Gradient refractive index optical fiber collimation-based QEPAS probe gas sensor and detection method

Abstract

The invention discloses a QEPAS probe gas sensor based on gradient refractive index optical fiber collimation and a detection method. The sensor comprises a gradient refractive index optical fiber collimator used for receiving laser and performing collimation; the first acoustic microresonator is used for guiding the collimated laser beam and the to-be-detected gas to excite and generate a photoacoustic signal and enhancing the intensity of the photoacoustic signal; the right-angle reflecting mirror is used for receiving the collimated laser beam output by the first acoustic micro-resonator and turning over the propagation direction of the collimated laser beam; the second acoustic micro-resonator is used for guiding the collimated laser beam after the propagation direction is turned to continue to be excited with the gas to be detected to generate a photoacoustic signal and enhancing the intensity of the photoacoustic signal; the quartz tuning fork is used for detecting a photoacoustic signal generated by excitation and converting the photoacoustic signal into a current signal; and the signal modulation and demodulation system is used for demodulating and calculating the gas concentration according to the current signal. According to the invention, while the size of the probe is greatly reduced, the convenience of optical path installation and the long-term stability of optical performance are greatly improved through the integrated design.

Description

Technical Field

[0001] The invention relates to the field of gas sensor detection technology, and in particular to a QEPAS probe gas sensor based on gradient refractive index optical fiber collimation and a detection method. Background Art

[0002] In recent years, trace gas sensors have been widely used in many fields such as environmental monitoring, industrial safety, and medical diagnosis. They can be divided into two categories according to the measurement principle: spectroscopy and non-spectroscopy. Non-spectroscopy includes electrochemical method, semiconductor method, chromatography, mass spectrometry, etc. Among them, electrochemical method and semiconductor method have the characteristics of low price and small size, and can realize real-time monitoring, but generally have the disadvantages of poor sensitivity and lifespan, and weak resolution of gases with similar chemical properties; chromatography and mass spectrometry have high sensitivity, usually up to ppb level, but their equipment is very expensive, and the measurement requires complex operation procedures, which are usually used for laboratory offline analysis.

[0003] Spectroscopic methods mainly include tunable diode laser absorption spectroscopy (TDLAS), cavity enhanced absorption spectroscopy (CEAS), photothermal spectroscopy (LITES), photoacoustic spectroscopy (PAS), etc. TDLAS and CEAS are direct absorption spectroscopy techniques with advantages such as fast response and high sensitivity. LITES uses a quartz tuning fork (QTF) as an optical detector in the TDLAS device, and generates a current signal based on the thermoelastic deformation caused by laser energy absorption, which has a wider response bandwidth than traditional photodetectors. However, its sensitivity is related to the optical path length, and the multiple reflection gas chamber makes the equipment expensive and bulky, and its practical application is subject to certain restrictions; PAS is based on the photoacoustic effect and only needs to detect local acoustic wave signals, so the sensor has a smaller size. Among them, quartz enhanced photoacoustic spectroscopy (QEPAS) technology, as an improvement on PAS technology, is a more compact and lower-cost alternative, and is increasingly attracting the attention of researchers.

[0004] QEPAS technology was first proposed by Kosterex's team in 2002. It is a highly sensitive optical measurement technology for trace gases. Its basic principle is to use a modulated narrow-linewidth laser to irradiate the sample to be measured. When the wavelength of the light is consistent with the characteristic absorption peak of the target gas, the molecular group will be periodically heated, which will cause the thermal expansion and contraction of the gas medium, and generate an acoustic signal whose intensity is positively correlated with the concentration of the target molecule; then, a quartz tuning fork (QTF) is introduced as a microphone, and its narrowband and high Q value characteristics (up to 10,000 to 13,000 in air) are used to achieve weak sound pressure measurement, and the target gas concentration is inverted. In order to more effectively achieve acoustic coupling and improve the intensity of the photoacoustic signal, it is necessary to set the laser modulation frequency to be consistent with the resonant frequency of the QTF, and adjust the collimator to guide the laser beam through the acoustic microresonator (MR), and then pass over the QTF cantilever at a close distance. This process converts the photoacoustic signal into the piezoelectric current of the quartz crystal. Among them, MR is generally a capillary hollow tube made of rigid materials. There are many forms of spatial combination structures between it and QTF, which can be roughly divided into coaxial and off-axis. The coaxial mode requires that the collimated light beam passes through the gap between the two cantilevers of QTF while passing through MR. This structure can achieve strong photoacoustic coupling and increase the detection sensitivity by 17 to 35 times compared with the case without MR. However, since the gap between the cantilevers of the standard commercial 30.72kHz tuning fork is less than 0.3mm, the beam collimation accuracy is required to be high, which is not conducive to the mass production of sensors. In 2009, Liu proposed the QEPAS off-axis structure, which placed the QTF on the side of the MR. The acoustic wave signal is coupled with the QTF through the opening on the side of the MR. In this way, the requirements for beam quality and structural accuracy are greatly reduced, and the detection sensitivity can also be increased by 12 to 25 times compared with the case without MR.

