Fabry-Perot interferometer measuring system based on cascade connection of three polar plates and double cavities

Abstract

The invention provides a Fabry-Perot interferometer measurement system based on three-pole-plate double-cavity cascade, and relates to the technical field of optical precision measurement, the system comprises an optical fiber coupling laser light source with an optical path protection device, an optical fiber collimator, an F-P interferometer and an optical fiber coupling optical detector, and adopts a double-interference-cavity cascade structure of three parallel pole plates; wherein the three parallel polar plates consist of a first fixed plate, a middle movable plate and a second fixed plate; a first interference cavity is formed between the first fixed plate and the middle movable plate, and a second interference cavity is formed between the middle movable plate and the second fixed plate. The limitation of a traditional single cavity is broken through, a double-interference-cavity cascade system is constructed by introducing the middle movable polar plate, the introduction of the middle polar plate not only expands a single cavity into two interference cavities which work cooperatively, but also realizes dynamic coupling modulation of the optical path difference through the mechanical mobility of the polar plate.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical precision measurement, in particular to a Fabry-Perot (FP) interferometer system based on three parallel polar plates, and more particularly to a Fabry-Perot interferometer measurement system based on three-polar-plate double-cavity cascade. BACKGROUND

[0002] The traditional Fabry-Perot (F-P) interferometer relies on two parallel mirrors to form a single interference cavity, and its function is limited by the single parameter of optical path modulation and the sensitivity bottleneck. In the prior art, the single-cavity structure can only realize single parameter measurement by changing the cavity length or the incident light wavelength, such as obtaining spectral information by scanning the cavity length, or monitoring the cavity length change by fixing the wavelength. However, when facing complex physical quantity coupling scenarios (such as pressure and temperature changes at the same time, multi-component medium refractive index calculation), the single-cavity structure is difficult to realize multi-parameter decoupling, and the measurement error will be significantly amplified due to environmental interference (such as mechanical vibration, thermal expansion). In addition, the optical path adjustment range of the single cavity is limited, and for the measurement of small pressure (such as below 1 Pa) or low refractive index difference (such as 10 -5 RIU level), the insufficient signal-to-noise ratio makes it difficult to improve the resolution. SUMMARY

[0003] Therefore, in view of the problems of single optical path modulation and low sensitivity existing in the prior art F-P interferometer, the purpose of the present application is to provide a Fabry-Perot interferometer measurement system based on three-polar-plate double-cavity cascade, which breaks through the limitation of the traditional single cavity, and constructs a double-interference-cavity cascade system by introducing an intermediate movable polar plate. The introduction of the intermediate polar plate not only expands the single chamber into two interference cavities working cooperatively, but also realizes the dynamic coupling modulation of the optical path difference through the mechanical movability of the polar plate.

[0004] To achieve the above purpose, the present application provides the following technical solutions:

[0005] Based on the above purpose, the application provides a Fabry-Perot interferometer measurement system based on three-plate double-cavity cascade, which comprises a fiber-coupled laser light source with a light path protection device, a fiber collimator, an F-P interferometer and a fiber-coupled light detector, and adopts a double-interference-cavity cascade structure of three parallel plates; wherein the three parallel plates are composed of a first fixed plate, an intermediate movable plate and a second fixed plate; a first interference cavity is formed between the first fixed plate and the intermediate movable plate, and a second interference cavity is formed between the intermediate movable plate and the second fixed plate; when the intermediate movable plate is displaced by the pressure difference, the cavity length of the first interference cavity will be shortened by a corresponding displacement amount, while the cavity length of the second interference cavity will be lengthened by the same displacement amount, thereby realizing the reverse change of the optical path of the double cavities. This reverse change of the optical path will cause the difference modulation of the optical path difference in the two interference cavities: the optical path difference of the first interference cavity decreases with the shortening of the cavity length, and the optical path difference of the second interference cavity increases with the lengthening of the cavity length, and the change amounts of the two are equal in size and opposite in direction. Based on the Fabry-Perot interference principle, the change of the optical path difference will directly cause the movement of the interference fringes or the periodic change of the transmitted light intensity. The superimposed signal of the double-cavity interference light intensity is collected by the fiber-coupled light detector, and the displacement amount corresponding to the pressure difference can be calculated by combining the double-cavity phase coupling model, so as to finally realize the high-precision measurement of the target physical quantity.

[0006] As a further scheme of the application, the first fixed plate is a magnesium trifluoride (MgF2) substrate with a thickness of 2 mm, and the surface has an optical precision of λ / 10 after ion beam polishing; wherein the transmission surface facing the first interference cavity is coated with an anti-reflection film with a center wavelength reflectivity of <0.5%, and the incident surface (facing the fiber collimator side) maintains a high flatness optical surface after polishing but is not coated. In this design, the anti-reflection film is only located on the transmission surface (i.e. the contact surface between the first fixed plate and the first interference cavity), which serves to reduce the interface reflection loss when the laser enters the first interference cavity from the magnesium trifluoride substrate, and to ensure that more light energy is transmitted into the first interference cavity to participate in multi-beam interference; while the incident surface is not coated, the low refractive index difference between magnesium trifluoride and air (MgF2 refractive index ≈1.38, air ≈1.0) is used to naturally reduce the reflectivity, and at the same time, the internal multi-interface interference introduced by double-sided coating is avoided.

[0007] Wherein, when λ=532nm, the flatness ≤53.2nm, the magnesium trifluoride substrate has an absorption coefficient <0.01dB / cm in the wavelength range of 200-1100nm from ultraviolet to near infrared, and as a transmission window for high-power laser, this characteristic can effectively avoid the thermal deformation caused by material absorption when high-power laser is incident, and ensure the stability of the optical precision of the substrate; at the same time, the combination of low absorption and high flatness ensures that the laser enters the first interference cavity in a nearly undistorted state, providing high-quality incident light conditions for subsequent high-precision measurement of double-cavity cascade interference.

[0008] As a further scheme of the present application, the intermediate movable plate is a single-crystal silicon micromechanical structure prepared by a photolithography and deep etching process to form a cantilever beam array support structure, the substrate thickness of the intermediate movable plate is 100 μm, the length of a single cantilever beam is 500 μm, the width is 50 μm, and the thickness is 2 μm, the elastic coefficient is optimized to k=0.1 N / m through finite element simulation, one end of the cantilever beam is fixed to the frame, and the other end is connected to the intermediate electrode plate body to form an elastic support system.

