CN116124702A - Photoacoustic cell resonance characteristic measurement device and method based on sweep frequency modulation - Google Patents
Photoacoustic cell resonance characteristic measurement device and method based on sweep frequency modulation Download PDFInfo
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- 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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Abstract
The invention provides a device and a method for measuring resonance characteristics of a photoacoustic cell based on sweep frequency modulation, wherein the device comprises the following components: the system comprises a lock-in amplifier, a data acquisition card, a calculation processing unit, a laser driver, a laser, an electro-optic modulator, a collimator, a pitching adjusting frame, a photoacoustic cell, a membraneless optical microphone and a temperature controller. The complex modulation function of the sine signal superposition sawtooth signal in the traditional measurement is realized by utilizing the frequency sweep modulation, the requirement on modulating the signal is reduced, the frequency range of the frequency sweep sine signal output by the data acquisition card is adjustable, and the modulation can be carried out in a large range.
Description
Technical Field
The invention relates to the technical field of photoacoustic spectroscopy gas detection, in particular to a photoacoustic cell resonance characteristic measurement device and method based on sweep frequency modulation.
Background
Trace gas detection has been widely used in the fields of environmental pollution monitoring, industrial production and emission measurement, medical diagnosis, agriculture, food safety, and the like; photoacoustic spectroscopy becomes an important branch in the field of trace gas detection because of the advantages of high sensitivity, high selectivity, zero background signal, real-time online monitoring and the like. The photoacoustic cell serving as the core of the photoacoustic spectroscopy system directly determines the accuracy and the detection limit of gas detection; however, the resonance characteristics of the practical photoacoustic cell often have a certain deviation from the theoretically designed parameters; machining errors, mounting errors of the microphone, and ambient temperature variations all affect the resonant characteristics of the photoacoustic cell, such as shift of formants, change in amplitude of resonance signals, and the like. The resonant characteristic of the photoacoustic cell can be measured to evaluate the design, processing and assembling effects of the photoacoustic cell, and the photoacoustic cell can be calibrated, so that the measurement accuracy is improved. Therefore, it is important to measure the resonance characteristics of each photoacoustic cell.
The traditional method for measuring the resonance characteristics of the photoacoustic cell is to measure the amplitudes of photoacoustic signals at different frequencies by using a photoacoustic spectroscopy system, and fit the data points to obtain a resonance characteristic curve of the photoacoustic cell. The measuring method is time-consuming and labor-consuming, and if the processing effect is poor, multiple rounds of testing are needed, the approximate position of the formants is determined by large-scale measurement, and then the resonance characteristic curve is obtained by fine measurement. Although the measurement results are accurate, the time and labor-consuming data measurement and the complicated data processing are unfavorable for the mass production and the calibration in the use process of the photoacoustic cell. As disclosed in chinese patent CN112834430a, a gas detection device and method based on acoustic pulse excitation of a photoacoustic cell are disclosed, an excitation light source is modulated at a frequency deviating from the resonant frequency of the photoacoustic cell, the center frequency of a formant is calculated by measuring a beat signal, and an attenuation constant is calculated according to fitting of a plurality of measured data to obtain a quality factor, but in order to ensure the generation of the beat frequency, the method needs to know the nominal resonant frequency when leaving the factory in advance, and can only modulate within a range deviating from 10-200Hz from the nominal resonant frequency; in addition, the influence of the ambient temperature on the measurement result is not considered, and in many application scenes, the detection of the gas concentration often has a strong coupling relation with the temperature, in these occasions, the detection of the gas concentration can be performed in a wide temperature area, the temperature is taken as disturbance, the measurement of the photoacoustic spectrum system can be influenced from the aspects of gas spectral line broadening, photoacoustic signals, photoacoustic effects, acoustic microphones and the like, and the measurement result can be calibrated for the measurement of the temperature, so that the measurement accuracy is improved.
Disclosure of Invention
In view of this, the invention provides a device and a method for measuring resonance characteristics of a photoacoustic cell based on sweep frequency modulation, so as to solve the problem that in the existing method for measuring resonance characteristics of a photoacoustic cell, in order to ensure generation of beat frequency, the nominal resonance frequency in factory is required to be known in advance, and modulation can only be performed in a small range.
The technical scheme of the invention is realized as follows: in one aspect, the invention provides a photoacoustic cell resonance characteristic measurement device based on sweep frequency modulation, wherein the device comprises:
the system comprises a lock-in amplifier, a data acquisition card, a calculation processing unit, a laser driver, a laser, an electro-optic modulator, a collimator, a pitching adjusting frame, a photoacoustic cell, a membraneless optical microphone and a temperature controller;
the data acquisition card is electrically connected with the phase-locked amplifier and the electro-optical modulator and is used for outputting a frequency-sweeping sinusoidal signal to the electro-optical modulator and outputting a frequency-doubling frequency-sweeping sinusoidal signal to the phase-locked amplifier;
the laser driver is electrically connected with the laser and used for driving the laser to form stable laser output, and the laser wavelength is positioned at the center of the absorption spectrum line of the measuring gas;
the electro-optical modulator is connected with the laser and used for modulating the intensity of laser output by the laser according to the sweep frequency sinusoidal signal output by the data acquisition card;
the collimator is connected with the electro-optical modulator and is used for collimating modulated laser output by the electro-optical modulator;
the pitching adjusting frame is rotationally connected with the collimator, and is used for supporting the collimator and calibrating an output light path of modulated laser through pitching adjustment;
the photoacoustic cell is used for introducing measurement gas and receiving modulated laser collimated by the collimator to generate a photoacoustic effect;
the membraneless optical microphone is fixed in the middle of the resonant cavity of the photoacoustic cell, and is used for detecting sound signals generated by the photoacoustic effect and converting the sound signals into electric signals;
the phase-locked amplifier is electrically connected with the membraneless optical microphone and is used for phase-locking the sound signal collected by the membraneless optical microphone according to the frequency-doubled frequency-swept sinusoidal signal output by the data collection card, extracting the amplitude of the frequency-doubled component of the sound signal and outputting the amplitude to the data collection card;
the temperature controller is connected with the photoacoustic cell and used for controlling the temperature of the photoacoustic cell;
and the calculation processing unit is electrically connected with the data acquisition card and is used for calculating the resonance characteristic and the environmental temperature of the photoacoustic cell according to the amplitude value acquired by the data acquisition card.
