A multi-point gas concentration detection method and detection device for eliminating the influence of dynamic loss

Abstract

The present invention discloses a multi-point gas concentration detection method and detection device for eliminating the influence of dynamic loss. The method is to connect a photoacoustic cell of the same model in parallel to each gas chamber to be tested as a reference gas chamber. The gas concentration of the reference gas chamber is known. The second harmonic signals output by the two gas chambers are ratioed. It can be seen from the ratio that the gas concentration of the gas chamber to be tested and the ratio of the second harmonic signals of the two gas chambers are in a linear relationship. The above relationship can be used to directly obtain the concentration information of the gas chamber to be tested from the gas concentration of the reference gas chamber and the ratio of the second harmonic signals of the two gas chambers. The method can not only realize multi-point gas concentration detection, but also eliminate the influence of optical fiber bending loss and the absorption loss caused by the front-end gas chamber when the first gas chamber to be tested detects the gas concentration of the second gas chamber to be tested, and the first gas chamber to be tested and the second gas chamber to be tested detect the gas concentration of the third reference gas chamber.

Description

Technical Field

[0001] The present invention relates to the technical field of gas concentration detection, and in particular to a multi-point gas concentration detection method and a detection device thereof for eliminating the influence of dynamic loss. Background Art

[0002] Fiber optic gas sensing technology is a technology with broad prospects in the current sensing field. Fiber optic gas sensors have specific applications in many industries with their high sensitivity, high precision, and fast response speed. In particular, chemical plant explosions have occurred frequently in recent years, and a large amount of toxic and harmful gases have seriously threatened the ecological environment and human health. Therefore, the detection of some harmful trace gases is particularly important to ensure our human safety. Not only that, in mining, oil and gas pipelines, and some corporate operation workshops, the detection of some trace gases requires not only high precision, but also distributed multi-point detection to ensure safety during operations. However, in the process of multi-point gas detection, it is easy to be affected by the external environment and itself, resulting in fiber loss, resulting in low gas detection accuracy.

[0003] Fiber loss is an important indicator of fiber transmission quality. There are many factors that cause fiber loss. In multi-point gas detection, the dynamic loss caused by light bending and the absorption of the detection chamber at the front end of the series gas chamber will cause the light signal to gradually weaken after being transmitted through the optical fiber. At the same time, the light signal will also gradually weaken, and the background signal in the output signal of the subsequent phase-locked amplifier will be too large, affecting the detection accuracy and stability. Summary of the invention

[0004] In order to overcome the defects in the above-mentioned prior art, the present invention provides a multi-point gas concentration detection method and a detection device thereof which have reasonable structural design, simple operation and can effectively eliminate the influence of dynamic loss.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A method for detecting gas concentration at multiple points to eliminate the influence of dynamic loss comprises the following steps:

[0007] (a) First, two waveforms output by a signal generator are superimposed to form a modulated light and a laser beam is emitted through a laser collimator. The laser beam is connected to a first coupler and is divided into two beams. The two beams pass through a first collimator and a second collimator respectively and enter a first gas chamber to be measured and a first reference gas chamber and then pass through a photoacoustic resonant cavity. The modulated light is absorbed by the gas, and the sound waves generated by the photoacoustic effect resonate on the wall of the photoacoustic resonant cavity, thereby enhancing the sound wave signal. The signal is detected by a microphone, and the microphone converts the sound signal into an electrical signal for output;

[0008] (b) The laser beam is collected by the third collimator and connected to the second coupler to be divided into two beams again. The beams pass through the fifth collimator and the sixth collimator respectively and enter the second gas chamber to be measured and the second reference gas chamber, and then pass through the photoacoustic resonant cavity. The modulated light is absorbed by the gas, and the sound waves generated by the photoacoustic effect resonate on the wall of the photoacoustic resonant cavity. The sound wave signal is detected by the microphone, and the microphone converts the sound signal into an electrical signal for output;

[0009] (c) The laser beam is collected by the seventh collimator and connected to the third coupler to be divided into two beams again. The beams pass through the ninth collimator and the tenth collimator respectively and enter the third gas chamber to be tested and the third reference gas chamber, and then pass through the photoacoustic resonant cavity. The modulated light is absorbed by the gas, and the sound waves generated by the photoacoustic effect resonate on the wall of the photoacoustic resonant cavity. The sound wave signals are detected by the microphone, and the microphone converts the sound signals into electrical signals for output.

[0010] (d) The microphones of the first gas chamber to be tested, the second gas chamber to be tested, the third gas chamber to be tested, and the first reference gas chamber, the second reference gas chamber, and the third reference gas chamber are connected to an analog switch to control the signal output of each optical path; the other end of the analog switch is connected to a preamplifier circuit to convert the current signal into a voltage signal, which is extracted by a phase-locked amplifier, and finally the harmonic signal is collected by a data acquisition system and transmitted to a computer for data processing.