[0009] As a further scheme of the present application, when the intermediate electrode plate is subjected to a double-cavity pressure difference ΔP=P1-P2, a displacement Δx in a direction perpendicular to the plane of the electrode plate is generated, and the displacement amount and the pressure difference are in a linear relationship as follows:

[0010]

[0011] Wherein, A is the effective force area of the electrode plate, and k is the total elastic coefficient of the cantilever beam.

[0012] As a further scheme of the present application, the second fixed plate is a silicon substrate, and a multilayer dielectric high-reflection film is plated on the surface facing the second interference cavity, which is prepared by alternately depositing Al2O3 / SiO2, and only forms a high-reflection interface on the surface facing the second interference cavity, with a reflectivity of >98% in the 400-1000 nm wavelength band, wherein the high-reflection film is prepared by an electron beam evaporation process, and the thickness of each film is monitored in real time by a quartz crystal oscillator.

[0013] As a further scheme of the present application, the cavity length of the first interference cavity is L1, and the cavity length of the second interference cavity is L2; when coherent light with a wavelength of λ is vertically incident on the first fixed plate, the transmitted light enters the first interference cavity, multiple-beam interference occurs between the first fixed plate and the intermediate movable plate, the surface of the intermediate movable plate facing the first cavity has a transmittance (T1) and a reflectance (R1), which satisfy T1+R1=1, and the multiple reflections of the light in the first cavity form an equal inclination interference transmitted light intensity distribution that satisfies the F-P interference formula:

[0014]

[0015] Wherein, is the phase difference in the first cavity, n1 is the refractive index of the medium in the cavity,

[0016] As a further scheme of the present application, the transmitted light of the first cavity enters the second interference cavity after passing through the intermediate movable plate, the rear surface of the intermediate movable plate facing the second cavity has a transmittance T2 and a reflectance R2, and the reflectance of the second fixed plate is R3, and the double-cavity phase is: Wherein, φ1 and φ2 are the surface reflection phases of the electrode plates, Δx is the displacement of the intermediate electrode plate, and Δn=n1-n2 is the difference in the refractive index of the medium of the double cavity. is the mechanical coupling coefficient, A is the effective stress area of the plate, and k is the total elastic coefficient of the cantilever beam;

[0017] The interference light intensity in the second cavity is:

[0018]

[0019] wherein, E is a second-order unit matrix.

[0020] As a further scheme of the present application, the total transmission light intensity after the double-cavity cascade is:

[0021]

[0022] wherein γ is an optical path coupling constant, and is related to the plate spacing.

[0023] As a further scheme of the present application, the total transmission light intensity I total (λ) = I2(λ), the spectral characteristics are determined by the double-cavity phases δ1 and δ2, the double cavity satisfies the resonance condition 2n1L1 = mλ and 2n2L2 = mλ, the total transmission reaches a peak value, narrowband filtering effect is formed, when the double-cavity length is misaligned, the transmission spectrum is broadened or a multi-peak structure appears, which can be used for spectral analysis, wherein m is an integer.

[0024] As a further scheme of the present application, when measuring the refractive index of a medium, the first cavity is filled with the medium to be measured, and the second cavity is vacuum, the stress birefringence effect and the film layer dispersion parameter are introduced, and the refractive index calculation formula is:

[0025]

[0026] wherein B is the stress birefringence coefficient of the intermediate plate, is the film layer dispersion parameter, is the cavity length change rate;

[0027] If the first cavity is filled with the medium to be measured, the refractive index n; the second cavity is vacuum n2 = 1; the initial cavity length L1 = L2 = L, the resonance condition is: 2nL = mλ1, 2L = mλ2, and by combining them, n = λ2 / λ1; when the refractive index of the medium changes Δn, the resonance wavelength shifts The cavity length error is eliminated by double-cavity wavelength difference measurement, and the resolution reaches

[0028] As a further scheme of the present application, when the double-cavity lengths are equal and filled with the same medium, the double-cavity cascade compresses the transmission peak bandwidth, and the transmission peak half-width satisfies:

[0029] The bandwidth is compressed to sub-nanometer level by increasing the reflectivity or the cavity length.

[0030] Compared with the prior art, the application provides a Fabry-Perot interferometer measurement system based on a triple-plate double-cavity cascade, which has the following beneficial effects:

[0031] The application realizes synchronous measurement of pressure and refractive index through a double-cavity cascade structure, solves the problem that a traditional single-cavity F-P cannot separate multiple physical quantity coupling, directly offsets common-mode noise through a differential modulation mechanism (L1 cavity shortening Δx and L2 cavity lengthening Δx), amplifies the signal through double-cavity optical path superposition, increases the stripe movement amount compared with the traditional single-cavity, eliminates the cavity length drift error through double-cavity resonance wavelength difference solving, and greatly improves the sensitivity.

[0032] In the application, the double-cavity phase coupling compresses the transmission peak half-width, reduces the actual measurement bandwidth, meets the demand of high-resolution spectral analysis, independently adjusts the double-cavity length through a piezoelectric ceramic, covers the full wavelength band of 400-1000 nm in the wavelength tuning range, realizes multi-channel parallel detection through a wavelength-position mapping algorithm, and breaks through the 1 nm bottleneck in spectral resolution. The Fabry-Perot interferometer measurement system based on a triple-plate double-cavity cascade in the application realizes high-precision measurement of pressure and refractive index through a double-interference cavity cascade structure, and is suitable for applications in multiple fields such as aerospace pressure monitoring, biomedical refractive index analysis, and spectral instrument filtering.

[0033] These aspects or other aspects of the application will be more apparent in the following description of the embodiments. It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the application. BRIEF DESCRIPTION OF DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the application or the related art, the following will briefly introduce the drawings needed to be used in the exemplary embodiments or the related art description. The drawings are used to provide further understanding of the application, and constitute a part of the specification, and are used to explain the application together with the embodiments of the application, and do not constitute a limitation of the application. In the drawings:

[0035] Figure 1 It is a general structure schematic diagram of a Fabry-Perot interferometer measurement system based on a triple-plate double-cavity cascade of the application.

