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

Abstract

The application discloses a kind of based on dual-frequency wavelength modulation and demodulation multi-gas synchronous detection device and method, it is related to gas detection field, the device includes light source module, photoacoustic signal detection module and data processing module;Wherein, light source module exports excitation light beam, and excitation light beam is irradiated to the gas chamber of photoacoustic signal detection module;Excitation light beam is the light beam after being modulated by one sawtooth signal and two different frequency sine waves;Photoacoustic signal detection module receives the photoacoustic signal generated after each to-be-measured gas in gas chamber and excitation light beam interact and is converted into piezoelectric signal;Data processing module carries out second harmonic demodulation to piezoelectric signal, obtains the second harmonic data after demodulation, further inversion to-be-measured gas concentration.The application is based on sawtooth signal and superimposes two different frequency sine wave signals, expands the frequency distribution of modulation signal, can simultaneously accurately detect the concentration of multiple absorption line adjacent gas.

Description

TECHNICAL FIELD

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

[0002] Gas detection technology plays an important role in environmental monitoring, industrial control, medical diagnosis and other fields, especially in the scenes of greenhouse gas emission, dangerous gas leakage and breath analysis, the accurate detection of gas concentration is crucial. Wavelength modulation spectroscopy (WMS) as a non-contact optical gas detection method has been widely used in accurate measurement of gas concentration. The traditional wavelength modulation technology applies a sawtooth wave to the laser light source, and superimposes a single sinusoidal modulation signal on the sawtooth wave, drives the wavelength of the laser to periodically scan around the target gas absorption peak, and extracts the signal related to the gas concentration through the second harmonic detection technology. This technology has high sensitivity, low noise, fast response and other advantages, and can effectively detect single gas or specific components in gas mixture.

[0003] However, when multiple gases with very close absorption lines or multiple gases with resonance absorption need to be detected, the traditional wavelength modulation method has certain limitations. Simple superposition of a single sinusoidal modulation signal on a sawtooth wave cannot effectively distinguish the absorption peaks of multiple gases at the same time, especially when these absorption peaks are relatively dense in the same wavelength range, the signal resolution capability decreases, resulting in reduced measurement accuracy. The traditional modulation technology is difficult to accurately distinguish and measure the concentration of each component gas when facing complex environmental gas mixtures. SUMMARY

[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 concentration of multiple gases with closely adjacent absorption lines at the same time.

[0005] To achieve the above purpose, the present 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 a gas chamber of the photoacoustic signal detection module; the gas 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 sinusoidal waves with different frequencies;

[0008] The photoacoustic signal detection module is configured to receive a photoacoustic signal generated after the gas in the gas chamber interacts with the excitation light beam, and convert the photoacoustic signal into a piezoelectric signal.

[0009] The data processing module is configured to perform second harmonic demodulation on the piezoelectric signal output by the photoacoustic signal detection module, obtain demodulated second harmonic data, and inverse 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; and the synchronous 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 configured to output the sawtooth wave signal; the second function generator is configured to output a first sinusoidal modulation signal; and the third function generator is configured to output a second sinusoidal modulation signal; the frequencies of the first sinusoidal modulation signal and the second sinusoidal modulation signal are different.

[0013] The light source temperature controller is configured to regulate the temperature of the excitation light beam output by the excitation light source.

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

[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 connected to the signal input ends of the first lock-in amplifier and the second lock-in amplifier, respectively; and the communication ports of the first lock-in amplifier and the second lock-in amplifier are connected to the communication ports of the computer, respectively.

[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 to perform second harmonic demodulation on the amplified piezoelectric signal at the first demodulation frequency to obtain the 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 to perform second harmonic demodulation on the amplified piezoelectric signal at the second demodulation frequency to obtain the 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 to receive the first demodulated harmonic signal and the second demodulated harmonic signal, and to invert the concentration of the gas to be measured based on 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 the fundamental frequency of the photoacoustic signal detection module;

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

[0024] Optionally, the first demodulation frequency and the second demodulation frequency are matched with the resonant frequency of the photoacoustic signal detection module.

