Multi-mode optical fiber gas sensing device and method

Abstract

The application relates to the technical field of gas detection, in particular to a multimode optical fiber gas sensing device and method, a pump laser, pump light with wavelength modulation on a gas absorption peak of a to-be-detected gas; a hollow core optical fiber filled with the to-be-detected gas, receives the input pump light, and generates thermal changes based on the photothermal effect and temperature rise; a multimode optical fiber wound on the outer wall of the hollow core optical fiber, generates a thermal-optic effect, the fiber parameters of the multimode optical fiber are changed, and then the disturbance of the fiber mode phase is caused; a probe laser generates probe light input into the multimode optical fiber, and the output end of the multimode optical fiber forms a real-time speckle pattern; an image sensor detects the real-time speckle pattern; a signal processor extracts a characteristic parameter representing the change degree of the real-time speckle pattern, and obtains the gas concentration of the to-be-detected gas according to the corresponding relationship between the characteristic parameter and the gas concentration of the to-be-detected gas. The application converts the measurement object into the spatial distribution of light intensity through speckle, and realizes high-sensitivity and high-selectivity detection of specific gas.

Description

Technical Field

[0001] This invention relates to the field of gas detection technology, specifically to a multimode fiber optic gas sensing device and method. Background Technology

[0002] In the field of gas detection, there is a significant demand for highly sensitive and selective detection of specific gas components in environmental monitoring, industrial safety, and production process control. Traditional direct absorption spectroscopy techniques, such as tunable diode laser absorption spectroscopy (TDLAS), are based on the principle of spectral absorption and are currently one of the mainstream gas detection technologies. They operate on the principle that gas molecules exhibit characteristic absorption of light at specific wavelengths. By measuring the intensity attenuation of the laser light after passing through the target gas, the gas concentration can be deduced using the Lambert-Beer law.

[0003] However, the sensitivity of traditional direct absorption spectroscopy is often limited by the effective absorption optical path. To improve detection sensitivity, long optical path gas cells (such as White's cell) are usually required to increase the interaction distance between light and gas, but this makes the system structure complex, bulky, and less stable. Summary of the Invention

[0004] To address the aforementioned problems, the present invention aims to provide a multimode fiber gas sensing device; a further objective of the present invention is to provide a multimode fiber gas sensing method.

[0005] Multimode fiber gas sensing device, including, A pump laser is used to output pump light whose wavelength is modulated onto the absorption peak of the gas being measured. A hollow optical fiber is filled with the gas to be tested. The hollow optical fiber receives the input pump light, and a thermal change is generated based on the photothermal effect as the temperature rises. A multimode fiber is wound around the outer wall of the hollow fiber. Based on the thermal change, a thermo-optic effect is generated. Under the thermo-optic effect, the fiber parameters of the multimode fiber change, thereby causing a perturbation of the fiber mode phase. A probe laser generates probe light, which is input into the multimode optical fiber, and a real-time speckle pattern is formed at the output end of the multimode optical fiber. An image sensor detects the real-time speckle pattern output from the multimode optical fiber; The signal processor compares the real-time speckle pattern with a reference speckle pattern, extracts feature parameters characterizing the degree of change in the real-time speckle pattern, and determines the gas concentration of the gas to be measured based on the correspondence between the feature parameters and the gas concentration of the gas to be measured.

[0006] The multimode fiber gas sensing device of the present invention uses methane gas as the gas to be measured, a CCD camera as the image sensor, near-infrared light as the detection light, a helium-neon laser as the detection laser, and a hollow fiber with a diameter of 10 micrometers to 100 micrometers.

[0007] The multimode fiber gas sensing device of the present invention includes a pump laser that is a quantum cascade laser. The laser output from the pump laser passes through a focusing lens and then enters an optical resonant cavity composed of a pair of parallel cavity mirrors. The hollow fiber is located inside the optical resonant cavity. The wavelength of the pump light is set at the strong absorption peak of the gas to be measured, at 3.3 μm.