[0036] Figure 2 It is a comparison diagram of a traditional F-P and the F-P optical path in the Fabry-Perot interferometer measurement system based on a triple-plate double-cavity cascade of the application.

[0037] Figure 3 It is a structure schematic diagram of an F-P interferometer in the Fabry-Perot interferometer measurement system based on a triple-plate double-cavity cascade of the application.

[0038] Figure 4 A wavelength resolution comparison chart in a Fabry-Perot interferometer measurement system based on a triple-plate double-cavity cascade for an embodiment of the application.

[0039] Figure 5 A multi-wavelength response comparison chart in a Fabry-Perot interferometer measurement system based on a triple-plate double-cavity cascade for an embodiment of the application.

[0040] Figure 6 An interference fringe contrast comparison chart in a Fabry-Perot interferometer measurement system based on a triple-plate double-cavity cascade for an embodiment of the application.

[0041] Figure 7 A pressure response characteristic comparison curve chart in a Fabry-Perot interferometer measurement system based on a triple-plate double-cavity cascade for an embodiment of the application.

[0042] Figure 8 A refractive index measurement accuracy comparison column chart in a Fabry-Perot interferometer measurement system based on a triple-plate double-cavity cascade for an embodiment of the application. DETAILED DESCRIPTION

[0043] Hereinafter, the present application will be further described with reference to the drawings and specific embodiments, and it should be noted that the following described embodiments or technical features between each of the embodiments or technical features can be combined in any manner to form new embodiments without conflict.

[0044] In order to make the objectives, technical solutions and advantages of the present application clearer, further detailed description will be made to the embodiments of the present application with reference to the drawings and in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and should not be used to limit the present application.

[0045] It should be noted that all the expressions of "first" and "second" in the embodiments of the present application are used to distinguish two non-identical entities or non-identical parameters with the same name, and it can be seen that "first" and "second" are only used for the convenience of description and should not be understood as a limitation of the embodiments of the present application. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, the process, method, system, product or device inherently includes other steps or units.

[0046] The technical solutions in the embodiments of the present application will be described clearly and completely in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0047] The flowchart shown in the drawing is only an example, and does not necessarily include all the contents and operations / steps, nor does it necessarily execute in the order described. For example, some operations / steps can be further decomposed, combined or partially merged, so the actual execution order can be changed according to the actual situation.

[0048] Some embodiments of the present application will be described in detail below with reference to the accompanying drawings. The following examples and features in the examples can be combined with each other without conflict.

[0049] In view of the problems of single optical path modulation and low sensitivity of the existing F-P interferometer, the present application proposes a Fabry-Perot interferometer measurement system based on three-plate double-cavity cascade, which breaks through the limitation of traditional single cavity and realizes high-precision physical quantity measurement by introducing an intermediate movable plate to construct a double-interference-cavity cascade system. The introduction of the intermediate plate not only expands the single chamber into two cooperating interference cavities, but also realizes the dynamic coupling modulation of the optical path difference through the mechanical movability of the plate. For example, when external pressure acts on the double cavity, the displacement of the intermediate plate will cause the length of the two chambers to change in opposite directions (one chamber shortens and the other chamber lengthens). This differential structure can effectively suppress common-mode interference (such as overall expansion caused by temperature), and at the same time, through the superposition effect of the double-cavity optical path difference, the signal response is amplified, thereby significantly improving the measurement accuracy and anti-interference ability.

[0050] Referring to Figures 1 to 8 As shown in the drawing, the embodiment of the present application provides a Fabry-Perot interferometer measurement system based on three-plate double-cavity cascade, which includes a fiber-coupled laser light source 1 with an optical path protection device, a fiber collimator 2, an F-P interferometer 3 and a fiber-coupled light detector 4. The system adopts a double-interference-cavity cascade structure of three parallel plates, breaks through the limitation of traditional single cavity, and realizes high-precision physical quantity measurement. The three parallel plates are composed of a first fixed plate, an intermediate movable plate and a second fixed plate. The first interference cavity is formed between the first fixed plate and the intermediate movable plate, and the second interference cavity is formed between the intermediate movable plate and the second fixed plate. When the intermediate movable plate is displaced by the pressure difference, the cavity length of the first interference cavity will shorten by a corresponding displacement, while the cavity length of the second interference cavity will lengthen by the same displacement, thereby realizing the reverse change of the double-cavity optical path. This reverse change of the optical path will cause the optical path difference in the two interference cavities to be differentially modulated: the optical path difference of the first interference cavity decreases with the shortening of the cavity length, and the optical path difference of the second interference cavity increases with the lengthening of the cavity length, and the change amounts of the two are equal in size and opposite in direction. Based on the Fabry-Perot interference principle, the change of the optical path difference will directly cause the movement of the interference fringes or the periodic change of the transmitted light intensity. The fiber-coupled light detector acquires the superimposed signal of the double-cavity interference light intensity, and combines the double-cavity phase coupling model to calculate the displacement corresponding to the pressure difference, and finally realizes the high-precision measurement of the target physical quantity.

[0051] In this embodiment, the first fixed plate is made of magnesium fluoride (MgF2) substrate with a thickness of 2 mm, and the surface is polished by ion beam polishing process to achieve optical accuracy of λ / 10 (λ is 532 nm, and the flatness is about 53.2 nm). The transmission surface facing the first interference cavity is coated with an antireflection film with a center wavelength reflectivity of <0.5%, and the incident surface (facing the optical fiber collimator side) maintains the high flatness optical surface after polishing but is not coated. In this design, the antireflection film is only on the transmission surface (i.e. the contact surface between the first fixed plate and the first interference cavity), which reduces the interface reflection loss when the laser enters the first interference cavity from the magnesium fluoride substrate, and ensures that more light energy is transmitted into the first interference cavity to participate in multi-beam interference. The incident surface is not coated, which naturally reduces the reflectivity by taking advantage of the low refractive index difference between magnesium fluoride and air (MgF2 refractive index ≈1.38, air ≈1.0), while avoiding the introduction of internal multi-interface interference by double-sided coating. MgF2 material is selected because it has low absorption coefficient (<0.01 dB / cm) and stable optical uniformity in the ultraviolet to near-infrared wavelength range (200-1100 nm), making it suitable as a transmission window for high-power lasers.