On the basis of the technical scheme, preferably, a first analog output channel of the data acquisition card is connected with a voltage signal input end of the electro-optic modulator and is used for outputting a sweep frequency sinusoidal signal to the electro-optic modulator; the second analog output channel of the data acquisition card is connected with the reference signal input end of the lock-in amplifier and is used for outputting the frequency-doubled frequency-swept sine signal to the lock-in amplifier; the two analog output channels of the data acquisition card are kept in phase.
On the basis of the technical scheme, preferably, the laser is a narrow linewidth laser.
On the basis of the technical scheme, preferably, the photoacoustic cell comprises a resonant cavity, two buffer cavities, a gas inflow channel, a gas outflow channel and two side end windows;
one side end window is opposite to the collimator and is communicated with one side of the gas inflow channel and one side of the resonant cavity through a buffer cavity;
the other side end window is communicated with the other side of the gas outflow channel and the resonant cavity through the other buffer cavity;
and a membraneless optical microphone is fixed at the opening of the upper end of the middle part of the resonant cavity.
On the basis of the above technical solution, preferably, the membraneless optical microphone is used for measuring sound signals by changing the refractive index of air by sound pressure.
On the basis of the technical scheme, preferably, the temperature controller is used for controlling the temperature of the photoacoustic cell, adjusting the temperature according to a certain interval, and establishing a correlation database of resonant characteristics of the photoacoustic cell and the temperature.
On the basis of the above technical solution, preferably, the calculation processing unit is configured to perform smoothing processing on the extracted amplitude signal, convert a time axis into a frequency axis according to a relationship between time and frequency of the swept frequency sinusoidal signal, perform lorentz fitting on the processed data to obtain a resonance characteristic curve of the photoacoustic cell, and calculate an environmental temperature according to the established correlation database of the resonance characteristic and the temperature of the photoacoustic cell.
On the basis of the technical scheme, preferably, the frequency range of the sweep frequency sinusoidal signal output by the data acquisition card is adjustable.
On the basis of the technical scheme, preferably, the period of the sweep frequency sinusoidal signal output by the data acquisition card is adjustable.
In another aspect, the present invention provides a photoacoustic cell resonance characteristic measurement method based on sweep frequency modulation, which adopts the photoacoustic cell resonance characteristic measurement device based on sweep frequency modulation as described above, wherein the method comprises the following steps:
s101, outputting a sweep frequency sinusoidal signal to an electro-optical modulator through a first analog output channel of a data acquisition card; the second analog output channel of the data acquisition card outputs a frequency doubling sweep sine signal to the reference signal input end of the lock-in amplifier;
s102, a laser driver drives a laser to form stable laser output, and the laser wavelength is positioned at the center of an absorption spectrum line of the measuring gas;
s103, the electro-optical modulator carries out sweep frequency intensity modulation on laser output by the laser according to a sweep frequency sinusoidal signal output by the data acquisition card;
s104, collimating the modulated laser output by the electro-optical modulator through a collimator, and passing through measurement gas which is introduced into the photoacoustic cell in advance to generate a photoacoustic effect;
s105, a membraneless optical microphone positioned in the middle of the resonant cavity of the photoacoustic cell collects sound signals generated by the photoacoustic effect and converts the sound signals into electric signals;
s106, the phase-locking amplifier performs phase locking on the electric signal output by the membraneless optical microphone and the frequency-doubling frequency-sweeping sinusoidal signal output by the data acquisition card, and extracts the amplitude of the frequency-doubling component of the photoacoustic signal at each frequency position;
s107, the data acquisition card acquires amplitude signals and transmits the amplitude signals to the calculation processing unit, the calculation processing unit carries out smoothing processing on the amplitude signals output by the phase-locked amplifier, a time axis is converted into a frequency axis according to the relation between the time and the frequency of the frequency-sweeping sinusoidal signals, and Lorentz fitting is carried out on the processed data to obtain a resonance characteristic curve of the photoacoustic cell;
s108, calculating the environmental temperature according to the measured resonance characteristic curve of the photoacoustic cell and a correlation database of the resonance characteristic and the temperature of the photoacoustic cell.
