First-harmonic-based photoacoustic spectroscopy self-correction method, gas detection method and gas detection system

Abstract

The present disclosure relates to a first harmonic-based photoacoustic spectrum self-correction method, a gas detection method and a gas detection system. The self-correction method comprises: obtaining a first harmonic signal of an electrical signal; fitting a baseline of the first harmonic signal based on a signal size of the first harmonic signal in a non-absorption region, the baseline being expressed as an n-order polynomial function with time as the independent variable and signal size as the dependent variable; subtracting the baseline from the first harmonic signal to obtain a fluctuation part signal of the first harmonic signal, and determining a peak-to-peak value of the fluctuation part signal and a reference time point; substituting the reference time point into the baseline to obtain an offset value corresponding to the reference time point and calculating a ratio between the peak-to-peak value and the offset value to obtain a normalized ratio, the normalized ratio being used for detecting a gas concentration of a gas. Thus, a more perfect and more accurate photoacoustic spectrum self-correction method using the first harmonic signal can be realized, which is more suitable for a resonance device and can realize more accurate gas concentration detection.

Description

Technical Field

[0001] This disclosure relates to the field of photoacoustic spectroscopy gas detection, and in particular to a photoacoustic spectroscopy self-calibration method based on the first harmonic, a gas detection method, and a gas detection system. Background Technology

[0002] Gas detection plays a crucial role in many fields. Among various gas detection technologies, photoacoustic spectroscopy (PAS) is a spectroscopic gas detection technique based on the photoacoustic effect, possessing characteristics such as no background, low gas consumption, high sensitivity, and a wide dynamic range. The basic principle of photoacoustic spectroscopy is that light with a wavelength matching the gas absorption peak is absorbed when passing through the gas and converted into a heat source through a non-radiative transition, thereby exciting an acoustic signal. By detecting the acoustic signal using a microphone, the gas concentration can be determined.

[0003] Photoacoustic spectroscopy can be divided into non-resonant photoacoustic spectroscopy and resonant photoacoustic spectroscopy. Non-resonant photoacoustic spectroscopy typically utilizes a small-volume non-resonant photoacoustic cavity and microphone, with uniform sound pressure distribution within the cavity, operating at frequencies on the order of approximately 10 Hz, and is susceptible to low-frequency noise. Resonant photoacoustic spectroscopy utilizes high-frequency modulation and demodulation and resonant amplification effects to achieve a higher signal-to-noise ratio. Based on the resonant amplification method, the three most common types of resonant photoacoustic spectroscopy are resonant cavity photoacoustic spectroscopy, cantilever beam enhanced photoacoustic spectroscopy, and quartz tuning fork enhanced photoacoustic spectroscopy. Resonant cavity photoacoustic spectroscopy uses a special cavity shape to create standing wave resonance within the cavity to amplify the signal, typically operating at frequencies on the order of kHz. Cantilever beam enhanced photoacoustic spectroscopy uses a cantilever beam to detect the sound signal and amplifies the signal through the resonance of the cantilever beam itself, also typically operating at frequencies on the order of kHz. Quartz tuning fork enhanced photoacoustic spectroscopy utilizes the resonance of the quartz tuning fork itself to amplify the sound signal generated between the two arms of the tuning fork, typically operating at the standard resonant frequency of commercially available quartz tuning forks, i.e., 32.7 kHz.

[0004] In photoacoustic spectroscopy, a class of homogeneous interference factors can alter the magnitude of the acoustic signal, ultimately affecting the accuracy of measurements. These interference factors include light intensity fluctuations (drifts in laser output intensity, changes in attenuation loss in the optical path), frequency mismatch (drifts in modulation and demodulation frequencies, changes in the system's resonant frequency due to differences in gas properties), and changes in microphone sensitivity due to aging. These homogeneous interference factors are wavelength-independent, do not change the shape of the signal, but only its magnitude, equivalent to multiplying the acoustic signal by a coefficient, thus affecting the concentration inversion results.

[0005] To address this issue, a background signal can be artificially introduced into the backgroundless photoacoustic spectrum. This background signal can then be used to normalize the measurement signal, resulting in a new signal independent of uniformity interference factors such as light intensity fluctuations and frequency mismatches, thus achieving self-calibration. One method for normalization using the background signal is the 2f / 1f method commonly used in Tunable Diode Laser Absorption Spectroscopy (TDLAS). This method uses the first harmonic signal (background signal) of the original signal to normalize the second harmonic signal (measurement signal), obtaining a 2f / 1f signal with self-calibration characteristics. Here, the first harmonic signal is the component of the original signal whose frequency equals the modulation frequency; the second harmonic signal is the component of the original signal whose frequency equals twice the modulation frequency.

[0006] In existing technologies, graphite sheets can be placed inside a non-resonant cavity as solid absorbers to introduce background signals, and the 2f / 1f method can be used to achieve normalization self-calibration. Since a non-resonant cavity has no resonant frequency, it can ensure simultaneous measurement of the second and first harmonic signals, but this reduces the system's signal-to-noise ratio.

[0007] Existing technologies can also introduce background signals using solid absorption in resonant photoacoustic spectroscopy systems, all employing the 2f / 1f method for normalization self-calibration. While resonant photoacoustic spectroscopy systems can easily achieve higher signal-to-noise ratios, they only amplify signal components with frequencies equal to the resonant frequency, suppressing signal components at other frequencies, making it difficult to simultaneously and efficiently acquire the first and second harmonic signals. Although existing technologies can acquire the first and second harmonics at different times through time-division modulation and demodulation, this reduces measurement speed and lacks simultaneity. Existing technologies can also acquire the first and second harmonics simultaneously and ensure that the second harmonic matches the resonant frequency through design, but this sacrifices the sensitivity of the first harmonic measurement.

[0008] Existing technologies can also utilize solid absorption on the surface of the tuning fork to introduce background signals in a quartz tuning fork-enhanced photoacoustic spectroscopy system, and normalized self-calibration is achieved using the first harmonic signal. Compared to the 2f / 1f method, this method only involves one frequency component and is more suitable for resonant photoacoustic spectroscopy. However, the 1f signal processing method in this technology ignores many factors and is not perfect or specific enough. For example, it does not consider the baseline shape and its formation, thus resulting in inaccuracy, and it does not consider the influence of gas absorption on solid-state acoustic signals. In addition, since this system uses a quartz tuning fork to enhance photoacoustic spectroscopy, the tiny size of the tuning fork makes optical path adjustment more difficult, and the fixed high resonant frequency of the tuning fork also makes data processing more complex.

[0009] In summary, the accuracy of photoacoustic spectra is affected by uniformity interference factors such as light intensity fluctuations, frequency mismatch, and microphone sensitivity variations. Among existing normalization self-calibration methods, there is a lack of a suitable and more accurate method that uses only the first harmonic for resonant systems. Summary of the Invention

[0010] In view of this, this disclosure proposes a photoacoustic spectral self-calibration method, a gas detection method, and a gas detection system based on the first harmonic, which can achieve a more complete and accurate photoacoustic spectral self-calibration suitable for resonant devices and using the first harmonic signal, as well as achieve accurate gas concentration detection.

[0011] According to one aspect of this disclosure, a photoacoustic spectral self-calibration method is provided, comprising: acquiring a first harmonic signal of an electrical signal, the electrical signal being obtained by converting an acoustic signal, the acoustic signal being excited by incident light being absorbed and converted into heat when it passes through a gas and strikes a solid absorber; fitting a baseline of the first harmonic signal based on the signal magnitude of the first harmonic signal in the non-absorption region, the baseline being represented as an nth-degree polynomial function with time as the independent variable and signal magnitude as the dependent variable, n≥2, the non-absorption region including the time range during which the gas does not absorb incident light; and matching the first harmonic signal with the signal magnitude of the solid absorber. Subtracting the baseline yields the fluctuating portion of the first harmonic signal, and the peak-to-peak value and reference time point of the fluctuating portion are determined. The reference time point includes the time point corresponding to the center of the gas absorption peak, and the peak-to-peak value includes the difference between the maximum and minimum values ​​of the fluctuating portion signal. Substituting the reference time point into the baseline yields the bias value corresponding to the reference time point, and the ratio between the peak-to-peak value and the bias value is calculated to obtain a normalized ratio. The bias value includes the signal magnitude corresponding to the baseline at the center of the absorption peak, and the normalized ratio is used to detect the gas concentration of the gas.

