CN217466666U - A multi-point gas detection device based on photothermal effect and wavelength division multiplexing interferometer - Google Patents

Abstract

A multipoint gas detection device based on a photothermal effect and a wavelength division multiplexing interferometer comprises a wavelength scanning laser, a circulator, a coupler, an optical fiber delay ring, a plurality of detection units, an optical fiber splitter, a filter, a chopper, a space collimating device, an erbium-doped optical fiber amplifier, a pumping laser, a photoelectric detector, a data acquisition card and a computer system; the circulator is connected with one input end of the coupler, two output ends of the coupler are respectively connected with the optical fiber delay ring and the detection unit, and the detection units are arranged in parallel and connected with the optical fiber branching unit; the circulator is also respectively connected with the wavelength scanning laser and the photoelectric detector; the optical fiber branching device is connected with the filter, the filter is connected with the output end of the space collimating device, and the middle of the space collimating device is provided with the chopper; the input end of the spatial collimating device is connected with the output end of the erbium-doped fiber amplifier, and the input end of the erbium-doped fiber amplifier is connected with the pump laser; the photoelectric detector is connected with a data acquisition card, and the data acquisition card is connected with a computer system.

Description

A multi-point gas detection device based on photothermal effect and wavelength division multiplexing interferometer

technical field

The utility model relates to the technical field of gas detection in a wide range, in particular to a multi-point gas detection device based on a photothermal effect and a wavelength division multiplexing interferometer.

Background technique

Multi-point (quasi-distributed) or distributed gas detection methods can accurately measure gas concentrations at different locations over a wide range. Leak detection is of great significance in long-distance pipeline gas transmission [1,2], large-scale underground mine gas monitoring [3], and urban road gas pollution detection [4-6]. At present, multi-point gas detection is mainly based on optical fiber multiplexing technology [7], including space division multiplexing (SDM) [3, 8, 9], time division multiplexing (TDM) [10-12], frequency division multiplexing (FDM) [13,14] and wavelength division multiplexing (WDM) [15–17].

For SDM [3, 8, 9], multiple gas sensors can share a single laser, but require multiple photodetectors to detect multiple signals. TDM [10–12] usually uses a wavelength-scanning pulsed laser source, and different time delays of the pulsed light can distinguish multiple gas sensors, but the spatial resolution is usually difficult to improve. FDM [13, 14] is mainly based on frequency-modulated continuous wave (FMCW), in order to locate the gas cell at different positions in space, an intensity-modulated swept laser is usually used, but the system and signal processing are relatively complex. WDM [15-17] often use multiple FBGs matching gas absorption peaks to distinguish multiple gas sensors by different wavelengths, so the signal crosstalk between multiple sensors is generally small. However, the number of detections is limited by the number of gas absorption peaks, and the absorption coefficients of each absorption peak of the gas are different, resulting in significant differences in the signal-to-noise ratio for each gas cell. In addition, the wavelength of FBGs often drifts due to environmental influences, which can lead to measurement errors.

Furthermore, stronger phosgene interactions have been demonstrated based on hollow-core photonic bandgap fiber (HC-PBF) gas cells [18–20]. A distributed gas detection system based on this hollow-core fiber and photothermal (PT) interferometry has also been reported [21], with a spatial resolution of about 30 m. The system uses a double-pulse heterodyne optical time domain reflectometry (OTDR) to detect gas-induced phase changes. However, the detection distance, spatial resolution and response speed are limited by the processing and fabrication of hollow core fibers (for example, fs-laser is required to fabricate multiple microchannels along HC-PBF and precisely control the hole spacing), which increases the engineering cost and affects the actual Stability in application.

references

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[3] Z.Wang,T.Chang,X.Zeng,H.Wang,L.Cheng,C.Wu,J.Chen,Z.Luo,and H.-L.Cui,"Fiber optic multipoint remote methane sensing system based on pseudodifferential detection,"Optics and Lasers in Engineering 114,50-59(2019).

[4] B. Culshaw and A. Kersey, "Fiber-optic sensing: A historical perspective," Journal of lightwave technology 26, 1064-1078 (2008).