On the basis of the above technical solution, preferably, in step S108, the database of correlation between resonant characteristics of the photoacoustic cell and temperature is obtained by calibrating resonant characteristics of the photoacoustic cell, and the method includes the following steps:
s201, outputting a sweep frequency sinusoidal signal to an electro-optical modulator through a first analog output channel of a data acquisition card; the second analog output channel of the data acquisition card outputs a frequency doubling sweep sine signal to the reference signal input end of the lock-in amplifier;
s202, a laser driver drives a laser to form stable laser output, and the laser wavelength is positioned at the center of an absorption spectrum line of the measuring gas;
s203, the electro-optical modulator carries out sweep frequency intensity modulation on laser output by the laser according to a sweep frequency sinusoidal signal output by the data acquisition card;
s204, controlling the temperature of the photoacoustic cell to be a constant value by a temperature controller;
s205, collimating the modulated laser output by the electro-optical modulator through a collimator, and passing through measurement gas which is introduced into the photoacoustic cell in advance to generate a photoacoustic effect;
s206, a membraneless optical microphone positioned in the middle of the resonant cavity of the photoacoustic cell collects sound signals generated by the photoacoustic effect and converts the sound signals into electric signals;
s207, performing phase locking on the electric signal output by the membraneless optical microphone and the frequency-doubled frequency-swept sinusoidal signal output by the data acquisition card by using a phase locking amplifier, and extracting the amplitude of the frequency-doubled component of the photoacoustic signal at each frequency position;
s208, the data acquisition card acquires amplitude signals and transmits the amplitude signals to the calculation processing unit, the calculation processing unit carries out smoothing processing on the amplitude signals output by the phase-locked amplifier, a time axis is converted into a frequency axis according to the relation between the time and the frequency of the frequency-sweeping sinusoidal signals, and Lorentz fitting is carried out on the processed data to obtain a resonance characteristic curve of the photoacoustic cell;
s209, setting the temperature of the temperature controller at certain temperature intervals, repeating the steps S204-S208, and establishing a correlation database of the resonance characteristics and the temperature of the photoacoustic cell.
On the basis of the above technical solution, preferably, the time axis is converted into a frequency axis according to the relationship between the time and the frequency of the frequency-swept sinusoidal signal, and the calculation formula is as follows:
wherein ,
for the frequency of the frequency doubling component of the photoacoustic signal resonating in the resonant cavity at time t +.>
For the initial frequency of the swept sine signal, +.>
The termination frequency of the sweep frequency sinusoidal signal is T, and the period of the sweep frequency sinusoidal signal is T.
On the basis of the above technical solution, preferably, the lorentz fitting is performed on the processed data, and a calculation formula is as follows:
wherein ,
for the frequency of the frequency doubling component of the photoacoustic signal resonating in the resonant cavity, +.>
To be at frequency +.>
Where A, B is a constant peak value of the doubling signal generated in the photoacoustic cell, +.>
For spectral line center frequency>
For a uniformly widened line width A, B, < >>
、/>
Fitting the data.
Compared with the prior art, the photoacoustic cell resonance characteristic measuring device and method based on sweep frequency modulation have the following beneficial effects:
(1) The complex modulation function of the sine signal superposition sawtooth signal in the traditional measurement is realized by utilizing the sweep frequency modulation, the requirement on modulating the signal is reduced, the frequency range of the sweep frequency sine signal output by the data acquisition card is adjustable, and the modulation can be carried out in a large range;
(2) By measuring resonance characteristics before measuring gas concentration by the photoacoustic spectrometry system, the measurement accuracy and repeatability of the photoacoustic spectrometry system are improved, the maintenance times of the system are reduced, and popularization and use of the photoacoustic spectrometer are promoted; the environment temperature can be synchronously measured while the resonance characteristic of the photoacoustic cell is measured, so that the measurement result of the gas concentration is corrected, and the measurement accuracy of the photoacoustic spectroscopy system is improved; the measuring speed is high, the accuracy is high, complicated data measurement and processing are avoided, and the large-scale production, testing and calibration of the photoacoustic cell are facilitated;
(3) Compared with a modulation mode of directly modulating the laser internally by current and temperature, the laser external modulation method has higher stability and more accurate modulation, can reduce noise, improves measurement accuracy, and does not have the problem that an internal modulation photoacoustic spectrum system is accompanied with intensity modulation during wavelength modulation;
(4) The sound collecting device is improved, sound signals are measured through the membraneless optical microphone, mechanical activity and resonance are not involved, the ultra-wide frequency measuring range is achieved, the ultra-high stability and the ultra-low noise floor are achieved, the bandwidth defect of a microphone is overcome, the measuring range of resonance characteristics is widened, the photoacoustic cells of various structures and resonance characteristics can be measured, and the photoacoustic cells with high resonance frequency can be matched, so that the system works at a high frequency and noise is reduced.
Drawings
In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, it being obvious that the drawings in the following description are only some embodiments of the invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a schematic diagram of a photoacoustic cell resonance characteristic measurement device based on sweep frequency modulation;
FIG. 2 is a schematic flow chart of a method for measuring resonance characteristics of a photoacoustic cell based on sweep frequency modulation;
FIG. 3 is a graph of the dispersion points and fitting of resonance characteristics measured by the conventional photoacoustic spectroscopy;
fig. 4 is a graph of resonance characteristics measured by the sweep modulation-based photoacoustic cell resonance characteristic measurement method of the present invention.