[0011] (e) By making a ratio between the collected harmonic signal of the first gas chamber to be tested and the first reference gas chamber, it can be obtained that the gas concentration of the gas chamber to be tested and the ratio of the second harmonic signal of the two gas chambers are in a linear relationship. The above relationship can be used to directly calculate the concentration information of the first gas chamber to be tested from the gas concentration of the first reference gas chamber and the ratio of the second harmonic signal of the two gas chambers; by making a ratio between the harmonic signal of the second gas chamber to be tested and the second reference gas chamber, the concentration information of the second gas chamber to be tested can be directly calculated from the gas concentration of the second reference gas chamber and the ratio of the second harmonic signal of the two gas chambers; then by making a ratio between the harmonic signal of the third gas chamber to be tested and the third reference gas chamber, the concentration information of the third gas chamber to be tested can be calculated from the gas concentration of the third reference gas chamber and the ratio of the second harmonic signal of the two gas chambers; it can realize multi-point gas concentration detection and eliminate the influence of optical fiber bending loss and the absorption loss caused by the front-end gas chamber when the first gas chamber to be tested detects the gas concentration of the second gas chamber to be tested, and the first gas chamber to be tested and the second gas chamber to be tested detects the gas concentration of the third reference gas chamber.

[0012] Furthermore, the two waveforms output by the signal generator in step (a) are a sine wave and a sawtooth wave.

[0013] Furthermore, to ensure the signal strength of the gas chamber to be tested, the splitting ratios of the first coupler, the second coupler, and the third coupler described in steps (a) to (c) are all 99:1, that is, the light beam entering the gas chamber to be tested is 99%, and the light beam entering the reference gas chamber is 1%.

[0014] The present invention also provides a multi-point gas concentration detection device designed to eliminate the influence of dynamic loss for realizing the above detection method, comprising a signal generator, a voltage adder, a laser transmitter, a first coupler, a second coupler, a third coupler, an analog switch, a preamplifier circuit, a phase-locked amplifier, a data acquisition system and a computer system, wherein the signal output end of the signal generator is connected to the synchronization signal input end of the voltage adder, and the other end of the synchronization signal output end of the signal generator is connected to the phase-locked amplifier, the output end of the voltage adder is connected to the laser transmitter, and the output end of the laser transmitter is connected to the first coupler, the first coupler divides the optical path into optical path one and optical path two according to a fixed coupling ratio, and the optical path one is provided with a first to-be-detected gas containing unknown concentration information. air chamber, optical path two is provided with a first reference air chamber containing fixed concentration information; a first collimator is provided on the side wall of the first air chamber to be tested, and a third collimator is provided on the other side wall corresponding to the first collimator; a second collimator is provided on the side wall of the first reference air chamber, and a fourth collimator is provided on the other side wall corresponding to the second collimator; the first air chamber to be tested is connected to the second coupler through the third collimator, and the second coupler divides the output light from the first air chamber to be tested into optical path three and optical path four according to a fixed coupling ratio, optical path three is provided with a second air chamber to be tested containing unknown concentration information, optical path four is provided with a second reference air chamber containing fixed concentration information, a fifth collimator is provided on the side wall of the second air chamber to be tested, and the fifth collimator is provided with the fifth collimator. A seventh collimator is arranged on the other side wall corresponding to the collimator; a sixth collimator is arranged on the side wall of the second reference gas chamber, and an eighth collimator is arranged on the other side wall corresponding to the sixth collimator; the second gas chamber to be measured is connected to the third coupler through the seventh collimator, and the third coupler divides the output light of the second gas chamber to be measured into an optical path five and an optical path six again according to a fixed coupling ratio, the optical path five is provided with a third gas chamber to be measured containing unknown concentration information, and the optical path six is ​​provided with a third reference gas chamber containing fixed concentration information, a ninth collimator is arranged on the side wall of the third gas chamber to be measured, and an eleventh collimator is arranged on the other side wall corresponding to the ninth collimator; a tenth collimator is arranged on the side wall of the third reference gas chamber, and the other side wall corresponding to the tenth collimator A twelfth collimator is arranged on one side wall; the first gas chamber to be tested, the second gas chamber to be tested, and the third gas chamber to be tested are photoacoustic cells of exactly the same model and size as the first reference gas chamber, the second reference gas chamber, and the third reference gas chamber, and a photoacoustic resonant cavity and a microphone are arranged in the photoacoustic cell, and the photoacoustic resonant cavity is cylindrical, and the microphones of the first gas chamber to be tested, the second gas chamber to be tested, and the third gas chamber to be tested and the first reference gas chamber, the second reference gas chamber, and the third reference gas chamber are connected to an analog switch, and the signal output end of the analog switch is connected to a preamplifier circuit, and the output end of the preamplifier circuit is connected to a phase-locked amplifier, and the signal extracted by the phase-locked amplifier is collected and processed by a data acquisition system, and the signal output end of the data acquisition system is connected to a computer system.

[0015] Furthermore, the laser emitter is a butterfly distributed feedback semiconductor laser.

[0016] Furthermore, the first air chamber to be tested and the first reference air chamber, the second air chamber to be tested and the second reference air chamber, and the third air chamber to be tested and the third reference air chamber are respectively packaged in a shell for easy fixation; and each air chamber is provided with an air inlet and an air outlet.