[5]C.Pijolat,C.Pupier,M.Sauvan,G.Tournier,and R.Lalauze,"Gasdetection for automotive pollution control,"Sensors and Actuators B:Chemical59,195-202(1999).

[6] M.A.Moeckli, M.Fierz, and M.W.Sigrist, "Emission factors for ethene and ammonia from a tunnel study with a photoacoustic trace gas detection system," Environmental science&technology 30, 2864-2867 (1996).

[7] C.K.Kirkendall and A.Dandridge,"Overview of high performancefibre-optic sensing,"Journal of Physics D:Applied Physics 37,R197(2004).

[8] G. Stewart, C. Tandy, D. Moodie, M. Morante, and F. Dong, "Design of a fibreoptic multi-point sensor for gas detection," Sensors and Actuators B: Chemical 51, 227-232 (1998 ).

[9] S.B.Schoonbaert, D.R.Tyner, and M.R.Johnson, "Remote ambient methane monitoring using fiber-optically coupled optical sensors," Applied Physics B119, 133-142 (2015).

[10]W.Jin,"Performance analysis of a time-division-multiplexed fiber-optic gas-sensor array by wavelength modulation of a distributed-feedbacklaser,"Applied optics 38,5290-5297(1999).

[11]C.Floridia,J.B.Rosolem,J.P.V.Fracarolli,F.R.Bassan,R.S.Penze,L.M.Pereira,and M.A.C.da Motta Resende,"Evaluation of EnvironmentalInfluences on a Multi-Point Optical Fiber Methane Leak Monitoring System,"Remote Sensing 11,1249 (2019).

[12]C.Sun,Y.Chen,G.Zhang,F.Wang,G.Liu,and J.Ding,"Multipoint remotemethane measurement system based on spectrum absorption and reflective TDM,"IEEE Photonics Technology Letters 28,2487- 2490 (2016).

[13]H.Ho,W.Jin,H.Yu,K.Chan,C.Chan,and M.Demokan,"Experimentaldemonstration of a fiber-optic gas sensor network addressed by FMCW,"IEEEphotonics technology letters 12,1546- 1548 (2000).

[14]F.Ye,L.Qian,and B.Qi,"Multipoint chemical gas sensing using frequency-shifted interferometry,"Journal of Lightwave Technology 27,5356-5364(2009).

[15] Y. Zhang, M. Zhang, and W. Jin, "Multi-point, fiber-optic gas detection with intra-cavity spectroscopy," Optics Communications 220, 361-364 (2003).

[16]M.Lu,K.Nonaka,H.Kobayashi,J.Yang,and L.Yuan,"Quasi-distributedregion selectable gas sensing for long distance pipeline maintenance,"Measurement Science and Technology 24,095104(2013).

[17]H.Zhang,Y.Lu,L.Duan,Z.Zhao,W.Shi,and J.Yao,"Intracavityabsorption multiplexed sensor network based on dense wavelength divisionmultiplexing filter,"Optics Express 22,24545-24550(2014 ).

[18] A. Cubillas, M. Silva-Lopez, J. Lazaro, O. Conde, M. Petrovich, and J. Lopez-Higuera, "Methane detection at 1670-nm band using a hollow-corephotonic bandgap fiber and a multiline algorithm , "Optics Express 15, 17570-17576 (2007).

[19]W.Jin,Y.Cao,F.Yang,and H.L.J.N.c.Ho,"Ultra-sensitive all-fibrephotothermal spectroscopy with large dynamic range,"6,6767(2015).

[20] J.Li,H.Yan,H.Dang,and F.Meng,"Structure design and application of hollow core microstructured optical fiber gas sensor: A review,"Optics&LaserTechnology 135,106658(2021).

[21] Y.Lin,F.Liu,X.He,W.Jin,M.Zhang,F.Yang,H.L.Ho,Y.Tan,and L.Gu,"Distributed gas sensing with optical fibre photothermal interferometry," Optics express 25, 31568-31585 (2017).

