Fabry-Perot optical fiber photoacoustic gas sensor, manufacturing method and sensing system

Abstract

The invention discloses a Fabry-Perot optical fiber photoacoustic gas sensor, a manufacturing method and a sensing system. The sensor comprises an anti-resonance hollow-core optical fiber, one end of the anti-resonance hollow-core optical fiber is in butt joint with the single-mode optical fiber, the other end of the anti-resonance hollow-core optical fiber is coupled with the acoustic sensing thin film, and the length of the anti-resonance hollow-core optical fiber is centimeter The acoustic sensing film is adhered to one side surface of the ceramic sleeve to form a reflecting surface of the Fabry-Perot interferometer; the tail end of the anti-resonance hollow-core optical fiber and the end face of the single-mode optical fiber form another reflecting surface, and the anti-resonance hollow-core optical fiber and the single-mode optical fiber jointly form a long-cavity Fabry-Perot interferometer; the ceramic sleeve is also used for installing a ceramic ferrule. The ceramic ferrule is used for installing the anti-resonance hollow-core optical fiber. The sensing system comprises a long-cavity Fabry-Perot interference probe, a photoacoustic cell, a pumping excitation module, a photoacoustic detection module and a signal control and processing module. According to the invention, high-sensitivity, low-cost and anti-interference acoustic signal detection and gas concentration detection can be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photoacoustic spectroscopy gas sensing, and in particular to a long-cavity Fabry-Pérot (FP) fiber-optic photoacoustic gas sensor based on an anti-resonant hollow core fiber (AR-HCF), a manufacturing method thereof, and a sensing system, which are suitable for high-sensitivity, anti-interference gas concentration detection and noise analysis scenarios. BACKGROUND

[0002] In photoacoustic spectroscopy gas detection, a fiber-optic microphone as a core acoustic detection unit usually adopts a Fabry-Pérot interference structure. In the prior art, the fiber-optic microphone usually adopts an additional reflective diaphragm or a cantilever beam to form a FP cavity with the end face of the fiber. The cavity length is usually in the range of tens to hundreds of microns. Such a short-cavity structure results in a large free spectral range (FSR) (nanometer level). In order to obtain a complete interference period, a spectral demodulation device with a wide spectral range and a high scanning speed needs to be used. However, a high-resolution spectrometer is expensive and has a low scanning speed (Hz level). Although a scheme based on a tunable laser or a high-speed detector array can achieve a scanning speed of KHz, the structure is complex and the cost is high, which limits its application in portable and low-cost systems.

[0003] In addition, the traditional FP interferometer is easily affected by the cavity length drift caused by the change in the ambient temperature, resulting in signal crosstalk. In the prior art, although the temperature interference can be suppressed by phase-locked amplification and other methods, the problem has not been fundamentally solved from the perspective of the fiber structure.

[0004] Therefore, there is an urgent need for a new long-cavity fiber FP sensor that has the characteristics of high sensitivity, low cost, anti-interference capability, and easy demodulation. SUMMARY

[0005] The present application aims to provide a long-cavity Fabry-Pérot fiber-optic photoacoustic gas sensor based on an anti-resonant hollow core fiber (AR-HCF) and a manufacturing method and a sensing system thereof. By constructing a centimeter-level long-cavity FP structure, combining the anti-temperature crosstalk characteristics and low numerical aperture advantages of the AR-HCF, high-sensitivity, low-cost, and anti-interference acoustic signal detection and gas concentration detection are achieved.

[0006] The technical solutions of the present application are as follows: A Fabry-Pérot fiber-optic photoacoustic gas sensor comprises: an anti-resonant hollow core fiber (AR-HCF) coupled at one end to a single-mode fiber and at the other end to an acoustic-sensitive film, wherein the length of the anti-resonant hollow core fiber is centimeter-level; the acoustic-sensitive film is adhered to one side of a ceramic sleeve to form one reflective surface of a Fabry-Pérot interferometer; The end of the anti-resonant hollow core fiber and the end face of the single mode fiber form another reflecting surface, and the two form a long cavity Fabry-Perot interferometer together. The ceramic sleeve is also used for mounting the ceramic ferrule, and further fixed by glue. The ceramic ferrule is used for mounting the anti-resonant hollow core fiber, and further fixed by glue.

