On-chip infrared gas detection system and method based on wavelength conversion

Abstract

The invention relates to the technical field of infrared gas detection, in particular to an on-chip infrared gas detection system and method based on wavelength conversion. According to the technology, wavelength conversion is achieved based on the chalcogenide waveguide nonlinear optical four-wave mixing effect, then infrared gas detection is achieved, a middle-infrared band optical signal corresponding to target gas characteristic absorption is converted into a near-infrared band optical signal through an FWM, a mature near-infrared detector is matched, high-sensitivity detection of target gas is achieved, and high-sensitivity detection of the target gas is achieved. Therefore, the technical bottlenecks of large size, high cost and low-temperature refrigeration of intermediate infrared detection hardware are broken through; in combination with a low-power CW pumping source, the power consumption of the whole machine and the weight of equipment can be effectively reduced, and the requirements of outdoor, mine and other portable scenes are met; the chalcogenide waveguide is convenient for on-chip integration, and the whole device is high in integration level; according to the system, the'pumping-signal light 'wavelength combination is optimized for each gas to be detected, so that the multi-component gas can be flexibly detected.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of infrared gas detection, and more particularly to an on-chip infrared gas detection system and method based on wavelength conversion. BACKGROUND

[0002] The infrared spectrum detection technology of gas is based on the Lambert-Beer law, and the qualitative and quantitative analysis is achieved by identifying the characteristic absorption peaks of gas molecules in a specific infrared band. The mid-infrared band (2-20 μm) is the "fingerprint absorption region" of most gas molecules, and the absorption peaks in this band have the characteristics of narrow line width, strong selectivity and high absorption coefficient, which can effectively distinguish multi-component gases and is an ideal band for high-precision gas detection. Based on this characteristic, the current detection technology of mid-infrared gas is mainly realized by the following two methods: (1) Direct mid-infrared detection technology: directly using a mid-infrared detector (such as indium antimonide (InSb), mercury cadmium telluride (HgCdTe), quantum well infrared detector (QWIP), etc.) to receive the mid-infrared light signal after gas absorption, and combining a spectrometer (such as a Fourier transform infrared spectrometer, a grating spectrometer) to realize wavelength resolution. The advantage of this technology is that it does not require wavelength conversion and can directly utilize the strong absorption characteristics of mid-infrared, thus having high theoretical detection sensitivity. It has been preliminarily applied in industrial leakage monitoring, environmental air quality detection, petroleum and chemical process control, etc. (2) Indirect wavelength conversion detection technology: Considering the technical limitations of mid-infrared detectors, some schemes attempt to convert the mid-infrared band light signal into a more easily detected band (such as near-infrared, visible light) through nonlinear optical effects, and then use mature near-infrared / visible light detectors (such as silicon (Si) detectors, indium gallium arsenide (InGaAs) detectors) to realize detection. The existing wavelength conversion technologies mainly include: ① Raman scattering conversion: using the Raman scattering effect of gas or solid medium to convert mid-infrared light into near-infrared light with wavelength shift, but the conversion efficiency is very low (usually <0.1%); ② Optical parametric oscillator (OPO): high-power pulsed pumping utilizes the second-order nonlinear effect of a nonlinear crystal to generate signal light and idler light, thereby converting the energy of the pump light into mid-infrared signal light. The reverse conversion from mid-infrared to near-infrared can also be realized. This conversion usually requires a resonant cavity to enhance the signal, and oscillation output occurs when the pump power exceeds the threshold value. ③ Difference frequency effect (DFG): using the second-order nonlinear effect of a nonlinear medium to generate difference frequency with two pump lights of different frequencies, which can generate mid-infrared light in the forward direction, or convert mid-infrared signal to near-infrared light in the reverse direction. It is an important research direction for mid-infrared integrated conversion in recent years, and the latest technical progress focuses on the periodic thin film lithium niobate (TFLN) platform. However, a high temperature environment of 150-190°C is required to suppress water adsorption loss, which additionally increases the power consumption of the temperature control module (>5W) and cannot adapt to room temperature portable scenarios.