[0008] The multimode fiber gas sensing device of the present invention includes a normalized correlation coefficient between the real-time speckle pattern and the reference speckle pattern as a characteristic parameter. The formula for calculating the normalized correlation coefficient is as follows:

[0009] in, The normalized correlation coefficient is... For reference, the light intensity distribution of the speckle pattern, The light intensity distribution of the real-time speckle pattern. To reference the grayscale of the speckle pattern, To reference the average gray level of the speckle pattern, The grayscale value of the real-time speckle pattern. This represents the average gray level of the real-time speckle pattern.

[0010] The multimode fiber gas sensing device of the present invention includes, as a characteristic parameter, the contrast of the real-time speckle pattern.

[0011] Where K is the contrast of the real-time speckle pattern. , , where is the standard deviation of light intensity. The average light intensity For reference, the contrast of the speckle pattern.

[0012] The multimode fiber gas sensing device of the present invention perturbs the fiber mode phase of the multimode fiber. for: ,in, Let λ be the phase change of the m-th fiber propagation mode, and λ be the wavelength of the probe light. Let m be the refractive index corresponding to the m-th fiber propagation mode. This refers to the change in length of the multimode optical fiber. The initial length of the multimode optical fiber is given. .

[0013] The multimode fiber gas sensing device of the present invention includes a pre-trained neural network model for training the feature parameters and establishing the correspondence between the feature parameters and the gas concentration of the gas to be measured.

[0014] The multimode fiber gas sensing method includes the following steps: Step S1: The wavelength output by the pump laser is modulated onto the absorption peak of the gas to be tested and incident into a hollow optical fiber filled with the gas to be tested. The hollow optical fiber undergoes thermal changes due to the increase in temperature caused by the photothermal effect. Step S2: The multimode fiber wound around the outer wall of the hollow optical fiber induces a thermo-optic effect based on the thermal change. Under the thermo-optic effect, the fiber parameters of the multimode fiber change, thereby causing a disturbance in the fiber mode phase. Step S3: The probe light generated by the probe laser is injected into the multimode fiber, and the image sensor detects the real-time speckle pattern output by the multimode fiber. Step S4: Compare the speckle pattern with the reference speckle pattern, extract the feature parameters characterizing the degree of change of the speckle pattern, and obtain the gas concentration of the gas to be tested based on the correspondence between the feature parameters and the gas concentration of the gas to be tested.

[0015] The multimode fiber gas sensing method of the present invention establishes a correspondence between the feature parameters and the gas concentration of the gas to be measured by training a pre-trained neural network model on the feature parameters.

[0016] The multimode fiber gas sensing method of the present invention includes a normalized correlation coefficient between the real-time speckle pattern and the reference speckle pattern as a characteristic parameter. The formula for calculating the normalized correlation coefficient is as follows:

[0017] in, The normalized correlation coefficient is... For reference, the light intensity distribution of the speckle pattern, The light intensity distribution of the real-time speckle pattern. To reference the grayscale of the speckle pattern, To reference the average gray level of the speckle pattern, The grayscale value of the real-time speckle pattern. The average gray level of the real-time speckle pattern; and / or, The characteristic parameters include the contrast of the real-time speckle pattern.

[0018] Where K is the contrast of the real-time speckle pattern. , , where is the standard deviation of light intensity. The average light intensity For reference, the contrast of the speckle pattern.

[0019] Beneficial effects: This invention utilizes the high sensitivity characteristics of speckle in multimode fiber and combines it with hollow fiber to form a dual-fiber structure for gas sensing. It avoids the difficulty of directly measuring the attenuation of weak light intensity in traditional direct absorption spectroscopy. By converting the measurement object into the spatial distribution of light intensity through speckle, it achieves high sensitivity and high selectivity detection of specific gases. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the multimode fiber gas sensing device of the present invention; Figure 2 This is an example diagram of the multimode fiber optic gas sensing device of the present invention; Figure 3 This is a schematic flowchart of the multimode fiber gas sensing method of the present invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0023] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0024] Reference Figure 1 , Figure 2 A multimode fiber optic gas sensing device, including, Pump laser 10 is used to output pump light with wavelength modulated to the absorption peak of the gas to be measured; Hollow-core optical fiber 20 is filled with the gas to be measured. Hollow-core optical fiber 20 receives the input pump light, and thermal changes are generated based on the photothermal effect as the temperature rises. Multimode fiber 40 is wound around the outer wall of hollow fiber 20. Based on thermal changes, a thermo-optic effect is generated. Under the thermo-optic effect, the fiber parameters of the multimode fiber change, which in turn causes a perturbation of the fiber mode phase. The probe laser 30 generates probe light, which is input into the multimode fiber, and the output end of the multimode fiber forms a real-time speckle pattern. Image sensor 50 detects the real-time speckle pattern output by multimode fiber 40; The signal processor 60 compares the real-time speckle pattern with the reference speckle pattern, extracts the feature parameters that characterize the degree of change in the real-time speckle pattern, and obtains the gas concentration of the gas to be measured based on the correspondence between the feature parameters and the gas concentration of the gas to be measured.