Detailed Description
The following description of the embodiments of the present invention will clearly and fully describe the technical aspects of the embodiments of the present invention, and it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the present invention without making any inventive effort, are intended to fall within the scope of the present invention.
Example 1
As shown in fig. 1, the photoacoustic cell resonance characteristic measuring device based on sweep frequency modulation of the present invention comprises:
the system comprises a lock-in amplifier 1, a data acquisition card 2, a calculation processing unit 3, a laser driver 4, a laser 5, an electro-optical modulator 6, a collimator 7, a pitching adjusting frame 8, a photoacoustic cell 9, a membraneless optical microphone 10 and a temperature controller 11;
the data acquisition card 2 is electrically connected with the lock-in amplifier 1 and the electro-optical modulator 6 and is used for outputting a frequency sweep sinusoidal signal to the electro-optical modulator 6 and outputting a frequency doubling frequency sweep sinusoidal signal to the lock-in amplifier 1;
the laser driver 4 is electrically connected with the laser 5, and is used for driving the laser 5 to form stable laser output, and the laser wavelength is positioned at the center of the absorption spectrum line of the measuring gas;
the electro-optical modulator 6 is connected with the laser 5 and is used for modulating the intensity of laser output by the laser 5 according to the sweep frequency sinusoidal signal output by the data acquisition card 2;
the collimator 7 is connected with the electro-optical modulator 6 and is used for collimating modulated laser output by the electro-optical modulator 6;
the pitching adjusting frame 8 is rotatably connected with the collimator 7, and is used for supporting the collimator 7 and calibrating an output light path of modulated laser through pitching adjustment;
the photoacoustic cell 9 is used for introducing measurement gas and receiving modulated laser collimated by the collimator 7 to generate a photoacoustic effect;
the membraneless optical microphone 10 is fixed in the middle of the resonant cavity of the photoacoustic cell 9, and is used for detecting sound signals generated by the photoacoustic effect and converting the sound signals into electric signals;
the lock-in amplifier 1 is electrically connected with the membraneless optical microphone 10, and is used for performing lock-in on the sound signal collected by the membraneless optical microphone 10 according to the frequency-doubled frequency-swept sinusoidal signal output by the data collection card 2, extracting the amplitude of the frequency-doubled component of the sound signal and outputting the amplitude to the data collection card 2;
the temperature controller 11 is connected with the photoacoustic cell 9 and is used for controlling the temperature of the photoacoustic cell 9;
the calculation processing unit 3 is electrically connected with the data acquisition card 2, and is used for calculating the resonance characteristic and the ambient temperature of the photoacoustic cell 9 according to the amplitude value acquired by the data acquisition card 2.
The device can measure the resonance characteristic of the photoacoustic cell before the photoacoustic spectrometry system measures the concentration of gas (such as acetylene), is beneficial to improving the measurement accuracy and repeatability of the photoacoustic spectrometry system, reduces the maintenance times of the system and promotes popularization and use of the photoacoustic spectrometer; and when the resonance characteristic of the photoacoustic cell is measured, the environment temperature can be synchronously measured, so that the measurement result of the subsequent gas concentration is corrected, and the measurement accuracy of the photoacoustic spectroscopy system is improved.
The first analog output channel of the data acquisition card 2 is connected with the voltage signal input end of the electro-optical modulator 6 and is used for outputting a sweep frequency sinusoidal signal to the electro-optical modulator 6; the second analog output channel of the data acquisition card 2 is connected with the reference signal input end of the lock-in amplifier 1 and is used for outputting the frequency-doubled frequency-swept sinusoidal signal to the lock-in amplifier 1; the two analog output channels of the data acquisition card 2 are kept in phase.
The first analog output channel of the data acquisition card is connected with the voltage signal input end of the electro-optic modulator and is used for outputting a frequency sweep sinusoidal signal with the frequency of 250-1500Hz and the period of 60s to the electro-optic modulator so as to realize the frequency sweep intensity modulation of outputting stable laser to the laser; the second analog output channel of the data acquisition card is connected with the reference signal input end of the phase-locked amplifier and is used for outputting a frequency-doubled frequency-swept sinusoidal signal with the period of 60s and 500-3000Hz to the phase-locked amplifier as a reference signal for signal extraction; two paths of analog output channels of the data acquisition card need to be kept in phase, so that the frequency between two paths of signals always keeps a two-time relationship.
Wherein the laser 5 is a narrow linewidth laser.
The laser is a narrow linewidth laser, and can be a DFB laser, a quantum cascade laser or an interband cascade laser, etc., so that the interference of introducing impurity gas is avoided.
Wherein the photoacoustic cell 9 comprises a resonant cavity, two buffer cavities, a gas inflow channel, a gas outflow channel and two side end windows;
one side end window is opposite to the collimator 7 and is communicated with one side of the gas inflow channel and the resonant cavity through a buffer cavity;
the other side end window is communicated with the other side of the gas outflow channel and the resonant cavity through the other buffer cavity;
the upper end opening of the middle part of the resonant cavity is fixedly provided with a membraneless optical microphone 10.
The photoacoustic cell comprises a resonant cavity, two buffer cavities, a gas inflow channel, a gas outflow channel and two side end windows. The upper end opening of the middle part of the resonant cavity is fixedly provided with a membraneless optical microphone, the two buffer cavities are arranged on two sides of the resonant cavity, and the gas inflow channel and the gas outflow channel are respectively arranged on the two buffer cavities; the two side end windows are plated with an antireflection film, so that the light energy utilization efficiency is improved.
