Laser fiber methane gas detection system and method based on dual-wavelength OTDR

Abstract

The invention discloses a laser optical fiber methane gas detection system and method based on a dual-wavelength OTDR. The system comprises: a signal generator, a temperature controller, a plurality of lasers, a first optical switch, an optical circulator, a second optical switch, a plurality of couplers, a plurality of gas absorption chambers, a plurality of optical fiber reflectors, a photoelectric conversion tube, an absorption peak feedback module, an operational amplifier, a signal processing circuit and a display module. The signal generator is respectively connected to the laser and the operational amplifier; the temperature controller is respectively connected to the laser and the absorption peak feedback module; the laser is also connected to the first optical switch, which is connected to the second optical switch via the optical circulator; the second optical switch is also connected in series with the plurality of couplers, the plurality of gas absorption chambers and the plurality of optical fiber reflectors; the photoelectric conversion tube is respectively connected to the optical circulator, the absorption peak feedback module and the operational amplifier; the operational amplifier is also connected to the signal processing circuit, which is also connected to the display module.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas sensing, and in particular relates to a laser optical fiber methane gas detection system and method based on a dual-wavelength OTDR. Background Art

[0002] Methane is a flammable and explosive gas and a major component of natural gas, gas, and biogas. Due to the frequent occurrence of methane safety incidents, there is a real need for sensors that can monitor methane leaks in real time. Current electrical methane sensors are susceptible to poisoning, require calibration, and are not inherently safe.

[0003] The method of methane monitoring using fiber-optic transmission laser combined with a gas cell is inherently safe, highly sensitive, has good insulation, is resistant to electromagnetic interference, has a simple structure, and can be used for long-distance continuous monitoring. It can monitor in harsh environments such as high temperature, strong electromagnetic, and chemical corrosion, and is highly practical.

[0004] Based on the principles of gas molecular absorption spectroscopy, existing technology employs OTDR technology, incorporating the Beer-Lambert law, to measure gas concentration by detecting laser absorption losses in the gas chamber at different locations within the optical fiber. Furthermore, the reflected light from the fiber optic reflector, instead of Rayleigh scattered light, is used as the detection signal, increasing the intensity of the detected optical signal by two orders of magnitude and improving the signal-to-noise ratio. Since methane only absorbs the characteristic 1653.72nm wavelength and not the 1650nm wavelength, these two light sources are used as OTDR light sources to measure the loss of the return light from the fiber optic reflector connected in series with the gas chamber. An optical switch is used to control the optical path selection state. When methane is present in the gas chamber, the measured loss results will differ. This difference in loss allows the location of the gas chamber containing methane and its concentration to be determined. Because the wavelengths of the two lasers used are very similar, all losses, other than those caused by methane, generated during the optical fiber transmission process are nearly identical. Summary of the Invention

[0005] The present invention aims to solve the deficiencies of the prior art and provides the following solutions:

[0006] A dual-wavelength OTDR-based laser fiber methane gas detection system includes: a signal generator, a temperature controller, several lasers, a first optical switch, an optical circulator, a second optical switch, several couplers, several gas absorption chambers, several fiber reflectors, a photoelectric conversion tube, an absorption peak feedback module, an operational amplifier, a signal processing circuit, and a display module;

[0007] The signal generator is connected to the laser and the operational amplifier respectively;

[0008] The temperature controller is connected to the laser and the absorption peak feedback module respectively;

[0009] The laser is also connected to the first optical switch, and the first optical switch is connected to the second optical switch via the optical circulator;

[0010] The second optical switch is further connected in series with a plurality of the couplers, a plurality of the gas absorption chambers and a plurality of the fiber reflectors;

[0011] The photoelectric conversion tube is connected to the optical circulator, the absorption peak feedback module and the operational amplifier respectively;

[0012] The operational amplifier is further connected to the signal processing circuit, and the signal processing circuit is further connected to the display module.

[0013] Preferably, the signal generator is used to generate a modulated optical signal;

[0014] The temperature controller is used to adjust the temperature of the laser so that the laser generates laser light of a target wavelength;

[0015] The photoelectric conversion tube is used to perform photoelectric conversion, converting the optical signal carrying the information of the gas to be detected into a photocurrent;

[0016] The signal processing circuit is used to obtain concentration information of the gas to be detected based on the photocurrent.

