Multi-gas synchronous detection device and method based on dual-frequency wavelength modulation and demodulation

Abstract

The invention discloses a multi-gas synchronous detection device and method based on dual-frequency wavelength modulation and demodulation, and relates to the field of gas detection, the device comprises a light source module, a photoacoustic signal detection module and a data processing module; wherein the light source module outputs an excitation light beam and irradiates the excitation light beam into a gas chamber of the photoacoustic signal detection module; the excitation light beam is a light beam modulated by a sawtooth wave signal and two sine waves with different frequencies; the photoacoustic signal detection module receives photoacoustic signals generated after each gas to be detected in the gas chamber interacts with the excitation light beam and converts the photoacoustic signals into piezoelectric signals; and the data processing module performs secondary harmonic demodulation on the piezoelectric signal to obtain demodulated secondary harmonic data, and further inverts the concentration of the gas to be detected. According to the invention, two sine wave signals with different frequencies are superposed on the basis of a sawtooth wave signal, the frequency distribution of a modulation signal is expanded, and the concentration of gas adjacent to various absorption lines can be accurately detected at the same time.

Description

Technical Field

[0001] The present application relates to the field of gas detection, and in particular to a multi-gas synchronous detection device and method based on dual-frequency wavelength modulation and demodulation. Background Art

[0002] Gas detection technology plays an important role in environmental monitoring, industrial control, and medical diagnosis. In particular, accurate detection of gas concentration is crucial in scenarios such as greenhouse gas emissions, hazardous gas leaks, and breath analysis. Wavelength Modulation Spectroscopy (WMS), as a non-contact optical gas detection method, has been widely used for accurate measurement of gas concentration. Traditional wavelength modulation technology applies a sawtooth wave to the laser light source and superimposes a single sinusoidal modulation signal on the sawtooth wave to drive the laser wavelength to periodically scan near the absorption peak of the target gas, and extracts signals related to gas concentration through second harmonic detection technology. This technology has the advantages of high sensitivity, low noise, and fast response, and can effectively detect specific components in a single gas or gas mixture.

[0003] However, when it is necessary to detect multiple gases with very close absorption lines or multiple gases with resonant absorption, the traditional wavelength modulation method has certain limitations. It is difficult to effectively distinguish the absorption peaks of multiple gases at the same time by superimposing a simple sawtooth wave with a single sinusoidal modulation signal, especially when these absorption peaks are densely distributed in the same wavelength range, the signal resolution decreases, resulting in reduced measurement accuracy. When faced with complex environmental gas mixtures, traditional modulation technology is difficult to accurately distinguish and measure the concentration of each component gas. Summary of the invention

[0004] The purpose of the present application is to provide a multi-gas synchronous detection device and method based on dual-frequency wavelength modulation and demodulation, which can accurately detect the concentrations of multiple gases with adjacent absorption lines at the same time.

[0005] To achieve the above objectives, this application provides the following solutions:

[0006] In a first aspect, the present application provides a multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation, comprising: a light source module, a photoacoustic signal detection module and a data processing module;

[0007] The light source module is used to output an excitation light beam, and irradiate the excitation light beam into the air chamber of the photoacoustic signal detection module; the air chamber contains at least one gas to be detected; the excitation light beam is a light beam modulated by a sawtooth wave signal and two sine waves of different frequencies;

[0008] The photoacoustic signal detection module is used to receive the photoacoustic signal generated by the interaction between the gas to be measured in the gas chamber and the excitation light beam, and convert the photoacoustic signal into a piezoelectric signal;

[0009] The data processing module is used to perform second harmonic demodulation on the piezoelectric signal output by the photoacoustic signal detection module to obtain the demodulated second harmonic data and to invert the concentration of the gas to be measured according to the demodulated second harmonic data to obtain the concentration of the gas to be measured.

[0010] Optionally, the light source module includes an excitation light source, a light source temperature controller, a first function generator, a second function generator and a third function generator;

[0011] The output end of the light source temperature controller is connected to the temperature control input end of the excitation light source; the signal output end of the first function generator is connected to the current scanning input end of the excitation light source; the signal output ends of the second function generator and the third function generator are both connected to the current modulation input end of the excitation light source; the synchronization signal output ends of the second function generator and the third function generator are connected to the signal input end of the data processing module;

[0012] The first function generator is used to output the sawtooth wave signal; the second function generator is used to output a first sinusoidal modulation signal; the third function generator is used to output a second sinusoidal modulation signal; the first sinusoidal modulation signal and the second sinusoidal modulation signal have different frequencies;

[0013] The light source temperature controller is used to adjust the temperature of the excitation light beam output by the excitation light source;

[0014] The excitation light source is used to output the excitation light beam and irradiate the excitation light beam into the air chamber of the photoacoustic signal detection module under the simultaneous driving of the light source temperature controller, the first function generator, the second function generator and the third function generator.

