CN112304872A - A tuning fork sound and heat integrated enhanced trace gas detection system and detection method thereof - Google Patents
A tuning fork sound and heat integrated enhanced trace gas detection system and detection method thereofDownload PDFInfo
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- G01N29/022—Fluid sensors based on microsensors, e.g. quartz crystal-microbalance [QCM], surface acoustic wave [SAW] devices, tuning forks, cantilevers, flexural plate wave [FPW] devices
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Abstract
Description
Technical Field
The invention relates to the technical field of gas detection, in particular to a tuning fork acoustic-thermal integrated enhanced trace gas detection system.
The invention also relates to a detection method of the tuning fork acoustic-thermal integrated enhanced trace gas detection system.
Background
With the progress and development of social economy, people have stronger environmental awareness and more important environmental problems. The rapid and accurate detection of gas concentration is receiving wide attention in various industries. As an important part of environmental monitoring, the gas detection technology can be applied to atmospheric gas detection, and can be applied to human daily life, industrial development and aerospace technology. In the construction of aerospace field in China, the inside and outside of aircrafts, rockets and the like need to be strictly detected by trace gas before launching so as to ensure the normal operation of equipment and the life safety of workers; in the exploitation and transportation of coal, petroleum and natural gas, the real-time monitoring of combustible gases such as methane, acetylene and the like is also very important; the emission of harmful gases such as industrial waste gas and automobile exhaust also makes gas detection an important aspect. Because of the low content of these gases, the mixture in air is not easily perceived. Therefore, there is an urgent need to develop a gas detection system with high detection accuracy to solve the above-mentioned existing technical problems.
Disclosure of Invention
In order to overcome the defects in the prior art and further improve the detection precision and the detection limit, the invention provides a tuning fork acoustic-thermal integrated enhanced trace gas detection system, which applies a method of combining the photoacoustic spectroscopy and the photothermal spectroscopy, enhances harmonic signals by increasing the absorption optical path of gas in a gas chamber, then superposes the second harmonic amplitudes of the photoacoustic signals and the photothermal signals detected by a quartz tuning fork, and finally obtains the minimum detection limit of the gas to be obviously enhanced.
The invention also provides a detection method of the tuning fork acoustic-thermal integrated enhanced trace gas detection system.
In order to achieve the technical purpose, the invention adopts the technical scheme that: a tuning fork acoustic-thermal integrated enhanced trace gas detection system comprises a laser source, an isolator, a wavelength division multiplexer, an erbium-doped optical fiber, a circulator, an absorption gas chamber, a gas sensor, a coupler, a preamplifier, a phase-locked amplifier, a data acquisition system and a computer system, wherein the laser source is connected with the isolator, and then the isolator is connected with the wavelength division multiplexer to form a resonant cavity; the wavelength division multiplexer is connected with the erbium-doped optical fiber in the resonant cavity, the erbium-doped optical fiber is connected with the circulator, the circulator is connected with the absorption gas chamber, the absorption gas chamber is connected with the gas sensor, the gas sensor is connected with the coupler, and then the coupler is connected with the wavelength division multiplexer; the output end of the electric signal of the gas sensor is connected with the input end of the pre-amplification circuit, so that the current signal output by the gas sensor is converted into a voltage signal through the pre-amplification circuit; the output end of the pre-amplification circuit is connected with the input end of a phase-locked amplifier, and the phase-locked amplifier is used for demodulating a voltage signal; the output end of the phase-locked amplifier is connected with a data acquisition system, and the data acquisition system is used for acquiring the demodulated voltage signal data, transmitting the data to a computer system through an optical fiber and carrying out signal processing and gas concentration calculation; the tuning fork sound-heat integrated enhanced trace gas detection system further comprises a signal generator module, a piezoelectric ceramic driver and a fiber grating, wherein the signal generator module is connected with the piezoelectric ceramic driver, the piezoelectric ceramic driver is connected with the fiber grating, and the fiber grating is connected with the circulator.
As a further improvement of the invention, the laser source adopts a 980nm high-stability single-mode pump source. The pumping source has stable wavelength and high output power, and ensures the rapidness and intuition of the light source in operation and control.
As a further improvement of the invention, the wavelength division multiplexer adopts an 980/1550nm wavelength division multiplexer, and the 980/1550nm wavelength division multiplexer has insertion loss of 0.2db and isolation of 14.8db at a wavelength of 980 nm; the wavelength at 1550nm is 0.2db of insertion loss, 32db of isolation and 22.8db of extinction ratio, and the polarization characteristic and low loss of the full polarization-maintaining optical fiber system are guaranteed. The 980/1550nm wavelength division multiplexer can transmit 980nm wave band pump light to erbium doped fiber for amplification, thereby jumping to 1550nm wave band.