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

[0026] Optionally, the formula for calculating the optical power of the excitation beam from 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 only with the sawtooth signal applied; a(v0) represents the absorption coefficient corresponding to the initial wave number v0; a'(v0) represents the first derivative of a(v0); a''(v0) represents the second derivative of a(v0); Δv1 represents the wave number variation amplitude related to the frequency of the first sinusoidal modulation signal; Δv2 represents the wave number variation 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; and L represents the absorption path length.

[0029] Optionally, the calculation formula of the first demodulation harmonic signal is as follows:

[0030]

[0031] The calculation formula of the second demodulation harmonic signal is as follows:

[0032]

[0033] In the formula, represents the first demodulation harmonic signal; represents the second demodulation 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 application provides a multi-gas synchronous detection method based on dual-frequency wavelength modulation and demodulation, which is applied to the multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation.

[0036] The light source module outputs an excitation light beam, and the excitation light beam is irradiated into a gas chamber of the photoacoustic signal detection module; the gas chamber contains at least one to-be-detected gas; the excitation light beam is a light beam modulated by a sawtooth signal and two sinusoidal waves with different frequencies;

[0037] The photoacoustic signal detection module receives a photoacoustic signal generated by the interaction between the to-be-detected gas in the gas chamber and the excitation light beam, and converts the photoacoustic signal into a piezoelectric signal;

[0038] The data processing module performs second harmonic demodulation on the piezoelectric signal output by the photoacoustic signal detection module, obtains demodulated second harmonic data, and inversely calculates the concentration of the to-be-detected gas according to the demodulated second harmonic data, to obtain the concentration of the to-be-detected gas.

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

[0040] The application provides a multi-gas synchronous detection device and method based on dual-frequency wavelength modulation and demodulation, the device comprising 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 of different frequencies; the photoacoustic signal detection module receives a photoacoustic signal generated after the interaction between a gas to be detected 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 to obtain second harmonic data after demodulation, and further inverses the concentration of the gas to be detected. By superimposing two sine wave signals of different frequencies on the basis of the sawtooth wave signal, compared with superimposing a single sine wave signal on the basis of the sawtooth wave signal, the frequency distribution of the modulation signal can be expanded to a wider range, so that multiple gases with closely adjacent absorption lines can be detected at the same time, and the synchronous and accurate detection of multiple gas concentrations is realized. BRIEF DESCRIPTION OF DRAWINGS

[0041] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative effort on the basis of these drawings.

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

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

[0044] Figure 3 A flow schematic diagram of a multi-gas synchronous detection method based on dual-frequency wavelength modulation and demodulation is provided for Embodiment 2 of the present application.

[0045] Figure 4 A waveform simulation diagram and an actual waveform detection diagram obtained by using the dual-frequency wavelength modulation technology of the present application are provided for Embodiment 2 of the present application.

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

[0047] Reference signs: 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 lock-in amplifier, 9 - second lock-in amplifier, 10 - computer. DETAILED DESCRIPTION

[0048] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0049] The above purposes, features and advantages of the present application will be more apparent and understandable. The present application will be described in further detail below with reference to the drawings and specific embodiments.

[0050] Embodiment 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 mismatched modulation signal frequency may result in insufficient detection sensitivity for weak absorption lines. The traditional wavelength modulation technology with a single modulation method is difficult to detect multiple gas absorption peaks at the same time in one scan. In order to improve the detection accuracy and meet the demand of multi-gas detection, the present application proposes an improved wavelength modulation technology combined with photoacoustic spectroscopy technology. Specifically, a multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation is proposed, as shown in FIG. 1. The multi-gas synchronous detection device includes a light source module, a photoacoustic signal detection module 6 and a data processing module. Figure 1

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

[0053] The photoacoustic signal detection module 6 is used to receive the photoacoustic signal generated after 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 be selected from instruments for photoacoustic signal detection such as a photoacoustic cell, a quartz tuning fork and a cantilever beam.

[0054] ​The data processing module is configured to perform second harmonic demodulation on the piezoelectric signal output by the photoacoustic signal detection module 6, to obtain demodulated second harmonic data, and to inverse 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.

