Mid-Infrared Gas Sensor Based on Conical Sub-Micron Grating-Groove Waveguide

Abstract

The present invention discloses a mid-infrared gas sensor based on a tapered sub-micron grating-slot waveguide, which includes a lower cladding, a first tapered grating array and a second tapered grating array. The first tapered grating array and the second tapered grating array are arranged on the upper surface of the lower cladding. The first tapered grating array is located on the front side of the second tapered grating array. The first tapered grating array is formed by 5566 identical first core layer waveguides evenly spaced in sequence from left to right. The second tapered grating array is formed by 5566 identical second core layer waveguides evenly spaced in sequence from left to right. Both the first core layer waveguide and the second core layer waveguide are tapered waveguides, and the upper and lower sides of the first core layer waveguide and the second core layer waveguide are both isosceles trapezoids. The advantages are high sensitivity, low detection threshold and fast response time.

Description

Technical Field

[0001] The present invention relates to a mid-infrared gas sensor, and in particular to a mid-infrared gas sensor based on a tapered sub-micron grating-groove waveguide. Background Art

[0002] Due to the development of emerging industrial technologies, frequent human activities have brought more serious environmental pollution problems. With the steady improvement of people's awareness of health and safety, effective on-line monitoring and accurate and rapid early warning of toxic, harmful, flammable and explosive gases have become the most basic guarantee for the safety of people's lives and property. At the same time, the high-precision requirements of technologies such as medical detection, deep-sea operation, and space exploration have directly led to a major development trend of developing integrated, miniaturized, and high-precision sensing devices.

[0003] As a device for detecting gases, gas sensors have many application scenarios, which are not only reflected in environmental quality monitoring, but also self-evident in energy extraction and clinical medicine. There are many types of gas sensors commonly used on the market, which are usually divided into: catalytic combustion type, semiconductor type, thermal conductivity detection type, electrochemical type and optical type according to the principle. Most of these traditional types of gas sensors, such as catalytic combustion type gas sensors, semiconductor type gas sensors, thermal conductivity detection type gas sensors and electrochemical type gas sensors, are based on various physical properties and chemical reactions. Their contact measurement methods often perform poorly in terms of long-term stability, have a high calibration frequency, and the service life of their built-in sensitive elements is short. As a new type of gas sensor, the optical type gas sensor cooperates with a light source and a photodetector to sense changes in optical wave parameters such as light intensity, frequency, polarization and phase to achieve the purpose of detecting gases. Compared with other several traditional types of gas sensors, the optical type gas sensor adopts a non-contact measurement method and has characteristics such as anti-electromagnetic interference and no cross-sensitivity, which just make up for the deficiencies of the above several traditional gas sensors in terms of short life and poor stability. Among the existing optical type gas sensors, the gas sensor based on spectral absorption has developed most rapidly. The spectral absorption type gas sensor is based on the measurement and analysis of the characteristic absorption spectra of substance molecules at different wavelengths. The type and concentration of gases can be defined by measuring the transmission intensity of spectral lines. With the development of micro-nano photon devices, the spectral absorption type gas sensor has gradually been converted from a traditional long-path optical fiber platform to a small-size optical waveguide platform, and has the characteristics of easy miniaturization and integrability. However, in the current existing research, the performance parameters such as sensitivity, detection threshold, and response time of the spectral absorption type waveguide gas sensor still need to be improved. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a mid-infrared gas sensor based on a tapered sub-micron grating-groove waveguide, which has high sensitivity, a low detection threshold, and a fast response time.

