Dual-step Fabry-Perot static interference Fourier transform spectrometer

Abstract

The present invention relates to the field of spectral imaging, and particularly to a double-step Fabry-Perot static interference Fourier transform spectrometer, which includes a light source, a collimating mirror, a Fabry-Perot array interference 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 interference system is used to modulate the target light field into an interference light field array, which includes two stepped lenses with orthogonal step directions, and the highest steps of the two stepped lenses are closely attached, so that an air gap with a two-dimensional increment change in the shape of a checkerboard is formed between the two stepped lenses; the beam reducing optical system is used to image the interference light field modulated by the Fabry-Perot array interference system 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 dual-step Fabry-Perot static interference 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 wide range of applications, and 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. This moving mirror splits the input beam, generating a time-varying optical path difference between the two beams. Under the illumination of monochromatic light, the detector's response to this time-varying optical path difference is a sine signal that changes with the optical path difference. By precisely measuring the change in the optical path difference, usually using a reference laser signal, the wavelength information of the incident light is 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 is large in volume, heavy in weight, difficult to integrate, and 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 advantages such as 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. 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 sine curve, and the spectrum can be restored through Fourier transform. However, too low a coating reflectivity will result in a decrease in fringe contrast, leading to interference failure. Summary of the Invention

[0004] In view of this, the present invention aims to provide a dual-step Fabry-Perot static interference Fourier transform spectrometer to solve the technical problems that the traditional Fourier transform infrared spectrometer mainly uses a time-modulated Michelson interferometer system. The Michelson interferometer system needs to use a beam splitter to split and combine the incident beam, and it uses a high-precision moving mirror drive system to finely scan the optical path difference, resulting in a large volume, heavy weight, difficult integration, and extremely vulnerable to the external environment.

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

[0006] A double-step Fabry-Perot static interference Fourier transform spectrometer, comprising a light source, a collimator, a Fabry-Perot array interference 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 collimator and is used to emit a light beam;

[0008] The collimator is used to collimate the light beam emitted by the light source into a parallel light beam;

[0009] The Fabry-Perot array interference system is located in the parallel optical path of the collimator. The Fabry-Perot array interference system is used to modulate the target light field to form an interference light field array. The Fabry-Perot array interference system includes a first stepped lens and a second stepped lens with orthogonal stepped directions. The first stepped lens and the second stepped lens are respectively used as the front interference plate and the rear interference plate of the Fabry-Perot array interference system. The number of steps of the first stepped lens is M, and the number of steps of the second stepped lens is N. The step height difference between any two adjacent steps of the first stepped lens is d, and the step height difference between any two adjacent steps of the second stepped lens is Md. The highest step of the first stepped lens is closely attached to the highest step of the second stepped lens, so that an air gap with a two-dimensional increasing change in the shape of a checkerboard is formed between the first stepped lens and the second stepped lens. The thickness of each region of the air gap corresponds to an optical path difference. The first stepped lens, the air gap and the second stepped lens form an FP resonator array;

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

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

[0012] Further, the first stepped lens and the second stepped lens respectively include a stepped substrate. When the stepped substrate uses a high refractive index medium, the upper surface of the stepped substrate of the high refractive index medium is not coated and is used as the reflection interface of the FP resonator, and an antireflection film is deposited on the lower surface of the stepped substrate of the high refractive index medium; when the stepped substrate uses 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 is used as the reflection interface of the FP resonator, and an antireflection film is deposited on the lower surface of the stepped substrate of the low refractive index medium.

[0013] Furthermore, the low refractive index medium adopts quartz, calcium fluoride, magnesium fluoride or sapphire dielectric materials; the high refractive index medium adopts silicon or germanium dielectric materials; the high refractive index dielectric film adopts a silicon film or a germanium film.