[0055] The peak value of the demodulated second harmonic signal 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 concentration of the gas, the concentration information of the gas to be measured can be obtained by calibrating the concentration of the detection device in advance by using a gas with a known concentration. As shown in FIG. 2, it is a second harmonic signal obtained by simultaneously detecting the gas to be measured using a single quartz tuning fork. According to the different peak positions of the second harmonic signal, the concentration information of the gas under different wavelengths with similar absorption line distances can be detected. Figure 2

[0056] In this embodiment, by superimposing two sine wave signals of different frequencies (for example, f1 and f2), the frequency distribution of the modulation signal can be expanded to a wider range, so that multiple gases with closely adjacent absorption lines can be detected simultaneously. The absorption peaks of different gases can be effectively distinguished by extracting different harmonic signals (such as 2f, 4f, etc.). Since the absorption peaks 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 with the temperature control input end of the excitation light source 1; the signal output end of the first function generator 3 is connected with 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 with the current modulation input end of the excitation light source 1; and the synchronous signal output ends of the second function generator 4 and the third function generator 5 are connected with the signal input end of the data processing module.

[0059] The first function generator 3 is configured to output the sawtooth wave signal; the second function generator 4 is configured to output a first sinusoidal modulation signal; and the third function generator 5 is configured to output a 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 controlled in temperature by the light source temperature controller 2, to ensure the temperature of the output light beam of the excitation light source 1.

[0060] ​When the multi-gas synchronous detection device is used for gas detection, the voltage amplitude of the sawtooth wave signal is controlled to make the wavelength of the output light beam of the excitation light source 1 continuously scan around the target absorption line of the gas to be detected, and to ensure that the wavelength of the output light beam slowly passes through the target absorption line of the gas to be detected.

[0061] The two different frequency sine modulation signals emitted by the second function generator 4 and the third function generator 5 are simultaneously loaded onto the excitation light source 1 together with the sawtooth wave scanning signal output by the first function generator 3 to modulate the wavelength of the output light beam of the excitation light source 1. The modulated output light beam is adjusted in propagation direction to determine the optical path of the output light beam of the excitation light source 1 to be incident on the photoacoustic signal detection module 6. After interacting with the gas to be detected in the gas chamber of the photoacoustic signal detection module 6, a photoacoustic signal is generated. The gas molecules absorb the optical energy of the output light beam of the excitation light source 1 to jump to a high-energy state, and an acoustic wave is generated in the collision de-excitation relaxation process. The acoustic wave pushes the vibrating arm of the detector such as a quartz tuning fork of the photoacoustic signal detection module 6 to vibrate, and the photoacoustic signal is converted into a piezoelectric signal by the piezoelectric effect of the quartz tuning fork. Therefore, the excitation light source 1 is used to output the excitation light beam under the simultaneous driving of the light source temperature controller 2, the first function generator 3, the second function generator 4, and the third function generator 5, and to irradiate the excitation light beam into the gas chamber of the photoacoustic signal detection module 6. The photoacoustic signal detection module 6 is used to receive the photoacoustic signal generated after the gas to be detected in the gas chamber interacts with the excitation light beam, and to convert the photoacoustic signal into a piezoelectric signal. The excitation light source 1 uses a tunable laser source. As an example, a DFB laser can be used, which can be tuned near the H2O gas absorption line 7306.73 cm -1 .

[0062] As an optional embodiment, the frequency f1 of the first sine modulation signal is half of the fundamental frequency of the photoacoustic signal detection module 6. The frequency f2 of the second sine 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. Therefore, f1 = 1441.9 Hz, and f2 = 8919.75 Hz.

[0063] As an optional embodiment, 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 with the signal output end of the photoacoustic signal detection module 6, the signal output end of the signal amplifier 7 is connected with the signal input end of the first lock-in amplifier 8 and the second lock-in amplifier 9 respectively, and the communication ports of the first lock-in amplifier 8 and the second lock-in amplifier 9 are connected with the communication ports of the computer 10.

[0065] The reference signal input end of the first lock-in amplifier 8 is connected with the synchronous signal output end of the third function generator 5, and the reference signal input end of the second lock-in amplifier 9 is connected with the synchronous signal output end of the second function generator 4.

[0066] The signal amplifier 7 is used for amplifying the piezoelectric signal output by the photoacoustic signal detection module 6 and sending 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 for receiving the amplified piezoelectric signal and the synchronous signal output by the third function generator 5, and performing second harmonic demodulation on the amplified piezoelectric signal at a first demodulation frequency to obtain a first demodulation harmonic signal.