[0005] Since the optical path required by QEPAS technology is very short, the volume of the gas chamber is greatly reduced, making it easier to miniaturize the photoacoustic detector. To further extend this advantage, researchers often choose more compact optical components, such as a customized miniature gradient refractive index lens (G-lens) for the collimator, with a diameter of about 1.8mm, a collimation distance of about 20mm, and a central spot diameter of 0.2mm. In 2018, Ma Yufei's team used G-lens combined with 3D printing technology to produce a 29×15×8mm micro-detection unit, but the inherent optical path calibration difficulties and low long-term optical stability of QEPAS technology could not be avoided in the production. In 2018, He Ying et al. proposed an all-fiber structure QEPAS based on micro-nano optical fibers. This structure uses the potential wave effect of micro-nano optical fibers to excite photoacoustic signals, eliminating the need to build a spatial optical path and avoiding the problem of beam collimation, but its photoacoustic conversion efficiency is low, and it is necessary to introduce an erbium-doped optical amplifier to significantly increase the optical power to maintain sufficient detection sensitivity, so that the gas absorption peak is limited to the C / L band. In 2021, Martino De Carlo and others proposed a solution to bond the light source used in QEPAS and the optical elements used for beam transmission together and semi-integrate them with a tuning fork. This solution encapsulates the photoacoustic core of the sensor in a single module and has high stability. However, this solution is difficult to implement and is currently in the theoretical verification stage.

[0006] Therefore, how to design a QEPAS beam collimation method and gas sensor with smaller size and higher optical stability is a technical problem that needs to be solved urgently. Summary of the invention

[0007] In view of the above-mentioned deficiencies of the prior art, the technical problem to be solved by the present invention is: how to provide a QEPAS probe gas sensor and detection method based on gradient refractive index fiber collimation, wherein the gradient refractive index fiber collimator (GRIN-FC) and the acoustic microresonator (MR) are integrated into one by mechanical coupling, and the GRIN-FC and the quartz tuning fork (QTF) form a double MRs off-axis parallel arrangement structure, which greatly reduces the volume of the probe and greatly improves the convenience of optical path installation and the long-term stability of optical performance through integrated design, realizing a photoacoustic detection channel (PADC) size of only 0.56 cm 3 The QEPAS probe gas sensor design.

[0008] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0009] QEPAS probe gas sensor based on graded-index fiber collimation, including:

[0010] A graded refractive index fiber collimator is used to receive and collimate the laser light emitted by the laser to form a collimated laser beam;

[0011] The first acoustic microresonator is used to guide the collimated laser beam into the sensor to excite the gas to generate a photoacoustic signal, and amplify the acoustic signal through acoustic wave resonance to enhance the intensity of the photoacoustic signal;

[0012] a right-angle reflector, used for receiving the collimated laser beam output by the first acoustic microresonator and flipping its propagation direction;

[0013] The second acoustic microresonator is used to guide the collimated laser beam after flipping the propagation direction to continue to excite the gas to be measured inside the sensor to generate a photoacoustic signal, and further enhance the intensity of the photoacoustic signal by amplifying the acoustic signal through acoustic wave resonance;

[0014] A quartz tuning fork is disposed between the first acoustic microresonator and the second acoustic microresonator, and is used to detect the photoacoustic signal generated when the first acoustic microresonator and the second acoustic microresonator transmit laser light, and convert the photoacoustic signal into a current signal;

[0015] The signal modulation and demodulation system is used to demodulate and calculate the gas concentration based on the current signal converted by the quartz tuning fork.

[0016] Preferably, the graded-index optical fiber collimator comprises a single-mode optical fiber, a coreless optical fiber and a graded-index multimode optical fiber connected end to end in sequence.

[0017] Preferably, the maximum length of the coreless optical fiber is Where Φ is the core diameter of the graded-index multimode fiber, and θ is the divergence angle of the single-mode fiber.

[0018] Preferably, the length L1 of the coreless optical fiber is 400-520 μm; the length L2 of the gradient-index multimode optical fiber is 1410-1530 μm; and the minimum spot diameter of the gradient-index optical fiber collimator is w<0.39 mm.

[0019] Preferably, the gradient refractive index fiber collimator is inserted into the first acoustic microresonator through a packaging structure and packaged and fixed;

[0020] The packaging structure includes a first stainless steel capillary and a second stainless steel capillary; the inner side of the first stainless steel capillary is nested on the outer plastic sheath of the gradient refractive index optical fiber collimator; the inner side of the second stainless steel capillary is nested on the cladding of the gradient refractive index optical fiber collimator; an end of the outer side of the second stainless steel capillary close to the first stainless steel capillary is nested with the inner side of the first stainless steel capillary; an end of the outer side of the second stainless steel capillary away from the first stainless steel capillary is nested with the inner side of the first acoustic microresonator;

[0021] The optical fiber in the graded refractive index optical fiber collimator passes through the first stainless steel capillary and the second stainless steel capillary in sequence and then enters the first acoustic microresonator.

[0022] Preferably, both the first acoustic microresonator and the second acoustic microresonator are provided with sound transmission holes for realizing acoustic coupling with a quartz tuning fork, so that when the collimated laser beam passes through the first acoustic microresonator and the second acoustic microresonator, it can pass over the cantilever of the quartz tuning fork and amplify the photoacoustic signal generated by the excitation, and then convert the photoacoustic signal generated by the excitation into a current signal through the quartz tuning fork.

[0023] Preferably, the signal modulation and demodulation system comprises:

[0024] A digital-to-analog conversion circuit is used to control the intensity and modulation frequency of the laser and generate the modulation signal required to drive the laser;

[0025] A transimpedance amplifier, used to convert the current signal output by the quartz tuning fork into a voltage signal;

[0026] A differential analog-to-digital converter, used for collecting the voltage signal converted by the transimpedance amplifier and converting it into a digital signal;

[0027] The fully digital lock-in amplifier is used to demodulate the second harmonic amplitude with concentration information from the collected voltage signal, and multiply the second harmonic amplitude with the calibration coefficient to obtain the gas concentration.

[0028] Preferably, it also includes:

[0029] The micro photodiode is used to receive the laser emitted by the second acoustic microresonator and monitor the optical path fault through the laser.

