Low-fineness Fabry-Perot interference spatial modulation Fourier transform spectrometer

Abstract

The invention relates to the field of spectral imaging, in particular to a low-fineness Fabry-Perot interference spatial modulation Fourier transform spectrometer, which comprises a light source, a collimating mirror, a Fabry-Perot array interference system, a beam shrinkage optical system and an area array detector, and is characterized in that the light source is used for emitting a light beam; the collimating mirror is used for collimating light beams emitted by the light source into parallel light beams; the Fabry-Perot array interference system comprises a parallel flat plate and a checkerboard step mirror, the height of steps is distributed in a checkerboard-shaped two-dimensional decreasing mode, the step height differences between every two adjacent steps are the same, the highest step is tightly attached to the parallel flat plate, and the highest step is tightly attached to the checkerboard step mirror. An air gap with the thickness changing in a two-dimensional progressive increase mode is formed between the parallel flat plate and the checkerboard step mirror, and the thickness of each area of the air gap corresponds to one optical path difference; and the beam shrinking optical system is used for imaging the modulated interference light field onto the area array detector to obtain an interference image array. The invention has the advantages of small volume, light weight, compact structure, static state, stability, reliability and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of spectral imaging, and in particular relates to a low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer. Background Art

[0002] Fourier transform spectroscopy is a widely used measurement method for obtaining infrared spectra of target scenes. Its application range is very broad and has extended from the laboratory to the field of remote sensing. 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 a Michelson interferometer system, a moving mirror is used to sample the interference pattern in time. The moving mirror generates a time-varying optical path difference between the two beams by splitting the input beam. Under the illumination of monochromatic light, the detector responds to this time-varying optical path difference as a sinusoidal signal that changes with the optical path difference. By accurately measuring the change in the optical path difference, a reference laser signal is usually used to recover the wavelength information of the incident light from the sampled interference signal. The illumination of multiple wavelengths produces an added synthetic 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 perform fine scanning of the optical path difference, the Fourier transform spectrometer based on the Michelson interferometer system is large in size, heavy in weight, difficult to integrate, and easily affected by the external environment. It is difficult to adapt to the application needs of new scientific and technological fields such as drones, micro-nano satellites, etc.

[0003] Compared with the Michelson interferometer system, the Fabry-Perot interferometer system has the advantages of compact structure, small size and light weight, so it is particularly suitable for applications such as drone-mounted and micro-nano satellite-mounted. Traditional Fabry-Perot interferometer systems are usually used as very narrow bandpass filters, and narrowband filtering is achieved by using very high reflectivity coatings. If a lower reflectivity coating is used on the surface of the Fabry-Perot cavity, the spectral transmission is essentially a sinusoidal curve, and the spectrum can be restored by Fourier transform. However, if the reflectivity of the coating is too low, the fringe contrast will be reduced, resulting in interference failure. Summary of the invention

[0004] In view of this, the present invention aims to provide a low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer to solve the technical problems of the existing Fourier transform spectrometer based on the Michelson interferometer system, such as large size, heavy weight, great integration difficulty and being easily affected by the external environment.

[0005] To achieve the above object, the technical solution created by the present invention is implemented as follows: A low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer comprises a light source, a collimator, a Fabry-Perot array interferometer system, a beam reduction optical system and a planar array detector; wherein: The light source is located on the object focal plane of the collimator and is used to emit a light beam; The collimator is used to collimate the light beam emitted by the light source into a parallel beam; The Fabry-Perot array interferometer system is located in the parallel optical path of the collimator. The Fabry-Perot array interferometer system includes a parallel plate and a checkerboard step mirror. The parallel plate serves as a front interferometer of the Fabry-Perot array interferometer system, and the checkerboard step mirror serves as a rear interferometer of the Fabry-Perot array interferometer system. The number of steps of the checkerboard step mirror is M×N, and the step heights are distributed in a two-dimensional decreasing manner in a checkerboard shape. The step height difference between any two adjacent steps is the same. The highest step fits tightly with the parallel plate, so that an air gap with a thickness that changes in a two-dimensional increasing manner is formed between the parallel plate and the checkerboard step mirror. The thickness of each area of ​​the air gap corresponds to an optical path difference. The parallel plate, the air gap and the checkerboard step mirror form a FP resonant cavity array. The beam reduction optical system is located in the exit direction of the Fabry-Perot array interferometer system and adopts a double telecentric optical path structure to image the interference light field modulated by the Fabry-Perot array interferometer system onto a planar array detector to obtain an interference image array.

