Gas detection method and system based on all-fiber oscillation interference absorption spectrum

Abstract

The invention relates to the field of gas detection, in particular to a gas detection method and system based on an all-fiber oscillation interference absorption spectrum. High-frequency modulation is introduced through piezoelectric ceramics (PZT) to generate a stable oscillation interference absorption spectrum, so that an efficient gas detection method without laser intensity calibration, baseline fitting and background signal deduction is realized, the system adopts a 10Hz triangular wave signal to scan laser current, and the PZT is modulated through a 1KHz sine wave signal, so that the accuracy of the gas detection is improved. The gas concentration or absorption coefficient is inversed by analyzing the envelope of the oscillation interference absorption spectrum and calculating and demodulating the contrast degree (V) of the oscillation interference absorption spectrum by using the oscillation interference absorption spectrum to optimize the sensitivity and linear range of detection, so that the accuracy of the calculation result is improved, and the method can be applied to a more severe environment; the problems of laser intensity fluctuation, baseline drift and background signal interference of a traditional laser absorption spectrum technology under severe environment interference are solved.

Description

Technical Field

[0001] The present invention relates to the field of gas detection, and specifically to a gas detection method and system based on all-fiber oscillating interference absorption spectroscopy. Background Art

[0002] Laser Absorption Spectroscopy (LAS), as a non-invasive, in-situ, highly selective and high-time-resolution gas sensing technology, is widely used in many fields such as environmental monitoring, industrial process control, greenhouse gas monitoring, etc. The LAS technology can provide accurate measurement of gas concentration by measuring the absorption of gas to laser with specific wavelength.

[0003] However, the traditional laser absorption spectroscopy technology faces some challenges in practical applications. Especially under the interference of harsh environment, its performance is limited. The specific problems are mainly manifested in the following aspects: Laser intensity fluctuation: The traditional LAS technology needs to frequently calibrate the laser intensity to compensate for the fluctuation of the light source, which may lead to a decrease in measurement accuracy in dynamic monitoring and long-term use; Baseline drift: Factors such as environmental temperature and humidity may cause baseline drift, and complex baseline fitting algorithms are required for correction, which increases the complexity of data processing and the requirement for real-time performance; Background signal interference: In a complex environment, the interference of background signals will affect the accuracy of measurement results, and an additional background signal subtraction mechanism is required, which restricts the adaptability and stability of the system. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the present invention provides a gas detection method and system based on all-fiber oscillating interference absorption spectroscopy, which solves the problems of laser intensity fluctuation, baseline drift and background signal interference existing in the traditional laser absorption spectroscopy technology under the interference of harsh environment.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A gas detection method based on all-fiber oscillating interference absorption spectroscopy, the gas detection method specifically includes the following steps:

[0007] S1. Obtain a stable oscillating interference signal by laser wavelength scanning and PZT modulation;

[0008] S2. Set the splitting ratio K value of the interferometer and introduce the gas to be detected into the interferometer;

[0009] S3. Obtain the oscillating interference absorption spectrum of the gas to be detected, respectively fit its upper envelope and lower envelope, and extract the constructive interference signal and destructive interference signal;

[0010] S4. Calculate the contrast of the oscillating interference absorption spectrum based on the constructive interference signal and the destructive interference signal;

[0011] S5. Invert the concentration and absorption coefficient of the gas to be detected based on the contrast and the splitting ratio K value.

[0012] Preferably, in step S1, the laser wavelength is scanned by tuning the output wavelength of the laser with a triangular wave signal of 10 Hz.

[0013] Preferably, in step S1, the PZT modulation is caused by applying a sine wave signal of 1 kHz to cause high-frequency phase modulation.

[0014] Preferably, in step S1, the laser is a continuously tunable laser with a central wavelength in the range of 1648 nm - 1655 nm.

[0015] Preferably, in step S2, the interferometer is a Mach-Zehnder interferometer, where the absorption arm is composed of an anti-resonant hollow fiber, the reference arm is composed of a standard single-mode fiber, and is modulated by a piezoelectric ceramic PZT.