Wherein the membraneless optical microphone 10 is used for measuring sound signals by changing the refractive index of air by sound pressure.
The membraneless optical microphone changes the refractive index of air through sound pressure, so as to measure sound signals, does not involve mechanical activity and resonance, has ultra-wide frequency measurement range and stability, has the measurement range of 10Hz-2MHz, and widens the resonance characteristic measurement range of the photoacoustic cell.
Wherein, the temperature controller 11 is used for controlling the temperature of the photoacoustic cell 9, adjusting the temperature according to a certain interval, and establishing a correlation database of the resonant characteristic of the photoacoustic cell and the temperature.
The heating and refrigerating module of the temperature controller is adhered to the photoacoustic cell and used for controlling the temperature of the photoacoustic cell, and the temperature is adjusted from 10 ℃ to 50 ℃ according to the interval of 5 ℃, so that the device can conveniently establish a correlation database of the resonance characteristic and the temperature of the photoacoustic cell when the resonance characteristic of the photoacoustic cell is calibrated; when the photoacoustic spectroscopy system actually measures the gas concentration, a temperature controller is not needed.
The computing unit 3 is configured to perform smoothing processing on the extracted amplitude signal, convert a time axis into a frequency axis according to a relationship between time and frequency of the swept frequency sinusoidal signal, perform lorentz fitting on the processed data to obtain a resonance characteristic curve of the photoacoustic cell 9, and calculate an environmental temperature according to an established correlation database of the resonance characteristic and the temperature of the photoacoustic cell.
The computing processing unit is used for carrying out smoothing processing on the extracted amplitude signals, converting a time axis into a frequency axis according to the relation between the time and the frequency of the sweep frequency sinusoidal signals, carrying out Lorentz fitting on the data to obtain a resonance characteristic curve of the photoacoustic cell, and calculating the environmental temperature according to a measured resonance characteristic and temperature correlation database.
The frequency range of the sweep frequency sinusoidal signal output by the data acquisition card 2 is adjustable.
The frequency range of the sweep frequency sinusoidal signal output by the data acquisition card is adjustable, a plurality of resonance modes of the photoacoustic cell can be measured simultaneously by expanding the frequency range, and the frequency resolution of the transverse axis of the resonance characteristic curve can be improved by reducing the frequency range, namely the measurement accuracy of the resonance frequency is improved.
The period of the sweep frequency sinusoidal signal output by the data acquisition card 2 is adjustable.
The period of the sweep frequency sinusoidal signal output by the data acquisition card is adjustable, the measurement accuracy of the resonance frequency can be improved by increasing the period, and the measurement speed can be accelerated by reducing the period.
The photoacoustic cell resonance characteristic measuring device based on sweep frequency modulation in the embodiment can measure resonance characteristics before the photoacoustic spectrometry system measures gas concentration, is beneficial to improving measurement accuracy and repeatability of the photoacoustic spectrometry system, reduces maintenance times of the system and promotes popularization and use of a photoacoustic spectrometer; the environment temperature can be synchronously measured while the resonance characteristic of the photoacoustic cell is measured, so that the measurement result of the gas concentration is corrected, and the measurement accuracy of the photoacoustic spectroscopy system is improved; the measuring speed is high, the accuracy is high, complicated data measurement and processing are avoided, and the large-scale production, testing and calibration of the photoacoustic cell are facilitated.
Example two
As shown in fig. 2, a method for measuring resonance characteristics of a photoacoustic cell based on sweep frequency modulation is provided, which employs the device for measuring resonance characteristics of a photoacoustic cell based on sweep frequency modulation as described in the first embodiment, wherein the method comprises the following steps:
s101, outputting a sweep frequency sinusoidal signal to an electro-optical modulator 6 through a first analog output channel of a data acquisition card 2; the second analog output channel of the data acquisition card 2 outputs a frequency doubling sweep sine signal to the reference signal input end of the lock-in amplifier 1;
the first analog output channel of the data acquisition card outputs a frequency sweep sine signal with the frequency of 250-1500Hz and the period of 60s to the electro-optic modulator; the second analog output channel of the data acquisition card outputs a frequency-doubled frequency-swept sine signal with the period of 60s at 500-3000Hz to the reference signal input end of the lock-in amplifier.
S102, a laser driver 4 drives a laser 5 to form stable laser output, and the laser wavelength is positioned at the center of an absorption spectrum line of the measuring gas;
the laser driver drives the laser through a current and temperature controlled resistor to form a stable laser output with the laser wavelength centered (1531.58 nm) in the absorption line of the measurement gas (e.g., acetylene).