[0006] Furthermore, the two opposite surfaces of the parallel plate and the checkerboard step mirror are respectively used as reflection interfaces, and the two reflection interfaces and the air gap form a FP resonant cavity, and the reflectivity of the two reflection interfaces is 30% to 40%.

[0007] Furthermore, the parallel plate adopts a high refractive index plate, and an anti-reflection film is evaporated on the surface of the high refractive index plate away from the checkerboard step mirror, and the surface of the high refractive index plate facing the checkerboard step mirror is not coated, or the parallel plate adopts a low refractive index plate, and an anti-reflection film is evaporated on the surface of the low refractive index plate away from the checkerboard step mirror, and a high refractive index dielectric film is evaporated on the surface of the low refractive index plate facing the checkerboard step mirror and serves as a reflection interface of the FP resonant cavity.

[0008] Furthermore, the high refractive index plate is made of silicon or germanium dielectric material, and the low refractive index plate is made of quartz, calcium fluoride, magnesium fluoride or sapphire dielectric material.

[0009] Furthermore, the checkerboard step mirror includes a step substrate, the step substrate adopts a high refractive index medium, the upper surface of the step substrate of the high refractive index medium is not coated and serves as a reflection interface of the FP resonant cavity, and an anti-reflection film is evaporated on the lower surface of the step substrate of the high refractive index medium; or, the step substrate adopts a low refractive index medium, a high refractive index medium film is evaporated on the upper surface of the step substrate of the low refractive index medium and serves as a reflection interface of the FP resonant cavity, and an anti-reflection film is evaporated on the lower surface of the step substrate of the low refractive index medium.

[0010] Furthermore, the low refractive index medium is made of quartz, calcium fluoride, magnesium fluoride or sapphire; the high refractive index medium is made of silicon or germanium; and the high refractive index medium film is made of silicon film or germanium film.

[0011] Furthermore, the step heights of the chessboard step mirror are distributed in a two-dimensional decreasing manner in a chessboard shape. m Direction d The step length decreases successively, and the height increases along n Direction Md The step size decreases successively. m , n ) steps height D ( m , n )for: D ( m , n )=-[( n -1) M + m -1] d .

[0012] Furthermore, the thickness of the air gap is distributed in a two-dimensional increasing manner in a checkerboard pattern. m Direction d The step length increases successively, and the thickness increases along n Direction Md The step size increases successively. m , n ) steps corresponding to the thickness of the air gap of the FP resonant cavity h ( m , n )for: h ( m , n )=[( n -1) M + m -1] d .

[0013] Furthermore, for the incident broadband spectrum, the step height difference d satisfies the following relationship: ; Among them, λ min is the minimum wavelength of the broadband spectrum, θ max is the incident angle of the light with the maximum field angle in the FP resonant cavity, J is the spectral harmonic order, and n0 is the refractive index of the medium in the FP resonant cavity; For an incident narrowband spectrum, the step height difference d satisfies the following relationship: ; Among them, 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, which is less than or equal to Any integer of .

[0014] Furthermore, the preparation process of the step substrate is as follows: S1: providing a plane substrate with a lateral width of a and a longitudinal width of b, and polishing and cleaning the plane substrate; S2: The planar substrate is longitudinally etched to form a longitudinal two-step structure, with an etching depth of NMd / 2 and an etching width of b / 2; S3: The longitudinal two-step structure is longitudinally etched to form a longitudinal four-step structure, the etching depth is NMd / 4, the etching width is b / 4, and step S3 is repeated to ensure 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; S4: performing transverse etching on 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: The lateral two-step structure is laterally etched to form a lateral four-step structure, with an etching depth of Md / 4 and an etching width of a / 4. Step S5 is repeated to ensure 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.

