Gas concentration measurement method based on wavelength correction open ring-down cavity absorption spectrum

Abstract

The invention provides a gas concentration measurement method based on a wavelength correction open ring-down cavity absorption spectrum. Tuned continuous laser is incident into the ring-down cavity to realize mode matching with the optical ring-down cavity, the acquisition module synchronously obtains a laser control signal and a transmission light intensity signal, the matched wavelength is corrected according to the corresponding relation between the current for controlling laser wavelength tuning and the laser wavelength, and accurate absorption spectrum abscissa data is obtained; the method comprises the following steps: carrying out linear processing on an exponential decay signal generated by each mode matching, calculating by using discrete Fourier transform to obtain initial light intensity and an initial offset value, carrying out iterative solution on a linear model to obtain exponential decay time, obtaining ordinate data of an absorption spectrum, and fitting and extracting the gas concentration by using the absorption spectrum. According to the method, the ring-down signal linearization model is iteratively solved through dynamic correction of the mode matching wavelength of the ring-down cavity, so that the influence of noise on the concentration measurement precision is reduced, and the method has a wide application prospect in the field of trace gas concentration measurement.

Description

Technical Field

[0001] The invention provides a gas concentration measurement method based on wavelength-corrected open ring-down cavity absorption spectrum, belonging to the technical field of trace gas concentration measurement. Background Art

[0002] Trace gases are gases present in very low concentrations, usually less than one part per million (10 -6 ), and its concentration detection and quantitative analysis technology are of great value in biomedicine, environmental monitoring and industrial production. Existing trace gas concentration detection technologies mainly include chemical detection methods and optical detection methods. Chemical detection methods require contact measurement, while optical detection methods are generally non-contact measurements. Among them, Cavity Ring-Down Spectroscopy (CRDS) uses multiple reflections of lasers in an optical ring-down cavity to increase the equivalent absorption light path by thousands of times, effectively reducing the lower limit of measurement, while also shortening the length of the actual optical path, which is more conducive to the miniaturization of high-sensitivity gas concentration measurement systems and has high practical application value.

[0003] In the cavity ring-down absorption spectroscopy technique, according to the type of laser light source used, it is divided into pulsed laser cavity ring-down absorption spectroscopy (Pulsed CRDS) and continuous laser cavity ring-down absorption spectroscopy (Continuous Wave Cavity Ring-Down Spectroscopy, CW CRDS). Pulsed laser cavity ring-down absorption spectroscopy uses pulsed laser as the input light source, with a simple optical path, easy operation, and no need for optical switches and other devices. Continuous laser cavity ring-down absorption spectroscopy uses continuous laser as the input light source, with high spectral resolution, and requires an optical switch to turn off the laser. In 1988, O'Keefe and Anthony published a paper titled "Cavity ring-down optical spectrometer for absorption measurements using pulsed laser sources" in "Review of Scientific Instruments" Vol. 59, No. 12, pp. 2544-2551. In this paper, the sensitivity of this method is about 2×10 -7 cm -1In 1997, Dai Dongxu et al. published a paper titled “Real-time Cavity Ring-Down Spectroscopy with High Repetition Rate” on pages 481-486 of Volume 6 of the Journal of Chemical Physics. They built a signal acquisition and data processing system based on pulsed laser ring-down cavity absorption spectroscopy and measured the vibration spectrum of water vapor at 620-665nm. When the equivalent optical path was 1170m, the system achieved a 5×10 -8 cm -1 In 1999, van Zee et al. published “Pulsed, single-mode cavity ringdown spectroscopy” in Applied Optics, Vol. 38, No. 18, pp. 3951-3960. They proposed that when only one longitudinal mode is excited, the ring-down signal is a single exponential decay signal, achieving the minimum sensitivity of PulsedCRDS. In 2006, Dube et al. published “Aircraft instrument for simultaneous, in situ measurement of NO3 and N2O5 via pulsed cavity ring-down spectroscopy” in Review of Scientific Instruments, Vol. 77, No. 3, p. 289. They designed an online small portable measurement system with an equivalent optical path of 55 km to measure nitrogen trioxide in the atmosphere, reaching 4×10 -11 cm -1In 2013, Lamoureux et al. published an article titled “HCN quantitative measurement in a laminar low pressure flame at 1036nm using pulsed CRDS technique” in Proceedings of the Combustion Institute, Vol. 34, No. 2, pp. 3557-3564. They used pulsed CRDS technique to measure the integrated spectral absorbance of HCN near the R(7) transition line and determined the distribution of HCN in the flame. In 2023, Bai Bing et al. published an article titled "Experimental Study on Accurate Quantitative Measurement of Flame OH Concentration by CRDS-CARS-PLIF Technology" in Spectroscopy and Spectral Analysis, Vol. 43, No. 12, pp. 3955-3962. They selected a 308.6nm laser to build a pulsed laser ring-down cavity absorption spectroscopy system, combined with laser-induced fluorescence spectroscopy and coherent anti-Stokes Raman scattering spectroscopy to achieve high-precision measurement of the OH radical concentration in the planar flame burner, and found that the OH radical concentration reached its maximum value when the height from the furnace surface was 6mm. The pulsed laser ring-down cavity absorption spectroscopy technology system has a simple experimental setup, a wide wavelength range, and does not require complex control systems such as optical switches. However, the cost of the pulsed light source is high, the output beam quality is poor, and the energy coupling efficiency is low, resulting in a low signal-to-noise ratio in the measurement. Due to the large bandwidth of the pulsed light, the spectral resolution of the measurement system is low, and it is difficult to achieve single-mode excitation in the optical cavity, which limits the measurement accuracy of the system.