[0017] Preferably, the modulated optical signal is a narrow pulse rectangular wave with adjustable pulse width, a pulse width of 30-160ns, and a duty cycle of 2‰.

[0018] Preferably, the temperature controller includes a KW-DFB semiconductor laser control module, which controls the temperature of the laser through internally integrated TEC and NTC to tune the output wavelength of the laser.

[0019] Preferably, the laser comprises two DFB semiconductor lasers, and the laser wavelengths are 1650 nm and 1653.7 nm respectively;

[0020] The 1650nm wavelength laser is used to measure the loss of all light passing through the optical fiber except methane gas, and the 1653.7nm wavelength laser is used to measure the loss of all light passing through the optical fiber including methane gas.

[0021] Preferably, the first optical switch switches the optical path every first preset time period, so that the laser light with a wavelength of 1650 nm or the laser light with a wavelength of 1653.7 nm is transmitted along the optical fiber, and a loss test is performed on the optical path;

[0022] The second optical switch switches the optical path every second preset time period, so that the optical fiber sensing network is alternately connected to the device to detect different optical paths connected to the air chamber.

[0023] Preferably, the splitting ratio of the coupler is 1:n-(p-1), wherein n is the number of the gas absorption chambers, and p is the position sequence number of the gas absorption chamber relative to the optical circulator.

[0024] Preferably, the length of the gas absorption chamber is 5 to 100 cm;

[0025] The gas absorption chamber includes two collimators, one of which expands the light in the optical fiber and outputs it, and the other collimator couples the light passing through the gas chamber into the optical fiber, and the optical path is reversible.

[0026] Preferably, the optical fiber reflector is a reflector with a wavelength of 1600nm to 1680nm, and a reflectivity greater than 90%.

[0027] The present invention also provides a laser fiber methane gas detection method based on a dual-wavelength OTDR, which is applied to any of the above-mentioned detection systems and includes the following steps:

[0028] When the OTDR measures the optical fiber loss at the wavelength of the characteristic absorption peak of methane, the measured first loss includes: the optical fiber loss, the loss of each optical fiber connector, the methane loss in each gas absorption chamber, and the gas loss other than methane in the gas absorption chamber;

[0029] When the OTDR measures the optical fiber loss at a wavelength other than the characteristic absorption peak of methane, the measured second loss includes: the optical fiber loss, the loss of each optical fiber connector, and the gas loss in the gas absorption chamber except methane;

[0030] Subtracting the first loss from the second loss to obtain a light loss value of methane in each of the gas absorption chambers for 1653.72 nm light;

[0031] The light loss value is calibrated with the methane concentration to obtain the methane concentration in each of the gas absorption chambers.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] (1) The present invention introduces an absorption peak feedback module and a calibration gas chamber. First, the laser emitter is modulated using a carrier signal and a temperature controller. Then, the absorption peak feedback module calculates the ratio of the upper and lower amplitudes of the signal after photoelectric conversion to the lowest point of the absorption peak and feeds it back to the temperature controller. The temperature controller automatically adjusts the operating temperature of the laser emitter according to the proportional relationship to automatically lock the gas absorption peak, thereby greatly improving the stability and automation of the detection device.

[0034] (2) The coupler with a specific splitting ratio of the present invention makes the light intensity distributed to each gas chamber nearly the same, thereby significantly increasing the laying distance of the sensing fiber and the number of monitoring points;

[0035] (3) The two laser wavelengths used in the present invention are 1653.7nm and 1650nm respectively. The difference between the two is the relatively accurate absorption loss of the methane gas molecules in the environment after eliminating environmental factors and system noise. The methane gas concentration in the test environment is then inverted by light intensity. Because the wavelengths of the two lasers are very close, the system can assume that the various losses generated during the transmission process of the two are close to each other, thereby improving the accuracy of the detection results. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0037] Figure 1 Schematic diagram of the system structure of an embodiment of the present invention.