[0052] The middle movable plate is a single crystal silicon micromechanical structure, and a cantilever beam array support structure is prepared by photolithography and deep etching process. The specific parameters are: substrate thickness 100 μm, single cantilever beam length 500 μm, width 50 μm, thickness 2 μm, and elastic coefficient optimized to 0.1 N / m through finite element simulation. Among them, one end of the cantilever beam is fixed to the frame, and the other end is connected with the middle electrode plate main body to form an elastic support system. The above design enables the middle electrode plate to produce displacement (Δx) in the direction perpendicular to the plane of the electrode plate when subjected to the pressure difference (ΔP=P1-P2) of the double cavities, and the displacement amount is linearly related to the pressure difference where A is the effective stress area of the electrode plate, k is the total elastic coefficient of the cantilever beam, is the partial derivative of the pressure, dp' is the pressure infinitesimal in the integral, and p is the pressure. The use of single crystal silicon not only ensures the high stiffness of the mechanical structure (Young's modulus 169 GPa), but also is compatible with standard MEMS process, facilitating large-scale integrated manufacturing.

[0053] The second fixed plate is a silicon substrate, and the surface facing the second interference cavity is coated with a multilayer dielectric high-reflection film (Al2O3 / SiO2 deposited alternately). Through optical design, a reflectivity of >98% is achieved in the wavelength range of 400-1000 nm. The high-reflection film is prepared by electron beam evaporation process, and the thickness of each layer is monitored in real time by a quartz crystal oscillator to ensure phase matching of the film system. For example, for a center wavelength λ0=632.8 nm, the high-reflection film is composed of 15 pairs of quarter-wave layers, and the theoretical reflectivity can reach 99.8%. The high-reflection characteristic of the second fixed plate ensures that the light undergoes multiple reflections in the second interference cavity, enhancing the intensity of the interference signal, and at the same time providing the necessary optical feedback for the cascade of the double cavities.

[0054] The application is based on a three-plate double-cavity cascade Fabry-Perot interferometer measurement system, and the specific optical path process is as follows:

[0055] The coherent light (wavelength λ) is emitted by a fiber-coupled laser light source 1, collimated by a fiber collimator 2, and then vertically incident to a first fixed plate. When the coherent light (wavelength λ) is vertically incident to the first fixed plate, the transmitted light enters a first interference cavity (cavity length L1) and multiple-beam interference occurs between the first fixed plate and an intermediate movable plate. The surface (front surface) of the intermediate movable plate facing the first cavity has a transmittance (T1) and a reflectivity (R1) (satisfying T1+R1=1), and the multiple reflections of light in the first cavity form an equal inclination interference. The coherent light first enters the first interference cavity (cavity length L1), and multiple-beam interference occurs between the first fixed plate and the intermediate movable plate, and the transmitted light intensity satisfies the F-P interference formula:

[0056]

[0057] Where I1(λ) is the transmitted light intensity of the first interference cavity, I0 is the incident light, φ1 is the phase difference in the first cavity, n1 is the refractive index of the medium in the cavity, φ1 is the reflection phase of the surface of the first interference cavity, τ1 is the transmittance of the surface of the intermediate movable plate facing the first cavity, r1 is the reflectivity of the surface of the intermediate movable plate facing the first cavity, a and b can be any number.

[0058] The first cavity transmitted light enters the second interference cavity (cavity length L2) after passing through the intermediate movable plate, and interferes again between the rear surface of the intermediate movable plate and the second fixed plate (high reflectivity R3≈1), wherein the transmittance of the surface of the intermediate movable plate facing the second cavity (rear surface) is T2, the reflectivity is R2, and the reflectivity of the second fixed plate is R3. Considering the coupling effect of the displacement Δx of the intermediate plate and the refractive index difference Δn=n1-n2 of the double cavity, the mechanical coupling coefficient is defined as (A is the effective force area of the plate, and k is the total elastic coefficient of the cantilever beam), and the double-cavity phase is: Where φ1 and φ2 are the reflection phases of the surfaces of the first and second cavities, respectively. Combining the displacement of the intermediate plate and the coupling effect of the refractive index, the transmitted interference light intensity in the second cavity is:

[0059]

[0060] Where I2(λ) is the transmitted light intensity of the second interference cavity, and dl is the cavity length element in the integral,

[0061] E is a second-order unit matrix, is the phase difference in the second cavity, and n2 is the refractive index of the medium in the second cavity. The total transmitted light after the double-cavity cascade is received by the optical fiber coupled light detector 4, and the physical quantity is calculated by analyzing the spectral characteristics (such as the transmission peak position, half-width, etc.). The total transmitted light intensity after the double-cavity cascade is: where γ is the optical path coupling constant, which is related to the distance between the polar plates, and γ is.

[0062] The total transmitted light intensity I after the double-cavity cascade is total (λ) = I2(λ), and the spectral characteristics are determined by the double-cavity phases δ1 and δ2. When the double cavity satisfies the resonance condition 2n1L1 = mλ and 2n2L2 = mλ (m is an integer), the total transmittance reaches a peak, forming a narrow-band filtering effect; when the double-cavity length is misaligned, the transmission spectrum is broadened or multiple peaks appear, which can be used for spectral analysis.

[0063] Referring to Figure 3 The system of the present application is a double-interference-cavity cascade structure constructed by three parallel polar plates, Figure 3 is a detailed view of the F-P interferometer structure. The first fixed plate is made of magnesium fluoride (MgF2) substrate with a thickness of 2 mm. The surface is polished by ion beam polishing process to achieve an optical accuracy of λ / 10 (when λ is 532 nm, the flatness is about 53.2 nm). The transmission surface facing the first interference cavity is coated with an antireflection film, so that the reflectivity at the center wavelength is less than 0.5%. This design can ensure that the incident light enters the first interference cavity with the lowest loss. MgF2 material has low absorption coefficient (<0.01 dB / cm) and stable optical uniformity in the ultraviolet to near-infrared wavelength range (200-1100 nm), which is very suitable as a transmission window for high-power laser.

