A trace gas detection system based on anti-resonant hollow core fiber mode interferometer

Abstract

The application provides a trace gas detection system based on an anti-resonant hollow core fiber mode interferometer, and relates to the technical field of optical sensing, and comprises: a transmission module comprising a gas sensor, which is used for transmitting pump light and detection light, and reflecting the refractive index change caused by the photo-thermal effect on the phase change of the detection light through a photo-thermal interference (PTI) sensing technology; a gas chamber module, which is used for taking the fiber core as a gas chamber as a place for light and gas interaction; and a signal detection module comprising a photoelectric detector, which is used for converting the detection light into an electric signal. The application takes the air fiber core of the AR-HCF as a gas chamber, constructs a mode interference type sensor based on the AR-HCF micro unit, takes the air fiber core of the AR-HCF as a gas chamber, uses a femtosecond laser to open a hole on the side of the AR-HCF, increases the rate of air entering the fiber core, and demodulates the phase change generated when the gas spectrum absorption is demodulated through a phase-sensitive fiber interferometer, so that the gas composition and concentration information are obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical sensing technology, in particular to a trace gas detection system based on anti-resonant hollow core fiber mode interferometer. BACKGROUND

[0002] In recent years, with the rapid development of science and technology and economy, people's living standards have improved significantly, but the speed of resource consumption has accelerated, and environmental pollution problems have become increasingly serious. The toxic and harmful gases produced in the industrial production process have increased sharply, not only leading to frequent gas poisoning accidents, but also seriously polluting the atmospheric environment. Especially the waste gas such as sulfur dioxide and carbon monoxide discharged by power plants, steel plants and fertilizer plants is the main cause of air pollution and acid rain formation, which damages the ecological balance, leads to land acidification and nutrient loss, and seriously damages forests. At the same time, the flammable and explosive gas generated by internal faults of the transformer in the power system also threatens the performance and service life of the equipment. Therefore, developing a real-time harmful gas monitoring system, developing a new type of gas sensor, and timely detecting the content of harmful gases in the environment air has important significance for protecting national property, reducing mine casualties, optimizing industrial system design and atmospheric environment monitoring.

[0003] As a device that collects gas information and converts it into an easily identifiable signal, gas sensors play a key role in environmental monitoring and industrial safety. However, traditional gas sensors have poor anti-interference ability, low sensitivity and insufficient accuracy. Optical fiber gas sensors have unique advantages by measuring optical properties related to the measured gas to calculate the type and concentration of the gas. Among them, the gas sensor based on spectral absorption type hollow photonic crystal fiber (HC-PBF) has attracted much attention due to its high sensitivity. However, HC-PBF also has some defects in application, such as high loss when fused with single-mode optical fiber, high cost and non-compact structure due to long gas chamber length, and transmission loss and physical damage caused by side opening. SUMMARY

[0004] The technical problem to be solved by the present application is to provide a trace gas detection system and method based on anti-resonant hollow core fiber mode interferometer, which constructs a gas chamber on ARHCF and obtains composition and concentration information by demodulating the phase change caused by gas spectral absorption through a phase-sensitive fiber interferometer.

[0005] To solve the above technical problems, the technical scheme of the present application is as follows:

[0006] A trace gas detection system based on anti-resonant hollow core fiber mode interferometer, comprising:

[0007] The light source module comprises a pump light source and a probe light source, the pump light source is used to excite the photothermal effect of the measured gas, and the probe light source is used to generate single-wavelength laser far away from the gas absorption peak.

[0008] The transmission module comprises a gas sensor, and is used to transmit the pump light and the probe light and reflect the refractive index change caused by the photothermal effect to the phase change of the probe light through a photothermal interference type (PTI) sensing technology.

[0009] The gas chamber module is used to take the core as a gas chamber for the light and gas interaction.

[0010] The signal detection module comprises a photodetector, and is used to convert the probe light into an electrical signal.

[0011] The demodulation module comprises a lock-in amplifier, and is used to amplify and detect the electrical signal carrying the gas concentration information.

[0012] Further, the pump light source is used to excite the photothermal effect of the measured gas, and the probe light source is used to generate single-wavelength laser far away from the gas absorption peak, comprising:

[0013] The pump light source is a DFB laser modulated by sawtooth signals and sinusoidal signals generated by a waveform generator, and the output wavelength of the pump light of the DFB laser is tuned through external triggering, so as to excite the photothermal effect of the measured gas.