[0038] Description of reference numerals:

[0039] 1. Signal generator; 2. Temperature controller; 3. First laser; 4. Second laser; 5. First optical switch; 6. Optical circulator; 7. Second optical switch; 8. First coupler; 9. First gas absorption chamber; 10. First fiber optic reflector; 11. Second coupler; 12. Second gas absorption chamber; 13. Second fiber optic reflector; 14. Third coupler; 15. Third gas absorption chamber; 16. Third fiber optic reflector; 17. Fourth coupler; 18. Fourth gas absorption chamber; 19. Fourth fiber optic reflector; 20. Photoelectric converter; 21. Absorption peak feedback module; 22. Operational amplifier; 23. Signal processing circuit; 24. Display module; 901. First collimator; 902. Second collimator; 1201. Third collimator; 1202. Fourth collimator; 1501. Fifth collimator; 1502. Sixth collimator; 1801. Seventh collimator; 1802. Eighth collimator. DETAILED DESCRIPTION

[0040] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0041] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0042] First, let's introduce the detection principle of this invention: According to molecular spectroscopy theory, different gas molecules have different internal structures and different intermolecular interactions, so different gas molecules have different spectral absorption ranges. When the laser emits light of a specific wavelength, and the energy of the photon is exactly equal to the difference between the energy levels of the gas molecules, the photon will be absorbed by the gas molecule, and the absorbed gas molecule will undergo an energy level transition. The expression is:

[0043] △E=E2-E1=hυ

[0044] Where h is Planck's constant, υ is the frequency of light, and E1 and E2 are the energies of the initial and final energy states, respectively.

[0045] Beer-Lambert law: When a laser outputs a frequency of λ and a light intensity of I0, and passes through a gas to be measured, if the wavelength range of the laser coincides with the spectral absorption range of the gas molecules, the gas molecules will absorb part of the light intensity according to molecular spectroscopy theory, and the output light intensity I and the input light intensity I0 satisfy the Beer-Lambert law:

[0046] I=I0e -α(λ)LC

[0047] Where α(λ) represents the absorption coefficient, α(λ) = NSf(λ,λ0); N represents the density of gas molecules per unit volume and per unit pressure, in mol·cm -3 ·atm -1 ; S represents the intensity of the gas absorption line; f(λ,λ0) is the linear function of the absorption line.

[0048] According to the absorption spectrum of methane molecules, methane molecules have a high absorption coefficient for lasers with a wavelength of 1653.7nm, but do not absorb lasers with a wavelength of 1650nm. Since the two wavelengths are very close, the system can consider the various losses generated during the transmission of the two to be infinitely close.

[0049] The signal processing circuit collects the signals of the 1650nm laser transmitter and the 1653.7nm laser transmitter, and the number of collection times is no less than 30 times. Through superposition and averaging noise reduction, the reflected signals are well collected.

[0050] Example 1

[0051] In this embodiment, if Figure 1As shown, the laser fiber methane gas detection system based on dual-wavelength OTDR includes: a signal generator 1, a temperature controller 2, several lasers, a first optical switch 5, an optical circulator 6, a second optical switch 7, several couplers, several gas absorption chambers, several fiber reflectors, a photoelectric conversion tube 20, an absorption peak feedback module 21, an operational amplifier 22, a signal processing circuit 23 and a display module 24.

[0052] The signal generator 1 is connected to the laser and the operational amplifier 22 respectively; the temperature controller 2 is connected to the laser and the absorption peak feedback module 21 respectively; the laser is also connected to the first optical switch 5, which is connected to the second optical switch 7 via the optical circulator 6; the second optical switch 7 is also connected in series with several couplers, several gas absorption chambers, and several fiber optic reflectors; the photoelectric conversion tube 20 is respectively connected to the optical circulator 6, the absorption peak feedback module 21, and the operational amplifier 22; the operational amplifier 22 is also connected to the signal processing circuit 23, and the signal processing circuit 23 is also connected to the display module 24.

[0053] Specifically, the signal generator 1 is used to generate a modulated light signal; the temperature controller 2 is used to adjust the temperature of the laser so that the laser generates laser light of the target wavelength; the photoelectric conversion tube 20 is used to perform photoelectric conversion to convert the light signal carrying information about the gas to be detected into a photocurrent; and the signal processing circuit 23 is used to obtain the concentration information of the gas to be detected based on the photocurrent.

[0054] The modulated optical signal is a narrow pulse rectangular wave with adjustable pulse width, a pulse width of 30 to 160 ns, and a duty cycle of 2‰.

[0055] The temperature controller 2 includes a KW-DFB semiconductor laser control module, which controls the temperature of the laser through the internally integrated TEC and NTC to tune the output wavelength of the laser.

