Miniature methane detection system

Abstract

The invention discloses a miniature methane detection system, and belongs to the technical field of combustible gas leakage monitoring. According to the invention, multiplexing and alignment-free of transmitting / receiving optical paths are realized through the optical path multiplexing module, a mechanical adjustment step is eliminated, and the installation and adjustment time is greatly reduced; dual suppression of stray light is achieved through cooperation of the single-mode gating optical fiber and the optical path multiplexing module, and the signal-to-noise ratio of the system is increased to be larger than or equal to 60 dB; the wavelength locking module is combined with a temperature locking algorithm, so that the wavelength stability is remarkably improved, and baseline drift and noise interference are inhibited; through all-fiber welding and single board integration, the vibration resistance of the system is improved, and the size of the system is reduced, so that the technical effect of miniature handheld is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of combustible gas leakage monitoring, and particularly relates to a micro methane detection system. BACKGROUND

[0002] At present, methane detectors include three types of electrochemical, catalytic combustion and laser absorption. The electrochemical sensor detects the concentration of methane by measuring the current generated by the reaction of methane and electrolyte, which is mature in technology, low in cost, but has a short service life and needs to be calibrated regularly. The catalytic combustion sensor utilizes the phenomenon of flameless combustion of methane under the action of a catalyst to realize concentration measurement, which has high sensitivity and low cost, but has the problems of cross-gas interference and the need for regular replacement of catalyst. The laser absorption sensor utilizes the characteristic absorption of methane molecules to inverse the concentration, which has the advantages of long service time, good selectivity, high sensitivity, fast detection speed and strong anti-interference ability, and can realize non-contact measurement by combining with remote sensing technology, and is particularly suitable for long-distance and wide-range methane leakage inspection in scenes such as gas transmission pipelines and urban pipe networks, so the methane sensor combining absorption spectrum technology and laser remote sensing technology is the research focus in the current public safety field.

[0003] In recent years, with more and more attention paid to the inspection work of urban pipelines and residential areas, the demand for micro handheld devices suitable for short-distance remote sensing (<10 meters) has begun to grow rapidly. However, methane detection systems are all aimed at long-distance remote sensing requirements, so there is a large space for structural optimization. In addition, the coaxial system lens is difficult to process, and the off-axis system installation structure is complex, which is seriously inconsistent with the low-cost development direction of micro handheld devices. The remote sensing scheme based on an optical fiber adopted by the present application utilizes mature devices in the field of optical communication to realize methane gas detection, greatly simplifies the device structure, processing difficulty and assembly steps without significantly affecting the detection performance, and provides a new structural scheme for large-scale industrialization of micro handheld methane detectors. SUMMARY

[0004] In view of the above problems, a micro methane detection system is provided, which realizes the technical effect of dynamically locking the 1653.72nm wavelength absorption line of the composite light source module by referring to the reference cell (providing an absolute wavelength reference) of the reference cell detector and combining the temperature locking algorithm of the PID to adjust the current output of the detection light source TEC in real time; through the signal detector, the reference cell detector (double detector synchronous acquisition) and the dynamic differential absorption algorithm, the system time-varying noise is eliminated, so that the noise signals such as light source fluctuation and environmental noise are eliminated, and the stability and detection precision of the system are improved.

[0005] To achieve the above purpose, the technical scheme adopted by the present application is.

[0006] The micro methane detection system comprises a composite light source module, a wavelength locking module for dynamically locking the wavelength of the composite light source module, an optical path multiplexing module for coaxially multiplexing and unidirectionally transmitting the emitting light path and the receiving light path, a single-mode gating optical fiber for selectively transmitting the 1653.72 nm wavelength and eliminating the stray light interference, a composite collimating module for collimating the outgoing composite detection light and receiving the diffuse reflection signal light, a signal detector and a signal processing module; the first port of the optical path multiplexing module is connected with the composite light source module, the second port is connected with the composite collimating module to coaxially emit the composite detection light and receive the diffuse reflection signal light, and the third port is connected with the signal detector through the single-mode gating optical fiber; the wavelength locking module realizes the wavelength locking of the composite detection light emitted by the composite light source module through a temperature locking algorithm. The composite detection light emitted by the composite light source module passes through the optical path multiplexing module and the composite collimating module in sequence and is emitted to the gas to be detected, and the diffuse reflection signal light containing the signal of the gas to be detected is output after reflection; the diffuse reflection signal light is received by the composite collimating module and is output to the signal processing module in sequence through the optical path multiplexing module, the single-mode gating optical fiber and the signal detector; and the concentration of the gas to be detected is inversely calculated by the signal processing module through a dynamic differential absorption algorithm.