SUMMARY OF THE INVENTION

The purpose of this utility model is to solve the above-mentioned problems in the prior art, and to provide a multi-point gas detection device based on photothermal effect and wavelength division multiplexing interferometer. The wavelengths are separated, and different wavelengths can be multiplexed with the same linear Sagnac interferometer, and the intensity of the detected photothermal phase can be deduced to the gas concentration. Since the wavelength selection has nothing to do with the gas absorption peak, the number of measurement points can be flexibly adjusted according to the bandwidth of the light source and the bandwidth of the wavelength division multiplexer. Using the data acquisition card to Fourier transform the signal and superimpose it in the frequency domain, the minimum detection limit of the gas obtained at the end will be significantly improved. By synthesizing the gas concentration data at each wavelength, the gas concentration at different locations to be measured can be accurately evaluated.

To achieve the above object, the utility model adopts the following technical solutions:

A multi-point gas detection device based on photothermal effect and wavelength division multiplexing interferometer, including wavelength scanning laser, circulator, coupler, fiber delay ring, multiple detection units, fiber splitter, filter, chopper , space collimation device, erbium-doped fiber amplifier, pump laser, photodetector, data acquisition card and computer system;

The circulator is a three-port fiber optic circulator, the circulator is connected to one input end of the coupler, and the two output ends of the coupler are respectively connected to the fiber delay ring and the detection unit, and the detection unit is arranged in parallel and connected to the optical fiber. The splitter is connected; the circulator is also connected to the wavelength scanning laser and the photodetector respectively;

The optical fiber splitter is connected with the filter, the filter is connected with the output end of the spatial collimation device, and a chopper is placed in the middle of the spatial collimation device; the input end of the spatial collimation device is connected with the dopant. The output end of the erbium fiber amplifier is connected, and the input end of the erbium-doped fiber amplifier is connected with the pump laser;

The photodetector is connected to a data acquisition card, the data acquisition card is connected to a computer system, and the data acquisition card is used to acquire the demodulated voltage signal data and transmit it to the computer system for signal processing and gas concentration calculation.

The detection unit includes a wavelength division multiplexer, an absorption gas chamber, a gas sensor and a fiber grating in sequence; the incident end of the wavelength division multiplexer and the output end of the coupler or the reflection end of the wavelength division multiplexer of the previous detection unit The transmission end of the wavelength division multiplexer is connected with the absorption gas chamber, the absorption gas chamber is connected with the gas sensor, the gas sensor is connected with the fiber grating, and the fiber grating is connected with the fiber splitter.

The gas sensor includes a first collimator and a second collimator, the first collimator is arranged at the front end of the absorption air chamber, and the second collimator is arranged at the rear end of the absorption air chamber; the first collimator is arranged at the front end of the absorption air chamber; The laser light from the collimator passes through the absorption chamber and enters the second collimator for transmission to the fiber grating.

The wavelength scanning laser is a C-band wavelength tunable laser.

The splitting ratio of the coupler is 50:50.

The pump laser is a semiconductor laser, and the gas absorption peak is aligned by tuning the wavelength.

The space collimation device includes a first collimator and a second collimator, and the laser light from the first collimator enters the second collimator through a chopper and is transmitted to the filter.

Compared with the prior art, the beneficial effects obtained by the technical solution of the present utility model are:

The utility model realizes multi-point gas detection through the wavelength division multiplexing interferometer. Different from the traditional wavelength division multiplexing method, the utility model is not limited by the gas absorption peak to affect the number of measurement points, nor is it limited by the absorption coefficient of the gas absorption peak to affect the signal-to-noise ratio of multi-point signals. The traditional Mach-Zehnder Interferometer (MZI) often changes the length difference between the two arms due to the disturbance of the external environment, which causes the working point to move and causes the phase fading phenomenon of the interferometer. In the linear Sagnac interferometer, The two beams of interference light are transmitted in the same single-mode fiber, so the optical path difference is theoretically zero, which can overcome the phase fading caused by the change of the optical path difference between the two arms and reduce the interference of the environment. The first-order photothermal signal is collected by a low-cost acquisition card, and superimposed and averaged in the frequency domain for signal processing, which can effectively improve the signal-to-noise ratio and the minimum detection limit, and is more suitable and effective in multi-point applications.

Description of drawings

FIG. 1 is a schematic diagram of the overall structure of the present invention.

Detailed ways

In order to make the technical problems, technical solutions and beneficial effects to be solved by the present utility model more clear and comprehensible, the present utility model will be further described in detail below with reference to the accompanying drawings and embodiments.