[0015] Optionally, the data processing module includes a signal amplifier, a first lock-in amplifier, a second lock-in amplifier and a computer;

[0016] The signal input end of the signal amplifier is connected to the signal output end of the photoacoustic signal detection module, the signal output end of the signal amplifier is respectively connected to the signal input ends of the first lock-in amplifier and the second lock-in amplifier, and the communication ports of the first lock-in amplifier and the second lock-in amplifier are respectively connected to the communication ports of the computer;

[0017] The reference signal input terminal of the first lock-in amplifier is connected to the synchronization signal output terminal of the third function generator; the reference signal input terminal of the second lock-in amplifier is connected to the synchronization signal output terminal of the second function generator;

[0018] The signal amplifier is used to amplify the piezoelectric signal output by the photoacoustic signal detection module, and send the amplified piezoelectric signal to the first lock-in amplifier and the second lock-in amplifier;

[0019] The first lock-in amplifier is used to receive the amplified piezoelectric signal and the synchronization signal output by the third function generator, and perform second harmonic demodulation on the amplified piezoelectric signal at a first demodulation frequency to obtain a first demodulated harmonic signal;

[0020] The second lock-in amplifier is used to receive the amplified piezoelectric signal and the synchronization signal output by the second function generator, and perform second harmonic demodulation on the amplified piezoelectric signal at a second demodulation frequency to obtain a second demodulated harmonic signal; the demodulated second harmonic data includes the first demodulated harmonic signal and the second demodulated harmonic signal;

[0021] The computer is used for receiving the first demodulated harmonic signal and the second demodulated harmonic signal, and inverting the concentration of the gas to be measured according to the first demodulated harmonic signal and the second demodulated harmonic signal to obtain the concentration of the gas to be measured.

[0022] Optionally, the frequency of the first sinusoidal modulation signal is half of the fundamental frequency of the photoacoustic signal detection module;

[0023] The frequency of the second sinusoidal modulation signal is half of the high-order overtone frequency of the photoacoustic signal detection module.

[0024] Optionally, the first demodulation frequency and the second demodulation frequency match the resonance frequency of the photoacoustic signal detection module.

[0025] Optionally, the first demodulation frequency is a high-order overtone frequency of the photoacoustic signal detection module; and the second demodulation frequency is a fundamental frequency of the photoacoustic signal detection module.

[0026] Optionally, the optical power calculation formula of the excitation light beam of the excitation light source is:

[0027]

[0028] In the formula, P(t) represents the optical power of the excitation light beam of the excitation light source; P0 represents the optical power of the excitation light beam to which only the sawtooth wave signal is applied; α(v0) represents the absorption coefficient corresponding to the initial wave number v0; α′(v0) represents the first-order derivative of α(v0); α”(v0) represents the second-order derivative of α(v0); Δv1 represents the amplitude of the wave number change related to the frequency of the first sinusoidal modulation signal; Δv2 represents the amplitude of the wave number change related to the frequency of the second sinusoidal modulation signal; f1 represents the frequency of the first sinusoidal modulation signal; f2 represents the frequency of the second sinusoidal modulation signal; and L represents the absorption path length.

[0029] Optionally, a calculation formula for the first demodulated harmonic signal is:

[0030]

[0031] The calculation formula of the second demodulated harmonic signal is:

[0032]

[0033] In the formula, represents a first demodulated harmonic signal; represents the second demodulated harmonic signal.

[0034] Optionally, the photoacoustic signal detection module includes a photoacoustic cell detector, a quartz tuning fork detector and a cantilever beam detector.

[0035] In a second aspect, the present application provides a multi-gas synchronous detection method based on dual-frequency wavelength modulation and demodulation, which is applied to a multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation as described in any one of the above items, and the multi-gas synchronous detection method based on dual-frequency wavelength modulation and demodulation includes:

[0036] Control the light source module to output an excitation light beam, and irradiate the excitation light beam into the air chamber of the photoacoustic signal detection module; the air chamber contains at least one gas to be detected; the excitation light beam is a light beam modulated by a sawtooth wave signal and two sine waves of different frequencies;

[0037] Using a photoacoustic signal detection module to receive a photoacoustic signal generated by the interaction between the gas to be measured in the gas chamber and the excitation light beam, and converting the photoacoustic signal into a piezoelectric signal;

[0038] The data processing module is used to perform second harmonic demodulation on the piezoelectric signal output by the photoacoustic signal detection module to obtain demodulated second harmonic data, and the concentration of the gas to be measured is inverted according to the demodulated second harmonic data to obtain the concentration of the gas to be measured.