[0007] Preferably, the composite light source module comprises a detection light source, an indication light source, a beam splitter and a beam combiner.

[0008] Preferably, the detection light source outputs the detection light of the wavelength of 1653.72 nm, which enters the beam combiner through the beam splitter and is combined with the indication light of the wavelength of 632.8 nm output by the indication light source to form the composite detection light.

[0009] Preferably, the beam splitter adopts a 1:99 1x2 optical fiber beam splitter to split the detection light into 1% reference light and 99% detection light.

[0010] Preferably, the beam combiner adopts a 50:50 optical fiber beam combiner.

[0011] Preferably, the optical path multiplexing module adopts a non-reciprocal three-port optical fiber circulator, the back isolation of the second port to the first port is greater than or equal to 50 dB, the second port to the third port is filtered to remove the stray light of non-methane absorption wavelength through the narrowband transmission characteristic of 1653.7±0.5 nm, and the signal-to-noise ratio is greater than or equal to 60 dB.

[0012] Preferably, the composite collimating module comprises a fiber collimator one at the coaxial emitting end and a fiber collimator two at the receiving end.

[0013] Preferably, the wavelength locking module comprises a reference gas cell detector and a detection light source TEC; the reference gas cell detector comprises a front-end sealed reference gas cell containing a standard methane gas concentration and a rear-end coupled photodetector.

[0014] Preferably, the temperature locking algorithm comprises the following steps: Step 1: the reference light is output to the photodetector after passing through the reference gas cell in front of the reference light detector, and the light intensity collected by the reference cell detector is : ; wherein, is the wavelength of the detection light output by the detection light source; L is the length of the reference gas cell; is the incident light intensity; is the absorption coefficient; Step 2: the light intensity collected by the reference cell detector is denoised and normalized to obtain the light intensity normalized absorption signal : ; wherein, is the dark background of the reference light detector; is the baseline when there is no absorption; Step 3: the second derivative of the light intensity normalized absorption signal is calculated to obtain the position of the absorption peak wavelength : ; wherein, the arg min operator represents returning the independent variable that makes the function minimum ; Step 4: the deviation between the position of the absorption peak wavelength and the set position : : ; Step 5: the output voltage of the software PID is calculated : ; wherein, K p , K d , K i is the parameter of the PID; the deviation between the position of the absorption peak wavelength and the set position ; is the data acquisition interval time; the driving current of the detection light source TEC is finely adjusted through the output voltage to realize temperature locking and wavelength drift suppression of the detection light source output wavelength.

[0015] ​Preferably, the dynamic differential absorption algorithm includes the following steps: S1: Output voltage signals of the signal detector and the reference cell detector V sig and V ref Represented as: ; in, R sig This is the resistance value of the transimpedance amplifier in the signal path; R ref The reference impedance amplifier value; The incident light intensity in the signal path; The incident light intensity is used as a reference for the optical path; This is time-varying noise in the system, related to laser intensity fluctuations and external background absorption noise. The absorption coefficient of the gas in the signal path; The absorption coefficient of the reference gas; S2: Output voltage signal V sig and V ref Light intensity normalization was performed separately for the reference path and signal path to eliminate the influence of differences in incident light intensity and current-voltage conversion coefficients on the absorption signal, thus obtaining the normalized absorption signal of the signal path. and normalized reference path absorption signal : ; S3: Absorb signal by calculating the normalized signal path and normalized reference path absorption signal The ratio of [value] to eliminate system time-varying noise The path integral concentration of methane gas was obtained. CL : ; in, S ( T The absorption line for methane gas is strong. C For the gas concentration in the signal path; L For signal path absorption optical path; The absorption coefficient of the reference gas; The absorption coefficient of the gas in the signal path; L ref The reference path absorption optical path.

[0016] By adopting the above technical solution, the present invention has the following beneficial effects.

