CN110044824B - Quartz tuning fork-based dual-spectrum gas detection device and method - Google Patents
Quartz tuning fork-based dual-spectrum gas detection device and method Download PDFInfo
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- CN110044824B CN110044824B CN201910372601.8A CN201910372601A CN110044824B CN 110044824 B CN110044824 B CN 110044824B CN 201910372601 A CN201910372601 A CN 201910372601A CN 110044824 B CN110044824 B CN 110044824B
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
- G01N2021/1704—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids in gases
Abstract
The invention discloses a quartz tuning fork-based dual-spectrum gas detection device and a quartz tuning fork-based dual-spectrum gas detection method. The invention utilizes the resonance characteristic and the piezoelectric effect of the quartz tuning fork, simultaneously uses the quartz tuning fork as the optical signal detector and the acoustic signal detector, realizes a double-spectrum technology for simultaneously acquiring a direct absorption spectrum signal and a photoacoustic spectrum signal, provides correction for the photoacoustic spectrum through the gas concentration information inverted by the direct absorption spectrum signal, and respectively uses the corrected photoacoustic spectrum and the direct absorption spectrum for the analysis of samples with low concentration and high concentration, thereby fully utilizing the light intensity of laser and realizing the measurement of the gas concentration in a wide dynamic range.
Description
Technical Field
The invention relates to the technical field of photoelectric or spectral measurement, in particular to a quartz tuning fork-based dual-spectrum gas detection device and method.
Background
Laser spectroscopy, which is an optical analysis means, has characteristics of high resolution and selectivity, no secondary pollution and no destruction, and is widely used for atmospheric environment monitoring, industrial process control, combustion diagnosis, respiratory gas composition diagnosis, and the like. At present, the laser spectroscopy technology is divided into the following from the technical principle: high-precision optical cavity-based spectroscopy (e.g., cavity ring-down spectroscopy, cavity enhancement spectroscopy, or integrated cavity spectroscopy), modulation spectroscopy (e.g., wavelength/frequency modulation, magnetic rotation modulation spectroscopy), direct absorption spectroscopy, and photoacoustic spectroscopy. In general, the spectrum technology based on the high-precision optical cavity can realize the effective absorption optical path of a hundred-meter to kilometer level, and has extremely high sensitivity, but the optical system is relatively complex and has high cost and technical requirements. The modulation spectrum technology is combined with the corresponding modulation technology, so that the method cannot directly acquire related information, and the related physical quantity information can be acquired only through a correction process. Direct absorption spectroscopy, as a calibration-free spectroscopic analysis method, can achieve different levels of sensitivity by selecting a single-pass cell or a multiple reflection type absorption cell. The photoacoustic spectrum has the advantages of wavelength response bandwidth limitation, wide dynamic range, simple system structure and small volume. With the development of Photoacoustic signal detection technology, researchers at the university of rice in 2002 reported a novel Photoacoustic spectrum based on a Quartz tuning fork, namely Quartz tuning fork enhanced Photoacoustic spectrum (QEPAS). Researchers at the university of Finland Turkish library in 2003 report a novel Photoacoustic spectrum based on Michelson interference technology, and the Michelson interference method is used to extract Photoacoustic signals, namely Cantilever-enhanced Photoacoustic Spectroscopy (CEPAS). In contrast, the sensitivity of CEPAS is higher, but the overall structure of the system is relatively complex; the QEPAS replaces a traditional microphone with a quartz tuning fork as an acoustic signal detector, so that the volume of the photoacoustic spectroscopy system is more miniaturized, and the sensitivity is also greatly improved. Therefore, in recent years, the QEPAS spectroscopy technology is more popular with the vast spectrum researchers at home and abroad.
In a conventional spectroscopic system, a semiconductor photodetector is usually used to realize the conversion of optical telecommunications, and the semiconductor photodetector has a limited response bandwidth to optical wavelengths according to the difference of semiconductor materials. The absorption spectrum detects the weak attenuation of light intensity in the process of the related action of light and substances, and the photoelectric detector is burnt out when the light intensity of a light source is too strong and useless; in the photoacoustic spectroscopy, pressure waves (i.e. acoustic waves) caused by energy released in a radiationless relaxation process during interaction between light and a substance are detected, and the size of a photoacoustic signal is in direct proportion to the optical power of an incident light source in principle.
