CN113267453B - Passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system and detection method thereof - Google Patents
Passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system and detection method thereof Download PDFInfo
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- CN113267453B CN113267453B CN202110339342.6A CN202110339342A CN113267453B CN 113267453 B CN113267453 B CN 113267453B CN 202110339342 A CN202110339342 A CN 202110339342A CN 113267453 B CN113267453 B CN 113267453B
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Abstract
The invention discloses a passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system, which comprises: in the gas chambers of the excitation laser module, the passive tuning fork and the target gas, the excitation laser module generates laser with specified wavelength of three specified modulation frequencies, the target gas and the laser act in the gas chambers to generate acoustic wave signals with three frequencies, after the acoustic wave signals are subjected to resonance enhancement, the passive tuning fork generates reverse phase resonance with different resonance frequencies, the distance between the interference type optical fiber sensing module and the vibrating arm of the passive tuning fork is changed, and the interference type optical fiber sensing module collects the phase difference change of interference light caused by the distance change; the gas information analysis module extracts three second harmonic signals corresponding to modulation frequencies from the phase difference change, so that concentration information of three target gases is obtained, and the gas information analysis module has the advantages of high sensitivity, small size, uncharged front end, intrinsic safety, electromagnetic interference resistance, high temperature resistance and the like.
Description
Technical Field
The invention belongs to the technical field of photoacoustic spectroscopy, and particularly relates to a passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system and a detection method thereof.
Background
The multi-gas online measurement has important application in the fields of environmental monitoring, industrial control, agricultural production, bioengineering, medical diagnosis and the like. Quartz enhanced photoacoustic spectroscopy is an indirect absorption spectroscopy technique with many advantages: the appearance is compact, and the cost is low; a photoelectric detector is not used, and the excitation laser wavelength is not limited; the detection sensitivity is not greatly related to the interaction length of light and a sample, but is in direct proportion to the laser power and the sensitivity of the acoustic sensor; the zero background characteristic allows the amplifier to operate over a large dynamic range with a zero signal as a reference.
However, the quartz tuning fork cannot identify the excited molecules from which the detected acoustic waves come, and the traditional quartz enhanced photoacoustic spectroscopy cannot realize synchronous gas monitoring. Even if one laser wavelength can cover two target gases, the laser must not switch wavelengths between the two gases, causing a delay in detection. The quartz enhanced photoacoustic spectrum adopting the customized low-frequency quartz tuning fork realizes double-gas synchronous detection by using fundamental frequency and one-time pan frequency, but the one-time pan frequency is usually higher and is not matched with the gas molecule relaxation rate. Meanwhile, no matter a commercial standard quartz tuning fork (32.768 kHz) or a customized quartz tuning fork is adopted, the quartz enhanced photoacoustic spectroscopy needs low-noise prepositive amplification on weak piezoelectric current, and cannot be applied to special environments such as strong electromagnetic interference, high temperature, inflammability, explosiveness and the like.
Disclosure of Invention
The invention provides a passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system, and aims to solve the problems.
The invention is realized in this way, a passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system, the system comprising:
the device comprises an excitation laser module, a passive tuning fork, a gas chamber, an optical fiber micro-vibration sensing module and a gas information analysis module, wherein the passive tuning fork is arranged in the gas chamber, and the gas chamber is filled with gas to be detected containing target gas 1, target gas 2 and target gas 3;
the excitation laser module is used for simultaneously generating three paths of laser 1, laser 2 and laser 3 with specified modulation frequency and specified wavelength, and transmitting the three paths of laser 1, laser 2 and laser 3 to a gap between two vibrating arms of the passive tuning fork, the target gas 1 and the laser 1 act to generate a sound wave signal 1, the target gas 2 and the laser 2 act to generate a sound wave signal 2, the target gas 3 and the laser 3 act to generate a sound wave signal 3, the sound wave signal 1, the sound wave signal 2 and the sound wave signal 3 are transmitted to the passive tuning fork, so that the passive tuning fork simultaneously generates reverse phase resonance with different resonance frequencies in the front and rear directions, the distance from the fiber micro-vibration sensing module to the vibrating arms of the passive tuning fork in the front and rear vibration directions is changed, the fiber micro-vibration sensing module respectively collects phase difference changes of interference light caused by the three distance changes, and the gas information analysis module further simultaneously calculates the gas concentrations of the target gas 1, the target gas 2 and the target gas 3;
the wavelength of the laser 1 is the wavelength on the absorption line of the target gas 1, the modulation frequency of the laser 1 is half of the minimum inverse resonance frequency of the passive tuning fork in the front-back direction, the wavelength of the laser 2 is the wavelength on the absorption line of the target gas 2, the modulation frequency of the laser 2 is slightly smaller than half of the minimum inverse resonance frequency of the passive tuning fork in the front-back direction, the wavelength of the laser 3 is the wavelength on the absorption line of the target gas 3, and the modulation frequency of the laser 3 is slightly larger than half of the minimum inverse resonance frequency of the passive tuning fork in the front-back direction.