[0005] The technical solution adopted by the present invention to solve the above technical problems is as follows: A mid-infrared gas sensor based on a conical sub-micron grating-groove waveguide, comprising a lower cladding, a first conical grating array and a second conical grating array. The lower cladding is a rectangular waveguide made of calcium fluoride. The length direction of the lower cladding is along the left-right direction, the width direction is along the front-back direction, and the thickness direction is along the up-down direction. The thickness of the lower cladding is greater than or equal to 6 μm. The first conical grating array and the second conical grating array are arranged on the upper surface of the lower cladding. The first conical grating array is located on the front side of the second conical grating array. The first conical grating array is formed by evenly spaced distribution of m identical first core layer waveguides in order from left to right. The value of m is 5566. The first core layer waveguide is a conical waveguide made of silicon. The front end face of the first core layer waveguide is rectangular. The long side of the front end face of the first core layer waveguide is along the left-right direction, and the wide side is along the up-down direction. The length of the long side of the front end face of the first core layer waveguide is 784 nm, and the width of the wide side of the front end face of the first core layer waveguide is 6 μm. The long side of the rear end face of the first core layer waveguide is along the left-right direction, and the wide side is along the up-down direction. The length of the long side of the rear end face of the first core layer waveguide is 980 nm, and the width of the wide side of the front end face of the first core layer waveguide is 6 μm. The first core layer waveguide has four side faces. The distance between the front end face and the rear end face of the first core layer waveguide is 1.6 μm. The upper and lower side faces of the first core layer waveguide are both trapezoids with an upper base of 784 nm, a lower base of 980 nm, and a height of 1.An isosceles trapezoid with a length of 6 μm. The left and right sides of the first core layer waveguide are both rectangular. The wide side directions of the left and right sides of the first core layer waveguide are along the up and down directions. The widths of the wide sides of the left and right sides of the first core layer waveguide are both 6 μm. The two long sides of the left side of the first core layer waveguide coincide with one corresponding waist on the upper side of the first core layer waveguide on the left and one corresponding waist on the lower side of the first core layer waveguide on the left. The two long sides of the right side of the first core layer waveguide coincide with one corresponding waist on the upper side of the first core layer waveguide on the right and one corresponding waist on the lower side of the first core layer waveguide on the right. The front end faces of m first core layer waveguides are in the same plane, and the rear end faces of m first core layer waveguides are in the same plane. The plane where the front end faces of m first core layer waveguides are located is parallel to the front end face of the lower cladding. Among m first core layer waveguides, the distance between the left-right symmetry plane of the leftmost first core layer waveguide and the left end face of the lower cladding is greater than or equal to 980 nm, and the distance between the left-right symmetry plane of the rightmost first core layer waveguide and the right end face of the lower cladding is greater than or equal to 980 nm. The distance between the left-right symmetry planes of adjacent two first core layer waveguides is 1960 nm. The second tapered grating array is formed by m identical second core layer waveguides evenly spaced in order from left to right. The value of m is 5566. The second core layer waveguide is a tapered waveguide made of silicon. The rear end face of the second core layer waveguide is rectangular. The long side of the rear end face of the second core layer waveguide is along the left-right direction, and the wide side is along the up and down direction. The length of the long side of the rear end face of the second core layer waveguide is 784 nm, and the width of the wide side of the rear end face of the second core layer waveguide is 6 μm. The long side of the front end face of the second core layer waveguide is along the left-right direction, and the wide side is along the up and down direction. The length of the long side of the front end face of the second core layer waveguide is 980 nm, and the width of the wide side of the front end face of the second core layer waveguide is 6 μm. The distance between the front end face and the rear end face of the second core layer waveguide is 1.6 μm. The second core layer waveguide has four side faces. The upper and lower side faces of the second core layer waveguide are both trapezoids with an upper base of 784 nm, a lower base of 980 nm, and a height of 1.An isosceles trapezoid with a length of 6 μm. The left and right sides of the second core layer waveguide are both rectangles. The wide side directions of the left and right sides of the second core layer waveguide are along the up and down directions. The widths of the wide sides of the left and right sides of the second core layer waveguide are both 6 μm. The two long sides of the left side of the second core layer waveguide coincide with the left waist of the upper side of the second core layer waveguide and the left waist of the lower side of the second core layer waveguide one by one. The front end faces of m second core layer waveguides are in the same plane, and the rear end faces of m second core layer waveguides are in the same plane. The plane where the rear end faces of m second core layer waveguides are located is parallel to the rear end face of the lower cladding. Among m second core layer waveguides, the distance between the left-right symmetry plane of the leftmost second core layer waveguide and the left end face of the lower cladding is greater than or equal to 980 nm, and the distance between the left-right symmetry plane of the rightmost second core layer waveguide and the right end face of the lower cladding is greater than or equal to 980 nm. The distance between the left-right symmetry planes of adjacent two second core layer waveguides is 1960 nm; the distance between the plane where the front end faces of m second core layer waveguides are located and the plane where the rear end faces of m first core layer waveguides are located is 120 nm; the distance between the plane where the front end faces of m first core layer waveguides are located and the front end face of the lower cladding is equal to the distance between the plane where the rear end faces of m second core layer waveguides are located and the rear end face of the lower cladding, and this distance is greater than or equal to 11.55 μm.