[0012] In one possible implementation, acquiring the first harmonic signal of the electrical signal includes: demodulating the electrical signal using a quadrature digital lock-in amplifier to obtain an initial first harmonic signal, wherein the initial first harmonic signal is the modulus of the first harmonic signal; identifying the bounce effect region of the initial first harmonic signal, and inverting the signal magnitude within the bounce effect region to obtain the first harmonic signal, wherein the bounce effect region includes a time range defined by two time points where the signal magnitude is 0 and the derivatives are opposite.

[0013] In one possible implementation, the incident light is generated by a tunable laser, the driving current of which includes a signal current obtained by superimposing a low-frequency triangular scanning signal with a high-frequency sinusoidal modulation signal; the low-frequency triangular scanning signal is used to scan the center wavelength of the incident light across the entire gas absorption peak, and the non-absorption region includes the time range during which the center wavelength of the incident light is not within the gas absorption peak and the gas does not absorb the incident light; the gas absorbance of the incident light is less than 0.05.

[0014] According to another aspect of this disclosure, a gas detection method is provided, comprising: determining a normalized ratio corresponding to a gas to be detected using the self-calibration method; and obtaining the gas concentration of the gas to be detected based on the ratio-concentration relationship calibration curve corresponding to the gas to be detected and the normalized ratio corresponding to the gas to be detected, wherein the ratio-concentration relationship calibration curve is a curve obtained by first using the self-calibration method to obtain standard normalized ratios at different standard concentrations and then linearly fitting the standard normalized ratios at different standard concentrations.

[0015] According to another aspect of this disclosure, a gas detection system is provided, comprising: a laser modulation device, a tunable laser, a resonant device, a microphone, and a computing device; the laser modulation device is used to output a driving current to the tunable laser, the driving current comprising a signal current obtained by superimposing a low-frequency triangular scanning signal with a high-frequency sinusoidal modulation signal; the tunable laser is used to output incident light into the resonant cavity of the resonant device under the modulation of the driving current, the resonant cavity containing a gas to be detected and a solid absorber, wherein the incident light is absorbed by the gas to be detected and converted into a heat source when it passes through the solid absorber and is converted into a sound signal; the microphone is used to collect the sound signal and convert the sound signal into an electrical signal; the computing device is used in the gas detection method to obtain the gas concentration of the gas to be detected.

[0016] In one possible implementation, the system further includes a device-type lock-in amplifier for demodulating the electrical signal converted by the microphone to obtain the first harmonic signal of the electrical signal and sending the first harmonic signal to the computing device; or, the system further includes a data acquisition card for acquiring the electrical signal converted by the microphone and sending the electrical signal to the computing device, wherein the computing device is equipped with a quadrature digital lock-in amplifier to demodulate the electrical signal to obtain the first harmonic signal of the electrical signal; wherein the demodulation frequency used to demodulate the electrical signal is the same as the resonant frequency of the resonant device.

[0017] In one possible implementation, the resonant cavity includes a T-shaped resonant cavity, which includes a buffer cavity and a resonant tube. The first end of the resonant tube is connected to the first end of the buffer cavity, and the second end of the resonant tube is a closed end. The second end of the buffer cavity is sealed with a window that can transmit spatial light, and the incident light enters from the second end of the buffer cavity. The solid absorber is disposed at the closed end of the resonant tube, and the microphone is disposed at the closed end of the resonant tube, so that the solid absorber and the microphone are located at the antinode of the first-order longitudinal resonance mode of the T-shaped resonant cavity.

[0018] In one possible implementation, the tunable laser includes a fiber laser or a spatial laser; the fiber laser is used to generate fiber light, and the spatial laser is used to generate spatial light; when the tunable laser uses the fiber laser, the system further includes a fiber collimator, and a positioning slot for the fiber collimator is provided at the entrance of the resonant cavity of the resonant device; the fiber collimator is disposed in the positioning slot at the entrance of the resonant cavity, and the entrance of the resonant cavity includes the second end of the buffer cavity of the T-shaped resonant cavity; the fiber laser and the fiber collimator are connected by an optical fiber to transmit the fiber light output by the fiber laser, and the fiber collimator is used to convert the fiber light transmitted by the optical fiber into spatial light and inject it into the resonant cavity; wherein, the incident light includes the spatial light.

[0019] In one possible implementation, the microphone comprises a conventional microphone, or a cantilever microphone; where the microphone employs the cantilever microphone, the resonant frequency of the cantilever microphone matches the resonant frequency of the resonant device.

[0020] In one possible implementation, the laser modulation device includes: a first signal generator, a superimposed generator, and a laser controller; the first signal generator generates a low-frequency triangular scanning signal and a high-frequency sinusoidal modulation signal and inputs them to the superimposed generator, wherein the modulation frequency of the high-frequency sinusoidal modulation signal is the same as the resonant frequency of the resonant device; the superimposed generator superimposes the low-frequency triangular scanning signal and the high-frequency sinusoidal modulation signal to obtain a driving signal and inputs it to the laser controller; the laser controller outputs a driving current corresponding to the driving signal to the tunable laser to modulate the wavelength and intensity of the output of the tunable laser.

[0021] In one possible implementation, the laser modulation device includes: a second signal generator and a laser controller; the second signal generator is used to generate a driving signal that superimposes a low-frequency triangular scanning signal onto a high-frequency sinusoidal modulation signal and inputs it to the laser controller, wherein the modulation frequency of the high-frequency sinusoidal modulation signal is the same as the resonance frequency of the resonant device; the laser controller is used to output a driving current corresponding to the driving signal to the tunable laser to modulate the wavelength and light intensity output by the tunable laser.

[0022] According to various aspects of this disclosure, by fitting the baseline with an nth-order polynomial function based on the signal magnitude of the first harmonic signal in the non-absorption region, the accuracy of the baseline fitting can be improved. Subtracting the baseline from the first harmonic signal yields the accurate fluctuating portion of the first harmonic signal. The peak-to-peak value of the fluctuating portion signal and the bias value corresponding to the absorption peak center on the baseline are then obtained. Since the peak-to-peak value is proportional to both gas concentration and light intensity, while the bias value is only proportional to light intensity, calculating the ratio of the peak-to-peak value to the bias value is equivalent to correcting the peak-to-peak value using the bias value. This yields a normalized ratio that is not affected by light intensity fluctuations and is proportional to the concentration. Thus, a photoacoustic spectral self-calibration method suitable for resonant systems and using only the first harmonic signal is realized. This is beneficial for realizing a photoacoustic spectral system that is unaffected by uniformity factors such as light intensity fluctuations, frequency mismatch, and microphone sensitivity changes. In other words, it is beneficial for achieving accurate gas concentration detection and reducing the impact of various uniformity interference factors on gas concentration detection.

[0023] Other features and aspects of this disclosure will become clear from the following detailed description of exemplary embodiments with reference to the accompanying drawings. Attached Figure Description

[0024] The accompanying drawings, which are included in and form part of this specification, illustrate exemplary embodiments, features, and aspects of this disclosure together with the specification and serve to explain the principles of this disclosure.

[0025] Figure 1 A schematic diagram is shown of a first harmonic signal obtained by numerical simulation using a self-written program according to an embodiment of the present disclosure.

[0026] Figure 2 A flowchart illustrating a photoacoustic spectral self-calibration method according to an embodiment of the present disclosure is shown.

[0027] Figure 3 A schematic diagram is shown of a 1000 ppm methane first harmonic signal obtained using an orthogonal digital lock-in amplifier according to an embodiment of the present disclosure.

[0028] Figure 4 A schematic diagram of the fluctuating portion of a first harmonic signal according to an embodiment of the present disclosure is shown.

[0029] Figure 5 A flowchart of a gas detection method according to an embodiment of the present disclosure is shown.

[0030] Figure 6 A schematic diagram showing the calibration curve of normalized ratio versus concentration according to an embodiment of the present disclosure is provided.

[0031] Figure 7 A block diagram of a gas detection system according to an embodiment of the present disclosure is shown.

[0032] Figure 8 A schematic diagram of a gas detection system according to an embodiment of the present disclosure is shown.

[0033] Figure 9 A schematic diagram of another gas detection system according to an embodiment of the present disclosure is shown.

[0034] Figure 10 A schematic diagram of another gas detection system according to an embodiment of the present disclosure is shown.