[0007] The acoustic sensitive film is a graphene film, a metal film, a silicon film or a polymer film.

[0008] The numerical aperture of the anti-resonant hollow core fiber is less than 0.1, which is used for improving the light precision and interference signal quality between the acoustic sensitive film.

[0009] The manufacturing method of the sensor comprises the following steps: 1) Preparing the acoustic sensitive film and transferring it to one side of the second ceramic ferrule sleeve; 2) Inserting the first ceramic ferrule into the other side of the second ceramic ferrule sleeve and fixing it by glue; 3) Mounting the anti-resonant hollow core fiber in the first ceramic ferrule, adjusting the relative position of the end face of the anti-resonant hollow core fiber and the acoustic sensitive film under the microscope and real-time spectrum monitoring, and fixing it by glue after maximizing the contrast of the reflected spectrum; 4) Inserting the other side of the anti-resonant hollow core fiber into the second ceramic ferrule and fixing it by glue; 5) Inserting the single mode fiber into the third ceramic ferrule and fixing it by glue; 6) Mounting the second ceramic ferrule and the third ceramic ferrule into the two sides of the second ceramic ferrule sleeve to realize butt joint, adjusting the coupling gap between the anti-resonant hollow core fiber mounted in the second ceramic ferrule and the single mode fiber mounted in the third ceramic ferrule to realize the best interference signal, and fixing the second ceramic ferrule, the third ceramic ferrule and the second ceramic ferrule sleeve by glue.

[0010] The acoustic sensitive film is a graphene film, which is separated from a nickel substrate by chemical etching, treated by FeCl3 solution and washed, and then transferred into the ceramic sleeve.

[0011] A Fabry-Perot fiber photoacoustic gas sensing system comprises: A long cavity Fabry-Perot interferometer probe comprising the sensor; A photoacoustic cell for containing the gas to be measured; A pump excitation module for receiving a first synchronization signal from a signal control and processing module and outputting a modulated pump light to excite the gas to generate acoustic waves; The photoacoustic detection module comprises a detection light excitation module, a fiber circulator and a photodetector, receives a second synchronization signal from the signal control and processing module and outputs modulated detection light; The signal control and processing module is used for generating a synchronization signal, receiving a photodetector output signal, performing cross-correlation analysis, filter processing and harmonic extraction, and outputting a gas concentration response.

[0012] The signal control and processing module further comprises a temperature disturbance extraction module used for extracting a low-frequency disturbance signal and calibrating the gas concentration response to realize temperature crosstalk suppression.

[0013] The signal control and processing module acquires a cross-correlation peak value index drift signal by cross-correlation analysis of adjacent frame interference spectra, and extracts a target acoustic signal by integral operation and phase-locked amplification technology.

[0014] The pump excitation module comprises a DFB laser with a working wavelength of 1653.7 nm; The detection light excitation module comprises a DFB laser with a working wavelength of 1550 nm.

[0015] An optical photoacoustic gas detection method, comprising the following steps: a) Modulating pump light to excite a gas to be measured to generate sound waves; b) Modulating detection light to be injected into a sensor through a fiber circulator and reflected back by an interference probe; c) A photodetector converts the reflected light into an electrical signal; d) A signal control and processing module performs cross-correlation analysis on the electrical signal to obtain a peak value index drift of adjacent frame spectra; e) An integral operation and phase-locked amplification are performed on the drift signal to extract sound wave information; f) After filter processing and harmonic extraction, a gas concentration response is obtained, and a temperature disturbance signal can be extracted simultaneously for calibration.

[0016] The present application has the following advantages: 1. A centimeter-level AR-HCF is used to construct a long-cavity FP interferometer, and the free spectral range is extremely small (μm level), so that a low-cost wavelength tunable semiconductor laser can complete spectral scanning and demodulation, replacing an expensive spectrometer; 2. The low thermal expansion coefficient of the AR-HCF hollow core structure significantly improves the temperature stability and reduces the temperature crosstalk; 3. The low numerical aperture characteristic of the AR-HCF helps to accurately align the light with the diaphragm, improving the quality of the interference signal; 4. The cross-correlation and phase-locked amplification technology are combined to realize high signal-to-noise ratio and high sensitivity acoustic signal detection; 5. The application is suitable for photoacoustic spectroscopy gas detection and weak acoustic fingerprint monitoring, and expands the application range. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 : Structure diagram of long-cavity Fabry-Perot photoacoustic gas sensor. Figure 1 In the application, the anti-resonant hollow core fiber 100, the first reflecting surface 101, the second reflecting surface 102, the first ceramic ferrule sleeve 1031, the second ceramic ferrule sleeve 1032, the first ceramic ferrule 1041, the second ceramic ferrule 1042 and the third ceramic ferrule 1043, and the single-mode optical fiber 105.