Referring to FIG. 1 , this embodiment provides a multi-point gas detection device based on photothermal effect and wavelength division multiplexing interferometer, including a wavelength scanning laser 1 , a circulator 2 , a photodetector 3 , a data acquisition card 4 , and a computer system 5 , coupler 6, fiber delay loop 7, first wavelength division multiplexer 8, second wavelength division multiplexer 9, third wavelength division multiplexer 10, first gas sensor 11, second gas sensor 12, Three gas sensors 13 , first absorption gas chamber 14 , second absorption gas chamber 15 , third absorption gas chamber 16 , first three fiber grating 17 , second fiber grating 18 , third fiber grating 19 , fiber splitter 20 , filter 21 , space collimation device 22 , chopper 23 , erbium-doped fiber amplifier 24 , pump laser 25 .

The wavelength scanning laser 1 is connected to the circulator 2, the circulator 2 is connected to an input end of the coupler 6, and the two output ends of the coupler 6 are respectively connected to the input end of the fiber delay ring 7 and the first wavelength division multiplexer 8. The reflection end of the first wavelength division multiplexer 8 is connected to the incident end of the second wavelength division multiplexer 9, and the reflection end of the second wavelength division multiplexer 9 is incident with the third wavelength division multiplexer 10. The first, second, and third wavelength division multiplexers 8 to 10 are respectively connected to the first, second, and third gas sensors 11 to 13. The first, second, and third gas sensors 11 to 13 The third gas sensors 11-13 are respectively connected to the absorption gas chambers 14-16, the first, second and third gas sensors 11-13 are respectively connected with the first, second and third fiber gratings 17-19, The first, second, and third fiber gratings 17 to 19 are respectively connected to a fiber splitter 20, and the fiber splitter 20 is connected to a filter 21, and the filter 21 is connected to the output end of the space collimation device 22. Connected, the space collimation device 22 is used for optical path collimation in space; a chopper 23 is fixedly placed in the middle of the space collimator 22, and the input end of the space collimation device 22 is output with the erbium-doped fiber amplifier 24 The erbium-doped fiber amplifier 24 is used to amplify the optical power, and the input end of the erbium-doped fiber amplifier 24 is connected with the pump laser 25;

The circulator 2 is connected to the photodetector 3, and the photodetector 3 is connected to the data acquisition card 4, which is used to collect the demodulated voltage signal data, and then transmit it through the usb data line To the computer system 5, signal processing and gas concentration calculation are performed.

The wavelength scanning laser 1 adopts a wavelength tunable laser in the C-band (1525-1565 nm). The laser has stable and tunable wavelength and narrow line width, which ensures that the light source can be operated quickly and intuitively. In the experiment, the wavelengths of 1543.07nm, 1558.90nm and 1555.70nm were fixed respectively to realize the detection of the three absorption gas chambers.

The splitting ratio of the coupler is 50:50, that is, it contains two optical paths, and the proportion of the laser light in the two optical paths is 50% and 50%; the first optical path is transmitted through the fiber to the fiber delay loop and back to the coupler , the second optical path is transmitted to the first wavelength division multiplexer through the optical fiber.

The circulator 2 is a three-port fiber circulator. When the laser is incident from the port 201 of the circulator 2, it exits from the port 202, and when the laser is incident from the port 202 of the circulator 2, it exits from the port 203. The operating wavelength of the optical fiber circulator 2 is in the C-band, the isolation is at least greater than 40dB, and the maximum insertion loss is not more than 1.1dB.

The center wavelengths of the first, second and third wavelength division multiplexers 8 to 10 are respectively 1542.92nm, 1555.74nm and 1559.02nm, the bandwidth is 0.8nm, the channel isolation interval is 100GHz, and the isolation is greater than 30dB.

The first, second, and third gas sensors 11-13 include a first collimator 1101, a second collimator 1102, a third collimator 1201, a fourth collimator 1202, and a fifth collimator 1301 , the sixth collimator 1302, the first collimator 1101 is arranged at the front end of the first absorption gas chamber 11, the outgoing laser passes through the first absorption gas chamber 11 and then enters the second collimator 1102 and is transmitted to the first fiber grating 17. The third collimator 1201, the fourth collimator 1201, the fifth collimator 1301, and the sixth collimator 1302 are respectively placed in the front and rear ends of the second absorption air chamber 12 and the third absorption air chamber 13 and transmitted to the The second fiber grating 18 and the third fiber grating 19 .