[0035] Figure 11 A schematic diagram of another gas detection system according to an embodiment of the present disclosure is shown.

[0036] Figure 12 A schematic diagram of another gas detection system according to an embodiment of the present disclosure is shown.

[0037] Figure 13 A schematic diagram showing the results of multiple gas concentration tests of 1000 ppm methane standard gas according to an embodiment of the present disclosure.

[0038] Figure 14 A schematic diagram of a verification system according to an embodiment of the present disclosure is shown.

[0039] Figure 15 This illustrates the process of gradually increasing the light intensity attenuation using an optical fiber attenuator according to an embodiment of the present disclosure, V pp V bias A diagram illustrating the changes in Ratio.

[0040] Figure 16 The diagram illustrates that, according to an embodiment of the present disclosure, the resonant cavity is filled with air, and the modulation / demodulation frequency f is 820 Hz. mod The diagrams show the concentration inversion results using Ratio and Vpp at 830Hz and 820Hz, respectively.

[0041] Figure 17 A schematic diagram showing the measured first harmonic signal at three gas concentrations (20ppm, 50ppm, and 100ppm) according to embodiments of the present disclosure is provided.

[0042] Figure Labels

[0043] exist Figure 8 , Figure 9 , Figure 10 , Figure 11 , Figure 12 In the diagram: 701 is the laser modulation device, 7011 is the first signal generator, 7012 is the superposition device, 7013 is the laser controller, 7014 is the second signal generator, 702 is the tunable laser, 703 is the resonant device, 7031 is the buffer cavity, 7032 is the resonant tube, 7033 is the window, 7034 is the solid absorber, 7035 is the air inlet pipe, 7036 is the air outlet pipe, 7037 is the pressure gauge, 7038 is the switching valve, 704 is the microphone, 705 is the computing device, 706 is the fiber collimator, 707 is the fiber, 708 is the device-type lock-in amplifier, and 709 is the data acquisition card. Detailed Implementation

[0044] Various exemplary embodiments, features, and aspects of this disclosure will now be described in detail with reference to the accompanying drawings. The same reference numerals in the drawings denote elements that have the same or similar functions. Although various aspects of the embodiments are shown in the drawings, they are not necessarily drawn to scale unless specifically indicated otherwise.

[0045] The term “exemplary” as used herein means “serving as an example, embodiment, or illustration.” Any embodiment illustrated herein as “exemplary” is not necessarily to be construed as superior to or better than other embodiments.

[0046] In this document, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A alone, A and B simultaneously, and B alone. Furthermore, the term "at least one" in this document means any combination of at least two of any one or more elements. For example, including at least one of A, B, and C can mean including any one or more elements selected from the set consisting of A, B, and C.

[0047] It should be understood that the terms "first," "second," etc., in the claims, specification, and drawings of this disclosure are used to distinguish different objects, rather than to describe a specific order. The terms "comprising" and "including" as used in the specification and claims of this disclosure indicate the presence of the described features, integrals, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components, and / or collections thereof.

[0048] Furthermore, to better illustrate this disclosure, numerous specific details are set forth in the following detailed description. Those skilled in the art will understand that this disclosure can be practiced without certain specific details. In some instances, methods, means, components, and circuits well known to those skilled in the art have not been described in detail in order to highlight the main points of this disclosure.

[0049] To address the issues of uniformity interference factors such as light intensity fluctuations, frequency mismatch, and microphone sensitivity variations in photoacoustic spectroscopy, this disclosure proposes a photoacoustic spectral self-calibration method adapted to resonant photoacoustic spectral systems. This self-calibration method is based on a more comprehensive theoretical analysis of the first harmonic signal to better reflect actual conditions. Furthermore, it provides a gas detection method and system based on resonant cavity-based first harmonic self-calibration, outlining the selection and design of the resonant cavity and the hardware system design scheme.

[0050] The following is passed Figures 1 to 6 This disclosure provides a detailed description of the photoacoustic spectral self-calibration method and the gas detection method according to embodiments of the present invention. Figures 7 to 12 The gas detection system according to embodiments of this disclosure will be described in detail.

[0051] Before introducing the photoacoustic spectral self-calibration method of this disclosure, the conditions for proposing the photoacoustic spectral self-calibration method based on the first harmonic are described as follows: Condition 1: A solid absorber (such as a graphite sticker) exists in the gas detection system used to implement the self-calibration method. The absorption spectrum of the solid absorber is flat and can convert the absorbed light intensity into a heat source to excite the background sound signal; Condition 2: A tunable laser is used as the light source. The driving current applied to the tunable laser is a signal current of low-frequency triangular scanning superimposed with high-frequency sinusoidal modulation. Applying constant temperature control to the tunable laser will cause the wavelength and light intensity output by the tunable laser to change according to the law of the driving current, so as to generate a harmonic signal; Condition 3: After applying the modulation described in condition 2 to the tunable laser, the center wavelength scanning range of the tunable laser should cover a complete absorption peak or absorption peak group of the gas to be measured and include the non-absorption regions on both sides, so as to obtain a baseline from the first harmonic; Condition 4, the gas absorbs light relatively little, that is, it satisfies the small absorption approximation condition (absorbance < 0.05). In trace gas detection applications, this condition is easy to achieve, especially for photoacoustic spectroscopy with a large dynamic range; Condition 5, using the same demodulation frequency as the high-frequency sinusoidal modulation frequency in condition 2 to demodulate the acoustic signal to obtain the first harmonic, can make the modulation and demodulation frequency equal to the resonance frequency of the resonant photoacoustic spectroscopy system, so as to maximize the acoustic signal and be detected efficiently.

[0052] Under the above conditions, the first harmonic can be theoretically derived. The derivation process considers the actual nonlinear relationship between the laser output intensity and the driving current, rather than simply treating it as an ideal linear relationship; the derivation process also does not ignore the influence of gas absorption on the acoustic signal generated by the solid absorber. The derivation results show that the first harmonic signal consists of two parts: the first part is the baseline part (the baseline part can be understood as the first harmonic signal obtained when the gas concentration is 0 or the light is not absorbed by the gas at all), which is only proportional to the light intensity and is independent of gas absorption; the second part is the fluctuation part, which is proportional to the light intensity and the gas concentration. The derivation results also give the shape characteristics of the first harmonic. For the baseline part, considering the nonlinear relationship between the laser output intensity and the current, this embodiment proposes that the baseline part is not a constant value, but a slightly curved oblique line, which can be fitted by an nth-degree polynomial function; for the fluctuation part, when the gas absorption peak is a single absorption peak, the shape of the fluctuation part is a double-peak structure with one positive and one negative peak. In other cases, the shape of the fluctuation part is more complex. For example, Figure 1 This diagram illustrates the first harmonic signal obtained through numerical simulation using a self-written program. The absorption peak used in the simulation is the isolated absorption peak of methane (CH4) at 1651 nm. The horizontal axis represents time, and the vertical axis represents signal magnitude. Figure 1 As shown, the shape of the first harmonic signal is a slightly curved baseline superimposed with a wavy portion. This simulation result is consistent with the aforementioned conclusion, and subsequent verification also confirms its agreement with experimental results.

[0053] Based on the above conditions and the theoretical analysis results of the first harmonic, Figure 2 A flowchart illustrating a photoacoustic spectral self-calibration method according to an embodiment of the present disclosure is shown. Figure 2 As shown, the self-calibration method includes steps S11 to S14.

[0054] In step S11, the first harmonic signal of the electrical signal is acquired. The electrical signal is obtained by converting the acoustic signal. The acoustic signal is generated when the incident light passes through the gas and strikes the solid absorber, and is absorbed and converted into a heat source.

[0055] In practical applications, the incident light is generated by a tunable laser (also known as a tunable semiconductor laser). The driving current of the tunable laser consists of a signal current obtained by superimposing a low-frequency triangular scanning signal onto a high-frequency sinusoidal modulation signal. The low-frequency triangular scanning signal is a periodically changing signal with a waveform resembling a triangle. It drives the tunable semiconductor laser, causing its output center wavelength to increase or decrease approximately linearly with time, thus scanning the absorption peaks or absorption peak groups of the gas. The high-frequency sinusoidal modulation signal is a periodic sine wave with a frequency much higher than the low-frequency scanning signal. It is superimposed on the low-frequency triangular scanning signal to modulate the output of the tunable laser at high frequency. When the high-frequency sinusoidal modulation signal is superimposed on the low-frequency triangular scanning signal, the output of the tunable laser is simultaneously modulated by both signals. Specifically, both the output wavelength and light intensity change according to the low-frequency triangular scanning + high-frequency sinusoidal modulation pattern. The wavelength modulation causes the gas absorption of light to change over time. Due to the characteristics of the tunable semiconductor laser, the light intensity also changes in the same way when the wavelength is modulated. The triangular scanning signal causes the center wavelength of the incident light to change slowly over time and scans the center wavelength of the incident light across the entire gas absorption peak, while the sinusoidal modulation signal causes the wavelength to change rapidly and slightly around the center wavelength.