[0017] Furthermore, the laser beam of the optical path one enters the first gas chamber to be measured through the first collimator, passes through the photoacoustic resonant cavity of the first gas chamber to be measured, and the laser beam is collected by the third collimator; the laser beam of the optical path two enters the first reference gas chamber through the second collimator, passes through the photoacoustic resonant cavity of the first reference gas chamber, and the laser beam is collected by the fourth collimator.

[0018] Furthermore, the laser beam of the optical path three enters the second gas chamber to be measured through the fifth collimator, passes through the photoacoustic resonant cavity of the second gas chamber to be measured, and the laser beam is collected by the seventh collimator; the laser beam of the optical path four enters the second reference gas chamber through the sixth collimator, passes through the photoacoustic resonant cavity of the second reference gas chamber, and the laser beam is collected by the eighth collimator.

[0019] Furthermore, the laser beam of the optical path five enters the third gas chamber to be tested through the ninth collimator, passes through the photoacoustic resonant cavity of the third gas chamber to be tested, and the laser beam is collected by the eleventh collimator; the laser beam of the optical path six enters the third reference gas chamber through the tenth collimator, passes through the photoacoustic resonant cavity of the third reference gas chamber, and the laser beam is collected by the twelfth collimator.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention proposes a method for detecting gas concentration at multiple points to eliminate the influence of dynamic loss. The method compares the collected harmonic signals of the first gas chamber to be tested with the first reference gas chamber, and can obtain a linear relationship between the gas concentration of the gas chamber to be tested and the ratio of the second harmonic signals of the two gas chambers. The above relationship can be used to directly obtain the concentration information of the first gas chamber to be tested from the gas concentration of the first reference gas chamber and the ratio of the second harmonic signals of the two gas chambers. The second gas chamber to be tested is compared with the harmonic signal of the second reference gas chamber, and the concentration information of the second gas chamber to be tested is obtained from the gas concentration of the second reference gas chamber and the ratio of the second harmonic signals of the two gas chambers. The third gas chamber to be tested is compared with the harmonic signal of the third reference gas chamber, and the concentration information of the third gas chamber to be tested is obtained from the gas concentration of the third reference gas chamber and the ratio of the second harmonic signals of the two gas chambers. It can not only realize multi-point gas concentration detection, but also eliminate the influence of optical fiber bending loss and the absorption loss caused by the front-end gas chamber when the first gas chamber to be tested detects the gas concentration of the second gas chamber to be tested, and the first gas chamber to be tested and the second gas chamber to be tested detect the gas concentration of the third reference gas chamber. This method is technically more advantageous and highly feasible, and can greatly improve the measurement accuracy in multi-point gas detection.

[0022] By proposing a multi-point gas detection method that eliminates dynamic loss, it is found that the gas concentration in the gas chamber to be tested is linearly related to the ratio of the second harmonic signals of the two gas chambers. A multi-point gas concentration detection device that eliminates the influence of dynamic loss is designed. The device encapsulates the gas chamber to be tested and the reference gas chamber with a shell for fixation, and can also fix the curvature and length of the two optical fiber beams in the distance connected to the gas chamber to be tested and the reference gas chamber after the optical fiber enters the shell, which can effectively reduce the interference of the external environment on the internal gas chamber. The stability of the entire system detection is improved, it has high practical value, and it is highly feasible, which can greatly improve the measurement accuracy in multi-point gas detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the structure of the embodiment of the present invention.

[0024] In the figure: 1-signal generator, 2-voltage adder, 3-laser transmitter, 4-first coupler, 5-second coupler, 6-third coupler, 7-analog switch, 8-preamplifier circuit, 9-lock-in amplifier, 10-data acquisition system, 11-computer system, 12-optical path one, 13-optical path two, 14-first gas chamber to be tested, 15-first reference gas chamber, 16-first collimator, 17-second collimator, 18-third collimator, 19-fourth collimator, 20-optical path three, 21-optical path four, 22-second gas chamber to be measured, 23-second reference gas chamber, 24-fifth collimator, 25-sixth collimator, 26-seventh collimator, 27-eighth collimator, 28-optical path five, 29-optical path six, 30-third gas chamber to be measured, 31-third reference gas chamber, 32-ninth collimator, 33-tenth collimator, 34-eleventh collimator, 35-twelfth collimator, 36-photoacoustic resonant cavity, 37-microphone, 38-housing, 39-air inlet, 40-air outlet. DETAILED DESCRIPTION

[0025] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 creative work are within the scope of protection of the present invention.