[0014] Furthermore, the thickness of the air gap is distributed in a checkerboard-like two-dimensional increasing pattern, and the thickness increases sequentially along the m direction with a step size of d and increases sequentially along the n direction with a step size of Md . The thickness of the air gap of the FP resonator corresponding to the ( m , n )-th step is h ( m , n ) and is:

[0015] h ( m , n ) = ([( n -1) M + m -1) d .

[0016] Furthermore, for the incident broadband spectrum, the step height difference d of the first stepped lens satisfies the following relationship:

[0017] ;

[0018] where λ min is the minimum wavelength of the broadband spectrum, θ max is the incident angle corresponding to the light ray at the maximum field of view angle in the FP resonator, J is the spectral harmonic order, and n0 is the refractive index of the medium in the FP resonator;

[0019] For the incident narrowband spectrum, the step height difference d of the first stepped lens satisfies the following relationship:

[0020] ;

[0021] where λ S is the shortest wavelength of the narrowband spectrum, λ L is the longest wavelength of the narrowband spectrum, k is the spectral folding order, and an arbitrary integer less than or equal to is taken.

[0022] Furthermore, the preparation process of the step base of the first stepped lens is as follows:

[0023] A1: Provide a planar base with a transverse width of a and a longitudinal width of b, and polish and clean the planar base;

[0024] A2: Etch the planar substrate to form a two-step structure with an etching depth of Md / 2 and an etching width of a / 2;

[0025] A3: Etch the two-step structure to form a four-step structure with an etching depth of Md / 4 and an etching width of a / 4. Repeat step A3, 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 step height difference is d and the step width is a / M;

[0026] The preparation process of the step substrate of the second-step lens is as follows:

[0027] B1: Provide a planar substrate with a transverse width of a and a longitudinal width of b, and polish and clean the planar substrate;

[0028] B2: Etch the planar substrate to form a two-step structure with an etching depth of NMd / 2 and an etching width of b / 2;

[0029] B3: Etch the two-step structure to form a four-step structure with an etching depth of NMd / 4 and an etching width of b / 4. Repeat step B3, 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 step height difference is Md and the step width is b / N.

[0030] Furthermore, the beam shrinking optical system includes a beam shrinking objective lens, an aperture stop, and a beam shrinking image lens. The image-side focal plane of the beam shrinking objective lens coincides with the object-side focal plane of the beam shrinking image lens. The Fabry-Perot array interference system is located on the object-side focal plane of the beam shrinking objective lens, the area array detector is located on the image-side focal plane of the beam shrinking image lens, and the aperture stop is simultaneously located on the image-side focal plane of the beam shrinking objective lens and the object-side focal plane of the beam shrinking image lens. The beam shrinking objective lens is used to converge the interference light field modulated by the Fabry-Perot array interference system; the aperture stop is used to limit the beam aperture to achieve filtering of the beam numerical aperture; the beam shrinking image lens is used to collimate the beam after numerical aperture filtering into parallel light.

[0031] Furthermore, the area array detector is also used to perform image segmentation on the interference image array, segment the interference image array into interference image units corresponding to each FP resonator; and arrange the interference image units in the order of the thickness of each FP resonator to form an interference image data cube, and perform a discrete Fourier transform operation on the interference image data cube with the optical path difference as the axis to obtain a spectral image data cube that varies with wavelength.

[0032] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0033] The present invention constructs a Fabry - Perot array interference system by using two stepped lenses with medium - low reflectivity placed orthogonally, replacing the beam splitter and the precision moving mirror scanning mechanism in the traditional Fourier transform imaging spectrometer. This simplifies and lightens the opto - 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, and reliable. At the same time, the incident interfaces of the two stepped lenses adopt medium - low reflectivity, which can restore the spectrum through Fourier transform and reduce the difficulty of spectrum restoration in the traditional Fabry - Perot interference system. Through the coupled modulation transmission of the light field by the Fabry - Perot array interference system, synchronous measurement of the interference image sequence can be achieved, and then snapshot - type effective detection of the target scene image information and spectral information is realized, improving the real - time performance of multi - dimensional information detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0035] Figure 1 is a schematic structural diagram of a double - stepped Fabry - Perot static interference Fourier transform spectrometer according to Embodiment 1 of the present invention.