[0025] When a laser beam enters a multimode fiber, a random, granular pattern of light intensity distribution forms at its output end; this is called multimode fiber speckle. Any minute external disturbance, such as changes in pressure or temperature, will significantly alter the interference between modes, causing drastic changes in the speckle pattern. This high sensitivity makes it a unique "optical fingerprint," which can be used for sensing applications such as measuring pressure, refractive index, spectra, and computational imaging. For example, by analyzing speckle variations, the external physical quantities that caused these variations can be deduced, and even images transmitted through the fiber can be reconstructed.

[0026] This invention constructs a multimode fiber gas sensing device based on a dual-fiber structure with a thermally induced speckle effect. The basic principle is as follows: a section of multimode fiber 40 is tightly wound around a large-aperture hollow fiber 20 filled with the gas to be measured. A laser beam (pump light) with its wavelength tuned to a specific absorption peak of the gas to be measured is injected into the hollow fiber 20. Gas molecules absorb the light energy and generate heat, causing the temperature of the hollow fiber 20 to rise. The hollow fiber confines the light transmission within an air core with a diameter of approximately 10-100 micrometers. The interaction length between the light and the gas is the physical length of the fiber, achieving an extremely high optical path-to-volume ratio. This thermal change is then transferred to the multimode fiber 40 wound around the hollow fiber 20, inducing thermal expansion and a change in refractive index (thermo-optic effect) in the multimode fiber 40. These two effects together alter the phase relationship of the hundreds of modes transmitted in the multimode fiber 40, ultimately manifesting as a change in the speckle pattern at the output end. At this point, another visible light laser (probe light) is injected into the multimode fiber 40, and the changes in its output speckle are monitored in real time by the image sensor 50, thereby retrieving the gas concentration. High-sensitivity detection of gas concentration is achieved by analyzing the changes in speckle morphology.

[0027] This invention proposes a gas sensing device based on the thermal speckle effect of multimode fiber. Utilizing the high sensitivity of multimode fiber speckle, it combines it with a large-aperture hollow fiber 20 to form a dual-fiber structure for gas sensing. The gas absorption signal is converted into a thermal signal, which is then converted into an easily observable optical speckle signal through the multimode fiber 40. This avoids the difficulty of directly measuring the attenuation of weak light intensity in traditional direct absorption spectroscopy. By converting the measurement object into the spatial distribution of light intensity, high sensitivity and high selectivity detection of specific gases are achieved.

[0028] The multimode fiber gas sensing device of the present invention uses methane gas as the gas to be measured, the image sensor 50 is a CCD camera, the detection light is near-infrared light, the detection laser 30 is a helium-neon laser, and the diameter of the hollow fiber 20 is 10 micrometers to 100 micrometers.

[0029] The pump laser 10 is a quantum cascade laser. The laser output from the pump laser 10 passes through a focusing lens 11 and then enters an optical resonant cavity composed of a pair of parallel cavity mirrors 12. The hollow fiber 20 is located inside the optical resonant cavity. The pump light wavelength was set at the strong absorption peak of the gas to be measured, at 3.3 μm.

[0030] Combination Figure 1 A quantum cascade laser, acting as a pump laser 10, emits a laser beam with its wavelength tuned to the gas absorption peak. The laser beam is focused by a focusing lens 11 into an optical resonant cavity (FP cavity) composed of two highly reflective cavity mirrors 12, and coupled into a hollow fiber 20 filled with the gas to be measured. The laser beam passes through the hollow fiber 20 multiple times within the optical resonant cavity, continuously exciting the gas to absorb heat. The heat is transferred to the multimode fiber 40 through a double-fiber structure formed by the tight winding of the hollow fiber 20 and the multimode fiber 40, causing the multimode fiber 40 to thermally expand and change its refractive index. At this time, a helium-neon laser, acting as a probe laser 30, emits a visible red light beam that passes through another focusing lens 31 and enters the multimode fiber 40. The beam is then detected by an image sensor 50, and the concentration of the gas to be measured is ultimately inferred from the speckle changes displayed by the image sensor 50.