Further, the excitation laser module includes:
the device comprises a laser 1, a laser 2 and a laser 3, wherein the laser 1, the laser 2 and the laser 3 are respectively connected with the optical fiber collimator 1, the optical fiber collimator 2 and the optical fiber collimator 3 through optical fibers, and a crack microacoustic resonance tube 1 and a microacoustic resonance tube 2 are arranged between two vibrating arms of a passive tuning fork, the resonance frequency of the microacoustic resonance tube 1 is twice of the modulation frequency of the laser 2, and the resonance frequency of the microacoustic resonance tube 2 is twice of the modulation frequency of the laser 3;
the laser 1 generates laser 1 with specified modulation frequency and specified wavelength, the laser 1 is output to the optical fiber collimator 1, the collimated laser 1 emitted by the optical fiber collimator 1 directly enters a gap between two vibrating arms of the passive tuning fork, acts with the target gas 1 to generate an acoustic wave signal 1, and is directly transmitted to the passive tuning fork to cause the anti-phase resonance of the passive tuning fork in the front and back directions, and the resonance frequency of the passive tuning fork is equal to the modulation frequency of the laser 1;
the laser 2 generates laser 2 with specified modulation frequency and specified wavelength, the laser 2 is output to the optical fiber collimator 2, the collimated laser 2 emitted by the optical fiber collimator 2 is incident to the micro-acoustic resonance tube 1, an acoustic wave signal 2 generated by the action of the laser 2 and the target gas is amplified by the micro-acoustic resonance tube 1 and then transmitted to the passive tuning fork for resonance, so that the anti-phase resonance of the passive tuning fork in the front and back directions is caused, and the resonance frequency of the passive tuning fork is equal to the modulation frequency of the laser 2;
the laser 3 generates laser 3 with specified modulation frequency and specified wavelength, the laser 3 is output to the optical fiber collimator 3, the optical fiber is collimated, the collimated laser 3 emitted from the laser 3 is incident to the micro-acoustic resonator tube 2, an acoustic wave signal 3 generated by the action of the collimated laser 3 and the target gas is amplified by the micro-acoustic resonator tube 2 and then transmitted to the passive tuning fork for resonance, the anti-phase resonance of the passive tuning fork in the front and back direction is caused, and the resonance frequency of the passive tuning fork is equal to the modulation frequency of the laser 3.
Further, the passive tuning fork is connected by two vibrating arms and a base connecting the two vibrating arms, the vibrating arms are made of silicon, silicon dioxide, copper or aluminum, and the passive tuning fork generates reverse phase resonance with different vibration frequencies in the front and back directions based on the sound wave signal 1, the sound wave signal 2 and the sound wave signal 3.
Further, the gas cell includes:
the gas chamber body and locate incident window and exit window on the gas chamber body, incident window and exit window are arranged in the laser incidence direction, and the slit between two vibrating arms of passive tuning fork aligns the incident window for collimated laser is taken in from penetrating into the light window, sees through the slit between two vibrating arms of passive tuning fork.
Further, the optical fiber micro-vibration sensing module comprises:
the system comprises a detection light source, an optical fiber interferometer and a signal demodulation module, wherein the detection light source is connected with the optical fiber interferometer through an optical fiber, and the optical fiber interferometer is connected with the signal demodulation module through an optical fiber;
and the reflecting film is arranged on the passive tuning fork vibrating arm, the reflecting film is arranged in the vibration direction of the passive tuning fork vibrating arm, the detection light source outputs detection laser which is incident to the reflecting film through the optical fiber interferometer, the detection light enters the optical fiber interferometer through the reflection of the reflecting film, the distance between the passive tuning fork vibrating arm and the optical fiber interferometer is changed due to the vibration of the passive tuning fork vibrating arm in the front and back directions, the optical fiber interferometer collects the phase difference change of the interference light, and the signal demodulation module restores the phase change of the optical fiber interferometer.