[0056] As described above, the self-calibration method proposed in this disclosure is applicable to the small absorption approximation condition (i.e., the absorbance of the gas to the incident light can be less than 0.05). When the absorbance is large (i.e., the absorbance is greater than or equal to 0.05), known improvements in the art can be used. For example, the logarithm of the first harmonic signal can be taken before theoretical analysis and subsequent self-calibration processes to obtain a photoacoustic spectral self-calibration method that is not limited by the small absorption approximation condition. This disclosure does not limit this method.

[0057] In practical applications, based on the fundamental principle of photoacoustic effect, incident light is absorbed by both a gas and a solid absorber (such as graphite sticker), generating heat sources. These two heat sources then produce an acoustic signal. This acoustic signal is the superposition of the acoustic signal excited by the gas-absorbed incident light heat source and the background-carrying signal excited by the solid absorber-absorbed incident light heat source. This acoustic signal can be acquired and converted into an electrical signal using a microphone known in the art (such as a conventional microphone, or a cantilever microphone). The acoustic signal excited by the solid heat source (i.e., the solid absorber-absorbed incident light heat source) generates the baseline portion of the first harmonic signal (background signal), while the solid and gas heat sources (i.e., the gas-absorbed incident light heat source) together generate the fluctuating portion of the first harmonic signal (the measurement signal).

[0058] In one possible implementation, obtaining the first harmonic signal of the electrical signal may include: using a device-type lock-in amplifier known in the art to demodulate the electrical signal converted by the microphone to obtain the first harmonic signal of the electrical signal.

[0059] In one possible implementation, a quadrature digital lock-in amplifier (LLBIA) known in the art, implemented programmatically, can be used to demodulate the electrical signal. It is noted that if an LLBIA is used, the output will have the modulus of the first harmonic signal, which is not negative. This characteristic causes the first harmonic signal value to be automatically inverted when it is less than 0, producing a "bounce effect." Figure 3 A schematic diagram is shown illustrating the first harmonic signal of methane at 1000 ppm (ppm is volume concentration, representing parts per million), obtained using a quadrature digital lock-in amplifier. Figure 3 As shown, the initial first harmonic signal output by the quadrature digital lock-in amplifier is used. The initial first harmonic signal is the modulus of the first harmonic signal. The modulus of the first harmonic signal can be used to replace the first harmonic signal. Even under the condition of satisfying the small absorption approximation, due to the relatively strong absorption, the initial first harmonic signal also exhibits a "bounce effect" in the time range of about 2-2.5s (i.e. the time range corresponding to points a and b in the figure). This effect will affect the subsequent calculation of the peak-to-peak value Vpp and the normalized ratio.

[0060] Therefore, by identifying the bounce effect region and inverting the signal magnitude within that region, the normal first harmonic signal can be recovered. Figure 3 The diagram illustrates the normal first harmonic signal obtained by inverting the signal magnitude within the bounce effect region of the initial first harmonic signal. Specifically, acquiring the first harmonic signal of the electrical signal can include: demodulating the electrical signal using a quadrature digital lock-in amplifier to obtain the initial first harmonic signal; identifying the bounce effect region of the initial first harmonic signal and inverting the signal magnitude within the bounce effect region to obtain the first harmonic signal. The bounce effect region includes the time range defined by two time points where the signal magnitude is 0 and the derivatives are opposite. In other words, the bounce effect region corresponds to the time range where the first harmonic signal obtained using a device-type lock-in amplifier is negative, but is automatically inverted because the quadrature digital lock-in amplifier can only obtain moduli and cannot output negative values. For example... Figure 3 The time range between points a and b is the rebound effect region. This method ensures that the first harmonic signal obtained using a quadrature digital lock-in amplifier matches the first harmonic signal obtained by the lock-in amplifier device, avoiding the "rebound effect" that occurs at relatively high concentrations or with strong absorption. This simplifies the system while guaranteeing the largest possible dynamic range for concentration measurement under the small absorption approximation condition.

[0061] In step S12, based on the signal magnitude of the first harmonic signal in the non-absorption region, the baseline of the first harmonic signal is fitted. The baseline is represented as an nth-degree polynomial function with time as the independent variable and signal magnitude as the dependent variable, where n≥2. The non-absorption region includes the time range during which the gas does not absorb incident light.

[0062] As we know, the intensity-current and wavelength-current relationships of tunable lasers are not perfectly linear, exhibiting a certain degree of nonlinearity. Therefore, the actual output center wavelength only changes linearly with time. Consequently, an nth-order polynomial function is needed to more accurately fit the baseline of the first harmonic.

[0063] For example, such as Figure 1 The first harmonic signal shown can be obtained by utilizing the non-absorption regions on both sides (i.e., Figure 1 The baseline of the first harmonic signal is obtained by fitting a quadratic polynomial function to the signal magnitude within the time range before point c and after point d (i.e., the time range where the center wavelength of the triangular scan is outside the gas absorption peak). Using an nth-order polynomial function to fit the baseline takes into account the non-ideal linear relationship between the output intensity and current of the tunable laser, and does not treat the baseline as a constant. Therefore, a more accurate baseline of the first harmonic signal can be obtained, which is beneficial for obtaining the accurate fluctuating portion of the first harmonic signal.

[0064] In practical applications, the non-absorption and absorption regions of the first harmonic signal can be determined by pre-specifying time points (for example, the time range before 1.5 seconds and after 3.5 seconds can be designated as the non-absorption region, and the time range between 1.5 seconds and 3.5 seconds as the absorption region). Alternatively, relevant algorithms can be designed to identify the non-absorption and absorption regions, and this disclosure does not limit this approach. As described above, the low-frequency triangular scanning signal can be used to scan the center wavelength of the incident light across the entire gas absorption peak. Therefore, the non-absorption region can specifically include the time range in which the center wavelength corresponding to the incident light is not within the gas absorption peak and the gas does not absorb the incident light.

[0065] As we know, the electrical signal converted from a sound signal can be either a voltage signal or a current signal. When the electrical signal is a voltage signal, the magnitude of the first harmonic signal can be expressed as a volt, that is, the unit can be volt (V), millivolt (mV), microvolt (μV), etc. When the electrical signal is a current signal, the magnitude of the first harmonic signal can be expressed as an ampere, that is, the unit is ampere (A), milliampere (mA), microampere (μA), etc.

[0066] In practical applications, when the driving current of the tunable laser is in other forms, the nonlinear effect between light intensity and current is still considered. Other known functional forms in the art can be used to fit the baseline, and this disclosure does not limit this.

[0067] In step S13, the first harmonic signal is subtracted from the baseline to obtain the fluctuating portion of the first harmonic signal within the absorption region, and the peak-to-peak value and reference time point of the fluctuating portion signal are determined. The absorption region includes the time range during which the gas absorbs the incident light, the reference time point includes the time point corresponding to the center of the gas absorption peak (i.e., the time point corresponding to when the center wavelength of the triangular scan is located at the center of the absorption peak), and the peak-to-peak value includes the difference between the maximum and minimum values ​​of the fluctuating portion signal.

[0068] Subtracting the baseline from the first harmonic signal can be understood as subtracting the baseline from the first harmonic signal to obtain the fluctuating portion of the first harmonic signal within the absorption region. The absorption region can also be understood as the time range containing the fluctuating portion of the signal. Figure 1 The time range between point c and point d.