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

[0069] The computer 10 is used for receiving the first demodulation harmonic signal and the second demodulation harmonic signal, and inversely calculating the concentration of the to-be-tested gas according to the first demodulation harmonic signal and the second demodulation harmonic signal to obtain the concentration of the to-be-tested gas.

[0070] The demodulation frequencies of the first lock-in amplifier 8 and the second lock-in amplifier 9 are matched with the resonance frequency of the photoacoustic signal detection module 6, corresponding to the modulation frequencies of two sinusoidal modulation signals, the first demodulation frequency is the high-order overtone frequency of the photoacoustic signal detection module 6, and the second demodulation frequency is the fundamental frequency of the photoacoustic signal detection module 6.

[0071] The first lock-in 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. It processes the piezoelectric signal converted from the photoacoustic signal using second harmonic demodulation under the fundamental frequency condition of the photoacoustic signal detection module 6. Simultaneously, the second lock-in amplifier 9 receives the amplified piezoelectric signal output from the transimpedance preamplifier and the synchronization signal provided by the second function generator 4. It processes the piezoelectric signal converted from the photoacoustic signal using second harmonic demodulation under the high-order overtone condition of the photoacoustic signal detection module 6. The demodulated second harmonic data from the first lock-in amplifier 8 and the second lock-in amplifier 9 are transmitted to the computer 10. 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 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 by the second harmonic demodulation method under the fundamental frequency condition of the photoacoustic signal detection module 6, and the piezoelectric signal converted from the photoacoustic signal is processed by the second harmonic demodulation method under the high-order overtone condition of the photoacoustic signal detection module 6. When extracting the second harmonic signal, the absorption peaks of various gases are better distinguished, thus improving the sensitivity and accuracy of the signal.

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

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

[0075]

[0076] In equation (1), A scan T represents the amplitude of the sawtooth wave. scan The period of the sawtooth wave; t%T scan Representing time t with respect to T scan The remainder is taken; A1 and A2 are the amplitudes of the two sine waves; f1 and f2 are the frequencies of the two sine waves, i.e., the frequencies of the first and second sinusoidal modulation signals; cos(2πf1t) and cos(2πf2t) represent the two sinusoidal signals; the sawtooth wave part passes through... It can be described as linearly rising to its maximum value and then immediately returning to its 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 with two sinusoidal waves, the change of wavenumber (cm -1 ) can be converted to the change of wavenumber by the change of current of the modulation signal. Assuming that the wavelength modulation of the laser is linearly related to the input current, and the relationship between wavenumber v and wavelength λ is:

[0078]

[0079] The expression of the total change of wavenumber can be derived from the change of wavenumber caused by the change of current as follows:

[0080] v(I) = v0 + Δv(I) (3)

[0081] where v0 represents the initial wavenumber, and Δv(I) represents the change of wavenumber caused by the change of current I total (t) through the sawtooth wave and the sinusoidal wave modulation, the relationship between the wavenumber and the current can be defined by the rate of change of the wavenumber with respect to the current as follows:

[0082]

[0083] where represents the rate of change of the wavenumber with respect to the current; and the current is represented by the superposition of the sawtooth wave and the sinusoidal wave as follows:

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

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

[0086]

[0087] The expressions of I1(t) and I2(t) are defined as follows:

[0088] I1(t) = m1 cos(2πf1t), (7)

[0089] I2(t) = m2 cos(2πf2t) (8)

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

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

[0092] v(t) = v0 + Δv1 cos(2πf1t) + Δv2 cos(2πf2t) (9)

[0093] where v(t) is the wave number as a function of time, i.e., the relationship between the wave number and time t, and Δv1 and Δv2 are the wave number variation amplitudes associated with the two modulation frequencies, respectively.

[0094] The absorption of the gas can be expressed by the Beer-Lambert law, and thus the expression of the light intensity as a function of time when the laser passes through the gas sample 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 absorption coefficient as a function of time, and L is the absorption path length.