[0031] Combination Figure 2 Pump laser 10 emits a laser beam with a wavelength tuned to the gas absorption peak. The laser beam is incident on hollow fiber 20 filled with the gas to be measured, exciting the gas to absorb heat. The heat is transferred to multimode fiber 40 through the double fiber structure formed by the tight winding of hollow fiber 20 and multimode fiber 40, causing the multimode fiber 40 to thermally expand and change its refractive index. At this time, probe laser 30 emits a laser beam that passes through the multimode fiber and is detected by image sensor 50. Finally, the concentration of the gas to be measured is inferred from the speckle change displayed by image sensor 50.

[0032] This invention incident pump light onto a hollow optical fiber 20 filled with the gas to be measured. The hollow optical fiber 20 undergoes a thermal change due to the photothermal effect. Based on the Lambert-Beer law, the temperature change caused by the photothermal effect is as follows:

[0033] in, This represents the temperature change of the hollow-core optical fiber 20. To detect the wavelength of light, The absorption coefficient is... Let C be the power of the pump light and C be the gas concentration of the gas to be measured. That is, given the pump light power, the gas chamber structure, and the environmental conditions, the gas concentration and the temperature change of the hollow fiber satisfy a linear or quasi-linear relationship.

[0034] The thermal changes in the hollow optical fiber 20 are transmitted to the multimode optical fiber 40 wound around it, causing thermal expansion and refractive index changes in the multimode optical fiber 40 (thermo-optic effect). This alters the fiber parameters, resulting in changes in the effective optical path length and propagation constant of the multimode optical fiber 40, thereby causing perturbations in the fiber mode phase.

[0035]

[0036]

[0037] in, This represents the change in refractive index of the multimode fiber 40 due to the thermo-optical effect. This represents the change in length of the multimode fiber 40 caused by thermal expansion. The thermal changes in multimode fiber and hollow fiber. Proportional, where k is the thermal conduction attenuation coefficient. Hollow-core optical fiber is a flexible waveguide that can be tightly wound with multimode optical fiber to form a dual-fiber structure, so k can be approximated as 1. The heat change of hollow-core optical fiber is simplified as follows: .

[0038] in, Let λ be the phase change of the m-th fiber propagation mode, and λ be the wavelength of the probe light, i.e. the wavelength of the light wave propagating in the fiber. Let m be the refractive index corresponding to the m-th fiber propagation mode. This is the change in length of the multimode optical fiber, caused by the thermal expansion of the fiber. This represents the initial length of the multimode fiber, i.e., the fiber length before the temperature change.

[0039] Probe light is injected into the multimode fiber 40, forming a speckle pattern at the output end of the multimode fiber 40. The speckle pattern is then acquired by the image sensor 50. The perturbation of the fiber mode phase will eventually cause a change in the speckle statistics.

[0040] This invention compares the current real-time speckle pattern with a reference speckle pattern to extract characteristic parameters that characterize the degree of speckle variation. The characteristic parameters of the speckle exhibit a monotonic relationship with temperature changes caused by gas absorption, thus establishing a calibrable one-to-one correspondence between the speckle characteristic parameters and the gas concentration. Based on this correspondence, the concentration of the gas to be measured is calculated.

[0041] Under conditions where there is no analyte gas or its concentration is known, a reference speckle pattern is obtained as a baseline, and the current real-time speckle pattern is compared. Compared with reference speckle pattern By comparing the parameters, we can extract characteristic parameters that can characterize the degree of speckle variation.

[0042] The characteristic parameters include the normalized correlation coefficient between the real-time speckle pattern and the reference speckle pattern. The formula for calculating the normalized correlation coefficient is as follows:

[0043] in, The normalized correlation coefficient is... For reference, the light intensity distribution of the speckle pattern, The light intensity distribution of the real-time speckle pattern. To reference the grayscale of the speckle pattern, To reference the average gray level of the speckle pattern, The grayscale value of the real-time speckle pattern. The average gray level of the real-time speckle pattern. Normalized correlation coefficient. It measures the similarity between the reference speckle map and the real-time speckle map, with a value range of [-1, 1]. The closer the value is to 1, the more similar the two images are.