[0018] Figure 2 : Structure diagram of sensor manufacturing process.

[0019] Figure 3 : Overall structure diagram of sensing system. Figure 3 In the application, the long-cavity Fabry-Perot interference probe 1, the signal control and processing module 2, the pump excitation module 3, the photoacoustic cell 4, the photoacoustic detection module 5, the gas concentration signal 6, the first synchronization signal 201, the second synchronization signal 202, the detection light excitation module 501 (including the detection light driving component 5011 and the detection light laser component 5012), the optical fiber circulator 502, and the photodetector 503.

[0020] Figure 4 : Real sensor sample photo.

[0021] Figure 5 (a): Sensor frequency response.

[0022] Figure 5 (b): Sound pressure test results at 1700 Hz.

[0023] Figure 6 (a): Sensing output curve of different concentrations of methane gas.

[0024] Figure 6 (b): Detection limit. DETAILED DESCRIPTION

[0025] The application will be described in detail below in conjunction with the drawings and examples, but should not be understood as limiting the application.

[0026] As shown in the drawings, the sensor comprises: Figure 1 an anti-resonant hollow core fiber (AR-HCF) 100 which is connected to the single-mode optical fiber 105 at one end and coupled to the sound-sensitive sensing film at the other end, and has a length of 1-20 centimeters, preferably 8 centimeters; a sound-sensitive sensing film, preferably graphene, metal film, silicon film or polymer film, which is adhered to one side of the ceramic sleeve to form the first reflecting surface 101 of the FP cavity; a sound-sensitive sensing film, preferably graphene, metal film, silicon film or polymer film, which is adhered to one side of the ceramic sleeve to form the first reflecting surface 101 of the FP cavity; The end of the AR-HCF forms a second reflecting surface 102 with the end face of the single-mode optical fiber 105, and the two together constitute a long-cavity FP interferometer. The ceramic sleeve and the ceramic ferrule are used to fix the AR-HCF and the sound-sensitive sensing film, and after precise alignment, the sound-sensitive sensing film is fixed by using ultraviolet curing glue. The overall structure of the sensor is an exogenous FP cavity, that is, the two reflecting surfaces are located at the two ends of the AR-HCF, and a stable interference is formed through air gap or small gap coupling.

[0027] As shown in Figure 1 , 2 The method for manufacturing the sensor comprises the following steps: 1) Preparing a sound-sensitive sensing film and transferring it to one side of a first ceramic ferrule sleeve 1031; 2) Inserting a first ceramic ferrule 1041 into the other side of the first ceramic ferrule sleeve 1031 and fixing it with glue; 3) Installing the anti-resonant hollow core fiber 100 in the first ceramic ferrule 1041, adjusting the relative position of the end face of the anti-resonant hollow core fiber 100 and the sound-sensitive sensing film under a microscope and real-time spectrum monitoring, and fixing it with glue after maximizing the contrast of the reflected spectrum; 4) Inserting the other side of the anti-resonant hollow core fiber 100 into a second ceramic ferrule 1042 and fixing it with glue; 5) Inserting a single-mode optical fiber 105 into a third ceramic ferrule 1043 and fixing it with glue; 6) Installing the second ceramic ferrule 1042 and the third ceramic ferrule 1043 into the two sides of a second ceramic ferrule sleeve 1032 to realize butt joint; adjusting the coupling gap between the anti-resonant hollow core fiber 100 installed in the second ceramic ferrule 1042 and the single-mode optical fiber 105 installed in the third ceramic ferrule 1043 to realize the best interference signal, and then fixing the second ceramic ferrule 1042, the third ceramic ferrule 1043 and the second ceramic ferrule sleeve 1032 with glue.