[0016] Preferably, in step S2, it specifically includes the following steps:

[0017] S21. Adjust the optical path difference of the Mach-Zehnder interferometer to a quasi-zero state;

[0018] S22. Obtain the oscillating interference absorption spectrum of the Mach-Zehnder interferometer without gas input, and calculate the splitting ratio K value;

[0019] S23. Introduce the gas to be detected into the interferometer.

[0020] Preferably, in step S4, the calculation formula for the contrast is:

[0021]

[0022] In the above formula, V represents the contrast, I M and I m respectively represent the constructive interference signal and the destructive interference signal, I 1 is the output optical intensity of the reference arm after attenuation by the standard single-mode fiber in the case of no gas input to the Mach-Zehnder interferometer, I 2 is the output optical intensity of the absorption arm after attenuation by the anti-resonant hollow fiber in the case of no gas input to the Mach-Zehnder interferometer, α is the absorption coefficient, C is the gas concentration, L is the effective absorption optical path length, K is the splitting ratio K value, and A is the absorbance of the gas.

[0023] The technical solution also provides a system for implementing a gas detection method based on all-fiber oscillating interference absorption spectroscopy. The system includes a processor and a memory. The memory is used to store a computer program, and when the computer program is executed by the processor, it implements the gas detection method based on all-fiber oscillating interference absorption spectroscopy.

[0024] Compared with the prior art, the present invention provides a gas detection method and system based on all-fiber oscillating interference absorption spectroscopy, having the following beneficial effects:

[0025] 1. The present invention introduces high-frequency modulation through a piezoelectric ceramic (PZT) to generate a stable oscillating interference absorption spectrum, thereby realizing efficient gas detection without laser intensity calibration, baseline fitting, and background signal subtraction. The system uses a 10 Hz triangular wave signal to scan the laser current and modulates the PZT with a 1 kHz sine wave signal to optimize the detection sensitivity and linear range. By analyzing the envelope of the oscillating interference absorption spectrum, the contrast (V) of the oscillating interference absorption spectrum is calculated and demodulated, thereby inversely calculating the gas concentration or absorption coefficient, improving the accuracy of the calculation result.

[0026] 2. The present invention suppresses the random phase drift of the interferometer through active modulation, enabling the alternate generation of constructive interference and destructive interference, thereby stably generating an oscillating interference signal and ensuring signal stability and high precision during the gas concentration detection process.

[0027] 3. The present invention respectively fits the upper and lower envelopes of the oscillating interference absorption spectrum to obtain the constructive interference signal and destructive interference signal of the oscillating signal. This process can effectively extract the key signals in the oscillating interference absorption spectrum and provide basic data for subsequent gas concentration inversion. By fitting the envelope signal, the influence of laser intensity fluctuations is eliminated, improving signal stability.

[0028] 4. The present invention calculates the contrast V according to the constructive interference signal and destructive interference signal of the oscillating interference absorption spectrum. This parameter can reflect the clarity of the interference pattern and the interference intensity, thereby revealing the change in gas concentration. Through this process, laser intensity normalization can be achieved, eliminating errors caused by light source fluctuations. Description of the Drawings

[0029] The drawings described herein are used to provide a further understanding of the present application, and constitute a part of the present application. The schematic embodiments of the present application and their descriptions are used to explain the present application, and do not constitute an improper limitation of the present application. In the drawings:

[0030] Figure 1 is a flowchart of the gas detection method based on all-fiber oscillating interference absorption spectroscopy of the present invention;

[0031] Figure 2 Schematic diagram of the oscillating interference absorption spectrum of the present invention;

[0032] Figure 3 Comparison of the absorption coefficient obtained from the experiment of the present invention with the database. Detailed implementation manners

[0033] To make the above objects, features and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific implementation manners. Thereby, the implementation process of how the present application uses technical means to solve technical problems and achieve technical effects can be fully understood and implemented accordingly.

[0034] Those of ordinary skill in the art can understand that all or part of the steps in the following implementation methods can be completed by instructing relevant hardware through a program. Therefore, the present application can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk memories, CD-ROMs, optical memories, etc.) containing computer-usable program codes.