[0003] Although the existing mid-infrared gas detection technology has the advantage of high selectivity in theory, in practical application, especially for the demand of portability and low-cost detection, there are still the following difficult-to-overcome technical bottlenecks. For direct mid-infrared detection technology, the performance of the existing mid-infrared detector is severely dependent on the working environment: in order to suppress the dark current and improve the signal-to-noise ratio, such detector needs to operate under the condition of liquid nitrogen low temperature (77K) or deep low temperature (<40K), and must be matched with a complex refrigeration system (such as a dewar flask, a micro-refrigerator), which leads to a large volume of detection equipment (usually the weight is more than 5kg), high power consumption, high cost (the cost of a single equipment is more than 100,000 yuan), and the existence of the refrigeration system limits the long-term stability of the equipment (such as the dewar flask needs to be regularly supplemented with liquid nitrogen, and the service life of the micro-refrigerator is only 5000-10000 hours). For the existing indirect wavelength conversion detection technology, there are also some key problems: due to the small Raman scattering cross section (10 -30 cm 2 order of magnitude), effective Raman scattering conversion usually needs to rely on high-power pump light source (such as kilowatt laser), which not only has high energy consumption, but also is easy to cause thermal damage to the medium, and the scattering direction is random, which cannot constrain the light path on the chip, and the integration is poor; the optical parametric oscillation is usually realized in a bulk nonlinear crystal, which cannot be integrated on the chip due to the large volume of the crystal. The difference frequency based on the periodic thin film lithium niobate needs strict phase matching, which requires precise polarization period and complex dispersion design, greatly increasing the process difficulty. In addition, the existing wavelength conversion technology cannot cover the absorption peaks of multiple components at the same time, so as to realize the detection of multiple gases. SUMMARY

[0004] The purpose of the present application is to overcome the shortcomings of the prior art infrared detection equipment, such as high energy consumption, poor integration and inability to detect multiple gases, and to provide a wavelength conversion-based on-chip infrared gas detection system and method, which effectively reduces the energy consumption, has high integration and can flexibly detect multiple component gases.

[0005] To solve the above technical problems, the technical scheme adopted by the present application is: The present application provides a wavelength conversion-based on-chip infrared gas detection system, which comprises: A mid-infrared light source module for emitting mid-infrared signal light at the wavelength of the absorption peak of the gas to be detected; A gas absorption module comprising a gas chamber with an internal cavity, wherein the mid-infrared signal light and the gas to be detected are introduced into the gas chamber, so that the gas to be detected can fully absorb the mid-infrared signal light; A FWM wavelength conversion module for converting the mid-infrared signal light absorbed by the gas to be detected into near-infrared idler light; A near-infrared detection module for receiving the near-infrared idler light, converting the optical signal into an electrical signal and calculating the gas concentration; The FWM wavelength conversion module comprises: A tunable pump source is used to generate TE polarized pump light with a preset output wavelength and power. A first polarization controller is connected to the tunable pump source and used to adjust the polarization state of the pump light. A second polarization controller is connected to the output end of the gas absorption module and used to adjust the polarization state of the mid-infrared signal light absorbed by the target gas. A coupler is connected to the output ends of the first polarization controller and the second polarization controller, and the output end of the coupler is connected to the chalcogenide waveguide. The chalcogenide waveguide is used to receive the combined pump light and mid-infrared signal light and realize wavelength conversion based on the nonlinear optical four-wave mixing effect.

[0006] The wavelength conversion-based on-chip infrared gas detection system of the application realizes wavelength conversion and infrared gas detection based on the nonlinear optical four-wave mixing effect (FWM) of the chalcogenide waveguide. The core is to convert the mid-infrared waveband light signal corresponding to the target gas feature absorption into a near-infrared waveband light signal through FWM, adapt a mature near-infrared detector, and realize high-sensitivity detection of the target gas, thereby breaking through the technical bottleneck of large hardware volume, high cost, and the need for low-temperature refrigeration in mid-infrared detection. In the application, the FWM effect is used to convert the mid-infrared signal into a near-infrared signal, which is directly adapted to a near-infrared detection module operating at room temperature without any refrigeration module. Combined with a low-power CW pump source, the power consumption and weight of the entire device can be effectively reduced to meet the needs of portable scenarios such as outdoor and mine.

[0007] Further, by adjusting the dispersion of the chalcogenide waveguide, the dispersion can reach a preset target dispersion, and the target dispersion can meet the conditions required for phase matching of the nonlinear optical four-wave mixing effect to realize cross-band wavelength conversion.

[0008] Further, the dispersion of the chalcogenide waveguide is adjusted by adjusting the size of the chalcogenide waveguide.

[0009] Further, the chalcogenide waveguide comprises, from bottom to top, a substrate, a lower cladding layer, a GeSbS waveguide core layer, and an upper cladding layer, and the refractive indices of the upper cladding layer and the lower cladding layer are less than that of the GeSbS waveguide core layer.

[0010] Further, the material of the GeSbS waveguide core layer comprises Ge 25 Sb 10 S 65 .