[0015] Furthermore, the beam reduction optical system includes a beam reduction objective, an aperture diaphragm and a beam reduction image lens. The image-side focal plane of the beam reduction objective coincides with the object-side focal plane of the beam reduction image lens. The Fabry-Perot array interference system is located on the object-side focal plane of the beam reduction objective, the planar array detector is located on the image-side focal plane of the beam reduction image lens, and the aperture diaphragm is located on both the image-side focal plane of the beam reduction objective and the object-side focal plane of the beam reduction image lens. The beam reduction objective is used to converge the interference light field modulated by the flat Fabry-Perot array interference system; the aperture diaphragm is used to limit the aperture of the light beam to achieve filtering of the numerical aperture of the light beam; and the beam reduction image lens is used to collimate the light beam after numerical aperture filtering into parallel light.

[0016] Furthermore, the planar array detector is also used to perform image segmentation on the interference image array, dividing the interference image array into interference image units corresponding to each FP resonant cavity, integrating each interference image unit, and then arranging them in order of the thickness of each FP resonant cavity to form an interference image sequence, and performing discrete Fourier transform operation on the interference image sequence with the optical path difference as the axis to obtain a restored spectrum.

[0017] Compared with the prior art, the invention can achieve the following beneficial effects: The present invention adopts a chessboard step mirror with medium and low reflectivity to construct a Fabry-Perot array interferometer system, replaces the beam splitter and precision moving mirror scanning mechanism in the traditional Fourier transform imaging spectrometer, simplifies and lightens the optical-mechanical structure of the Fourier transform imaging spectrometer, improves the stability and robustness of the Fourier transform imaging spectrometer, and has the advantages of small size, light weight, compact structure, static, stable, reliable, etc. At the same time, the incident interface of the chessboard step mirror adopts medium and low reflectivity, and the spectrum can be restored by Fourier transform, which reduces the difficulty of spectrum restoration in the traditional Fabry-Perot interferometer system. Through the coupled modulation transmission of the light field by the Fabry-Perot array interferometer system, the synchronous measurement of the interference image sequence can be realized, thereby realizing the snapshot-type effective detection of the target scene image information and the spectral information, and improving the real-time performance of multi-dimensional information detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The drawings constituting part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments and descriptions of the present invention are used to explain the present invention and do not constitute an improper limitation on the present invention. In the drawings: Figure 1 It is a schematic diagram of the structure of the low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer described in Example 1 of the present invention.

[0019] Figure 2 It is a schematic diagram of the structure of the checkerboard step mirror described in Example 1 of the present invention.

[0020] Figure 3 Schematic diagram of the spatial distribution of the thickness of the air gap between the parallel plate and the checkerboard step mirror described in Example 1 of the present invention.

[0021] Figure 4 It is a schematic diagram of the preparation process of the step substrate described in Example 1 of the present invention.

[0022] Figure 5 It is a schematic diagram of the detection process of the interference image array by the beam reduction optical system described in Example 1 of the present invention.

[0023] Figure 6 It is a schematic diagram of the restoration process of the interference image array to spectrum as described in Example 1 of the present invention.

[0024] Figure 7 It is a schematic diagram of the structure of the checkerboard step mirror described in Example 2 of the present invention.

[0025] Description of the reference numerals of Example 1: light source 10 , collimating lens 20 , parallel plate 30 , checkerboard step mirror 40 , step substrate 401 , anti-reflection film 402 , focus reduction objective lens 50 , aperture stop 60 , focus reduction image lens 70 , and planar array detector 80 .

[0026] Description of the reference numerals of Example 2: stepped substrate 403 , anti-reflection film 404 , high refractive index dielectric film 405 . DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the invention more clear, the invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described here are only used to explain the invention and do not constitute a limitation of the invention.

[0028] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0029] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are 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 therefore cannot be understood as a limitation on 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 indicating the number of technical features indicated. Thus, features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0030] In the description of the invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installation", "connection" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the invention can be understood according to specific circumstances.

[0031] The present invention will be described in detail below with reference to the drawings and in combination with embodiments.