Aiming at the problems in the current direct absorption spectrum and photoacoustic spectrum technologies, the invention utilizes the piezoelectric effect and the resonance characteristic of a quartz tuning fork and simultaneously uses the piezoelectric effect and the resonance characteristic as an optical signal detector and an acoustic signal detector, thereby realizing a double-spectrum technology capable of simultaneously acquiring a direct absorption spectrum signal and a resonance enhanced optical and acoustic spectrum signal. The direct absorption spectrum with low sensitivity is used for measuring high-concentration samples, and the resonance enhanced photoacoustic spectrum signal with high sensitivity is used for measuring low-concentration samples. The method can overcome the defects or shortcomings of a spectral system only adopting one spectral method, can fully utilize the luminous intensity of an incident light source, is not limited by the wavelength of the incident light source, and has the characteristics of full-wave-band response, wide detectable concentration range and the like. So far, no research has been found on a method for simultaneously acquiring the two spectral signals by using a quartz tuning fork and an application thereof.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a double-spectrum detection device and a double-spectrum detection method which simultaneously use a quartz tuning fork as an optical signal detector for direct absorption spectrum and an acoustic signal detector for photoacoustic spectrum.
In order to achieve the technical purpose, the technical scheme of the invention is as follows:
a dual-spectrum gas detection device based on a quartz tuning fork is characterized by comprising a wavelength tunable laser, a focusing collimating lens, an aperture-tunable square diaphragm, a sample cell, a light-transmitting window, a first tuning fork, an air inlet and outlet, a second tuning fork, a first pre-amplification circuit, a second pre-amplification circuit, a laser control module, an analog-to-digital conversion module, a digital-to-analog conversion module and a computer control unit, wherein the aperture-tunable square diaphragm is arranged on the sample cell; the signal output end of the computer control unit is connected with the digital-to-analog conversion module, the signal output end of the digital-to-analog conversion module is connected with the laser control module, and the laser control module is in control connection with the wavelength tunable laser; the two ends of the sample cell are respectively provided with holes and embedded with a light-transmitting window sheet, the sample cell is provided with an air inlet and an air outlet, gas molecules to be detected are filled in the sample cell, the types of the gas molecules are determined according to the working wavelength of the wavelength tunable laser, and the first tuning fork and the second tuning fork are respectively arranged inside and outside the sample cell; the focusing collimating lens, the aperture-adjustable square diaphragm and the two light-transmitting window sheets of the sample cell are sequentially arranged on a light beam emergent light path of the wavelength-tunable laser, the first tuning fork and the second tuning fork are two quartz tuning forks with the same resonance frequency, the two quartz tuning forks are arranged in parallel but are not completely coaxial, and the side surfaces of the vibrating arms of the two tuning forks are perpendicular to an incident light beam; the signal input ends of the first pre-amplification circuit and the second pre-amplification circuit are respectively connected with the first tuning fork and the second tuning fork, the signal output ends of the first pre-amplification circuit and the second pre-amplification circuit are connected with an analog-to-digital conversion module, and the signal output end of the analog-to-digital conversion module is connected with the computer control unit.
The quartz tuning fork-based double-spectrum gas detection device is characterized in that the computer control unit comprises an analog signal output module and a two-path spectrum signal analysis processing algorithm module, wherein the analog signal output module and the two-path spectrum signal analysis processing algorithm module are written by Labview software.