[0004] Continuous laser cavity ring-down absorption spectroscopy uses a continuous laser as the light source. Since the line width of a continuous laser is narrow (generally less than 50MHz) and the wavelength tuning interval is also small, it can obtain measurement results with higher spectral resolution and better wavelength reproducibility. There are three common operation strategies for interrupting the input of the laser in the continuous laser cavity ring-down absorption spectroscopy. The first is to use a fast optical switch; the second is to quickly modulate the wavelength or amplitude of the continuous laser; the third is to quickly modulate the cavity length of the optical ring-down cavity. In the fast scanning continuous wave optical cavity ring-down absorption spectroscopy, every time the longitudinal mode resonance frequency of the cavity sweeps across the fixed laser frequency of the monochromatic light source, the optical power in the cavity increases, achieving mode matching. In 2000, He Yabai et al. published "Ringdown and cavity-enhanced absorption spectroscopy using a continuous-wave tunable diode laser and a rapidly swept optical cavity" in Chemical Physics Letters, Vol. 319, No. 1, pp. 131-137. They used continuous-wave lasers to eliminate the need for fast optical switches and simplified the system structure. They used a single-mode tunable diode laser to record the weak combination band spectrum of carbon dioxide diluted in nitrogen in the 1.55-micron near-infrared region. In 2000, Levenson et al. published "Frequency-switched heterodyne cavity ringdown spectroscopy" in Optics Letters, Vol. 25, No. 12, pp. 920-922. They suddenly shifted the wavelength of the laser coupled to the optical ring-down cavity. The radiation emitted from the input port of the excited cavity beat together with the reflection of the frequency-shifted input on the detector surface. When the beat frequency stabilized, the decay time of the generated optical heterodyne signal could be used to measure the absorption spectrum in the cavity with a sensitivity close to the quantum limit.In 2011, Huang Haifeng et al. published the Sensitivity Limit of Rapidly Swept Continuous Wave Cavity Ring-Down Spectroscopy in The Journal of Physical Chemistry A, Vol. 115, No. 34, pp. 9411-9421. They analyzed the accuracy of the spectrum measured by the swept cavity or swept frequency system through formulas and simulations, and used the Monte Carlo simulation method for processing. The results showed that the theoretical signal-to-noise ratio and sensitivity of the swept frequency CRDS are much lower than the laser attenuated by an external modulator, and the slower the wavelength modulation rate, the more obvious the effect. For continuous laser scanning ring-down cavity absorption spectroscopy, the optical path structure is complex and coupling is difficult. The laser wavelength needs to be calibrated to obtain an accurate absorption spectrum.