On the other hand, the invention also provides a gas concentration detection method of the passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system, which specifically comprises the following steps:
s1, respectively determining the wavelengths of laser 1, laser 2 and laser 3 based on absorption spectral lines of target gas 1, target gas 2 and target gas 3 to be detected in a gas chamber;
s2, respectively determining the modulation frequencies of the laser 1, the laser 2 and the laser 3 based on the minimum inverse resonance frequency of the passive tuning fork in the front and back directions, wherein the modulation frequency of the laser 1 is half of the minimum inverse resonance frequency of the passive tuning fork in the front and back directions, the modulation frequency of the laser 2 is slightly smaller than half of the minimum inverse resonance frequency of the passive tuning fork in the front and back directions, and the modulation frequency of the laser 3 is slightly larger than half of the minimum inverse resonance frequency of the passive tuning fork in the front and back directions;
s3, exciting the laser module to simultaneously output laser 1, laser 2 and laser 3 with corresponding modulation frequencies and wavelengths, enabling the laser 1, the laser 2 and the laser 3 to be incident to a slit between two vibrating arms of the passive tuning fork and respectively act with the target gas 1, the target gas 2 and the target gas 3 in the gas chamber, and simultaneously generating a sound wave signal 1, a sound wave signal 2 and a sound wave signal 3;
s4, directly transmitting the sound wave signal 1 to a passive tuning fork, amplifying the sound wave signal 2 and the sound wave signal 3 through two micro-acoustic resonance tubes and transmitting the sound wave signal and the sound wave signal to the passive tuning fork, and simultaneously generating three-resonance-frequency anti-phase vibration in the front and back directions by the passive tuning fork based on the sound wave signal 1, the sound wave signal 2 and the sound wave signal 3 to cause the distance from the fiber interferometer to a vibrating arm of the passive tuning fork to change;
and S5, respectively collecting interference light phase difference changes caused by corresponding distance changes by the optical fiber interferometer, and extracting second harmonic signals through a phase-locked amplifier to further obtain the gas concentrations of the target gas 1, the target gas 2 and the target gas 3.
The all-fiber three-gas detection photoacoustic spectroscopy system with the enhanced passive tuning fork resonance, disclosed by the invention, has the advantages of high sensitivity, small volume, easiness in networking, uncharged front end, intrinsic safety, electromagnetic interference resistance, high temperature resistance and the like, and is suitable for various trace gas measurement scenes.
Drawings
Fig. 1 is a schematic structural diagram of a passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system according to an embodiment of the present invention;
fig. 2 is a schematic structural diagram of a passive tuning fork according to an embodiment of the present invention;
fig. 3 is a schematic diagram illustrating an arrangement of micro-acoustic resonator tubes according to an embodiment of the present invention;
FIG. 4 is a schematic structural diagram of a gas cell provided in an embodiment of the present invention;
FIG. 5 is a schematic structural diagram of a fiber interferometer in an interferometric fiber sensing module according to an embodiment of the present invention;
fig. 6 is a relationship between normalized output amplitude and modulation frequency of the passive tuning fork and the two micro-acoustic resonator tubes according to the embodiment of the present invention;
1. the optical fiber acoustic resonance device comprises an excitation laser module, 11 laser devices 1 and 12 laser devices 2 and 13 laser devices 3 and 14 optical fiber collimators 1 and 15 optical fiber collimators 2 and 16 optical fiber collimators 3 and 17 optical fibers 1 and 18 optical fibers 2 and 19 optical fibers 3 and 20 micro acoustic resonance tubes 1 and 21 micro acoustic resonance tubes 2 and 2 passive tuning forks, 3 air chambers, 31 air chamber bodies, 32 optical windows, 33 air inlets, 34 air outlets, 35 optical fiber access holes, 4 interference type optical fiber sensing modules, 41 detection light sources, 42 optical fiber interferometers, 8978 zx8978 dB optical fiber couplers, 422 optical fiber Faraday rotating mirrors, 423 optical fiber Faraday rotators, 43 signal demodulation modules, 44 optical fibers, 5 gas information analysis modules, 51 signal generators, 52 phase-locked amplifiers, 53 data acquisition cards, 54 computers, 55 summers, 56 modulation signal lines, 57 clock signal lines, 58 clock signal lines.
Detailed Description
The following detailed description of the embodiments of the present invention will be given in order to provide those skilled in the art with a more complete, accurate and thorough understanding of the inventive concept and technical solutions of the present invention.
Fig. 1 is a schematic structural diagram of an all-fiber three-gas detection photoacoustic spectroscopy system with enhanced passive tuning fork resonance according to an embodiment of the present invention, and for convenience of illustration, only the parts related to the embodiment of the present invention are shown.