[0097] Considering the variation of the absorption coefficient α(v(t)), the wave number v can be approximated as a Taylor expansion:

[0098]

[0099] Using expanding the cos 2 (2πft) term, formula (11) can be converted to:

[0100]

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

[0102] P(t) = P0 + P1 cos(2πf1t) + P2 cos(2πf2t) (13)

[0103] where P(t) is the transmitted light power, P0 represents the light power of the excitation light beam when only the sawtooth signal is applied, P1 cos(2πf1t) represents the superimposed light power when the first sinusoidal modulation signal is applied, and P2 cos(2πf2t) represents the superimposed light power when the second sinusoidal modulation signal is applied. According to the Lambert-Beer law, the following formula can be obtained in combination with the light intensity variation expression (13):

[0104]

[0105] In the formula, α(v0) represents the absorption coefficient corresponding to the initial wave number v0; α'(v0) represents the first derivative of α(v0); and α"(v0) represents the second derivative of α(v0).

[0106] Thus, formula (14) is the optical power of the output light beam of the excitation light source 1 under the simultaneous driving of the light source temperature controller 2, the first function generator 3, the second function generator 4, and the third function generator 5.

[0107] The laser current is modulated at double frequencies f1, f2, and the generated second harmonic signals are demodulated at 2 times the frequency by a phase-locked amplifier, respectively, so that the first demodulation harmonic signal and the second demodulation harmonic signal are:

[0108]

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

[0110]

[0111] These intermodulation terms appear at frequencies (2πf1+2πf2) and (2πf1-2πf2), thereby interfering with the demodulation signal of the second harmonic, resulting in the appearance of additional frequency components in the demodulation signal, which affects the analysis result of the signal. However, since the photoacoustic spectroscopy technology performs second harmonic signal demodulation at the resonance frequency (base frequency, high-order overtone frequency, etc.) of the photoacoustic signal detector, although there are intermodulation terms, the demodulation result of the second harmonic signal demodulation at the resonance frequency (base frequency, high-order overtone frequency, 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 sinusoidal modulation signals of different frequencies are superimposed on the basis of the traditional sawtooth wave modulation, so that multiple gas absorption peaks can be effectively detected within the same modulation period, which is particularly suitable for detecting gases adjacent to the absorption line. Compared with the traditional single sinusoidal modulation technology, the resolution capability for different absorption peaks can be significantly enhanced, and in a complex gas environment, the concentrations of each component gas can be effectively distinguished and measured. At the same time, by using a double-frequency phase-locked demodulation technology, the harmonic signals corresponding to the two modulation frequencies are extracted respectively, real-time separation and accurate demodulation of complex gas signals are realized, and multiple gas components with close absorption lines can be accurately detected. The present application not only improves the resolution capability for gas detection with close absorption lines, but also optimizes the sensitivity and accuracy of simultaneous detection of multiple gases, and is particularly suitable for multi-component gas analysis in a complex gas environment, and for isotopic detection with close absorption lines, when the single-frequency wavelength modulation detection technology cannot well distinguish the signals. The present application 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, applied to the multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation described in Example 1. The method includes:

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

[0116] S2: The photoacoustic signal detection module 6 is used to receive the photoacoustic signal generated by the interaction between the gas to be tested in the gas chamber and the excitation beam, and the photoacoustic signal is converted into a piezoelectric signal.

[0117] S3: The data processing module performs second harmonic demodulation on the piezoelectric signal output by the photoacoustic signal detection module 6 to obtain the demodulated second harmonic data, and inverts 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 testing, that is, before performing 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 actual output waveform of the excitation light source 1. Figure 4 (a) in the figure is a waveform simulation diagram of single-frequency (1441.9Hz) wavelength modulation; Figure 4 (b) in the figure is a waveform simulation diagram of dual-frequency (1441.9Hz+8919.75Hz) wavelength modulation; Figure 4 (c) in the figure is the actual waveform of the sawtooth wave superimposed on a single-frequency (1441.9Hz) sine wave as observed using an oscilloscope; Figure 4 In the diagram, (d) represents the actual waveform of the sawtooth wave superimposed with a dual-frequency (1441.9Hz + 8919.75Hz) sine wave as observed using an oscilloscope.