[0030] The QEPAS probe gas sensor detection method based on graded refractive index optical fiber collimation is implemented based on the QEPAS gas probe sensor of the present invention, comprising:

[0031] S1: Receive the laser light emitted by the laser through the graded refractive index fiber collimator and collimate it to form a collimated laser beam;

[0032] S2: guiding the collimated laser beam to transmit inside the sensor through the first acoustic microresonator, and exciting the gas to be measured to generate a photoacoustic signal;

[0033] S3: receiving the collimated laser beam transmitted by the first acoustic microresonator through a right-angle mirror and flipping its propagation direction;

[0034] S4: The collimated laser beam is guided again through the second acoustic microresonator to transmit inside the sensor, and continues to excite the gas to generate a photoacoustic signal;

[0035] S5: detecting the photoacoustic signal generated when the first acoustic microresonator and the second acoustic microresonator transmit laser light by using a quartz tuning fork disposed between the first acoustic microresonator and the second acoustic microresonator, and converting the photoacoustic signal into a current signal;

[0036] S6: Calculate the gas concentration based on the demodulation of the current signal converted by the quartz tuning fork.

[0037] Preferably, in step S2, the method for preparing a gradient refractive index optical fiber collimator comprises:

[0038] S201: splicing the single-mode optical fiber and the coreless optical fiber to obtain a first spliced ​​optical fiber;

[0039] S202: cutting the coreless optical fiber in the first fusion-spliced ​​optical fiber to a length of L1;

[0040] S203: fusing the coreless optical fiber in the cut first fusion-splice optical fiber with the graded-index multimode optical fiber to obtain a second fusion-splice optical fiber;

[0041] S204: cutting the graded-index multimode optical fiber in the second fusion-spliced ​​optical fiber to a length of L2 to obtain a graded-index optical fiber collimator;

[0042] S205: Use a beam quality analyzer to test the collimation performance of the cut gradient refractive index fiber collimator.

[0043] Compared with the prior art, the QEPAS gas probe method based on graded refractive index fiber collimation in the present invention has the following advantages:

[0044] Beneficial effects:

[0045] In the QEPAS (quartz enhanced photoacoustic spectroscopy) gas sensor of the present invention, a graded refractive index fiber collimator (GRIN-FC) and an acoustic microresonator (MR) are integrated into one by mechanical coupling. At the same time, since a quartz tuning fork (QTF) is arranged between the first acoustic microresonator and the second acoustic microresonator, the GRIN-FC, QTF and MR form a double MRs off-axis parallel arrangement structure. While greatly reducing the volume of the probe, the convenience of optical path installation and the long-term stability of optical performance are greatly improved through the integrated design, and a photoacoustic detection channel (PADC) size of only 0.56 cm is achieved. 3 The QEPAS probe gas sensor design is a new and unique QEPAS probe gas sensor. Compared with the traditional QEPAS gas sensor in which the G-lens and MR are installed separately, the QEPAS gas sensor structure of the present invention is not only more compact, but also has better long-term mechanical stability. In addition, the optical axis calibration process of the laser beam and the MR is eliminated, which greatly simplifies the installation and debugging procedures of the QEPAS probe gas sensor.

[0046] The present invention utilizes the combined shearing and fusion technology of single-mode optical fiber, coreless optical fiber and gradient refractive index optical fiber to design a gradient refractive index optical fiber collimator (GRIN-FC) specifically for QEPAS. The diameter of GRIN-FC is only 125μm, thereby realizing the miniaturization design of QEPAS probe sensor. At the same time, the present invention theoretically analyzes the beam transmission model of GRIN-FC, and with the goal of minimizing the single-wavelength far-field spot, determines the optimal fusion length of the coreless optical fiber and the gradient refractive index optical fiber in GRIN-FC through numerical simulation, and the effective collimation distance exceeds 20mm, thereby improving the measurement accuracy of the QEPAS probe gas sensor.

[0047] The present invention utilizes multi-stage stainless steel capillaries to mechanically couple GRIN-FC to the interior of an acoustic microresonator (MR), and realizes an integrated package of GRIN-FC and MR without optical axis calibration through a multi-tube nested coupling method; at the same time, GRIN-FC and a quartz tuning fork constitute a dual MRs parallel off-axis QEPAS probe-type gas measurement structure with a size as low as 16mm×3.5mm, which greatly compresses the volume of the sensor while maintaining a high sensitivity.

[0048] Experimental data show that the structure of the QEPAS probe gas sensor designed in the present invention not only significantly reduces the volume of the sensor (<0.56cm 3 ), and by optimizing the MR length and the photoacoustic coupling aperture, the sensor has good measurement sensitivity, and the second harmonic signal intensity is increased by 1.72 times compared with the traditional single MR G-lens structure QEPAS sensor. The MDL of the QEPAS probe gas sensor for water vapor measurement reaches 0.73ppm, demonstrating its superior performance in trace gas detection. The miniaturization and commercialization of the QEPAS probe gas sensor in the present invention provides a new technical path, and its improved portability can be widely used in various fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to make the purpose, technical solution and advantages of the invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0050] Figure 1 and Figure 2 This is a structural schematic diagram and actual picture of the QEPAS probe gas sensor based on gradient refractive index fiber collimation.

[0051] Figure 3 Schematic diagram of the structure of the graded refractive index fiber collimator (GRIN-FC).

[0052] Figure 4 Middle: (a) is the 3D contour map of w with respect to L1 and L2; (b) is the relationship between w and L1 length on curve K.

[0053] Figure 5 In the figure: (a) is the actual picture of GRIN-FC; (b) is the integrated packaging method of GRIN-FC and MR; (c) is the actual picture of GRIN-FC; (d) is the relationship between the spot diameter and the propagation distance.