[0035] Laser absorption spectroscopy (LAS) is widely used in the field of gas detection, including chemical analysis, meteorological monitoring, and breath gas analysis, etc. Especially tunable diode laser absorption spectroscopy (TDLAS), as a highly sensitive quantitative analysis technology, uses a semiconductor diode laser to scan the absorption spectral lines of the target gas by periodically adjusting the injection current (such as a sawtooth wave or a triangular wave), so as to measure the gas concentration. However, due to the inherent characteristics of semiconductor lasers, the laser wavelength and output intensity will change simultaneously. Even without gas absorption, the detector will record a signal reflecting the change in laser intensity, forming a baseline, which requires baseline correction, making the extraction of absorbance values more complex. The traditional direct absorption spectroscopy (DAS) method usually realizes baseline correction by polynomially fitting the non-absorbing region in the absorption spectrum, but this method is easily interfered by the overlap of absorption lines of multi-component gases, especially with poor reliability in harsh environments. Wavelength modulation spectroscopy (WMS) modulates the laser injection current at a high frequency to detect higher harmonics, thereby reducing the sensitivity to baseline fluctuations. However, WMS is still an indirect detection technology and cannot directly obtain the actual absorption spectrum.

[0036] To overcome these limitations, the present invention provides a gas detection method based on all-fiber oscillating interference absorption spectroscopy. This method combines the advantages of TDLAS with the application of anti-resonant hollow fiber (AR-HCF) in the near-infrared wavelength range and integrates a Mach-Zehnder interferometer (MZI). The AOIAS generates a stable oscillating interference signal through a high-frequency modulated piezoelectric ceramic (PZT), enabling gas detection without laser intensity calibration and baseline fitting, without background signal subtraction. This technology can directly measure the gas concentration accurately and has excellent sensitivity and linear range. In addition, the adjustable K value supports optimizing sensitivity and linearity in a wide concentration range, such as Figure 1 shown, and the gas detection method specifically includes the following steps:

[0037] S1. Obtain a stable oscillating interference signal through laser wavelength scanning and PZT modulation. First, the wavelength of the laser is scanned by a 10 Hz triangular wave signal, which adjusts the wavelength range of the laser output light. In addition, a 1 KHz sine wave signal is used to modulate the piezoelectric ceramic (PZT), introducing high-frequency phase modulation to suppress the random phase drift of the interferometer through active modulation, causing constructive and destructive interference to alternate, thereby generating a stable oscillating interference signal. The modulation of the PZT not only adjusts the oscillation frequency of the oscillating interference absorption spectrum but also achieves high-precision control of the interference signal. In addition, the operating temperature and current of the laser are controlled by an integrated DFB laser driver. With a fixed driving temperature, only the driving current is changed, and the laser scans a wavelength range of 2 nm. However, both temperature and current can affect the laser wavelength, so the theoretical optimal range is around 1653 nm. After actual measurement, it is found that the laser is a continuously tunable laser with a center wavelength in the range of 1648 nm - 1655 nm, ensuring signal stability and high precision during gas concentration detection.

[0038] S2. Set the splitting ratio K value of the interferometer and introduce the gas to be detected into the interferometer. The interferometer is a Mach-Zehnder interferometer (MZI), where the absorption arm consists of an anti-resonant hollow fiber (AR-HCF) with an effective optical path length of 55 cm, and the reference arm is composed of a standard single-mode fiber (SMF) and is modulated by a piezoelectric ceramic PZT to generate a stable oscillating interference signal. The PZT, as a phase modulator, generates an oscillating interference absorption spectrum by adjusting the optical path difference (OPD) between the two arms, enabling gas detection without laser intensity calibration and baseline fitting, without background signal subtraction. For the setting of the splitting ratio K value, to better achieve the purpose of gas detection without laser intensity calibration and baseline fitting and without background signal subtraction, in step S2, it specifically includes the following steps:

[0039] S21. Adjust the optical path difference of the Mach-Zehnder interferometer to a quasi-zero state. Due to the refractive index difference between the SMF and the AR-HCF, through precise calculation, the optical path difference of the MZI is adjusted to quasi-zero, which ensures that the light entering each arm from the fiber beam splitter can maintain a specific ratio of their respective output light intensities after attenuation by the SMF and the AR-HCF. This specific ratio of the light intensity is the splitting ratio K value.