[0011] Further, the gas absorption module further comprises a temperature control unit for controlling the temperature in the gas chamber.

[0012] Further, after the mid-infrared light enters the gas chamber, the mid-infrared light and the gas to be detected are subjected to characteristic absorption according to the Lambert-Beer law.

[0013] Further, the near-infrared detection module comprises a photodiode and a signal processing unit, the signal processing unit comprises an amplifier, a data acquisition card and a concentration calculation unit, the photodiode converts the received near-infrared idler light into an electric signal, then the electric signal is amplified by the amplifier to output an analog signal, the data acquisition card converts the analog signal into a digital signal and transmits the digital signal to the concentration calculation unit, and the concentration calculation unit calculates the concentration of the gas to be detected according to a pre-stored "concentration-light intensity" calibration curve and the Lambert-Beer law.

[0014] The application also provides an on-chip infrared gas detection method based on wavelength conversion, which adopts the above-mentioned on-chip infrared gas detection system based on wavelength conversion and comprises the following steps: System initialization: the wavelength of the output mid-infrared light of the mid-infrared light source module is set to the wavelength of the gas to be detected, and the wavelength of the pump source is adjusted to a preset value; System calibration: a plurality of groups of standard concentrations of the gas to be detected are sequentially introduced into the clean gas chamber, the near-infrared idler light corresponding to each group of standard concentrations is collected by the near-infrared detection module, and the "concentration-light intensity" data is substituted into the Lambert-Beer law to obtain a calibration curve, which is stored in the near-infrared detection module; Gas detection: the residual gas in the gas chamber is emptied, the gas to be detected is introduced into the gas chamber, and the current idler light is collected by the near-infrared detection module, and the concentration of the gas to be detected is calculated by substituting the calibration curve.

[0015] Further, by synchronously adjusting the wavelength of the pump source and the wavelength of the mid-infrared signal light, the concentration of different gases can be detected.

[0016] Compared with the prior art, the application has the following beneficial effects: The on-chip infrared gas detection system and method based on wavelength conversion of the application realize high-sensitivity detection of target gas through the innovative architecture of "mid-infrared absorption + sulfide waveguide FWM wavelength conversion + room-temperature near-infrared detection", thereby breaking through the technical bottleneck of large hardware volume, high cost and the need for low-temperature refrigeration in mid-infrared detection; the application does not need low-temperature refrigeration, greatly reduces the equipment volume and power consumption, has high on-chip integration and low process complexity; and the application can flexibly detect multi-component gas and has balanced detection sensitivity. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 Structure diagram of the wavelength conversion based on-chip infrared gas detection system of the present application; Figure 2 Structure diagram of the FWM wavelength conversion module of the present application; Figure 3 Structure diagram of the chalcogenide waveguide of the present application; Figure 4 Preparation flowchart of the chalcogenide waveguide of the present application; Figure 5 CE thermodynamic diagram of the pump wavelength-signal wavelength-conversion efficiency in the second embodiment of the present application; Figure 6 Conversion efficiency spectrum under different pump wavelengths in the third embodiment of the present application.

[0018] In the drawings: 100, mid-infrared light source module; 200, gas absorption module; 300, FWM wavelength conversion module; 310, pump source; 320, first polarization controller; 330, second polarization controller; 340, coupler; 350, chalcogenide waveguide; 351, substrate; 352, lower cladding layer; 353, core layer; 354, upper cladding layer; 400, near-infrared detection module. DETAILED DESCRIPTION

[0019] The present application will be further described below in conjunction with specific embodiments. Among them, the drawings are only used for illustrative description, and the representation is only a schematic diagram, and cannot be understood as a limitation on the present application; in order to better illustrate the embodiments of the present application, some components in the drawings will be omitted, enlarged or reduced, and do not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings can be omitted.