[0028] The sensing system comprises: a long-cavity Fabry-Perot interferometric probe (i.e. the above-mentioned sensor); a photoacoustic cell containing a gas to be measured; a pump excitation module outputting modulated pump light to excite the gas to generate sound waves; a photoacoustic detection module comprising a detection light excitation module, a fiber loop, and a photodetector; a signal control and processing module for generating a synchronization signal, receiving the output signal of the photodetector, and performing cross-correlation analysis, filtering processing, and harmonic extraction, and finally outputting a gas concentration response.

[0029] Signal processing method The interference spectrum sequence is collected, the cross-correlation algorithm is used to process the adjacent frame spectrum, the cross-correlation peak value index drift is obtained, the drift signal is integrated, the FP cavity length change is obtained, the target signal is extracted by using the phase-locked amplification technology, the low-frequency temperature crosstalk is suppressed, the sound wave information is further extracted by the band-pass filtering, the gas concentration response is obtained through the harmonic extraction, and the temperature disturbance signal can be synchronously extracted for calibration.

[0030] Example 1: Sensor fabrication As shown in Figure 2 An 8 cm long AR-HCF segment and two reflective surfaces are used to form a long-cavity FP sensor. When the light propagates in the gas core region of the AR-HCF, two reflective interfaces are formed at the end of the single-mode fiber (SMF) and the transferred graphene film surface.

[0031] 1) The graphene film is first separated from the nickel substrate by chemical etching and transferred to one side of the first ceramic ferrule sleeve 1031; 2) The first ceramic ferrule 1041 is inserted from the other side of the first ceramic ferrule sleeve 1031 and fixed with ultraviolet curing glue; 3) The anti-resonant hollow fiber 100 is installed in the first ceramic ferrule 1041, and the relative position of its end face and the sound-sensitive sensing film is adjusted under a microscope and real-time spectrum monitoring, so that the reflection spectrum contrast is maximized, and then fixed with ultraviolet curing glue; 4) The other side of the anti-resonant hollow fiber 100 is inserted into the second ceramic ferrule 1042 and fixed with ultraviolet curing glue; 5) The single-mode fiber 105 is inserted into the third ceramic ferrule 1043 and fixed with ultraviolet curing glue; 6) The second ceramic ferrule 1042 and the third ceramic ferrule 1043 are installed into the two sides of the second ceramic ferrule sleeve 1032 to realize butt joint; the coupling gap between the anti-resonant hollow fiber 100 installed in the second ceramic ferrule 1042 and the single-mode fiber 105 installed in the third ceramic ferrule 1043 is adjusted to realize the best interference signal, and then the second ceramic ferrule 1042, the third ceramic ferrule 1043 and the second ceramic ferrule sleeve 1032 are fixed with ultraviolet curing glue.

[0032] Example 2: Sensing system As shown in Figure 3As shown, the signal control and processing module 2 can set and adjust the first synchronization signal 201 and the second synchronization signal 202 output by the data module; the pump excitation module 3 can receive the first synchronization signal 201 from the signal control and processing module 2 and output the corresponding modulated pump light; when the gas to be detected exists in the photoacoustic cell 4, the acoustic pressure field can be formed in the cavity under the action of the modulated pump light; the photoacoustic detection module 5 further includes a detection light excitation module 501 (including a detection light driving part 5011 and a detection light laser part 5012), a fiber loop 502, and a photodetector 503. The detection light excitation module can receive the second synchronization signal 202 from the signal control and processing module 2 and output the corresponding modulated detection light, and the output modulated detection light is connected to the long-cavity Fabry-Perot interference probe 1 through the fiber loop 502; the light reflected by the long-cavity Fabry-Perot interference probe 1 passes through the fiber loop 502 and is converted into an electrical signal by the photodetector 503. The photodetector 503 can input the electrical signal to the signal control and processing module 2 to realize data acquisition, and finally the signal control and processing module 2 processes the input electrical signal collected and outputs the gas concentration response 6.