[0014] The probe light source is single-wavelength probe light output, so as to generate single-wavelength laser far away from the gas absorption peak.

[0015] Further, the core device of the gas sensor is a mode interferometer based on ARHCF, and the mode interferometer comprises a single-mode fiber (SMF), a graded-index multimode fiber (GIF) and an anti-resonant hollow core fiber (ARHCF).

[0016] Further, the gas sensor is used to transmit the pump light and the probe light, comprising:

[0017] The pump light and the probe light output by the light source enter the ARHCF through the SMF and the GIF, so as to excite high-order modes.

[0018] According to the high-order modes and the fundamental mode, the mode interferometer reflects the phase change of the probe light to the refractive index change caused by the photothermal effect through The interference between the fundamental mode and the high-order mode is used to obtain the phase change relationship of the refractive index, wherein n FM and n HM are effective refractive indexes of the fundamental mode and the high-order mode, respectively, and n extis the medium refractive index in the anti-resonant hollow core photonic bandgap fiber mode interferometer ARHC-PBF core, λ is the wavelength of the light wave, L is the length of the ARHCF, (2k+1)π represents the phase difference term, k is an integer.

[0019] Further, the preparation process of the mode interferometer is:

[0020] The SMF is fused with the GIF;

[0021] The GIF is cut off at a distance of 245m from the fusion point by using a precision cutting system, and two SMF-GIF samples are prepared;

[0022] The cut sample is fused with the ARHCF;

[0023] The ARHCF is cut off at a distance of 4.3cm from the fusion point by using a precision cutting system;

[0024] The cut sample is fused with the SMF-GIF sample prepared in the second step;

[0025] A hole is opened on the side of the ARHCF fiber, which specifically includes: connecting a spectrometer and a light source at both ends of the interferometer; placing the ARHCF fiber under a 20x objective lens, adjusting the focusing position, and the focusing position is located between the two holes of the ARHCF fiber core; setting the femtosecond laser energy and the laser irradiation time; during the irradiation process, the spectrum is observed, if the loss increases, it proves that the position of the hole has damaged the air hole of the core, and the sample preparation fails.

[0026] Further, the change of refractive index caused by the photothermal effect is reflected on the phase change of the probe light through the photothermal interference type (PTI) sensing technology, which includes:

[0027] The light paths of the probe light and the pump light are coaxial and collinear;

[0028] The wavelength of the pump light is modulated to excite the photothermal effect of the pump light;

[0029] According to the photothermal effect, the change of the refractive index of the gas is converted into the change of the refractive index of the gas through the interferometer;

[0030] According to the light paths of the probe light and the pump light, the change of the refractive index is converted into the change of the phase of the probe light through the photothermal interference type (PTI) sensing technology.

[0031] Further, the photothermal interference type (PTI) sensing technology includes:

[0032] The wavelength of the pump light is modulated to obtain wavelength-modulated pump light;

[0033] According to the wavelength-modulated pump light, the measured gas is excited to produce a photothermal effect;

[0034] The photo-thermal effect is loaded to the probe laser through the interferometer to obtain the phase change of the probe light;

[0035] The probe laser is subjected to harmonic demodulation processing to convert the phase change into intensity change.

[0036] Further, the anti-resonant hollow core fiber is used as the place for light and gas interaction, and the fiber core is used as the gas chamber, comprising:

[0037] The air core of the ARHCF is used to construct the gas chamber, and the side of the air core is perforated;

[0038] The modulated pump light beam is incident to the gas chamber filled with target gas molecules;

[0039] The target gas molecules are subjected to characteristic absorption on the pump light to obtain the change of various physical quantities of the gas molecules;

[0040] According to the analysis of the change of the physical quantities, the change amount of the refractive index of the gas is obtained;

[0041] According to the change amount of the refractive index, the optical path change of the probe light passing through the gas chamber is obtained;

[0042] According to the optical path change, the phase change result of the probe light is obtained through the optical interferometer;

[0043] The phase change result of the probe light is inversed to obtain the sample concentration and temperature information.

[0044] Further, the probe light is converted into an electric signal, comprising:

[0045] The pump light and the probe light are subjected to filtering together to obtain the probe light, and the probe light is input into the photodetector to be converted into an electric signal.

[0046] Further, the electric signal carrying the gas concentration information is amplified and detected, comprising:

[0047] The electric signal carrying the gas information is input into a lock-in amplifier to be amplified and detected to obtain the concentration information of the measured gas.