[0056] The laser includes two DFB semiconductor lasers: a first laser 3 and a second laser 4. The wavelength of the first laser 3 is 1650 nm, and the wavelength of the second laser 4 is 1653.7 nm. The laser with a wavelength of 1650 nm is used to measure the loss of all light except methane gas passing through the optical fiber, and the laser with a wavelength of 1653.7 nm is used to measure the loss of all light including methane gas passing through the optical fiber.

[0057] In this embodiment, the first optical switch 5 may be a 2×1 optical switch, which is program-controlled to switch the conduction states of the first laser 3 and the second laser 4. The optical path is switched every first preset time, such as half a minute, so that a laser with a wavelength of 1650 nm or a laser with a wavelength of 1653.7 nm is transmitted along the optical fiber to perform a loss test on the optical path. The second optical switch 7 may be a 3×1 optical switch, which switches the conduction states of the three sensing optical paths. The optical paths are switched every second preset time, such as one minute, so that the optical fiber sensing network is alternately connected to the device, and different optical paths connecting to the gas chamber are tested.

[0058] In this embodiment, four couplers are included: a first coupler 8, a second coupler 11, a third coupler 14, and a fourth coupler 17. The splitting ratio of the couplers is 1:n-(p-1), where n is the number of gas absorption chambers and p is the position of the gas absorption chamber relative to the optical circulator 6. In this embodiment, the splitting ratio of the first coupler 8 is 1:4, with output end 802 having a light intensity ratio of 4 and output end 803 having a light intensity ratio of 1. The splitting ratio of the second coupler 11 is 1:3, with output end 1102 having a light intensity ratio of 3 and output end 1103 having a light intensity ratio of 1. The splitting ratio of the third coupler 14 is 1:2, with output end 1402 having a light intensity ratio of 2 and output end 1403 having a light intensity ratio of 1. The splitting ratio of the fourth coupler 17 is 1:1, with output ends 1702 and 1703 being the two output ends of the fourth coupler 17.

[0059] The gas absorption chamber has a length of 5 to 100 cm and includes two collimators, one of which expands and outputs light from the optical fiber, while the other couples the light passing through the chamber back into the optical fiber. The optical path is reversible. In this embodiment, four gas absorption chambers are included: a first gas absorption chamber 9, a second gas absorption chamber 12, a third gas absorption chamber 15, and a fourth gas absorption chamber 18. The first gas absorption chamber 9 includes a first collimator 901 and a second collimator 902, the second gas absorption chamber 12 includes a third collimator 1201 and a fourth collimator 1202, the third gas absorption chamber 15 includes a fifth collimator 1501 and a sixth collimator 1502, and the fourth gas absorption chamber 18 includes a seventh collimator 1801 and an eighth collimator 1802. The coupling insertion loss between the first collimator 901 and the second collimator 902, the coupling insertion loss between the third collimator 1201 and the fourth collimator 1202, the coupling insertion loss between the fifth collimator 1501 and the sixth collimator 1502, and the coupling insertion loss between the seventh collimator 1801 and the eighth collimator 1802 are all less than 0.5dB, and the difference between the coupling insertion losses at each location does not exceed 0.1dB.

[0060] The fiber optic reflector is a reflector with a wavelength of 1600 nm to 1680 nm and a reflectivity greater than 90%. In this embodiment, the fiber optic reflector includes: a first fiber optic reflector 10 , a second fiber optic reflector 13 , a third fiber optic reflector 16 and a fourth fiber optic reflector 19 .