[0069] For example, Figure 1 Subtracting the baseline of the first harmonic signal shown yields the following result. Figure 4 The fluctuating portion of the first harmonic signal shown can be used to find the peak values ​​(i.e., the maximum and minimum values) of the fluctuating portion, thus obtaining peak-to-peak values ​​that are proportional to both gas concentration and light intensity (e.g.,...). Figure 4 Peak-to-peak value V of the fluctuating signal pp ); and, a reference time point (such as ) can be obtained by finding the time point where the ordinate of the fluctuation part in the middle of the double peak is 0 (that is, the time point where the signal magnitude is 0). Figure 4 The time point corresponding to point e in the middle is the reference time point. This reference time point corresponds to the center of the gas absorption peak. Specifically, the reference time point is a point with a special shape in the fluctuating signal (e.g., a zero-crossing point between positive and negative double peaks). It corresponds to the moment when the center wavelength of the low-frequency triangular scan of the laser is located at the center of the absorption peak. Alternatively, the reference time point can be manually specified. For example, the middle value of the absorption region (e.g., the middle value of 1.5 seconds and 3.5 seconds, 2.5 seconds) can be specified as the reference time point. This embodiment of the present disclosure does not limit the method of determining the reference time point.

[0070] In step S14, the reference time point is substituted into the baseline to obtain the bias value corresponding to the reference time point, and the ratio between the peak value and the bias value is calculated to obtain the normalized ratio. The bias value includes the signal magnitude of the baseline corresponding to the absorption peak center, and the normalized ratio is used to detect the gas concentration of the gas.

[0071] This involves substituting the reference time point into the baseline, that is, substituting the reference time point into the nth-degree polynomial function corresponding to the baseline, to obtain the signal magnitude of the baseline corresponding to the reference time point (i.e., the baseline signal magnitude corresponding to the center wavelength of the triangular scan being at the center of the absorption peak), which is also the bias value V that is only proportional to the light intensity. biasSince the reference time point corresponds to the center of the gas absorption peak, this bias value can also be understood as the signal magnitude corresponding to the center of the absorption peak. For example, Figure 4 Point e corresponds to Figure 1 Point e in the equation is equivalent to... Figure 1 Find the ordinate corresponding to point e and obtain the offset value.

[0072] Then, the ratio between the peak-to-peak value and the bias value can be calculated, that is, the normalized ratio Ratio = V. pp / V bias As mentioned above, peak-to-peak value V pp The bias value V is directly proportional to both gas concentration and light intensity. bias Proportional only to light intensity, therefore Ratio = V is calculated. pp / V bias This allows us to obtain a normalized ratio that is not affected by light intensity fluctuations and is proportional only to the concentration. This is equivalent to using the bias value of the first harmonic signal itself to correct the peak-to-peak value, which means achieving normalized self-calibration of the photoacoustic spectrum.

[0073] Understandably, since the normalized ratio is proportional to the gas concentration, it can be used for gas concentration calibration and inversion; at the same time, the normalized ratio is independent of uniformity interference factors such as light intensity, has self-correction capability, and can achieve accurate gas concentration detection.

[0074] According to the self-calibration method of this disclosure, by fitting the baseline with an nth-order polynomial function based on the signal magnitude of the first harmonic signal in the non-absorption region, the accuracy of the baseline fitting can be improved. Subtracting the baseline from the first harmonic signal yields the accurate fluctuating portion of the first harmonic signal. The peak-to-peak value of the fluctuating portion signal and the bias value corresponding to the absorption peak center on the baseline are then obtained. Since the peak-to-peak value is proportional to both gas concentration and light intensity, while the bias value is only proportional to light intensity, calculating the ratio of the peak-to-peak value to the bias value is equivalent to correcting the peak-to-peak value using the bias value. This yields a normalized ratio that is not affected by light intensity fluctuations and is proportional to the concentration. This achieves a photoacoustic spectral self-calibration method suitable for resonant systems that uses only the first harmonic signal. This is beneficial for achieving a photoacoustic spectral system that is unaffected by uniformity factors such as light intensity fluctuations, frequency mismatch, and microphone sensitivity changes. In other words, it is beneficial for achieving accurate gas concentration detection and reducing the impact of various uniformity interference factors on gas concentration detection.

[0075] Based on the self-calibration method provided in the above embodiments of this disclosure, the embodiments of this disclosure also provide Figure 5 The flowchart of the gas detection method shown is as follows: Figure 5 As shown, the gas detection method includes:

[0076] Step S21: Using the self-calibration method provided in the above-described embodiments of the present disclosure, the normalized ratio value corresponding to the gas to be detected is determined. That is, the acoustic signal in step S11 can be generated by the absorption of incident light when it passes through the gas to be detected and the solid absorber and is converted into a heat source, thereby obtaining the normalized ratio value corresponding to the gas to be detected.

[0077] Step S22: Based on the ratio-concentration relationship calibration curve corresponding to the gas to be detected and the normalized ratio corresponding to the gas to be detected, the gas concentration of the gas to be detected is obtained. The ratio-concentration relationship calibration curve is obtained by using the self-calibration method provided in the above-described embodiments of this disclosure to obtain the standard normalized ratio under different standard concentrations and then performing linear fitting on the standard normalized ratio under different standard concentrations.

[0078] As described above, the normalized ratio is directly proportional to the gas concentration and can be used for gas concentration calibration and inversion. In practical applications, a series of standard gases with different concentrations can be used, and the normalized ratios of the standard gases at different standard concentrations can be obtained using the self-calibration method of this disclosure. Then, a ratio-concentration relationship calibration curve can be fitted, thus completing the gas concentration calibration. It should be understood that different types of gases can have their own corresponding ratio-concentration relationship calibration curves calibrated separately. In actual measurement, the normalized ratio corresponding to the gas to be detected can be obtained using the self-calibration method of this disclosure, and the gas concentration of the gas to be detected can be calculated using the ratio-concentration relationship calibration curve corresponding to that gas, thus completing the gas concentration inversion.

[0079] For example, such as Figure 6 As shown, the calibration curve of the normalized ratio (Ratio) versus concentration and the corresponding linear regression equation are displayed. Linear fit goodness R 2 The peak-to-peak value V was 0.99995; and the peak-to-peak value V was also displayed. pp The calibration curve with concentration and the corresponding linear regression equation Linear fit goodness R 2 Reaching 0.99994 further demonstrates the accuracy of the baseline fitting in the embodiments of this disclosure, as well as the accuracy of gas concentration calibration and inversion using the normalized ratio. Traditional photoacoustic spectroscopy typically uses peak-to-peak value Vt. pp Traditional methods for calibrating and inverting gas concentrations are easily affected by uniformity interference factors such as light intensity. However, the gas detection method of this disclosure reduces the impact of uniformity interference factors such as light intensity on gas concentration detection and improves the accuracy of gas concentration detection by using a normalized ratio (Ratio) that is directly proportional to the concentration.

[0080] Based on the above-described photoacoustic spectral self-calibration method and gas detection method, embodiments of this disclosure also provide, for example... Figure 7 The diagram shown is a block diagram of a gas detection system, such as Figure 7 As shown, the system includes:

[0081] The laser modulation device 701, the tunable laser 702, the resonant device 703, the microphone 704, and the computing device 705.

[0082] The laser modulation device 701 is used to output a driving current to the tunable laser 702. The driving current includes the signal current obtained by superimposing a low-frequency triangular scanning signal with a high-frequency sinusoidal modulation signal.

[0083] A tunable laser 702 is used to output incident light into the resonant cavity (not shown in the figure) of a resonant device 703 under the modulation of a driving current. The resonant cavity contains a gas to be detected and a solid absorber. When the incident light passes through the gas to be detected and strikes the solid absorber, it is absorbed and converted into a heat source, thereby exciting an acoustic signal. The acoustic signal is amplified by the resonance of the resonant cavity.

[0084] Microphone 704 is used to collect sound signals and convert them into electrical signals;

[0085] The computing device 705 is used to execute the gas detection method of the present disclosure embodiment described above to obtain the gas concentration of the gas to be detected.

[0086] In practical applications, the laser modulation device 701, the tunable laser 702, the resonant device 703, and the microphone 704 can each be hardware devices known in the art, as long as they can achieve their respective required functions. The computing device 705 can be equipped with software programs for implementing the gas detection method of the above-described embodiments of this disclosure, so that the above-described gas detection method can be executed in the computing device 705. The computing device 705 can be a computer, server, terminal, tablet computer, laptop computer, etc., and this disclosure does not limit this.

[0087] In one possible implementation, embodiments of this disclosure also provide Figure 8 The gas detection system shown, such as Figure 8 As shown, the laser modulation device 701 may include: a first signal generator 7011, a superimposed generator 7012, and a laser controller 7013;

[0088] The first signal generator 7011 is used to generate a low-frequency triangular scanning signal (CH1) and a high-frequency sinusoidal modulation signal (CH2) and input them to the superimposed unit 7012. The modulation frequency of the high-frequency sinusoidal modulation signal is the same as the resonance frequency of the resonant device.