The system comprises: the device comprises an excitation laser module, a passive tuning fork, a gas chamber, an optical fiber micro-vibration sensing module and a gas information analysis module, wherein the passive tuning fork is arranged in the gas chamber, and the gas chamber is filled with gas to be detected containing target gas 1, target gas 2 and target gas 3;
the excitation laser module is used for simultaneously generating three paths of laser 1, laser 2 and laser 3 with specified modulation frequency and specified wavelength, and transmitting the three paths of laser 1, laser 2 and laser 3 to a gap between two vibrating arms of the passive tuning fork, the target gas 1 and the laser 1 act to generate a sound wave signal 1, the target gas 2 and the laser 2 act to generate a sound wave signal 2, the target gas 3 and the laser 3 act to generate a sound wave signal 3, and the sound wave signal 1, the sound wave signal 2 and the sound wave signal 3 are transmitted to the passive tuning fork, so that the passive tuning fork simultaneously generates reverse phase resonance with different frequencies in the front and rear directions, the distance from the fiber micro-vibration sensing module to the vibrating arms of the passive tuning fork in the front and rear vibration directions is changed, the fiber micro-vibration sensing module respectively collects phase difference changes of interference light caused by the three distance changes, and the gas information analysis module further simultaneously calculates the gas concentrations of the target gas 1, the target gas 2 and the target gas 3;
the wavelength of the laser 1 is the wavelength on the absorption line of the target gas 1, the modulation frequency of the laser 1 is half of the minimum inverse resonance frequency of the passive tuning fork in the front-back direction, the wavelength of the laser 2 is the wavelength on the absorption line of the target gas 2, the modulation frequency of the laser 2 is slightly smaller than half of the minimum inverse resonance frequency of the passive tuning fork in the front-back direction, the wavelength of the laser 3 is the wavelength on the absorption line of the target gas 3, the modulation frequency of the laser 3 is slightly larger than half of the minimum inverse resonance frequency of the passive tuning fork in the front-back direction, the three modulation frequencies are arranged in an equal difference mode, and the difference value is 1-3 Hz.
The acoustic wave signal 1, the acoustic wave signal 2 and the acoustic wave signal 3 all enable the passive tuning fork to generate reverse phase resonances with different amplitudes in the front-back direction, namely the passive tuning fork generates reverse phase resonances with three vibration amplitudes, wherein the vibration amplitude of the passive tuning fork caused by the acoustic wave signal 1 is larger than that of the acoustic wave signal 2 and that of the acoustic wave signal 3, the resonance frequencies of the three reverse phase resonances are all consistent with the resonance frequencies of the corresponding acoustic wave signals, and the reverse resonance refers to that the vibration phases of the two vibrating arms are 180 degrees apart.
In an embodiment of the invention, the excitation laser module comprises:
the resonant frequency of the micro-acoustic resonant tube 1 is twice the modulation frequency of the laser 2, and the resonant frequency of the micro-acoustic resonant tube 2 is twice the modulation frequency of the laser 3, as shown in fig. 3;
the laser 1 generates laser 1 with specified modulation frequency and specified wavelength, the laser 1 is output to the optical fiber collimator 1, the collimated laser 1 emitted by the optical fiber collimator 1 directly enters a gap between two vibrating arms of the passive tuning fork, acts with the target gas 1 to generate an acoustic wave signal 1, and is directly transmitted to the passive tuning fork to cause the anti-phase resonance of the passive tuning fork in the front and back directions, and the resonance frequency of the passive tuning fork is equal to the modulation frequency of the laser 1;
the laser 2 generates laser 2 with specified modulation frequency and specified wavelength, the laser 2 is output to the optical fiber collimator 2, the collimated laser 2 emitted by the optical fiber collimator 2 is incident to the micro-acoustic resonance tube 1, an acoustic wave signal 2 generated by the action of the laser 2 and the target gas is amplified by the micro-acoustic resonance tube 1 and then transmitted to the passive tuning fork for resonance, so that the anti-phase resonance of the passive tuning fork in the front and back directions is caused, and the resonance frequency of the passive tuning fork is equal to the modulation frequency of the laser 2;
the laser 3 generates laser 3 with specified modulation frequency and specified wavelength, the laser 3 is output to the optical fiber collimator 3, the collimated laser 3 emitted by the optical fiber collimator 3 is incident to the micro-acoustic resonance tube 2, an acoustic signal 3 generated by the action of the laser 3 and a target gas is amplified by the micro-acoustic resonance tube 2 and then transmitted to the passive tuning fork for resonance, and the anti-phase resonance of the passive tuning fork in the front and back directions is caused, wherein the resonance frequency of the passive tuning fork is equal to the modulation frequency of the laser 3, and the micro-acoustic resonance tube is composed of micro metal or ceramic tubes.