[0040] S22. Obtain the oscillating interference absorption spectrum of the Mach-Zehnder interferometer without gas input, and calculate the splitting ratio K value. The calculation formula for the splitting ratio K value is:

[0041]

[0042] In the above formula, K represents the splitting ratio K value, I 1 is the output light intensity of the reference arm after attenuation by the standard single-mode fiber in the case of no gas input to the Mach-Zehnder interferometer, and I 2 is the output light intensity of the absorption arm after attenuation by the anti-resonant hollow fiber in the case of no gas input to the Mach-Zehnder interferometer.

[0043] S23. Introduce the gas to be detected into the interferometer.

[0044] S3. Obtain the oscillating interference absorption spectrum of the gas to be detected, respectively fit its upper envelope and lower envelope, and extract the constructive interference signal and the destructive interference signal. First, use an envelope fitting algorithm, such as the least squares method or other suitable fitting algorithms, to fit the upper and lower envelopes of the interference signals in the collected oscillating interference absorption spectrum. Then, according to the fitting results, extract the constructive interference signal and the destructive interference signal, so as to obtain complete interference signal data. For the fitted signal envelope signals, whether it is the upper envelope or the lower envelope, they are time series signals. Each time point or data point of the upper envelope is a constructive interference signal, and the same is true for the lower envelope. Each time point of the time series signal corresponds to a wavelength, which makes each point of the envelope correspond to a different wavelength. This is similar to the wavelength scanning in the direct absorption spectrum. This process can effectively extract the key signals in the oscillating interference absorption spectrum and provide basic data for the subsequent gas concentration inversion. By fitting the envelope signal, the influence of laser intensity fluctuation is eliminated, and the signal stability is improved. As Figure 2 shown, it is a schematic diagram of the oscillating interference absorption spectrum.

[0045] S4. Calculate the contrast of the oscillating interference absorption spectrum according to the constructive interference signal and the destructive interference signal;

[0046] In spectroscopy, the absorbance A is defined as:

[0047] A = αCL

[0048] When gas absorption occurs in the absorption arm of the MZI, the change in the output light intensity can be described by Lambert-Beer's law:

[0049] I 2 e -αCL = I 1 Ke -αCL = I 1 e ln ( K ) -A

[0050] In the MZI, when constructive and destructive interference occur to the coupled light, the maximum and minimum light intensities of the output are given by the following formulas:

[0051]

[0052] By high-frequency modulating the PZT, the light in the absorption arm and the reference arm will rapidly and alternately undergo constructive and destructive interference at the coupling point, thereby generating an oscillatory interference absorption spectrum. The contrast of this oscillatory interference absorption spectrum can be expressed by the following formula:

[0053]

[0054] In the above formula, V represents the contrast, I M and I m respectively represent the constructive interference signal and the destructive interference signal. I 1 is the output light intensity of the reference arm after attenuation by a standard single-mode fiber in the case of a Mach-Zehnder interferometer without gas input. I 2 is the output light intensity of the absorption arm after attenuation by an anti-resonant hollow fiber in the case of a Mach-Zehnder interferometer without gas input. α is the absorption coefficient, C is the gas concentration, L is the effective absorption optical path length, K is the splitting ratio K value, A is the absorbance of the gas. The contrast V is calculated from the ratio of the constructive interference signal I M and the destructive interference signal I m of the oscillatory interference absorption spectrum. This parameter can reflect the clarity and interference intensity of the oscillatory interference absorption spectrum, and thus reveal the change in gas concentration. Through this process, the normalization of the laser intensity can be achieved, eliminating the error caused by the light source fluctuation, making the signal only related to the gas concentration.

