Low-Finesse Fabry-Perot Interferometric Spatial Modulation Fourier Transform Spectrometer

Abstract

The present invention relates to the field of spectral imaging, and particularly to a Fabry-Perot interferometric spatial modulation Fourier transform spectrometer with low finesse, which includes a light source, a collimating mirror, a Fabry-Perot array interferometric system, a beam-reducing optical system, and a planar array detector. The light source is used to emit a light beam; the collimating mirror is used to collimate the light beam emitted by the light source into a parallel light beam; 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 manner in a checkerboard pattern, the step height differences between adjacent two steps are 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 between the parallel flat plate and the checkerboard stepped mirror, and the thickness of each region of the air gap corresponds to an optical path difference respectively; the beam-reducing optical system is used to image the modulated interference light field onto the planar array detector 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 Fabry-Perot interferometric spatial modulation Fourier transform 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 light beams through the beam splitting of the input light beam. Under the illumination of monochromatic light, the response of the detector to this time-varying optical path difference is a sine signal that changes 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 lead to a decrease 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 Fabry-Perot interferometric 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 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 Fabry-Perot interferometric spatial modulation Fourier transform spectrometer, comprising a light source, a collimating mirror, a Fabry-Perot array interferometric system, a beam-reducing optical system and a planar array detector; wherein,

[0007] The light source is located on the object focal plane of the collimating mirror and is used for emitting a light beam;

[0008] The collimating mirror is used for collimating the light beam emitted by the light source into a parallel light beam;

[0009] The Fabry-Perot array interferometric system is located in the parallel optical path of the collimating mirror. The Fabry-Perot array interferometric system includes a parallel flat plate and a checkerboard stepped mirror. The parallel flat plate serves as the front interferometric plate of the Fabry-Perot array interferometric system, and the checkerboard stepped mirror serves as the rear interferometric plate of the Fabry-Perot array interferometric system. The number of steps of the checkerboard stepped mirror is M×N, and the step heights are distributed in a two-dimensional decreasing manner in a checkerboard pattern. The step height difference between any two adjacent steps is the same. The highest step is closely attached to the parallel flat plate, so that an air gap with a two-dimensional increasing thickness is formed between the parallel flat plate and the checkerboard stepped mirror. The thickness of each region of the air gap corresponds to an optical path difference. The parallel flat plate, the air gap and the checkerboard stepped mirror form an FP resonator array;

[0010] The beam-reducing optical system is located in the outgoing direction of the Fabry-Perot array interferometric system and adopts a double telecentric optical path structure. It is used for imaging the interference light field modulated by the Fabry-Perot array interferometric system onto the planar array detector to obtain an interference image array.

[0011] Further, the two surfaces of the parallel flat 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 reflectivities of the two reflection interfaces are 30% - 40%.

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

[0013] Further, the high-refractive-index flat plate adopts a silicon or germanium dielectric material, and the low-refractive-index flat 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. 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. 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 is made of quartz, calcium fluoride, magnesium fluoride or sapphire medium material; the high refractive index medium is made of silicon or germanium medium material; the high refractive index medium film is made of a silicon film or a germanium film.

[0016] Further, the step heights of the checkerboard stepped mirror are distributed in a two-dimensional decreasing pattern in the shape of a checkerboard. The height decreases successively with a step size of m in the d direction, and decreases successively with a step size 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, the thickness of the air gap is distributed in a two-dimensional increasing pattern in the shape of a checkerboard. The thickness increases successively with a step size of m in the d direction, and increases successively with a step size 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] Further, 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 field of view 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] Further, 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 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;

[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 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.