S103, the electro-optical modulator 6 carries out sweep frequency intensity modulation on the laser output by the laser 5 according to the sweep frequency sinusoidal signal output by the data acquisition card 2;
s104, the modulated laser output by the electro-optical modulator 6 is collimated by the collimator 7 and passes through the measurement gas which is introduced into the photoacoustic cell 9 in advance to generate a photoacoustic effect;
s105, a membraneless optical microphone 10 positioned in the middle of a resonant cavity of the photoacoustic cell 9 collects sound signals generated by the photoacoustic effect and converts the sound signals into electric signals;
s106, the phase-locked amplifier 1 performs phase locking on the electric signal output by the membraneless optical microphone 10 and the frequency-doubled frequency-swept sinusoidal signal output by the data acquisition card 2, and extracts the amplitude of the frequency-doubled component of the photoacoustic signal at each frequency position;
s107, the data acquisition card 2 acquires amplitude signals and transmits the amplitude signals to the calculation processing unit 3, the calculation processing unit 3 carries out smoothing processing on the amplitude signals output by the lock-in amplifier 1, a time axis is converted into a frequency axis according to the relation between the time and the frequency of a sweep frequency sinusoidal signal, and then Lorentz fitting is carried out on the processed data to obtain a resonance characteristic curve of the photoacoustic cell 9;
compared with the traditional photoacoustic spectrometry method (fig. 3), the photoacoustic cell resonance characteristic measurement method (fig. 4) based on sweep frequency modulation of the embodiment has the advantages that the measurement speed is greatly increased, the accuracy is high, and scanning of a larger frequency range can be performed.
S108, calculating the environment temperature (24.8 ℃) according to the measured resonance characteristic curve of the photoacoustic cell 9 and a correlation database of the resonance characteristic of the photoacoustic cell and the temperature.
The method can measure the resonance characteristic of the photoacoustic cell before the photoacoustic spectrometry system measures the gas concentration, is beneficial to improving the measurement accuracy and repeatability of the photoacoustic spectrometry system, reduces the maintenance times of the system, and promotes popularization and use of the photoacoustic spectrometer; and when the resonance characteristic of the photoacoustic cell is measured, the environment temperature can be synchronously measured, so that the measurement result of the subsequent gas concentration is corrected, and the measurement accuracy of the photoacoustic spectroscopy system is improved.
In step S108, the database of correlation between the resonant characteristic of the photoacoustic cell and the temperature is obtained by calibrating the resonant characteristic of the photoacoustic cell, and the method comprises the following steps:
s201, outputting a sweep frequency sinusoidal signal to an electro-optical modulator 6 through a first analog output channel of the data acquisition card 2; the second analog output channel of the data acquisition card 2 outputs a frequency doubling sweep sine signal to the reference signal input end of the lock-in amplifier 1;
the first analog output channel of the data acquisition card outputs a frequency sweep sine signal with the frequency of 250-1500Hz and the period of 60s to the electro-optic modulator; the second analog output channel of the data acquisition card outputs a frequency-doubled frequency-swept sine signal with the period of 60s at 500-3000Hz to the reference signal input end of the lock-in amplifier.
S202, a laser driver 4 drives a laser 5 to form stable laser output, and the laser wavelength is positioned at the center of an absorption spectrum line of the measuring gas;
the laser driver drives the laser through a current and temperature controlled resistor to form a stable laser output with the laser wavelength centered (1531.58 nm) in the absorption line of the measurement gas (e.g., acetylene).
S203, the electro-optical modulator 6 carries out sweep frequency intensity modulation on the laser output by the laser 5 according to the sweep frequency sinusoidal signal output by the data acquisition card 2;
s204, controlling the temperature of the photoacoustic cell 9 to be a constant value by the temperature controller 11;
s205, collimating the modulated laser output by the electro-optical modulator 6 by using a collimator 7, and passing through measurement gas which is introduced into the photoacoustic cell 9 in advance to generate a photoacoustic effect;
s206, a membraneless optical microphone 10 positioned in the middle of the resonant cavity of the photoacoustic cell 9 collects sound signals generated by the photoacoustic effect and converts the sound signals into electric signals;
s207, the phase-locked amplifier 1 performs phase locking on the electric signal output by the membraneless optical microphone 10 and the frequency-doubled frequency-swept sinusoidal signal output by the data acquisition card 2, and extracts the amplitude of the frequency-doubled component of the photoacoustic signal at each frequency position;
s208, the data acquisition card 2 acquires amplitude signals and transmits the amplitude signals to the calculation processing unit 3, the calculation processing unit 3 carries out smoothing processing on the amplitude signals output by the lock-in amplifier 1, a time axis is converted into a frequency axis according to the relation between the time and the frequency of the sweep frequency sinusoidal signals, and then Lorentz fitting is carried out on the processed data to obtain a resonance characteristic curve of the photoacoustic cell 9;
s209, setting the temperature of the temperature controller 11 at certain temperature intervals, repeating the steps S204-S208, and establishing a correlation database of the resonance characteristics and the temperature of the photoacoustic cell.
The temperature controller adjusts the temperature of the photoacoustic cell from 10 ℃ to 50 ℃ at intervals of 5 ℃, and repeats steps S204-S208 to establish a correlation database of resonant characteristics of the photoacoustic cell and the temperature.
The time axis is converted into a frequency axis according to the relation between the time and the frequency of the frequency sweep sinusoidal signal, and the calculation formula is as follows:
wherein ,
for the frequency of the frequency doubling component of the photoacoustic signal resonating in the resonant cavity at time t +.>
For the initial frequency of the swept sine signal, +.>
The termination frequency of the sweep frequency sinusoidal signal is T, and the period of the sweep frequency sinusoidal signal is T.