Referring to fig. 2, the passive tuning fork does not need to supply power and generate current, and is connected to two vibrating arms and a base connecting the two vibrating arms, the vibrating arms are made of silicon, silicon dioxide, copper or aluminum, the passive tuning fork generates three-frequency front-back resonance based on the acoustic signal 1, the acoustic signal 2 and the acoustic signal 3, and the sensitivity S of the interferometric fiber optic spectroscopy system based on the passive tuning fork can be expressed as follows:
in the formula: q and f 0 The quality factor and the fundamental mode resonance frequency of the passive tuning fork are respectively, P is the power of the excitation laser, alpha is the absorption coefficient of the target gas, and R is the sensitivity of the interferometric fiber sensing module. The sensitivity S of the system is in direct proportion to the Q value of the passive tuning fork, the power of the excitation laser, the absorption coefficient of the target gas and the sensitivity of the interference type optical fiber sensing module, and is in inverse proportion to the resonance frequency f of the passive tuning fork 0 。
The width, the thickness and the length of a vibrating arm of the passive tuning fork are w, T and L, the distance between the two vibrating arms is s, the outer side of one vibrating arm is plated with a reflecting film, the width of the film is w, and the film is close to the top of the outer side of the vibrating arm and used for reflecting detection light; the passive tuning fork works in an anti-phase resonance mode, the two vibrating arms move in anti-phase mode, and environmental noise interference is resisted, and the relation between the resonance frequency of the passive tuning fork and the parameters thereof is as follows:
in the formula: m is a group of eff And K eff Respectively, the effective mass and effective elastic coefficient of the passive tuning fork, where M eff ＝0.24267ρLTw，K eff ＝0.2575T 3 wE/L 3 And E and ρ are the Young's modulus and density of the passive tuning fork, respectively. As shown in the formula (2): the resonant frequency of the tuning fork can be changed by adjusting the material of the passive tuning fork and the thickness and the length of the vibrating arm. Meanwhile, the width of the passive tuning fork is reasonably increased, and the efficiency of transmitting the photoacoustic energy to the tuning fork can be increased.
Referring to fig. 3, a schematic diagram of an arrangement of a passive tuning fork and two micro-acoustic resonator tubes is described, by way of example, the two micro-acoustic resonator tubes are arranged vertically, and the emitting direction is aligned with the slit between the arms of the passive tuning fork, the micro-acoustic resonator tubes 1 and 2 are made of, for example, brass, and the resonant frequencies of the micro-acoustic resonator tubes 1 and 2 can be expressed as follows:
in the formula: v is the speed of sound in the carrier gas, l 1 And l 2 ，r 1 And r 2 The length and the inner radius of the micro-acoustic resonator tube 1 and the micro-acoustic resonator tube 2, respectively. The resonant frequency of the micro-acoustic resonance tube can be adjusted by changing the length or the inner radius of the micro-acoustic resonance tube so that f 1 、f 0 And f 2 Arranged in an equal difference, the difference can be selected to be 1-3 Hz.
The optical fiber micro-vibration sensing module comprises:
the system comprises a detection light source, an optical fiber interferometer and a signal demodulation module, wherein the detection light source is connected with the optical fiber interferometer through an optical fiber, and the optical fiber interferometer is connected with the signal demodulation module through an optical fiber; and the reflection film is arranged on the passive tuning fork vibrating arm and is arranged in the anti-phase vibration direction of the passive tuning fork vibrating arm. Referring to fig. 4, the optical fiber interferometer includes a 3dB optical fiber coupler, an optical fiber faraday rotator and a reflective film on the passive tuning fork, wherein the end surface of the optical fiber faraday rotator is processed by reflection elimination, the detection light output by the detection light source enters the optical fiber interferometer through the optical fiber, the detection light emitted from the optical fiber faraday rotator is directed at the reflective film on the vibrating arm of the passive tuning fork, the detection light is reflected by the reflective film, the vibration of the vibrating arm of the passive tuning fork causes the distance change from the optical fiber faraday rotator to the vibrating arm of the passive tuning fork, and further causes the phase difference change of the output interference light of the optical fiber interferometer, and the signal demodulation module restores the phase change of the optical fiber interferometer.