[0039] According to the specific embodiments provided in this application, this application discloses the following technical effects:

[0040] The present application provides a multi-gas synchronous detection device and method based on dual-frequency wavelength modulation and demodulation, the device includes a light source module, a photoacoustic signal detection module and a data processing module; wherein the light source module outputs an excitation light beam, and irradiates the excitation light beam into the gas chamber of the photoacoustic signal detection module; the excitation light beam is a light beam modulated by a sawtooth signal and two sine waves of different frequencies; the photoacoustic signal detection module receives the photoacoustic signal generated by the interaction between the gas to be measured in the gas chamber and the excitation light beam and converts the photoacoustic signal into a piezoelectric signal; the data processing module performs second harmonic demodulation on the piezoelectric signal, obtains the demodulated second harmonic data, and further inverts the concentration of the gas to be measured. The present invention superimposes two sine wave signals of different frequencies on the basis of the sawtooth wave signal, and compared with superimposing a single sine wave signal on the basis of the sawtooth wave signal, the frequency distribution of the modulated signal can be expanded to a wider range, thereby simultaneously detecting multiple gases with adjacent absorption lines, and realizing synchronous and accurate detection of multiple gas concentrations. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0042] Figure 1 A schematic diagram of the structure of a multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation provided in Example 1 of the present application;

[0043] Figure 2 A schematic diagram of a second harmonic signal obtained when a single quartz tuning fork is used to simultaneously detect a gas to be tested at different frequencies using the dual-frequency wavelength modulation signal demodulation technology of the present application provided in Example 1 of the present application;

[0044] Figure 3 A schematic flow chart of a multi-gas synchronous detection method based on dual-frequency wavelength modulation and demodulation provided in Example 2 of the present application;

[0045] Figure 4 A waveform simulation diagram and an actual waveform detection diagram of the dual-frequency wavelength modulation technology of the present application provided in Example 2 of the present application;

[0046] Figure 5 A schematic diagram of the wave number change of the excitation light source after the modulation signal is loaded onto the excitation light source using the dual-frequency wavelength modulation technology of the present application provided in Example 2 of the present application.

[0047] Figure numerals: 1-excitation light source, 2-light source temperature controller, 3-first function generator, 4-second function generator, 5-third function generator, 6-photoacoustic signal detection module, 7-signal amplifier, 8-first phase-locked amplifier, 9-second phase-locked amplifier, 10-computer. DETAILED DESCRIPTION

[0048] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0049] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0050] Example 1

[0051] In the prior art, a single modulation frequency is difficult to effectively distinguish multiple gas absorption peaks at the same time, especially when the distance between the absorption lines is close, the overlap of signals leads to a decrease in detection accuracy; insufficient modulation depth or mismatch of the modulation signal frequency may lead to insufficient detection sensitivity for weaker absorption lines; and traditional wavelength modulation technology with a single modulation method is difficult to detect multiple gas absorption peaks simultaneously in a single scan. In order to improve detection accuracy and meet the needs of multi-gas detection, this application proposes a technology that combines an improved wavelength modulation technology with a photoacoustic spectroscopy technology. Specifically, a multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation is proposed, such as Figure 1 As shown, the multi-gas synchronous detection device includes: a light source module, a photoacoustic signal detection module 6 and a data processing module.

[0052] The light source module is used to output an excitation light beam and irradiate the excitation light beam into the air chamber of the photoacoustic signal detection module 6; the air chamber contains at least one gas to be detected; the excitation light beam is a light beam modulated by a sawtooth wave signal and two sine waves of different frequencies. As an example, the gas to be detected can be H2O.

[0053] The photoacoustic signal detection module 6 is used to receive the photoacoustic signal generated by the interaction between the gas to be measured in the gas chamber and the excitation light beam, and convert the photoacoustic signal into a piezoelectric signal. The photoacoustic signal detection module can select instruments for photoacoustic signal detection such as a photoacoustic cell, a quartz tuning fork, and a cantilever beam.

[0054] The data processing module is used to perform second harmonic demodulation on the piezoelectric signal output by the photoacoustic signal detection module 6 to obtain the demodulated second harmonic data and to invert the concentration of the gas to be measured based on the demodulated second harmonic data to obtain the concentration of the gas to be measured.

[0055] The peak value of the second harmonic signal obtained after demodulation corresponds to the concentration information of the gas to be measured. Since the piezoelectric signal output by the photoacoustic signal detection module 6 is proportional to the gas concentration, the concentration information of the gas to be measured can be obtained by calibrating the detection device with a gas of known concentration in advance. Figure 2 As shown in FIG. 1 , it is the second harmonic signal obtained when a single quartz tuning fork is used to simultaneously detect the gas to be tested. According to the different peak positions of the second harmonic signal, the gas concentration information at different wavelengths with similar absorption line distances can be detected.

[0056] In this embodiment, by superimposing two sinusoidal wave signals of different frequencies (such as f1 and f2), the frequency distribution of the modulated signal can be extended to a wider range, thereby simultaneously detecting multiple gases with adjacent absorption lines. The absorption peaks of different gases can be effectively distinguished by extracting different harmonic signals (such as 2f, 4f, etc.). Since the absorption peak of each gas can be distinguished by its unique frequency response, the use of dual-frequency wavelength modulation technology can simultaneously obtain the concentration information of multiple gases in one scan, greatly improving the efficiency of gas detection.