[0032] The invention provides a low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer, comprising a light source, a collimator, a Fabry-Perot array interferometer system, a beam reduction optical system and a planar array detector; wherein the light source is located on the object focal plane of the collimator, and is used to emit a light beam; the collimator is used to collimate the light beam emitted by the light source into a parallel light beam; the Fabry-Perot array interferometer system is located in the parallel light path of the collimator, and the Fabry-Perot array interferometer system comprises a parallel plate and a checkerboard step mirror, wherein the parallel plate serves as a front interferometer of the Fabry-Perot array interferometer system, the checkerboard step mirror serves as a rear interferometer of the Fabry-Perot array interferometer system, and the stage of the checkerboard step mirror The number of steps is M×N, and the step heights are distributed in a two-dimensional decreasing manner in a checkerboard shape. The step height difference between any two adjacent steps is the same, and the highest step fits tightly with the parallel plate, so that an air gap with a thickness that changes in a two-dimensional increasing manner is formed between the parallel plate and the checkerboard step mirror. The thickness of each area of ​​the air gap corresponds to an optical path difference, and the parallel plate, the air gap and the checkerboard step mirror constitute a FP resonant cavity array; the beam reduction optical system is located in the exit direction of the Fabry-Perot array interferometer system, and adopts a double telecentric optical path structure to image the interference light field modulated by the Fabry-Perot array interferometer system onto the area array detector to obtain an interference image array.

[0033] The light beam emitted by the light source is collimated into parallel light by a collimator, and then incident on a Fabry-Perot array interferometer system based on a checkerboard step mirror, forming interference in the FP resonant cavity of the Fabry-Perot array interferometer system. The Fabry-Perot array interferometer system consists of a parallel plate and a checkerboard step mirror. The checkerboard step mirror is arranged in a two-dimensional checkerboard shape by multiple steps. Each step of the checkerboard step mirror has a specific thickness, corresponding to a FP resonant cavity. Therefore, the Fabry-Perot array interferometer system is actually an FP resonant cavity array composed of multiple FP resonant cavities. By closely fitting the surface of the highest step of the parallel plate and the checkerboard step mirror, an air gap with a two-dimensional linear increasing thickness in a checkerboard shape is formed between the parallel plate and the checkerboard step mirror. The parallel plate and the checkerboard step mirror are made of silicon or germanium materials with high refractive index, so that the reflection interface on both sides of the air gap has a medium-low reflectivity, forming a low-fineness Fabry-Perot interferometer array, so that the spectrum can be demodulated by Fourier transform. The step structure of the checkerboard step mirror causes the thickness of the air gap to vary in two-dimensional discrete linear fashion, and different air gap thicknesses correspond to different optical path differences, thereby modulating the optical path difference of the target light field incident therein, resulting in changes in interference intensity. Therefore, the parallel light collimated by the collimator is modulated by the Fabry-Perot array interference system to form an interference light field array, and each interference light field unit corresponds to a specific optical path difference. The interference light field modulated by the Fabry-Perot array interference system is projected onto the surface array detector at a certain reduction ratio through the beam reduction optical system. The beam reduction optical system adopts a double telecentric optical path structure, and obtains an interference pattern array corresponding to the optical path difference array on the surface array detector, and can demodulate the spectral information by performing a discrete Fourier transform on the interference pattern array, thereby realizing snapshot measurement of the target scene image information and spectral information. The present invention reduces the size of the Fourier transform infrared imaging spectrometer, and has the advantages of compact structure, static, stable, and reliable.

[0034] Example 1 like Figure 1-Figure 6 As shown, the low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer provided in Example 1 of the present invention includes a light source 10, a collimating lens 20, a parallel plate 30, a checkerboard step mirror 40, a focus reduction objective lens 50, an aperture stop 60, a focus reduction image lens 70 and a planar array detector 80 (using devices such as HgCdTe or InSb).

[0035] The light beam emitted by the light source 10 is first collimated into a parallel light beam by a collimator 20, and then incident on the Fabry-Perot array interferometer system, forming interference in the FP resonant cavity of the Fabry-Perot array interferometer system. The interference plate of the Fabry-Perot array interferometer system is composed of a parallel plate 30 and a checkerboard step mirror 40. The parallel plate 30 serves as the front interferometer of the Fabry-Perot array interferometer system, and the checkerboard step mirror 40 serves as the rear interferometer of the Fabry-Perot array interferometer system. The checkerboard step mirror 40 includes a step base 401, and the multiple steps of the step base 401 are arranged in a two-dimensional checkerboard shape. Each step has a specific thickness, which corresponds to an FP resonant cavity. The two reflection interfaces of the Fabry-Perot array interferometer system use medium reflectivity to form a low-fineness interference cavity, which is close to double-beam interference, so that spectral restoration can be performed through Fourier transform. The parallel plate 30 and the highest step of the checkerboard step mirror 40 are tightly fitted together to form an air gap with a thickness that changes in a two-dimensional incremental manner between the two, thereby forming an optical path difference that changes with the spatial position, and achieving zero optical path difference at the optical contact position of the two opposite surfaces of the parallel plate 30 and the checkerboard step mirror 40.