[0036] Figure 2 is a schematic structural diagram of the first stepped lens according to Embodiment 1 of the present invention.

[0037] Figure 3 is a schematic diagram of the spatial distribution of the air - gap thickness between the first stepped lens and the second stepped lens according to Embodiment 1 of the present invention.

[0038] Figure 4 is a schematic diagram of the preparation process of the stepped substrate of the first stepped lens according to Embodiment 1 of the present invention.

[0039] Figure 5 is a schematic diagram of the detection process of the interference image array by the beam - reducing optical system according to Embodiment 1 of the present invention.

[0040] Figure 6 is a schematic diagram of the restoration process from the interference image array to the spectrum according to Embodiment 1 of the present invention.

[0041] Figure 7 is a schematic structural diagram of the first stepped lens according to Embodiment 2 of the present invention.

[0042] Description of reference numerals in Embodiment 1: light source 10, collimator 20, first stepped lens 30, second stepped lens 40, stepped substrate 301, antireflection film 302, beam reducing objective lens 50, aperture stop 60, beam reducing image lens 70, area array detector 80.

[0043] Description of reference numerals in Embodiment 2: stepped substrate 303, antireflection film 304, high refractive index dielectric film 305. Detailed implementation manners

[0044] 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 in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, rather than limiting the present invention.

[0045] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments may be combined with each other.

[0046] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. They 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. Therefore, it 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 indicating 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 "plurality" is two or more.

[0047] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected" 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 situations.

[0048] The present invention will be described in detail below with reference to the drawings and in conjunction with the embodiments.

[0049] The present invention provides a double-step Fabry-Perot static interference Fourier transform spectrometer, which includes a light source, a collimating mirror, a Fabry-Perot array interference system, a beam-reducing optical system, and a planar array detector; wherein, the light source is located on the object-side 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 interference system is located in the parallel optical path of the collimating mirror, and the Fabry-Perot array interference system modulates the target light field to form an interference light field array; the beam-reducing optical system is located in the outgoing direction of the Fabry-Perot array interference system and adopts a double telecentric optical path structure, and is used to image the interference light field modulated by the Fabry-Perot array interference system onto the planar array detector to obtain an interference image array.

[0050] The Fabry-Perot array interference system includes a first stepped lens and a second stepped lens with orthogonal stepped directions. The first stepped lens and the second stepped lens serve as the front interference plate and the rear interference plate of the Fabry-Perot array interference system respectively. The number of steps of the first stepped lens is M, and the number of steps of the second stepped lens is N. The step height difference between any two adjacent steps of the first stepped lens is d, and the step height difference between any two adjacent steps of the second stepped lens is Md. The highest step of the first stepped lens is closely attached to the highest step of the second stepped lens, so that an air gap with a two-dimensional increasing change in the shape of a checkerboard is formed between the first stepped lens and the second stepped lens. The thickness of each region of the air gap corresponds to an optical path difference respectively. The first stepped lens, the air gap, and the second stepped lens form an FP resonator array.

[0051] 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 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 two stepped lenses. The two stepped lenses are placed face to face, and the stepped directions are orthogonal to each other. The highest step surfaces of the two stepped lenses are in close contact, so that an air gap with a two-dimensional checkerboard-like variation in thickness is formed between the two stepped lenses. The two stepped lenses are made of silicon or germanium materials with a high refractive index, so that the reflection interfaces on both sides of the air gap have medium and low reflectivities, forming a low-finesse Fabry-Perot interference array, and thus the spectral demodulation can be realized through Fourier transform. By reasonably designing the structural parameters of the double stepped lens, the thickness of the air gap increases linearly in a two-dimensional discrete manner, and the thicknesses of different regions of the air gap correspond to different optical path differences, thus forming a spatial modulation of the optical path difference to modulate the optical path difference of the target light field incident thereon and generate 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 at a certain reduction ratio through the beam reduction optical system. 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.