[0064] The middle movable plate is made of single crystal silicon micro-mechanical structure, and the cantilever beam array support structure is prepared by photolithography and deep etching process. The substrate thickness is 100 μm, the length of a single cantilever beam is 500 μm, the width is 50 μm, and the thickness is 2 μm. The elastic coefficient is optimized to 0.1 N / m through finite element simulation. One end of the cantilever beam is fixed to the frame, and the other end is connected to the middle polar plate main body to form an elastic support system. When subjected to a pressure difference between the double cavities, the middle polar plate can produce displacement in the direction perpendicular to the polar plate plane, and the displacement is linearly related to the pressure difference. Single crystal silicon not only ensures the high stiffness of the mechanical structure (Young's modulus 169 GPa), but also is compatible with standard MEMS process, facilitating large-scale integrated manufacturing.

[0065] The second fixed plate is a silicon substrate coated with a multilayer dielectric high-reflectance film (Al203 / Si02 deposited alternately), which, through optical design, can achieve a reflectance of >98% in the 400-1000 nm wavelength band. The high-reflectance film is prepared using an electron beam evaporation process, and the thickness of each layer is monitored in real time by a quartz crystal oscillator to ensure phase matching of the film system. For example, for a center wavelength λ0= 632.8 nm, the high-reflectance film is composed of 15 pairs of quarter-wave layers, and the theoretical reflectance can reach 99.8%. The high-reflectance characteristics of the second fixed plate can ensure that the light undergoes multiple reflections within the second interference cavity, enhancing the intensity of the interference signal, while providing the necessary optical feedback for the double-cavity cascade.

[0066] In combination Figure 3The light path characteristics of the structure of the first fixed plate (MgF2 coated with an anti-reflection film), the intermediate movable plate (Si), the second fixed plate (Si coated with a high-reflection film), and the double interference cavity (the first cavity is located between the first fixed plate and the intermediate movable plate, and the second cavity is located between the intermediate movable plate and the second fixed plate) are described in detail as follows: After collimation, the 532 nm laser is positively incident on the outer surface of the MgF2 substrate (the leftmost interface without a film in the figure), and due to the refractive index difference between MgF2 (refractive index about 1.38) and air, about 3.5% of the light is directly reflected at this outer surface, forming stray light propagating in the reverse direction along the incident direction, which is the first light, and this light does not participate in the interference cavity effect and is the initial source of system stray reflection; the light that transmits through the outer surface of the MgF2 continues to propagate inside the substrate, and when it reaches the inner surface of the MgF2 (coated with an anti-reflection film, facing the first interference cavity), only a very weak light is reflected at this surface due to the anti-reflection film designed to suppress reflection (typical reflectivity <0.5%), and these lights propagate back and forth in the MgF2 substrate, some of which are transmitted back into the air, and some are reflected again, and due to the extremely low energy and not entering the interference cavity, they have no substantial impact on the measurement, which is the second light; the light that transmits through the MgF2 anti-reflection film enters the first interference cavity (cavity length L1) and is normally incident on the front surface of the Si movable plate (the interface facing the first interference cavity, whose reflectivity is determined by the refractive index difference between Si and the medium in the cavity), and part of the light is positively reflected at this front surface, returning to the first interference cavity along the original path, and forming multi-beam interference with the subsequent transmitted light in the first interference cavity (which is the core of the first cavity Fabry-Perot interference), which is the main contribution light constituting the first interference cavity signal, which is the third light; the light that transmits through the front surface of the movable plate enters the Si substrate (refractive index about 3.42) and then propagates to the back surface of the movable plate (the interface facing the second interference cavity, whose reflectivity is determined by the refractive index difference between Si and the medium in the second cavity), and part of the light is negatively reflected at this back surface, propagating back and forth in the Si plate, and due to the Si plate thickness of only 100 μm, the light intensity will decay with the increase of the number of reflections, which will weakly couple the light intensity of the first and second interference cavities, but will have minimal interference with the main interference signal, which is the fourth light; the light that transmits through the back surface of the movable plate enters the second interference cavity (cavity length L2) and is normally incident on the high-reflection film surface of the Si substrate (facing the second interference cavity, using an Al2O3 / SiO2 film system, with a reflectivity >98% in the 400-1000 nm wavelength band), and most of the light will be strongly reflected at this high-reflection film surface, returning to the second interference cavity along the original path, and forming a high-contrast multi-beam interference with the transmitted light (which is the core of the second cavity Fabry-Perot interference), directly determining the modulation depth of the total transmitted light intensity after the double-cavity cascade, which is the fifth light;The extremely weak light (accounting for less than 2%) through the high-reflection film enters the inside of the second Si substrate, propagates to the outer surface of the second fixed plate (facing away from the second interference cavity, without a film layer), and is scattered and reflected due to the difference in refractive index between Si and air. After the light goes back and forth in the second Si plate, part of the light is transmitted back to the second interference cavity. However, due to the extremely low energy, the light is only the internal scattered light of the substrate, and does not affect the high-precision measurement of the system, which is the sixth light. Overall, the third light (the light reflected by the front surface of the first cavity) and the fifth light (the light reflected by the high-reflection film of the second cavity) are the signal lights of the double-cavity cascade interference, which carry the cavity length change information by means of multi-beam interference. The remaining four lights are scattered lights, which are weak in energy or do not participate in the interference cavity in the propagation path, and thus the interference with the measurement signal is suppressed, thereby ensuring that the system realizes high-sensitivity and high-stability displacement or pressure measurement through double-cavity cascade. The pressure device 1 and the pressure device 2 are key components for realizing pressure difference modulation in the Fabry-Perot interferometer measurement system based on the three-pole double-cavity cascade, and their functions are closely related to the propagation and interference characteristics of the six lights. Specifically, the pressure device 1 acts on the first fixed plate (the magnesium fluoride substrate coated with an anti-reflection film), and the pressure device 2 acts on the second fixed plate (the silicon substrate coated with a high-reflection film). By applying controllable pressure to the two interference cavities (the first interference cavity is located between the first fixed plate and the intermediate movable plate, and the second interference cavity is located between the intermediate movable plate and the second fixed plate), a pressure difference is formed between the two cavities. According to the principle described in the document, the intermediate movable plate will produce a displacement Δx in the direction perpendicular to the plane of the pole plate under the action of the pressure difference. This displacement will cause the cavity length L1 of the first interference cavity to shorten by Δx, and at the same time, the cavity length L2 of the second interference cavity will lengthen by Δx, thereby realizing the reverse change of the optical path of the double cavity. The change in the cavity length directly affects the light rays involved in the core interference among the six lights. The optical path difference of the third light (the light reflected by the front surface of the intermediate movable plate) in the first interference cavity is 2n1L1 (n1 is the refractive index of the medium in the first cavity). When L1 shortens by Δx, the optical path difference will decrease by 2n1Δx, causing the interference fringes of the first interference cavity to move. The optical path difference of the fifth light (the light reflected by the high-reflection film surface of the second fixed plate) in the second interference cavity is 2n2L2 (n2 is the refractive index of the medium in the second cavity). When L2 lengthens by Δx, the optical path difference will increase by 2n2Δx, causing the interference fringes of the second interference cavity to move in the opposite direction. The interference signal change of these two lights is the core basis for the system to realize the measurement of physical quantities such as pressure and refractive index.Among the rest four lights, the first light (incident light and the first fixed plate outer surface reflected light) and the second light (the first fixed plate internal reflection light) belong to stray reflection light, because they do not enter the interference cavity or the energy is weak, their propagation is not affected by the change of cavity length, the fourth light (intermediate movable plate internal reflection light) although it propagates in the movable plate, but because the movable plate thickness is only 100 μm and the energy attenuates with reflection, the interference to the core interference signal is minimal, the sixth light (second fixed plate internal reflection light) is because the energy through the high reflection film is extremely low (<2%), even when the cavity length changes, it only exists as a stray light inside the substrate, and has no substantial impact on the measurement, while the reverse change of the double cavity optical path can further suppress the interference of these stray lights to the interference signal through the differential modulation mechanism, ensuring that the system can realize high-precision measurement through the interference characteristics of the third light and the fifth light.