Referring to fig. 5, in the embodiment of the present invention, the gas chamber includes: the air chamber body, the incident window and the exit window which are arranged in the laser incidence direction, the air inlet and the air outlet which are arranged at the top of the air chamber body, and the optical fiber inlet and outlet hole which is arranged at the bottom of the air chamber body are sealed by sealing rubber. The incident window and the exit window comprise but are not limited to a calcium fluoride optical window, a magnesium fluoride optical window, a cesium iodide optical window and a quartz optical window, the air inlet is arranged on the upper portion of the air chamber body and used for inputting target gas, the air outlet is arranged on the upper portion of the air chamber body and used for discharging the target gas, and the optical fiber inlet and outlet holes are arranged on the lower portion of the air chamber body and sealed by adopting a sealant.
Referring to fig. 6, the relationship between the normalized output amplitude and the modulation frequency of the passive tuning fork and the two micro-acoustic resonator tubes is described. The bandwidth of the passive tuning fork is about 2Hz to 6Hz, the bandwidth of the micro-acoustic resonator tube is about 20 Hz to 200Hz, and the Q value of the passive tuning fork is far larger than that of the micro-acoustic resonator tube. The resonant frequency of the passive tuning fork is f 0 The modulation frequency of the laser 1 is 1/2*f 0 The modulation frequency of the laser 2 is 1/2*f 1 The modulation frequency of the laser 3 is 1/2*f 2 ，f 1 <f 0 <f 2 The resonance frequencies of the two micro-acoustic resonance tubes are respectively f 1 And f 2 ，f 1 、f 0 And f 2 And (5) arranging in an equal difference mode. The target gas 1 acts with the laser 1 to generate an acoustic signal 1, which directly acts on the passive tuning fork and causes resonanceThe target gas 2 and the laser 2 act to generate a sound wave signal 2, the target gas 3 and the laser 3 act to generate a sound wave signal 3, and the sound wave signal 2 and the sound wave signal 3 are amplified by the micro-acoustic resonance tube and then transmitted to the tuning fork to cause the passive tuning fork to vibrate. Since the passive tuning fork resonance enhancement amplitude is not as large as that of the acoustic signal 1 due to the acoustic signal 2 and the acoustic signal 3, but after the two micro-acoustic resonance tubes are amplified, the output of the two micro-acoustic resonance tubes is obviously improved, so that the three-gas detection has high sensitivity.
In conclusion, the all-fiber three-gas detection photoacoustic spectroscopy system with the resonance enhancement function of the passive tuning fork provided by the invention picks up the vibration signal of the passive tuning fork by resonance enhancement and detection of the phase change of the interference light output by the fiber interferometer, recovers the photoacoustic signal generated by the action of the excitation laser and the trace gas, and further extracts the second harmonic signal.
The invention provides a gas concentration detection method of a passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system, which specifically comprises the following steps:
s1, respectively determining the wavelengths of laser 1, laser 2 and laser 3 based on absorption spectral lines of target gas 1, target gas 2 and target gas 3 to be detected in a gas chamber;
s2, respectively determining the modulation frequencies of the laser 1, the laser 2 and the laser 3 based on the minimum inverse resonance frequency of the passive tuning fork in the front and back directions, wherein the modulation frequency of the laser 1 is half of the minimum inverse resonance frequency of the passive tuning fork in the front and back directions, the modulation frequency of the laser 2 is slightly smaller than half of the minimum inverse resonance frequency of the passive tuning fork in the front and back directions, and the modulation frequency of the laser 3 is slightly larger than half of the minimum inverse resonance frequency of the passive tuning fork in the front and back directions;
s3, exciting the laser module to simultaneously output laser 1, laser 2 and laser 3 with corresponding modulation frequencies and wavelengths, enabling the laser 1, the laser 2 and the laser 3 to be incident to a slit between two vibrating arms of the passive tuning fork and respectively act with the target gas 1, the target gas 2 and the target gas 3 in the gas chamber, and simultaneously generating a sound wave signal 1, a sound wave signal 2 and a sound wave signal 3;
s4, directly transmitting the acoustic signal 1 to a passive tuning fork, amplifying the acoustic signal 2 and the acoustic signal 3 through two micro-acoustic resonance tubes respectively, and transmitting to the passive tuning fork, wherein the passive tuning fork simultaneously generates three-frequency anti-phase vibration in the front and back directions based on the acoustic signal 1, the acoustic signal 2 and the acoustic signal 3, so that the distance between the fiber interferometer and the vibrating arm of the passive tuning fork is changed;
and S5, respectively collecting interference light phase difference changes caused by corresponding distance changes by the optical fiber interferometer, and extracting second harmonic signals through a lock-in amplifier to further obtain the gas concentrations of the target gas 1, the target gas 2 and the target gas 3.