[0033] The signal processing process of the signal control and processing module 2 further includes a cross-correlation analysis part, a filter processing part, and a harmonic extraction part. The collected spectral information is processed by the cross-correlation analysis part, the disturbance condition of the acoustic sensitive film can be obtained, the disturbance information of different frequencies is demodulated, and the influence of temperature crosstalk on the response of the long-cavity Fabry-Perot interference probe 1 is solved from the root. The demodulated disturbance information can be extracted by the filter processing part, and the disturbance information at high frequency sawtooth frequency is extracted, and the gas concentration response is obtained through the harmonic extraction part. The filter processing part can also extract low-frequency disturbance information to obtain temperature disturbance. The obtained temperature disturbance can also calibrate the gas concentration response, thereby solving the influence of temperature disturbance on the photoacoustic signal.

[0034] Example 3: Methane gas detection A real long-cavity fiber microphone (as shown in Figure 4 ).

[0035] Acoustic wave sensing performance calibration: To obtain the frequency response of the fiber optic microphone sample, the output voltage of the audio sweep frequency generator was fixed at 0.3 V, and the output frequency was adjusted from 800 Hz to 3700 Hz. The sensor response at a certain frequency was obtained by averaging the peak-to-peak values ​​of 15 consecutive cross-correlation peak index drift signals. Furthermore, to avoid noise interference, a bandwidth filter of ±200 Hz was applied to the cross-correlation peak drift data at each frequency during acquisition. After demodulation, the acquired acoustic signals were used to calculate the peak-to-peak values ​​of three consecutive cross-correlation peak index drift points using Matlab, resulting in the sensor sample frequency response test data shown in Figure 5(a). The frequency response test results indicate that the sensor sample has the maximum response at the resonant frequency of 1700 Hz. Therefore, during photoacoustic spectroscopy detection, a photoacoustic cell corresponding to the resonant frequency should be designed to form a standing wave within the cell for resonant detection of the photoacoustic spectral signal.

[0036] To quantify the linear response characteristics of the fiber optic microphone sample, a stepped sound pressure level (SPL) test was conducted at the sensor's resonant frequency of 1700 Hz. Calibration SPL signals ranging from 16.6 to 103.8 mPa were generated using a loudspeaker audio sweep frequency generator. Interference spectral sequences from the sensor output were acquired, and a cross-correlation algorithm was used to extract the peak index drift between adjacent images. The mean peak-to-peak value of the drift points under each SPL set was calculated (to eliminate random jitter errors), ultimately yielding the sensor input-output curve shown in Figure 5(b).

[0037] Equipment parameters used in methane sensor performance calibration: 1. The laser operating wavelength of the pump excitation module is 1653.7 nm DFB laser; 2. The laser operating wavelength of the detection light excitation module is 1550 nm DFB laser; 3. Sampling rate of the signal control and processing module: 250 MHz.

[0038] The final performance metrics achieved are: The sensitivity is 0.00177 ppm; (as shown in Figure 6(a)) The detection limit is 2.72 ppm (when the integration time is 300 s, as shown in Figure 6(b)).

[0039] The core innovations of this invention are threefold: A) It achieves a long cavity structure with an extremely small FSR, enabling the replacement of expensive OSA spectrometers with low-cost tunable semiconductor lasers for spectral scanning and demodulation.

[0040] B) It utilizes the inherent properties of AR-HCF, where the natural gas core structure provides inherent stability against temperature crosstalk.

[0041] C) The lower NA of AR-HCF allows for the formation of low-divergence beams within the cavity, improving the quality of the interference signal. This design integrates a unique fiber platform, long cavity technology, and cost-effective demodulation scheme into a unified and innovative acoustic sensing platform.

[0042] This invention can be applied to the fields of photoacoustic spectroscopy gas detection and noise analysis, such as listening to the faint characteristic acoustic patterns generated by pipeline leaks.

[0043] The embodiments described above can be further combined or replaced, and these embodiments are merely descriptions of preferred embodiments of the present invention, not limitations on the concept and scope of the present invention. Various changes and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the inventive concept are all within the protection scope of the present invention. The protection scope of the present invention is given by the appended claims and any equivalents.