[0020] The same or similar reference numerals in the drawings of the embodiments of the present application correspond to the same or similar components; in the description of the present application, it is understood that if the orientation or positional relationship indicated by the terms "upper", "lower", "left", "right" and the like is based on the orientation or positional relationship shown in the drawings, only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore the positional relationship described in the drawings is only used for illustrative description, and cannot be understood as a limitation on the present application, for those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0021] Embodiment one The present embodiment is a first embodiment of a wavelength conversion based on-chip infrared gas detection system, as shown in Figure 1 and Figure 2As shown, it comprises the middle infrared light source module 100, the gas absorption module 200, the FWM wavelength conversion module 300, and the near-infrared detection module 400 connected in sequence; each module is connected in series according to the "signal flow direction", and the specific path is as shown in Figure 1 As shown: the middle infrared light source module 100→the gas absorption module 200→the FWM wavelength conversion module 300→the near-infrared detection module 400, wherein the FWM wavelength conversion module 300 is the core, and needs to access the middle infrared signal light (from the gas absorption module 200) and the pump light (independent pump source 310) at the same time, generates the near-infrared idler light through the nonlinear effect, and finally completes the signal acquisition and concentration calculation by the near-infrared detection module 400. In this embodiment, the structure and function of each module are as follows: (1) The middle infrared light source module 100: used for emitting the middle infrared signal light of the absorption peak wavelength of the gas to be detected; a tunable quantum cascade laser (QCL) can be selected.

[0022] (2) The gas absorption module 200: comprising a gas chamber with a cavity inside and a temperature control unit, the middle infrared signal light and the gas to be detected are introduced into the gas chamber, so that the gas to be detected can fully absorb the middle infrared signal light. In this module, the middle infrared signal light and the gas to be detected are fully utilized to extract concentration information: after the middle infrared light enters the gas chamber, it is absorbed according to the Lambert-Beer law with the gas to be detected, and the change of the light intensity after absorption directly reflects the gas concentration information; at the same time, the temperature control unit is used to maintain the temperature of the gas chamber, so as to avoid the temperature fluctuation from causing the absorption peak of the gas to shift, and ensure the accuracy of the concentration detection.

[0023] (3) The FWM wavelength conversion module 300: used for converting the middle infrared signal light absorbed by the gas to be detected into near-infrared idler light. In this embodiment, as shown in Figure 2 The FWM wavelength conversion module 300 comprises: The tunable pump source 310: used for generating TE polarized pump light with a preset output wavelength and power; in this embodiment, a tunable 2um band CW thulium fiber pump source 310 is selected; The first polarization controller 320: connected with the tunable pump source 310, used for adjusting the polarization state of the pump light; The second polarization controller 330: connected with the output end of the gas absorption module 200, used for adjusting the polarization state of the middle infrared signal light absorbed by the gas to be detected; The coupler 340: the input end of the coupler 340 is connected with the output end of the first polarization controller 320 and the second polarization controller 330 respectively, and the output end of the coupler 340 is connected with the GeSbS chalcogenide waveguide 350; The chalcogenide waveguide 350 is used for receiving the combined pump light and mid-infrared signal light, and wavelength conversion is realized based on the nonlinear optical four-wave mixing effect. Figure 3 As shown in FIG. 5, the chalcogenide waveguide 350 includes, from bottom to top, a substrate 351, a lower cladding layer 352, a GeSbS waveguide core layer 353, and an upper cladding layer 354. The refractive index of the upper cladding layer 354 and the lower cladding layer 352 is less than that of the GeSbS waveguide core layer 353. The material of the GeSbS waveguide core layer 353 is a chalcogenide glass of GeSbS. 25 Sb 10 S 65 .

[0024] In this embodiment, the propagation loss of the GeSbS chalcogenide waveguide 350 is less than 0.5 dB / cm. The waveguide dispersion is adjusted by adjusting the width and thickness of the chalcogenide waveguide 350, so as to satisfy the FWM phase matching condition and realize effective conversion of the cross-band wavelength. The tunable 2-um-band CW thulium fiber pump source 310 is used to generate TE polarized pump light with a certain output power. The first polarization controller 320 is used to adjust the polarization state of the pump light, so as to ensure that the polarization state of the pump light is consistent with that of the mid-infrared signal light. The coupler 340 is used to combine the pump light and the signal light, and then realize low-loss connection between the input / output optical fiber and the chalcogenide waveguide 350 through the inverted tapered waveguide.

[0025] (4) The near-infrared detection module 400: the core function is to receive the near-infrared idler light, convert the optical signal into an electrical signal, and calculate the gas concentration. The components include a commercial InGaAs avalanche photodiode (APD) with a response range of 1.0-1.6 um, and a signal processing unit mainly including a low-noise amplifier (LNA), a data acquisition card (DAQ), and a concentration calculation unit (embedded chip with built-in Lambert-Beer law fitting algorithm for real-time calculation of gas concentration). After the InGaAs avalanche photodiode converts the received near-infrared idler light into a weak electrical signal (current ≈ nA level), the electrical signal is connected to the low-noise amplifier through a shielded wire for amplification (to avoid noise interference), and an analog signal with an output amplitude of Mv level is output. Then the data acquisition card converts the analog signal into a digital signal, which is transmitted to the concentration calculation unit. The concentration calculation unit calls the pre-stored “concentration-light intensity” calibration curve (obtained by standard gas calibration), and calculates the concentration of the gas to be detected according to the Lambert-Beer law. Finally, the concentration value is displayed in real time on the display screen.