[0017] (1) The present application solves the technical problems of off-axis system installation and adjustment complexity (> 2 hours), low yield (< 60%) of coaxial hole structure, realizes the elimination of mechanical adjustment steps of the transmitting / receiving optical path without alignment, greatly reduces the installation and adjustment time; through the synergistic effect of single-mode gating optical fiber and optical path multiplexing module, the technical problems of high insertion loss (> 30%) of traditional optical filter and environmental stray light interference signal-to-noise ratio are solved, and the signal-to-noise ratio is improved to ≥60dB; through the wavelength locking module combined with the temperature locking algorithm, the technical effect of high measurement error caused by laser wavelength drift and background noise interference concentration inversion accuracy is solved, the wavelength stability is significantly improved, and the baseline drift and noise interference are suppressed; through all-fiber fusion and single-board integration, the technical problems of poor mechanical stability of free space optical path and large EMI interference of multi-circuit board are solved, the anti-vibration performance of the system is significantly improved, and the system volume is reduced, thereby realizing the technical effect of miniaturization and hand-held.

[0018] (2) The present application realizes coaxial optical path multiplexing through a three-port optical fiber circulator, the isolation of the second port to the first port of the optical fiber circulator is ≥50dB, combined with single-mode gating optical fiber, forming double noise suppression at the physical level of absorption spectrum, the signal-to-noise ratio of the system is ≥60dB, which is 20dB better than the traditional system; through the adoption of dynamic differential absorption algorithm, the time-varying noise is eliminated through reference / signal double-path normalization mathematics, combined with reference cell detector to realize fine adjustment of the driving current of the detection light source TEC, realize temperature locking and wavelength drift suppression of the output wavelength of the detection light source, thereby realizing the technical effect of dynamic compensation and locking of the output wavelength of the composite light source module.

[0019] (3) The present application constructs a double noise suppression barrier through the optical fiber circulator (physical isolation back reflection ≥50dB) and the single-mode gating optical fiber (selectively passing 1653.72±0.5nm wavelength laser), effectively eliminating the influence of noise; through all-fiber fusion of the composite collimator (fiber fusion eliminates free space interface), the coaxial deviation of the composite collimator is ≤0.1mrad, the mechanical vibration sensitivity of the system is significantly reduced, the system frequency drift is significantly reduced, and the technical effects of handheld miniaturization, anti-vibration and high stability of the equipment are realized.

[0020] (4) The present application realizes the technical effect of dynamic locking of the 1653.72nm wavelength absorption line of the composite light source module through the reference cell of the reference cell detector (providing absolute wavelength reference) combined with the temperature locking algorithm of PID to adjust the current output of the detection light source TEC in real time; through the signal detector, reference cell detector (double detectors synchronously collect) and dynamic differential absorption algorithm, the time-varying noise of the system is eliminated, thereby eliminating noise signals such as light source fluctuation and environmental noise, and improving the stability and detection accuracy of the system.

[0021] (5) The application realizes coaxial light path multiplexing, single-mode gating fiber and dynamic differential absorption algorithm to cooperatively suppress noise through the optical fiber circulator, and realizes the technical effects of miniaturization, zero installation and adjustment, 60dB signal-to-noise ratio and high precision of the system in combination with wavelength locking of the reference gas cell, and breaks through the technical problems of 'precision-volume-cost' in the field of gas spectrum detection; in addition, the application simplifies the structure of the miniature methane remote meter by using the mature optical fiber collimator, beam splitter or optical fiber circulator, realizes multiplexing of the detection light path, indication light path and collection light path, greatly reduces the cost of the device, simplifies the installation and adjustment steps, and provides a new structure scheme for short-distance and miniature methane remote measurement. BRIEF DESCRIPTION OF DRAWINGS

[0022] The making and using of the preferred embodiments of the present application will now be discussed in detail. It should be appreciated that the present application provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the application and do not limit the scope of the application.

[0023] Figure 1 Structure diagram of the application Figure 1 .

[0024] Figure 2 Structure diagram of the application Figure 2 .

[0025] Figure 3 Structure diagram of the application Figure 3 .

[0026] The making and using of the preferred embodiments of the present application will now be discussed in detail. It should be appreciated that the present application provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the application and do not limit the scope of the application.

[0027] The application realizes emission / reception light path multiplexing and alignment-free through a light path multiplexing module, eliminates mechanical adjustment steps, and greatly reduces the installation and adjustment time; through the cooperation of single-mode gating fiber and light path multiplexing module, the system signal-to-noise ratio is improved to ≥60dB; through the wavelength locking module combined with the temperature locking algorithm, the wavelength stability is significantly improved, and the baseline drift and noise interference are suppressed; through all-fiber fusion and single-board integration, the anti-vibration performance of the system is improved, and the system volume is reduced, thereby realizing the technical effect of miniaturization and hand-held.