[0057] As an optional implementation, the light source module includes an excitation light source 1 , a light source temperature controller 2 , a first function generator 3 , a second function generator 4 and a third function generator 5 .

[0058] The output end of the light source temperature controller 2 is connected to the temperature control input end of the excitation light source 1; the signal output end of the first function generator 3 is connected to the current scanning input end of the excitation light source 1; the signal output ends of the second function generator 4 and the third function generator 5 are both connected to the current modulation input end of the excitation light source 1; the synchronization signal output ends of the second function generator 4 and the third function generator 5 are connected to the signal input end of the data processing module.

[0059] The first function generator 3 is used to output the sawtooth wave signal; the second function generator 4 is used to output the first sinusoidal modulation signal; and the third function generator 5 is used to output the second sinusoidal modulation signal. The frequencies of the first sinusoidal modulation signal and the second sinusoidal modulation signal are different. The excitation light source 1 is temperature-controlled under the control of the light source temperature controller 2 to ensure the temperature of the output light beam of the excitation light source 1.

[0060] When using the multi-gas synchronous detection device for gas detection, by controlling the voltage amplitude of the sawtooth wave signal, the wavelength of the output light beam of the excitation light source 1 is continuously scanned near the target absorption line of the gas to be detected, and it is ensured that the output light beam wavelength slowly passes through the target absorption line of the gas to be detected.

[0061] The two sinusoidal modulation signals of different frequencies emitted by the second function generator 4 and the third function generator 5 and the sawtooth scanning signal output by the first function generator 3 are simultaneously loaded onto the excitation light source 1, and the wavelength of the output beam of the excitation light source 1 is modulated. The propagation direction of the modulated output beam is adjusted to determine the optical path of the output beam of the excitation light source 1, so as to be incident on the photoacoustic signal detection module 6, and the photoacoustic signal is generated after interacting with the gas to be measured in the air chamber of the photoacoustic signal detection module 6. The gas molecules absorb the light energy of the output beam of the excitation light source 1 and transition to a high energy state, and generate sound waves in the collision de-excitation relaxation process. The sound waves drive the detector of the photoacoustic signal detection module 6, such as the arm of a quartz tuning fork, to vibrate, and the piezoelectric effect of the quartz tuning fork converts the photoacoustic signal into a piezoelectric signal. Therefore, the excitation light source 1 is used to output the excitation light beam under the simultaneous drive of the light source temperature controller 2, the first function generator 3, the second function generator 4 and the third function generator 5, and irradiate the excitation light beam into the air chamber of the photoacoustic signal detection module 6. The photoacoustic signal detection module 6 is used to receive the photoacoustic signal generated by the interaction between the gas to be tested and the excitation light beam in the gas chamber, and to convert the photoacoustic signal into a piezoelectric signal. The excitation light source 1 adopts a tunable laser source. As an example, a DFB laser can be used, which can be used at the H2O gas absorption line 7306.73cm -1 Tuning nearby.

[0062] As an optional implementation, the frequency f1 of the first sinusoidal modulation signal is half of the fundamental frequency of the photoacoustic signal detection module 6. The frequency f2 of the second sinusoidal modulation signal is half of the high-order overtone frequency of the photoacoustic signal detection module 6. If the photoacoustic signal detection module 6 uses a quartz tuning fork, its fundamental frequency is 2883.8 Hz, and the first overtone is 17839.5 Hz, then, f1 = 1441.9 Hz, f2 = 8919.75 Hz.

[0063] As an optional implementation, the data processing module includes a signal amplifier 7, a first lock-in amplifier 8, a second lock-in amplifier 9 and a computer 10. As an example, the signal amplifier 7 is a transimpedance preamplifier.

[0064] The signal input end of the signal amplifier 7 is connected to the signal output end of the photoacoustic signal detection module 6, and the signal output end of the signal amplifier 7 is respectively connected to the signal input ends of the first phase-locked amplifier 8 and the second phase-locked amplifier 9, and the communication ports of the first phase-locked amplifier 8 and the second phase-locked amplifier 9 are respectively connected to the communication ports of the computer 10.

[0065] The reference signal input terminal of the first lock-in amplifier 8 is connected to the synchronization signal output terminal of the third function generator 5 ; the reference signal input terminal of the second lock-in amplifier 9 is connected to the synchronization signal output terminal of the second function generator 4 .

[0066] The signal amplifier 7 is used to amplify the piezoelectric signal output by the photoacoustic signal detection module 6, and send the amplified piezoelectric signal to the first lock-in amplifier 8 and the second lock-in amplifier 9;

[0067] The first lock-in amplifier 8 is used to receive the amplified piezoelectric signal and the synchronization signal output by the third function generator 5, and perform second harmonic demodulation on the amplified piezoelectric signal at a first demodulation frequency to obtain a first demodulated harmonic signal.

