CO2 gas sensor with cascaded long-period waveguide grating structure and method of use

Abstract

The present invention discloses a CO2 gas sensor based on a cascaded long-period waveguide grating structure, comprising a silicon substrate, a waveguide lower cladding, a strip waveguide core layer and a waveguide upper cladding arranged in sequence from bottom to top, a first long-period waveguide grating and a second long-period waveguide grating are arranged on the upper surface of the strip waveguide core layer, the strip waveguide core layer is made of a polymer material, the waveguide lower cladding is made of a polymer material, and the waveguide upper cladding is made of a composite gas-sensitive material of a polymer and a metal-organic framework material that has strong selective adsorption for CO2 gas. The present invention also discloses a method for using a CO2 gas sensor based on a cascaded long-period waveguide grating structure. The present invention solves the problems of time-consuming ZIF-8 production on the surface of an optical waveguide, high requirements for ZIF-8 film uniformity, and low sensor sensitivity, and at the same time utilizes the highly sensitive response of the characteristic wavelength of the interferometer output spectrum trough to the refractive index disturbance of the external environment to achieve highly sensitive and rapid measurement of CO2 gas.

Description

Technical Field

[0001] The present invention relates to a gas sensor, in particular to a CO2 gas sensor based on a cascaded long-period waveguide grating structure and a use method thereof, belonging to the field of optical waveguide sensing technology. Background Art

[0002] CO2 is a well-known gas that is colorless and odorless. However, when its concentration is too high indoors (2000ppm to 5000ppm), it can cause negative effects on the human body, including headaches, drowsiness, increased heart rate, and decreased concentration. Furthermore, rising CO2 levels in the atmosphere contribute to the greenhouse effect, indirectly impacting our lives. Therefore, high-sensitivity, high-detection-limit monitoring of CO2 concentrations is of practical significance.

[0003] Among CO2 gas sensors, the main types currently being researched are semiconductor gas sensors, electrochemical gas sensors, and optical gas sensors. The most common semiconductor CO2 gas sensor utilizes a redox reaction between gas and a metal oxide film on the semiconductor surface, causing a change in the sensor's conductivity to detect CO2. While operating at low temperatures and offering high sensitivity, it is susceptible to environmental influences and suffers from poor gas selectivity. Electrochemical CO2 gas sensors reduce CO2 gas at a sensitive electrode and detect CO2 gas concentration via the output potential. Compared to semiconductor CO2 gas sensors, electrochemical CO2 gas sensors offer better gas selectivity, but the solid electrolyte manufacturing process is complex and their lifespan is relatively short. Among optical CO2 gas sensors, the non-dispersive infrared absorption method uses the infrared absorption spectrum corresponding to the gas (the absorption wavelength for CO2 gas is 4.26 microns) to perform qualitative and quantitative analysis. Due to its advantages of good stability, good selectivity, fast response, and long lifespan, it is widely used, but humidity significantly affects its performance.

[0004] Metal-organic frameworks (MOFs) have emerged as promising sensing materials due to their high porosity and selective gas adsorption capacity. ZIF-8, in particular, exhibits excellent thermal and chemical stability, a large surface area, highly selective adsorption of CO₂, and insensitivity to humidity. Sensors that selectively detect CO₂ by growing ZIF-8 on the surface of optical waveguides offer advantages such as compact size, immunity to electromagnetic interference, ease of integration, high sensitivity, and low cost. However, fabricating ZIF-8 on an optical waveguide surface and modulating light within the waveguide via an evanescent field to measure the refractive index or gas concentration outside the waveguide presents challenges such as time-consuming sensor fabrication, high ZIF-8 film uniformity requirements, and low sensor sensitivity. Summary of the Invention

[0005] The purpose of the present invention is to provide a CO2 gas sensor based on a cascaded long-period waveguide grating structure. A Mach-Zehnder interferometer is formed by two cascaded long-period waveguide gratings, and a polymer and metal-organic framework composite gas-sensitive material is used as the sensing arm of the interferometer. This solves the problems of time-consuming ZIF-8 production on the optical waveguide surface, high requirements for ZIF-8 film uniformity, and low sensor sensitivity. At the same time, the characteristic wavelength of the interferometer output spectrum trough is used to respond highly sensitively to the refractive index disturbance of the external environment, thereby achieving highly sensitive and rapid measurement of CO2 gas.