[0006] Compared with the prior art, the advantages of the present invention are that an infrared gas sensor is formed by a lower cladding, a first tapered grating array, and a second tapered grating array. The first tapered grating array and the second tapered grating array are arranged on the upper surface of the lower cladding. The first tapered grating array is located in front of the second tapered grating array. The first tapered grating array is formed by 5566 completely identical first core layer waveguides evenly spaced in order from left to right. The second tapered grating array is formed by 5566 completely identical second core layer waveguides evenly spaced in order from left to right. The first core layer waveguide and the second core layer waveguide are both tapered waveguides. The upper and lower sides of the first core layer waveguide and the second core layer waveguide are both isosceles trapezoids. While effectively increasing the light-matter interaction region at the first tapered grating array and the second tapered grating array, the energy density of the light-matter interaction is enhanced, thereby obtaining a relatively large dimensionless parameter of the light-matter interaction. And as the light-matter interaction is enhanced, correspondingly more light participates in sensing, so that the sensing sensitivity and the sensing detection threshold are further improved. Compared with the same type of sensor devices, the length of the effective optical path is further reduced, and thus a faster response time is obtained. Therefore, the present invention has a high sensitivity, a low detection threshold, and a fast response time. Description of the Drawings

[0007] Figure 1Left view of the mid-infrared gas sensor based on a tapered sub-micron grating-slot waveguide according to the present invention;

[0008] Figure 2 Top view of the mid-infrared gas sensor based on a tapered sub-micron grating-slot waveguide according to the present invention;

[0009] Figure 3 Graph showing the variation of the sensitivity of the mid-infrared gas sensor based on a tapered sub-micron grating-slot waveguide according to the present invention with the dimensionless parameter of the light-matter interaction;

[0010] Figure 4 Graph showing the variation of the detection threshold of the mid-infrared gas sensor based on a tapered sub-micron grating-slot waveguide according to the present invention with the dimensionless parameter of the light-matter interaction;

[0011] Figure 5 Response time graph of the mid-infrared gas sensor based on a tapered sub-micron grating-slot waveguide according to the present invention. Detailed implementation manners

[0012] The present invention will be further described in detail below in conjunction with the embodiments with reference to the drawings.

[0013] Embodiment: As Figure 1 and Figure 2 shown, a mid-infrared gas sensor based on a tapered sub-micron grating-slot waveguide includes a lower cladding 1, a first tapered grating array 2 and a second tapered grating array 3. The lower cladding 1 is a rectangular waveguide made of calcium fluoride. The length direction of the lower cladding 1 is along the left-right direction, the width direction is along the front-back direction, and the thickness direction is along the up-down direction. The thickness of the lower cladding 1 is greater than or equal to 6 μm. The first tapered grating array 2 and the second tapered grating array 3 are disposed on the upper surface of the lower cladding 1, and the first tapered grating array 2 is located on the front side of the second tapered grating array 3;