[0089] The superimposed unit 7012 is used to superimpose a low-frequency triangular scanning signal with a high-frequency sinusoidal modulation signal to obtain a driving signal, which is then input to the laser controller 7013.

[0090] The laser controller 7013 is used to output a drive current (LDC) corresponding to the drive signal to the tunable laser 702 to modulate the wavelength and intensity of the incident light output by the tunable laser 702.

[0091] The driving current can be understood as the signal current obtained by superimposing a low-frequency triangular scanning signal with a high-frequency sinusoidal modulation signal. The laser controller 7013 can also simultaneously output a temperature control current (TEC) to control the temperature of the tunable laser 702, thus providing stable temperature control. It should be understood that the signal generator 7011, the superimposed unit 7012, and the laser controller 7013 can all be devices known in the art, as long as they can achieve the required functional interfaces. This disclosure does not limit the scope of these devices.

[0092] In one possible implementation, such as Figure 8 As shown, the resonant cavity in the resonant device 703 can be a T-shaped resonant cavity. The T-shaped resonant cavity includes a buffer cavity 7031 and a resonant tube 7032. The first end of the resonant tube 7032 is connected to the first end of the buffer cavity 7031. The second end of the resonant tube 7032 is a closed end. The second end of the buffer cavity 7031 is sealed with a window 7033 that can transmit spatial light (such as infrared light). The incident light enters from the second end of the buffer cavity 7031. A solid absorber 7034 is disposed at the closed end of the resonant tube 7032, and a microphone 704 is disposed at the closed end of the resonant tube 7032, so that the solid absorber 7034 and the microphone 704 are located at the antinode of the first-order longitudinal resonance mode of the T-shaped resonant cavity.

[0093] The resonance device 703 is further equipped with an inlet pipe 7035, an outlet pipe 7036, and a pressure gauge 7037. The inlet pipe 7035 is used to input the gas to be tested into the resonance cavity, the outlet pipe 7036 is used to discharge the gas to be tested from the resonance cavity, and the pressure gauge 7037 is used to detect the gas pressure in the resonance cavity. The inlet pipe 7035 and the outlet pipe 7036 may also be equipped with switching valves 7038 to control the entry and exit of the gas to be tested. It should be understood that the inclusion of an inlet pipe 7035, an outlet pipe 7036, and a pressure gauge 7037 on the resonance device 703 is one possible implementation provided by this embodiment. In practice, the inlet pipe and the outlet pipe can be combined into a single inlet / outlet pipe, or the pressure gauge can be omitted. Those skilled in the art can customize the hardware structure included in the resonance device 703 based on the teachings of this disclosure, and this embodiment does not limit this.

[0094] The T-shaped resonant cavity allows incident light to enter through the entrance port (window 7033) at the second end of the buffer cavity 7031. The gas to be detected within the resonant cavity is absorbed and converted into a first heat source, which then strikes the solid absorber 7034, generating a second heat source. The acoustic signals excited by the two heat sources are converted into electrical signals by the microphone 704. The antinode of the first-order longitudinal resonance mode of the T-shaped resonant cavity is located at the closed end of the resonant tube 7032. Therefore, by selecting a T-shaped resonant cavity and placing the solid absorber 7034 at the closed end, it is ensured that the heat sources from both the solid and the gas are efficiently coupled to the resonance mode of the resonant device 703, maximizing the acoustic signal of the photoacoustic spectrum in principle. In particular, placing the microphone 704 at the closed end of the resonant tube 7032 allows it to acquire the maximum acoustic signal at the antinode of the resonance mode of the T-shaped resonant cavity, enabling the computing device 705 to effectively execute the self-calibration method of this embodiment.

[0095] It should be understood that the above-mentioned T-shaped resonant cavity is one possible implementation provided by the embodiments of this disclosure. In fact, the resonant device 703 can also adopt any type of resonant cavity in the art. For example, a classic H-shaped resonant cavity can also be adopted. In the H-shaped resonant cavity, its own resonant tube wall can be used as a solid absorber to absorb incident light and generate background sound signal. At this time, the microphone 704 can be placed at the midpoint of the resonant tube of the H-shaped resonant cavity to obtain the maximum sound signal at the antinode of the resonance mode of the H-shaped resonant cavity.

[0096] In one possible implementation, microphone 704 can be a conventional microphone or a cantilever microphone. When microphone 704 is a cantilever microphone, the resonant frequency of the cantilever microphone matches the resonant frequency of the resonant device 703, allowing for further amplification of the sound signal through the cantilever resonance of the microphone, thus achieving double resonant amplification and improving sensitivity. It should be understood that the resonant device 703 can be machined with a mounting structure for microphone 704 to facilitate mounting microphone 704 at the antinodes of the resonant cavity to collect sound signals.

[0097] In one possible implementation, the tunable laser 702 can be a fiber laser; the fiber laser is used to generate fiber light; such as Figure 8As shown, when the tunable laser 702 is a fiber laser, the gas detection system also includes a fiber collimator 706. The resonant cavity of the resonant device 703 is also provided with a positioning groove for the fiber collimator 706. The fiber collimator 706 is set at the positioning groove at the entrance of the resonant cavity (such as the positioning groove of the entrance of a T-shaped resonant cavity or the entrance of an H-shaped resonant cavity) to ensure that once the fiber collimator 706 is inserted into the positioning groove, the desired optical path can be obtained, eliminating the need for optical path adjustment. Since the entrance port of the T-shaped resonant cavity is also the second end of the buffer cavity 7031 of the T-shaped resonant cavity, the entrance port of the resonant cavity includes the second end of the buffer cavity 7031 of the T-shaped resonant cavity. That is, an optical fiber collimator 706 can also be provided outside the window 7033 at the second end of the buffer cavity of the T-shaped resonant cavity. The fiber laser 702 and the optical fiber collimator 706 are connected by an optical fiber 707 to transmit the fiber light output by the fiber laser 702. The optical fiber collimator 706 is used to convert the fiber light transmitted by the optical fiber 707 into spatial light and shoot it into the resonant cavity (such as shooting into the T-shaped resonant cavity through the window 7033 at the second end of the buffer cavity 7031 of the T-shaped resonant cavity); wherein, the incident light includes spatial light. It should be understood that when the tunable laser 702 is a fiber laser, the resonant device 703 can be fabricated with a mounting structure for the fiber collimator 706 (that is, a fiber interface is reserved at the entrance of the resonant cavity) to install the fiber collimator 706. This ensures that when the fiber 707 and the fiber collimator 706 are connected to the resonant cavity, the optical path inside the cavity is automatically established, eliminating the need for optical path adjustment.

[0098] In one possible implementation, the tunable laser 702 can also be a spatial light laser (such as a mid-infrared laser), used to generate spatial light (such as mid-infrared light); it should be understood that the spatial light generated by the spatial light laser can be directly incident into the resonant cavity through the entrance port (e.g., the spatial light generated by the spatial light laser can be directly incident into the T-shaped resonant cavity through the window 7033 at the second end of the buffer cavity 7031 of the T-shaped resonant cavity), therefore, as Figure 9 In another gas detection system shown, the exit port of the spatial laser 702 can be directly aligned with the entrance port of the resonant cavity (such as the second end of the buffer cavity 7031 of the T-shaped resonant cavity) to facilitate the emission of incident light into the resonant cavity.