[0054] Figure 6 In the figure: (a) is a schematic diagram of the structure of the signal modulation and demodulation system; (b) is a physical diagram of the signal modulation and demodulation system.

[0055] Figure 7 MR parameter optimization: (a) is the hole diameter d; (b) is the length L MR .

[0056] Figure 8 Different collimator and MR combination structures: (a) is a bare tuning fork; (b) is a single resonant tube GRIN-FC; (c) is a double resonant tube GRIN-FC; (d) is a single resonant tube Grin-lens.

[0057] Fig. 9 Figure 2 shows the second harmonic signal diagrams of the four structures.

[0058] Fig.10 It is the noise sequence of the double-resonance tube GRIN-FC structure. DETAILED DESCRIPTION

[0059] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but only represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the scope of protection of the present invention.

[0060] The following is a further detailed description through specific implementation methods:

[0061] Example:

[0062] This embodiment discloses a QEPAS (quartz enhanced photoacoustic spectroscopy) gas sensor and detection method based on gradient refractive index fiber collimation.

[0063] like Figure 1 and Figure 2As shown, the QEPAS probe gas sensor based on gradient refractive index fiber collimation includes:

[0064] A graded refractive index fiber collimator (GRIN-FC) is used to receive the laser light emitted by the laser and collimate it to form a collimated laser beam;

[0065] In this embodiment, the gradient refractive index fiber collimator is used to ensure that the laser maintains a small divergence angle during the transmission process and effectively passes through the entire sensing system.

[0066] A first acoustic microresonator (MR1) is used to guide a collimated laser beam into the interior of the sensor to excite the gas to generate a photoacoustic signal, and to amplify the acoustic signal through acoustic wave resonance to enhance the intensity of the photoacoustic signal;

[0067] In this embodiment, the first acoustic microresonator enhances the intensity of the photoacoustic signal generated by the interaction between the laser and the gas molecules to be measured through its acoustic resonance characteristics.

[0068] a right-angle reflector (RP), used for receiving the collimated laser beam outputted by the first acoustic microresonator and flipping its propagation direction, so that the collimated laser beam after flipping the propagation direction can enter the second acoustic microresonator;

[0069] In this embodiment, the size of the right-angle reflector is 2 mm×2 mm×2 mm.

[0070] The second acoustic microresonator (MR2) is used to guide the collimated laser beam after flipping the propagation direction to continue to excite the gas to be measured inside the sensor to generate a photoacoustic signal, and further enhance the intensity of the photoacoustic signal by amplifying the acoustic signal through acoustic wave resonance;

[0071] In this embodiment, the second acoustic microresonator further enhances the intensity of the photoacoustic signal generated by the interaction between the laser and the gas molecules to be measured through its acoustic resonance characteristics.

[0072] A quartz tuning fork (QTF) is disposed between the first acoustic microresonator and the second acoustic microresonator, and is used to detect the photoacoustic signal generated when the first acoustic microresonator and the second acoustic microresonator transmit laser light, and convert the photoacoustic signal into a current signal;

[0073] In this embodiment, the quartz tuning fork is a commercial standard tuning fork with a size of 5.6 mm × 1.5 mm × 0.36 mm and a resonant frequency of 30.72 kHz. The original welding base is removed, and the original metal wire is replaced with a thinner enameled copper wire to reduce the thickness. The quartz tuning fork is placed between the two microacoustic resonators and placed horizontally.

[0074] The signal modulation and demodulation system is used to demodulate and calculate the gas concentration based on the current signal converted by the quartz tuning fork.

[0075] A micro photodiode (PD) is used to receive the laser emitted by the second acoustic microresonator and monitor optical path faults through the laser.

[0076] In this embodiment, the structure of the entire QEPAS gas sensor uses a 3D printed plastic bracket to fix the relative position of the QTF and MRs, where the distance between the MR and the QTF is 0.1mm, and the center of the sound transmission hole is 0.7mm away from the top of the tuning fork. The entire sensor is a 16mm×3.5mm cylindrical probe structure with a volume of only 0.56cm 3 , which is only 6.2% of the volume of the compact photoacoustic detection structure reported recently. In addition, a stainless steel sleeve (ID=3.8 mm) containing a vent hole can be added to the outside to provide protection for the photoacoustic sensing area.

[0077] This embodiment also discloses a QEPAS probe gas sensor detection method based on gradient refractive index optical fiber collimation, which is implemented based on a QEPAS gas probe sensor and includes:

[0078] S1: Receive the laser light emitted by the laser through the graded refractive index fiber collimator and collimate it to form a collimated laser beam;

[0079] S2: guiding the collimated laser beam to transmit inside the sensor through the first acoustic microresonator, and exciting the gas to be measured to generate a photoacoustic signal;

[0080] S3: receiving the collimated laser beam transmitted by the first acoustic microresonator through a right-angle mirror and flipping its propagation direction;

[0081] S4: The collimated laser beam is guided again through the second acoustic microresonator to transmit inside the sensor, and continues to excite the gas to generate a photoacoustic signal;

[0082] S5: detecting the photoacoustic signal generated when the first acoustic microresonator and the second acoustic microresonator transmit laser light by using a quartz tuning fork disposed between the first acoustic microresonator and the second acoustic microresonator, and converting the photoacoustic signal into a current signal;

[0083] S6: Calculate the gas concentration based on the demodulation of the current signal converted by the quartz tuning fork.