The center wavelengths of the first, second, and third fiber gratings 17 to 19 are respectively 1543.00 nm, 1555.69 nm and 1558.93 nm, the bandwidth is 0.2 nm, and the reflectivity is greater than 90%.

The pump laser 25 is a semiconductor laser, which is aligned with the gas (acetylene) absorption peak (1530.38nm) by tuning the wavelength, the center wavelength of the filter is 1530.33nm, and the 3dB bandwidth is about 0.8nm.

The space collimator structure 22 includes a first collimator 2201, a second collimator 2202 and a U-shaped metal plate. The first collimator 2201 is glued on the U-shaped metal plate, and the second collimator The straightener 2202 is glued to the U-shaped metal plate. The laser light from the first collimator 2201 enters the second collimator 2202 through the chopper 23 and is transmitted to the filter 21 . The chopper 20 realizes the intensity modulation of the gas absorption by the pump laser by adjusting the chopping frequency.

After the data acquisition card 4 is connected to the computer system 5, software programming is used for signal processing, Fourier transform is performed on the photothermal signal in the time domain in real time, and the frequency domain signal is accumulated and averaged in real time, so as to optimize the signal-to-noise ratio and improve the minimum detection rate. limited purpose.

The detection steps of the embodiment of the present utility model are as follows:

Start the pump laser 25, align the absorption peak of the gas to be tested (acetylene) at 1530.38 nm, turn on the erbium-doped fiber amplifier 24, amplify the power of the pump laser, turn on the chopper 23, set the modulation frequency of the chopper, and realize the pumping The intensity of the laser 25 is modulated, and the pump laser 25 enters the first, second, and third absorption gas chambers 14-16 after passing through the fiber splitter 20, and the gas in the first, second, and third absorption gas chambers 14-16 After being modulated by the periodic intensity of the laser, the local periodic exotherm causes periodic changes in the local effective refractive index. Start the wavelength scanning laser 1, the fixed wavelength is 1543.07nm, the laser enters the input port of the circulator 2, and enters the linear Sagnac interferometer through the output port of the circulator 2. The optical paths of the two interfering beams at the first wavelength are as follows:

The light path of the first beam passes through the coupler 6 → the first wavelength division multiplexer 8 → the first collimator 1101 of the first gas sensor → the first absorption gas chamber 14 → the second collimator 1102 of the first gas sensor → The first fiber grating 17→the second collimator 1102 of the first gas sensor→the first absorption gas chamber 14→the first collimator 1101 of the first gas sensor→the first wavelength division multiplexer 8→coupler 6→ Fiber delay ring 7 → circulator 2;

The optical path of the second beam passes through the coupler 6 → the optical fiber delay ring 7 → the first wavelength division multiplexer 8 → the first collimator 1101 of the first gas sensor → the first absorption gas chamber 14 → the second Collimator 1102→first fiber grating 17→second collimator 1102 of the first gas sensor→first absorption gas chamber 14→first collimator 1101 of the first gas sensor→first wavelength division multiplexer 8 → Coupler 6 → Circulator 2.

Since the gas in the first absorption gas cell 14 is modulated by the pump laser, the change of the local refractive index will cause the change of the phase. When the two interfering beams pass through the absorption gas cell, they will carry the phase information of the gas concentration information. Finally, the two beams of interference light enter the photodetector 3 through the circulator 2. The photodetector 3 converts the interference light intensity signal into a voltage signal, which is then collected by the data acquisition card 4, and then transmitted to the computer system 5 for signal processing. and gas concentration calculations.