[0099] In one possible implementation, such as Figure 8As shown, the gas detection system also includes a device-type lock-in amplifier 708, used to demodulate the electrical signal (CH3) converted by the microphone 704, obtain the first harmonic signal of the electrical signal, and send the first harmonic signal to the computing device 705, so that the computing device 705 can determine the gas concentration of the gas to be detected based on the first harmonic signal demodulated by the device-type lock-in amplifier 708. The demodulation frequency of the device-type lock-in amplifier 708 is equal to the aforementioned high-frequency sinusoidal modulation frequency, that is, the demodulation frequency used to demodulate the electrical signal is the same as the resonant frequency of the resonant device 703, so as to be able to demodulate the first harmonic signal of the electrical signal. When the device-type lock-in amplifier 708 is used, the signal generator 7011 can also send a synchronization signal (TTL1) to the device-type lock-in amplifier 708 while outputting CH1 and CH2. TTL1 and CH1 are periodically synchronized, which can provide the device-type lock-in amplifier 708 with the start time information of a cycle to notify the device-type lock-in amplifier 708 to synchronously start signal demodulation. Furthermore, a reference channel is also required between the signal generator 7011 and the device-type lock-in amplifier 708 to input a reference signal to the device-type lock-in amplifier 708. This reference signal is the high-frequency sinusoidal modulation signal (CH2) output by the signal generator 708. In other words, the signal generator 7011 inputs a reference signal with the same phase as the high-frequency sinusoidal modulation signal (CH2) to the device-type lock-in amplifier 708. This reference signal is used by the device-type lock-in amplifier 708 to demodulate the electrical signal (CH3) converted by the microphone 704. That is, the device-type lock-in amplifier 708 demodulates the electrical signal (CH3) based on the reference signal (i.e., CH2).

[0100] In one possible implementation, when the resonant frequency of the resonant device 703 is low, the phase delay between wavelength modulation (FM) and intensity modulation (IM) of the tunable laser can be ignored. In this case, the modulus of the first harmonic signal can be used to replace the first harmonic signal. Therefore, in the gas detection system, either a device-based lock-in amplifier or a program-written quadrature digital lock-in amplifier can be used to demodulate the electrical signal, simplifying the hardware structure of the gas detection system. The first harmonic signal obtained by both methods contains a baseline component and a fluctuation component, which can be used to solve the normalization ratio (Ratio) to achieve normalization and self-calibration, and to detect the gas concentration.

[0101] Therefore, when using a quadrature digital lock-in amplifier, such as Figure 10 and Figure 11The two gas detection systems shown may further include a data acquisition card 709, used to acquire the electrical signal (CH3) converted by the microphone 704 and send it to a computing device 705. The computing device 705 also houses a quadrature digital lock-in amplifier (QLAA) to demodulate the electrical signal, obtaining its first harmonic signal. The demodulation frequency of the QLAA is equal to the aforementioned high-frequency sinusoidal modulation frequency; that is, the demodulation frequency used to demodulate the electrical signal is the same as the resonant frequency of the resonant device 703, enabling the demodulation of the first harmonic signal. When using the QLAA, the signal generator 7011 can also send a synchronization signal (TTL1) to the data acquisition card 709 while outputting CH1 and CH2. TTL1 and CH1 are periodically synchronized, providing the data acquisition card 709 with the start time information of a cycle to notify it to synchronously start signal acquisition.

[0102] Considering that in practice, there might be signal generators that can directly output a low-frequency triangular scanning signal superimposed on a high-frequency sinusoidal modulation signal as a drive signal, therefore, in one possible implementation, such as Figure 12 As shown, the laser modulation device 701 may further include: a second signal generator 7014 and a laser controller 7013; the second signal generator 7014 is used to generate a drive signal that superimposes a low-frequency triangular scanning signal onto a high-frequency sinusoidal modulation signal and inputs it to the laser controller 7013, wherein the modulation frequency of the high-frequency sinusoidal modulation signal is the same as the resonance frequency of the resonant device 703; the laser controller 7013 is used to output a drive current corresponding to the drive signal to the tunable laser 702 to modulate the wavelength and intensity of the output of the tunable laser. The second signal generator 7014 may be a signal generator known in the art capable of directly outputting a drive signal that superimposes a low-frequency triangular scanning signal onto a high-frequency sinusoidal modulation signal.

[0103] As described above, a "bounce effect" occurs when an orthogonal digital lock-in amplifier demodulates an electrical signal. Therefore, the gas detection method deployed in the computing device 705 can recover the normal first harmonic signal by identifying the bounce effect region and inverting the signal magnitude within the bounce effect region. The specific implementation method can be referred to step S11 of the self-calibration method in the above-described embodiment of the present disclosure, and will not be elaborated here.

[0104] In practical applications, the gas detection system described above can be used to perform a single gas detection on the gas to be detected to obtain the gas concentration. In order to improve the accuracy of gas concentration detection, multiple gas detections can be performed to obtain multiple gas concentrations, and the average value of the multiple gas concentrations can be calculated as the gas concentration detected by the gas to be detected. This disclosure does not limit this aspect. Figure 13The results of multiple gas concentration detections of 1000 ppm methane standard gas using the system of this embodiment are shown, yielding an average inversion result of 1013.45 ppm with a relative error of only about 1.3%. This demonstrates that the gas detection system of this embodiment can achieve relatively accurate gas concentration detection.

[0105] To verify the beneficial effects of the photoacoustic spectral self-calibration method, gas detection method, and gas detection system proposed in the embodiments of this disclosure, based on the above... Figure 10 The gas detection system shown in this disclosure adopts... Figure 14 The verification system is shown to validate the beneficial effects of the above methods and systems, such as... Figure 14 As shown, the verification system is in Figure 10 The gas detection system shown is configured with an optical fiber attenuator 141, an optical fiber coupler 142, a photodetector 143, and an oscilloscope 144.

[0106] The photoacoustic spectral self-calibration method and gas detection method based on the first harmonic proposed in this disclosure can make the photoacoustic spectral system immune to uniformity factors such as light intensity fluctuations, frequency mismatch, and microphone sensitivity variations. Utilizing Figure 14 The verification system shown can obtain Figure 15 and Figure 16 The experimental results shown include, Figure 15 The process of gradually increasing the optical intensity attenuation using fiber optic attenuator 141 (states A to F) and maintaining the optical intensity without attenuation (state G, where the attenuator is removed) demonstrates the influence of V. pp and V bias The ratio decreases as the incident light intensity decreases and reaches its maximum value when there is no attenuation, but the ratio value remains basically unchanged throughout the process, which reflects the ability of the ratio value to resist light intensity fluctuations. Figure 16 This demonstrates the modulation / demodulation frequency f when the resonant cavity is filled with air and the resonant frequency is 820Hz. mod The concentration inversion results (i.e., the detected gas concentration) using the Ratio value and Vpp were obtained at 830Hz and 820Hz respectively. The average concentration retrieved using the Ratio value remained essentially unchanged (from 2.248 to 2.207), while the concentration retrieved using Vpp showed a larger change (from 1.958 to 2.259). bias The presence of a significant step demonstrates the resistance to frequency mismatch when using the Ratio value for gas concentration detection. These beneficial effects demonstrate the resistance of the self-calibration method and gas detection method of the present disclosure to homogeneity interference factors in the resonant photoacoustic spectrum.

[0107] The photoacoustic spectral self-calibration method and gas detection method based on the first harmonic proposed in this disclosure use only the first harmonic signal for self-calibration (i.e., the 1f / 1f method). Compared with the existing 2f / 1f method, it is more suitable for resonant devices with only one resonant frequency. It can fully utilize the acoustic amplification and high signal-to-noise ratio advantages of resonant photoacoustic spectroscopy without time-division multiplexing or sacrificing a frequency component. One-hour continuous measurement results of 5ppm methane standard gas show that the 1σ standard deviation is less than 0.4ppm, and the Allan variance reaches 3.5ppb at 104s; the measurement experiment of methane in air obtained an average value of 2.2ppm. These results demonstrate the system's high sensitivity and high signal-to-noise ratio.

[0108] The photoacoustic spectral self-calibration method based on the first harmonic proposed in this disclosure takes into account the nonlinearity between the actual light intensity and current of the laser, and uses an nth-order polynomial function to fit the baseline of the first harmonic signal, which ensures the accuracy of the baseline fitting and helps to improve the accuracy of concentration detection. Figure 1 The simulation results of the first harmonic shape shown in the figure are consistent with Figure 17 The shape characteristics of the first harmonic signal measured at the three gas concentrations (20ppm, 50ppm, and 100ppm) shown in the figure are consistent, and it is visible Figure 1 The baseline obtained by the second-order fitting fits well with the first harmonic in the non-absorbing region.

[0109] This disclosure presents a gas detection system based on a resonant cavity. In this system, a solid absorber is placed at the closed end of the resonant tube of a T-shaped resonant cavity. This ensures that heat sources from both the solid and the gas are effectively coupled to the cavity's resonant mode, thereby increasing the signal and improving sensitivity. Compared to commercially available quartz tuning forks, the resonant cavity is larger, and optical path adjustment is simpler. When using a fiber optic light source, only the fiber needs to be plugged in to establish the optical path, eliminating the need for optical path adjustment. Furthermore, using a resonant cavity allows for a lower resonant frequency, at which point the FM-IM phase delay of the laser is negligible. Therefore, an orthogonal digital lock-in amplifier can be used to demodulate the signal, simplifying the system.