[0048] The above scheme of the present application at least has the following beneficial effects:

[0049] The trace gas detection system based on the anti-resonant hollow core fiber mode interferometer provided by the embodiment of the present application can indirectly detect the trace existence of the gas, realize high-sensitivity trace gas detection, and achieve several ppm level detection capability; the length of the gas chamber is significantly shortened to dozens of centimeters, the whole sensor structure is more compact, the AR-HCF has a larger fiber core diameter, the interaction efficiency between the gas and the light field is improved, the system can more quickly reflect the change of the gas concentration, and the system has strong detection capability for flammable and explosive gases such as methane and acetylene; the gas sensing unit adopts an all-fiber structure, has the advantages of small size, anti-electromagnetic interference, and low cost; the holes are accurately opened between two adjacent cladding holes on the AR-HCF fiber, the smooth entry of the gas is ensured, the periodic structure of the fiber is avoided from being damaged, the transmission loss is reduced, the gas entry rate into the gas chamber is improved, and the light-gas interaction time is greatly shortened; the single-mode fiber and the AR-HCF are realized through the fiber fusion process and parameter setting, and low-loss and high-strength fusion is realized. BRIEF DESCRIPTION OF DRAWINGS

[0050] Figure 1 is a flowchart of a trace gas detection system based on an anti-resonant hollow core fiber mode interferometer provided by an embodiment of the present application.

[0051] Figure 2 is a schematic diagram of a gas detection system based on an anti-resonant hollow core fiber mode interferometer provided by an embodiment of the present application.

[0052] Figure 3 is a schematic diagram of a photothermal interference type (PTI) sensing technology provided by an embodiment of the present application.

[0053] Figure 4 is a schematic diagram of a mode interferometer based on an ARHCF and an ARHCF end surface provided by an embodiment of the present application. DETAILED DESCRIPTION

[0054] Exemplary embodiments of the present disclosure will be described in greater detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided so that the present disclosure can be more thoroughly understood and the scope of the present disclosure can be accurately conveyed to those skilled in the art.

[0055] As shown in Figure 1 , the embodiment of the present application proposes a trace gas detection system based on an anti-resonant hollow core fiber mode interferometer, including the following steps:

[0056] The light source module 11 includes a pump light source and a detection light source, the pump light source is used to excite the photothermal effect of the measured gas, and the detection light source is used to generate a single-wavelength laser far away from the gas absorption peak;

[0057] The transmission module 12 comprises a gas sensor, and is used for transmitting pump light and probe light, and reflecting the refractive index change caused by the photo-thermal effect on the phase change of the probe light through a photo-thermal interference (PTI) sensing technology;

[0058] The gas chamber module 13 is used for taking the fiber core as a gas chamber, and taking the anti-resonant hollow core fiber as a place for light and gas interaction;

[0059] The signal detection module 14 comprises a photodetector, and is used for converting the probe light into an electric signal;

[0060] The demodulation module 15 comprises a lock-in amplifier, and is used for amplifying and detecting the electric signal carrying the gas concentration information.

[0061] In the embodiment of the present application, the DFB laser is used as the pump light source, which can provide stable and single-frequency laser output, and effectively excite the photo-thermal effect of the measured gas; the probe light source generates single-wavelength laser far from the gas absorption peak, which avoids the direct attenuation of the probe light caused by the gas absorption, and ensures the stable transmission of the probe light and the accurate measurement of the phase change; through the photo-thermal interference sensing technology, the refractive index change caused by the photo-thermal effect of the gas is converted into the phase change of the probe light, and the non-contact and high-sensitivity gas concentration measurement is realized; the anti-resonant hollow core fiber is used as the gas chamber, and the fiber core is a cavity structure, which can accommodate the measured gas; meanwhile, the transmission characteristics of the fiber make the light and the gas fully interact, and enhance the detection effect; the photodetector efficiently converts the probe light into an electric signal, improves the response speed and detection precision of the system, the lock-in amplifier is specially used for amplifying and detecting the weak electric signal carrying the gas concentration information, effectively filters out the noise, improves the signal-to-noise ratio, and ensures the accurate measurement of the gas concentration.