[0044] By acquiring reference speckle patterns before and after perturbation, and performing cross-correlation analysis on both, the speckle cross-correlation coefficient is calculated to characterize the degree of speckle pattern variation with gas concentration. The change in the cross-correlation coefficient of the speckle patterns quantitatively characterizes the degree of speckle evolution, thereby calculating the concentration of the gas to be measured. Furthermore, by changing the gas concentration parameter, cross-correlation analysis is performed on speckle patterns obtained under different conditions to obtain the relationship between the speckle cross-correlation coefficient and gas concentration.

[0045] Simulation results show that the speckle cross-correlation coefficient exhibits a monotonic trend with increasing gas concentration, indicating that the speckle cross-correlation coefficient has good sensitivity and discriminative ability to changes in gas concentration, thus verifying the feasibility of using the speckle cross-correlation coefficient as a parameter for characterizing gas concentration.

[0046] The characteristic parameters also include the contrast of the real-time speckle pattern.

[0047] Where K is the contrast of the real-time speckle pattern. , , where is the standard deviation of light intensity. The average light intensity For reference, the contrast of the speckle pattern.

[0048] The contrast K of the real-time speckle pattern is defined as the ratio of the speckle pattern under the conditions of gas absorption and non-absorption. It is a dimensionless quantity that measures the intensity of the fluctuation of bright and dark particles in the speckle pattern. Standard deviation of light intensity Average light intensity reflects the magnitude of spatial fluctuations in light intensity. This represents the average light intensity within a local area. A high contrast K value in a real-time speckle pattern indicates that the speckle particles are clear and have distinct light and dark areas, corresponding to a static or slowly changing scattering medium; a low contrast K value in a real-time speckle pattern indicates that the speckle is "blurred" due to motion or disturbance, corresponding to a dynamic or strongly disturbed scattering medium.

[0049] The characteristic parameters of speckle patterns exhibit a monotonic relationship with temperature changes caused by gas absorption, thereby establishing a calibrable one-to-one correspondence between the speckle characteristic parameters and the gas concentration. Based on this correspondence, the gas concentration of the gas to be measured can be calculated.

[0050] By performing image analysis on the speckle pattern output from multimode optical fiber, at least one feature parameter that can characterize the degree of speckle variation is extracted. The feature parameter includes, but is not limited to, speckle cross-correlation coefficient, speckle contrast, or a combination thereof. The feature parameter is input into a neural network model for training, so that the neural network model establishes a mapping relationship between the speckle feature parameter and the gas concentration, thereby realizing the quantitative inversion of the gas concentration.

[0051] In the numerical model, the speckle pattern output from a multimode fiber is considered as the spatial interference result of the coherent superposition of multiple propagation modes in the multimode fiber. Each propagation mode has a different propagation constant and initial phase, and their coherent superposition forms a stable speckle distribution at the output end.

[0052] The complex amplitude distribution at the output end of a multimode fiber can be represented as a superposition of multiple propagation modes, and its mathematical expression is as follows:

[0053] in This represents the spatial pattern distribution corresponding to the m-th propagation mode. For mode amplitude, The initial random phase; The phase perturbation caused by concentration is a local quantity that describes the phase perturbation of a single propagation mode due to thermal effects (thermal expansion + thermo-optical effect).

[0054] The speckle intensity distribution at the output end of the multimode fiber can be further obtained from the complex amplitude distribution, and its expression is:

[0055] When the concentration of the external gas changes, gas absorption causes local temperature variations, which in turn alter the effective refractive index of each propagation mode in the multimode fiber, resulting in random phase perturbations in each mode. These phase perturbations disrupt the original coherent superposition relationship, leading to decorrelation changes in the output speckle pattern.