A dual-spectrum gas detection method based on a quartz tuning fork is characterized by comprising the following steps:
[01] a laser modulation signal or a pulse driving voltage signal written by Labview software in the computer control unit is converted into an analog signal through a digital-to-analog conversion module, and the analog signal or the analog signal is finally input to the wavelength tunable laser through a laser control module to realize the wavelength tuning and modulation output of the laser;
[02] modulated laser emitted by the wavelength tunable laser is converted into parallel beams through a focusing collimating lens, and the spot size of the parallel beams is controlled and adjusted by an aperture-adjustable square diaphragm and then coupled into a sample cell;
[03] in the sample cell, the light spot of the incident light beam is larger than the width of the side face of the tuning fork vibrating arm, so that part of light of the incident light beam is vertically incident on the side face of the first tuning fork vibrating arm, and the other part of light directly enters the side face of the second tuning fork vibrating arm after passing through the sample cell;
[04] because the modulation frequency of the wavelength tunable laser is the same as the resonance frequency of the quartz tuning fork, part of energy of an incident beam penetrating through the sample cell is absorbed by the charged molecules to be detected, a photoacoustic signal is formed through a non-radiative relaxation process, the first tuning fork is excited to resonate, and the light beam incident to the side surface of the vibrating arm of the first tuning fork simultaneously causes the first tuning fork to resonate through the action of light pressure, so that two different signal sources simultaneously excite the first tuning fork to resonate, and finally a superposed signal of two resonance enhanced signals is generated;
[05] in addition, the incident light beam passing through the sample cell also causes the second tuning fork to resonate through the action of light pressure;
[06] piezoelectric current can be induced by the piezoelectric effect of the quartz tuning fork, and the piezoelectric current generated by the resonance of the first tuning fork and the second tuning fork is amplified and converted into voltage signals through the first pre-amplification circuit and the second pre-amplification circuit with low noise and high precision respectively;
[07] the two paths of signals are converted into digital signals by an analog-to-digital conversion module and then input into a double-spectrum signal analysis processing algorithm module compiled by Labview software in a computer control unit for relevant processing;
[08] the double-spectrum signal analysis processing Algorithm module firstly carries out spectrum analysis on a time domain signal output by the quartz tuning fork through a Fast Fourier Transform Algorithm (FFTA), then calculates amplitudes of two paths of spectrum signals at a resonant frequency by combining an extreme value Algorithm, and finally respectively obtains a resonance enhanced superposition spectrum corresponding to the first tuning fork and a direct absorption spectrum corresponding to the second tuning fork according to a corresponding relation between the emission wavelength of the laser and the amplitudes of the two paths of spectrum signals;
[09] before the double-spectrum detection device is used for measuring an unknown-concentration analyte, firstly, a concentration value of the analyte is inverted by using a direct absorption spectrum corresponding to the second tuning fork, then, a resonance enhanced superposition spectrum corresponding to the first tuning fork is corrected by using the concentration value, and a response constant of the photoacoustic cell is calculated;
[10] and finally, respectively utilizing the direct absorption spectrum and the photoacoustic spectrum to analyze a high-concentration or low-concentration sample, and selecting a corresponding detection method according to the concentration range of the analyte to be detected.
The invention has the advantages that:
the invention utilizes the resonance characteristic and the piezoelectric effect of the quartz tuning fork, simultaneously uses the quartz tuning fork as the optical signal detector and the acoustic signal detector, realizes a double-spectrum technology for simultaneously acquiring a direct absorption spectrum signal and a photoacoustic spectrum signal, provides correction for the photoacoustic spectrum through the gas concentration information inverted by the direct absorption spectrum signal, and respectively uses the corrected photoacoustic spectrum and the direct absorption spectrum for analyzing samples with low concentration and high concentration. In addition, the invention uses software algorithm to replace the use of hardware devices such as a signal generator, a phase-locked amplifier, an optical power meter and the like in the traditional spectrum method, and the system is more compact and has lower cost.
Drawings
Fig. 1 is a schematic structural diagram of an embodiment of a quartz tuning fork-based dual-spectrum gas detection device.
In the figure: the device comprises a wavelength tunable laser 1, a focusing collimating lens 2, an aperture tunable square diaphragm 3, a sample cell 4, a light-transmitting window 5, a first tuning fork 6, an air inlet 7-1, an air outlet 7-2, a second tuning fork 8, a first pre-amplification circuit 9, a second pre-amplification circuit 10, a laser control module 11, an analog-to-digital conversion module 12-1, a digital-to-analog conversion module 12-2 and a computer control unit 13.
FIG. 2 is a waveform diagram of the modulation signal used by the apparatus of the present invention and the time domain signal and the frequency domain signal output by the tuning fork.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments.