[0032] Further, the beam reduction optical system includes a beam reduction objective lens, an aperture stop, and a beam reduction image lens. The image-side focal plane of the beam reduction objective lens 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 lens, and the area array detector is located on the image-side focal plane of the beam reduction image lens. The aperture stop is simultaneously located on the image-side focal plane of the beam reduction objective lens and the object-side focal plane of the beam reduction image lens. The beam reduction objective lens is used to converge the interference light field modulated by the flat Fabry-Perot array interference system; the aperture stop is used to limit the beam aperture and achieve filtering of the beam numerical aperture; the beam reduction image lens is used to collimate the beam filtered by the numerical aperture into parallel light.

[0033] Further, 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, integrate each interference image unit, and then arrange them in the thickness order of each FP resonator to form an interference image sequence. Taking the optical path difference as the axis, perform a discrete Fourier transform operation on the interference image sequence to obtain the restored spectrum.

[0034] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0035] The present invention uses a checkerboard step mirror with medium and low reflectivity to construct a Fabry-Perot array interference system, replacing the beam splitter and precision moving mirror scanning mechanism in the traditional Fourier transform imaging spectrometer, simplifying and lightening the optomechanical structure of the Fourier transform imaging spectrometer, and improving the stability and robustness of the Fourier transform imaging spectrometer. It has the advantages of small volume, light weight, compact structure, static, stable, and reliable. At the same time, the incident interface of the checkerboard step mirror uses medium and low reflectivity, and the spectrum can be restored through Fourier transform, reducing the difficulty of spectrum restoration in the traditional Fabry-Perot interference system. Through the coupling modulation transmission of the light field by the Fabry-Perot array interference system, synchronous measurement of the interference image sequence can be realized, and then snapshot and effective detection of the target scene image information and spectrum information can be achieved, improving the real-time performance of multi-dimensional information detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments and descriptions of the present invention are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0037] Figure 1 is a schematic structural diagram of a low-finesse Fabry-Perot interference spatial modulation Fourier transform spectrometer according to Embodiment 1 of the present invention.

[0038] Figure 2 is a schematic structural diagram of the checkerboard step mirror according to Embodiment 1 of the present invention.

[0039] Figure 3 It is a schematic diagram of the spatial distribution of the air gap thickness between the parallel flat plate and the checkerboard stepped mirror according to Embodiment 1 of the present invention.

[0040] Figure 4 It is a schematic diagram of the preparation process of the stepped substrate according to Embodiment 1 of the present invention.

[0041] Figure 5 It is a schematic diagram of the detection process of the interference image array by the beam-shrinking optical system according to Embodiment 1 of the present invention.

[0042] Figure 6 It is a schematic diagram of the restoration process from the interference image array to the spectrum according to Embodiment 1 of the present invention.

[0043] Figure 7 It is a schematic diagram of the structure of the checkerboard stepped mirror according to Embodiment 2 of the present invention.

[0044] Explanation of the reference numerals in Embodiment 1: light source 10, collimating mirror 20, parallel flat plate 30, checkerboard stepped mirror 40, stepped substrate 401, antireflection film 402, beam-shrinking objective lens 50, aperture stop 60, beam-shrinking image lens 70, area array detector 80.

[0045] Explanation of the reference numerals in Embodiment 2: stepped substrate 403, antireflection film 404, high refractive index dielectric film 405. Detailed implementation manners

[0046] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to 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.

[0047] 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.

[0048] 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, the 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 specified, the meaning of "a plurality" is two or more.

[0049] In the description of the present invention, it should be noted that unless otherwise clearly specified and defined, the terms "mounted", "connected", and "coupled" should 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 it 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.

[0050] The present invention will be described in detail below with reference to the drawings and in conjunction with embodiments.