[0084] In the QEPAS (quartz enhanced photoacoustic spectroscopy) gas sensor of the present invention, a graded refractive index fiber collimator (GRIN-FC) and an acoustic microresonator (MR) are integrated into one by mechanical coupling. At the same time, since a quartz tuning fork (QTF) is arranged between the first acoustic microresonator and the second acoustic microresonator, the GRIN-FC, QTF and MR form a double MRs off-axis parallel arrangement structure. While greatly reducing the volume of the probe, the convenience of optical path installation and the long-term stability of optical performance are greatly improved through the integrated design, and a photoacoustic detection channel (PADC) size of only 0.56 cm is achieved. 3 The QEPAS probe gas sensor design is a new and unique QEPAS probe gas sensor. Compared with the traditional QEPAS gas sensor in which the G-lens and MR are installed separately, the QEPAS gas sensor structure of the present invention is not only more compact, but also has better long-term mechanical stability. In addition, the optical axis calibration process of the laser beam and the MR is eliminated, which greatly simplifies the installation and debugging procedures of the QEPAS probe gas sensor.

[0085] The present invention utilizes the combined shearing and fusion technology of single-mode optical fiber, coreless optical fiber and gradient refractive index optical fiber to design a gradient refractive index optical fiber collimator (GRIN-FC) specifically for QEPAS. The diameter of GRIN-FC is only 125μm, thereby realizing the miniaturization design of QEPAS probe sensor. At the same time, the present invention theoretically analyzes the beam transmission model of GRIN-FC, and with the goal of minimizing the single-wavelength far-field spot, determines the optimal fusion length of the coreless optical fiber and the gradient refractive index optical fiber in GRIN-FC through numerical simulation, and the effective collimation distance exceeds 20mm, thereby improving the measurement accuracy of the QEPAS probe gas sensor.

[0086] The present invention utilizes multi-stage stainless steel capillaries to mechanically couple GRIN-FC to the interior of an acoustic microresonator (MR), and realizes an integrated package of GRIN-FC and MR without optical axis calibration through a multi-tube nested coupling method; at the same time, GRIN-FC and a quartz tuning fork constitute a dual MRs parallel off-axis QEPAS probe-type gas measurement structure with a size as low as 16mm×3.5mm, which greatly compresses the volume of the sensor while maintaining a high sensitivity.

[0087] Experimental data show that the structure of the QEPAS probe gas sensor designed in the present invention not only significantly reduces the volume of the sensor (<0.56cm 3), and by optimizing the MR length and the photoacoustic coupling aperture, the sensor has good measurement sensitivity, and the second harmonic signal intensity is increased by 1.72 times compared with the traditional single MR G-lens structure QEPAS sensor. The MDL of the QEPAS probe gas sensor for water vapor measurement reaches 0.73ppm, demonstrating its superior performance in trace gas detection. The miniaturization and commercialization of the QEPAS probe gas sensor in the present invention provides a new technical path, and its improved portability can be widely used in various fields.

[0088] In order to better introduce the technical solution of the present invention, this embodiment is described through the following parts.

[0089] 1. Graded Index Fiber Collimator (GRIN-FC)

[0090] The preparation technology of GRIN-FC was first proposed by Swanson et al. in 2002 and applied to compact optical coherence tomography (OCT) systems. Subsequently, many scholars have continuously improved it and achieved a lot of results in the field of microscopic imaging. However, these studies are mainly focused on the optimization of multi-wavelength imaging performance, while the application in the QEPAS system needs to focus on enhancing its single-wavelength long-range light collimation capability, and there are relatively few related discussions. The core of GRIN-FC is a gradient-index multimode fiber (GIF) with a radially decreasing refractive index. The gradient refractive index allows the light beam transmitted in the fiber to converge periodically, with a period spacing of p = 2π / g. g is the gradient refractive index distribution constant, which is provided by the manufacturer. If the incident light is a point light source located at the center of the GIF end face, and the GIF length is equal to an integer multiple of 1 / 2p, the light beam passing through the GIF can be converted into a spatially collimated laser beam. However, the laser output from the end face of a single-mode fiber (SMF) is a Gaussian beam. When a single lens pair is used for collimation, the divergence angle of the beam is not only affected by the diffraction of the lens aperture, but is also inversely proportional to the focal length of the collimating lens.

[0091] Therefore, in this embodiment, a GIF with a relatively large core diameter of 105 μm (cladding diameter of 125 μm) is selected as the collimator fiber to reduce the diffraction effect. A coreless fiber (CLF) is inserted between the SMF and the GIF as a focal length extension medium. Figure 3 As shown, the graded-index fiber collimator includes a single-mode fiber (SMF), a coreless fiber (CLF) and a graded-index multimode fiber (GIF) connected end to end in sequence.

[0092] Figure 3 In the equation, L1 is the length of CLF, L2 is the length of GIF, Φ is the core diameter of GIF, and θ is the divergence angle of SMF. If the equivalent focal length of GIF is defined as F = L1 + 1 / 2L2, when F takes the maximum value, CLF should expand the output beam width of SMF to close to the core diameter of GIF, that is, At the same time, affected by the change of F, L2 will no longer be equal to an integer multiple of 1 / 2p, and the calculation of its accurate length is relatively complicated.