[0068] The second lock-in amplifier 9 is used to receive the amplified piezoelectric signal and the synchronization signal output by the second function generator 4, and perform second harmonic demodulation on the amplified piezoelectric signal at a second demodulation frequency to obtain a second demodulated harmonic signal. The demodulated second harmonic data includes the first demodulated harmonic signal and the second demodulated harmonic signal.

[0069] The computer 10 is used for receiving the first demodulated harmonic signal and the second demodulated harmonic signal, and inverting the concentration of the gas to be measured according to the first demodulated harmonic signal and the second demodulated harmonic signal to obtain the concentration of the gas to be measured.

[0070] The demodulation frequencies of the first phase-locked amplifier 8 and the second phase-locked amplifier 9 match the resonance frequency of the photoacoustic signal detection module 6, corresponding to the modulation frequencies of the two sinusoidal modulation signals. The first demodulation frequency is the high-order overtone frequency of the photoacoustic signal detection module 6; the second demodulation frequency is the fundamental frequency of the photoacoustic signal detection module 6.

[0071] The first phase-locked amplifier 8 receives the amplified piezoelectric signal output from the transimpedance preamplifier (signal amplifier 7) and the synchronization signal provided by the third function generator 5, and processes the piezoelectric signal converted from the photoacoustic signal under the fundamental frequency condition of the photoacoustic signal detection module 6 by the second harmonic demodulation method; at the same time, the second phase-locked amplifier 9 receives the amplified piezoelectric signal output from the transimpedance preamplifier and the synchronization signal provided by the second function generator 4, and processes the piezoelectric signal converted from the photoacoustic signal under the high-order overtone condition of the photoacoustic signal detection module 6 by the second harmonic demodulation method; the demodulated second harmonic data of the first phase-locked amplifier 8 and the second phase-locked amplifier 9 are respectively transmitted to the computer 10, and the concentration of the gas to be measured is inverted by the LabView software detection program configured in the computer 10 to obtain the concentration of the gas to be measured.

[0072] In this embodiment, dual-frequency sinusoidal wave modulation not only improves the multidimensionality of the modulated signal, but also optimizes the demodulation algorithm. The piezoelectric signal converted from the photoacoustic signal is processed under the fundamental frequency condition of the photoacoustic signal detection module 6 by the second harmonic demodulation method. The piezoelectric signal converted from the photoacoustic signal is processed under the high-order overtone condition of the photoacoustic signal detection module 6 by the second harmonic demodulation method. When extracting the second harmonic signal, the absorption peaks of various gases can be better distinguished, thereby improving the sensitivity and accuracy of the signal.

[0073] The laser excitation method of the dual-frequency wavelength modulation technology used in multi-gas detection and the demodulation principle of the photoacoustic signal are theoretically derived. The corresponding derivation process is as follows:

[0074] When the current of the laser diode of the excitation light source 1 is modulated in the form of a sine wave, the wavelength and the laser power are modulated because there is a linear relationship between the wavelength and the laser power and the injection current. First, a complete expression for the output waveform change of the excitation light source 1 under the driving condition of superimposing two sine waves on a sawtooth wave is established:

[0075]

[0076] In formula (1), A scan is the amplitude of the sawtooth wave; T scan is the period of the sawtooth wave; t%T scan represents the time t versus T scan Take the remainder; A1 and A2 are the amplitudes of the two sine waves; f1 and f2 are the frequencies of the two sine waves, namely the frequency of the first sine modulation signal and the frequency of the second sine modulation signal; cos(2πf1t) and cos(2πf2t) are represented as two sine wave signals; the sawtooth wave part is represented by It is described as rising linearly to the maximum value and then immediately returning to the minimum value. The sine wave part is two sine waves of different frequencies superimposed on the sawtooth wave.

[0077] When the laser wavelength is modulated by a sawtooth wave superimposed on two sine waves, the wave number (wavenumber, cm -1 ) can be converted into a change in wave number by the current change of the modulation signal. Assume that the wavelength modulation of the laser is linearly related to the input current, and the relationship between the wave number v and the wavelength λ is:

[0078]

[0079] According to the wave number change caused by the current change, the expression of the total wave number change can be derived as follows:

[0080] v(I)=v0+Δv(I) (3)

[0081] In the formula, v0 represents the initial wave number, Δv(I) represents the current I total (t) The wave number changes caused by sawtooth wave and sine wave modulation. The relationship between wave number and current can be defined by the rate of change of wave number relative to current:

[0082]

[0083] In the formula, Indicates the rate of change of wave number with current; the current is represented by the superposition of sawtooth wave and sine wave as follows:

[0084] I total (t) = I scan (t)+I1(t)+I2(t) (5)

[0085] In formula (5), I scan (t) represents the current change of sawtooth wave, I1(t) and I2(t) are the current changes of two sine waves. scan The expression of (t) is:

[0086]

[0087] The expressions for defining I1(t) and I2(t) are:

[0088] I1(t)=m1 cos(2πf1t), (7)

[0089] I2(t)=m2 cos(2πf2t) (8)

[0090] In the formula, m scan , m1, and m2 are the modulation current depths of the sawtooth wave, the superimposed first sine wave, and the superimposed second sine wave respectively.