[0005] CRDS measures the decay time of light in a cavity and is not affected by changes in light intensity, which promotes the application of CRDS in the detection of trace gas concentrations. However, the accuracy and sensitivity of CRDS are affected by the accuracy of the decay time extraction during the gas concentration measurement process. In 2008, Michael et al. published "Discrete sums for the rapid determination of exponential decay constants" in "Review of Scientific instruments", volume 79, issue 2, page 023108. They used a linear regression method based on sums, using discrete sums instead of integrals to quickly extract decay time constants from discrete exponential data and increase the rate of the entire experiment. In 2011, Kallapur et al. published "Nonlinear estimation of ring-down time for a Fabry-Perot optical cavity" in "Optics Express" Vol. 19, No. 7, pp. 6377-6386. They applied the recursive equation of discrete-time Kalman filtering to estimate the Fabry-Perot cavity decay time constant of cavity ring-down absorption spectroscopy (CRDS). Their estimated value converged well to a small range of the expected value of the cavity decay time constant. In 2018, Zhang Weipeng et al. published an article titled "Sensitivity improvement by optimized optical switching and curve fitting in a cavity ring-down spectrometer" in Applied Optics, Vol. 57, No. 29, pp. 8487-8493. They adopted a realistic non-exponential curve fitting method, taking into account the switching speed and delayed triggering of the optical switch, thereby reducing the Allan variance of the measurement results and improving sensitivity.In 2023, He Xing et al. published "The Extreme Learning Machine for the Extraction of Mono-Exponential Decay Time" in IEEE Photonics Journal, Vol. 15, No. 2, p. 8500306. They used the extreme learning machine (ELM) neural network method to efficiently extract the decay time and obtain a nonlinear fitting model in a noisy mono-exponential system. After training with a data set, the extreme learning machine method extracts the decay time with high precision and accuracy without relying on iterations and preset initial values. In 2024, Song Yushuo et al. published "A CNN-assisted mid-infrared high-sensitivity exhaled ammonia sensor based on cavity ring-down spectroscopy" in Sensors and Actuators B: Chemical, Vol. 401, p. 135071. They introduced convolutional neural network (CNN) into the processing of cavity ring-down absorption spectroscopy technology to perform filtering and decay time extraction algorithms. By using the best trained model to read and identify the experimental ring-down signal, the ring-down signal can be extracted at high speed and with high precision without manual intervention.

[0006] When measuring the concentration of the target gas in an open optical path, the extinction effect of the aerosol should also be considered compared to the gas concentration measurement in a closed gas chamber. Since the aerosol in the path has an additional extinction effect on the laser in addition to absorption, the attenuation rate will be accelerated and the exponential decay time will become smaller. In the current aerosol abnormal extinction processing method (a gas temperature and concentration measurement method based on an open optical path adjustable ring-down cavity, application number: 202311006512.4), the abnormal values ​​in the multiple decay time data at a single wavelength obtained after a long-term measurement are eliminated, and then the influence on the absorption rate is eliminated by time domain analysis. The result of the exponential decay time directly determines the final result of the absorption spectrum. The key to measuring trace gas concentrations using cavity ring-down absorption spectroscopy is to extract the exponential decay time efficiently, quickly and accurately.

[0007] Based on the above background, the present invention proposes a method for measuring gas concentration based on wavelength-corrected open ring-down cavity absorption spectrum, tunes continuous laser to be incident into the optical ring-down cavity, realizes mode matching with the optical ring-down cavity, calculates the initial light intensity and initial value of bias according to discrete Fourier transform, processes the exponential decay signal into a linearized model, and solves the exponential decay time by iteratively solving the linearized model. According to the corresponding relationship between the laser control current and the laser wavelength, the matching wavelength is corrected to obtain the continuous laser absorption spectrum, that is, the gas concentration. Summary of the invention

[0008] The present invention proposes a gas concentration measurement method based on wavelength-corrected open ring-down cavity absorption spectrum, which belongs to the technical field of trace gas concentration measurement; the measurement system includes a tunable diode laser 101, an acousto-optic modulator 102, a collimator 103, an optical ring-down cavity 104, a photodetector 105, an acquisition module 106, a parameter extraction module 107 and a threshold detection feedback circuit 108, etc.