[0119] Before detection, i.e. before step S1 is performed, the wavelength of the excitation light source 1 loaded with different frequency sine waves is detected by the spectrometer, respectively, and the wavelength variation of the excitation light source 1 after the superposition of a sawtooth wave, a sine wave and two sine waves is determined, the wavelength variation of the scanning to the absorption line of the gas to be detected 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 detected are recorded, so that only the required current and temperature need to be input during actual detection to perform the detection process. Figure 5 (a) in FIG. 1 is a wavelength variation diagram of the excitation light source 1 with the change of temperature and current after the superposition of a single frequency (1441.9 Hz) sine wave on the excitation light source 1; Figure 5 (b) in FIG. 1 is a wavelength variation diagram of the excitation light source 1 with the change of temperature and current after the superposition of a single frequency (8919.75 Hz) sine wave on the excitation light source 1; Figure 5 (c) in FIG. 1 is a wavelength variation diagram of the excitation light source 1 with the change of temperature and current after the superposition of two frequencies (1441.9 Hz+8919.75 Hz) sine waves on the excitation light source 1. In order to compare the effects of the three modulation results on the output wavelength of the excitation light source 1, the currents and temperatures required by the three modulation methods when the excitation light source 1 is controlled to a certain fixed wave number are monitored and controlled, as shown in (d) in FIG. 1, the excitation light source 1 is controlled to output 7306.73 cm Figure 5 -1 When the control temperature rises, the current decreases, but the control current required by the double modulation technology is higher than that of the conventional modulation technology when the control temperature is higher than 23℃.

[0120] The technical features of the above embodiments can be combined arbitrarily, and in order to make the description concise, all possible combinations of the technical features in the above embodiments are not described, but as long as the combinations of the technical features do not exist contradictory, they should be considered as the scope of the present application.

[0121] The principles and implementation modes of the present application are described by using specific examples, and the above embodiment is only used to help understand the method and its core idea; meanwhile, for those skilled in the art, according to the idea of the present application, the specific implementation mode and application range can be changed. In conclusion, the content of the present application should not be understood as a limitation.​

Claims

1. A multi-gas simultaneous 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 for outputting an excitation light beam and irradiating the excitation light beam into a gas chamber of the photoacoustic signal detection module; the gas chamber contains at least one gas to be detected; the excitation light beam is a light beam modulated by superposition of a sawtooth wave signal and two sine waves with different frequencies; the frequency of the first sine modulation signal is half of the fundamental frequency of the photoacoustic signal detection module; the frequency of the second sine modulation signal is half of the high-order overtone frequency of the photoacoustic signal detection module; The photoacoustic signal detection module is used for receiving a photoacoustic signal generated after the gas to be detected in the gas chamber interacts with the excitation light beam and converting the photoacoustic signal into a piezoelectric signal; The data processing module is used for amplifying the piezoelectric signal output by the photoacoustic signal detection module, performing second harmonic demodulation on the amplified piezoelectric signal at a first demodulation frequency and a second demodulation frequency respectively, obtaining demodulated second harmonic data, and inversely calculating the concentration of the gas to be detected according to the demodulated second harmonic data to obtain the concentration of the gas to be detected; the demodulated second harmonic data comprises a first demodulation harmonic signal and a second demodulation harmonic signal; 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; The light source module comprises 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 with the temperature control input end of the excitation light source; the signal output end of the first function generator is connected with 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 connected with the current modulation input end of the excitation light source; and the synchronous signal output ends of the second function generator and the third function generator are connected with the signal input end of the data processing module; The first function generator is used for outputting the sawtooth wave signal; the second function generator is used for outputting the first sine modulation signal; and the third function generator is used for outputting the second sine modulation signal; the frequencies of the first sine modulation signal and the second sine modulation signal are different; The light source temperature controller is used for regulating the temperature of the excitation light beam output by the excitation light source; The excitation light source is used for outputting the excitation light beam under the simultaneous driving of the light source temperature controller, the first function generator, the second function generator and the third function generator, and irradiating the excitation light beam into the gas chamber of the photoacoustic signal detection module.