As a further improvement of the present invention, the gas sensor is a single tuning fork acoustic-thermal integrated sensor, and the structure of the gas sensor includes a quartz tuning fork, a first collimator, a second collimator, and a third collimator, wherein the first collimator is disposed at a front end of the quartz tuning fork, and the second collimator and the third collimator are disposed at a rear end of the quartz tuning fork.
As a further improvement of the present invention, the laser light from the first collimator enters the second collimator through the gap between the two tuning fork arms, and then is transmitted to the coupler through the second collimator.
As a further improvement of the invention, the splitting ratio of the coupler is 1:9, namely the coupler comprises two light paths, and the laser ratios of the two light paths are respectively 10% and 90%; wherein, the light path with the laser ratio of 10 percent is transmitted into a third collimator through the optical fiber, so that the laser on the third collimator irradiates on the tuning fork arm of the quartz tuning fork; the optical path with the laser ratio of 90% is transmitted into the wavelength division multiplexer through the optical fiber, so that the circulation of the optical path is performed.
As a further improvement of the present invention, the absorption cell is a long optical path absorption cell, and the minimum detection limit is increased by increasing the absorption optical path in the cell. Enhancing the harmonic signals generated by the absorption of light by the gas in the absorption gas cell.
As a further improvement of the invention, in order to realize the scanning and modulation of the gas absorption wavelength, the signal generation module provides a modulation frequency for the fiber grating, the piezoelectric ceramic driver controls the fiber grating to realize the scanning and modulation of the wavelength, the reflection wavelength of the fiber grating enters the absorption gas chamber through the circulator, and the modulation frequency of the fiber grating is one half of the resonance frequency of the quartz tuning fork due to the realization of the gas concentration demodulation through the second harmonic, wherein the reflection wavelength of the fiber grating corresponds to the central wavelength of the gas (acetylene) absorption. The whole operation is to control the piezoelectric ceramic driver through the signal generation module, and then control the fiber grating through the piezoelectric ceramic driver, so as to realize the scanning and modulation of the gas absorption wavelength.
The method combines the photoacoustic spectrometry and the photothermal spectrometry, adds the absorption gas chamber to enhance the harmonic signal, then superposes the photoacoustic signal detected by the quartz tuning fork and the second harmonic amplitude of the photothermal signal, and finally obtains the minimum detection limit of the gas which can be obviously enhanced. The photoacoustic spectrum signal is irrelevant to the absorption optical path, so that the chance of improving the signal intensity by a long optical path is lost; and the photothermal spectrum demodulation is complex, the stability is not high, and the factors limit the minimum detection limit to be further improved. The photoacoustic spectrometry is realized on the basis of photoacoustic effect, a harmonic signal generated by light absorption of gas is converted into an acoustic signal, and the concentration of the gas is deduced through detection of the acoustic signal, so that the influence of background noise is eliminated, and the minimum detection limit of the gas is improved; meanwhile, the quartz tuning fork can also realize detection of photo-thermal signals, and if the photo-thermal signals are detected again on the basis of the traditional photo-acoustic gas sensing system, the detected signal intensity can be obviously enhanced. Meanwhile, the second harmonic photothermal signal can directly reflect the concentration of gas, eliminate background noise in the system and improve the measurement precision by improving the laser power and the effective action length of the gas and the light. Therefore, by using the method of combining the photoacoustic spectrum and the photothermal spectrum, the background noise can be eliminated, the signal intensity can be obviously enhanced, and the minimum detection limit of the gas is improved.