[0036] The two surfaces of the checkerboard step mirror 40 opposite to the parallel plate 30 serve as reflection interfaces, and together with the air gap, form a distributed FP resonant cavity. Each FP resonant cavity has a specific thickness, and thus has a fixed optical path difference. Light incident on the reflection interfaces on both sides of the FP resonant cavity will be reflected multiple times in the FP resonant cavity, and interference will occur between the light beams that pass through the FP resonant cavity at different times. The intensity of the interference signal emitted by the light beam through the FP resonant cavity depends on the thickness of the air gap between the two reflection interfaces, the reflectivity of the reflection interface, the incident angle of the light, and the refractive index of the medium in the FP resonant cavity between the reflection interfaces on both sides. The medium in the FP resonant cavity can be air or vacuum, or a low refractive index medium. The present invention is described using an air embodiment. The reflection interface of a traditional Fabry-Perot interference system usually adopts a very high reflectivity to produce a narrow transmission band, while the reflection interface of the Fabry-Perot array interference system in the present invention adopts a medium reflectivity to produce a quasi-cosine interference signal that can be demodulated by Fourier transform.

[0037] The interference image signal formed by the FP resonant cavity corresponding to the (m, n)th step in the checkerboard step mirror 40 of the Fabry-Perot array interferometer system can be expressed as: ; Among them, R1 and R2 are the reflectivities of the two reflection interfaces of the FP resonant cavity; ν is the wave number, ν=1 / λ, λ is the wavelength, and δ(m,n) is the optical path difference of the FP resonant cavity corresponding to the (m,n)th step, and its expression is: ; Wherein, n0 is the refractive index of the medium in the FP resonant cavity. When the FP resonant cavity is air, n0=1; θ is the incident angle of the light in the FP resonant cavity, corresponding to different field angles of the target scene; h(m,n) is the gap thickness of the FP resonant cavity, which varies with the spatial position due to the checkerboard step structure.

[0038] 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 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, but the interference signal is an extremely narrow comb function. The traditional Fabry-Perot interferometer uses a reflection interface with a very high reflectivity and uses resonance to produce very narrow interference features. These very narrow comb-shaped interference signals will produce side lobes in the Fourier transform spectrum, which will take away the signal from the main frequency and cause the restored spectrum to be distorted. In order to suppress the spectral distortion caused by harmonic signals and reduce the interference caused by the side lobe signal extending 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 is less than or equal to twice the minimum wavelength, that is, 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, medium-wave infrared 3μm-5μm, and long-wave infrared 8μm-14μm, they can basically meet the application requirements.

[0039] Therefore, the present invention uses a reflective interface with medium reflectivity to generate a cosine-like interference signal. At this time, the spectral fineness is low, and the checkerboard step mirror 40 is close to double-beam interference. The interference pattern function can be approximately expressed as: ; Therefore, the spectral information of the incident light signal can be restored from the interference signal through Fourier transform.

[0040] 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 also reduce the modulation of the interference fringes. Therefore, in order to achieve effective demodulation of the spectrum through Fourier transform, it is necessary to balance the fringe modulation and spectral sidelobe suppression. By balancing the interference pattern modulation and the cosine degree of the interference signal, the reflectivity of the reflection interface is determined, the spectral sidelobes are minimized and the interference pattern modulation is maximized. In other words, by optimizing the reflectivity of the reflection interface, the maximum trade-off between the interference fringe modulation and the cosine characteristic is achieved. In the present invention, the reflectivity of the two reflection interfaces of the FP resonant cavity is set at 30% to 40%, which can achieve the best trade-off between the interference pattern modulation and the cosine characteristic. Therefore, each FP resonant cavity in the present invention is a low-fineness Fabry-Perot cavity that modulates the interference intensity of the interference image under a given optical path difference.