Examples
As shown in fig. 1, a dual-spectrum detection device based on a quartz tuning fork comprises a wavelength tunable laser 1, a focusing collimating lens 2, an aperture-tunable square diaphragm 3, a sample cell 4, a light-transmitting window 5, a first tuning fork 6, an air inlet 7-1, an air outlet 7-2, a second tuning fork 8, a first pre-amplification circuit 9, a second pre-amplification circuit 10, a laser control module 11, an analog-to-digital conversion module 12-1, a digital-to-analog conversion module 12-2, and a computer control unit 13; the signal output end of the computer control unit 13 is connected with the digital-to-analog conversion module 12-2, the signal output end of the digital-to-analog conversion module 12-2 is connected with the laser control module 11, and the laser control module 11 is in control connection with the wavelength tunable laser 1; two ends of a sample cell 4 are respectively provided with a hole and embedded with a light-transmitting window 5, the sample cell 4 is provided with a gas inlet 7-1 and a gas outlet 7-2, gas molecules to be detected are filled in the sample cell 4, the types of the gas molecules are determined according to the working wavelength of the wavelength tunable laser 1, and a first tuning fork 6 and a second tuning fork 7 are respectively arranged inside and outside the sample cell 4; the focusing collimating lens 2, the aperture-adjustable diaphragm 3 and the two light-transmitting window sheets 5 of the sample cell 4 are sequentially arranged on a light beam emergent light path of the wavelength tunable laser 1, the first tuning fork 6 and the second tuning fork 8 are two quartz tuning forks with the same resonant frequency, the two tuning forks are arranged in parallel but are not completely coaxial, and the side surfaces of the vibrating arms of the two tuning forks are both vertical to incident light beams; the signal input ends of the first pre-amplification circuit 9 and the second pre-amplification circuit 10 are respectively connected with the first tuning fork 6 and the second tuning fork 7, the signal output ends of the first pre-amplification circuit 9 and the second pre-amplification circuit 10 are connected to the analog-to-digital conversion module 12-1, and the signal output end of the analog-to-digital conversion module 12-1 is connected with the computer control unit 13.
The computer control unit 13 comprises an analog signal output module written by Labview software and a two-way spectral signal analysis processing algorithm module.
A quartz tuning fork-based double-spectrum gas detection method comprises the following specific operation steps:
[01] laser modulation signals or pulse driving voltage signals written by Labview software in the computer control unit 13 are changed into analog signals through the digital-to-analog conversion module 12-2, and then are input into the wavelength tunable laser 1 through the laser control module 11 to realize the tuning and modulation output of the laser wavelength, as shown in the upper panel of FIG. 2;
[02] modulated laser emitted by the wavelength tunable laser 1 is converted into parallel beams through a focusing collimating lens 2, and the size of light spots of the parallel beams is controlled and adjusted by an aperture-adjustable square diaphragm 3 and then coupled into a sample cell 4;
[03] in the sample cell 4, the light spot of the incident light beam is larger than the width of the side surface of the tuning fork vibrating arm, so that part of light of the incident light beam is vertically incident on the side surface of the first tuning fork 6 vibrating arm, and the other part of light directly enters the side surface of the second tuning fork 8 vibrating arm after passing through the sample cell 4;
[04] because the modulation frequency of the laser light source is the same as the resonance frequency of the quartz tuning fork, part of energy of an incident beam penetrating through the sample cell 4 is absorbed by the charged molecules to be detected, a photoacoustic signal is formed through a non-radiative relaxation process, the first tuning fork 6 is excited to resonate, and the light beam incident to the side face of the vibrating arm of the first tuning fork 6 also causes the first tuning fork 6 to resonate through the action of light pressure, so that two different signal sources can simultaneously excite the first tuning fork 6 to resonate, and finally a 'superposed signal' of two resonance enhanced signals is generated;
[05] in addition, the incident light beam passing through the sample cell 4 also causes the second tuning fork 8 to resonate through the optical pressure effect;
[06] piezoelectric current can be induced by the piezoelectric effect of the quartz tuning fork, and the piezoelectric current generated by the resonance of the first tuning fork 6 and the second tuning fork 8 is amplified and converted into voltage signals through the low-noise and high-precision first pre-amplification circuit 8 and the second pre-amplification circuit 9 respectively;
[07] the two paths of signals are converted into digital signals by the analog-to-digital conversion module 12-1 and then input into a double-spectrum signal analysis processing algorithm module written by Labview software in the computer control unit 13 for relevant processing;
[08] the dual-spectrum signal analysis processing Algorithm module firstly performs spectrum analysis (as shown in a lower panel of fig. 2) on a time domain signal (as shown in a middle panel of fig. 2) output by the quartz tuning fork through a Fast Fourier Transform Algorithm (FFTA), and then calculates amplitudes of two paths of spectrum signals at the resonant frequency by combining an extreme value Algorithm. Finally, respectively obtaining a superposition spectrum corresponding to the first tuning fork 6 and a direct absorption spectrum corresponding to the second tuning fork 8 according to the corresponding relation between the emission wavelength of the laser and the amplitudes of the two paths of spectrum signals;
[09] before the device is used for measuring an analyte with unknown concentration, the concentration value of the analyte is inverted by using the direct absorption spectrum corresponding to the second tuning fork 8, the resonance enhanced 'superimposed spectrum' corresponding to the first tuning fork 6 is corrected by using the concentration value, and the response constant of the photoacoustic cell is calculated.