[0026] Specifically, the essence of the FWM effect is that the pump light, the mid-infrared signal light, and the near-infrared idler light exchange energy in the third-order nonlinear medium, and in this process, the energy conservation ( , i.e. where ωp, ωs, ωi and λp, λs, λi represent the angular frequencies and wavelengths of the pump light, mid-infrared signal light and near-infrared idler light, respectively, and phase matching =0) two core conditions, where, represents the total phase mismatch parameter, 2γPp represents the nonlinear phase mismatch from the phase modulation and cross-phase modulation, represents the linear phase mismatch, where and are the second-order dispersion and fourth-order dispersion of the propagation constant β with respect to the angular frequency ω at the pump frequency ωp. Since 2γPp is always greater than 0, to achieve phase matching, Δβ must be negative, which has the following three cases: (1) <0, <0; (2) >0, <0; (3) <0, >0; the latter two cases can be unified as <0; for the (1) case, there is only one phase matching point, and only narrowband wavelength conversion near the phase matching frequency can be achieved; and when <0, the compensation effect of the high-order dispersion ( ) on the low-order dispersion ( ) can offset the rapid growth of the linear phase mismatch with the wavelength detuning (ωs-ωp), so that the phase matching condition is established in a wider wavelength range, breaking the limitation of a single phase matching point, and thus realizing long-distance wavelength conversion. Therefore, in the embodiment, the core of the FWM wavelength conversion module 300 is to realize the target dispersion by adjusting the waveguide structure, so as to realize effective long-distance wavelength conversion from the mid-infrared band to the near-infrared band.

[0027] The working process of the system provided in the embodiment includes: (1) System initialization: start the power supply of each module, adjust the output wavelength of the tunable QCL to the target gas characteristic absorption peak (such as methane 3270 nm) and the wavelength of the pump source 310, and the room temperature control unit stabilizes the temperature of the quartz gas chamber to 25°C; adjust the polarization state of the pump light using the polarization controller, so that the polarization overlap degree with the mid-infrared signal light is >90%; (2) Sampling and detection of the gas to be detected: after emptying the gas chamber, the sampling pump injects the gas to be detected into the gas chamber, and stays for 6 seconds to ensure uniform gas diffusion; the near-infrared detection module 400 collects the current idler light intensity, and calculates the gas concentration according to the "Lambert-Beer law"; (3) Result output and system reset: the concentration calculation unit display screen displays the detection concentration (such as "methane concentration: XX ppm"); (4) The air chamber is emptied through the exhaust port, ready for the next detection, and then the above steps are repeated.

[0028] In this embodiment, a preparation method is provided for the GeSbS chalcogenide waveguide 350, as shown in Figure 3 and Figure 4 , comprising the following steps: Step 1: Clean the substrate 351, and then deposit the lower cladding layer 352 on the substrate 351; Step 2: Grow the GeSbS chalcogenide thin film by thermal evaporation method; Step 3: Spin-coat positive photoresist 400 nm, after exposure and development, the pattern is transferred to the photoresist layer, and then the pattern is transferred from the photoresist layer to the GeSbS chalcogenide thin film layer by reactive ion etching, to obtain the waveguide core layer 353; Step 4: Perform oxygen plasma treatment on the etched GeSbS chalcogenide thin film layer to remove the residual photoresist after etching; Step 5: Deposit the upper cladding layer 354 on the waveguide surface that has been subjected to oxygen plasma treatment, and perform polishing treatment, to finally obtain the mid-infrared cross-waveband wavelength conversion chip, i.e. the chalcogenide waveguide 350.

[0029] This embodiment solves the core bottleneck of the existing mid-infrared gas detection technology in volume, power consumption, cost and integration through the innovative architecture of "mid-infrared absorption + chalcogenide waveguide 350 FWM wavelength conversion + room temperature near-infrared detection", and the specific advantages are as follows: ① No need for cryogenic refrigeration, greatly reducing equipment volume and power consumption: the existing direct mid-infrared detection technology relies on liquid nitrogen or deep cryogenic refrigeration system, resulting in equipment weight exceeding 5 kg, high power consumption and the need for regular maintenance; while the present application converts the mid-infrared signal into a near-infrared signal through FWM effect, directly adapting to the InGaAs avalanche photodiode working at room temperature, without any refrigeration module. Combined with a low-power CW pump source 310, the overall power consumption can be controlled to be <10W, and the equipment weight can be reduced to <1kg, meeting the needs of portable scenarios such as outdoor and mine.