[0056] The above phase perturbation is modeled as a random variable related to temperature change, and its statistical properties can be expressed as follows:

[0057] in, This represents the amount of temperature change caused by a change in gas concentration. This is the phase perturbation coefficient; It is a single propagation mode phase perturbation The variance of the speckle pattern is used to quantify the intensity of its random fluctuations. It measures the intensity of phase noise caused by various small random factors (such as local temperature fluctuations and stress inhomogeneity) when there is no external disturbance. The larger the variance, the more unstable the phase and the more "fuzzy" the speckle pattern.

[0058] One specific embodiment of the present invention includes a neural network model for training feature parameters and establishing a correspondence between the feature parameters and the gas concentration of the gas to be measured.

[0059] By performing image analysis on the speckle pattern output from multimode optical fiber, at least one feature parameter that can characterize the degree of speckle variation is extracted. The feature parameter includes, but is not limited to, speckle cross-correlation coefficient, speckle contrast, or a combination thereof. The feature parameter is then input into a neural network model for training, enabling the neural network model to establish a mapping relationship between the speckle feature parameter and the gas concentration, thereby achieving quantitative inversion of the gas concentration.

[0060] In one specific embodiment, the speckle cross-correlation coefficient is used as a feature parameter input into a neural network model for training. The neural network model learns the mapping relationship between the speckle feature parameters and the gas concentration, thereby achieving quantitative inversion of the gas concentration. The relationship can be expressed as:

[0061] Where C represents the gas concentration. The cross-correlation coefficient of speckle patterns. This is a nonlinear mapping function learned by a neural network.

[0062] Numerical simulation results show that by introducing a neural network to perform regression analysis on the speckle characteristic parameters, the effects of system nonlinearity and noise disturbance can be effectively compensated, and high-precision prediction of gas concentration can be achieved, thus verifying the effectiveness and feasibility of the method.

[0063] The correspondence between characteristic parameters and gas concentration is as follows: ,in, C represents the gas concentration of the gas to be measured. for Another representation or normalized form of represents the similarity between two images, where ΔT is the temperature change caused by the photothermal effect. Phase change in multimode fiber refers to the change in the overall phase of light relative to a reference state (such as the initial moment or reference gas state) as light propagates through the fiber. It is primarily caused by changes in the refractive index of the fiber's environment, which in turn are affected by factors such as temperature, pressure, and gas concentration. The phase change in multimode fiber is directly proportional to the change in optical path difference ΔL, i.e. , where λ is the wavelength of the probe light.

[0064] The temperature change of the hollow fiber 20 refers to the change in temperature of the gas inside the hollow fiber 20 or the wall of the hollow fiber 20 relative to the reference state, i.e., ΔT = T_final_state - T_initial_state. It may be caused by ambient temperature fluctuations, heating caused by gas absorbing laser energy, or external heating / cooling, etc.

[0065] At The core difference lies in the different scales of description. Let be the phase change of the m-th propagation mode in the optical fiber. The total phase change, usually without a subscript, represents the total or representative phase change of all modes in the optical fiber. It is a holistic quantity and is the physical quantity that the sensing link ultimately needs to observe.

[0066] Reference Figure 3 A multimode fiber gas sensing method includes the following steps: Step S1: The wavelength output by the pump laser is modulated onto the absorption peak of the gas to be tested and incident into a hollow optical fiber filled with the gas to be tested. The hollow optical fiber undergoes thermal changes due to the temperature rise caused by the photothermal effect. Step S2: The multimode fiber wound around the outer wall of the hollow fiber is based on the thermo-optic effect caused by thermal changes. Under the thermo-optic effect, the fiber parameters of the multimode fiber change, which in turn causes the fiber mode phase to be disturbed. Step S3: The probe light generated by the probe laser is injected into the multimode fiber, and the image sensor detects the real-time speckle pattern output by the multimode fiber. Step S4: Compare the speckle pattern with the reference speckle pattern, extract the characteristic parameters that characterize the degree of change in the speckle pattern, and obtain the gas concentration of the gas to be measured based on the correspondence between the characteristic parameters and the gas concentration of the gas to be measured.

[0067] This invention is based on the thermal speckle effect of multimode fiber and the thermal absorption effect of gas. It utilizes the flexible waveguide of hollow fiber and the extremely high optical path-to-volume ratio to construct a highly sensitive and selective dual-fiber gas sensing device and method. It cleverly converts the gas absorption signal into a thermal signal, and then converts it into an easily observable optical speckle signal through multimode fiber. By combining the thermal speckle effect of multimode fiber with the thermal effect of gas, it achieves highly sensitive and selective gas detection, effectively avoiding the difficulty of directly measuring the attenuation of weak light intensity in traditional spectral absorption technology.