[0041] For infrared materials, silicon and germanium materials have a higher refractive index and thus have a relatively moderate reflectivity. For silicon material, its refractive index is 3.4 and its surface reflectivity is 30%; for germanium material, its refractive index is 4.0 and its surface reflectivity is 36%. Therefore, the parallel plate 30 is made of a high refractive index medium such as silicon or germanium, and an anti-reflection film is evaporated on the surface of the parallel plate 30 away from the checkerboard step mirror 40, and no film is applied on the surface of the parallel plate 30 facing the checkerboard step mirror 40. The step substrate 401 can be made of a high refractive index medium material such as silicon or germanium, and the surface of the step substrate 401 facing the parallel plate 50 is not coated, so that the reflective interface has a medium-low reflectivity to achieve low fineness. The surface of the step substrate 401 away from the parallel plate 50 is evaporated with an anti-reflection film 402 to ensure high transmittance, and the highest step of the step substrate 401 is in contact with the uncoated side of the parallel plate 30 to achieve zero optical path difference on the optical contact surface.

[0042] An air gap with increasing thickness is formed between the checkerboard step mirror 40 and the parallel plate 30, thereby forming a spatially distributed modulation of the optical path difference. The height of each step is complementary to the thickness of the air gap of the FP resonant cavity. Assuming that the step height difference of the checkerboard step mirror 40 is d, and the number of steps is M×N, in order to form continuous optical path difference sampling, the thickness of the air gap of the FP resonant cavity corresponding to the (m,n)th step is: .

[0043] 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 high-order sidelobe signals, the sampled signal cutoff frequency is extended to J Subharmonics (generally, J For the incident broadband spectrum, the step height difference d should satisfy the following relationship: ; Among them, λ min is the minimum wavelength of the broadband spectrum, θ max is the incident angle of the maximum field angle light in the FP resonant cavity, J is the spectral harmonic order, and n0 is the refractive index of the medium in the FP resonant cavity.

[0044] For incident narrowband spectra, especially for gas detection, in order to achieve high spectral resolution, the step height difference d should satisfy the following relationship: ; Among them, 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, which is less than or equal to Any integer of .

[0045] The step base 601 is composed of M×N steps. The height difference between any two adjacent steps in one direction of the step base 601 is constant d, and the height difference between any two adjacent steps in another direction of the step base 601 is constant Md. The step base 601 is manufactured by etching. The manufacturing process is as follows: S1: providing a plane substrate with a lateral width of a and a longitudinal width of b, and polishing and cleaning the plane substrate.

[0046] S2: The planar substrate is longitudinally etched to form a longitudinal two-step structure, with an etching depth of NMd / 2 and an etching width of b / 2.

[0047] S3: The longitudinal two-step structure is longitudinally etched to form a longitudinal four-step structure, with an etching depth of NMd / 4 and an etching width of b / 4. Step S3 is repeated to ensure 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.

[0048] S4: performing transverse etching on 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.

[0049] S5: The lateral two-step structure is laterally etched to form a lateral four-step structure, with an etching depth of Md / 4 and an etching width of a / 4. Step S5 is repeated to ensure 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. Finally, a step substrate 401 with a step height difference of d and a step number of M×N is manufactured.

[0050] The interference light field modulated by the Fabry-Perot array interference system enters the beam reduction optical system, which reduces the light field at a certain magnification and then incidents it on the area array detector to form an interference pattern array. The interference pattern array has M×N interference pattern units, each of which corresponds to a specific step of the checkerboard step mirror 40, that is, each interference pattern unit corresponds to a specific FP resonant cavity.

[0051] The beam reduction optical system adopts a double telecentric optical path structure, which is composed of a beam reduction objective lens 50, an aperture stop 60, and a beam reduction image lens 70. The beam reduction objective lens 50 serves as the objective lens of the beam reduction optical system, and the beam reduction image lens 70 serves as the eyepiece of the beam reduction optical system. The image side focal plane of the beam reduction objective lens 50 coincides with the object side focal plane of the beam reduction image lens 70. The checkerboard step mirror 40 is located on the object side focal plane of the beam reduction objective lens 50. The area array detector 80 is located on the image side focal plane of the beam reduction image lens 70. The aperture stop 60 is located on the image side focal plane of the beam reduction image lens 70. 0 is located on the image-side focal plane of the focusing objective 50 and the object-side focal plane of the focusing mirror 70. The focusing objective 50 is used to converge the interference light field modulated by the flat Fabry-Perot array interferometer system. The aperture diaphragm 60 is used to limit the light beam aperture to achieve filtering of the numerical aperture of the light beam. The focusing mirror 70 is used to collimate the light beam after numerical aperture filtering into parallel light, thereby projecting the interference light field onto the area array detector 80 to obtain a set of interference pattern arrays.