[0014] The first tapered grating array 2 is formed by evenly spaced distribution of m completely identical first core layer waveguides 4 in the order from left to right. The value of m is 5566. The first core layer waveguide 4 is a tapered waveguide made of silicon. The front end face of the first core layer waveguide 4 is rectangular, with the long side of the front end face along the left-right direction and the short side along the up-down direction. The length of the long side of the front end face of the first core layer waveguide 4 is 784 nm, and the width of the short side of the front end face is 6 μm. The long side of the rear end face of the first core layer waveguide 4 is along the left-right direction, and the short side is along the up-down direction. The length of the long side of the rear end face of the first core layer waveguide 4 is 980 nm, and the width of the short side of the front end face is 6 μm. The first core layer waveguide 4 has four side faces. The distance between the front end face and the rear end face of the first core layer waveguide 4 is 1.6 μm. The upper and lower side faces of the first core layer waveguide 4 are both isosceles trapezoids with the upper base of 784 nm, the lower base of 980 nm, and the height of 1.6 μm. The left and right side faces of the first core layer waveguide 4 are both rectangular. The short side direction of the left and right side faces of the first core layer waveguide 4 is along the up-down direction, and the width of the short side of the left and right side faces is 6 μm. The two long sides of the left side face of the first core layer waveguide 4 coincide with the left waist of the upper side face and the left waist of the lower side face of the first core layer waveguide 4 one by one. The two long sides of the right side face of the first core layer waveguide 4 coincide with the right waist of the upper side face and the right waist of the lower side face of the first core layer waveguide 4 one by one. The front end faces of the m first core layer waveguides 4 are in the same plane, and the rear end faces of the m first core layer waveguides 4 are in the same plane. The plane where the front end faces of the m first core layer waveguides 4 are located is parallel to the front end face of the lower cladding 1. Among the m first core layer waveguides 4, the distance L1 between the left-right symmetry plane of the leftmost first core layer waveguide 4 and the left end face of the lower cladding 1 is greater than or equal to 980 nm, and the distance L2 between the left-right symmetry plane of the rightmost first core layer waveguide 4 and the right end face of the lower cladding 1 is greater than or equal to 980 nm. The distance L3 between the left-right symmetry planes of two adjacent first core layer waveguides 4 is 1960 nm;

[0015] The second tapered grating array 3 is formed by evenly spaced distribution of m completely identical second core layer waveguides 5 in order from left to right. The value of m is 5566. The second core layer waveguide 5 is a tapered waveguide made of silicon. The rear end face of the second core layer waveguide 5 is rectangular, with the long side of the rear end face of the second core layer waveguide 5 along the left - right direction and the short side along the up - down direction. The length of the long side of the rear end face of the second core layer waveguide 5 is 784 nm, and the width of the short side of the rear end face of the second core layer waveguide 5 is 6 μm. The long side of the front end face of the second core layer waveguide 5 is along the left - right direction, and the short side is along the up - down direction. The length of the long side of the front end face of the second core layer waveguide 5 is 980 nm, and the width of the short side of the front end face of the second core layer waveguide 5 is 6 μm. The distance between the front end face and the rear end face of the second core layer waveguide 5 is 1.6 μm. The second core layer waveguide 5 has four side faces. The upper and lower side faces of the second core layer waveguide 5 are both isosceles trapezoids with the upper base of 784 nm, the lower base of 980 nm, and the height of 1.6 μm. The left and right side faces of the second core layer waveguide 5 are both rectangular. The short side direction of the left and right side faces of the second core layer waveguide 5 is along the up - down direction, and the width of the short side of the left and right side faces of the second core layer waveguide 5 is 6 μm. The two long sides of the left side face of the second core layer waveguide 5 coincide with one waist on the left side of the upper side face and one waist on the left side of the lower side face of the second core layer waveguide 5 respectively. The front end faces of the m second core layer waveguides 5 are in the same plane, and the rear end faces of the m second core layer waveguides 5 are in the same plane. The plane where the rear end faces of the m second core layer waveguides 5 are located is parallel to the rear end face of the lower cladding 1. Among the m second core layer waveguides 5, the distance L4 between the left - right symmetry plane of the second core layer waveguide 5 at the leftmost end and the left end face of the lower cladding 1 is greater than or equal to 980 nm, and the distance L5 between the left - right symmetry plane of the second core layer waveguide 5 at the rightmost end and the right end face of the lower cladding 1 is greater than or equal to 980 nm. The distance L6 between the left - right symmetry planes of adjacent two second core layer waveguides 5 is 1960 nm;