[0030] ② High on-chip integration and low process complexity: the existing optical parametric oscillation technology relies on bulk nonlinear crystals, which cannot be integrated with other optical modules for miniaturization, while the periodic thin film lithium niobate TFLN difference frequency technology requires high-temperature control and high-precision polarization process, resulting in poor compatibility between modules. The scheme of this embodiment uses thermal evaporation + electron beam lithography to prepare a GeSbS rectangular waveguide, which has simple preparation process, no need for high-temperature control or polarization treatment, is compatible with CMOS process, suitable for mass production, and has small chip footprint and high integration.

[0031] ③Flexible detection of multi-component gas, and balanced detection sensitivity: In the case of fixed waveguide parameters, the wavelength combination of "pump-signal light" is optimized for each gas in this embodiment, and the conversion efficiency is stable at -30 to -40 dB, thereby realizing balanced detection of different gases.

[0032] Embodiment Two This embodiment is a first embodiment of a wavelength conversion-based on-chip infrared gas detection method. This embodiment uses the detection system provided in Embodiment One. In this embodiment, methane gas detection is taken as an example, and the specific steps include the following.

[0033] I. Preparation before experiment: The core components of each module and the corresponding parameters need to be prepared, as follows: ①The mid-infrared light source module 100 uses a tunable quantum cascade laser QCL, whose output wavelength covers the range of 2.6-4.5 pm, the output power is 15 mW, the polarization state is TE polarization, and the wavelength stability is controlled at ±0.1 nm / h, which ensures accurate matching of the characteristic absorption peaks of different gases.

[0034] ②The gas absorption module 200 is equipped with a quartz gas chamber with a volume of 15 mL, which is matched with a temperature control unit composed of a patch heating sheet and a temperature sensor. The temperature control target is 25℃, and the temperature control accuracy can reach ±0.1℃. At the same time, a gas sampling pump with a flow rate of 100 mL / min is configured for the injection of the gas to be detected and the cleaning of the gas chamber.

[0035] ③The core of the FWM wavelength conversion module 300 is a GeSbS chalcogenide waveguide 350. The cross section of the chalcogenide waveguide 350 is shown in Figure 3 from top to bottom, respectively, are a substrate 351, a lower cladding layer 352, a Ge 25 Sb 10 S 65 core layer 353, an upper cladding layer 354. The selection of the waveguide upper and lower cladding layer 352 materials in this embodiment has less restrictive conditions, only needs to satisfy that the refractive index is lower than that of the Ge 25 Sb 10 S 65 core layer 353, and the absorption in the near-infrared-mid-infrared waveband (1um~5um) range is as low as possible, thereby effectively reducing the transmission loss of the overall waveguide. The materials that meet the requirements are CaF2(1~9um), sapphire Sapphire(1~5.5um), silicon nitride Si3N4(1~7um), and magnesium fluoride CaF2(1~7.7um). In particular, since the transparent window of SiO2 is 1~3.6um, for gases with absorption peaks below 3.6um, SiO2 can still be selected as the cladding layer. After the material is determined, the width and thickness of the waveguide core layer 353 need to be scanned and optimized to control the zero dispersion wavelength to be located near 2um, so as to ensure that the Ge25 Sb 10 S 65 The core layer 353 has a stable composition, and its thickness is fixed at 850 nm. Therefore, only the width of the core layer 353 needs to be scanned to optimize the dispersion characteristics of the chalcogenide waveguide 350. For example, when the width of the chalcogenide waveguide 350 is selected as 2.6 μm, the corresponding pump wavelength-signal wavelength-conversion efficiency (CE) thermal diagram is shown below. Figure 5 As shown, under a 2µm pump at 250mW, the absorption wavelength of methane at 3270nm can be converted to 1440.5nm in the near-infrared band, with a conversion efficiency (CE) of -35dB. After simulation, the results are then... Figure 4 The process flow shown is used to fabricate the corresponding chalcogenide waveguide 350 chip.

[0036] ④ The near-infrared detection module 400 uses a commercially available InGaAs avalanche photodiode with a response range of 1.0-1.6μm, and a low-noise amplifier and data acquisition card with the lowest possible noise figure to ensure accurate acquisition and amplification of weak electrical signals. It is also equipped with 99.999% pure nitrogen for cleaning the gas chamber and removing residual gas.