[0026] Reference Figure 1 As shown, a method for detecting gas concentration at multiple points to eliminate the influence of dynamic loss includes the following steps:

[0027] (a) First, two waveforms output by a signal generator are superimposed to form a modulated light and a laser beam is emitted through a laser collimator. The laser beam is connected to a first coupler and is divided into two beams. The two beams pass through a first collimator and a second collimator respectively and enter a first gas chamber to be measured and a first reference gas chamber and then pass through a photoacoustic resonant cavity. The modulated light is absorbed by the gas, and the sound waves generated by the photoacoustic effect resonate on the wall of the photoacoustic resonant cavity, thereby enhancing the sound wave signal. The signal is detected by a microphone, and the microphone converts the sound signal into an electrical signal for output;

[0028] (b) The laser beam is collected by the third collimator and connected to the second coupler to be divided into two beams again. The beams pass through the fifth collimator and the sixth collimator respectively and enter the second gas chamber to be measured and the second reference gas chamber, and then pass through the photoacoustic resonant cavity. The modulated light is absorbed by the gas, and the sound waves generated by the photoacoustic effect resonate on the wall of the photoacoustic resonant cavity. The sound wave signal is detected by the microphone, and the microphone converts the sound signal into an electrical signal for output;

[0029] (c) The laser beam is collected by the seventh collimator and connected to the third coupler to be divided into two beams again. The beams pass through the ninth collimator and the tenth collimator respectively and enter the third gas chamber to be tested and the third reference gas chamber, and then pass through the photoacoustic resonant cavity. The modulated light is absorbed by the gas, and the sound waves generated by the photoacoustic effect resonate on the wall of the photoacoustic resonant cavity. The sound wave signals are detected by the microphone, and the microphone converts the sound signals into electrical signals for output.

[0030] (d) The microphones of the first gas chamber to be tested, the second gas chamber to be tested, the third gas chamber to be tested, and the first reference gas chamber, the second reference gas chamber, and the third reference gas chamber are connected to an analog switch to control the signal output of each optical path; the other end of the analog switch is connected to a preamplifier circuit to convert the current signal into a voltage signal, which is extracted by a phase-locked amplifier, and finally the harmonic signal is collected by a data acquisition system and transmitted to a computer for data processing.

[0031] (e) By making a ratio between the collected harmonic signal of the first gas chamber to be tested and the first reference gas chamber, it can be obtained that the gas concentration of the gas chamber to be tested and the ratio of the second harmonic signal of the two gas chambers are in a linear relationship. The above relationship can be used to directly calculate the concentration information of the first gas chamber to be tested from the gas concentration of the first reference gas chamber and the ratio of the second harmonic signal of the two gas chambers; by making a ratio between the harmonic signal of the second gas chamber to be tested and the second reference gas chamber, the concentration information of the second gas chamber to be tested can be directly calculated from the gas concentration of the second reference gas chamber and the ratio of the second harmonic signal of the two gas chambers; then by making a ratio between the harmonic signal of the third gas chamber to be tested and the third reference gas chamber, the concentration information of the third gas chamber to be tested can be calculated from the gas concentration of the third reference gas chamber and the ratio of the second harmonic signal of the two gas chambers; it can realize multi-point gas concentration detection and eliminate the influence of optical fiber bending loss and the absorption loss caused by the front-end gas chamber when the first gas chamber to be tested detects the gas concentration of the second gas chamber to be tested, and the first gas chamber to be tested and the second gas chamber to be tested detects the gas concentration of the third reference gas chamber.

[0032] Furthermore, the two waveforms output by the signal generator in step (a) are a sine wave and a sawtooth wave.

[0033] Furthermore, to ensure the signal strength of the gas chamber to be tested, the splitting ratios of the first coupler, the second coupler, and the third coupler described in steps (a) to (c) are all 99:1, that is, the light beam entering the gas chamber to be tested is 99%, and the light beam entering the reference gas chamber is 1%.