Claims

1. A Fabry-Perot fiber optic photoacoustic gas sensor, characterized in that, include: An anti-resonant hollow fiber (AR-HCF) with one end connected to a single-mode fiber and the other end coupled to an acoustically sensitive thin film, wherein the length of the anti-resonant hollow fiber is on the order of centimeters. The acoustically sensitive thin film is adhered to one side of the ceramic sleeve to form a reflective surface of the Fabry-Perot interferometer. The end of the anti-resonant hollow fiber and the end face of the single-mode fiber form another reflecting surface, which together form a long-cavity Fabry-Perot interferometer. The ceramic sleeve is also used to install the ceramic insert and is further secured with adhesive. The ceramic ferrule is used to install the anti-resonant hollow optical fiber and is further fixed by adhesive.

2. The sensor according to claim 1, characterized in that, The acoustically sensitive film is a graphene film, a metal film, a silicon film, or a polymer film.

3. The sensor according to claim 1, characterized in that, The numerical aperture of the anti-resonant hollow fiber is less than 0.1, which is used to improve the optical alignment accuracy and interference signal quality between the fiber and the acoustically sensitive thin film.

4. A method for manufacturing a sensor as described in any one of claims 1 to 3, characterized in that, Includes the following steps: 1) Prepare an acoustically sensitive thin film and transfer it to one side of the second ceramic ferrule sleeve; 2) Insert the first ceramic insert from the other side of the second ceramic insert sleeve and secure it with glue; 3) Install the anti-resonant hollow fiber in the first ceramic ferrule, and adjust the relative position of its end face and the acoustically sensitive film under microscope and real-time spectral monitoring to maximize the contrast of the reflection spectrum, and then fix it with glue. 4) Insert the other side of the anti-resonant hollow fiber into the second ceramic ferrule and fix it with glue; 5) Insert the single-mode fiber into the third ceramic ferrule and secure it with glue; 6) Install the second and third ceramic ferrules into both sides of the second ceramic ferrule sleeve to achieve docking; adjust the coupling gap between the anti-resonant hollow fiber installed in the second ceramic ferrule and the single-mode fiber installed in the third ceramic ferrule to achieve the best interference signal, and then use glue to fix the second ceramic ferrule, the third ceramic ferrule and the second ceramic ferrule sleeve.

5. The manufacturing method according to claim 4, characterized in that, The acoustically sensitive film is a graphene film, which is separated from the nickel substrate by chemical etching, treated with FeCl3 solution and rinsed before being transferred into a ceramic sleeve.

6. A Fabry-Perot fiber optic photoacoustic gas sensing system, characterized in that, include: A long-cavity Fabry-Perot interferometer probe, comprising the sensor according to any one of claims 1-3; A photoacoustic cell is used to contain the gas to be measured. The pump excitation module is used to receive the first synchronization signal from the signal control and processing module and output modulated pump light to excite the gas to generate sound waves. The photoacoustic detection module includes a detection light excitation module, an optical fiber circulator, and a photodetector. It receives a second synchronization signal from the signal control and processing module and outputs modulated detection light. The signal control and processing module is used to generate synchronization signals, receive output signals from photodetectors, perform cross-correlation analysis, filtering, harmonic extraction, and output gas concentration response.

7. The sensing system according to claim 6, characterized in that, The signal control and processing module also includes a temperature disturbance extraction module, which is used to extract low-frequency disturbance signals and calibrate the gas concentration response to achieve temperature crosstalk suppression.

8. The sensing system according to claim 6, characterized in that, The signal control and processing module obtains the cross-correlation peak index drift signal by cross-correlation analysis of the interference spectrum of adjacent frames, and then extracts the target acoustic signal by integration operation and lock-in amplification technology.

9. The sensing system according to claim 6, characterized in that, The pump excitation module includes a DFB laser with an operating wavelength of 1653.7 nm; The probe light excitation module includes a DFB laser with a working wavelength of 1550 nm.

10. A photoacoustic gas detection method, applied to the sensing system of claim 6, characterized in that, Includes the following steps: a) Modulate the pump light to excite the gas under test to generate sound waves; b) The modulated probe light is injected into the sensor via an optical fiber circulator and reflected back by the interferometric probe; c) The photodetector converts the reflected light into an electrical signal; d) The signal control and processing module performs cross-correlation analysis on the electrical signal to obtain the peak index shift of the spectrum between adjacent frames; e) Perform integration and lock-in amplification on the drift signal to extract acoustic information; f) After filtering and harmonic extraction, the gas concentration response is obtained, and the temperature disturbance signal can be extracted simultaneously for calibration.

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