The Lorentz fitting is carried out on the processed data, and a calculation formula is as follows:
wherein ,
for the frequency of the frequency doubling component of the photoacoustic signal resonating in the resonant cavity, +.>
To be at frequency +.>
Where A, B is a constant peak value of the doubling signal generated in the photoacoustic cell, +.>
For spectral line center frequency>
For a uniformly widened line width A, B, < >>
、/>
Fitting the data.
The photoacoustic cell resonance characteristic measurement method based on sweep frequency modulation in the embodiment can measure resonance characteristics before the photoacoustic spectrometry system measures gas concentration, is beneficial to improving measurement accuracy and repeatability of the photoacoustic spectrometry system, reduces maintenance times of the system and promotes popularization and use of a photoacoustic spectrometer; the environment temperature can be synchronously measured while the resonance characteristic of the photoacoustic cell is measured, so that the measurement result of the gas concentration is corrected, and the measurement accuracy of the photoacoustic spectroscopy system is improved; the measuring speed is high, the accuracy is high, complicated data measurement and processing are avoided, and the large-scale production, testing and calibration of the photoacoustic cell are facilitated.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, alternatives, and improvements that fall within the spirit and scope of the invention.
Claims (10)
1. The utility model provides a photoacoustic cell resonance characteristic measuring device based on sweep frequency modulation which characterized in that: the device comprises:
the system comprises a lock-in amplifier (1), a data acquisition card (2), a calculation processing unit (3), a laser driver (4), a laser (5), an electro-optic modulator (6), a collimator (7), a pitching adjusting frame (8), a photoacoustic cell (9), a membraneless optical microphone (10) and a temperature controller (11);
the data acquisition card (2) is electrically connected with the phase-locked amplifier (1) and the electro-optical modulator (6) and is used for outputting a frequency-sweeping sinusoidal signal to the electro-optical modulator (6) and outputting a frequency-doubling frequency-sweeping sinusoidal signal to the phase-locked amplifier (1);
the laser driver (4) is electrically connected with the laser (5) and is used for driving the laser (5) to form stable laser output, and the laser wavelength is positioned at the center of the absorption spectrum line of the measuring gas;
the electro-optical modulator (6) is connected with the laser (5) and is used for modulating the intensity of laser output by the laser (5) according to the sweep frequency sinusoidal signal output by the data acquisition card (2);
the collimator (7) is connected with the electro-optical modulator (6) and is used for collimating modulated laser output by the electro-optical modulator (6);
the pitching adjusting frame (8) is rotationally connected with the collimator (7) and is used for supporting the collimator (7) and calibrating an output light path of modulated laser through pitching adjustment;
the photoacoustic cell (9) is used for introducing measurement gas and receiving modulated laser collimated by the collimator (7) to generate a photoacoustic effect;
the membraneless optical microphone (10) is fixed in the middle of the resonant cavity of the photoacoustic cell (9) and is used for detecting sound signals generated by the photoacoustic effect and converting the sound signals into electric signals;
the phase-locked amplifier (1) is electrically connected with the membraneless optical microphone (10) and is used for phase-locking the sound signal collected by the membraneless optical microphone (10) according to the frequency-doubled frequency-swept sinusoidal signal output by the data collection card (2), extracting the amplitude of the frequency-doubled component of the sound signal and outputting the amplitude to the data collection card (2);
the temperature controller (11) is connected with the photoacoustic cell (9) and is used for controlling the temperature of the photoacoustic cell (9);
the calculation processing unit (3) is electrically connected with the data acquisition card (2) and is used for calculating the resonance characteristic and the ambient temperature of the photoacoustic cell (9) according to the amplitude acquired by the data acquisition card (2).
2. A sweep modulation based photoacoustic cell resonance characteristic measuring apparatus according to claim 1, wherein: the first analog output channel of the data acquisition card (2) is connected with the voltage signal input end of the electro-optical modulator (6) and is used for outputting a sweep frequency sinusoidal signal to the electro-optical modulator (6); the second analog output channel of the data acquisition card (2) is connected with the reference signal input end of the phase-locked amplifier (1) and is used for outputting the frequency-doubled frequency-swept sinusoidal signal to the phase-locked amplifier (1); the two analog output channels of the data acquisition card (2) are kept in phase.
3. A sweep modulation based photoacoustic cell resonance characteristic measuring apparatus according to claim 1, wherein: the laser (5) is a narrow linewidth laser.
4. A sweep modulation based photoacoustic cell resonance characteristic measuring apparatus according to claim 1, wherein: the photoacoustic cell (9) comprises a resonant cavity, two buffer cavities, a gas inflow channel, a gas outflow channel and two side end windows;
one side end window is opposite to the collimator (7) and is communicated with one side of the gas inflow channel and the resonant cavity through a buffer cavity;
the other side end window is communicated with the other side of the gas outflow channel and the resonant cavity through the other buffer cavity;
a membraneless optical microphone (10) is fixed at the opening of the upper end of the middle part of the resonant cavity.
5. A sweep modulation based photoacoustic cell resonance characteristic measuring apparatus according to claim 1, wherein: the temperature controller (11) is used for controlling the temperature of the photoacoustic cell (9), adjusting the temperature at certain intervals and establishing a correlation database of resonant characteristics of the photoacoustic cell and the temperature.