[0062] In a specific embodiment of the present application, a system is built as shown in Figure 2The gas detection system is composed of a light source part, a transmission part, a gas chamber part, a signal detection part and a demodulation part. The waveform generator outputs sawtooth signals and sine signals, and the signals obtained by adding the two signals are input to a DFB laser as pump light. The wavelength of the laser output by the externally triggered and tuned DFB laser is determined by the absorption wavelength of the detected gas. The specific wavelength laser is amplified in the EDFA through an isolator, and the background noise of the EDFA is filtered out by a filter 1 and input to the interferometer sensing unit through a coupler. When the gas chamber is filled with the detected gas, the pump light and the detected gas interact in the interferometer sensing unit. The detection light source outputs single-wavelength laser, and the wavelength is the principle gas absorption wavelength. The laser is input to the interferometer sensing unit through an isolator and a coupler. The interaction between the pump laser and the gas causes the environment in the interferometer to change, and the change information is loaded into the detection light by the sensing unit. The pump light and the detection light pass through the filter 2 together, the pump light is filtered out, and the detection light is input to the photodetector to be converted into an electrical signal. Finally, the signal carrying the gas concentration information is input to the lock-in amplifier for amplification and detection, and the concentration information of the measured gas is displayed in the computer through a specific program.

[0063] In a preferred embodiment of the present application, the pump light source is used to excite the photo-thermal effect of the measured gas, and the detection light source is used to generate single-wavelength laser far away from the gas absorption peak, which comprises:

[0064] The pump light source is a DFB laser modulated by sawtooth signals and sine signals generated by a waveform generator, and a specific wavelength pump light is output by externally triggering and tuning the distributed feedback DFB laser to excite the photo-thermal effect of the measured gas.

[0065] The detection light source is single-wavelength detection light output to generate single-wavelength laser far away from the gas absorption peak.

[0066] In the embodiment of the present application, the DFB laser can accurately output specific wavelength pump light by externally triggering and tuning. The DFB laser has a built-in grating structure, which can achieve smooth and tunable control of the wavelength, accurately control the wavelength of the pump light, and ensure that the photo-thermal effect effectively occurs on the target gas. The single-wavelength detection light far away from the gas absorption peak helps to reduce background noise and interference, improve the signal-to-noise ratio of the detection signal, and improve the dynamic range of the system, so that the system can simultaneously detect high-concentration and low-concentration measured gas.

[0067] In a preferred embodiment of the present application, the core device of the gas sensor is a mode interferometer based on ARHCF.

[0068] In the embodiment of the present application, the AR-HCF has extremely low dispersion, optical nonlinearity and extremely high laser damage threshold, through the combination of SMF, GIF and AR-HCF, the mode interferometer can utilize the mode transmission characteristics in different optical fibers to produce interference effect, the GIF can accept more light, and its refractive index gradually changes along the fiber axis, which helps to reduce the propagation loss of light in the fiber and improve the transmission efficiency and capacity. The mode coupling between GIF and SMF and AR-HCF produces a complex interference pattern, and the hollow core structure of AR-HCF minimizes the interaction of light with the fiber material during transmission. By utilizing the characteristics of ARHCF hollow core, the measured gas can be 100% overlapped with the transmitted light field, reducing the light transmission loss and improving the optical thermal pumping efficiency.

[0069] In a specific embodiment of the present application, the optical fiber used includes SMF, wherein the core and cladding diameters are 9m and 125m respectively, GIF, wherein the core and cladding are 62.5m and 125m respectively, and ARHCF, wherein the core and cladding diameters are 45m and 245m respectively; when connected, as shown in the structure of the mode interferometer, a length of L ARHCF is fused at both ends with a length of 245m gradient index multimode fiber, and finally the GIF is fused at both ends with a single mode fiber, and the microchannel connects the surface of the optical fiber and the air core of the ARHCF. Figure 4

[0070] In a preferred embodiment of the present application, the gas sensor is used for transmitting pump light and probe light, which includes:

[0071] The pump light and probe light output by the light source pass through the SMF and GIF and enter the ARHCF to excite high-order modes;

[0072] According to the high-order modes and the fundamental mode, the mode interferometer produces The interference between the fundamental mode and the high-order mode to obtain the phase change relationship of the refractive index, wherein n FM and n HM are the effective refractive indexes of the fundamental mode and the high-order mode respectively, n ext is the medium refractive index in the core of the ARHC-PBF, λ is the wavelength of the light wave, L is the length of the ARHCF, and (2k+1)π represents the term of the phase difference, and k is an integer.