[0067] In the aspect of high-precision pressure measurement, the original F-P interferometer is often difficult to accurately measure due to insufficient sensitivity and easy interference when facing complex environment pressure measurement. The F-P interferometer system based on three parallel polar plates relies on the double-interference cavity structure. When external pressure is applied, the intermediate movable plate will displace due to the pressure difference on both sides, causing the double-interference cavity optical path difference to change. When external pressure acts on the interferometer system, the first fixed plate and the second fixed plate transmit the pressure to the double-interference cavity through the precise driving device, and the intermediate movable plate displaces Δx due to the pressure difference on both sides. The cantilever beam support structure of the intermediate movable plate makes its displacement and the pressure difference satisfy Hooke's law where A is the effective stress area of the polar plate, and k is the total elastic coefficient of the cantilever beam (formed by multiple cantilever beams in parallel). The temperature interference compensation and double-cavity optical path difference feedback are introduced, and the pressure calculation formula is: where k is the elastic coefficient of the cantilever beam, is the thermal expansion caused by temperature, and α Si are the thermal expansion coefficients of the first fixed plate (MgF2) and the intermediate polar plate (single crystal silicon) respectively, ΔT is the change of environmental temperature, is the real-time phase difference, is the optical path difference feedback coefficient. At this time, the length of the first interference cavity is shortened from L1 to L1-Δx, and the length of the second interference cavity is extended from L2 to L2+Δx, and the double-cavity optical path difference changes respectively and total optical path difference change If the double cavities are filled with the same medium (n1=n2=n), then Δδ total =0, and the displacement caused by pressure is differentially compensated by the double cavities; if one side is vacuum (n1=n, n2=1), then By monitoring the number of interference fringes moving combined with Hooke's law, the pressure calculation formula can be obtained By selecting different filling medium (changing n) or adjusting the cantilever elastic coefficient k, the sensitivity and range of pressure measurement can be flexibly adjusted.

[0068] In the medium refractive index measurement, the traditional F-P interferometer is easily disturbed by environmental factors such as temperature during measurement, and it is difficult to realize accurate measurement of small changes in refractive index. The new system is based on the principle of double-cavity resonance wavelength difference. When the first cavity is filled with the medium to be measured and the second cavity is vacuum, the refractive index of the medium can be accurately calculated by measuring the resonance wavelength and establishing the correlation. The refractive index solving formula is: B is the stress birefringence coefficient of the middle electrode plate, is the film dispersion parameter (determined by the properties of the antireflection film / high reflection film material), is the cavity length change rate. When the refractive index of the medium changes slightly, the refractive index change and the cavity length drift can also be effectively separated by the double-cavity data decoupling algorithm. Assuming that the first cavity is filled with the medium to be measured (refractive index n), the second cavity is vacuum (n2=1), and the initial cavity length L1=L2=L, the resonance condition is: 2nL=mλ1, 2L=mλ2, and n=λ2 / λ1 can be obtained by combining. When the refractive index of the medium changes Δn, the resonance wavelength shifts The cavity length error can be eliminated by double-cavity wavelength difference measurement, and the resolution reaches

[0069] As shown in Figure 2 , the traditional F-P interferometer (left) adopts a double-plate single-cavity structure, and the optical path modulation is single; the present application (right) cascades a three-plate double-cavity, when the middle electrode plate is displaced Δx under the action of pressure difference, the first cavity is shortened to L1-Δx, and the second cavity is lengthened to L2+Δx, forming a differential modulation of optical path difference, effectively suppressing common-mode interference and enhancing signal response. As shown in Figure 7 , the traditional F-P interferometer has insufficient sensitivity due to the single-cavity structure, while the present application significantly improves the pressure measurement performance through the double-cavity differential effect: if the double-cavity is filled with the same medium (n1=n2=n), the common-mode interference (such as temperature expansion) is cancelled; if one side is vacuum (n2=1), the total optical path difference changes Δ(nL1-L2)=nΔx, and the number of interference fringes moves Combined with Hooke's law, the pressure solving formula is obtained: Figure 7 It is shown that the fringe movement amount of the present application is increased by about 50% compared with the traditional scheme under the same pressure, and the resolution can reach below 1Pa.