[0034] The present invention also provides a multi-point gas concentration detection device designed to eliminate the influence of dynamic loss for realizing the above detection method, comprising a signal generator 1, a voltage adder 2, a laser transmitter 3, a first coupler 4, a second coupler 5, a third coupler 6, an analog switch 7, a preamplifier circuit 8, a phase-locked amplifier 9, a data acquisition system 10 and a computer system 11, wherein the signal output end of the signal generator 1 is connected to the synchronization signal input end of the voltage adder 2, the other end of the synchronization signal output end of the signal generator 1 is connected to the phase-locked amplifier 9, the output end of the voltage adder 2 is connected to the laser transmitter 3, and the output end of the laser transmitter 3 is connected to the first coupler 8. The first coupler 4 is connected to the optical path 12 and the second coupler 5 according to a fixed coupling ratio. The optical path 12 is provided with a first gas chamber 14 to be tested containing unknown concentration information, and the optical path 13 is provided with a first reference gas chamber 15 containing fixed concentration information; a first collimator 16 is provided on the side wall of the first gas chamber 14 to be tested, and a third collimator 18 is provided on the other side wall corresponding to the first collimator 16; a second collimator 17 is provided on the side wall of the first reference gas chamber 15, and a fourth collimator 19 is provided on the other side wall corresponding to the second collimator 17; the first gas chamber 14 to be tested is connected to the second coupler 5 through the third collimator 18 The second coupler 5 divides the output light from the first gas chamber 14 to be tested into a third light path 20 and a fourth light path 21 according to a fixed coupling ratio. The third light path 20 is provided with a second gas chamber 22 to be tested containing unknown concentration information, and the fourth light path 21 is provided with a second reference gas chamber 23 containing fixed concentration information. A fifth collimator 24 is provided on the side wall of the second gas chamber 22 to be tested, and a seventh collimator 26 is provided on the other side wall corresponding to the fifth collimator 24; a sixth collimator 25 is provided on the side wall of the second reference gas chamber 23, and an eighth collimator 27 is provided on the other side wall corresponding to the sixth collimator 25; the second gas chamber 22 to be tested is connected to the reference gas chamber 23 through the seventh collimator. The collimator 26 is connected to the third coupler 6. The third coupler 6 divides the output light of the second gas chamber 22 to be tested into a fifth light path 28 and a sixth light path 29 according to a fixed coupling ratio. The fifth light path 28 is provided with a third gas chamber 30 to be tested containing unknown concentration information. The sixth light path 29 is provided with a third reference gas chamber 31 containing fixed concentration information. A ninth collimator 32 is provided on the side wall of the third gas chamber 30 to be tested, and an eleventh collimator 34 is provided on the other side wall corresponding to the ninth collimator 32. A tenth collimator 33 is provided on the side wall of the third reference gas chamber 31, and a twelfth collimator 35 is provided on the other side wall corresponding to the tenth collimator 33.The first gas chamber to be tested 14, the second gas chamber to be tested 22, the third gas chamber to be tested 30 and the first reference gas chamber 15, the second reference gas chamber 23, the third reference gas chamber 31 are photoacoustic cells of the same model and size, and a photoacoustic resonant cavity 36 and a microphone 37 are arranged in the photoacoustic cell. The photoacoustic resonant cavity 36 is cylindrical. The first gas chamber to be tested 14, the second gas chamber to be tested 22, the third gas chamber to be tested 30 and the microphone 37 of the first reference gas chamber 15, the second reference gas chamber 23, the third reference gas chamber 31 are connected to an analog switch 7. The signal output end of the analog switch 7 is connected to a preamplifier circuit 8, and the output end of the preamplifier circuit 8 is connected to a phase-locked amplifier 9. The signal extracted by the phase-locked amplifier 9 is collected and processed by a data acquisition system 10, and the signal output end of the data acquisition system 10 is connected to a computer system 11. ;

[0035] Furthermore, the laser emitter 3 is a butterfly distributed feedback semiconductor laser.

[0036] Furthermore, the first test air chamber 14 and the first reference air chamber 15, the second test air chamber 22 and the second reference air chamber 23, and the third test air chamber 30 and the third reference air chamber 31 are respectively packaged with a housing 38 for easy fixation; each air chamber is provided with an air inlet 39 and an air outlet 40.

[0037] Furthermore, the laser beam of the optical path 12 enters the first gas chamber to be measured 14 through the first collimator 16, passes through the photoacoustic resonant cavity 36 of the first gas chamber to be measured 14, and the laser beam is collected by the third collimator 18; the laser beam of the optical path 2 13 enters the first reference gas chamber 15 through the second collimator 17, passes through the photoacoustic resonant cavity 36 of the first reference gas chamber 15, and the laser beam is collected by the fourth collimator 19.

[0038] Furthermore, the laser beam of the optical path three 20 enters the second gas chamber to be tested 22 through the fifth collimator 24, passes through the photoacoustic resonant cavity 36 of the second gas chamber to be tested 22, and the laser beam is collected by the seventh collimator 26; the laser beam of the optical path four 21 enters the second reference gas chamber 23 through the sixth collimator 25, passes through the photoacoustic resonant cavity 36 of the second reference gas chamber 23, and the laser beam is collected by the eighth collimator 27.

[0039] Furthermore, the laser beam of the optical path five 28 enters the third gas chamber to be tested 30 through the ninth collimator 32, passes through the photoacoustic resonant cavity 36 of the third gas chamber to be tested 30, and the laser beam is collected by the eleventh collimator 34; the laser beam of the optical path six 29 enters the third reference gas chamber 31 through the tenth collimator 33, passes through the photoacoustic resonant cavity 36 of the third reference gas chamber 31, and the laser beam is collected by the twelfth collimator 35.

[0040] The multi-point gas concentration detection method for eliminating the influence of dynamic loss provided by the present invention divides the second harmonic amplitude of the gas chamber to be measured by the second harmonic amplitude of the reference gas chamber. Theoretical derivation shows that the gas concentration of the gas chamber to be measured is linearly related to the ratio of the second harmonic signals of the two gas chambers. The above relationship can be used to directly obtain the concentration information of the gas chamber to be measured from the gas concentration of the reference gas chamber and the ratio of the second harmonic signals of the two gas chambers. The above multi-point optical fiber gas sensing system is connected and the gas to be measured is filled into the gas chamber.

[0041] During multi-point gas sensing detection, the photoacoustic signal is collected and transmitted by the signal processing units at each level, and the second harmonic of the signal is extracted through the phase-locked amplifier. The second harmonic formula can be expressed as follows:

[0042] S 2f =S m ·CFP·cos(ψ)·H 2 (v 0 )

[0043] Among them, S2f is the second harmonic signal extracted by the phase-locked amplifier, Sm is the microphone sensitivity, C is the target gas concentration, F is the response frequency of the photoacoustic cell, P is the input light power, ψ is the phase difference between the phase-locked amplifier and the laser amplitude modulation, and H2 (cm-1) is the size of the second harmonic obtained by the Fourier cosine series expansion of the absorption profile at the center of the target gas absorption line.