[0110] The method and system proposed in this disclosure can address the "bounce effect" that occurs when using a quadrature digital lock-in amplifier by proposing a correction method (i.e., inverting the signal magnitude within the bounce effect region). This can expand the dynamic range of gas concentration detection in the system when using a digital lock-in amplifier, simplifying the gas detection system while enabling the detection of larger gas concentrations under small absorption approximation conditions.

[0111] The flowcharts and block diagrams in the accompanying drawings illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present disclosure. In this regard, each block in a flowchart or block diagram may represent a module, segment, or portion of an instruction containing one or more executable instructions for implementing a specified logical function. In some alternative implementations, the functions marked in the blocks may occur in a different order than those marked in the drawings. For example, two consecutive blocks may actually be executed substantially in parallel, and they may sometimes be executed in reverse order, depending on the functions involved. It should also be noted that each block in the block diagrams and / or flowcharts, and combinations of blocks in the block diagrams and / or flowcharts, may be implemented using a dedicated hardware-based system that performs the specified function or action, or using a combination of dedicated hardware and computer instructions, which is not limited by the embodiments of the present disclosure.

[0112] The various embodiments of this disclosure have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A photoacoustic spectral self-calibration method, characterized in that, include: The first harmonic signal of an electrical signal is acquired by converting an acoustic signal. The acoustic signal is generated when incident light passes through a gas and strikes a solid absorber, where it is absorbed and converted into a heat source. Based on the signal magnitude of the first harmonic signal in the non-absorption region, the baseline of the first harmonic signal is fitted. The baseline is represented as an nth-degree polynomial function with time as the independent variable and signal magnitude as the dependent variable, where n≥2. The non-absorption region includes the time range during which the gas does not absorb incident light. Subtract the first harmonic signal from the baseline to obtain the fluctuating portion of the first harmonic signal, and determine the peak-to-peak value and reference time point of the fluctuating portion signal. The reference time point includes the time point corresponding to the center of the gas absorption peak, and the peak-to-peak value includes the difference between the maximum and minimum values ​​of the fluctuating portion signal. Substituting the reference time point into the baseline, the bias value corresponding to the reference time point is obtained, and the ratio between the peak value and the bias value is calculated to obtain the normalized ratio. The bias value includes the signal magnitude of the absorption peak center corresponding to the baseline. The normalized ratio is used to detect the gas concentration of the gas.

2. The method according to claim 1, characterized in that, The acquisition of the first harmonic signal of the electrical signal includes: The electrical signal is demodulated using an orthogonal digital lock-in amplifier to obtain an initial first harmonic signal, wherein the initial first harmonic signal is the modulus of the first harmonic signal; The bounce effect region of the initial first harmonic signal is identified, and the signal magnitude within the bounce effect region is inverted to obtain the first harmonic signal. The bounce effect region includes the time range defined by two time points where the signal magnitude is 0 and the derivatives are opposite.

3. The method according to claim 1, characterized in that, The incident light is generated by a tunable laser, and the driving current of the tunable laser includes a signal current obtained by superimposing a low-frequency triangular scanning signal with a high-frequency sinusoidal modulation signal. The low-frequency triangular scanning signal is used to scan the center wavelength of the incident light across the entire gas absorption peak. The non-absorption region includes the time range in which the center wavelength of the incident light is not within the gas absorption peak and the gas does not absorb the incident light. The gas has an absorptivity of less than 0.05 for incident light.

4. A gas detection method, characterized in that, include: Using the self-calibration method according to any one of claims 1 to 3, determine the normalization ratio value corresponding to the gas to be detected; Based on the ratio-concentration relationship calibration curve corresponding to the gas to be detected and the normalized ratio corresponding to the gas to be detected, the gas concentration of the gas to be detected is obtained. The ratio-concentration relationship calibration curve is a curve obtained in advance by using the self-calibration method according to any one of claims 1 to 3 to obtain the standard normalized ratio under different standard concentrations and then performing linear fitting on the standard normalized ratio under different standard concentrations.

5. A gas detection system, characterized in that, include: Laser modulation devices, tunable lasers, resonant devices, microphones, and computing devices; The laser modulation device is used to output a driving current to the tunable laser, the driving current including a signal current obtained by superimposing a low-frequency triangular scanning signal with a high-frequency sinusoidal modulation signal. The tunable laser is used to output incident light into the resonant cavity of the resonant device under the modulation of the driving current. The resonant cavity contains a gas to be detected and a solid absorber. When the incident light passes through the gas to be detected and strikes the solid absorber, it is absorbed and converted into a heat source, thereby exciting an acoustic signal. The microphone is used to collect the acoustic signal and convert the acoustic signal into an electrical signal; The computing device is used to execute the gas detection method according to claim 4 to obtain the gas concentration of the gas to be detected.

6. The system according to claim 5, characterized in that, The system further includes a device-type lock-in amplifier for demodulating the electrical signal converted by the microphone to obtain the first harmonic signal of the electrical signal and sending the first harmonic signal to the computing device; or... The system also includes a data acquisition card for acquiring the electrical signal converted by the microphone and sending the electrical signal to the computing device. The computing device is equipped with an orthogonal digital lock-in amplifier to demodulate the electrical signal to obtain the first harmonic signal of the electrical signal. The demodulation frequency used to demodulate the electrical signal is the same as the resonant frequency of the resonant device.

7. The system according to claim 5, characterized in that, The resonant cavity includes a T-shaped resonant cavity, which includes a buffer cavity and a resonant tube. The first end of the resonant tube is connected to the first end of the buffer cavity, and the second end of the resonant tube is a closed end. The second end of the buffer cavity is sealed with a window that can transmit spatial light, and the incident light enters from the second end of the buffer cavity. The solid absorber is disposed at the closed end of the resonant tube, and the microphone is disposed at the closed end of the resonant tube, so that the solid absorber and the microphone are located at the antinode of the first-order longitudinal resonance mode of the T-shaped resonant cavity.

8. The system according to claim 5 or 7, characterized in that, The tunable laser includes a fiber laser or a spatial laser; the fiber laser is used to generate fiber light, and the spatial laser is used to generate spatial light. When the tunable laser uses the fiber laser, the system further includes a fiber collimator, and the resonant cavity of the resonant device is provided with a positioning groove for the fiber collimator at the entrance. The fiber collimator is disposed in the positioning groove at the entrance of the resonant cavity, and the entrance of the resonant cavity includes the second end of the buffer cavity of the T-shaped resonant cavity. The fiber laser and the fiber collimator are connected by an optical fiber to transmit the fiber light output by the fiber laser. The fiber collimator is used to convert the fiber light transmitted by the fiber into spatial light and inject it into the resonant cavity; wherein, the incident light includes the spatial light.

9. The system according to claim 5 or 7, characterized in that, The microphone includes a conventional microphone, or a cantilever beam microphone; When the microphone uses the cantilever beam microphone, the resonant frequency of the cantilever beam microphone matches the resonant frequency of the resonant device.

10. The system according to claim 5, characterized in that, The laser modulation device includes: a first signal generator, a superimposed generator, and a laser controller; The first signal generator is used to generate a low-frequency triangular scanning signal and a high-frequency sinusoidal modulation signal and input them to the superimposed unit, wherein the modulation frequency of the high-frequency sinusoidal modulation signal is the same as the resonance frequency of the resonance device; The superimposed device is used to superimpose the low-frequency triangular scanning signal onto the high-frequency sinusoidal modulation signal to obtain a driving signal, which is then input to the laser controller. The laser controller is used to output a drive current corresponding to the drive signal to the tunable laser to modulate the wavelength and light intensity output by the tunable laser.

11. The system according to claim 5, characterized in that, The laser modulation device includes: a second signal generator and a laser controller; The second signal generator is used to generate a driving signal that superimposes a low-frequency triangular scanning signal onto a high-frequency sinusoidal modulation signal and inputs it to the laser controller, wherein the modulation frequency of the high-frequency sinusoidal modulation signal is the same as the resonance frequency of the resonance device; The laser controller is used to output a drive current corresponding to the drive signal to the tunable laser to modulate the wavelength and light intensity output by the tunable laser.

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