[0052] Embodiment 1

[0053] As Figures 1 - 6 shown, the double stepped Fabry-Perot static interference Fourier transform spectrometer provided by Embodiment 1 of the present invention for creation includes a light source 10, a collimating mirror 20, a first stepped lens 30, a second stepped lens 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).

[0054] The light beam emitted by the light source 10 is first collimated into a parallel beam by the collimating mirror 20, and then incident into the Fabry-Perot array interference system, where interference is formed in the FP resonator of the Fabry-Perot array interference system. The interference plate of the Fabry-Perot array interference system is composed of a first stepped lens 40 and a second stepped lens 50. The first stepped lens 40 serves as the front interference plate of the Fabry-Perot array interference system, and the second stepped lens 50 serves as the rear interference plate of the Fabry-Perot array interference system. The first stepped lens 40 and the second stepped lens 50 are made of high refractive index materials such as silicon or germanium, and a low-finesse interference cavity is formed by using the medium reflectivity of Fresnel reflection to make it close to double-beam interference, so that spectral restoration can be performed through Fourier transform. The stepped surface of the first stepped lens 40 is orthogonal to the stepped surface of the second stepped lens 50, and the surfaces of their highest steps are closely attached, so that an air gap with a two-dimensional increasing change in thickness in a checkerboard pattern is formed between the first stepped lens 40 and the second stepped lens 50. Therefore, an optical path difference that changes with the spatial position is formed, and a zero optical path difference is achieved at the position where the first stepped lens 40 and the second stepped lens 50 are optically in contact on the surface.

[0055] Taking the stepped surfaces of the first stepped lens 40 and the second stepped lens 50 as the reflection interfaces, a distributed FP resonator is formed with the air gap. Each FP resonator has a specific thickness, so it has a fixed optical path difference. The light incident on the reflection interfaces on both sides of the FP resonator will be reflected multiple times in the FP resonator, and interference will occur between the light beams passing through the FP resonator different times. The intensity of the interference signal of the light beam exiting through the FP resonator depends on the thickness of the air gap between the two reflection interfaces, the reflectivity of the reflection interfaces, the incident angle of the light, and the refractive index of the medium in the FP resonator between the two reflection interfaces. The medium in the FP resonator can be air or vacuum, or a low refractive index medium. In this invention, the air embodiment is used for illustration. The reflection interfaces of the traditional Fabry-Perot interference system usually use a very high reflectivity to produce a narrow transmission band, while the reflection interfaces of the Fabry-Perot array interference system in this invention use a medium reflectivity to produce a quasi-cosine interference signal that can be demodulated by Fourier transform.

[0056] The interference image signal formed by the FP resonator corresponding to the m-th row step of the first stepped lens 40 and the n-th column step of the second stepped lens 50 of the Fabry-Perot array interference system can be expressed as:

[0057] ;

[0058] where R1 and R2 are the reflectivities of the two reflection interfaces of the FP resonator; ν is the wave number, ν = 1 / λ, λ is the wavelength, and δ(m,n) is the optical path difference of the FP resonator corresponding to the (m,n)-th step, and its expression is:

[0059] ;

[0060] 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 air gap thickness of the FP resonator, which varies with different spatial positions due to the checkerboard step structure.