[0070] From the optical filtering and spectral analysis function, the original F-P interferometer has a wide filtering bandwidth and limited spectral resolution, which is difficult to meet the high resolution requirement when dealing with complex spectrum. The double-cavity cascade structure of the new system realizes phase coupling through the optical path difference, which compresses the half-height width of the transmission peak by about 30% compared with the single cavity. At the same time, through the independent adjustment of the cavity length by the piezoelectric ceramic, the dynamic tuning of the transmission peak wavelength can be realized, covering a wide spectral range. In spectral analysis applications, the light source is divided into multiple channels using the "wavelength-position" mapping, combined with the detector array and interpolation algorithm, the spectral resolution below 1nm can be realized. When the double-cavity length is equal and filled with the same medium, the cascade system is equivalent to a high-precision filter, and the transmission peak half-height width (FWHM) is: Where F is the double-cavity phase coupling factor, m is the interference order (integer), and σ is the root mean square of the cavity length jitter caused by mechanical vibration. By increasing the reflectivity or cavity length, the bandwidth can be compressed to sub-nanometer level.

[0071] As shown in Figure 8 , the traditional F-P is easily disturbed by temperature, while the present application realizes high-resolution solution based on the principle of double-cavity resonance wavelength difference: the first cavity is filled with the medium to be measured (refractive index n), the second cavity is vacuum (n2=1), the initial cavity length L1=L2=L, the resonance condition is: 2nL=m1λ1, 2L=m2λ2, and by solving the equations, n=λ2 / λ1, the cavity length error is eliminated. When the refractive index changes Δn, the resonance wavelength shifts Combined with the double-cavity data decoupling algorithm, the resolution can reach 10 -5 RIU level.

[0072] As shown in Figure 4 , Figure 5 , the traditional F-P filter has a wide bandwidth (half-height width of about 3nm), while the present application compresses the spectral width through double-cavity phase coupling: when the double-cavity length is equal and filled with the same medium, the equivalent high-precision filter has a transmission peak half-height width (FWHM) of: Where is the double-cavity phase coupling factor, and σ L is the root mean square of the cavity length jitter caused by mechanical vibration. When the double-cavity length is equal and filled with the same medium, by adjusting the cavity length independently through the piezoelectric ceramic, κ=1 can make the cascade system equivalent to a high-precision filter, and by increasing the reflectivity or cavity length, the bandwidth can be compressed to sub-nanometer level.

[0073] By increasing the reflectivity or cavity length, the bandwidth can be compressed to sub-nanometer level (such as Figure 4 , the spectral resolution of the present application is <1nm). By adjusting the cavity length independently through the piezoelectric ceramic, a wide spectral range (400-1000nm) can be covered, which is suitable for multi-component spectral analysis.

[0074] Figure 6For the interference fringe contrast comparison chart, the signal quality difference between the traditional F-P interferometer and the interferometer of the application is clearly shown by the curve comparison: the interference fringe contrast of the traditional single cavity structure rapidly attenuates with the increase of the optical path difference, and when the optical path difference exceeds 2l, the relative intensity is less than 500, while the application is based on the three-pole double-cavity cascade structure, and the relative intensity of the interference fringe contrast remains nearly 1000 when the optical path difference reaches 10l, which is due to the suppression effect of the double-cavity phase coupling on the common-mode interference, and the detection error of the spectral peak position can be effectively reduced, which provides a stable signal basis for the accurate identification of the fringe movement amount and the refractive index solution in pressure measurement, and also lays a foundation for the realization of the sub-nanometer resolution in spectral analysis.

[0075] The application realizes the synchronous measurement of pressure and refractive index through the double-cavity cascade structure, solves the problem that the traditional single-cavity F-P cannot separate the coupling of multiple physical quantities, directly offsets the common-mode noise by using the differential modulation mechanism (L1 cavity shortens by Dx, and L2 cavity lengthens by Dx), and also amplifies the signal by superimposing the optical paths of the double cavities, so that the fringe movement amount is higher than that of the traditional single cavity, the cavity length drift error is eliminated by calculating the wavelength difference of the double cavities, and the sensitivity is improved in a leap.

[0076] In the application, the double-cavity phase coupling compresses the half width of the transmission peak, reduces the actual measurement bandwidth, meets the demand of high-resolution spectral analysis, the piezoelectric ceramic independently adjusts the lengths of the double cavities, the wavelength tuning range covers the full wavelength band of 400-1000nm, the “wavelength-position” mapping algorithm realizes multi-channel parallel detection, and the spectral resolution breaks through the bottleneck of 1nm.

[0077] The above is the exemplary embodiment disclosed by the application, but it should be noted that various changes and modifications can be made without departing from the scope of the embodiment disclosed by the application defined in the claims. The functions, steps and / or actions of the method claims described herein do not need to be performed in any specific order. In addition, although the elements of the embodiment disclosed by the application can be described or claimed in singular form, they can also be understood as plural, unless explicitly limited as singular.

[0078] It should be understood that, unless the context clearly supports an exception, the singular form “one” used herein is intended to also include the plural form. It should also be understood that “and / or” used herein means any and all possible combinations of one or more associated listed items. The above embodiment serial numbers of the application are only for description, and do not represent the advantages and disadvantages of the embodiments.

[0079] It should be understood by those of ordinary skill in the art that the above discussion of any of the embodiments is merely exemplary in nature and not intended to imply limitations on the scope of the disclosure, including the claims; nor have the technical features been combined in any way in the above embodiments or combinations of different embodiments, and there are many other variations of the above aspects of the embodiments of the present application, which are not provided in detail in order to be brief. Therefore, any omissions, modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the embodiments of the present application shall be included in the scope of protection of the embodiments of the present application.

Claims

1. A Fabry-Perot interferometer measurement system based on a triplate dual-cavity cascade, characterized in that, The system comprises a fiber-coupled laser light source with a light path protection device, a fiber collimator, an F-P interferometer and a fiber-coupled light detector, and adopts a double-interference-cavity cascade structure of three parallel plates; wherein the three parallel plates are composed of a first fixed plate, an intermediate movable plate and a second fixed plate; a first interference cavity is formed between the first fixed plate and the intermediate movable plate, and a second interference cavity is formed between the intermediate movable plate and the second fixed plate; when the intermediate movable plate is displaced by a pressure difference, the cavity length of the first interference cavity will be shortened by a corresponding displacement amount, while the cavity length of the second interference cavity will be lengthened by the same displacement amount, realizing reverse changes of the optical path of the double cavities and differential modulation of the optical path difference in the two cavities: the optical path difference of the first interference cavity decreases with the shortening of the cavity length, and the optical path difference of the second interference cavity increases with the lengthening of the cavity length, and the change amounts of the two are equal in size and opposite in direction; based on the Fabry-Perot interference principle, the change of the optical path difference causes the movement of the interference fringes or the periodic change of the transmitted light intensity, the superimposed signal of the double-cavity interference light intensity is collected by the fiber-coupled light detector, and the displacement amount corresponding to the pressure difference is obtained by solving the double-cavity phase coupling model, so as to realize high-precision measurement of the target physical quantity.