[0093] Therefore, we estimate its length through numerical simulation and use the beam propagation method (BPM) of the optical waveguide design software RSoft CADLayout (Version 2020.03) to simulate the GRIN-FC structure and minimize the far-field 1 / e at 20 mm from the end face. 2 The spot diameter w is taken as the optimization target. The simulation parameters are set according to the actual optical system: the wavelength of the light source is set to 1392nm, corresponding to the characteristic peak of water vapor absorption; the fiber cladding diameter is 125μm; the SMF is Corning SMF-28; the CLF is the CL0-125-0-250 model of YOFC, with a refractive index of 1.444. Then, the simulation calculates the change characteristics of w at different lengths of CLF and GIF, such as Figure 4 As shown in (a), it can be seen that there is a minimum gradient curve K when w changes with L1 and L2. On this curve, w is inversely proportional to L1, which is consistent with the collimation characteristics of the Gaussian beam. However, it is calculated that L1 cannot exceed 520μm, otherwise it will cause CLF light leakage, such as Figure 4 (b) As shown in the figure, the length of the coreless optical fiber is finally determined to be L1 = 400-520 μm; the length of the graded-index multimode optical fiber is L2 = 1410-1530 μm; and the minimum spot diameter of the graded-index optical fiber collimator is w < 0.39 mm.

[0094] Specifically, the preparation method of the gradient refractive index optical fiber collimator includes:

[0095] S201: splicing a single-mode optical fiber (SMF) and a coreless optical fiber (CLF) to obtain a first spliced ​​optical fiber;

[0096] S202: cutting the coreless optical fiber in the fusion-spliced ​​optical fiber to a length of L1;

[0097] S203: fusing the coreless optical fiber in the cut first fusion-splice optical fiber with a graded-index multimode optical fiber (GIF) to obtain a second fusion-splice optical fiber;

[0098] S204: Cut the graded-index multimode optical fiber in the second fusion-spliced ​​optical fiber to a length of L2 to obtain a graded-index optical fiber collimator; the final preparation result is as follows: Figure 5 (a) shown.

[0099] In this embodiment, the requirement for cutting accuracy is relatively high, and a precision translation stage is required to control the position of the optical fiber.

[0100] S205: Use a beam quality analyzer (Thorlabs BP209) to test the collimation performance of the cut (prepared) gradient refractive index fiber collimator.

[0101] In this embodiment, the collimation performance test results are as follows: Figure 5 As shown in (d), the GRIN-FC collimation performance is significantly improved compared to the direct output of single-mode fiber at a light source wavelength of 1392nm. At a collimation distance of 20mm, the spot diameter can be smaller than the MR inner diameter of 0.6mm, meeting the optical design requirements of the QEPAS system.

[0102] 2. Package Structure

[0103] Since the diameter of GRIN-FC is extremely small, it can be seamlessly inserted into the MR and fixed. Figure 5 (b) and Figure 5 (c) The packaging method of GRIN-FC and MR is shown. In this embodiment, the gradient refractive index fiber collimator is inserted into the first acoustic microresonator and packaged and fixed through the packaging structure; the structure uses three stainless steel capillaries (T1, T2 and MR) nested with each other and fixed with epoxy resin glue. Among them, T1 (inner diameter ID = 1.1mm, outer diameter OD = 1.3mm) is used to fix the outer plastic sheath of the optical fiber; T2 (inner diameter ID = 0.2mm, outer diameter OD = 0.5mm) is used to fix and protect the GRIN-FC cladding, while ensuring its precise horizontal centering in the MR; MR (inner diameter ID = 0.6mm, outer diameter OD = 0.8mm) includes an optional slotted window for observing the laser beam state, and a small sound transmission hole with a diameter of d to achieve acoustic coupling with the QTF.

[0104] The first acoustic microresonator and the second acoustic microresonator are both provided with sound transmission holes for realizing acoustic coupling with a quartz tuning fork, so that when a collimated laser beam (laser) passes through the first acoustic microresonator and the second acoustic microresonator, it can pass over the cantilever of the quartz tuning fork and amplify the photoacoustic signal generated by the excitation, and then convert the photoacoustic signal generated by the excitation into a current signal through the quartz tuning fork.

[0105] Specifically, the packaging structure includes a first stainless steel capillary (T1) and a second stainless steel capillary (T2); the inner side of the first stainless steel capillary is nested on the outer plastic sheath of the gradient refractive index optical fiber collimator; the inner side of the second stainless steel capillary is nested on the cladding of the gradient refractive index optical fiber collimator; the outer side of the second stainless steel capillary, one end close to the first stainless steel capillary, is nested with the inner side of the first stainless steel capillary; the outer side of the second stainless steel capillary, one end away from the first stainless steel capillary, is nested with the inner side of the first acoustic microresonator; the optical fiber in the gradient refractive index optical fiber collimator passes through the first stainless steel capillary and the second stainless steel capillary in sequence and then enters the first acoustic microresonator.

[0106] The present invention inserts the graded refractive index fiber collimator into the acoustic microresonator through a packaging structure and packages it. Compared with the traditional QEPAS system in which the G-lens and MR are installed separately, this structure is not only more compact, but also has better long-term mechanical stability. In addition, the optical axis calibration process of the laser beam and MR is eliminated, greatly simplifying the installation and debugging procedures of QEPAS.

[0107] 3. Signal modulation and demodulation system

[0108] In order to verify the measurement performance of the gas probe, the following Figure 6 (a) and Figure 6 (b) shows the QEPAS signal modulation and demodulation system. The system includes a laser constant temperature control and drive unit, a transimpedance amplifier (TIA), and a DDS signal generator, ADC, and a digital lock-in amplifier implemented by the digital-analog circuit on a single-chip microcomputer (MCU).

[0109] The MCU model is STMicroelectronics STM32H743. Its built-in dual-channel 12-bit 1Msps DAC is used to construct a DDS to output a 1Hz triangle wave and a sine wave of 1 / 2QTF resonance frequency to modulate and drive the laser, which are used to tune the laser wavelength and establish QTF photoacoustic resonance. The QTF converts the generated photoacoustic signal into a current signal, which is then converted into a voltage by a 10MΩ TIA, and then collected using the MCU's built-in 16-bit 1Msps ADC. Finally, the fully digital lock-in amplifier (DLIA) running on the MCU chip demodulates the amplitude of the second harmonic (2f) with concentration information, and multiplies it with the calibration coefficient to obtain the gas concentration. Since the system fully utilizes the on-chip resources of the MCU, the circuit cost is only about US$80.