[0009] The technical scheme of the present invention is as follows: a tunable diode laser 101 outputs a continuous laser whose wavelength changes with a time period, an acousto-optic modulator 102 performs switch control on the input laser, a collimator 103 converts the optical fiber light into spatial light, the laser is incident on an optical ring-down cavity 104, the gas absorbs the light, the laser is reflected multiple times between two reflectors, a photodetector 105 converts the transmitted light signal into an electrical signal, a collection module 106 synchronously collects the current control signal and the light intensity signal of the laser, and uploads the collected data to a parameter extraction module 107 for processing, after a threshold detection feedback circuit 108 detects an effective exponential decay signal, the acousto-optic modulator 102 is controlled to turn off the continuous laser input, the matching wavelength is corrected according to the corresponding relationship between the current control signal and the laser wavelength, the collected exponential decay light intensity signal is processed to obtain the decay time, the influence of the aerosol is removed according to the decay time data obtained under an open optical path, an absorption spectrum is obtained, and then the gas concentration is extracted.

[0010] After wavelength modulation, the tuned laser is incident into the ring-down cavity to achieve mode matching; the tunable diode laser 101 is controlled by adjusting the current parameter to generate a continuously tuned laser signal, and when the laser wavelength matches the longitudinal mode of the optical ring-down cavity 104, the transmitted light intensity is an exponential decay signal; since the cavity length of the optical ring-down cavity 104 is fixed, the laser wavelength that can achieve mode matching is also a fixed value, but due to the influence of environmental vibration factors, the cavity length will change slightly, resulting in a small gap between the actual mode-matched laser wavelength and the theoretical mode-matched laser wavelength, which in turn leads to a situation where the number of wavelength points is more than the theoretical distribution, and each laser wavelength that achieves mode matching needs to be corrected;

[0011] The current adjustment parameter signal of the laser and the light intensity signal transmitted by the optical ring-down cavity 104 are synchronously collected to correct the matching wavelength; the current adjustment parameter of the laser is controlled to have a one-to-one correspondence with the wavelength of the laser, which is calibrated in advance by an etalon, and the laser wavelength that achieves mode matching at this time is determined according to the current parameter result corresponding to the position where the light intensity signal rises rapidly;

[0012] The laser of a certain wavelength within the tuning range realizes mode matching in the optical ring-down cavity 104 to generate an exponentially decaying light intensity signal; the tunable diode laser 101 emits laser according to the set laser scanning waveform and scanning frequency, and the laser realizes mode matching in the optical ring-down cavity 104. The time domain signal collected by the acquisition module 106 is an exponentially decaying light intensity signal I(t), which is expressed as:

[0013]

[0014] Among them, the decay time I0 is the initial light intensity of the ring-down signal, t is the time, c is the speed of light, R is the reflectivity of the reflector, L is the distance between the two reflectors of the optical ring-down cavity 104, and α(v) is the coefficient of the spectral absorption rate, which is expressed as:

[0015]

[0016] Among them, bias The influence on the absorption rate at each laser wavelength point is consistent. ext is the aerosol extinction rate of the open light path;

[0017] A linear model of the exponential decay signal is established; the acquisition module 106 converts the exponential decay signal from an optical signal into a discrete electrical signal, and performs data processing in the parameter extraction module 107. At this time, the received time-domain discrete light intensity sequence is expressed as:

[0018]

[0019] Where x(n) is the sampling data time series of the exponential decay signal, t s is the sampling time, β is the exponential decay coefficient, satisfying β = 1 / τ, A0 is the bias, and the logarithm of formula (3) is converted into a linearized model, which is expressed as follows:

[0020]

[0021] The decay time is iteratively solved by the decay signal linearization model; the discrete Fourier transform of the time domain discrete sequence x(n) is performed, which is expressed as:

[0022]

[0023] Where X(k) is the discrete Fourier transform result, T is the total signal time, N is the total number of data points, and when k = 0, δ k The value is 1, when k>0, δ k The value is 0, and the ratio of the real part to the imaginary part of the frequency domain signal is expressed as:

[0024]

[0025] Select the discrete Fourier transform coefficient when k = 1, and calculate the exponential decay coefficient expression as follows:

[0026]