[0061] Furthermore, in this embodiment, the first laser 3 and the second laser 4 are connected to the input end 501 and the input end 502 of the first optical switch 5, respectively; the output end 503 of the first optical switch 5 is connected to the input end 601 of the optical circulator 6; the output end 602 of the optical circulator 6 is connected to the input end 701 of the second optical switch 7; the output end 702 of the second optical switch 7 is connected to the input end 801 of the first coupler 8; the output end 803 of the first coupler 8 is connected to the first collimator 901 of the first gas absorption chamber 9; the second collimator 902 of the first gas absorption chamber 9 is connected to the first fiber reflector 10; the output end 802 of the first coupler 8 is connected to the input end 1101 of the second coupler 11 via a 2 km sensing optical fiber; the output end 1103 of the second coupler 11 is connected to the third collimator 1201 of the second gas absorption chamber 12; the fourth collimator 1202 of the second gas absorption chamber 12 is connected to the second fiber reflector 13; and the output end 1103 of the second coupler 11 is connected to the third collimator 1201 of the second gas absorption chamber 12; 02 is connected to the input end 1401 of the third coupler 14 through a 2 km sensing optical fiber, the output end 1403 of the third coupler 14 is connected to the fifth collimator 1501 of the third gas absorption chamber 15, the sixth collimator 1502 of the third gas absorption chamber 15 is connected to the third fiber reflector 16, the output end 1402 of the third coupler 14 is connected to the input end 1701 of the fourth coupler 17 through a 2 km sensing optical fiber, the output end 1703 of the fourth coupler 17 is connected to the fourth gas absorption chamber 15 The seventh collimator 1801 of the absorption chamber 18 is connected, the eighth collimator 1802 of the fourth gas absorption chamber 18 is connected to the fourth fiber reflector 19, the return end 603 of the optical circulator 6 is connected to the input end 2001 of the photoelectric conversion tube 20, the output end 2002 of the photoelectric conversion tube 20 is connected to the inverting input end 2202 of the operational amplifier 22, the output end 2002 of the photoelectric conversion tube 20 is connected to the absorption peak feedback module 21, and the absorption peak feedback module 21 is connected to the temperature controller 2. The output end 2203 of the operational amplifier 22 is connected to the signal processing circuit 23, the signal processing circuit 23 is connected to the display module 24, and the non-inverting input end 2201 of the operational amplifier 22 is connected to the signal generator 1.

[0062] In this embodiment, the two collimators in the gas absorption chamber are fixedly connected at both ends of a stainless steel fixed tube with a length of 22 cm and an inner diameter of 2 cm. Two through holes are provided, each with a diameter of 0.5 cm. The sensing fiber can be a conventional single-mode fiber or a multimode fiber.

[0063] The following describes the workflow of the system in this embodiment:

[0064] Start the system, the signal generator 1 generates a pulse signal, activates the first laser 3 and the second laser 4, the temperature controller 2 controls the temperature of the first laser 3 and the second laser 4, and fine-tunes the wavelength of the first laser 3 and the second laser 4. The first optical switch 5 first switches to connect the first laser 3 to the optical circulator 6 and disconnects the second laser 4 from the optical circulator 6. The second optical switch 7 first switches to connect the optical circulator 6 to the first sensor network 702. The first laser passes through the input end 601 of the optical circulator 6, is emitted from the output end 602 of the optical circulator 6, is input through the input end 701 of the second optical switch 7, is output from the output end 702 of the second optical switch 7, and is input through the input end 801 of the first coupler 8. All the laser beams pass through the first coupler 8. The output end 803 of the coupler 8 is output, passes through the first collimator 901 into the first gas absorption chamber 9, and is emitted to the first fiber reflector 10 through the second collimator 902. It is reflected back by the first fiber reflector 10, passes through the first coupler 8 and the second optical switch 7, and is input from the output end 602 of the optical circulator 6. It returns through the return end 603 of the optical circulator 6. Another laser beam is input through the output end 802 of the first coupler 8 and the input end 1101 of the second coupler 11. One laser beam is output through the output end 1103 of the second coupler 11, passes through the third collimator 1201 into the second gas absorption chamber 12, and is emitted to the second fiber reflector 13 through the third collimator 1201. It is reflected back by the second fiber reflector 13 and passes through the second coupler 1 1, the first coupler 8, the second optical switch 7, input from the output end 602 of the optical circulator 6, return through the return end 603 of the optical circulator 6, another laser beam is input through the output end 1102 of the second coupler 11 and the input end 1401 of the third coupler 14, one laser beam is output through the output end 1403 of the third coupler 14, enters the third gas absorption chamber 15 through the fifth collimator 1501, is emitted to the third fiber reflector 16 through the sixth collimator 1502, is reflected by the third fiber reflector 16, passes through the third coupler 14, the second coupler 11, the first coupler 8, the second optical switch 7, input from the output end 602 of the optical circulator 6, return through the return end 603 of the optical circulator 6, and another laser beam is output through the third coupler 14. The output end 802 of the coupler 14 is input through the input end 1701 of the fourth coupler 17. A path of laser light is output through the output end 1703 of the fourth coupler 17, passes through the seventh collimator 1801, enters the fourth gas absorption chamber 18, passes through the eighth collimator 1802, is emitted to the fourth fiber optic reflector 19, is reflected back by the fourth fiber optic reflector 19, passes through the fourth coupler 17, the third coupler 14, the second coupler 11, the first coupler 8, and the second optical switch 7, and is input from the output end 602 of the optical circulator 6. The laser light returns through the return end 603 of the optical circulator 6. The laser light returned from the return end 603 of the optical circulator 6 passes through the input end 2001 of the photoelectric conversion tube 20, is converted into an electrical signal, and is output through the output end 2002 of the photoelectric conversion tube 20.One electrical signal, after being processed by the absorption peak feedback module 21, is fed into the temperature controller 2 to adjust the temperature of the first laser 3 and the second laser 4. The other electrical signal is fed into the inverting input 2201 of the operational amplifier 22. The pulse signal from the signal generator 1 is fed into the non-inverting input 2202 of the operational amplifier 22. After signal amplification, it is input into the signal processing circuit 23 via the output 2203 of the operational amplifier 22. The signal is subjected to noise reduction processing and inverted to obtain the gas concentration, which is then displayed on the display module 24. The data collected after the first laser 3 enters the device is collected.