[0052] Each interference pattern unit in the interference pattern array corresponds to a FP resonant cavity of a specific air gap thickness in the FP resonant cavity array, and thus corresponds to an inherent optical path difference δ. In the present invention, each interference pattern unit in the interference pattern array is composed of a certain number of pixels to form a super-pixel, and intensity modulated by the corresponding FP resonant cavity. The interference pattern array is subjected to image segmentation, and the interference pattern array is divided into interference pattern units corresponding to each specific FP resonant cavity. After the divided interference pattern units are integrated and arranged in the order of the thickness of the FP resonant cavity, an interference pattern sequence corresponding to each specific optical path difference δ is obtained. The interference pattern sequence is subjected to discrete Fourier transform operation along the optical path difference δ axis, and the restoration of spectral information is realized.

[0053] Example 2 The difference between Example 2 and Example 1 is only that the structure of the chessboard step mirror is different. Figure 7 As shown, the step substrate 403 of the checkerboard step mirror of Example 2 is made of a low refractive index transparent medium material such as quartz, calcium fluoride, magnesium fluoride, sapphire, etc. to ensure a higher transmittance at the interface, and a high refractive index medium film 405 such as silicon or germanium is evaporated on the step surface of the step substrate 403 so that the reflection interface has a medium-low reflectivity to achieve low fineness, and an anti-reflection film 404 is evaporated on the lower surface (i.e., the bottom plane) of the step substrate 403 to ensure high transmittance.

[0054] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps described in the disclosure of the present invention can be performed in parallel, sequentially or in different orders, as long as the desired results of the technical solution disclosed in the present invention can be achieved, and this document does not limit this.

[0055] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer, characterized in that: It includes a light source, a collimator, a Fabry-Perot array interferometer system, a beam reduction optical system and a planar array detector; wherein, The light source is located on the object focal plane of the collimator and is used to emit a light beam; The collimator is used to collimate the light beam emitted by the light source into a parallel beam; The Fabry-Perot array interference system is located in the parallel optical path of the collimator. The Fabry-Perot array interference system includes a parallel plate and a checkerboard step mirror. The parallel plate serves as a front interference plate of the Fabry-Perot array interference system, and the checkerboard step mirror serves as a rear interference plate of the Fabry-Perot array interference system. The number of steps of the checkerboard step mirror is M×N, and the step heights are distributed in a two-dimensional decreasing manner in a checkerboard shape. The step height difference between any two adjacent steps in one direction of the checkerboard step mirror is d, and the step height difference between any two adjacent steps in another direction of the checkerboard step mirror is Md. The highest step fits tightly with the parallel plate, so that an air gap with a thickness that changes in a two-dimensional increasing manner is formed between the parallel plate and the checkerboard step mirror. The thickness of each area of ​​the air gap corresponds to an optical path difference. The parallel plate, the air gap and the checkerboard step mirror form a FP resonant cavity array. The beam reduction optical system is located in the exit direction of the Fabry-Perot array interferometer system and adopts a double telecentric optical path structure to image the interference light field modulated by the Fabry-Perot array interferometer system onto the area array detector to obtain an interference image array.

2. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 1, characterized in that: The two opposite surfaces of the parallel plate and the chessboard step mirror are respectively used as reflection interfaces, and the two reflection interfaces and the air gap form a FP resonant cavity, and the reflectivity of the two reflection interfaces is 30%~40%.

3. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 1 or 2, characterized in that: A high refractive index plate is used as a parallel plate, and an anti-reflection film is evaporated on the surface of the high refractive index plate away from the checkerboard step mirror, and the surface of the high refractive index plate facing the checkerboard step mirror is not coated. Alternatively, a low refractive index plate is used as a parallel plate, and an anti-reflection film is evaporated on the surface of the low refractive index plate away from the checkerboard step mirror, and a high refractive index dielectric film is evaporated on the surface of the low refractive index plate facing the checkerboard step mirror and serves as the reflection interface of the FP resonant cavity.

4. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 3, characterized in that: The high refractive index plate is made of silicon or germanium dielectric material, and the low refractive index plate is made of quartz, calcium fluoride, magnesium fluoride or sapphire dielectric material.

5. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 1, characterized in that: The checkerboard step mirror comprises a step substrate, wherein the step substrate adopts a high refractive index medium, the upper surface of the step substrate of the high refractive index medium is not coated and serves as a reflection interface of the FP resonant cavity, and an anti-reflection film is evaporated on the lower surface of the step substrate of the high refractive index medium; or, the step substrate adopts a low refractive index medium, a high refractive index medium film is evaporated on the upper surface of the step substrate of the low refractive index medium and serves as a reflection interface of the FP resonant cavity, and an anti-reflection film is evaporated on the lower surface of the step substrate of the low refractive index medium.

6. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 5, characterized in that: The low refractive index medium adopts quartz, calcium fluoride, magnesium fluoride or sapphire medium material; the high refractive index medium adopts silicon or germanium medium material; and the high refractive index medium film adopts silicon film or germanium film.

7. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 1, characterized in that: The step height of the chessboard step mirror is distributed in a two-dimensional decreasing manner in a chessboard shape. m Direction d The step length decreases successively, and the height increases along n Direction Md The step size decreases successively. m , n ) steps height D ( m , n )for: D ( m , n )=-[( n -1) M + m -1] d 。 8. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 7, characterized in that: The thickness of the air gap is distributed in a two-dimensional increasing manner in a checkerboard pattern. m Direction d The step length increases successively, and the thickness increases along n Direction Md The step size increases successively. m , n ) steps corresponding to the thickness of the air gap of the FP resonant cavity h ( m , n )for: h ( m , n )=[( n -1) M + m -1] d 。 9. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 1, characterized in that: For an incident broadband spectrum, the step height difference d satisfies the following relationship: ； Among them, λ min is the minimum wavelength of the broadband spectrum, θ max is the incident angle of the light with the maximum field angle in the FP resonant cavity, J is the spectral harmonic order, and n0 is the refractive index of the medium in the FP resonant cavity; For an incident narrowband spectrum, the step height difference d satisfies the following relationship: ； Among them, 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, which is less than or equal to Any integer of .

10. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 5, characterized in that: The preparation process of the step substrate is: S1: providing a plane substrate with a lateral width of a and a longitudinal width of b, and polishing and cleaning the plane substrate; S2: The planar substrate is longitudinally etched to form a longitudinal two-step structure, with an etching depth of NMd / 2 and an etching width of b / 2; S3: The longitudinal two-step structure is longitudinally etched to form a longitudinal four-step structure, the etching depth is NMd / 4, the etching width is b / 4, and step S3 is repeated to ensure 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; S4: performing transverse etching on 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: The lateral two-step structure is laterally etched to form a lateral four-step structure, with an etching depth of Md / 4 and an etching width of a / 4. Step S5 is repeated to ensure 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.

11. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 1, characterized in that: The beam reduction optical system includes a beam reduction objective, an aperture diaphragm and a beam reduction image lens. The image side focal plane of the beam reduction objective coincides with the object side focal plane of the beam reduction image lens. The Fabry-Perot array interference system is located on the object side focal plane of the beam reduction objective, the area array detector is located on the image side focal plane of the beam reduction image lens, and the aperture diaphragm is located on both the image side focal plane of the beam reduction objective and the object side focal plane of the beam reduction image lens. The beam reduction objective is used to converge the interference light field modulated by the flat Fabry-Perot array interference system; the aperture diaphragm is used to limit the aperture of the light beam to achieve filtering of the numerical aperture of the light beam; and the beam reduction image lens is used to collimate the light beam after numerical aperture filtering into parallel light.

12. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 1, characterized in that: The planar array detector is also used to perform image segmentation on the interference image array, dividing the interference image array into interference image units corresponding to each FP resonant cavity, integrating each interference image unit, and then arranging them in the order of the thickness of each FP resonant cavity to form an interference image sequence. With the optical path difference as the axis, the interference image sequence is subjected to discrete Fourier transform operation to obtain a restored spectrum.

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