[0037] II. Experimental Procedure: Step 1: System Setup: Follow Figure 1 and Figure 2 Connect each module in sequence. Use a mid-infrared single-mode fiber (5μm core diameter) to connect the output of the tunable quantum cascade laser (QCL) to the inlet of the quartz gas cell. Use another mid-infrared fiber to connect the outlet of the gas cell to the inverted tapered coupler 340 of the FWM wavelength conversion module 300. Finally, use a near-infrared single-mode fiber to connect the waveguide output of the FWM wavelength conversion module 300 to the optical input of the InGaAs avalanche photodiode.

[0038] Step 2: System initialization and self-test: Turn on the power to the mid-infrared light source module 100, FWM wavelength conversion module 300 and near-infrared detection module 400 in sequence, check whether the status of each module is normal, and set the wavelength of the tunable quantum cascade laser (QCL) corresponding to the mid-infrared signal light to the methane absorption peak of 3270nm, and adjust the wavelength of the independent pump source 310 to 2µm (both can be tuned according to the gas being detected).

[0039] Step 3: System calibration: first, start the gas sampling pump and input nitrogen, and keep it for 5 minutes to clear the residual gas in the gas chamber; then input 0 ppm, 10 ppm, 20 ppm, 50 ppm and 100 ppm methane standard gas into the quartz gas chamber in turn, and keep each concentration point for 3 minutes to ensure the stability of the gas concentration in the chamber; then use the InGaAs avalanche photodiode and the data acquisition card to record the near-infrared idler light intensity corresponding to each concentration, collect 10 groups of data for each concentration, and take the average value; then substitute the "concentration-light intensity" data into the Lambert-Beer law to obtain the calibration curve, which is stored in the concentration calculation unit (embedded chip). For each change of gas, the calibration curve needs to be stored once, and for the detection of the same gas, it only needs to be detected the first time.

[0040] Step 4: Concentration measurement of the gas to be detected: first, input nitrogen for 5 minutes to clear the residual standard gas in the gas chamber until the data acquisition card shows that the light intensity is stable (fluctuation <1%); then inject the gas to be detected, methane, into the gas chamber through the sampling pump, and keep it for 3 minutes before closing the pump; the embedded chip will automatically collect the current idler light intensity, substitute it into the calibration curve to calculate the corresponding concentration, and display it in real time on the display screen, such as "methane concentration: 5.9 ppm (average value)".

[0041] ⑤ System maintenance and reset: after the detection is completed, input nitrogen to clear the gas chamber for 5 minutes, and close the sampling pump.

[0042] Example Three This embodiment is a second embodiment of a wavelength conversion-based on-chip infrared gas detection method. This embodiment is similar to Example Two, except that in this embodiment, the pump wavelength and the mid-infrared signal light wavelength are adjusted synchronously to measure the concentration of different gases, as shown in FIG. 2B. Figure 6 As shown in FIG. 2B, when the pump wavelengths are 1992 nm, 2000 nm, 2025 nm and 2027 nm, the absorption peak wavelengths of ethylene (C2H4), methane (CH4), ozone (O3) and propylene (C3H6) corresponding to 3245 nm, 3270 nm, 3360 nm and 3370 nm can be converted to the near-infrared 1.44 um band, so that they can be detected by the InGaAs avalanche photodiode, and the detection sensitivity of different gases can be consistent due to the balanced conversion efficiency.

[0043] In this embodiment, by synchronously adjusting the mid-infrared signal light wavelength of the quantum cascade laser and the wavelength of the pump source 310 according to the mid-infrared absorption peak of the gas to be detected, the FWM phase matching condition of different gases can be met without modifying the geometry parameters of the chalcogenide waveguide 350, so that the idler light always falls within the detection range of the InGaAs avalanche photodiode, and the conversion efficiency is balanced; multi-component detection is achieved.

[0044] In the specific contents of the foregoing specific embodiments, each technical feature can be combined arbitrarily without contradiction. In order to make the description simple, all possible combinations of the foregoing technical features are not described, but as long as the combinations of the technical features do not contradict, they should be considered as the scope of the present disclosure.

[0045] Obviously, the above embodiments of the present application are merely exemplary and are not intended to limit the embodiments of the present application. Based on the above description, other different forms of changes or variations can be made by those of ordinary skill in the art. Here, it is not necessary and impossible to exhaust all the embodiments. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application should be included in the protection scope of the claims of the present application.