[0065] After 30 seconds of signal collection, the first optical switch 5 switches to the second laser 4, connecting it to the optical circulator 6. The first laser 3 is disconnected from the optical circulator 6. The second optical switch 7 continues to switch to the first sensor network 702. Following the same steps as above, data is collected after the second laser emitter enters the device.

[0066] The data collected by the first laser 3 and the data collected by the second laser 4 are subtracted point by point to obtain the methane concentration of the first sensor network 702 after eliminating the influence of the detection environment factors on the methane concentration detection.

[0067] After another thirty seconds, the first optical switch 5 switches to the first laser 3, connecting it to the optical circulator 6. The second laser 4 is disconnected from the optical circulator 6. The second optical switch 7 switches to the second sensor network 703. Following the same steps as above, the data after the first laser 3 enters the device is collected.

[0068] Similarly, after another thirty seconds, the first optical switch 5 switches to the second laser 4, connecting it to the optical circulator 6. The first laser 3 is disconnected from the optical circulator 6. The second optical switch 7 continues to switch to the second sensor network 703. Following the same steps as above, the data after the second laser 4 enters the device is collected.

[0069] The data collected by the first laser 3 and the data collected by the second laser 4 are subtracted point by point to obtain the methane concentration of the second sensor network 703 after eliminating the influence of the detection environment factors on the methane concentration detection.

[0070] As described above, the methane concentration of the third sensor network 704 is obtained after eliminating the influence of the detection environment factors on the methane concentration detection.

[0071] Example 2

[0072] In this embodiment, a laser fiber methane gas detection method based on a dual-wavelength OTDR includes the following steps:

[0073] When the OTDR measures the optical fiber loss at the wavelength of the characteristic absorption peak of methane, the measured first loss includes: the optical fiber loss, the loss of each optical fiber connector, the methane loss in each gas absorption chamber, and the gas loss other than methane in the gas absorption chamber;

[0074] When the OTDR measures the optical fiber loss at a wavelength other than the characteristic absorption peak of methane, the measured second loss includes: the optical fiber loss, the loss of each optical fiber connector, and the gas loss in the gas absorption chamber except methane;

[0075] The first loss and the second loss are subtracted to obtain the optical loss value of methane in each gas absorption chamber for 1653.72 nm light;

[0076] The light loss value is calibrated with the methane concentration to obtain the methane concentration in each gas absorption chamber.