[0091] Therefore, formula (3) can be expressed as:

[0092] v(t)=v0+Δv1 cos(2πf1t)+Δv2cos(2πf2t) (9)

[0093] Among them, v(t) is the wave number changing with time, that is, the relationship between the wave number and time t, and Δv1 and Δv2 are the wave number change amplitudes related to the two modulation frequencies, respectively.

[0094] The absorption of gas can be expressed by the Beer-Lambert law. Therefore, when the laser passes through the gas sample, the expression of the change of light intensity over time is:

[0095] I(t)=I0e -α(v(t))L (10)

[0096] Where I(t) is the transmitted light intensity, I0 is the incident light intensity, α(v(t)) is the time-varying absorption coefficient, and L is the absorption path length.

[0097] Taking into account the variation of the absorption coefficient α(v(t)), the wave number v can be approximated by the Taylor expansion:

[0098]

[0099] use Expand cos 2 (2πft) term, formula (11) can be converted to:

[0100]

[0101] When the laser current is modulated at frequencies f1 and f2, optical power is generated as shown in the following equation:

[0102] P(t)=P0+P1cos(2πf1t)+P2cos(2πf2t) (13)

[0103] Where P(t) is the transmitted light power, P0 represents the light power of the excitation light beam when a single sawtooth wave signal is applied, P1cos(2πf1t) represents the superimposed light power when the first sinusoidal modulation signal is applied, and P2cos(2πf2t) represents the superimposed light power when the second sinusoidal modulation signal is applied. According to the Lambert-Beer law combined with the light intensity variation expression (13), we can obtain:

[0104]

[0105] In the formula, α(v0) represents the absorption coefficient corresponding to the initial wave number v0; α′(v0) represents the first-order derivative of α(v0); α”(v0) represents the second-order derivative of α(v0).

[0106] Therefore, formula (14) is the optical power of the output light beam of the excitation light source 1 when the light source temperature controller 2, the first function generator 3, the second function generator 4 and the third function generator 5 are driven simultaneously.

[0107] The laser current is modulated at dual frequencies f1 and f2, and the second harmonic signals generated are demodulated by the lock-in amplifier at twice the frequency. The first demodulated harmonic signal and the second demodulated harmonic signal for:

[0108]

[0109] However, when two sinusoidal modulation signals exist at the same time, intermodulation products (i.e., intermodulation terms) will be generated. The product terms in the modulation signal, such as cos(2πf1t)·cos(2πf2t), can be decomposed into:

[0110]

[0111] These intermodulation terms will appear at the frequencies (2πf1+2πf2) and (2πf1-2πf2), thereby interfering with the demodulation signal of the second harmonic, resulting in additional frequency components in the demodulation signal, affecting the analysis results of the signal. However, since the photoacoustic spectroscopy technology demodulates the second harmonic signal at the resonant frequency (fundamental frequency, high-order overtones, etc.) of the photoacoustic signal detector, although there are intermodulation terms, the demodulation result of the second harmonic signal demodulation at the resonant frequency (fundamental frequency, high-order overtones, etc.) of the photoacoustic signal detector can effectively avoid the influence of the intermodulation terms, and the accuracy of the demodulation result is relatively high.

[0112] In this embodiment, two sine wave modulation signals of different frequencies are superimposed on the traditional sawtooth wave modulation, so that multiple gas absorption peaks can be effectively detected in the same modulation period, which is particularly suitable for detecting gases with adjacent absorption lines. Compared with the traditional single sine modulation technology, it can significantly enhance the resolution of different absorption peaks, and effectively distinguish and measure the concentration of each component gas in a complex gas environment. At the same time, by adopting the dual-frequency phase-locked demodulation technology, by extracting the harmonic signals corresponding to the two modulation frequencies respectively, the real-time separation and accurate demodulation of the complex gas signal are realized, and the multiple gas components with close absorption lines can be accurately detected. The present application not only improves the resolution of gas detection with similar absorption lines, but also optimizes the sensitivity and accuracy of simultaneous detection of multiple gases. It is particularly suitable for multi-component gas analysis in a complex gas environment, and when the absorption lines of some isotope gases are close and the single-frequency wavelength modulation detection technology cannot distinguish the signals well, the isotope detection of gas absorption lines close to each other is widely used in gas analysis in the fields of environmental monitoring, industrial process control and medical diagnosis.

[0113] Example 2

[0114] like Figure 3 As shown, this embodiment provides a multi-gas synchronous detection method based on dual-frequency wavelength modulation and demodulation, which is applied to a multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation described in Example 1, and the method includes:

[0115] S1: Control the light source module to output an excitation light beam, and irradiate the excitation light beam into the air chamber of the photoacoustic signal detection module 6; the air chamber contains at least one gas to be detected; the excitation light beam is a light beam modulated by a sawtooth wave signal and two sine waves of different frequencies.