The invention provides a detection method of a tuning fork acoustic-thermal integrated enhanced trace gas detection system, which comprises the following steps:
starting a laser source to emit laser with the wavelength of 980nm, transmitting the laser into an isolator, enabling the laser to pass in a single direction and isolating the laser transmitted in the reverse direction due to the unidirectional isolation effect of the isolator, transmitting the laser with the wavelength of 980nm into a wavelength division multiplexer through the isolator, transmitting the laser with the wavelength of 980nm to erbium-doped optical fiber for amplification treatment, enabling the laser with the wavelength of 980nm to jump to the band of 1550nm, enabling the laser coming out of the erbium-doped optical fiber to enter an absorption gas chamber through a circulator, and enhancing harmonic signals generated by the absorption of the laser by gas in the absorption gas chamber; the second harmonic signal from the absorption gas chamber enters a gas sensor, the second harmonic signal in the gas sensor passes through a first collimator and penetrates through a gap between two tuning fork arms to enter a second collimator, and then the second harmonic signal is output through the second collimator to enter a coupler; the coupler is divided into two light paths according to the splitting ratio of 1:9, wherein the light path with the laser accounting for 90% enters the wavelength division multiplexer through optical fiber transmission to circulate the light path; a light path (namely, second harmonic generated by light absorption of gas in the absorption gas chamber) with the laser content of 10% is transmitted into a third collimator through an optical fiber, and is detected through a quartz tuning fork in a mode of irradiating on the tuning fork arms to generate a photo-thermal signal, meanwhile, the second harmonic generated by the photo-acoustic signal passing through a gap between the two tuning fork arms is also detected through the quartz tuning fork, and finally, the second harmonic integrated signal generated by the gas absorption in the absorption gas chamber and the gas sensor is realized through the detection of the photo-acoustic and photo-thermal signals; the current signal output from the quartz tuning fork is converted into a voltage signal through a preamplification circuit, then demodulated by a lock-in amplifier, acquired through a data acquisition system, and finally transmitted to a computer system for signal processing and gas concentration calculation.
As a further improvement of the invention, in order to realize the scanning and modulation of the gas absorption wavelength, the signal generation module provides a modulation frequency for the fiber grating, the piezoelectric ceramic driver controls the fiber grating to realize the scanning and modulation of the wavelength, the reflection wavelength of the fiber grating enters the absorption gas chamber through the circulator, and the modulation frequency of the fiber grating is one half of the resonance frequency of the quartz tuning fork due to the realization of the gas concentration demodulation through the second harmonic, wherein the reflection wavelength of the fiber grating corresponds to the central wavelength of the gas (acetylene) absorption. The whole operation is to control the piezoelectric ceramic driver through the signal generation module, and then control the fiber grating through the piezoelectric ceramic driver, so as to realize the scanning and modulation of the gas absorption wavelength.
Compared with the prior art, the invention has the beneficial effects that:
according to the invention, the long optical path gas chamber is added before the laser enters the gas sensor, the second harmonic generated by gas absorption in the gas chamber is detected by the quartz tuning fork in a photo-thermal mode, and the harmonic signal can be enhanced and the influence of background noise is eliminated simultaneously through the long absorption optical path. Realize optoacoustic and integrative signal detection of light and heat through quartz tuning fork, eliminate the background noise of system to utilize long optical path air chamber to strengthen the light and heat harmonic signal, compare traditional optoacoustic sensing system, increase the harmonic signal that long optical path air chamber gas absorption produced, detected signal obviously promotes. By adopting the single quartz tuning fork photoacoustic photothermal integrated sensor in the laser resonant cavity and utilizing the characteristics of high power density and reciprocating of the laser resonant cavity, the minimum detection limit of the sensing system can be improved again.
Drawings
Fig. 1 is a schematic diagram of the structure provided by the present invention.
In the figure: 1-laser source, 2-isolator, 3-wavelength division multiplexer, 4-erbium doped fiber, 5-circulator, 6-absorption gas chamber, 7-gas sensor, 8-coupler, 9-preamplifier, 10-phase-locked amplifier, 11-data acquisition system, 12-computer system, 13-signal generator module, 14-piezoelectric ceramic driver, 15-fiber grating, 701-quartz tuning fork, 702-first collimator, 703-second collimator, 704-third collimator.
Detailed Description
The invention is described and illustrated in detail below with reference to the drawings and examples so that the advantages and features of the invention may be more readily understood by those skilled in the art, and the scope of the invention is more clearly defined.
Referring to fig. 1, the tuning fork acoustic-thermal integrated enhanced trace gas detection system provided by the invention comprises alaser source 1, anisolator 2, a wavelength division multiplexer 3, an erbium-doped fiber 4, acirculator 5, an absorption gas chamber 6, a gas sensor 7, acoupler 8, apreamplifier 9, a lock-inamplifier 10, a data acquisition system 11 and acomputer system 12, wherein thelaser source 1 is connected with theisolator 2, and then theisolator 2 is connected with the wavelength division multiplexer 3 to form a resonant cavity; the wavelength division multiplexer 3 is connected with an erbium-doped fiber 4 in the resonant cavity, the erbium-doped fiber 4 is connected with acirculator 5, thecirculator 5 is connected with an absorption gas chamber 6, the absorption gas chamber 6 is connected with a gas sensor 7, the gas sensor 7 is connected with acoupler 8, and then thecoupler 8 is connected with the wavelength division multiplexer 3; the output end of the electrical signal of the gas sensor 7 is connected with the input end of thepre-amplification circuit 9, so that the current signal output by the gas sensor 7 is converted into a voltage signal through thepre-amplification circuit 9; the output end of thepre-amplification circuit 9 is connected with the input end of a phase-lockedamplifier 10, and the phase-lockedamplifier 10 is used for demodulating a voltage signal; the output end of the phase-lockedamplifier 10 is connected with a data acquisition system 11, the data acquisition system 11 is used for acquiring the demodulated voltage signal data, and then transmitting the data to acomputer system 12 through an optical fiber for signal processing and gas concentration calculation; the tuning fork sound-heat integrated enhanced trace gas detection system further comprises asignal generator module 13, a piezoelectricceramic driver 14 and afiber grating 15, wherein thesignal generator module 13 is connected with the piezoelectricceramic driver 14, the piezoelectricceramic driver 14 is connected with thefiber grating 15, and thefiber grating 15 is connected with thecirculator 5.