[0028] The application will be further described below with reference to the accompanying drawings. Figures 1-3 . Example 1

[0029] As Figure 1 The technical solution one of the application is that the wavelength of the detection light source is 1653.72nm of TO package laser, compared with DFB package structure, it has the advantages of small structure volume and low cost, and the purpose of selecting this laser is the core device of methane absorption spectrum. The wavelength of the indicating light source is 632.8nm of red light, which plays the role of indicating light path. The tail end of the detection light source and the indicating light source is directly coupled with optical fiber, and then passes through a fiber combiner to synthesize an optical fiber, which passes through a fiber collimator one and is output. The fiber combiner is a 50:50 fiber combiner, and the fiber collimator one is a short focal length fiber collimator, which is used for collimating the fine beam of the indicating light and the detection light. A single-mode gating optical fiber is coupled at the front end of the signal detector, which is used for selectively passing the diffuse reflection signal light of 1653.72nm and suppressing stray light.

[0030] The single-mode gating optical fiber is directly coupled at the input end of the fiber collimator two, and the fiber collimator two selects a medium or large aperture fiber collimator, which is used for diffuse reflection signal light collection. This technical solution mainly uses the fiber combiner, the fiber collimator and the fiber device to simplify the optical and mechanical structure and reduce the cost. The optical circulator adopts a non-reciprocal three-port optical circulator, the back isolation of the second port to the first port of the optical circulator is ≥50dB, the second port to the third port passes through the narrow-band transmission characteristic filter of 1653.7±0.5nm to filter out the non-methane absorption wavelength stray light, and the signal-to-noise ratio is ≥60dB.

[0031] The detection light source outputs the detection light of 1653.72nm to the combiner, and the indicating light source outputs the indicating light of 632.8nm to the combiner to form a composite detection light, which is output to the fiber collimator one, and is emitted from the fiber collimator one. After being reflected by the gas to be measured, the diffuse reflection signal light is received by the fiber collimator two, and the diffuse reflection signal light of 1653.72nm is selectively passed through the single-mode gating optical fiber, and the stray light is suppressed, and is output to the signal detector. The signal detector converts the diffuse reflection signal light containing the concentration information of the gas to be measured into an electrical signal and outputs it to the signal processing module. The signal processing module combines the dynamic differential absorption algorithm to calculate the concentration of the gas to be measured. Example 2

[0032] As Figure 2The second technical solution of the application is based on the first technical solution, and a reference cell detector is added, an input end of a detection light source is split into reference light and detection light through a 1:99 optical fiber beam splitter, the reference cell detector receives the reference light split by the optical fiber beam splitter, and a sealed reference gas cell containing a standard methane gas concentration is arranged at a front end of the reference cell detector, and a photodetector is coupled at a rear end of the reference cell detector. The reference light enters the reference gas cell and is subjected to spectral absorption of the methane gas, the reference light carrying the standard concentration methane gas information is collected by the rear-end coupled photodetector and is converted into an electrical signal, and the driving current of the TEC of the detection light source is accurately controlled through a temperature locking algorithm to realize temperature locking and wavelength drift suppression of the output wavelength of the detection light source, and finally realize wavelength locking of the output composite detection light of the composite light source module.

[0033] The optical circulator adopts a non-reciprocal three-port optical circulator, the back isolation of the second port to the first port of the optical circulator is greater than or equal to 50 dB, the second port to the third port is filtered to remove non-methane absorption wavelength stray light through a narrow-band transmission characteristic of 1653.7±0.5 nm, and the signal-to-noise ratio is greater than or equal to 60 dB. The indication light source is output through an optical fiber and combined with the detection light split by the optical fiber beam splitter, and the combined light is combined into a bundle of optical fibers through an optical fiber combiner to an optical fiber collimator one, the optical fiber collimator one is a short focal length optical fiber collimator, and is used for collimating the fine light beams of the indication light and the detection light. A single-mode gating optical fiber is coupled at a front end of the signal detector, and is used for selectively passing the 1653.72 nm detection light and suppressing stray light. The single-mode gating optical fiber is directly coupled to an input end of an optical fiber collimator two, the optical fiber collimator two is an optical fiber collimator with a medium or large aperture, and is used for collecting the diffuse reflection signal light.