[0055] S5. Invert the concentration and absorption coefficient of the gas to be detected based on the contrast and the splitting ratio K value. After obtaining the contrast and the splitting ratio K value, the concentration and absorption coefficient of the gas to be detected can be inversely calculated according to the calculation formula of the contrast. As Figure 3 shown, it is a comparison schematic diagram of the absorption coefficient of the gas obtained from the experiment of the present invention and the database.

[0056] Corresponding to the gas detection method based on all-fiber oscillating interference absorption spectroscopy provided in the above embodiments, this embodiment also provides a system for implementing the gas detection method. Since the all-fiber oscillating interference absorption spectroscopy-based gas detection system provided in this embodiment corresponds to the all-fiber oscillating interference absorption spectroscopy-based gas detection method provided in the above embodiments, the implementation manners of the foregoing all-fiber oscillating interference absorption spectroscopy-based gas detection method are also applicable to the all-fiber oscillating interference absorption spectroscopy-based gas detection system provided in this embodiment and will not be described in detail in this embodiment.

[0057] The system includes a processor and a memory. The memory is used to store a computer program, and when the computer program is executed by the processor, it implements the gas detection method based on all-fiber oscillating interference absorption spectroscopy.

[0058] The above embodiments have introduced the present invention in detail. Specific examples are used in this article to elaborate on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A gas detection method based on all-fiber oscillation interference absorption spectroscopy, characterized in that: The gas detection method specifically comprises the following steps: S1, obtain stable oscillation interference signal through laser wavelength scanning and PZT modulation; S2. Set the splitting ratio K value of the interferometer and pass the gas to be detected into the interferometer; S3, obtaining the oscillation interference absorption spectrum of the gas to be detected, fitting its upper envelope and lower envelope respectively, and extracting the constructive interference signal and the destructive interference signal; S4, calculating the contrast of the oscillation interference absorption spectrum according to the constructive interference signal and the destructive interference signal; S5. Invert the concentration and absorption coefficient of the gas to be detected according to the contrast ratio and the spectral ratio K value.

2. The gas detection method according to claim 1, characterized in that: In step S1, the laser wavelength is scanned by tuning the output wavelength of the laser through a 10 Hz triangle wave signal.

3. The gas detection method according to claim 1, characterized in that: In step S1 , the PZT is modulated by applying a 1 kHz sinusoidal wave signal to induce high frequency phase modulation.

4. The gas detection method according to claim 1, characterized in that: In step S1, the laser is a continuously tunable laser with a central wavelength between 1648 nm and 1655 nm.

5. The gas detection method according to claim 1, characterized in that: In step S2, the interferometer is a Mach-Zehnder interferometer, wherein the absorption arm is composed of an anti-resonant hollow-core optical fiber, the reference arm is composed of a standard single-mode optical fiber, and is modulated by a piezoelectric ceramic PZT.

6. The gas detection method according to claim 5, characterized in that: In step S2, the following steps are specifically included: S21, adjusting the optical path difference of the Mach-Zehnder interferometer to a quasi-zero state; S22, obtaining the oscillation interference absorption spectrum of the Mach-Zehnder interferometer when no gas is introduced, and calculating the splitting ratio K value; S23, passing the gas to be detected into the interferometer.

7. The gas detection method according to claim 5, characterized in that: In step S4, the calculation formula of contrast is: In the above formula, V represents the contrast, I M and I m They represent constructive interference signal and destructive interference signal respectively, I1 is the reference arm output light intensity after attenuation by standard single-mode fiber in the case of no gas input Mach-Zehnder interferometer, I2 is the absorption arm output light intensity after attenuation by antiresonant hollow-core fiber in the case of no gas input Mach-Zehnder interferometer, α is the absorption coefficient, C is the gas concentration, L is the effective absorption optical path length, K is the splitting ratio K value, and A is the absorbance of the gas.

8. A system for implementing the gas detection method based on all-fiber oscillation interference absorption spectroscopy according to any one of claims 1 to 7, characterized in that: It comprises a processor and a memory, wherein the memory is used to store a computer program, and when the computer program is executed by the processor, the gas detection method based on all-fiber oscillation interference absorption spectroscopy as described in any one of claims 1 to 7 is implemented.

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