[0006] The present invention is specifically achieved in this way:

[0007] A CO2 gas sensor based on a cascaded long-period waveguide grating structure comprises a silicon substrate, a waveguide lower cladding, a strip waveguide core layer and a waveguide upper cladding arranged in sequence from bottom to top;

[0008] The waveguide lower cladding and strip waveguide core are both made of polymer materials, and the waveguide upper cladding is made of a composite gas-sensitive material of polymer and metal-organic framework material with strong selective adsorption for CO2 gas.

[0009] A first long-period waveguide grating and a second long-period waveguide grating are provided on the upper surface of the strip waveguide core layer. The two cascaded long-period waveguide gratings form a Mach-Zehnder interferometer to measure CO2 concentration information.

[0010] A further approach is:

[0011] The polymer material used for the strip waveguide core layer is any one of SU-8 2005, EpoCore, EpoClad, OrmoCore, OrmoClad or NOA 73.

[0012] A further approach is:

[0013] The polymer material used for the waveguide lower cladding is any one of polydimethylsiloxane, polymethyl methacrylate, polyimide, fluorinated polyarylether or polycarbonate.

[0014] A further approach is:

[0015] The composite gas-sensing material of the polymer / metal-organic framework material of the upper cladding layer of the strip waveguide is a composite material of PDMS and ZIF-8, wherein the average size of the ZIF-8 particles is 40±10 nm, and the ZIF-8 / PDMS ratio is 1-5 wt%.

[0016] ZIF-8 possesses excellent thermal and chemical stability, a large specific surface area, highly selective adsorption of CO₂, and insensitivity to humidity. PDMS, in addition, is an excellent carrier for ZIF-8 due to its relatively low price, flexibility, chemical inertness, thermal stability, good permeability to CO₂, and excellent light transmittance. ZIF-8 and PDMS are simply mixed to form a ZIF-8 / PDMS composite gas-sensitive material, which acts as an optical transmission medium and effectively reduces optical transmission losses.

[0017] A further approach is:

[0018] The refractive index of the strip waveguide core layer is respectively greater than the refractive index of the upper cladding material and the lower cladding material.

[0019] A further approach is:

[0020] The strip waveguide is a single-mode waveguide.

[0021] A further approach is:

[0022] The transmittance of the first long-period waveguide grating and the second long-period waveguide grating are both 3 dB and their structural parameters are consistent.

[0023] The present invention also provides a method for using a CO2 gas sensor based on a cascaded long-period waveguide grating structure, comprising:

[0024] The light emitted by the broadband light source is partially coupled into the cladding mode at the first long-period waveguide grating through the strip waveguide core layer, forming independent transmission of the core fundamental mode and the cladding mode;

[0025] The light coupled to the upper cladding passes through the PDMS / ZIF-8 composite gas-sensitive material. When the external CO2 concentration changes, ZIF-8 has a highly selective adsorption of CO2, and the effective refractive index of the PDMS / ZIF-8 composite gas-sensitive material changes.

[0026] The light of the cladding mode is coupled into the core layer at the second long-period waveguide grating, and interferes with the light of the core layer's fundamental mode. The CO2 concentration information in the environment is obtained by the drift of the characteristic wavelength of the interferometer output spectrum trough.

[0027] The present invention has at least the following outstanding technical effects:

[0028] By arranging a first long-period waveguide grating and a second long-period waveguide grating on the upper surface of the strip waveguide core layer, a Mach-Zehnder interferometer is formed, thereby realizing the measurement of CO2 concentration information;

[0029] The strip waveguide core layer is made of polymer material, the waveguide lower cladding is made of polymer material, and the waveguide upper cladding is made of a composite gas-sensitive material of polymer and metal-organic framework material, so that the polymer and metal-organic framework composite gas-sensitive material serves as the sensing arm of the interferometer, thereby improving the sensitivity of the sensor;

[0030] Taking advantage of the good light transmittance of PDMS, the ZIF-8 / PDMS composite gas-sensing material solves the problems of severe light scattering and large transmission loss of ZIF-8.