[0061] It can be seen from the interference image signal formula that when the reflectivity of the reflection interface is very low, the interference signal is a cosine function, but the modulation degree of the interference fringes is also very low; when the reflectivity of the reflection interface is very high, the interference fringes have a very high modulation degree, but the interference signal is an extremely narrow comb function. The traditional Fabry - Perot interferometer uses a reflection interface with a very high reflectivity to generate very narrow interference characteristics through resonance. In the Fourier transform spectrum of these very narrow comb - shaped interference signals, side lobes will be generated. The side lobes 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 extension of side - lobe signals to the main - frequency signal, the maximum wave number of the incident spectrum should be less than or equal to twice the minimum wave number, that is, the maximum wavelength should be less than or equal to twice the minimum wavelength, which means that the wavelength bandwidth of the incident spectrum should be less than or equal to the minimum wavelength, or less than or equal to half of the maximum wavelength. For short - wave infrared 1μm - 2μm, mid - wave infrared 3μm - 5μm, and long - wave infrared 8μm - 14μm, the application requirements can basically be met.

[0062] 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 FP resonator is close to double - beam interference. The interference pattern function can be approximately expressed as:

[0063] ;

[0064] Therefore, the spectral information of the incident light signal can be restored from the interference signal through Fourier transform.

[0065] 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 also 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.

[0066] 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 first stepped lens 40 and the second stepped lens 50 are made of high - refractive - index media such as silicon or germanium. The stepped surfaces of the first stepped lens 40 and the second stepped lens 50 are not coated and serve as the reflection interfaces of the FP resonator. Only an antireflection film 402 is deposited on the planes of the first stepped lens 40 and the second stepped lens 50 to ensure high transmittance. The surfaces of the highest uncoated steps of the first stepped lens 40 and the second stepped lens 50 are orthogonally and tightly fitted to achieve zero optical path difference at the optical contact surface.

[0067] An air gap with an increasing thickness is formed between the first stepped lens 40 and the second stepped lens 50, thereby 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. To form continuous optical path difference sampling, let the step height difference between any two adjacent steps of the first stepped lens 40 be d, the number of steps be M, let the step height difference between any two adjacent steps of the second stepped lens 50 be Md, the number of steps be N, and the air gap thickness h(m,n) of the (m,n) - th FP resonator formed by the m - th step of the first stepped mirror and the n - th step of the second stepped mirror is:

[0068] 。

[0069] 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 high - order side - lobe signals, the sampling signal cut - off frequency is extended to J sub - harmonics (in general, JTaking 3 can meet the requirements). For the incident broadband spectrum, the step height difference d should satisfy the following relationship:

[0070] ;

[0071] where λ min is the minimum wavelength of the broadband spectrum, θ max is the incident angle corresponding to the light ray with the maximum field of view in the FP resonator, J is the spectral harmonic order, and n0 is the refractive index of the medium in the FP resonator.

[0072] 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:

[0073] ;

[0074] where λ S is the shortest wavelength of the narrowband spectrum, λ L is the longest wavelength of the narrowband spectrum, k is the spectral folding order, and any integer less than or equal to is taken.

[0075] The first stepped lens 40 includes a stepped substrate 401. The stepped substrate 401 has M steps, and the height difference between any two adjacent steps is a constant d. The stepped substrate 401 is fabricated by etching, and the fabrication process is as follows:

[0076] A1: Provide a planar substrate with a transverse width of a and a longitudinal width of b, and polish and clean the planar substrate.

[0077] A2: Etch the planar substrate to form a two-step structure with an etching depth of Md / 2 and an etching width of a / 2.

[0078] A3: Etch the two-step structure to form a four-step structure with an etching depth of Md / 4 and an etching width of a / 4. Repeat step A3 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 step height difference is d and the step width is a / M.

[0079] The second stepped lens 50 has N steps, and the height difference between any two adjacent steps is a constant Md. The stepped substrate of the second stepped lens 50 is also fabricated by etching, and the fabrication process is as follows:

[0080] B1: Provide a planar substrate with a transverse width of a and a longitudinal width of b, and polish and clean the planar substrate.

[0081] B2: Etch the planar substrate to form a two-step structure, with an etching depth of NMd / 2 and an etching width of b / 2.