[0073] ​In the embodiment of the present application, the gas sensing unit is a full-fiber structure, which has the advantages of small volume, resistance to electromagnetic interference, low cost and easy integration. The excitation of high-order modes increases the mode diversity in the interferometer, enhances the interference effect, and makes the measurement of phase change more sensitive and accurate. By utilizing the interference between the fundamental mode and the high-order mode, the mode interferometer can cover a wider range of refractive index changes. The relationship between the refractive index and the phase change provides a direct method for measuring the refractive index change of the medium, realizing high-precision measurement. By adjusting the length of the ARHCF, the refractive index of the core medium, and the wavelengths of the pump light and the probe light, the high sensitivity of the phase change to the refractive index has the potential to realize high-resolution measurement and accurately monitor small changes.

[0074] In a specific embodiment of the present application, the mode interferometer based on ARHCF is the core device of the gas sensor. The light output by the light source is input into the GIF through the single-mode optical fiber. After the light passes through the GIF, the mode field is enlarged, and then enters the ARHCF, exciting high-order modes. Finally, the output light is converged into the output SMF through the GIF. Since the core diameters of the single-mode optical fiber and the ARHCF do not match, the mode field difference is large, and the role of the GIF is to match the mode sizes of the two. When the modes satisfy the phase matching condition, the mode interferometer occurs in the interference between the fundamental mode and the high-order mode.

[0075] In a preferred embodiment of the present application, the preparation process of the mode interferometer is as follows:

[0076] First step: fuse the SMF and the GIF;

[0077] Second step: use a precision cutting system to cut the GIF at a distance of 245 m from the fusion point, and prepare two SMF-GIF samples;

[0078] Third step: fuse the cut sample with the ARHCF. In order to avoid the collapse of the air hole of the ARHCF, the fusion program needs to be manually set. The fusion machine parameters are set as follows: discharge time xx milliseconds, discharge current xx mA, and discharge position distance from the single-mode optical fiber end face x microns;

[0079] Fourth step: use a precision cutting system to cut the ARHCF at a distance of 4.3 cm from the fusion point;

[0080] Fifth step: fuse the cut sample with the SMF-GIF sample prepared in the second step;

[0081] Sixth step: drill holes on the side of the ARHCF fiber; the steps include:

[0082] Connect the spectrometer and the light source at both ends of the interferometer;

[0083] Place the ARHCF fiber under 20X objective, adjust the focus position (between the two holes of the ARHCF fiber core); the femtosecond laser energy is set to XX mW, and the laser irradiation time is xx seconds; during the irradiation process, the spectrum is observed, if the loss increases, it proves that the position of the hole hurts the air hole of the fiber core, and the sample preparation fails.

[0084] In the embodiment of the present application, the structure has ultra-high sensitivity, ultra-short gas chamber, and strong detection capability for flammable and explosive gases such as methane and acetylene; the unique ARHCF fiber side hole scheme improves the efficiency of gas entering the fiber and reduces the gas response time;

[0085] In a specific embodiment of the present application, the SMF is fused with the GIF, and the fusion program is set to the general multimode fusion program. The GIF is cut off at a distance of 245 m from the fusion point by using a precision cutting system; the two steps are repeated to prepare two SMF-GIF samples. The cut sample is fused with the ARHCF; since the fiber size is large and the core is air core, the general fusion program cannot automatically identify the ARHCF, so manual fusion is required. Adjust the fusion parameters to avoid the collapse of the air hole at the fusion point causing large fusion loss. The ARHCF is cut off at a distance of 4.3 cm from the fusion point by using a precision cutting system. The cut sample is fused with the SMF-GIF sample prepared in the second step, and the fusion steps are the same as in the third step. The ARHCF fiber is side-holed; the prepared sample is connected to a light source and a spectrometer at both ends, the purpose is to detect whether the periodic structure of the ARHCF fiber will be damaged during the drilling process; the ARHCF fiber is placed under a 20X objective, the focus position is adjusted so that the focus position is between the two holes of the ARHCF fiber core, the femtosecond laser energy is set to XX mW, and the laser irradiation time is xx seconds. During the irradiation process, the spectrum is observed, if the loss increases, it proves that the position of the hole hurts the air hole of the fiber core, and the sample preparation fails. The number of side holes on the 4.3 cm ARHCF fiber is kept at about 5.