[0051] The present invention provides a low-finesse Fabry-Perot interferometric spatial modulation Fourier transform spectrometer, which includes a light source, a collimating mirror, a Fabry-Perot array interferometric system, a beam reduction optical system, and a planar array detector; wherein, the light source is located on the object focal plane of the collimating mirror and is used to emit a light beam; the collimating mirror is used to collimate the light beam emitted by the light source into a parallel light beam; the Fabry-Perot array interferometric system is located in the parallel optical path of the collimating mirror, and the Fabry-Perot array interferometric system includes a parallel flat plate and a checkerboard stepped mirror. The parallel flat plate serves as the front interference plate of the Fabry-Perot array interferometric system, and the checkerboard stepped mirror serves as the rear interference plate of the Fabry-Perot array interferometric 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 is the same. The highest step is closely attached to the parallel flat plate, so that an air gap with a two-dimensional increasing thickness is formed between the parallel flat plate and the checkerboard stepped mirror. The thickness of each region of the air gap corresponds to an optical path difference. The parallel flat plate, the air gap, and the checkerboard stepped mirror form an FP resonator array; the beam reduction optical system is located in the outgoing direction of the Fabry-Perot array interferometric system and adopts a double telecentric optical path structure to image the interference light field modulated by the Fabry-Perot array interferometric system onto the planar array detector to obtain an interference image array.

[0052] The light beam emitted by the light source is collimated into parallel light by the collimating mirror, and then enters the Fabry-Perot array interference system based on the checkerboard stepped mirror, where interference is formed in the FP resonator of the Fabry-Perot array interference system. The Fabry-Perot array interference system consists of a parallel flat plate and a checkerboard stepped mirror. The checkerboard stepped mirror is composed of multiple steps arranged in a two-dimensional checkerboard shape. Each step of the checkerboard stepped mirror has a specific thickness, corresponding to an FP resonator. Therefore, the Fabry-Perot array interference system is actually an FP resonator array composed of multiple FP resonators. By tightly 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 shape 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 high refractive indices, so that the reflection interfaces on both sides of the air gap have medium and low reflectivities, forming a low-finesse Fabry-Perot interference array, and thus the spectral demodulation can be realized 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 incident target light field and generating a change in the interference intensity. Therefore, the parallel light collimated by the collimating mirror forms an interference light field array after being modulated by the Fabry-Perot array interference system, 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 area array detector by the beam reduction optical system at a certain reduction ratio. The beam reduction optical system adopts a double telecentric optical path structure, and an interference pattern array corresponding to the optical path difference array is obtained on the area array detector, and the spectral information can be demodulated by performing a discrete Fourier transform on the interference pattern array, realizing the 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 being structurally compact, static, stable, and reliable.

[0053] Embodiment 1

[0054] As Figures 1-6 shown, the low-finesse Fabry-Perot interference spatial modulation Fourier transform spectrometer provided by Embodiment 1 of the present invention for creation includes a light source 10, a collimating mirror 20, a parallel flat plate 30, a checkerboard stepped mirror 40, a beam reduction objective lens 50, an aperture stop 60, a beam reduction image lens 70, and an area array detector 80 (using devices such as HgCdTe or InSb).

[0055] The light beam emitted by the light source 10 is first collimated into a parallel light beam by the collimating mirror 20, and then enters the Fabry-Perot array interference system, where interference is formed in the FP resonant cavity of the Fabry-Perot array interference system. The interference plate of the Fabry-Perot array interference system consists of a parallel flat plate 30 and a checkerboard stepped mirror 40. The parallel flat plate 30 serves as the front interference plate of the Fabry-Perot array interference system, and the checkerboard stepped mirror 40 serves as the rear interference plate of the Fabry-Perot array interference system. The checkerboard stepped mirror 40 includes a stepped substrate 401, and the multiple steps of the stepped substrate 401 are arranged in a two-dimensional checkerboard shape. Each step has a specific thickness, thus corresponding to an FP resonant cavity. 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 flat plate 30 is closely attached to the highest step of the checkerboard stepped mirror 40, 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 a zero optical path difference is achieved at the optical contact positions of the two opposite surfaces of the parallel flat plate 30 and the checkerboard stepped mirror 40.