[0034] The detection light source outputs the detection light of 1653.72 nm through a detection light splitter, and the detection light is split into 99% detection light and 1% reference light, the reference light enters the reference gas cell at the front end of the reference cell detector through an optical fiber, is subjected to spectral absorption of the standard concentration methane gas in the reference gas cell, and then outputs the reference light containing the standard concentration methane gas signal to the rear-end photodetector, is converted into an electrical signal by the photodetector, and then the driving current of the TEC of the detection light source is accurately controlled through a temperature locking algorithm to realize wavelength locking of the output composite detection light of the composite light source module.

[0035] The probe light output is split by a beam splitter into 99%, and the probe light is mixed with the indicating light with a wavelength of 632.8 nm output by an indicating light source in a beam combiner according to a light intensity ratio of 50:50 to form a composite probe light output to a fiber collimator one. The composite probe light output is emitted from the fiber collimator one, reflected by the gas to be measured, and forms a diffuse reflection signal light which is received by a fiber collimator two and selectively passes through the diffuse reflection signal light with a wavelength of 1653.72 nm through a single-mode gating fiber, suppresses stray light, and is output to a signal detector. The signal detector converts the diffuse reflection signal light containing the concentration information of the gas to be measured into an electrical signal and outputs the electrical signal to a signal processing module. The signal processing module combines a dynamic differential absorption algorithm to calculate and obtain the concentration of the gas to be measured. Embodiment 3

[0036] As Figure 3 The third technical solution of the present application is based on the second technical solution, and a fiber circulator is added. The fiber circulator is a non-reciprocal three-port fiber circulator. The back isolation of the second port to the first port is greater than or equal to 50 dB. The second port to the third port filters out non-methane absorption wavelength stray light through a narrow band transmission characteristic of 1653.7±0.5 nm, and the signal-to-noise ratio is greater than or equal to 60 dB. The original composite fiber collimator composed of the fiber collimator one and the fiber collimator two is replaced by a fiber collimator. The fiber collimator is a medium or large aperture fiber collimator, which can collimate and emit the composite probe light and receive the diffuse reflection signal light reflected back at the same time. The other hardware components remain unchanged. The beam splitter is a 1:99 1x2 fiber beam splitter that splits the probe light into 1% reference light and 99% probe light. The beam combiner is a 50:50 fiber beam combiner. The composite collimation module is a medium or large aperture fiber collimator. The wavelength locking module includes a reference cell detector and a probe light source TEC. The reference cell detector includes a front-end sealed reference cell containing a standard methane gas concentration and a rear-end coupled photodetector.

[0037] The flow and principle of the technical solution adopted in this embodiment are as follows: the probe light source outputs probe light with a wavelength of 1653.72 nm. After the probe light passes through the beam splitter, it is split into reference light and probe light according to a splitting ratio of 1:99. The reference light enters the reference cell of the reference cell detector through the optical fiber. After the standard concentration methane gas in the reference cell is spectrally absorbed, the reference light containing the standard concentration methane gas information is output to the photodetector at the rear end of the reference cell detector. The photodetector converts the reference light into an electrical signal. The temperature locking algorithm dynamically calculates and feeds back the driving current of the probe light source TEC to achieve temperature locking and wavelength drift suppression of the output wavelength of the probe light source. The temperature of the probe light source TEC is adjusted to dynamically compensate and adjust the working temperature of the probe light source, and the working wavelength of the probe light source output is locked at 1653.72 nm.

[0038] The temperature locking algorithm comprises the following steps: step 1: the reference light is output to a photodetector after passing through a reference gas cell in front of the reference light detector, and the light intensity collected by the reference cell detector is ; ; Wherein, is the wavelength of the detection light output by the detection light source; L is the length of the reference gas cell; is the incident light intensity; is the absorption coefficient.

[0039] Step 2: the light intensity collected by the reference cell detector is denoised and normalized to obtain a light intensity normalized absorption signal : ; Wherein, is the dark background of the reference light detector; is the baseline when there is no absorption; Step 3: the second derivative of the light intensity normalized absorption signal is calculated to obtain the position of the absorption peak wavelength : ; Wherein, the arg min operator represents the independent variable that makes the function reach the minimum value ; Step 4: the deviation between the position of the absorption peak wavelength and the set position is calculated :

[0040] Step 5: the output voltage of the software PID is calculated : ; Wherein, K p , K d , K i is the parameter of the PID; the deviation between the position of the absorption peak wavelength and the set position ; is the data acquisition interval time; the driving current of the detection light source TEC is finely adjusted through the output voltage to realize temperature locking and wavelength drift suppression of the detection light source output wavelength.