[10] And finally, respectively utilizing the direct absorption spectrum and the photoacoustic spectrum to analyze a high-concentration or low-concentration sample, and selecting a corresponding detection method according to the concentration range of the analyte to be detected.
The direct absorption spectrum and photoacoustic spectroscopy principle are analyzed as follows:
principle of direct absorption spectroscopy
The light intensity of the light source changes along with the Lambert-beer law in the absorption process of the interaction process of the light and gas molecules:
I(v)＝I0(v)exp(-α(v)CL) (1)
in the above formula, I0(v) I (v) is the incident light intensity and the emergent light intensity respectively, alpha (v) is the molecular absorption coefficient at a specific wavelength v, C is the concentration of the gas molecules to be detected, and L is the effective absorption optical path of the interaction of light and substances. Molecular absorption coefficient alpha (v), molecular absorption line strength S (T), molecular number density N (T, P), and absorption line phi (v-v)0) The specific relationship is as follows:
α(v)＝φ(v-v0)·S(T)·N(T,P) (2)
wherein v is0For the molecular absorption center wavelength, T and P are the temperature and pressure of the sample, respectively. In the actual signal processing process, the absorption profile is numerically integrated, and the integrated absorption area is obtained as follows:
wherein the absorption line type satisfies the normalization condition:
therefore, the above formula can be further simplified as follows:
A＝S·N·L (4)
finally, the number or concentration of molecules to be measured can be inverted under the condition that experimental parameters (such as temperature, pressure, optical path and molecular spectral line parameters) are known, so that the direct absorption spectrum can be used as a direct method to invert the concentration information of the absorbent without frequent correction theoretically. Although direct absorption spectroscopy has the advantage of directly inverting the concentration of the absorber, its detection sensitivity is limited.
Photoacoustic spectroscopy principle of quartz tuning fork
Quartz tuning fork photoacoustic spectroscopy is commonly used in electronic clocks and watchesThe quartz tuning fork used as the acoustic signal sensor in the oscillator with frequency standard has small volume (length, width and thickness of 3700,600 and 300 μm respectively), low cost, and high quality factor (up to 10 in vacuum environment)5) The response bandwidth is narrow (typically a few Hz) and thus has good immunity to ambient noise. The expression of the photoacoustic signal generated in the photoacoustic spectrum of the quartz tuning fork is as follows:
where is the absorption coefficient of the alpha molecule, P is the incident light power, Q and f0Respectively, the quality factor and the resonance frequency of the quartz tuning fork. Therefore, the photoacoustic spectrum signal has obvious dependence on incident light power, and is an indirect spectroscopic method, and the measurement of unknown sample concentration can be realized only after the response constant of the photoacoustic cell is corrected by a sample with known concentration. Although the photoacoustic spectroscopy technology needs to be corrected, the photoacoustic spectroscopy technology based on the quartz tuning fork has good immune noise advantage due to the resonance characteristic of the quartz tuning fork, so that the detection sensitivity is higher than that of the traditional direct absorption spectroscopy.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be considered to be within the technical scope of the present invention, and the technical solutions and the inventive concepts thereof according to the present invention should be equivalent or changed within the scope of the present invention.