As a further improvement of the present invention, thelaser source 1 adopts a 980nm high-stability single-mode pump source. The pumping source has stable wavelength and high output power, and ensures the rapidness and intuition of the light source in operation and control.
As a further improvement of the invention, the wavelength division multiplexer 3 adopts an 980/1550nm wavelength division multiplexer, and the 980/1550nm wavelength division multiplexer has insertion loss of 0.2db and isolation of 14.8db at a wavelength of 980 nm; the wavelength at 1550nm is 0.2db of insertion loss, 32db of isolation and 22.8db of extinction ratio, and the polarization characteristic and low loss of the full polarization-maintaining optical fiber system are guaranteed. The 980/1550nm wavelength division multiplexer can transmit 980nm wave band pump light to erbium doped fiber for amplification, thereby jumping to 1550nm wave band.
As a further improvement of the present invention, the gas sensor 7 is a single tuning fork acousto-thermal integrated sensor, and the structure thereof includes aquartz tuning fork 701, afirst collimator 702, asecond collimator 703 and athird collimator 704, thefirst collimator 702 is disposed at the front end of thequartz tuning fork 701, and thesecond collimator 703 and thethird collimator 704 are disposed at the rear end of thequartz tuning fork 701.
As a further improvement of the present invention, the laser light from thefirst collimator 702 passes through the gap between the two tuning fork arms to enter thesecond collimator 703, and then is transmitted to thecoupler 8 through thesecond collimator 703.
As a further improvement of the present invention, the splitting ratio of thecoupler 8 is 1:9, that is, the coupler comprises two light paths, and the laser ratios of the two light paths are 10% and 90%, respectively; wherein, the optical path with the laser ratio of 10% is transmitted into thethird collimator 704 through the optical fiber, so that the laser on thethird collimator 704 irradiates on the tuning fork arm of thequartz tuning fork 701; the optical path with the laser ratio of 90% is transmitted into the wavelength division multiplexer 3 through the optical fiber, thereby performing circulation of the optical path.
As a further improvement of the present invention, the absorption gas cell 6 is a long optical path absorption gas cell for enhancing harmonic signals generated by absorption of light by the gas in the absorption gas cell 6. This is a phenomenon of absorption due to the interaction between light and gas according to the beer-lambert law, and the incident light intensity and the transmitted light intensity have the following relationship under the unsaturated absorption condition:
Iout(ν)＝I0(ν)exp(-αvCL)
wherein alpha isvIs the gas absorption coefficient in cm-1L represents the effective absorption optical path of the gas, in cm; c represents the gas volume percentage, and the unit is ppm; alpha is generally less than 0.05 under weak absorption conditions, so the incident light versus transmitted light can be expressed as:
Iout(ν)＝I0(ν)[1-αvCL]
therefore, the light path can play a role in enhancing harmonic signals after the gas absorption of the long-optical-path gas chamber.
As a further improvement of the present invention, in order to implement the scanning and modulation of the gas absorption wavelength, the signal generatingmodule 13 provides a modulation frequency for thefiber grating 15, the piezoelectricceramic driver 14 controls thefiber grating 15 to implement the scanning and modulation of the wavelength, the reflected wavelength of thefiber grating 15 enters the absorption gas chamber through the circulator, and since the gas concentration demodulation is implemented through the second harmonic, the modulation frequency of thefiber grating 15 is one half of the resonance frequency of thequartz tuning fork 701, wherein the reflected wavelength of thefiber grating 15 corresponds to the central wavelength absorbed by the gas (acetylene). The whole operation is that the piezoelectricceramic driver 14 is controlled by the signal generatingmodule 13, and then thefiber bragg grating 15 is controlled by the piezoelectricceramic driver 14, so that the scanning and modulation of the gas absorption wavelength are realized.