[0068] In one specific embodiment, a large-aperture hollow-core optical fiber 20 filled with methane (CH4) gas is used as the sensing core. A mid-infrared light source is employed, with its output wavelength set at the strong absorption peak of methane at 3.3 μm. When this laser light is incident on the hollow-core optical fiber 20, it is absorbed by the methane gas molecules, generating a photothermal effect that causes the temperature of the hollow-core optical fiber 20 to rise. A section of multimode optical fiber 40 is tightly wound around the outer wall of the large-aperture hollow-core optical fiber 20, which serves as a thermal sensing unit. This multimode fiber undergoes thermal expansion as the surface temperature of the large-aperture hollow-core optical fiber 20 increases, and simultaneously, a change in refractive index occurs due to the thermo-optical effect. These physical changes collectively perturb the phase relationship of multiple modes propagating in the optical fiber. Simultaneously, a near-infrared light beam is introduced into the multimode optical fiber 40 as the probe light, forming a speckle pattern at its output end. This speckle pattern is acquired using a CCD camera, and image processing, such as calculating the cross-correlation coefficient or contrast change of the speckle image, quantitatively characterizes the degree of speckle evolution, thereby retrieving the concentration of methane gas. In this embodiment, the system achieves high-sensitivity detection of methane gas concentration as low as 1 ppm.

[0069] To further verify the selectivity and reliability of the system, based on the above implementation method, the wavelength of the mid-infrared light source was scanned in the range of 3.14 μm to 3.55 μm. The system successfully obtained the absorption spectrum of methane in this wavelength range, and the measurement results were compared with standard spectral lines in the HITRAN database, showing good consistency. This result not only confirms that the method has the capability for spectral analysis, but also highlights its accuracy in gas identification and quantitative detection.

[0070] By utilizing the thermal speckle effect of multimode fiber, the light intensity attenuation signal caused by gas absorption is converted into spatial distribution changes in the speckle image. This invention effectively achieves signal conversion and amplification, significantly improving detection sensitivity. Benefiting from the dual function of multimode fiber as both a light-transmitting and sensing element, and the all-optical detection design concept, the structure is compact and easily miniaturized. Furthermore, using a CCD camera instead of expensive spectral analysis equipment significantly reduces system costs, while the rich spatial information contained in the speckle pattern provides great potential for further improving detection accuracy and achieving multi-parameter sensing through advanced algorithms. This invention demonstrates significant advantages in sensitivity, stability, cost, and functional scalability, providing an innovative path for the development of a new generation of high-performance gas sensors.

[0071] The description and accompanying drawings provide typical embodiments of specific structures for specific implementations. Other modifications are possible based on the spirit of the invention. While the above-described invention presents preferred embodiments, these are not intended to be limiting.

[0072] For those skilled in the art, various changes and modifications will undoubtedly be apparent after reading the above description. Therefore, the appended claims should be construed as covering all changes and modifications that encompass the true intent and scope of the invention. Any and all equivalent scope and content within the scope of the claims should be considered to remain within the intent and scope of the invention.

Claims

1. A multimode fiber optic gas sensing device, characterized in that, include, A pump laser is used to output pump light whose wavelength is modulated onto the absorption peak of the gas being measured. A hollow optical fiber is filled with the gas to be tested. The hollow optical fiber receives the input pump light, and a thermal change is generated based on the photothermal effect as the temperature rises. A multimode fiber is wound around the outer wall of the hollow fiber. Based on the thermal change, a thermo-optic effect is generated. Under the thermo-optic effect, the fiber parameters of the multimode fiber change, thereby causing a perturbation of the fiber mode phase. A probe laser generates probe light, which is input into the multimode optical fiber, and a real-time speckle pattern is formed at the output end of the multimode optical fiber. An image sensor detects the real-time speckle pattern output from the multimode optical fiber; The signal processor compares the real-time speckle pattern with a reference speckle pattern, extracts feature parameters characterizing the degree of change in the real-time speckle pattern, and determines the gas concentration of the gas to be measured based on the correspondence between the feature parameters and the gas concentration of the gas to be measured.