[0056] The two opposite surfaces of the checkerboard stepped mirror 40 facing the parallel flat plate 30 serve as reflection interfaces and form a distributed FP resonant cavity with the air gap. Each FP resonant cavity 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 resonant cavity will be reflected multiple times in the FP resonant cavity, and interference will occur between the light beams passing through the FP resonant cavity different numbers of times. The intensity of the interference signal of the light beam exiting through the FP resonant cavity 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 resonant cavity between the two reflection interfaces. The medium in the FP resonant cavity can be air or vacuum, or a low-refractive-index medium. In this invention, an air embodiment is used for illustration. The reflection interfaces of the traditional Fabry-Perot interference system usually use very high reflectivity to produce a narrow transmission band, while in the Fabry-Perot array interference system of this invention, the reflection interfaces use medium reflectivity to produce a quasi-cosine interference signal that can be demodulated by Fourier transform.

[0057] The interference image signal formed by the FP resonant cavity corresponding to the (m, n)th step in the checkerboard stepped mirror 40 of the Fabry-Perot array interference system can be expressed as:

[0058] ;

[0059] where 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:

[0060] ;

[0061] Among them, 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 of view angles in the target scenario; h(m,n) is the gap thickness of the FP resonator, which varies with different spatial positions due to the checkerboard step structure.

[0062] 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 by resonance. Sidelobes will be generated in the Fourier transform spectrum of these very narrow comb - shaped interference signals, and 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 signals 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 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.

[0063] Therefore, the present invention uses a reflection interface with a medium reflectivity to generate a quasi - cosine interference signal. At this time, the spectral fineness is relatively low, and the checkerboard step mirror 40 is close to double - beam interference. The interference pattern function can be approximately expressed as:

[0064] ;

[0065] Therefore, the spectral information of the incident light signal can be restored from the interference signal through Fourier transform.

[0066] 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 reflectivity will lead to a decrease in the modulation depth of the interference fringes. 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 to minimize the spectral side lobes and maximize 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, which modulates the interference intensity of the interference image at a given optical path difference.

[0067] For infrared materials, silicon and germanium materials have relatively high refractive indices and thus relatively moderate reflectivities. For silicon material, its refractive index is 3.4 and the surface reflectivity is 30%; for germanium material, its refractive index is 4.0 and the surface reflectivity is 36%. Therefore, the parallel plate 30 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 30 facing away from the checkerboard stepped mirror 40, and no film is deposited on the surface of the parallel plate 30 facing the checkerboard stepped mirror 40. The stepped substrate 401 can be made of a high - refractive - index medium material such as silicon or germanium. No film is deposited on the surface of the stepped substrate 401 facing the parallel plate 50, so that the reflection interface has a medium - low reflectivity to achieve low finesse. An anti - reflection film 402 is deposited on the surface of the stepped substrate 401 facing away from the parallel plate 50 to ensure high transmittance. The highest step of the stepped substrate 401 is in contact with the uncoated side of the parallel plate 30, and zero optical path difference is achieved at the optical contact surface.

[0068] An air gap with an increasing thickness is formed between the checkerboard stepped mirror 40 and the parallel plate 30, 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 stepped mirror 40 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:

[0069] 。

[0070] To achieve effective spectral restoration, the step height difference d should satisfy the Nyquist sampling theorem. To suppress the spectral aliasing caused by the Fourier transform of the high - order side - lobe signals, the signal cut - off frequency of the sampling 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:

[0071] ;

[0072] where λ min is the minimum wavelength of the broadband spectrum, θ max is the incident angle corresponding to the maximum field of view light in the FP resonator, J is the spectral harmonic order, and n0 is the refractive index of the medium in the FP resonator.

[0073] 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:

[0074] ;

[0075] 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 .

[0076] The step substrate 601 is composed of M×N steps. The height difference between any two adjacent steps in one direction of the step substrate 601 is a constant d, and the height difference between any two adjacent steps in the other direction of the step substrate 601 is a constant Md. It is fabricated by etching, and the fabrication process is as follows:

[0077] S1: Provide a planar substrate with a transverse width of a and a longitudinal width of b, and polish and clean the planar substrate.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] S5: Transversely etch the horizontal two-step structure to form a horizontal 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. Finally, a step base 401 with a step height difference of d and M×N steps is fabricated.