[0110] Specifically, the signal modulation and demodulation system includes:

[0111] A digital-to-analog conversion circuit is used to control the intensity and modulation frequency of the laser and generate the modulation signal required to drive the laser;

[0112] Transimpedance amplifier, used to convert the weak current signal output by the quartz tuning fork into a voltage signal for subsequent processing;

[0113] A differential analog-to-digital converter, used for collecting the voltage signal converted by the transimpedance amplifier and converting it into a digital signal;

[0114] The fully digital lock-in amplifier is used to demodulate the second harmonic amplitude with concentration information from the collected voltage signal, and multiply the second harmonic amplitude with the calibration coefficient to obtain the gas concentration.

[0115] 4. Experimental Description

[0116] In order to ensure the safety of the experimental process, water vapor was selected as the target measurement object in this experiment. Its absorption intensity at the characteristic peak of 1392.53nm is 1.50cm / mol. The average optical power of the laser is about 10mW and the line width is 3MHz. The temperature and humidity standard generator Rotronic RHPM550 is used to control the water vapor concentration and ambient temperature.

[0117] In the experiment, the parameters of MR were first tuned. A large number of studies have shown that the introduction of MR in the QEPAS system can effectively improve the intensity of the photoacoustic signal. Its characteristic parameters have a significant impact on the detection sensitivity, such as the inner diameter ID, length LMR, photoacoustic coupling hole size d, etc. Since the probe structure studied in this article is quite different from the traditional QEPAS sensor, it is necessary to perform experimental optimization on LMR and d, which have a greater impact on the sensitivity. The target gas is set as static air with constant temperature and humidity (25°C, 35RH), and the water vapor concentration is calculated to be 1.09%. First, fix LMR = 8mm, and test different d, such as Figure 7 As shown in (a), the optimal d value is 0.3 mm. Then, d = 0.3 mm is fixed, and LMR is tested in the range of 6 to 10 mm, and the optimal MR length LMR = 7.5 mm is obtained. Figure 7 (b) as shown.

[0118] In order to verify the detection performance of the QEPAS gas probe, the static air at a constant temperature of 25°C and 35RH is still used as the measurement target. The second harmonic amplitude of GRIN-FC in three measurement structures, namely, bare tuning fork, single MR, and dual MRs, is compared with the off-axis QEPAS structure composed of conventional G-lens. Figure 8 As shown. Among them, the nominal working distance of G-lens is 20mm and the optical waist diameter is 0.3mm. The results are shown in Fig. 9 As shown, it can be seen that GRIN-FC has a 1.26-fold sensitivity improvement compared to G-lens under single MR conditions; GRIN-FC dual MRs has a 1.72-fold performance improvement compared to G-lens single MR; dual MRs can improve sensitivity by 26 times compared to a bare tuning fork.

[0119] In order to obtain the ultimate detection limit (MDL) and normalized noise equivalent absorption coefficient (NNEA) of the QEPAS gas probe (double MRs structure under GRIN-FC) for water vapor, the central wavelength of the light source is adjusted to a position away from the water vapor absorption peak by adjusting the operating temperature of the laser. The signal obtained at this time can be considered as the background noise of the system. The DLIA equivalent integration time is set to about 250ms, and 1000 points are measured continuously at an output rate of 1Hz. The noise intensity sequence obtained is as follows: Fig.10As shown. The calculated noise standard deviation 1σ is about 0.1734mV. Similarly, the 1σ values ​​of the bare tuning fork, single resonant tube GRIN-FC, and single resonant tube Grin-lens are 0.1652mV, 0.1688mV, and 0.1774mV, respectively. According to the MDL calculation method in the preliminary study, the MDL can be obtained by calculating only the gas concentration and signal-to-noise ratio in the concentration. According to the test results of the 2f amplitude above, the 2f signal-to-noise ratio (SNR) of the structures are 605.32, 11315.17, 14994.23, and 8511.84, respectively. It can be concluded that the MDL of the bare tuning fork, single resonant tube GRIN-FC, double resonant tube GRIN-FC, and single resonant tube G-lens QEPAS system for water vapor (H2O) are approximately 21.56ppm, 0.98ppm, 0.73ppm, and 1.29ppm, respectively. Since the average optical power of the system is 10 mW and the detection bandwidth of the system is 10 Hz, the NNEA of each structure is 32.72×10-9cm-1·W·Hz-1 / 2, 2.32×10-9cm-1·W·Hz-1 / 2, 1.75×10-9cm-1·W·Hz-1 / 2, and 1.32×10-9cm-1·W·Hz-1 / 2, respectively, as shown in Table 1. In summary, compared with the traditional structure, the proposed gas probe maintains superior detection sensitivity while significantly reducing the volume.

[0120] Table 1 Comparison of performance of sensors of various structures

[0121]

[0122] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. QEPAS probe gas sensor based on graded refractive index fiber collimation, characterized in that: include: A graded refractive index fiber collimator is used to receive and collimate the laser light emitted by the laser to form a collimated laser beam; The first acoustic microresonator is used to guide the collimated laser beam into the sensor to excite the gas to generate a photoacoustic signal, and amplify the acoustic signal through acoustic wave resonance to enhance the intensity of the photoacoustic signal; a right-angle reflector, used for receiving the collimated laser beam output by the first acoustic microresonator and flipping its propagation direction; The second acoustic microresonator is used to guide the collimated laser beam after flipping the propagation direction to continue to excite the gas to be measured inside the sensor to generate a photoacoustic signal, and further enhance the intensity of the photoacoustic signal by amplifying the acoustic signal through acoustic wave resonance; A quartz tuning fork is disposed between the first acoustic microresonator and the second acoustic microresonator, and is used to detect the photoacoustic signal generated when the first acoustic microresonator and the second acoustic microresonator transmit laser light, and convert the photoacoustic signal into a current signal; The signal modulation and demodulation system is used to demodulate and calculate the gas concentration based on the current signal converted by the quartz tuning fork.