2. The multimode fiber optic gas sensing device according to claim 1, characterized in that, The gas to be tested is methane gas, the image sensor is a CCD camera, the detection light is near-infrared light, the detection laser is a helium-neon laser, and the diameter of the hollow optical fiber is 10 micrometers to 100 micrometers.

3. The multimode fiber optic gas sensing device according to claim 1, characterized in that, The pump laser is a quantum cascade laser. The laser output from the pump laser passes through a focusing lens and then enters an optical resonant cavity composed of a pair of parallel cavity mirrors. The hollow fiber is located inside the optical resonant cavity. The wavelength of the pump light is set at the strong absorption peak of the gas to be measured, at 3.3 μm.

4. The multimode fiber optic gas sensing device according to claim 1, characterized in that, The feature parameters include the normalized correlation coefficient between the real-time speckle pattern and the reference speckle pattern, and the formula for calculating the normalized correlation coefficient is as follows: ； in, The normalized correlation coefficient is... For reference, the light intensity distribution of the speckle pattern, The light intensity distribution of the real-time speckle pattern. To reference the grayscale of the speckle pattern, To reference the average gray level of the speckle pattern, The grayscale value of the real-time speckle pattern. This represents the average gray level of the real-time speckle pattern.

5. The multimode fiber optic gas sensing device according to claim 1, characterized in that, The characteristic parameters include the contrast of the real-time speckle pattern. ； Where K is the contrast of the real-time speckle pattern. , , where is the standard deviation of light intensity. The average light intensity. For reference, the contrast of the speckle pattern.

6. The multimode fiber optic gas sensing device according to claim 1, characterized in that, Perturbation of the fiber mode phase of the multimode fiber for: ,in, Let λ be the phase change of the m-th fiber propagation mode, and λ be the wavelength of the probe light. Let m be the refractive index corresponding to the m-th fiber propagation mode. This refers to the change in length of the multimode optical fiber. The initial length of the multimode optical fiber is given. .

7. The multimode fiber optic gas sensing device according to claim 1, characterized in that, It includes a pre-trained neural network model for training the feature parameters and establishing the correspondence between the feature parameters and the gas concentration of the gas to be measured.

8. A multimode fiber gas sensing method, characterized in that, Includes the following steps: Step S1: The wavelength output by the pump laser is modulated onto the absorption peak of the gas to be tested and incident into a hollow optical fiber filled with the gas to be tested. The hollow optical fiber undergoes thermal changes due to the increase in temperature caused by the photothermal effect. Step S2: The multimode fiber wound around the outer wall of the hollow optical fiber induces a thermo-optic effect based on the thermal change. Under the thermo-optic effect, the fiber parameters of the multimode fiber change, thereby causing a disturbance in the fiber mode phase. Step S3: The probe light generated by the probe laser is injected into the multimode fiber, and the image sensor detects the real-time speckle pattern output by the multimode fiber. Step S4: Compare the speckle pattern with the reference speckle pattern, extract the feature parameters characterizing the degree of change of the speckle pattern, and obtain the gas concentration of the gas to be tested based on the correspondence between the feature parameters and the gas concentration of the gas to be tested.

9. The multimode fiber gas sensing method according to claim 8, characterized in that, The feature parameters are trained using a pre-trained neural network model to establish a correspondence between the feature parameters and the gas concentration of the gas to be measured.

10. The multimode fiber gas sensing method according to claim 8, characterized in that, The feature parameters include the normalized correlation coefficient between the real-time speckle pattern and the reference speckle pattern, and the formula for calculating the normalized correlation coefficient is as follows: ； in, The normalized correlation coefficient is... For reference, the light intensity distribution of the speckle pattern, The light intensity distribution of the real-time speckle pattern. To reference the grayscale of the speckle pattern, To reference the average gray level of the speckle pattern, The grayscale value of the real-time speckle pattern. The average gray level of the real-time speckle pattern; and / or, The characteristic parameters include the contrast of the real-time speckle pattern. ； Where K is the contrast of the real-time speckle pattern. , , where is the standard deviation of light intensity. The average light intensity. For reference, the contrast of the speckle pattern.

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