[0086] In a preferred embodiment of the present application, the refractive index change caused by the photothermal effect is reflected on the phase change of the probe light through the photothermal interference type (PTI) sensing technology, which includes:

[0087] The light paths of the probe light and the pump light are coaxial and collinear;

[0088] The wavelength of the pump light is modulated to excite the photothermal effect of the pump light;

[0089] According to the photothermal effect, the change of the refractive index of the gas is converted into the change of the refractive index of the gas through the interferometer;

[0090] According to the light paths of the probe light and the pump light, the change of the refractive index is converted into the change of the phase of the probe light through the photothermal interference type (PTI) sensing technology.

[0091] In the embodiments of the present application, the coaxial and collinear optical path design ensures that the pump light and the probe light propagate on the same path, reducing the error caused by optical path deviation; by modulating the wavelength of the pump light, precise wavelength modulation can more effectively excite the photothermal effect of the pump light, improving the sensitivity of the change of the physical parameters of the gas; the photothermal effect allows the measurement to be performed without directly contacting the gas sample, reducing interference and pollution during the measurement process, and the photothermal effect is not limited to temperature change, but can be extended to the detection of pressure, concentration and other physical parameters, widening the application range of the sensing technology; the mode interferometer can accurately capture the interference phenomenon between the fundamental mode and the high-order mode, and convert the small refractive index change into a measurable phase change. The PTI technology uses the pump-probe principle, can monitor the change process of the physical parameters of the gas over time in real time, and has the potential to perform high-precision sensing in a long distance range in combination with the optical fiber transmission technology, and is suitable for remote monitoring and distributed sensing systems.

[0092] In a specific embodiment of the present application, the probe light, the pump light and the gas sample are coaxial and collinear, the wavelength of the pump light source is changed, the wavelength of the pump light is modulated, and the probe laser does not cause gas absorption due to being away from the gas absorption peak. The target gas molecules can produce characteristic absorption to the pump light, thereby causing the energy of the gas molecules to increase and the thermal motion to intensify, and further causing the temperature, pressure and density of the gas chamber to change, and the changes of the temperature, pressure and density of the gas chamber are manifested as changes of the refractive index of the gas in the optical aspect. Since the probe light and the pump light are coaxial, the change of the refractive index will cause the optical path of the probe light to change when the probe light passes through the optical interferometer in the gas chamber, and the optical interferometer has high sensitivity to the optical path difference, and a small change in the optical path will cause the interference light intensity of the optical interferometer to fluctuate; and according to the light paths of the probe light and the pump light, when the probe light passes through the same light path in the gas chamber, the cumulative phase is also modulated. The phase change of the probe light can be accurately detected by a sensitive interferometer interference measurement method.

[0093] In a preferred embodiment of the present application, the photothermal interference type (PTI) sensing technology comprises:

[0094] The wavelength of the pump light is modulated to obtain wavelength-modulated pump light;

[0095] The wavelength-modulated pump light excites the measured gas to produce a photothermal effect;

[0096] The photothermal effect loads the photothermal information to the probe laser through the interferometer to obtain the phase change of the probe light;

[0097] The probe laser is subjected to harmonic demodulation processing to convert the phase change into an intensity change.

[0098] In this embodiment of the invention, the pump light is wavelength modulated to shift the frequency of the photothermal signal to a higher frequency, thereby avoiding excessive noise at low frequencies. Harmonic demodulation is performed by a lock-in amplifier, and broadband noise is filtered out by a narrow-band low-pass filter, thereby obtaining a high signal-to-noise ratio. The sample concentration and temperature information are inverted by utilizing the changes in the probe light interference results. By combining the characteristics of photothermal spectroscopy and wavelength modulation techniques, the wavelength of the pump light is precisely controlled to selectively match the specific absorption peak of the target gas, significantly enhancing the photothermal effect and improving the detection sensitivity. The photothermal effect is converted into the phase change of the probe light by an interferometer. The phase information is more accurate and stable than the intensity information, which helps to improve the accuracy of the measurement. Harmonic demodulation processing converts the phase change into an intensity change, and the dynamic changes of gas parameters are monitored based on the phase.

[0099] In a specific embodiment of the present invention, such as Figure 3 The diagram shows the schematic of a photothermal interferometry (PTI) sensing system. The system consists of a probe light, a pump light, a photodetector, a laser driver, a lock-in amplifier, and a signal processing module. The gas being measured is excited by the pump light to produce a photothermal effect, and this photothermal information is loaded onto the probe laser via an interferometer. Only when the probe laser reaches the photodetector and is converted into an electrical signal, is the phase change converted into an intensity change through harmonic demodulation by the lock-in amplifier.