[0116] S2: using the photoacoustic signal detection module 6 to receive the photoacoustic signal generated by the interaction between the gas to be measured in the gas chamber and the excitation light beam, and converting the photoacoustic signal into a piezoelectric signal.

[0117] S3: using the data processing module to perform second harmonic demodulation on the piezoelectric signal output by the photoacoustic signal detection module 6 to obtain demodulated second harmonic data, and inverting the concentration of the gas to be measured based on the demodulated second harmonic data to obtain the concentration of the gas to be measured.

[0118] Before detection, that is, before executing step S1, the waveform loaded on the excitation light source 1 is simulated using python software, and the dual-frequency modulated waveform signal loaded on the excitation light source 1 is monitored using an oscilloscope. By comparing the simulated waveform and the monitored waveform, it is determined whether the waveform output by the excitation light source 1 is the required waveform, thereby adjusting the waveform actually output by the excitation light source 1. Figure 4 (a) is a waveform simulation diagram of single frequency (1441.9 Hz) wavelength modulation; Figure 4 (b) is a waveform simulation diagram of dual-frequency (1441.9Hz+8919.75Hz) wavelength modulation; Figure 4 (c) is the actual waveform of a sawtooth wave superimposed on a single frequency (1441.9 Hz) sine wave observed using an oscilloscope; Figure 4 (d) in the figure is the actual waveform of the sawtooth wave superimposed on a double-frequency (1441.9 Hz + 8919.75 Hz) sine wave observed using an oscilloscope.

[0119] Before detection, that is, before executing step S1, a spectrometer is used to perform wavelength detection on the excitation light source 1 after loading sine waves of different frequencies. The wavelength detection is used to determine the change in the output wavelength of the excitation light source 1 after applying a sawtooth wave alone, superimposing a sine wave, and superimposing two sine waves. The wavelength change when scanning to the absorption line of the gas to be measured is determined, and the current and temperature required to be loaded on the excitation light source 1 when scanning to the absorption line of the gas to be measured are recorded. Therefore, in actual detection, only the required current and temperature need to be input to execute the detection process. Figure 5 (a) is a graph showing the wavelength variation of the excitation light source 1 with temperature and current after a single frequency (1441.9 Hz) sine wave is superimposed on the excitation light source 1; Figure 5 (b) is a graph showing the wavelength variation of the excitation light source 1 with temperature and current after a single frequency (8919.75 Hz) sine wave is superimposed on the excitation light source 1; Figure 5 (c) is a graph showing the wavelength variation of the excitation light source 1 with temperature and current after the excitation light source 1 is superimposed with two sine waves of frequencies (1441.9Hz+8919.75Hz). In order to compare the effects of the three modulation results on the output wavelength of the excitation light source 1, the current and temperature required for the three modulation methods are monitored and controlled when the excitation light source 1 is at a certain fixed wave number, as shown in FIG. Figure 5 As shown in (d), the output of the excitation light source 1 is 7306.73 cm -1 In the three modulation cases at different wave numbers, the current shows a downward trend as the control temperature increases. However, when the control temperature is above 23°C, the required control current of the dual modulation technology is higher than that of the traditional modulation technology.

[0120] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0121] This article uses specific examples to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method and core ideas of this application. At the same time, for those skilled in the art, according to the ideas of this application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. A multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation, characterized in that: The multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation comprises: a light source module, a photoacoustic signal detection module and a data processing module; The light source module is used to output an excitation light beam, and irradiate the excitation light beam into the air chamber of the photoacoustic signal detection module; the air chamber contains at least one gas to be detected; the excitation light beam is a light beam modulated by a sawtooth wave signal and two sine waves of different frequencies; The photoacoustic signal detection module is used to receive the photoacoustic signal generated by the interaction between the gas to be measured in the gas chamber and the excitation light beam, and convert the photoacoustic signal into a piezoelectric signal; The data processing module is used to perform second harmonic demodulation on the piezoelectric signal output by the photoacoustic signal detection module to obtain the demodulated second harmonic data and to invert the concentration of the gas to be measured according to the demodulated second harmonic data to obtain the concentration of the gas to be measured.

2. The multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation according to claim 1 is characterized in that: The light source module includes an excitation light source, a light source temperature controller, a first function generator, a second function generator and a third function generator; The output end of the light source temperature controller is connected to the temperature control input end of the excitation light source; the signal output end of the first function generator is connected to the current scanning input end of the excitation light source; the signal output ends of the second function generator and the third function generator are both connected to the current modulation input end of the excitation light source; the synchronization signal output ends of the second function generator and the third function generator are connected to the signal input end of the data processing module; The first function generator is used to output the sawtooth wave signal; the second function generator is used to output a first sinusoidal modulation signal; the third function generator is used to output a second sinusoidal modulation signal; the first sinusoidal modulation signal and the second sinusoidal modulation signal have different frequencies; The light source temperature controller is used to adjust the temperature of the excitation light beam output by the excitation light source; The excitation light source is used to output the excitation light beam and irradiate the excitation light beam into the air chamber of the photoacoustic signal detection module under the simultaneous driving of the light source temperature controller, the first function generator, the second function generator and the third function generator.