Claims (3)
1. A dual-spectrum gas detection device based on a quartz tuning fork is characterized by comprising a wavelength tunable laser, a focusing collimating lens, an aperture-tunable square diaphragm, a sample cell, a light-transmitting window, a first tuning fork, an air inlet and outlet, a second tuning fork, a first pre-amplification circuit, a second pre-amplification circuit, a laser control module, an analog-to-digital conversion module, a digital-to-analog conversion module and a computer control unit, wherein the aperture-tunable square diaphragm is arranged on the sample cell; the signal output end of the computer control unit is connected with the digital-to-analog conversion module, the signal output end of the digital-to-analog conversion module is connected with the laser control module, and the laser control module is in control connection with the wavelength tunable laser; the two ends of the sample cell are respectively provided with holes and embedded with a light-transmitting window sheet, the sample cell is provided with an air inlet and an air outlet, gas molecules to be detected are filled in the sample cell, the types of the gas molecules are determined according to the working wavelength of the wavelength tunable laser, and the first tuning fork and the second tuning fork are respectively arranged inside and outside the sample cell; the focusing collimating lens, the aperture-adjustable square diaphragm and the two light-transmitting window sheets of the sample cell are sequentially arranged on a light beam emergent light path of the wavelength-tunable laser, the first tuning fork and the second tuning fork are two quartz tuning forks with the same resonance frequency, the two quartz tuning forks are arranged in parallel but are not completely coaxial, and the side surfaces of the vibrating arms of the two tuning forks are perpendicular to an incident light beam; the light spot of an incident beam adjusted by the square diaphragm with the adjustable aperture is larger than the width of the side face of the tuning fork vibrating arm; the signal input ends of the first pre-amplification circuit and the second pre-amplification circuit are respectively connected with the first tuning fork and the second tuning fork, the signal output ends of the first pre-amplification circuit and the second pre-amplification circuit are connected with an analog-to-digital conversion module, and the signal output end of the analog-to-digital conversion module is connected with the computer control unit.
2. The dual-spectrum gas detection device based on the quartz tuning fork of claim 1, wherein the computer control unit comprises an analog signal output module and a two-way spectrum signal analysis processing algorithm module which are written by Labview software.
3. A method for the quartz tuning fork based dual spectrum gas detection device according to claim 1, comprising the steps of:
[01] a laser modulation signal or a pulse driving voltage signal written by Labview software in the computer control unit is converted into an analog signal through a digital-to-analog conversion module, and the analog signal or the analog signal is finally input to the wavelength tunable laser through a laser control module to realize the wavelength tuning and modulation output of the laser;
[02] modulated laser emitted by the wavelength tunable laser is converted into parallel beams through a focusing collimating lens, and the spot size of the parallel beams is controlled and adjusted by an aperture-adjustable square diaphragm and then coupled into a sample cell;
[03] in the sample cell, the light spot of the incident light beam is larger than the width of the side face of the tuning fork vibrating arm, so that part of light of the incident light beam is vertically incident on the side face of the first tuning fork vibrating arm, and the other part of light directly enters the side face of the second tuning fork vibrating arm after passing through the sample cell;
[04] because the modulation frequency of the wavelength tunable laser is the same as the resonance frequency of the quartz tuning fork, part of energy of incident beams in the sample cell is absorbed by charged molecules to be detected, a photoacoustic signal is formed through a radiationless relaxation process, the first tuning fork is excited to resonate, and meanwhile, the light beams incident to the side surface of the vibrating arm of the first tuning fork also cause the first tuning fork to resonate through the action of light pressure, so that two different signal sources simultaneously excite the first tuning fork to resonate, and finally, a superposed signal of two resonance enhanced signals is generated;
[05] in addition, the incident light beam passing through the sample cell also causes the second tuning fork to resonate through the action of light pressure;
[06] piezoelectric current can be induced by the piezoelectric effect of the quartz tuning fork, and the piezoelectric current generated by the resonance of the first tuning fork and the second tuning fork is amplified and converted into voltage signals through the first pre-amplification circuit and the second pre-amplification circuit with low noise and high precision respectively;
[07] the two paths of signals are converted into digital signals by an analog-to-digital conversion module and then input into a double-spectrum signal analysis processing algorithm module compiled by Labview software in a computer control unit for relevant processing;
[08] the double-spectrum signal analysis processing algorithm module firstly carries out spectrum analysis on a time domain signal output by the quartz tuning fork through a fast Fourier transform algorithm, then calculates amplitudes of two paths of spectrum signals at a resonance frequency by combining an extreme value algorithm, and finally respectively obtains a superposition spectrum corresponding to the first tuning fork and a direct absorption spectrum corresponding to the second tuning fork according to a corresponding relation between the emission wavelength of the laser and the amplitudes of the two paths of spectrum signals;
[09] before the double-spectrum gas detection device is used for measuring an analyte with unknown concentration, firstly, a concentration value of the analyte is inverted by using a direct absorption spectrum corresponding to the second tuning fork, then, a resonance enhanced superposition spectrum corresponding to the first tuning fork is corrected by using the concentration value, and a response constant of the photoacoustic cell is calculated;
[10] and finally, respectively utilizing the direct absorption spectrum and the photoacoustic spectrum to analyze a high-concentration or low-concentration sample, and selecting a corresponding detection method according to the concentration range of the analyte to be detected.
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