[0044] During the transmission process, it will always be affected by itself or the external environment, resulting in fiber bending loss, fiber insertion loss, and the absorption of the front air chamber affects the gas detection in the back air chamber. The most direct manifestation of this part of dynamic loss is the weakening of the incident laser energy and the attenuation of the optical power. Then the second harmonic signal extracted by the phase-locked amplifier will also attenuate.

[0045] The present invention proposes a multi-point gas concentration detection method for eliminating dynamic loss in an optical fiber sensing system, that is, each gas chamber to be tested is connected in parallel with a photoacoustic cell of the same model and size as a reference gas chamber, and the two gas chambers are filled with the same gas at different concentrations, wherein the concentration of the reference gas chamber is known. The formula for the second harmonic ratio extracted from the two gas chambers can be expressed as:

[0046]

[0047] Let Sm·F·cos(ψ)=ω, then the above formula can be simplified to

[0048]

[0049] Because when the models of the photoacoustic cell and the microphone do not change, and the phase difference between the phase-locked amplifier and the laser amplitude modulation does not change artificially, the microphone sensitivity Sm, the response frequency F of the photoacoustic cell, and the phase difference ψ between the phase-locked amplifier and the laser amplitude modulation will not change, that is, the parameter ω can become a constant value, let λ be the ratio of the measured air chamber to the reference air chamber ω, that is, λ is also a constant

[0050]

[0051] Two photoacoustic cells are connected in parallel and connected through a fiber coupler. The splitting ratio of the fiber coupler is β:θ. Before passing through the coupling area, the optical power is I. The laser splits the optical path into two paths through the coupler. The two optical paths enter the gas chamber to be tested and the reference gas chamber respectively. Due to the different curvature and length of the optical fiber, after the coupling area is split, the two laser beams reach the two gas chambers through two optical fibers. The optical power loss ratio from the coupling area to the two gas chambers is i:j respectively. The optical power ratio reaching the two gas chambers has the following relationship:

[0052]

[0053] The laser enters the two air chambers from the coupling area, and its dynamic loss is mainly affected by the curvature and length of the optical fiber. The two air chambers are now encapsulated in a shell. The bending of the optical fiber before entering the shell has the same effect on the two air chambers, which can effectively reduce the interference of the external environment on the two air chambers. That is, the bending loss is mainly affected by the bending loss of the distance from entering the shell to the collimator of the two air chambers. Now the curvature and length of the two optical fibers in this distance are fixed, so the loss ratio i:j is a fixed ratio; at the same time, the coupler splitting ratio β:θ is also a fixed value, and there is the following relationship:

[0054]

[0055] At this time, η is a constant value

[0056] Substituting (3), (4), (5) into (1), we can obtain

[0057]

[0058] In this relationship, the second harmonic signals of the test chamber and the reference chamber can be selected and output by the analog switch and taken by the phase-locked amplifier. If the gas concentration of the reference chamber B is known, the concentration of the test chamber A can be calculated. By transforming equation (6), we can get:

[0059]

[0060] It can be seen from this formula that the gas concentration of the test chamber is linearly related to the ratio of the second harmonic signals of the two chambers. The above relationship can be used to directly calculate the concentration information of the test chamber from the gas concentration of the reference chamber and the ratio of the second harmonic signals of the two chambers.

[0061] The influence of optical power loss caused by the front-end air chamber absorption can also be eliminated by equation (7), because according to the Beer-Lambert law I out =I in ·e -αCL , I out is the transmitted light intensity, I in is the incident light intensity, α is the absorption coefficient, C is the gas concentration in the test chamber, and L is the length of the chamber. For the second test chamber and the second reference chamber, the front end passes through the same optical path. The optical path enters the first test chamber through the first coupler and then enters the second test chamber and the second reference chamber through the second coupler. After the optical path passes through the first test chamber, there will be an absorption loss effect on the second test chamber and the second reference chamber. This is reflected in the reduction of optical power. Because the absorption loss effect is caused by the first test chamber, the absorption loss effect on the second test chamber and the second reference chamber is the same. Then, through the ratio method in formula (7), the absorption loss component of the second test chamber and the second reference chamber is directly eliminated, thereby eliminating the absorption loss effect. For the third test chamber and the third reference chamber, the front end also passes through the same optical path. The optical path enters the first test chamber through the first coupler and then enters the second test chamber through the second coupler and finally reaches the third test chamber and the third reference chamber. After the optical path passes through the first gas chamber to be tested and the second gas chamber to be tested, there will be an absorption loss effect on the third gas chamber to be tested and the third reference gas chamber at the rear end. This is reflected in the reduction of optical power, because the absorption loss is caused by the first gas chamber to be tested and the second gas chamber to be tested, so the absorption loss of the third gas chamber to be tested and the third reference gas chamber are the same. Then, through the ratio method in formula (7), the absorption loss components of the second gas chamber to be tested and the second reference gas chamber are directly reduced, thereby eliminating the absorption loss effect.