Change and fix the scanning laser wavelength to the second wavelength, the laser will enter the second wavelength division multiplexer 9 from the reflection end of the first wavelength division multiplexer 8, enter the first collimator 1201 of the second gas sensor 12, and pass through the first wavelength division multiplexer 8. The second absorption gas chamber 15 enters the first collimator 1202 of the second gas sensor 12, and is reflected by the second fiber grating 18, and the gas concentration information of the second absorption gas chamber 15 can be obtained at the photodetector 3; change and fix When the wavelength of the scanning laser is the third wavelength, the laser will enter the third wavelength division multiplexer 10 from the second wavelength division multiplexer 9 , enter the first collimator 1301 of the third gas sensor 13 , and pass through the third absorption gas chamber 16 After entering the second collimator 1302 of the third gas sensor 13 and being reflected by the third fiber grating 19 , the gas concentration information of the third absorption gas chamber 16 can be obtained at the photodetector 3 .

By synthesizing the gas concentration data at each wavelength, the gas concentration at different locations to be measured can be accurately evaluated. In order to realize the extraction of the weak signal, after the data acquisition card 4 is connected to the computer system 5 through the usb, the labview software programming is used to carry out signal processing, the photothermal signal in the time domain is subjected to Fourier transform in real time, and the frequency domain signal is accumulated and averaged in real time. , effectively improve the signal-to-noise ratio and the minimum detection limit.

Claims (7)

1. The utility model provides a gaseous detection device of multiple spot based on photothermal effect and wavelength division multiplexing interferometer which characterized in that: the device comprises a wavelength scanning laser, a circulator, a coupler, an optical fiber delay ring, a plurality of detection units, an optical fiber splitter, a filter, a chopper, a space collimating device, an erbium-doped optical fiber amplifier, a pump laser, a photoelectric detector, a data acquisition card and a computer system;

the circulator is a three-port optical fiber circulator, the circulator is connected with one input end of the coupler, two output ends of the coupler are respectively connected with the optical fiber delay ring and the detection unit, and the detection units are arranged in parallel and connected with the optical fiber branching unit; the circulator is also respectively connected with the wavelength scanning laser and the photoelectric detector;

the optical fiber branching device is connected with a filter, the filter is connected with the output end of the space collimating device, and a chopper is arranged in the middle of the space collimating device; the input end of the spatial collimating device is connected with the output end of the erbium-doped fiber amplifier, and the input end of the erbium-doped fiber amplifier is connected with the pump laser;

the photoelectric detector is connected with a data acquisition card, the data acquisition card is connected with a computer system, and the data acquisition card is used for acquiring the demodulated voltage signal data and transmitting the data to the computer system for signal processing and gas concentration calculation.

2. The multi-point gas detection device based on photothermal effect and wavelength division multiplexing interferometer of claim 1, wherein: the detection unit comprises a sequential wavelength division multiplexer, an absorption gas chamber, a gas sensor and a fiber bragg grating; the incident end of the wavelength division multiplexer is connected with the output end of the coupler or the reflection end of the wavelength division multiplexer of the previous detection unit, the transmission end of the wavelength division multiplexer is connected with an absorption gas chamber, the absorption gas chamber is connected with a gas sensor, and the gas sensor is connected with the fiber bragg grating; the fiber grating is connected with the fiber splitter.

3. The multi-point gas detection device based on photothermal effect and wavelength division multiplexing interferometer of claim 2, wherein: the gas sensor comprises a first collimator and a second collimator, the first collimator is arranged at the front end of the absorption gas chamber, and the second collimator is arranged at the rear end of the absorption gas chamber; and laser emitted by the first collimator passes through the absorption gas chamber, enters the second collimator and is transmitted to the fiber grating.

4. The multi-point gas detection device based on photothermal effect and wavelength division multiplexing interferometer according to claim 1, wherein: the wavelength scanning laser is a laser with tunable wavelength in a C wave band.

5. The multi-point gas detection device based on photothermal effect and wavelength division multiplexing interferometer according to claim 1, wherein: the splitting ratio of the coupler is 50: 50.

6. The multi-point gas detection device based on photothermal effect and wavelength division multiplexing interferometer according to claim 1, wherein: the pump laser is a semiconductor laser and aims at a gas absorption peak through tuning wavelength.

7. The multi-point gas detection device based on photothermal effect and wavelength division multiplexing interferometer according to claim 1, wherein: the spatial collimating device comprises a first collimator and a second collimator, and laser emitted by the first collimator enters the second collimator through the chopper and is transmitted to the filter.

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