[0041] ​The probe light after being split by the beam splitter enters the beam combiner, and is combined with the indicating light output by the indicating light source at a wavelength of 632.8 nm at a ratio of 50:50 to form composite probe light. The composite probe light is emitted to the gas to be measured after passing through the optical fiber circulator first port, the optical fiber circulator second port, and the optical fiber collimator in sequence. After reflection, the diffuse reflection signal light containing the signal of the gas to be measured is received by the optical fiber collimator and is output to the signal detector along the optical fiber circulator second port, the optical fiber detector third port, and the single-mode gating optical fiber in sequence. The signal detector converts the electric signal into an electric signal and outputs it to the signal processing module. Finally, the signal processing module inversely calculates the concentration of the gas to be measured by the dynamic differential absorption algorithm.

[0042] The dynamic differential absorption algorithm includes the following steps: S1: the output voltage signals of the signal detector and the reference cell detector V sig and V ref are represented as: ; wherein, R sig is the resistance value of the signal path transimpedance amplifier; R ref is the resistance value of the reference path transimpedance amplifier; is the incident light intensity of the signal path; is the incident light intensity of the reference light path; is the system time-varying noise, which is related to the fluctuation of the laser intensity, external background absorption, and other noises; is the absorption coefficient of the gas in the signal path; is the absorption coefficient of the gas in the reference path; S2: the output voltage signals V sig and V ref are normalized respectively to eliminate the influence of the incident light intensity of the reference path and the signal path and the difference in the current-voltage conversion coefficient on the absorption signal, to obtain the normalized signal path absorption signal and the normalized reference path absorption signal : ;

[0043] S3: the ratio of the normalized signal path absorption signal and the normalized reference path absorption signal is calculated to eliminate the system time-varying noise , to obtain the path integral concentration of the methane gas CL : ; wherein, S T ​) is the absorption line strength of the methane gas; C ) is the gas concentration of the signal path; L ) is the absorption optical path of the signal path; ) is the absorption coefficient of the reference path gas; ) is the absorption coefficient of the signal path gas; L ref ) is the absorption optical path of the reference path.

[0044] The present application realizes optical path multiplexing through a fiber circulator, so that the transmitting optical path (composite probe light) and the receiving optical path (diffuse reflection signal light) are coaxial, eliminating the need for mechanical adjustment of the traditional beam splitter; the fiber circulator is connected with the following components to form a closed-loop optical path for one-way optical path transmission. The first port is connected with the beam combination end of the probe laser (1653.72 nm) and the red light indicating light source (632.8 nm); the second port is connected with a composite fiber collimator, which contains a short focal length collimator (transmitting) and a large aperture collimator (receiving); the third port is connected with a signal detector through a single-mode gating optical fiber, for receiving diffuse reflection signal light and suppressing noise interference of stray light.

[0045] The fiber collimator in the embodiment has both collimating effect of outgoing light and receiving diffuse reflection signal light, and the outgoing light and the receiving light are coaxial structures, which do not need separate light adjustment and the structure can be simpler compared with the off-axis structures of schemes 1 and 2. The core function of the fiber circulator is as follows: (1) optical path multiplexing and simplification: through the one-way transmission characteristic of the circulator, composite probe light transmission and diffuse reflection signal light reception share the same optical probe (fiber collimator), and the number of optical path components is reduced. (2) mechanical stability: the all-fiber fusion structure eliminates the micro-displacement sensitivity problem of free space optical path. (3) wavelength cooperative transmission: dual-wavelength compatibility: supports coaxial transmission of 1653.72 nm (methane absorption peak) and 632.8 nm (indicating light), realizes low-loss coupling through the broadband characteristic (630-1700 nm) of the circulator, and the power distribution of the red indicating light power is ≤5%, avoiding the saturation of the detector. (4) signal isolation and noise suppression: reverse isolation: the isolation degree of the second port to the first port of the fiber circulator is ≥50 dB, effectively suppressing the back reflection noise of the laser; ambient light suppression: the narrowband transmission characteristic (1653.7±0.5 nm) of the second port to the third port filters out non-methane absorption wavelength stray light, and the signal-to-noise ratio is improved to ≥60 dB.