[0062] It can be seen from formula (7) that the gas concentration value of the gas chamber to be tested is only related to the ratio of the second harmonic signals of the two gas chambers and the concentration value of the reference gas chamber. If the gas concentration value of the reference optical path gas chamber is fixed, and the gas concentration of the detection gas chamber can be obtained according to the ratio of the collected second harmonic signals of the gas chamber to be tested and the reference gas chamber, the analog switch can be used to control the output of multiple signals to achieve multi-point gas detection. At the same time, the influence of optical power loss caused by optical fiber bending and gas absorption in the front gas chamber and optical power fluctuation can be eliminated.

[0063] In multi-point gas detection, this method can eliminate dynamic effects including fiber bending loss and absorption loss of the gas chamber, achieving high measurement accuracy and long detection distance. At the same time, it reduces the number of connected optical fibers and improves the stability of the entire system detection, with the advantages of low cost and high practical value.

[0064] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for detecting gas concentration at multiple points to eliminate the influence of dynamic loss, characterized in that: The steps include: (a) First, two waveforms output by the signal generator are superimposed to form a modulated light and a laser beam is emitted through a laser collimator. The laser beam is connected to a first coupler and divided into two beams. The beams pass through the first collimator and the second collimator respectively and enter the first gas chamber to be measured and the first reference gas chamber and then pass through the photoacoustic resonant cavity. The modulated light absorbs energy from the gas, and the sound waves generated by the photoacoustic effect resonate on the wall of the photoacoustic resonant cavity, thereby enhancing the sound wave signal. The signal is detected by a microphone, and the microphone converts the sound signal into an electrical signal for output; (b) The laser beam is collected by the third collimator and connected to the second coupler to be divided into two beams again. The beams pass through the fifth collimator and the sixth collimator respectively and enter the second gas chamber to be measured and the second reference gas chamber, and then pass through the photoacoustic resonant cavity. The modulated light is absorbed by the gas, and the sound waves generated by the photoacoustic effect resonate on the wall of the photoacoustic resonant cavity. The sound wave signal is detected by the microphone, and the microphone converts the sound signal into an electrical signal for output; (c) The laser beam is collected by the seventh collimator and connected to the third coupler to be divided into two beams again. The beams pass through the ninth collimator and the tenth collimator respectively and enter the third gas chamber to be tested and the third reference gas chamber, and then pass through the photoacoustic resonant cavity. The modulated light is absorbed by the gas, and the sound waves generated by the photoacoustic effect resonate on the wall of the photoacoustic resonant cavity. The sound wave signals are detected by the microphone, and the microphone converts the sound signals into electrical signals for output. (d) The microphones of the first gas chamber to be tested, the second gas chamber to be tested, the third gas chamber to be tested, and the first reference gas chamber, the second reference gas chamber, and the third reference gas chamber are connected to an analog switch to control the signal output of each optical path; the other end of the analog switch is connected to a preamplifier circuit to convert the current signal into a voltage signal, which is extracted by a lock-in amplifier, and finally the harmonic signal is collected by a data acquisition system and transmitted to a computer for data processing; (e) By making a ratio between the collected harmonic signal of the first gas chamber to be tested and the first reference gas chamber, it can be obtained that the gas concentration of the gas chamber to be tested and the ratio of the second harmonic signal of the two gas chambers are in a linear relationship. The above relationship can be used to directly calculate the concentration information of the first gas chamber to be tested from the gas concentration of the first reference gas chamber and the ratio of the second harmonic signal of the two gas chambers; by making a ratio between the harmonic signal of the second gas chamber to be tested and the second reference gas chamber, the concentration information of the second gas chamber to be tested can be directly calculated from the gas concentration of the second reference gas chamber and the ratio of the second harmonic signal of the two gas chambers; then by making a ratio between the harmonic signal of the third gas chamber to be tested and the third reference gas chamber, the concentration information of the third gas chamber to be tested can be calculated from the gas concentration of the third reference gas chamber and the ratio of the second harmonic signal of the two gas chambers; it can realize multi-point gas concentration detection and eliminate the influence of optical fiber bending loss and the absorption loss caused by the front-end gas chamber when the first gas chamber to be tested detects the gas concentration of the second gas chamber to be tested, and the first gas chamber to be tested and the second gas chamber to be tested detect gas concentrations of the third reference gas chamber.

2. A method for detecting gas concentration at multiple points to eliminate the influence of dynamic loss according to claim 1, characterized in that: The two waveforms output by the signal generator described in step (a) are a sine wave and a sawtooth wave.

3. A method for detecting gas concentration at multiple points to eliminate the influence of dynamic loss according to claim 1, characterized in that: To ensure the signal strength of the gas chamber to be tested, the splitting ratios of the first coupler, the second coupler, and the third coupler described in steps (a) to (c) are all 99:1, that is, the light beam entering the gas chamber to be tested is 99%, and the light beam entering the reference gas chamber is 1%.