[0016] The distance L7 between the plane where the front end faces of the m second core layer waveguides 5 are located and the plane where the rear end faces of the m first core layer waveguides 4 are located is 120 nm. The distance L8 between the plane where the front end faces of the m first core layer waveguides 4 are located and the front end face of the lower cladding 1 is equal to the distance L9 between the plane where the rear end faces of the m second core layer waveguides 5 are located and the rear end face of the lower cladding 1, and this distance is greater than or equal to 11.55 μm.

[0017] A plane that is parallel to the left - right symmetry plane of the left - most first - core - layer waveguide in the first tapered grating array and is located to the left of the left - right symmetry plane of the left - most first - core - layer waveguide in the first tapered grating array, with a distance of 980 nm from the left - right symmetry plane of the left - most first - core - layer waveguide in the first tapered grating array is called the first plane. A plane that is parallel to the left - right symmetry plane of the right - most first - core - layer waveguide in the first tapered grating array and is located to the right of the left - right symmetry plane of the right - most first - core - layer waveguide in the first tapered grating array, with a distance of 980 nm from the left - right symmetry plane of the right - most first - core - layer waveguide in the first tapered grating array is called the second plane. The distance between the first plane and the second plane is defined as the sensing effective optical path length of the mid - infrared gas sensor of the present invention. According to the Beer - Lambert law, when the wavelength of the light source covers the characteristic absorption spectral line of the trace gas to be measured, the output light intensity shows a decreasing trend. By the selective absorption characteristics of different molecular substances, the composition and concentration of the substance can be determined. Among them, the attenuation of the light intensity at the output port can be expressed as:

[0018] I out =I in exp(-ηε g C g L0 - α int L0) (1)

[0019] Wherein, I out and I in are the output light intensity and the input light intensity respectively, ε g is the absorption coefficient of the target gas, C g is the concentration of the target gas, L0 represents the sensing effective optical path of the mid - infrared gas sensor, α int represents the intrinsic loss of the waveguide in the mid - infrared gas sensor, including absorption loss and scattering loss, and η is a dimensionless parameter representing the degree of light - matter interaction in the evanescent field. In a spectroscopic absorption - type gas sensor, the larger the dimensionless parameter of light - matter interaction, correspondingly, the more light participates in the sensing process, and the better the sensing performance.

[0020] In the conical sub-micron grating-slot waveguide mid-infrared gas sensor of the present invention, first, compared with traditional strip waveguides, ridge waveguides and other structures in the design of the first conical grating array and the second conical grating array, there are relatively large gaps between adjacent first core layer waveguides and between adjacent second core layer waveguides. Therefore, gas can be filled in the gaps, increasing the interaction region between light and matter, and thus enhancing the light-matter interaction. Secondly, for the gap structure formed by m first core layer waveguides and m second core layer waveguides, since its dielectric interface does not satisfy the condition of electric displacement continuity, the electric field is more strongly restricted in the formed low-refractive-index gap region. Moreover, the upper and lower sides of the first core layer waveguide and the second core layer waveguide are both designed as isosceles trapezoids, which causes the guided mode to shift towards the rear end face of the first core layer waveguide and the front end face of the second core layer during propagation, and then concentrates in the slit region formed between the m first core layer waveguides in the first conical grating array and the m second core layer waveguides in the second conical grating array, enhancing the light-matter interaction. Finally, both the first conical grating array and the second conical grating array are one-dimensional photonic crystal structures. The slow light effect generated by structural dispersion near the Brillouin zone boundary greatly slows down the light speed, allowing photons to fully interact with matter and achieve energy density gain, thereby enhancing the light-matter interaction. The conical sub-micron grating-slot waveguide mid-infrared gas sensor of the present invention can make the dimensionless parameter of the light-matter interaction reach 6.1516 at a wavelength of 7.7 μm.