[0031] The optical transmission medium of the waveguide upper cladding layer adopts a composite gas-sensitive material of PDMS / ZIF-8. Taking advantage of the good light transmittance of PDMS and its simple stirring production method, it solves the previous problem of integrating ZIF-8 on the waveguide surface, which is time-consuming and difficult to grow uniformly. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a basic structural diagram of the CO2 gas sensor of the present invention;

[0033] Figure 2 This is a top view of the strip waveguide core layer in the CO2 gas sensor;

[0034] In the figure: 1. silicon substrate, 2. waveguide lower cladding, 3. strip waveguide core layer, 4. waveguide upper cladding, 31. first long-period waveguide grating, 32. second long-period waveguide grating. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0036] Example 1

[0037] This embodiment provides a CO2 gas sensor based on a cascaded long-period waveguide grating structure, which achieves highly sensitive CO2 gas measurement.

[0038] like Figure 1 As shown in the figure, the CO2 gas sensor based on a cascaded long-period waveguide grating structure described in the present invention includes a silicon substrate 1, a waveguide lower cladding 2, a strip waveguide core 3, a waveguide upper cladding 4, and a first long-period waveguide grating 31 (LPWG1) and a second long-period waveguide grating 32 (LPWG2) on the strip waveguide core. The two cascaded long-period waveguide gratings form a Mach-Zehnder interferometer, thereby measuring CO2 concentration information.

[0039] The waveguide upper cladding layer is a composite gas-sensitive material composed of PDMS / ZIF-8, wherein the average ZIF-8 particle size is 40±10nm and the ZIF-8 / PDMS content is 1-5wt%. ZIF-8 has excellent thermal and chemical stability, a large specific surface area, high selective adsorption of CO2 gas, and insensitivity to humidity. In addition, PDMS material is an excellent carrier for ZIF-8 material due to its relatively low price, flexibility, chemical inertness, thermal stability, good CO2 permeability, and good light transmittance. ZIF-8 and PDMS are simply stirred and mixed to form the ZIF-8 / PDMS composite gas-sensitive material as an optical transmission medium, which can effectively reduce optical transmission loss.

[0040] The strip waveguide core layer is made of a polymer material such as SU-8 2005, EpoCore, EpoClad, OrmoCore, OrmoClad, or NOA 73. The lower cladding layer is made of a polymer material such as polydimethylsiloxane, polymethyl methacrylate, polyimide, fluorinated polyarylether, or polycarbonate. The refractive index of the strip waveguide core is greater than that of the polymer materials of the lower and upper cladding layers, confining light propagation in the core layer and ensuring single-mode operation.

[0041] Example 2

[0042] This embodiment provides a method for using a CO2 gas sensor based on a cascaded long-period waveguide grating structure, including:

[0043] The light emitted by the broadband light source is partially coupled into the cladding mode at the first long-period waveguide grating through the strip waveguide core, forming independent transmission of the core fundamental mode and the cladding mode. According to coupling theory, the phase matching condition for coupling at the first long-period waveguide grating is

[0044]

[0045] Among them, β co , β cl are the propagation constants of the core fundamental mode and cladding mode, respectively, and Λ is the period of the long-period waveguide grating. The relationship between the propagation constant and the central wavelength λ is

[0046]

[0047] in, are the effective refractive indices of the core fundamental mode and cladding mode, respectively. From formula (1) and formula (2), we can get

[0048]

[0049] Two cascaded long-period waveguide gratings form a Mach-Zehnder interferometer. The cladding mode transmitted in the cladding is coupled into the core at the second long-period waveguide grating and interferes with the core fundamental mode. The output intensity of the interference signal can be expressed by the following formula:

[0050]

[0051] Among them, I co with I cl The light intensity of the core fundamental mode and cladding mode respectively, is the phase difference between the core fundamental mode and the cladding mode, which can be expressed as:

[0052]

[0053] Among them L co is the transmission length of the core fundamental mode, L cl is the transmission length of the cladding mode. From formula (4) and formula (5), we can see that when When π, the output light intensity is destructive interference, and the output light intensity at this time is:

[0054]

[0055] Characteristic wavelength corresponding to the trough of the interferometer transmission spectrum:

[0056]

[0057] ZIF-8 exhibits highly selective adsorption of CO2 gas. When the ambient CO2 concentration changes, the adsorption of CO2 by the PDMS / ZIF-8 composite gas-sensing material causes a significant shift in the effective refractive index of the cladding mode, while the effective refractive index of the core mode changes minimally. Light from the cladding mode couples into the core at the second long-period waveguide grating, interfering with the core mode. By monitoring the shift in the characteristic wavelength of the interference spectrum trough, information about the ambient CO2 concentration can be obtained.

[0058] Although the present invention is described herein with reference to illustrative embodiments of the present invention, the above embodiments are merely preferred embodiments of the present invention, and the embodiments of the present invention are not limited to the above embodiments. It should be understood that those skilled in the art can design many other modifications and implementations, which will fall within the scope and spirit of the principles disclosed in this application.

Claims

1. A CO2 gas sensor based on a cascaded long-period waveguide grating structure, characterized by: The method comprises a silicon substrate, a waveguide lower cladding layer, a strip waveguide core layer and a waveguide upper cladding layer which are sequentially arranged from bottom to top; The waveguide lower cladding and strip waveguide core are both made of polymer materials, and the waveguide upper cladding is made of a composite gas-sensitive material of polymer and metal-organic framework material with strong selective adsorption for CO2 gas. The upper surface of the strip waveguide core layer is provided with a first long-period waveguide grating and a second long-period waveguide grating. The two cascaded long-period waveguide gratings form a Mach-Zehnder interferometer to measure CO2 concentration information. The composite gas-sensitive material of the polymer / metal-organic framework material of the waveguide upper cladding is a composite material of PDMS and ZIF-8; In the composite material of PDMS and ZIF-8, the average size of ZIF-8 particles is 40±10 nm, and the ZIF-8 / PDMS ratio is 1-5 wt %; the ZIF-8 and PDMS are simply stirred and mixed to form the composite material of PDMS and ZIF-8; The refractive index of the strip waveguide core layer is greater than the refractive index of the upper cladding material and the lower cladding material respectively; The transmittance of the first long-period waveguide grating and the second long-period waveguide grating are both 3 dB and their structural parameters are consistent.

2. The CO2 gas sensor based on the cascaded long-period waveguide grating structure according to claim 1, characterized in that: The polymer material used for the strip waveguide core layer is any one of SU-8 2005, EpoCore, EpoClad, OrmoCore, OrmoClad or NOA 73.

3. The CO2 gas sensor based on the cascaded long-period waveguide grating structure according to claim 1, characterized in that: The polymer material used for the waveguide lower cladding is any one of polydimethylsiloxane, polymethyl methacrylate, polyimide, fluorinated polyarylether or polycarbonate.

4. The CO2 gas sensor based on the cascaded long-period waveguide grating structure according to claim 1, characterized in that: The strip waveguide is a single-mode waveguide.

5. A method for using a CO2 gas sensor based on a cascaded long period waveguide grating structure according to any one of claims 1 to 4, characterized in that include: The light emitted by the broadband light source is partially coupled into the cladding mode at the first long-period waveguide grating through the strip waveguide core layer, forming independent transmission of the core fundamental mode and the cladding mode; The light coupled to the upper cladding passes through the PDMS / ZIF-8 composite gas-sensitive material. When the external CO2 concentration changes, ZIF-8 has a highly selective adsorption of CO2, and the effective refractive index of the PDMS / ZIF-8 composite gas-sensitive material changes. The light of the cladding mode is coupled into the core layer at the second long-period waveguide grating, and interferes with the light of the core layer's fundamental mode. The CO2 concentration information in the environment is obtained by the drift of the characteristic wavelength of the interferometer output spectrum trough.

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