4. A multi-point gas concentration detection device that eliminates the influence of dynamic loss, characterized in that: The invention comprises a signal generator, a voltage adder, a laser transmitter, a first coupler, a second coupler, a third coupler, an analog switch, a preamplifier circuit, a phase-locked amplifier, a data acquisition system and a computer system. The signal output end of the signal generator is connected to the synchronization signal input end of the voltage adder, and the other end of the synchronization signal output end of the signal generator is connected to the phase-locked amplifier. The output end of the voltage adder is connected to the laser transmitter, and the output end of the laser transmitter is connected to the first coupler. The first coupler divides the optical path into an optical path 1 and an optical path 2 according to a fixed coupling ratio. The optical path 1 is provided with a first gas chamber to be measured containing unknown concentration information, and the optical path 2 is provided with a first reference gas chamber containing fixed concentration information. The side wall of the first gas chamber to be measured is provided with a A first collimator is arranged on the side wall of the first reference gas chamber, and a third collimator is arranged on the other side wall corresponding to the first collimator; a second collimator is arranged on the side wall of the first reference gas chamber, and a fourth collimator is arranged on the other side wall corresponding to the second collimator; the first gas chamber to be measured is connected to the second coupler through the third collimator, and the second coupler divides the output light from the first gas chamber to be measured into a third optical path and a fourth optical path according to a fixed coupling ratio, the third optical path is provided with a second gas chamber to be measured containing unknown concentration information, the fourth optical path is provided with a second reference gas chamber containing fixed concentration information, a fifth collimator is arranged on the side wall of the second gas chamber to be measured, and a seventh collimator is arranged on the other side wall corresponding to the fifth collimator; a A sixth collimator is arranged on the side wall of the third gas chamber to be measured, and an eighth collimator is arranged on the other side wall corresponding to the sixth collimator; the second gas chamber to be measured is connected to the third coupler through the seventh collimator, and the third coupler divides the output light of the second gas chamber to be measured into optical path five and optical path six according to a fixed coupling ratio, and the optical path five is provided with a third gas chamber to be measured containing unknown concentration information, and the optical path six is ​​provided with a third reference gas chamber containing fixed concentration information, a ninth collimator is arranged on the side wall of the third gas chamber to be measured, and an eleventh collimator is arranged on the other side wall corresponding to the ninth collimator; a tenth collimator is arranged on the side wall of the third reference gas chamber, and a twelfth collimator is arranged on the other side wall corresponding to the tenth collimator; the first gas chamber to be measured and the second gas chamber to be measured are connected to the third coupler through the seventh collimator, and the third coupler divides the output light of the second gas chamber to be measured into optical path five and optical path six according to a fixed coupling ratio, and the optical path five is provided with a third gas chamber to be measured containing unknown concentration information, and the optical path six is ​​provided with a third reference gas chamber containing fixed concentration information, The air chamber, the third air chamber to be tested and the first reference air chamber, the second reference air chamber and the third reference air chamber are photoacoustic cells of exactly the same model and size. A photoacoustic resonant cavity and a microphone are arranged in the photoacoustic cell. The photoacoustic resonant cavity is cylindrical. The microphones of the first air chamber to be tested, the second air chamber to be tested, the third air chamber to be tested and the first reference air chamber, the second reference air chamber and the third reference air chamber are connected to an analog switch. The signal output end of the analog switch is connected to a preamplifier circuit, and the output end of the preamplifier circuit is connected to a phase-locked amplifier. The signal extracted by the phase-locked amplifier is collected and processed by a data acquisition system. The signal output end of the data acquisition system is connected to a computer system. The laser transmitter is a butterfly distributed feedback semiconductor laser.The first gas chamber to be measured and the first reference gas chamber, the second gas chamber to be measured and the second reference gas chamber, and the third gas chamber to be measured and the third reference gas chamber are respectively packaged with a shell for easy fixation; each gas chamber is provided with an air inlet and an air outlet; the laser beam of the optical path one enters the first gas chamber to be measured through the first collimator, passes through the photoacoustic resonant cavity of the first gas chamber to be measured, and the laser beam is collected by the third collimator; the laser beam of the optical path two enters the first reference gas chamber through the second collimator, passes through the photoacoustic resonant cavity of the first reference gas chamber, and the laser beam is collected by the fourth collimator; the laser beam of the optical path three enters the first reference gas chamber through the fifth collimator, and the laser beam passes through the fifth collimator. The laser beam of the optical path 4 enters the second gas chamber to be measured through the photoacoustic resonator of the second gas chamber to be measured, and the laser beam is collected by the seventh collimator; the laser beam of the optical path 4 enters the second reference gas chamber through the sixth collimator, passes through the photoacoustic resonator of the second reference gas chamber, and the laser beam is collected by the eighth collimator; the laser beam of the optical path 5 enters the third gas chamber to be measured through the ninth collimator, passes through the photoacoustic resonator of the third gas chamber to be measured, and the laser beam is collected by the eleventh collimator; the laser beam of the optical path 6 enters the third reference gas chamber through the tenth collimator, passes through the photoacoustic resonator of the third reference gas chamber, and the laser beam is collected by the twelfth collimator. ;

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