[0082] B3: Etch the two-step structure to form a four-step structure, with an etching depth of NMd / 4 and an etching width of b / 4. Repeat step B3, 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 step height difference is Md and the step width is b / N.

[0083] 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 at a certain magnification and then irradiates it onto the area array detector to form an interference pattern array. The interference pattern array has M×N interference pattern units, and each interference pattern unit corresponds to an FP resonator with a specific thickness, that is, respectively corresponding to a specific row step of the first - step mirror 40 and a specific column step of the second - step lens 50.

[0084] 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 Fabry - Perot array interference system 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 Fabry - Perot array interference system, and the aperture stop 60 is used to limit the beam aperture to 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.

[0085] 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.

[0086] Embodiment 2

[0087] The difference between Example 2 and Example 1 lies only in that the structures of the first stepped lens and the second stepped lens in Example 2 are different from those of the first stepped lens and the second stepped lens in Example 1. As Figure 7 shown, the step substrate 303 of the first stepped lens in Example 2 is made of a low refractive index transparent dielectric material such as quartz, calcium fluoride, magnesium fluoride, sapphire, etc. to ensure a high transmittance at the interface. A high refractive index dielectric film 305 such as silicon or germanium is deposited on the step surface of the step substrate 303 so that the reflection interface has a medium-low reflectivity to achieve low fineness. An antireflection film 304 is deposited on the lower surface (i.e., the bottom plane) of the step substrate 303 to ensure a high transmittance. The structure of the second stepped lens can be obtained in the same way.

[0088] It should be understood that various forms of the processes shown above can be used, with steps reordered, added, or deleted. For example, the steps recited in the disclosure of the present invention can be executed in parallel, sequentially, or in a different order, as long as the desired results of the technical solution disclosed in the present invention can be achieved, and no limitation is imposed herein.

[0089] 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 dual-step Fabry-Perot static interference Fourier transform spectrometer, characterized in that, It includes a light source, a collimating mirror, a Fabry - Perot array interference system, a beam - reducing optical system and a planar array detector; among them, The light source is located on the object - side 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 interference system is located in the parallel optical path of the collimating mirror. The Fabry - Perot array interference system is used to modulate the target light field to form an interference light field array. The Fabry - Perot array interference system includes a first stepped lens and a second stepped lens with orthogonal step directions. The first stepped lens and the second stepped lens are respectively used as the front interference plate and the rear interference plate of the Fabry - Perot array interference system. The number of steps of the first stepped lens is M, and the number of steps of the second stepped lens is N. The step - height difference between any two adjacent steps of the first stepped lens is d, and the step - height difference between any two adjacent steps of the second stepped lens is Md. The highest step of the first stepped lens is closely attached to the highest step of the second stepped lens, so that an air gap with a two - dimensional increment in a checkerboard pattern is formed between the first stepped lens and the second stepped lens. The thickness of each region of the air gap corresponds to an optical path difference. The first stepped lens, the air gap and the second stepped lens form an FP resonator array; The two opposite surfaces of the first stepped lens and the second stepped lens are respectively used as reflection interfaces. The two reflection interfaces and the air gap form an FP resonator, and the reflectivity of the two reflection interfaces is 30% - 40%; The first stepped lens and the second stepped lens respectively include a stepped substrate. When the stepped substrate uses a high - refractive - index medium, the upper surface of the stepped substrate of the high - refractive - index medium is not coated and is used as the reflection interface of the FP resonator, and an antireflection film is evaporated on the lower surface of the stepped substrate of the high - refractive - index medium; when the stepped substrate uses a low - refractive - index medium, a high - refractive - index medium film is evaporated on the upper surface of the stepped substrate of the low - refractive - index medium and is used as the reflection interface of the FP resonator, and an antireflection film is evaporated on the lower surface of the stepped substrate of the low - refractive - index medium; the low - refractive - index medium uses quartz, calcium fluoride, magnesium fluoride or sapphire dielectric materials; the high - refractive - index medium uses silicon or germanium dielectric materials; the high - refractive - index medium film uses a silicon film or a germanium film; For the incident broadband spectrum, the step - height difference d of the first stepped lens satisfies the following relationship: ； 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; For the incident narrow - band spectrum, the step - height difference d of the first stepped lens satisfies the following relationship: ； Among them, λ S is the shortest wavelength of the narrowband spectrum, and λ L is the longest wavelength of the narrowband spectrum, k is the spectral folding order, taking any integer less than or equal to ; The beam - reducing optical system is located in the outgoing direction of the Fabry - Perot array interference system and adopts a double - telecentric optical path structure. It is used to image the interference light field modulated by the Fabry - Perot array interference system onto the planar array detector to obtain an interference image array.