Claims

1. A wavelength conversion based on-chip infrared gas detection system, characterized in that, The system comprises: a mid-infrared light source module (100) for emitting mid-infrared signal light at a wavelength absorbed by the gas to be detected; a gas absorption module (200) comprising a gas chamber with a cavity, wherein the mid-infrared signal light and the gas to be detected are introduced into the gas chamber, so that the gas to be detected can fully absorb the mid-infrared signal light; a FWM wavelength conversion module (300) for converting the mid-infrared signal light absorbed by the gas to be detected into near-infrared idler light; a near-infrared detection module (400) for receiving the near-infrared idler light, converting the light signal into an electrical signal, and calculating the concentration of the gas. The FWM wavelength conversion module (300) comprises: a tunable pump source (310) for generating TE polarized pump light with a preset output wavelength and power; a first polarization controller (320) connected to the tunable pump source (310) for adjusting the polarization state of the pump light; a second polarization controller (330) connected to the output end of the gas absorption module (200) for adjusting the polarization state of the mid-infrared signal light absorbed by the gas to be detected; a coupler (340) with its input end connected to the output ends of the first and second polarization controllers (320, 330) and its output end connected to a chalcogenide waveguide (350); a chalcogenide waveguide (350) for receiving the combined pump light and mid-infrared signal light and realizing wavelength conversion based on the nonlinear optical four-wave mixing effect.

2. The wavelength conversion based on-chip infrared gas detection system of claim 1, wherein, By adjusting the dispersion of the chalcogenide waveguide (350), the dispersion can reach a preset target dispersion, which can meet the phase matching conditions required by the nonlinear optical four-wave mixing effect, so as to realize cross-band wavelength conversion.

3. The wavelength conversion based on-chip infrared gas detection system of claim 2, wherein, The dispersion of the chalcogenide waveguide (350) is adjusted by adjusting the size of the waveguide.

4. The wavelength conversion based on-chip infrared gas detection system of claim 2, wherein, The chalcogenide waveguide (350) comprises, from bottom to top, a substrate (351), a lower cladding layer (352), a GeSbS waveguide core layer (353), and an upper cladding layer (354), wherein the refractive indices of the upper and lower cladding layers (354, 352) are smaller than that of the GeSbS waveguide core layer (353).

5. The wavelength conversion based on-chip infrared gas detection system of claim 4, wherein, The material of the GeSbS waveguide core layer (353) comprises Ge 25 Sb 10 S 65 .

6. The wavelength conversion based on-chip infrared gas detection system of claim 1, wherein, The gas absorption module (200) further comprises a temperature control unit for controlling the temperature in the gas chamber.

7. The wavelength conversion based on-chip infrared gas detection system according to any one of claims 1 to 6, wherein, After the mid-infrared light enters the gas chamber, it will be absorbed by the gas to be detected according to the Lambert-Beer law.

8. The wavelength conversion based on-chip infrared gas detection system of claim 7, wherein, The near-infrared detection module (400) comprises a photodiode and a signal processing unit, wherein the signal processing unit comprises an amplifier, a data acquisition card, and a concentration calculation unit. The photodiode converts the received near-infrared idler light into an electrical signal, which is then amplified by the amplifier to output an analog signal. The data acquisition card converts the analog signal into a digital signal and transmits it to the concentration calculation unit. The concentration calculation unit calculates the concentration of the gas to be detected based on the pre-stored "concentration-light intensity" calibration curve and the Lambert-Beer law.

9. A wavelength conversion based on-chip infrared gas detection method, characterized in that, The wavelength conversion-based on-chip infrared gas detection system according to any one of claims 1-8 comprises the following steps: System initialization: the output of the mid-infrared light source module (100) wavelength is set to the measured gas absorption wavelength, adjust the wavelength of the tunable pump source (310) to the preset value; System calibration: a plurality of groups of standard concentration of the measured gas are sequentially introduced into the clean gas chamber, and the near-infrared detection module (400) is used to collect the corresponding near-infrared idler light of each group of standard concentration, and the "concentration-light intensity" data is substituted into the Lambert-Beer law to obtain the calibration curve and store it in the near-infrared detection module (400); Gas detection: empty the residual gas in the gas chamber, introduce the gas to be detected into the gas chamber, and use the near-infrared detection module (400) to collect the current idler light, and substitute it into the calibration curve to calculate the concentration of the measured gas.

10. The wavelength conversion based on-chip infrared gas detection method of claim 9, wherein, By synchronously adjusting the wavelength of the tunable pump source (310) and the wavelength of the mid-infrared signal light, the concentration detection of different gases is realized.

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