[0046] The application constructs a double noise suppression barrier by a fiber circulator (physical isolation back reflection is greater than or equal to 50 dB) and a single-mode gating optical fiber (selectively passing a 1653.72+ / -0.5 nm wavelength laser), effectively eliminating the influence of noise; through full-fiber fusion of a compound collimator (fiber fusion eliminates a free space interface), coaxial deviation of the compound collimator is less than or equal to 0.1 mrad, mechanical vibration sensitivity of the system is significantly reduced, system frequency drift is significantly reduced, and technical effects of handheld miniaturization, vibration resistance and high stability of the equipment are achieved.

[0047] In addition, due to the use of the fiber circulator, alignment-free of the detection light path, the indication light path and the collection light path can be achieved. Compared with the off-axis structure, the adjustment structure of the laser can be removed, the design and processing difficulty is reduced, and the product process steps are reduced; compared with the coaxial hole structure, the hole process is saved, and the product cost is lower. Through the one-way transmission characteristics of the fiber circulator, the traditional discrete emission / reception light path is integrated into a single coaxial channel, solving the calibration problem of multiple light paths; combined with wavelength multiplexing technology, the cooperative transmission of detection light, indication light and reference light is realized.

[0048] Due to the use of the fiber structure, flexible layout of the internal structure of the instrument can be achieved, and the complexity of the optical and mechanical structure is significantly reduced. In addition, flexible layout can realize single-board integration of circuit systems in a small space, reducing the design and processing cost of the electronic part of the instrument. Full-fiber connection avoids the complex mechanical structure of the free space light path. The use of the reference gas cell can greatly improve the stability of the methane measurement signal. The indication light, the detection light and the reference gas cell can be integrated into a module separately to adapt to the different schemes.

[0049] The optical fiber at the input end of the signal detector uses a single-mode gating optical fiber, which selectively passes the 1653.72 nm detection light by using the fundamental mode transmission characteristics of the single-mode optical fiber, and suppresses stray light, thereby replacing the function of the optical filter to reduce the cost. At the same time, the reverse isolation characteristics of the fiber circulator are used to suppress the back reflection of the laser, avoiding self-oscillation; the wideband transmission characteristics are compatible with multiple wavelengths, avoiding the cumulative insertion loss caused by the use of multiple optical filters.

[0050] The application realizes coaxial light path multiplexing, single-mode gating optical fiber and dynamic differential absorption algorithm cooperative noise suppression through the fiber circulator, and wavelength locking combined with the reference gas cell, to achieve the technical effects of miniaturization, zero installation and adjustment, 60 dB signal-to-noise ratio and high precision of the system, breaking through the technical difficulties of "precision-volume-cost" in the field of gas spectrum detection; in addition, the application uses the mature optical fiber collimator, beam splitter or fiber circulator to simplify the structure of the miniature methane remote meter, realize multiplexing of the detection light path, the indication light path and the collection light path, greatly reduce the cost of the device, and simplify the installation and adjustment steps, providing a new structure scheme for short-distance, miniature handheld methane remote measurement.

[0051] Although the present application has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the application as defined by the appended claims. Moreover, the specific embodiments described are not to be taken in a limiting sense but are merely for the purpose of illustrating the presently preferred embodiments of the application. It is therefore contemplated that the claims will cover process, machines, manufacture, composition of matter, means, methods, or steps similar, equivalent, or even subcombinations of the methods and steps described in the claims.

Claims

1. A miniature methane detection system, characterized in that: The system includes a composite light source module, a wavelength locking module for dynamically locking the output wavelength of the composite light source module, an optical path multiplexing module for coaxial multiplexing and unidirectional transmission of the transmitting and receiving optical paths, a single-mode gating fiber for selectively transmitting a 1653.72nm wavelength and eliminating stray light interference, a composite collimating module for collimating the emitted composite probe light and receiving diffuse reflection signal light, a signal detector, and a signal processing module. The first port of the optical path multiplexing module is connected to the composite light source module, the second port is connected to the composite collimating module for coaxial emission of the composite probe light and reception of diffuse reflection signal light, and the third port is connected to the signal detector via the single-mode gating fiber. The wavelength locking module uses a temperature-locking algorithm to lock the wavelength of the composite probe light output by the composite light source module. The composite light source module outputs a composite probe light that is sequentially transmitted through an optical path multiplexing module and a composite collimation module to the gas to be measured. After reflection, it outputs a diffuse reflection signal light containing the signal of the gas to be measured. The diffuse reflection signal light is received by the composite collimation module and sequentially transmitted through an optical path multiplexing module, a single-mode gating fiber, and a signal detector before being output to the signal processing module. The signal processing module uses a dynamic differential absorption algorithm to invert the concentration of the gas to be measured.