[0100] In a preferred embodiment of the present invention, an anti-resonant hollow fiber is used as the site of light-gas interaction, and the fiber core is used as a gas chamber, comprising:

[0101] A hole is made on the side of the ARHCF optical fiber;

[0102] The modulated pump beam is incident into a gas cell filled with target gas molecules;

[0103] The target gas molecules exhibit characteristic absorption of the pump light, thereby revealing changes in various physical quantities of the gas molecules;

[0104] The change in the refractive index of the gas is obtained by analyzing the changes in physical quantities.

[0105] The change in refractive index is used to obtain the change in optical path length of the probe light as it passes through the gas cell;

[0106] Based on the change in optical path, an optical interferometer is used to obtain the phase change result of the probe light;

[0107] By inverting the phase change results of the probe light, the sample concentration and temperature information can be obtained.

[0108] In the embodiment of the present application, the core of the ARHCF acts as a gas cell, which has a larger core and a faster gas exchange rate compared with the traditional HC-PBF, and the light and gas interact more fully, and the phase change generated by demodulating the gas spectrum absorption by the phase-sensitive fiber interferometer is obtained, and then the composition and concentration information is obtained;

[0109] In a specific embodiment of the present application, the wavelength-modulated pump beam is incident into a cell filled with gas, and the gas molecules are excited to a higher energy state before returning to the initial state by molecular collision. This process will cause the gas to be locally heated, thereby slightly changing the temperature, pressure and density of the target gas, and finally adjusting the refractive index of the gas. When the probe light passes through the same optical path in the gas cell, its cumulative phase is also modulated. The phase change of the probe light can be accurately detected by a sensitive interferometer. Finally, the phase change is converted into an intensity change by harmonic demodulation, and the phase change generated by demodulating the gas spectrum absorption by the phase-sensitive mode interferometer is obtained, and the composition and concentration information is obtained by inversion.

[0110] In a preferred embodiment of the present application, the probe light is converted into an electrical signal, comprising:

[0111] The pump light and the probe light pass through the filter together to obtain the probe light, and the probe light is input into the photodetector to be converted into an electrical signal.

[0112] In the embodiment of the present application, the filter can effectively filter out the pump light and its harmonic stray light, ensuring the purity of the probe light signal; the pump light usually has a high power density, which can directly or indirectly interfere with the probe light signal, and separating the pump light from the probe light through the filter can avoid such interference and ensure the authenticity of the probe light signal; although the intensity of the probe light is usually much lower than that of the pump light, the probe light signal is strengthened after filtering, so that the photodetector can more easily capture the signal, improving the detection sensitivity of the signal, and by filtering out unnecessary stray light, the filter helps to balance the ratio of signal and noise, making the signal easier to be recognized and extracted.

[0113] In a preferred embodiment of the present application, the electrical signal carrying the gas concentration information is amplified and detected, comprising:

[0114] The electrical signal carrying the gas information is input into the phase-locked amplifier for amplification and detection to obtain the concentration information of the measured gas.

[0115] In the embodiment of the present application, the lock-in amplifier can amplify signals of specific frequencies, effectively suppress noise and interference, stably amplify weak electrical signals, reduce errors caused by signal fluctuations, and make the measurement results more stable and reliable.

[0116] It should be noted that the system is a system corresponding to the above method, and all the implementation manners in the above method embodiments are applicable to this embodiment and can also achieve the same technical effects.

[0117] Embodiments of the present application also provide a computing device, comprising a processor and a memory storing a computer program, wherein the computer program is executed by the processor to perform the method described above. All the implementation manners in the above method embodiments are applicable to this embodiment and can also achieve the same technical effects.

[0118] Embodiments of the present application also provide a computer readable storage medium storing instructions, which, when executed on a computer, cause the computer to perform the method described above. All the implementation manners in the above method embodiments are applicable to this embodiment and can also achieve the same technical effects.