2. The dual frequency wavelength modulation and demodulation based multi-gas simultaneous detection device according to claim 1, wherein, The data processing module comprises 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 with the signal output end of the photoacoustic signal detection module, the signal output end of the signal amplifier is connected with the signal input end of the first lock-in amplifier and the second lock-in amplifier respectively, and the communication ports of the first lock-in amplifier and the second lock-in amplifier are connected with the communication ports of the computer respectively; The reference signal input end of the first lock-in amplifier is connected with the synchronous signal output end of the third function generator, and the reference signal input end of the second lock-in amplifier is connected with the synchronous signal output end of the second function generator; The signal amplifier is used for amplifying the piezoelectric signal output by the photoacoustic signal detection module and sending the amplified piezoelectric signal to the first lock-in amplifier and the second lock-in amplifier; The first lock-in amplifier is used for receiving the amplified piezoelectric signal and the synchronous signal output by the third function generator, and performing second harmonic demodulation on the amplified piezoelectric signal at a first demodulation frequency to obtain a first demodulation harmonic signal; The second lock-in amplifier is used for receiving the amplified piezoelectric signal and the synchronous signal output by the second function generator, and performing second harmonic demodulation on the amplified piezoelectric signal at a second demodulation frequency to obtain a second demodulation harmonic signal; the demodulated second harmonic data includes the first demodulation harmonic signal and the second demodulation harmonic signal; The computer is used for receiving the first demodulation harmonic signal and the second demodulation harmonic signal, and inversely calculating the concentration of the to-be-detected gas according to the first demodulation harmonic signal and the second demodulation harmonic signal to obtain the concentration of the to-be-detected gas.

3. The dual frequency wavelength modulation and demodulation based multi-gas simultaneous detection device according to claim 1, wherein, The light power calculation formula of the excitation light beam of the excitation light source is: ； wherein represents the optical power of the excitation light beam of the excitation light source; represents the optical power of the excitation light beam of the excitation light source applying the sawtooth signal alone; represents the initial wave number corresponding absorption coefficient; represents the first derivative of represents the second derivative of represents the wave number variation amplitude related to the frequency of the first sinusoidal modulation signal; represents the wave number variation amplitude related to the frequency of the second sinusoidal modulation signal; represents the frequency of the first sinusoidal modulation signal; represents the frequency of the second sinusoidal modulation signal; represents the absorption path length.​​ 4. The apparatus for simultaneous detection of multiple gases based on dual frequency wavelength modulation and demodulation according to claim 3, wherein, The calculation formula of the first demodulation harmonic signal is: ； The calculation formula of the second demodulation harmonic signal is: ； wherein represents the first demodulation harmonic signal; represents the second demodulation harmonic signal.

5. The dual frequency wavelength modulation and demodulation based multi-gas simultaneous detection apparatus according to claim 1, wherein, The photoacoustic signal detection module includes a photoacoustic cell detector, a quartz tuning fork detector and a cantilever beam detector.

6. A method for multi-gas synchronous detection based on dual-frequency wavelength modulation and demodulation, applied to the multi-gas synchronous detection device based on dual-frequency wavelength modulation and demodulation in any one of claims 1 to 5, characterized in that, The multi-gas synchronous detection method based on double-frequency wavelength modulation and demodulation includes: The light source module outputs an excitation light beam, and the excitation light beam irradiates into a gas chamber of the photoacoustic signal detection module; the gas chamber contains at least one to-be-detected gas; the excitation light beam is a light beam modulated by superposition of one sawtooth signal and two sine waves with different frequencies; the frequency of the first sine modulation signal is half of the fundamental frequency of the photoacoustic signal detection module; the frequency of the second sine modulation signal is half of the high-order overtone frequency of the photoacoustic signal detection module; The photoacoustic signal detection module receives the photoacoustic signal generated after the to-be-detected gas in the gas chamber interacts with the excitation light beam, and converts the photoacoustic signal into a piezoelectric signal; The signal amplifier is used for amplifying the piezoelectric signal output by the photoacoustic signal detection module and sending the amplified piezoelectric signal to the first lock-in amplifier and the second lock-in amplifier; The piezoelectric signal output by the photoacoustic signal detection module is amplified using a data processing module. Second harmonic demodulation is then performed on the amplified piezoelectric signal at a first demodulation frequency and a second demodulation frequency to obtain demodulated second harmonic data. The concentration of the gas to be measured is then calculated based on this demodulated second harmonic data. The demodulated second harmonic data includes a first demodulated harmonic signal and a second demodulated harmonic signal. The first demodulation frequency is the higher-order overtone frequency of the photoacoustic signal detection module, and the second demodulation frequency is the fundamental frequency of the photoacoustic signal detection module.

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