The method combines the photoacoustic spectrometry and the photothermal spectrometry, enhances the harmonic signal by increasing the absorption optical path of the gas in the gas chamber, then superposes the second harmonic amplitude of the photoacoustic signal and the photothermal signal detected by the quartz tuning fork, and finally obtains the minimum detection limit of the gas to be obviously enhanced. The photoacoustic spectrum signal is irrelevant to the absorption optical path, so that the chance of improving the signal intensity by a long optical path is lost; and the photothermal spectrum demodulation is complex, the stability is not high, and the factors limit the minimum detection limit to be further improved. The photoacoustic spectrometry is realized on the basis of photoacoustic effect, a harmonic signal generated by gas absorption is converted into an acoustic signal, and the concentration of the gas is deduced through the detection of the acoustic signal, so that the influence of background noise is eliminated, and the minimum detection limit of the gas is improved; meanwhile, the quartz tuning fork can also realize detection of photo-thermal signals, and if the photo-thermal signals are detected again on the basis of the traditional photo-acoustic gas sensing system, the detected signal intensity can be obviously enhanced. Meanwhile, the second harmonic photothermal signal can directly reflect the concentration of gas, eliminate background noise in the system and improve the measurement sensitivity by improving the laser power and the effective action length of the gas and the light. Therefore, by using the method of combining the photoacoustic spectrum and the photothermal spectrum, the background noise can be eliminated, the signal intensity can be obviously enhanced, and the minimum detection limit of the gas is improved.
The invention provides a detection method of a tuning fork acoustic-thermal integrated enhanced trace gas detection system, which comprises the following steps:
starting a laser source 1 to emit laser with the wavelength of 980nm, transmitting the laser into an isolator 2, enabling the laser to pass in a single direction and isolating the laser transmitted in the reverse direction due to the unidirectional isolation effect of the isolator 2, transmitting the laser with the wavelength of 980nm into an erbium-doped optical fiber 4 through the isolator 2 and amplifying the laser, enabling the laser with the wavelength of 980nm to jump to the wavelength of 1550nm, enabling the laser coming out of the erbium-doped optical fiber 4 to enter an absorption air chamber 6 through a circulator 5, and enhancing harmonic signals generated by the absorption of the laser by gas in the absorption air chamber 6; the second harmonic signal from the absorption gas chamber 6 enters the gas sensor 7, and in the gas sensor 7, the second harmonic signal passes through the first collimator 702 and enters the second collimator 703 through the gap between the two tuning fork arms, and then the second harmonic signal is output to the coupler 8 through the second collimator 703; the coupler 8 is divided into two light paths according to the splitting ratio of 1:9, wherein the light path with the laser accounting for 90% enters the wavelength division multiplexer 3 through optical fiber transmission to circulate the light path; a light path (namely, the second harmonic generated by the absorption of gas in the absorption gas chamber) with a laser content of 10% is transmitted into a third collimator 704 through an optical fiber, and is detected by a quartz tuning fork 701 in a mode of irradiating on the tuning fork arms to generate a photo-thermal signal, meanwhile, the second harmonic generated by the photo-acoustic signal passing through a gap between the two tuning fork arms is also detected by the quartz tuning fork 701, and finally, the second harmonic integrated signal generated by the absorption of gas in the absorption gas chamber 6 and the gas sensor 7 is realized through the detection of the photo-acoustic and photo-thermal signals; the current signal output from the quartz tuning fork 701 is converted into a voltage signal through the preamplifier circuit 9, then demodulated by the lock-in amplifier 10, and then data is acquired through the data acquisition system 11 and finally transmitted to the computer system 12 for signal processing and gas concentration calculation.
In order to realize the scanning and modulation of the gas absorption wavelength, the signal generation module provides modulation frequency for the fiber bragg grating, the laser input by the circulator controls the fiber bragg grating to realize the scanning and modulation of the wavelength through the piezoelectric ceramic driver, and the reflected wavelength of the fiber bragg grating enters the absorption gas chamber through the circulator. Because the gas concentration demodulation is realized by the second harmonic wave, the modulation frequency of the fiber grating is half of the resonance frequency of the quartz tuning fork, wherein the reflection wavelength of the fiber grating corresponds to the central wavelength absorbed by the gas. The whole operation is to control the piezoelectric ceramic driver through the signal generation module, and then control the fiber grating through the piezoelectric ceramic driver, so as to realize the scanning and modulation of the gas absorption wavelength.
The embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to the above embodiments, and various changes can be made thereto within the knowledge of those skilled in the art.
Claims (10)
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