[0119] The above is the preferred embodiment of the present application, it should be noted that for those skilled in the art, without departing from the principles of the present application, can make a number of improvements and refinements, these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A trace gas detection system based on a reverse-homocell optical fiber mode interferometer, characterized in that, include: The light source module includes a pump light source and a probe light source. The pump light source is used to excite the photothermal effect of the gas being measured, and the probe light source is used to generate a single-wavelength laser far from the gas absorption peak. The transmission module includes a gas sensor for transmitting pump light and probe light, and reflecting the refractive index change caused by the photothermal effect onto the phase change of the probe light; The air cell module is used to create a space where light and gas interact, with the fiber core serving as the air cell; The signal detection module includes a photodetector for converting the detection light into an electrical signal; The demodulation module includes a lock-in amplifier for processing the electrical signal to obtain gas information; The pump source is a DFB laser modulated by sawtooth and sinusoidal signals generated by a waveform generator, and outputs pump light of a specific wavelength through external triggering and tuning of the distributed feedback DFB laser to excite the photothermal effect of the gas being measured; the probe source is the output single-wavelength probe light to generate single-wavelength laser light far from the gas absorption peak; the gas sensor includes an ARHCF-based mode interferometer, which includes single-mode fiber (SMF), graded-index multimode fiber (GIF), and anti-resonant hollow fiber (ARHCF); Gas sensors are used to transmit pump light and probe light, including: The pump light and probe light output from the light source pass through a single-mode fiber (SMF) and a graded-index multimode fiber (GIF), and then enter an anti-resonant hollow fiber (ARHCF) to excite higher-order modes. According to the high order mode and the fundamental mode, the mode interferometer is prepared by interference between the fundamental mode and the high order mode to obtain the relationship between the refractive index and the phase change, wherein, and are the effective refractive indexes of the fundamental mode and the high order mode respectively, is the refractive index of the medium in the ARHC-PBF fiber core, and λ is the wavelength of the light wave, L is the length of the ARHCF, is a term representing the phase difference, and k is an integer; the preparation process of the mode interferometer is as follows: Fusing SMF and GIF; The GIF was cut at a distance of 245m from the weld point using a precision cutting system, and two SMF-GIF samples were prepared. The cut sample is then fused with ARHCF. ARHCF was cut at a distance of 4.3 cm from the weld point using a precision cutting system; The cut sample is fused with the SMF-GIF sample; A hole is made on the side of the ARHCF fiber.

2. The trace gas detection system based on anti-resonant hollow core fiber mode interferometer according to claim 1, characterized in that, The refractive index change caused by the photothermal effect is reflected in the phase change of the probe light, including: The optical paths of the probe light and pump light are determined to be coaxial and collinear. The wavelength of the pump light is modulated to excite the photothermal effect of the pump light; Based on the photothermal effect, it is converted into a change in the refractive index of the gas using an interferometer; Based on the optical paths of the probe light and pump light, the change in refractive index is converted into a change in the phase of the probe light.

3. The trace gas detection system based on anti-resonant hollow core fiber mode interferometer according to claim 2, characterized in that, Transforming changes in refractive index into changes in the phase of the probe light includes: The pump light is wavelength modulated to obtain wavelength-modulated pump light; The gas under test is excited by pump light modulated by wavelength to produce a photothermal effect; The photothermal effect is used to load photothermal information into the probe laser through an interferometer in order to obtain the phase change of the probe light; The probe laser is subjected to harmonic demodulation to convert phase changes into intensity changes.

4. The trace gas detection system based on anti-resonant hollow core fiber mode interferometer according to claim 3, characterized in that, Using anti-resonant hollow fiber as the site of light-gas interaction, with the fiber core serving as the gas chamber, including: An air core is constructed using ARHCF, and openings are made on its side. The modulated pump beam is incident into a gas cell filled with target gas molecules; The target gas molecules exhibit characteristic absorption of the pump light, thereby revealing changes in various physical quantities of the gas molecules; The change in the refractive index of the gas is obtained by analyzing the changes in physical quantities. According to the change amount of the refractive index, the optical path change when the probe light passes through the gas chamber is obtained; According to the optical path change, the phase change result of the probe light is obtained through the optical interferometer; The sample concentration and temperature information are obtained by inverting the phase change result of the probe light.

5. The trace gas detection system based on anti-resonant hollow core fiber mode interferometer according to claim 4, characterized in that, The probe light is converted into an electrical signal, including: The pump light and the probe light pass through the filter together to obtain the probe light, and the probe light is input into the photodetector to be converted into an electrical signal.

6. The trace gas detection system based on anti-resonant hollow core fiber mode interferometer according to claim 5, characterized in that, The phase-locked amplifier processes the electrical signal to obtain gas information, including: The electrical signal carrying the gas information is input into the phase-locked amplifier for amplification and detection to obtain the concentration information of the measured gas.

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