2. The dual-step Fabry-Perot static interference Fourier transform spectrometer according to claim 1, wherein The thickness of the air gap is distributed in a two-dimensional checkerboard pattern with an increasing step, and the thickness increases successively with a step size of m along the d direction, and the thickness increases successively with a step size of n along the Md direction. The thickness of the air gap of the FP resonator corresponding to the ( m , n )-th step is h ( m , n ) and is: h ( m , n )=[( n -1) M + m -1] d 。 3. The dual-step Fabry-Perot static interference Fourier transform spectrometer according to claim 1, characterized in that, The preparation process of the stepped substrate of the first stepped lens is as follows: A1: Provide a planar substrate with a transverse width of a and a longitudinal width of b, and polish and clean the planar substrate; A2: Etch the planar substrate to form a two - stepped structure with an etching depth of Md / 2 and an etching width of a / 2; A3: Etch the two-step structure to form a four-step structure with an etch depth of Md / 4 and an etch width of a / 4. Repeat step A3, ensuring that the etch depth each time is 1 / 2 of the previous etch and the etch width each time is 1 / 2 of the previous etch, until the step height difference is d and the step width is a / M. The preparation process of the step substrate of the second-step lens is as follows: B1: Provide a planar substrate with a transverse width of a and a longitudinal width of b, and polish and clean the planar substrate. B2: Etch the planar substrate to form a two-step structure with an etch depth of NMd / 2 and an etch width of b / 2. B3: Etch the two-step structure to form a four-step structure with an etch depth of NMd / 4 and an etch width of b / 4. Repeat step B3, ensuring that the etch depth each time is 1 / 2 of the previous etch and the etch width each time is 1 / 2 of the previous etch, until the step height difference is Md and the step width is b / N.

4. The dual-step Fabry-Perot static interference Fourier transform spectrometer according to claim 1, wherein The beam shrinking optical system includes a beam shrinking objective lens, an aperture stop, and a beam shrinking image lens. The image-side focal plane of the beam shrinking objective lens coincides with the object-side focal plane of the beam shrinking image lens. The Fabry-Perot array interference system is located on the object-side focal plane of the beam shrinking objective lens, the area array detector is located on the image-side focal plane of the beam shrinking image lens, and the aperture stop is simultaneously located on the image-side focal plane of the beam shrinking objective lens and the object-side focal plane of the beam shrinking image lens. The beam shrinking objective lens is used to converge the interference light field modulated by the Fabry-Perot array interference system; the aperture stop is used to limit the beam aperture to achieve filtering of the beam numerical aperture; the beam shrinking image lens is used to collimate the beam after numerical aperture filtering into parallel light.

5. The dual-step Fabry-Perot static interference Fourier transform spectrometer according to claim 1, wherein The area array detector is also used to perform image segmentation on the interference image array, segment the interference image array into interference image units corresponding to each FP resonator; and arrange the interference image units in the thickness order of each FP resonator to form an interference image data cube, and perform a discrete Fourier transform operation on the interference image data cube with the optical path difference as the axis to obtain a spectral image data cube that varies with wavelength.

Images