[0021] To verify the superiority of the conical sub-micron grating-slot waveguide mid-infrared gas sensor of the present invention, a differential detection structure is used here to test its sensing performance. Its composition includes a mid-infrared light source, an optical fiber, an optical gas cell and a photodetector. The light source uses a quantum cascade laser (Alpes Lasers) with an output power of 1 mW (7.7 μm, 1290 cm -1 ). The working spectral range of the photodetector (Horiba, DSS-MCT14-020L) is 3 μm - 12 μm, and the equivalent noise power and bandwidth are 5×10 -12 WHz -1 / 2 and 5 KHz respectively. The single-wavelength double-path differential detection structure can divide the input light of the laser source into two beams with a fixed energy ratio through a beam splitter (power splitter). One path passes through the optical gas cell filled with the gas to be measured for full reaction and is used as the signal optical path, and the other path passes through the reference gas cell filled with a balance gas (usually nitrogen) and then is connected to the photodetector as the reference optical path. The detection error caused by the light source fluctuation and the environmental noise existing in the light propagation process can be effectively eliminated through the reference optical path. Methane is used as the target gas for the test, and its binary diffusion coefficient in air is 0.208 cm 2 / s, and its absorption coefficient ε g, from the HITRAN spectroscopic database, with a value of 174 L·mol -1 ·cm -1 , the initial gas concentration C g is 50 ppm, the intrinsic loss α of the waveguide int is 4.0 dB / cm, and the ambient temperature and pressure are 298.15 K and 100 kPa respectively. Among them, the curve of the change in the dimensionless parameter of the light-matter interaction of the sensitivity of the mid-infrared gas sensor based on the tapered sub-micron grating-groove waveguide of the present invention is as shown in Figure 3 ; the curve of the detection threshold of the mid-infrared gas sensor based on the tapered sub-micron grating-groove waveguide of the present invention with respect to the change in the dimensionless parameter of the light-matter interaction is as shown in Figure 4 ; the curve of the response time of the mid-infrared gas sensor based on the tapered sub-micron grating-groove waveguide of the present invention is as shown in Figure 5 .

[0022] Sensitivity is defined as the change in the normalized output light intensity caused by the change in gas concentration. The greater the degree of light-matter interaction, the higher the corresponding sensing sensitivity. Analysis Figure 3It can be seen that the value of the sensitivity S of the mid-infrared gas sensor of the present invention gradually increases with the increase of the dimensionless parameter η of the light-matter interaction. At a wavelength of 7.7 μm, the dimensionless parameter of the light-matter interaction of the mid-infrared gas sensor of the present invention reaches 6.1516, and the sensing sensitivity is 19.0341. Compared with the same type of spectral absorption waveguide sensor for methane gas detection, the sensing sensitivity of the present invention is significantly improved compared with the sensing sensitivity value of 4.89 of the device disclosed in Document 1 (Pi, M. et al. Design of a mid-infrared suspended chalcogenide / silica-on-silicon slot-waveguide spectroscopic gas sensor with enhanced light-gas interaction effect. Sensors Actuators, B Chem. 297, 126732 (2019).) and the sensing sensitivity value of 7.151 of the device disclosed in Document 2 (Wang, Y. et al. Ultra-high-power-confinement-factor integrated mid-infrared gas sensor based on the suspended slot chalcogenide glass waveguide. Sensors Actuators B Chem. 347, (2021).).

[0023] The greater the degree of light-matter interaction, the more light will contact and react with the gas accordingly, and the smaller the sensing detection threshold. Analysis Figure 4 It can be seen that the detection threshold C of the mid-infrared gas sensor of the present invention minThe value gradually decreases as the dimensionless parameter η of the light-matter interaction increases. At a wavelength of 7.7 μm, the dimensionless parameter of the light-matter interaction of the mid-infrared gas sensor of the present invention reaches 6.1516, and the detection threshold is 0.18524 ppm. Compared with the same type of spectral absorption waveguide sensor for methane gas detection, the sensing detection threshold of the mid-infrared gas sensor of the present invention is lower than the detection thresholds of the devices disclosed in Document 3 (Gervais, A., Jean, P., Shi, W. & LaRochelle, S. Design of slow-light subwavelength grating waveguides for enhanced on-chip methane sensing by absorption spectroscopy. IEEE J. Sel. Top. Quantum Electron. 25, (2019).) which is 1.42 ppm and the detection threshold of the device disclosed in Document 4 (Xu, G. et al. Design and analysis of slow-light Bloch slot waveguides for on-chip gas sensing. J. Opt. Soc. Am. B 37, 257 (2020).) which is 5.88 ppm.