[0082] The interference light field modulated by the Fabry - Perot array interference system enters the beam - reducing optical system. The beam - reducing optical system reduces the light field by a certain magnification and then irradiates it onto the area array detector, forming an interference pattern array. The interference pattern array has M×N interference pattern units, and each interference pattern unit corresponds to a specific step of the checkerboard step mirror 40, that is, each interference pattern unit corresponds to a specific FP resonator.

[0083] The beam - reducing optical system adopts a double telecentric optical path structure, which consists of a beam - reducing objective lens 50, an aperture stop 60, and a beam - reducing image lens 70. The beam - reducing objective lens 50 serves as the objective lens of the beam - reducing optical system, and the beam - reducing image lens 70 serves as the eyepiece of the beam - reducing optical system. The image - side focal plane of the beam - reducing objective lens 50 coincides with the object - side focal plane of the beam - reducing image lens 70. The checkerboard step mirror 40 is located on the object - side focal plane of the beam - reducing objective lens 50, and the area array detector 80 is located on the image - side focal plane of the beam - reducing image lens 70. The aperture stop 60 is simultaneously located on the image - side focal plane of the beam - reducing objective lens 50 and the object - side focal plane of the beam - reducing image lens 70. The beam - reducing objective lens 50 is used to converge the interference light field modulated by the planar Fabry - Perot array interference system. The aperture stop 60 is used to limit the beam aperture and achieve filtering of the beam numerical aperture. The beam - reducing image lens 70 is used to collimate the beam after numerical aperture filtering into parallel light, so as to project the interference light field onto the area array detector 80 to obtain a set of interference pattern arrays.

[0084] Each interference pattern unit in the interference pattern array corresponds to an FP resonator with a specific air - gap thickness in the FP resonator array, and thus corresponds to an inherent optical path difference δ. In the present invention, each interference pattern unit in the interference pattern array consists of a certain number of pixels to form a super - pixel, and is intensity - modulated by the corresponding FP resonator. Perform image segmentation on the interference pattern array to divide the interference pattern array into interference pattern units corresponding to each specific FP resonator. Integrate the divided interference pattern units and arrange them in the order of the thickness of the FP resonator, and an interference pattern sequence corresponding to each specific optical path difference δ is obtained. Perform discrete Fourier transform operation on the interference pattern sequence along the optical path difference δ axis, and the restoration of spectral information is achieved.

[0085] Embodiment 2

[0086] The difference between Embodiment 2 and Embodiment 1 lies only in the different structures of the checkerboard step mirror, as Figure 7As shown, the step substrate 403 of the checkerboard step mirror in Embodiment 2 is made of a low refractive index transparent dielectric material such as quartz, calcium fluoride, magnesium fluoride, or sapphire to ensure a high transmittance at the interface. A high refractive index dielectric film 405 such as silicon or germanium is deposited on the step surface of the step substrate 403, so that the reflection interface has a medium and low reflectivity to achieve low fineness. An antireflection film 404 is deposited on the lower surface (i.e., the bottom plane) of the step substrate 403 to ensure a high transmittance.

[0087] It should be understood that various forms of the processes shown above can be used, steps can be reordered, added, or deleted. For example, the steps described 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.

[0088] 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 principles of the present invention shall be included within 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 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. The reflectivity of the two reflection interfaces is 30%~40%; 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 ; 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.

2. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 1, 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.

3. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 2, 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.

4. 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.

5. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 4, 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.

6. 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 。 7. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 6, 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 。 8. The low-fineness Fabry-Perot interferometer spatial modulation Fourier transform spectrometer according to claim 4, 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.

9. 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.

10. 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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