2. The miniature methane detection system as described in claim 1, characterized in that: The composite light source module includes a detection light source, an indicator light source, a beam splitter, and a beam combiner.

3. The miniature methane detection system as described in claim 2, characterized in that: The probe light output by the probe light source has a wavelength of 1653.72nm. After passing through the beam splitter, it enters the beam combiner and is combined with the indicator light output by the indicator light source with a wavelength of 632.8nm to form a composite probe light.

4. The miniature methane detection system as described in claim 3, characterized in that: The beam splitter uses a 1×2 fiber optic beam splitter with a 1:99 ratio to split the probe light into 1% reference light and 99% probe light.

5. The miniature methane detection system as described in claim 1, characterized in that: The combiner is a 50:50 fiber optic combiner.

6. The miniature methane detection system as described in claim 1, characterized in that: The optical path multiplexing module adopts a non-reciprocal three-port fiber optic circulator. The back-to-back isolation between the second port and the first port is ≥50dB. The second port to the third port filters out stray light of non-methane absorption wavelength through narrowband transmission characteristics of 1653.7±0.5nm, and the signal-to-noise ratio is ≥60dB.

7. The miniature methane detection system as described in claim 1, characterized in that: The composite collimation module includes a fiber collimator one at the coaxial transmitter end and a fiber collimator two at the receiver end.

8. The miniature methane detection system as described in claim 1, characterized in that: The wavelength locking module includes a reference gas cell detector and a detection light source TEC; the reference gas cell detector includes a front-end sealed reference gas cell containing a standard methane gas concentration and a rear-end coupled photodetector.

9. The miniature methane detection system as described in claim 1, characterized in that: The temperature locking algorithm includes the following steps: Step 1: The reference light is output to the photodetector after passing through the reference cell at the front end of the reference photodetector. The light intensity collected by the reference cell detector... for: in, To detect the wavelength of the probe light output from the probe light source; L For reference, the length of the gas pool; The incident light intensity; The absorption coefficient; Step 2: Measure the light intensity collected by the reference cell detector. Denoising and intensity normalization are performed to obtain the intensity-normalized absorption signal. : in, The dark background serves as a reference for the photodetector; This is the baseline when there is no absorption; Step 3: Calculate the second derivative of the normalized absorption signal to obtain the location of the absorption peak wavelength. : The `arg min` operator returns the argument that minimizes the function. ; Step 4: Calculate the location of the peak wavelength of the absorption peak. With set position deviation : Step 5: Calculate the output voltage of the software PID controller. for: in, K p , K d , K i These are the parameters for the PID; Location of absorption peak wavelength With set position Deviation; This refers to the data acquisition interval; measured by the output voltage. By finely adjusting the drive current of the probe light source TEC, temperature locking and wavelength drift suppression of the probe light source output wavelength can be achieved.

10. A miniature methane detection system as described in claim 1, characterized in that: The dynamic differential absorption algorithm includes the following steps: S1: Output voltage signal of the signal detector and the reference cell detector V sig and V ref Represented as: in, R sig This is the resistance value of the transimpedance amplifier in the signal path; R ref The reference impedance amplifier value; The incident light intensity in the signal path; The incident light intensity is used as a reference for the optical path; This is time-varying noise in the system, related to laser intensity fluctuations and external background absorption noise. The absorption coefficient of the gas in the signal path; The absorption coefficient of the reference gas; S2: Output voltage signal V sig and V ref Light intensity normalization was performed separately for the reference path and signal path to eliminate the influence of differences in incident light intensity and current-voltage conversion coefficients on the absorption signal, thus obtaining the normalized absorption signal of the signal path. and normalized reference path absorption signal : S3: Absorb signal by calculating the normalized signal path and normalized reference path absorption signal The ratio of [value] to eliminate system time-varying noise The path integral concentration of methane gas was obtained. CL : in, S ( T The absorption line for methane gas is strong. C For the gas concentration in the signal path; L For signal path absorption optical path; The absorption coefficient of the reference gas; The absorption coefficient of the gas in the signal path; L ref The reference path absorption optical path.

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