[0077] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. Laser fiber methane gas detection system based on dual-wavelength OTDR, characterized by: include: A signal generator, a temperature controller, several lasers, a first optical switch, an optical circulator, a second optical switch, several couplers, several gas absorption chambers, several fiber optic reflectors, a photoelectric conversion tube, an absorption peak feedback module, an operational amplifier, a signal processing circuit, and a display module; The signal generator is connected to the laser and the operational amplifier respectively; The temperature controller is connected to the laser and the absorption peak feedback module respectively; The temperature controller includes a KW-DFB semiconductor laser control module, which controls the temperature of the laser through internally integrated TEC and NTC to tune the output wavelength of the laser; The laser is also connected to the first optical switch, and the first optical switch is connected to the second optical switch via the optical circulator; The second optical switch is further connected in series with a plurality of the couplers, a plurality of the gas absorption chambers and a plurality of the fiber reflectors; The photoelectric conversion tube is connected to the optical circulator, the absorption peak feedback module and the operational amplifier respectively; The operational amplifier is further connected to the signal processing circuit, and the signal processing circuit is further connected to the display module; The absorption peak feedback module calculates the ratio of the upper and lower amplitudes of the signal after photoelectric conversion to the lowest point of the absorption peak and feeds it back to the temperature controller. The temperature controller automatically adjusts the operating temperature of the laser emitter according to the proportional relationship to automatically lock the gas absorption peak.

2. The laser fiber methane gas detection system based on dual-wavelength OTDR according to claim 1 is characterized in that: The signal generator is used to generate a modulated optical signal; The temperature controller is used to adjust the temperature of the laser so that the laser generates laser light of a target wavelength; The photoelectric conversion tube is used to perform photoelectric conversion, converting the optical signal carrying the information of the gas to be detected into a photocurrent; The signal processing circuit is used to obtain concentration information of the gas to be detected based on the photocurrent.

3. The laser fiber methane gas detection system based on dual-wavelength OTDR according to claim 2, characterized in that: The modulated optical signal is a narrow pulse rectangular wave with adjustable pulse width, a pulse width of 30 to 160 ns, and a duty cycle of 2‰.

4. The laser fiber methane gas detection system based on dual-wavelength OTDR according to claim 1, characterized in that: The laser comprises two DFB semiconductor lasers, and the laser wavelengths are 1650nm and 1653.7nm respectively; The 1650nm wavelength laser is used to measure the loss of all light passing through the optical fiber except methane gas, and the 1653.7nm wavelength laser is used to measure the loss of all light passing through the optical fiber including methane gas.

5. The laser fiber methane gas detection system based on dual-wavelength OTDR according to claim 4, characterized in that: The first optical switch switches the optical path at a first preset time interval, so that the laser light with a wavelength of 1650 nm or the laser light with a wavelength of 1653.7 nm is transmitted along the optical fiber, and a loss test is performed on the optical path; The second optical switch switches the optical path every second preset time period, so that the optical fiber sensing network is alternately connected to the device to detect different optical paths connected to the air chamber.

6. The laser fiber methane gas detection system based on dual-wavelength OTDR according to claim 1, characterized in that: The splitting ratio of the coupler is 1:n-(p-1), where n is the number of the gas absorption chambers and p is the position sequence number of the gas absorption chamber relative to the optical circulator.

7. The laser fiber methane gas detection system based on dual-wavelength OTDR according to claim 1, characterized in that: The length of the gas absorption chamber is 5 to 100 cm; The gas absorption chamber includes two collimators, one of which expands the light in the optical fiber and outputs it, and the other collimator couples the light passing through the gas chamber into the optical fiber, and the optical path is reversible.

8. The laser fiber methane gas detection system based on dual-wavelength OTDR according to claim 1, characterized in that: The optical fiber reflector is a reflector with a wavelength of 1600nm to 1680nm, and a reflectivity greater than 90%.

9. A laser fiber methane gas detection method based on dual-wavelength OTDR, the detection method being applied to the detection system according to any one of claims 1 to 8, characterized in that: The following steps are involved: When the OTDR measures the optical fiber loss at the wavelength of the characteristic absorption peak of methane, the measured first loss includes: the optical fiber loss, the loss of each optical fiber connector, the methane loss in each gas absorption chamber, and the gas loss other than methane in the gas absorption chamber; When the OTDR measures the optical fiber loss at a wavelength other than the characteristic absorption peak of methane, the measured second loss includes: the optical fiber loss, the loss of each optical fiber connector, and the gas loss in the gas absorption chamber other than methane; Subtracting the first loss from the second loss to obtain a light loss value of methane in each of the gas absorption chambers for 1653.72 nm light; The light loss value is calibrated with the methane concentration to obtain the methane concentration in each of the gas absorption chambers.

Images