6. A sweep modulation based photoacoustic cell resonance characteristic measuring apparatus according to claim 1, wherein: the computing processing unit (3) is used for carrying out smoothing processing on the extracted amplitude signals, converting a time axis into a frequency axis according to the relation between the time and the frequency of the sweep frequency sinusoidal signals, then carrying out Lorentz fitting on the processed data to obtain a resonance characteristic curve of the photoacoustic cell (9), and then computing the environmental temperature according to the established correlation database of the resonance characteristic and the temperature of the photoacoustic cell.
7. A sweep modulation based photoacoustic cell resonance characteristic measuring apparatus according to claim 1, wherein: the frequency range of the sweep frequency sinusoidal signal output by the data acquisition card (2) is adjustable.
8. A measurement method of a sweep modulation based photoacoustic cell resonance characteristic measurement apparatus according to any one of claims 1 to 7, wherein: the method comprises the following steps:
s101, outputting a sweep frequency sinusoidal signal to an electro-optical modulator (6) through a first analog output channel of a data acquisition card (2); a second analog output channel of the data acquisition card (2) outputs a frequency doubling sweep sine signal to a reference signal input end of the lock-in amplifier (1);
s102, a laser driver (4) drives a laser (5) to form stable laser output, and the laser wavelength is positioned at the center of an absorption spectrum line of the measuring gas;
s103, the electro-optical modulator (6) carries out sweep frequency intensity modulation on laser output by the laser (5) according to a sweep frequency sinusoidal signal output by the data acquisition card (2);
s104, collimating the modulated laser output by the electro-optical modulator (6) through a collimator (7), and passing through measurement gas which is introduced into the photoacoustic cell (9) in advance to generate a photoacoustic effect;
s105, a membraneless optical microphone (10) positioned in the middle of a resonant cavity of the photoacoustic cell (9) collects sound signals generated by the photoacoustic effect and converts the sound signals into electric signals;
s106, the phase-locking amplifier (1) performs phase locking on the electric signal output by the membraneless optical microphone (10) and the frequency-doubling frequency-sweeping sinusoidal signal output by the data acquisition card (2), and extracts the amplitude of the frequency-doubling component of the photoacoustic signal at each frequency position;
s107, the data acquisition card (2) acquires amplitude signals and transmits the amplitude signals to the calculation processing unit (3), the calculation processing unit (3) carries out smoothing processing on the amplitude signals output by the phase-locked amplifier (1), a time axis is converted into a frequency axis according to the relation between the time and the frequency of a frequency sweep sine signal, and Lorentz fitting is carried out on the processed data to obtain a resonance characteristic curve of the photoacoustic cell (9);
s108, calculating the environmental temperature according to the measured resonance characteristic curve of the photoacoustic cell (9) and a correlation database of the resonance characteristic of the photoacoustic cell and the temperature.
9. The measurement method according to claim 8, wherein: in the step S108, the database of correlation between the resonant characteristic of the photoacoustic cell and the temperature is obtained by calibrating the resonant characteristic of the photoacoustic cell, and the method comprises the following steps:
s201, outputting a sweep frequency sinusoidal signal to an electro-optical modulator (6) through a first analog output channel of a data acquisition card (2); a second analog output channel of the data acquisition card (2) outputs a frequency doubling sweep sine signal to a reference signal input end of the lock-in amplifier (1);
s202, a laser driver (4) drives a laser (5) to form stable laser output, and the laser wavelength is positioned at the center of an absorption spectrum line of the measuring gas;
s203, the electro-optical modulator (6) carries out sweep frequency intensity modulation on laser output by the laser (5) according to a sweep frequency sinusoidal signal output by the data acquisition card (2);
s204, controlling the temperature of the photoacoustic cell (9) to be a constant value by the temperature controller (11);
s205, collimating the modulated laser output by the electro-optical modulator (6) through a collimator (7), and passing through measurement gas which is introduced into the photoacoustic cell (9) in advance to generate a photoacoustic effect;
s206, a membraneless optical microphone (10) positioned in the middle of a resonant cavity of the photoacoustic cell (9) collects sound signals generated by the photoacoustic effect and converts the sound signals into electric signals;
s207, performing phase locking on an electric signal output by the membraneless optical microphone (10) and a frequency doubling frequency sweeping sinusoidal signal output by the data acquisition card (2) by a phase locking amplifier (1), and extracting the amplitude of a frequency doubling component of the photoacoustic signal at each frequency position;
s208, acquiring amplitude signals by a data acquisition card (2), transmitting the amplitude signals to a calculation processing unit 3, performing smoothing processing on the amplitude signals output by a phase-locked amplifier (1) by the calculation processing unit (3), converting a time axis into a frequency axis according to the relation between the time and the frequency of a frequency-sweeping sinusoidal signal, and performing Lorentz fitting on the processed data to obtain a resonance characteristic curve of a photoacoustic cell (9);
s209, setting the temperature of the temperature controller (11) at certain temperature intervals, repeating the steps S204-S208, and establishing a correlation database of the resonance characteristics and the temperature of the photoacoustic cell.
10. The measurement method according to claim 8, wherein: the time axis is converted into a frequency axis according to the relation between the time and the frequency of the sweep frequency sinusoidal signal, and the calculation formula is as follows:
wherein ,
for the frequency of the frequency doubling component of the photoacoustic signal resonating in the resonant cavity at time t +.>
For the initial frequency of the swept sine signal, +.>
The termination frequency of the sweep frequency sinusoidal signal is T, and the period of the sweep frequency sinusoidal signal is T. />
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