3. The multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation according to claim 2 is characterized in that: The data processing module includes a signal amplifier, a first lock-in amplifier, a second lock-in amplifier and a computer; The signal input end of the signal amplifier is connected to the signal output end of the photoacoustic signal detection module, the signal output end of the signal amplifier is respectively connected to the signal input ends of the first lock-in amplifier and the second lock-in amplifier, and the communication ports of the first lock-in amplifier and the second lock-in amplifier are respectively connected to the communication ports of the computer; The reference signal input terminal of the first lock-in amplifier is connected to the synchronization signal output terminal of the third function generator; the reference signal input terminal of the second lock-in amplifier is connected to the synchronization signal output terminal of the second function generator; The signal amplifier is used to amplify the piezoelectric signal output by the photoacoustic signal detection module, and send the amplified piezoelectric signal to the first lock-in amplifier and the second lock-in amplifier; The first lock-in amplifier is used to receive the amplified piezoelectric signal and the synchronization signal output by the third function generator, and perform second harmonic demodulation on the amplified piezoelectric signal at a first demodulation frequency to obtain a first demodulated harmonic signal; The second lock-in amplifier is used to receive the amplified piezoelectric signal and the synchronization signal output by the second function generator, and perform second harmonic demodulation on the amplified piezoelectric signal at a second demodulation frequency to obtain a second demodulated harmonic signal; the demodulated second harmonic data includes the first demodulated harmonic signal and the second demodulated harmonic signal; The computer is used for receiving the first demodulated harmonic signal and the second demodulated harmonic signal, and inverting the concentration of the gas to be measured according to the first demodulated harmonic signal and the second demodulated harmonic signal to obtain the concentration of the gas to be measured.

4. The multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation according to claim 3 is characterized in that: The frequency of the first sinusoidal modulation signal is half of the fundamental frequency of the photoacoustic signal detection module; The frequency of the second sinusoidal modulation signal is half of the high-order overtone frequency of the photoacoustic signal detection module.

5. The multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation according to claim 4 is characterized in that: The first demodulation frequency and the second demodulation frequency match the resonance frequency of the photoacoustic signal detection module.

6. The multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation according to claim 5, characterized in that: The first demodulation frequency is the high-order overtone frequency of the photoacoustic signal detection module; and the second demodulation frequency is the fundamental frequency of the photoacoustic signal detection module.

7. The multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation according to claim 6, characterized in that: The optical power calculation formula of the excitation light beam of the excitation light source is: In the formula, P(t) represents the optical power of the excitation light beam of the excitation light source; P0 represents the optical power of the excitation light beam of a single applied sawtooth wave signal; α(v0) represents the absorption coefficient corresponding to the initial wave number v0; α′(v0) represents the first-order derivative of α(v0); α″(v0) represents the second-order derivative of α(v0); Δv1 represents the wave number change amplitude related to the frequency of the first sinusoidal modulation signal; Δv2 represents the wave number change amplitude related to the frequency of the second sinusoidal modulation signal; f1 represents the frequency of the first sinusoidal modulation signal; f2 represents the frequency of the second sinusoidal modulation signal; L represents the absorption path length.

8. The multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation according to claim 7 is characterized in that: The calculation formula of the first demodulated harmonic signal is: The calculation formula of the second demodulated harmonic signal is: In the formula, represents a first demodulated harmonic signal; represents the second demodulated harmonic signal.

9. The multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation according to claim 1, characterized in that: The photoacoustic signal detection module comprises a photoacoustic cell detector, a quartz tuning fork detector and a cantilever beam detector.

10. A multi-gas synchronous detection method based on dual-frequency wavelength modulation and demodulation, applied to a multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation as claimed in any one of claims 1 to 9, characterized in that: The multi-gas synchronous detection method based on dual-frequency wavelength modulation and demodulation includes: Control the light source module to output an excitation light beam, and irradiate the excitation light beam into the air chamber of the photoacoustic signal detection module; the air chamber contains at least one gas to be detected; the excitation light beam is a light beam modulated by a sawtooth wave signal and two sine waves of different frequencies; Using a photoacoustic signal detection module to receive a photoacoustic signal generated by the interaction between the gas to be measured in the gas chamber and the excitation light beam, and converting the photoacoustic signal into a piezoelectric signal; The data processing module is used to perform second harmonic demodulation on the piezoelectric signal output by the photoacoustic signal detection module to obtain demodulated second harmonic data, and the concentration of the gas to be measured is inverted according to the demodulated second harmonic data to obtain the concentration of the gas to be measured.

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