Among them, λ S is the shortest wavelength of the narrowband spectrum, and λ L is the longest wavelength of the narrowband spectrum, k is the spectral folding order, taking any integer less than or equal to ; The detection system is located in the outgoing direction of the Fabry - Perot array interference system, and is used to image the interference light field modulated by the Fabry - Perot array interference system to obtain an interference image array.

2. The low-finesse snapshot image-plane interference imaging spectrometer according to claim 1, wherein The step heights of the checkerboard stepped mirror are distributed in a two-dimensional decreasing checkerboard pattern, with the height decreasing successively in steps of m in the d direction, and decreasing successively in steps of n in the Md direction. The height of the ( m , n )-th step D ( m , n ) is: D ( m , n )=-[( n -1) M + m -1] d 。 3. The low-finesse snapshot image-plane interference imaging spectrometer according to claim 2, wherein The thickness of the air gap is distributed in a two-dimensional checkerboard pattern with an increasing step, and the thickness increases sequentially with a step of m in the d direction, and the thickness increases sequentially with a step of n in the Md direction. The thickness of the air gap of the FP resonator corresponding to the ( m , n )-th step is h ( m , n ) and is: h ( m , n )=[( n -1) M + m -1] d 。 4. The low-finesse snapshot image-plane interference imaging spectrometer according to claim 1, characterized in that The preparation process of the stepped substrate is as follows: S1: Provide a planar substrate with a transverse width of a and a longitudinal width of b, and polish and clean the planar substrate. S2: Vertically etch the planar substrate to form a longitudinal two - step structure, with an etching depth of NMd / 2 and an etching width of b / 2. S3: Vertically etch the longitudinal two - step structure to form a longitudinal four - step structure, with an etching depth of NMd / 4 and an etching width of b / 4. Repeat step S3, ensuring that the etching depth each time is 1 / 2 of the previous etching depth and the etching width each time is 1 / 2 of the previous etching width, until the etching depth is Md and the etching width is b / N. S4: Horizontally etch the longitudinal four - step structure to form a transverse two - step structure, with an etching depth of Md / 2 and an etching width of a / 2. S5: Horizontally etch the transverse two - step structure to form a transverse four - step structure, with an etching depth of Md / 4 and an etching width of a / 4. Repeat step S5, ensuring that the etching depth each time is 1 / 2 of the previous etching depth and the etching width each time is 1 / 2 of the previous etching width, until the etching depth is d and the etching width is a / M.

5. The low-finesse snapshot image-plane interference imaging spectrometer according to claim 1, characterized in that, The telescopic system includes a telescopic objective lens, a field stop, and a collimating mirror; among them, The telescopic objective lens is used to collect the target light field and image the target light field at the field stop. The field stop is simultaneously located on the image - side focal plane of the telescopic objective lens and the object - side focal plane of the collimating mirror, and the field stop is used to limit the field of view of the imaging of the target light field. The collimating mirror is used to collimate the divergent light emitted by the target light field located at the field stop into parallel light.

6. The low-finesse snapshot image-plane interference imaging spectrometer according to claim 1, characterized in that The detection system includes a relay imaging mirror, a cold - screen diaphragm, and a area array detector; among them, The relay imaging mirror adopts an object - side telecentric optical path structure and is used to image the interference light field modulated by the checkerboard stepped mirror onto the area array detector. The cold - screen diaphragm is located on the image - side focal plane of the relay imaging mirror and is used to limit the object - side numerical aperture. The area array detector is located at the image plane of the relay imaging mirror and is used to perform photoelectric conversion on the interference light field to obtain an interference image array.

7. The low-finesse snapshot image-plane interference imaging spectrometer according to claim 6, characterized in that, The area array detector is also used to segment the interference image array, segment the interference image array into interference image units corresponding to each FP resonator; and arrange the interference image units in the thickness order of each FP resonator to form an interference image data cube, and perform a discrete Fourier transform operation on the interference image data cube with the optical path difference as the axis to obtain a spectral image data cube that varies with wavelength.

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