[0024] Analysis Figure 5It can be known that the effective optical path length of the mid-infrared gas sensor of the present invention is 1.090936 cm. When t > 4.0 s, the diffusion of the gas in the gas chamber is 99.9% completed and tends to be stable. Therefore, for methane gas, the response time required for sensing is 4.0 s. Compared with the same type of spectral absorption waveguide sensor for methane gas detection, the response time of the present invention is faster than the response time of 10 s of the device disclosed in Document 5 (Kumari, B., Varshney, R.K. & Pal, B.P. Design of chipscale silicon rib slot waveguide for sub-ppm detection of N2O gas at mid-IR band. Sensors Actuators, B Chem. 255, 3409–3416 (2018).) and the response time of 9 s of the device disclosed in Document 2 (Wang, Y. et al. Ultra-high-power-confinement-factor integrated mid-infrared gas sensor based on the suspended slot chalcogenide glass waveguide. Sensors Actuators B Chem. 347, (2021).).

[0025] In summary, in the tapered sub-micron grating-slot waveguide mid-infrared gas sensor of the present invention, the design of the first tapered grating array and the second tapered grating array increases the interaction region between light and matter. The upper and lower sides of the first core layer waveguide and the second core layer waveguide are both designed as isosceles trapezoids, making the light field more concentrated at the gap structure formed by the m first core layer waveguides and the m second core layer waveguides. The scheme that both the first tapered grating array and the second tapered grating array are one-dimensional photonic crystal structures effectively increases the energy density between light and matter, thereby increasing the light-matter interaction, and making the dimensionless parameter of the light-matter interaction reach 6.1516 at a wavelength of 7.7 μm. The differential detection structure eliminates the detection error caused by the light source fluctuation and the environmental noise existing in the light propagation process, and tests the sensing performance. The sensing sensitivity of the tapered sub-micron grating-slot waveguide mid-infrared gas sensor is 19.0341, the sensing detection threshold is 0.18524 ppm, and the response time is 4.0 s. Compared with the same type of spectral absorption waveguide sensor, it has higher sensitivity, lower detection threshold, and faster response time.