2. The triple-plate dual-cavity cascade Fabry-Perot interferometer based measurement system of claim 1, wherein, The first fixed plate is a magnesium fluoride substrate with a thickness of 2 mm, and the surface has an optical precision of λ / 10 after ion beam polishing; wherein the transmission surface facing the first interference cavity is coated with an anti-reflection film with a center wavelength reflectivity of <0.5%, and the incidence surface facing the fiber collimator side maintains a high flatness optical surface after polishing but is not coated, the incidence surface is not coated, the low refractive index difference between magnesium fluoride and air naturally reduces the reflectivity, and at the same time, the internal multi-interface interference introduced by double-sided coating is avoided, wherein when λ=532 nm, the flatness is ≤53.2 nm, and the absorption coefficient of the magnesium fluoride substrate is <0.01 dB / cm in the ultraviolet to near-infrared wavelength band of 200-1100 nm, serving as a transmission window for high-power laser.

3. The triple-plate dual-cavity cascade Fabry-Perot interferometer based measurement system of claim 2, wherein, The intermediate movable plate is a cantilever beam array support structure prepared by photolithography and deep etching process of single crystal silicon micromechanical structure, the substrate thickness of the intermediate movable plate is 100 μm, the length, width and thickness of a single cantilever beam are 500 μm, 50 μm and 2 μm respectively, and the elastic coefficient is optimized to k=0.1 N / m through finite element simulation, one end of the cantilever beam is fixed to the frame, the other end is connected to the intermediate plate main body, and an elastic support system is formed.

4. The triple-plate dual-cavity cascade Fabry-Perot interferometer based measurement system of claim 3, wherein, When the intermediate movable plate is subjected to a double-cavity pressure difference ΔP=P1-P2, the intermediate plate generates a displacement Δx in the direction perpendicular to the plate plane, and the displacement amount and the pressure difference are in a linear relationship as follows: Wherein A is the effective stress area of the plate, and k is the total elastic coefficient of the cantilever beam.

5. The triple-plate dual-cavity cascade Fabry-Perot interferometer based measurement system of claim 4, wherein, The second fixed plate is a silicon substrate, and the surface facing the second interference cavity is coated with a multilayer dielectric high-reflection film, which is made by alternately depositing Al2O3 / SiO2, only a high-reflection interface is formed on the surface facing the second interference cavity, and the reflectivity in the wavelength band of 400-1000 nm is >98%, wherein the high-reflection film is prepared by electron beam evaporation process, and the thickness of each layer is monitored in real time by a quartz crystal oscillator.

6. The triple-plate dual-cavity cascade Fabry-Perot interferometer based measurement system according to any one of claims 1 to 5, wherein The cavity length of the first interference cavity is L1, and the cavity length of the second interference cavity is L2; when coherent light with a wavelength of λ is vertically incident on the first fixed plate, the transmitted light enters the first interference cavity, multiple-beam interference occurs between the first fixed plate and the intermediate movable plate, the surface of the intermediate movable plate towards the first cavity has a transmittance (T1) and a reflectivity (R1), and T1+R1=1, and the multiple reflections of the light in the first cavity form an equal inclination interference transmitted light intensity distribution satisfying the F-P interference formula: wherein is the phase difference in the first cavity, n1is the refractive index of the medium in the cavity, 7. The triple-plate dual-cavity cascade Fabry-Perot interferometer based measurement system of claim 6, wherein, The transmitted light of the first cavity enters the second interference cavity after passing through the middle movable plate, the transmittance of the middle movable plate towards the rear surface of the second cavity is T2, the reflectance is R2, the reflectance of the second fixed plate is R3, and the double-cavity phase is: Wherein φ1, φ2 are the surface reflection phases of the pole plates, Δx is the displacement of the middle pole plate, Δn = n1-n2 is the refractive index difference of the double-cavity medium, is the mechanical coupling coefficient, A is the effective force area of the pole plate, and k is the total elastic coefficient of the cantilever beam. The interference light intensity in the second cavity is: I2(λ) = I1(λ) T2 2 R3 2 [E-2 Y2 Y3 cos(∮ C k·dl)] -1 wherein E is a second order identity matrix.

8. The triple-plate dual-cavity cascade Fabry-Perot interferometer based measurement system of claim 7, wherein, The total transmitted light intensity after the double-cavity cascade is: Wherein γ is an optical path coupling constant, which is related to the distance between the polar plates.

9. The triple-plate dual-cavity cascade Fabry-Perot interferometer based measurement system of claim 8, wherein, Total transmitted light intensity I after double-cavity cascade total (λ) = I2(λ), the spectral characteristics are determined by the double-cavity phases δ1 and δ2, when the double-cavity satisfies the resonance condition 2n1L1 = mλ and 2n2L2 = mλ, the total transmittance reaches a peak value, forming a narrow-band filtering effect, when the double-cavity length is misaligned, the transmission spectrum is broadened or a multi-peak structure appears, which can be used for spectral analysis, where m is an integer.

10. The triple-plate dual-cavity cascade Fabry-Perot interferometer based measurement system of claim 9, wherein, When measuring the refractive index of a medium, the first cavity is filled with the medium to be measured, the second cavity is vacuum, the stress birefringence effect and the film layer dispersion parameter are introduced, and the refractive index calculation formula is: Wherein, B is the stress birefringence coefficient of the intermediate electrode plate, is the film layer dispersion parameter, is the cavity length change rate; If the first cavity is filled with the medium to be measured, the refractive index is n; the second cavity is vacuum, n2=1; the initial cavity length is L1=L2=L, the resonance condition is: 2nL=mλ1, 2L=mλ2, and by combining them, n=λ2 / λ1; when the refractive index of the medium changes by Δn, the resonance wavelength shifts The cavity length error is eliminated by double-cavity wavelength difference measurement, and the resolution reaches

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