The invention has been described above with reference to the accompanying drawings, it is obvious that the invention is not limited to the specific implementation in the above-described manner, and it is within the scope of the invention to apply the inventive concept and solution to other applications without substantial modification.
Claims (4)
1. A passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system, the system comprising:
the device comprises an excitation laser module, a passive tuning fork, a gas chamber, an optical fiber micro-vibration sensing module and a gas information analysis module, wherein the passive tuning fork is arranged in the gas chamber, and the gas chamber is filled with gas to be detected containing target gas 1, target gas 2 and target gas 3;
the excitation laser module is used for simultaneously generating three paths of laser 1, laser 2 and laser 3 with specified modulation frequency and specified wavelength, and transmitting the three paths of laser 1, laser 2 and laser 3 to a gap between two vibrating arms of the passive tuning fork, the target gas 1 and the laser 1 act to generate a sound wave signal 1, the target gas 2 and the laser 2 act to generate a sound wave signal 2, the target gas 3 and the laser 3 act to generate a sound wave signal 3, the sound wave signal 1, the sound wave signal 2 and the sound wave signal 3 are transmitted to the passive tuning fork, so that the passive tuning fork simultaneously generates reverse phase resonance with different vibration frequencies in the front and rear directions, the distance from the fiber micro-vibration sensing module to the vibrating arms of the passive tuning fork in the front and rear vibration directions is changed, the fiber micro-vibration sensing module respectively collects phase difference changes of interference light caused by the three distance changes, and the gas information analysis module further simultaneously calculates the gas concentrations of the target gas 1, the target gas 2 and the target gas 3;
the wavelength of the laser 1 is the wavelength on the absorption line of the target gas 1, the modulation frequency of the laser 1 is half of the minimum inverse resonance frequency of the passive tuning fork in the front-back direction, the wavelength of the laser 2 is the wavelength on the absorption line of the target gas 2, the modulation frequency of the laser 2 is slightly smaller than half of the minimum inverse resonance frequency of the passive tuning fork in the front-back direction, the wavelength of the laser 3 is the wavelength on the absorption line of the target gas 3, and the modulation frequency of the laser 3 is slightly larger than half of the minimum inverse resonance frequency of the passive tuning fork in the front-back direction;
the passive tuning fork is connected by two vibrating arms and a base connected with the two vibrating arms, the vibrating arms are made of silicon, silicon dioxide, copper or aluminum, and the passive tuning fork generates reverse phase resonance with different vibration frequencies in the front and back directions based on the sound wave signal 1, the sound wave signal 2 and the sound wave signal 3;
the passive tuning fork has the advantages that the width, the thickness and the length of the vibrating arms are w, T and L, the distance between the two vibrating arms is s, the outer side of one vibrating arm is plated with a reflecting film, the width of the film is w, the width of the film is close to the top of the outer side of the vibrating arm and used for reflecting detection light, the passive tuning fork works in an antiphase resonance mode, the two vibrating arms perform antiphase motion, and the relation between the resonance frequency of the passive tuning fork and the parameters thereof is as follows:
in the formula: m eff And K eff Respectively, the effective mass and effective elastic coefficient of the passive tuning fork, where M eff ＝0.24267ρLTw，K eff ＝0.2575T 3 wE/L 3 And E and rho are respectively the Young modulus and the density of the passive tuning fork, and can be known by the formula: the thickness and the length of the passive tuning fork material and the vibrating arm can be changedThe resonant frequency of the tuning fork;
the optical fiber micro-vibration sensing module comprises: the system comprises a detection light source, an optical fiber interferometer and a signal demodulation module, wherein the detection light source is connected with the optical fiber interferometer through an optical fiber, and the optical fiber interferometer is connected with the signal demodulation module through an optical fiber;
and the reflecting film is arranged on the passive tuning fork vibrating arm, the reflecting film is arranged in the vibration direction of the passive tuning fork vibrating arm, the detection light source outputs detection laser which is incident to the reflecting film through the optical fiber interferometer, the detection light enters the optical fiber interferometer through the reflection of the reflecting film, the distance between the passive tuning fork vibrating arm and the optical fiber interferometer is changed due to the vibration of the passive tuning fork vibrating arm in the front and back directions, the optical fiber interferometer collects the phase difference change of the interference light, and the signal demodulation module restores the phase change of the optical fiber interferometer.