Claims

1. A mid-infrared gas sensor based on a conical sub-micron grating-groove waveguide, characterized in that It includes a lower cladding, a first tapered grating array, and a second tapered grating array. The lower cladding is a cuboid waveguide made of calcium fluoride. The length direction of the lower cladding is along the left-right direction, the width direction is along the front-back direction, and the thickness direction is along the up-down direction. The thickness of the lower cladding is greater than or equal to 6 µm. The first tapered grating array and the second tapered grating array are arranged on the upper surface of the lower cladding, and the first tapered grating array is located on the front side of the second tapered grating array; The first tapered grating array is formed by evenly spacing and distributing m completely identical first core layer waveguides in order from left to right. The value of m is 5566. The first core layer waveguide is a tapered waveguide made of silicon. The front end face of the first core layer waveguide is rectangular. The long side of the front end face of the first core layer waveguide is along the left-right direction, and the wide side is along the up-down direction. The length of the long side of the front end face of the first core layer waveguide is 784 nm, and the width of the wide side of the front end face of the first core layer waveguide is 6 µm. The long side of the rear end face of the first core layer waveguide is along the left-right direction, and the wide side is along the up-down direction. The length of the long side of the rear end face of the first core layer waveguide is 980 nm, and the width of the wide side of the rear end face of the first core layer waveguide is 6 µm. The first core layer waveguide has four side faces. The distance between the front end face and the rear end face of the first core layer waveguide is 1.6 µm. The upper and lower side faces of the first core layer waveguide are both isosceles trapezoids with an upper base of 784 nm, a lower base of 980 nm, and a height of 1.6 µm. The left and right side faces of the first core layer waveguide are both rectangular. The wide side direction of the left and right side faces of the first core layer waveguide is along the up-down direction, and the width of the wide side of the left and right side faces of the first core layer waveguide is both 6 µm. The two long sides of the left side face of the first core layer waveguide coincide with one corresponding waist of the upper side face of the first core layer waveguide on the left and one corresponding waist of the lower side face of the first core layer waveguide on the left. The two long sides of the right side face of the first core layer waveguide coincide with one corresponding waist of the upper side face of the first core layer waveguide on the right and one corresponding waist of the lower side face of the first core layer waveguide on the right. The front end faces of the m first core layer waveguides are in the same plane, the rear end faces of the m first core layer waveguides are in the same plane, and the plane where the front end faces of the m first core layer waveguides are located is parallel to the front end face of the lower cladding. Among the m first core layer waveguides, the distance between the left-right symmetry plane of the first core layer waveguide at the leftmost end and the left end face of the lower cladding is greater than or equal to 980 nm, and the distance between the left-right symmetry plane of the first core layer waveguide at the rightmost end and the right end face of the lower cladding is greater than or equal to 980 nm. The distance between the left-right symmetry planes of adjacent two first core layer waveguides is 1960 nm; The described second tapered grating array is formed by evenly spaced distribution of m identical second core layer waveguides in the order from left to right. The value of m is 5566. The second core layer waveguide is a tapered waveguide made of silicon. The rear end face of the second core layer waveguide is rectangular. The long side of the rear end face of the second core layer waveguide extends in the left-right direction, and the short side extends in the up-down direction. The length of the long side of the rear end face of the second core layer waveguide is 784 nm, and the width of the short side of the rear end face of the second core layer waveguide is 6 µm. The long side of the front end face of the second core layer waveguide extends in the left-right direction, and the short side extends in the up-down direction. The length of the long side of the front end face of the second core layer waveguide is 980 nm, and the width of the short side of the front end face of the second core layer waveguide is 6 µm. The distance between the front end face and the rear end face of the second core layer waveguide is 1.6 µm. The second core layer waveguide has four side faces. The upper and lower side faces of the second core layer waveguide are both isosceles trapezoids with an upper base of 784 nm, a lower base of 980 nm, and a height of 1.6 µm. The left and right side faces of the second core layer waveguide are both rectangular. The short side direction of the left and right side faces of the second core layer waveguide extends in the up-down direction. The width of the short side of the left and right side faces of the second core layer waveguide is both 6 µm. The two long sides of the left side face of the second core layer waveguide coincide with one corresponding waist on the upper side face on the left and one corresponding waist on the lower side face on the left of the second core layer waveguide. The front end faces of the m second core layer waveguides are in the same plane, and the rear end faces of the m second core layer waveguides are in the same plane. The plane where the rear end faces of the m second core layer waveguides are located is parallel to the rear end face of the lower cladding. Among the m second core layer waveguides, the distance between the left-right symmetry plane of the second core layer waveguide at the leftmost end and the left end face of the lower cladding is greater than or equal to 980 nm, and the distance between the left-right symmetry plane of the second core layer waveguide at the rightmost end and the right end face of the lower cladding is greater than or equal to 980 nm. The distance between the left-right symmetry planes of adjacent two second core layer waveguides is 1960 nm; The distance between the plane where the front end faces of the m second core layer waveguides are located and the plane where the rear end faces of the m first core layer waveguides are located is 120 nm; The distance between the plane where the front end faces of the m first core layer waveguides are located and the front end face of the lower cladding is equal to the distance between the plane where the rear end faces of the m second core layer waveguides are located and the rear end face of the lower cladding, and the distance between the plane where the front end faces of the m first core layer waveguides are located and the front end face of the lower cladding is greater than or equal to 11.55 µm.

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