2. The QEPAS probe gas sensor based on graded-index optical fiber collimation according to claim 1, characterized in that: The graded-refractive-index optical fiber collimator comprises a single-mode optical fiber, a coreless optical fiber and a graded-refractive-index multimode optical fiber which are connected end to end in sequence.

3. The QEPAS probe gas sensor based on graded-index optical fiber collimation according to claim 2, characterized in that: Maximum length of coreless fiber Where Φ is the core diameter of the graded-index multimode fiber, and θ is the divergence angle of the single-mode fiber.

4. The QEPAS probe gas sensor based on graded-index optical fiber collimation according to claim 3, characterized in that: The length L1 of the coreless optical fiber is 400-520 μm; the length L2 of the graded-index multimode optical fiber is 1410-1530 μm; and the minimum spot diameter of the graded-index optical fiber collimator is w<0.39 mm.

5. The QEPAS probe gas sensor based on gradient index optical fiber collimation according to claim 1, characterized in that: Inserting the graded refractive index fiber collimator into the first acoustic microresonator through the packaging structure and packaging and fixing it; The packaging structure includes a first stainless steel capillary and a second stainless steel capillary; the inner side of the first stainless steel capillary is nested on the outer plastic sheath of the gradient refractive index optical fiber collimator; the inner side of the second stainless steel capillary is nested on the cladding of the gradient refractive index optical fiber collimator; an end of the outer side of the second stainless steel capillary close to the first stainless steel capillary is nested with the inner side of the first stainless steel capillary; an end of the outer side of the second stainless steel capillary away from the first stainless steel capillary is nested with the inner side of the first acoustic microresonator; The optical fiber in the graded refractive index optical fiber collimator passes through the first stainless steel capillary and the second stainless steel capillary in sequence and then enters the first acoustic microresonator.

6. The QEPAS probe gas sensor based on graded-index optical fiber collimation according to claim 1, characterized in that: The first acoustic microresonator and the second acoustic microresonator are both provided with sound transmission holes for realizing acoustic coupling with a quartz tuning fork, so that when the collimated laser beam passes through the first acoustic microresonator and the second acoustic microresonator, it can pass over the cantilever of the quartz tuning fork to enhance the photoacoustic signal generated by the excitation, and then the photoacoustic signal generated by the excitation is converted into a current signal through the quartz tuning fork.

7. The QEPAS probe gas sensor based on graded-index optical fiber collimation according to claim 1, characterized in that: The signal modulation and demodulation system includes: A digital-to-analog conversion circuit is used to control the intensity and modulation frequency of the laser and generate the modulation signal required to drive the laser; A transimpedance amplifier, used to convert the current signal output by the quartz tuning fork into a voltage signal; A differential analog-to-digital converter, used for collecting the voltage signal converted by the transimpedance amplifier and converting it into a digital signal; The fully digital lock-in amplifier is used to demodulate the second harmonic amplitude with concentration information from the collected voltage signal, and multiply the second harmonic amplitude with the calibration coefficient to obtain the gas concentration.

8. The QEPAS probe gas sensor based on graded-index optical fiber collimation according to claim 1, characterized in that: Also includes: The micro photodiode is used to receive the laser emitted by the second acoustic microresonator and monitor the optical path fault through the laser.

9. QEPAS probe gas sensor detection method based on graded refractive index optical fiber collimation, characterized in that: The QEPAS gas probe sensor according to claim 1 is implemented as follows: S1: Receive the laser light emitted by the laser through the graded refractive index fiber collimator and collimate it to form a collimated laser beam; S2: guiding the collimated laser beam to transmit inside the sensor through the first acoustic microresonator, and exciting the gas to be measured to generate a photoacoustic signal; S3: receiving the collimated laser beam transmitted by the first acoustic microresonator through a right-angle mirror and flipping its propagation direction; S4: The collimated laser beam is guided again through the second acoustic microresonator to transmit inside the sensor, and continues to excite the gas to generate a photoacoustic signal; S5: detecting the photoacoustic signal generated when the first acoustic microresonator and the second acoustic microresonator transmit laser light by using a quartz tuning fork disposed between the first acoustic microresonator and the second acoustic microresonator, and converting the photoacoustic signal into a current signal; S6: Calculate the gas concentration based on the demodulation of the current signal converted by the quartz tuning fork.

10. The QEPAS probe gas sensor detection method based on graded-index optical fiber collimation according to claim 1, characterized in that: In step S2, the method for preparing a gradient refractive index optical fiber collimator includes: S201: splicing the single-mode optical fiber and the coreless optical fiber to obtain a first spliced ​​optical fiber; S202: cutting the coreless optical fiber in the first fusion-spliced ​​optical fiber to a length of L1; S203: fusing the coreless optical fiber in the cut first fusion-splice optical fiber with the graded-index multimode optical fiber to obtain a second fusion-splice optical fiber; S204: cutting the graded-index multimode optical fiber in the second fusion-spliced ​​optical fiber to a length of L2 to obtain a graded-index optical fiber collimator; S205: Use a beam quality analyzer to test the collimation performance of the cut gradient refractive index fiber collimator.

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