2. The passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system of claim 1, wherein the excitation laser module comprises:
the passive tuning fork comprises a laser 1, a laser 2 and a laser 3, wherein the laser 1, the laser 2 and the laser 3 are respectively connected with the optical fiber collimator 1, the optical fiber collimator 2 and the optical fiber collimator 3 through optical fibers, and a micro-acoustic resonance tube 1 and a micro-acoustic resonance tube 2 which are arranged at a gap between two vibrating arms of the passive tuning fork, the resonance frequency of the micro-acoustic resonance tube 1 is twice of the modulation frequency of the laser 2, and the resonance frequency of the micro-acoustic resonance tube 2 is twice of the modulation frequency of the laser 3;
the laser 1 generates laser 1 with specified modulation frequency and specified wavelength, the laser 1 is output to the optical fiber collimator 1, the collimated laser 1 emitted by the optical fiber collimator 1 directly enters a gap between two vibrating arms of the passive tuning fork, acts with the target gas 1 to generate an acoustic wave signal 1, and is directly transmitted to the passive tuning fork to cause the anti-phase resonance of the passive tuning fork in the front and back directions, and the resonance frequency of the passive tuning fork is equal to the modulation frequency of the laser 1;
the laser 2 generates laser 2 with specified modulation frequency and specified wavelength, the laser 2 is output to the optical fiber collimator 2, the collimated laser 2 emitted by the optical fiber collimator 2 is incident to the micro-acoustic resonance tube 1, an acoustic wave signal 2 generated by the action of the laser 2 and the target gas is amplified by the micro-acoustic resonance tube 1 and then transmitted to the passive tuning fork for resonance, so that the anti-phase resonance of the passive tuning fork in the front and back directions is caused, and the resonance frequency of the passive tuning fork is equal to the modulation frequency of the laser 2;
the laser 3 generates laser 3 with specified modulation frequency and specified wavelength, the laser 3 is output to the optical fiber collimator 3, the collimated laser 3 emitted by the optical fiber collimator 3 is incident to the micro-acoustic resonance tube 2, an acoustic wave signal 3 generated by the action of the laser 3 and the target gas is amplified by the micro-acoustic resonance tube 2 and then transmitted to the passive tuning fork for resonance, so that the anti-phase resonance of the passive tuning fork in the front and back directions is caused, and the resonance frequency of the passive tuning fork is equal to the modulation frequency of the laser 3.
3. The passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system of claim 1, wherein the gas cell comprises:
the gas chamber body and locate incident window and exit window on the gas chamber body, incident window and exit window are arranged in the laser incidence direction, and the slit between two vibrating arms of passive tuning fork aims at the incident window for collimated laser jets into from the incident window, sees through the slit between two vibrating arms of passive tuning fork.
4. A gas concentration detection method based on the passive tuning fork resonance enhanced all-fiber three-gas detection photoacoustic spectroscopy system of any one of claims 1 to 3, the method comprising the following steps:
s1, respectively determining the wavelengths of laser 1, laser 2 and laser 3 based on absorption spectral lines of target gas 1, target gas 2 and target gas 3 to be detected in a gas chamber;
s2, respectively determining the modulation frequencies of the laser 1, the laser 2 and the laser 3 based on the minimum inverse resonance frequency of the passive tuning fork in the front and back directions, wherein the modulation frequency of the laser 1 is half of the minimum inverse resonance frequency of the passive tuning fork in the front and back directions, the modulation frequency of the laser 2 is slightly smaller than half of the minimum inverse resonance frequency of the passive tuning fork in the front and back directions, and the modulation frequency of the laser 3 is slightly larger than half of the minimum inverse resonance frequency of the passive tuning fork in the front and back directions;
s3, exciting the laser module to simultaneously output laser 1, laser 2 and laser 3 with corresponding modulation frequencies and wavelengths, enabling the laser 1, the laser 2 and the laser 3 to be incident to a slit between two vibrating arms of the passive tuning fork and respectively act with the target gas 1, the target gas 2 and the target gas 3 in the gas chamber, and simultaneously generating a sound wave signal 1, a sound wave signal 2 and a sound wave signal 3;
s4, directly transmitting the acoustic signal 1 to a passive tuning fork, amplifying the acoustic signal 2 and the acoustic signal 3 through two micro-acoustic resonance tubes respectively, and transmitting the amplified acoustic signals to the passive tuning fork, wherein the passive tuning fork simultaneously generates three-frequency anti-phase vibration in the front and back directions based on the acoustic signal 1, the acoustic signal 2 and the acoustic signal 3, so that the distance from the fiber interferometer to a vibrating arm of the passive tuning fork is changed;
and S5, respectively collecting interference light phase difference changes caused by corresponding distance changes by the optical fiber interferometer, and extracting second harmonic signals through a phase-locked amplifier to further obtain the gas concentrations of the target gas 1, the target gas 2 and the target gas 3.
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