[0027] When calculating the initial light intensity, take the real part of the discrete Fourier transform coefficient when k=1. At this time, the coefficient of bias A0 is 0, and the initial light intensity is obtained:

[0028]

[0029] Take k = 0, then the discrete Fourier transform result is a real number, the imaginary part is 0, and the bias A0 is:

[0030]

[0031] Substitute the initial light intensity and bias data calculated by discrete Fourier transform into the linearization model, fit the linearization model to obtain the exponential decay coefficient, use the original light intensity signal to make a difference with the fitting result, calibrate the light intensity data, first correct the value of x(n) at n=1, obtain the calibration light intensity data x(n) after the first iteration, update the frequency domain signal X(K) according to discrete Fourier transform, recalculate the exponential decay coefficient according to formula (7), update the initial light intensity according to formula (8), update the bias according to formula (9), substitute it into the linearization model for iterative processing, correct the value of x(n) at n=2, and then correct the corresponding light intensity data point in each iteration, stop after reaching the set number of iterations, and obtain the exponential decay time;

[0032] The gas concentration is solved by fitting the ring-down cavity absorption spectrum; all exponential decay time results are processed according to formula (2), that is, the absorbance of a single wavelength point with the same offset is obtained. Due to the extinction effect of aerosol on the laser in the open optical path, it is necessary to process the multiple absorbance results corresponding to multiple mode matchings of a single wavelength point to remove the aerosol extinction rate α ext The influence on the absorption rate; the corrected laser wavelength is used as the horizontal coordinate, and the calculated absorption rate of a single wavelength point with the same offset is used as the vertical coordinate, that is, the absorption spectrum with offset d0 is obtained, and the two parameters of gas concentration and offset d0 are set as the unknown quantities to be determined, and the obtained absorption spectrum is fitted to finally obtain the gas concentration.

[0033] The advantages of the present invention are as follows: the laser emission signal is continuous light, the laser wavelength is periodically tuned with time, the laser wavelength signal and the ring-down cavity transmission signal are synchronously collected, and a continuous laser absorption spectrum is obtained after dynamically correcting the matching wavelength, thereby reducing the complexity of the system; the exponential decay signal is processed into a linearized model and iteratively solved, thereby reducing the influence of noise on the measurement accuracy and improving the accuracy of the system in measuring gas concentration. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a typical structural diagram of the method described in this patent, which consists of the following parts: tunable diode laser 101, acousto-optic modulator 102, collimator 103, optical ring-down cavity 104, photodetector 105, acquisition module 106, parameter extraction module 107, threshold detection feedback circuit 108

[0035] Figure 2 This is the flow chart of the gas concentration measurement method based on wavelength-corrected open ring-down cavity absorption spectrum of this patent

[0036] Figure 3 This is the calculation result of the gas concentration measurement method based on the wavelength-corrected open ring-down cavity absorption spectrum of this patent. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is further described below in conjunction with the accompanying drawings. The present invention provides a method for measuring gas concentration based on wavelength-corrected open ring-down cavity absorption spectrum. The measuring device is shown in the attached figure. Figure 1 As shown, the tunable diode laser 101 outputs a continuous laser whose wavelength changes with the time period, the acousto-optic modulator 102 controls the input laser on and off, the collimator 103 converts the fiber light into spatial light, the laser is incident on the optical ring-down cavity 104, the gas absorbs the light, the laser is reflected multiple times between the two reflectors, the photodetector 105 converts the transmitted light signal into an electrical signal, the acquisition module 106 synchronously acquires the current control signal and the light intensity signal of the laser, and uploads the acquired data to the parameter extraction module 107 for processing, and the threshold detection feedback circuit 108 controls the acousto-optic modulator 102 to turn off the continuous laser input after detecting the effective exponential decay signal. The matching wavelength is corrected according to the corresponding relationship between the current control signal and the laser wavelength, the acquired exponential decay light intensity signal is processed to obtain the decay time, and the influence of the aerosol is removed according to the decay time data obtained under the open optical path to obtain the absorption spectrum, and then the gas concentration is extracted.

[0038] The example was implemented under the following conditions: the experiment was carried out in an open environment at standard atmospheric pressure, the target gas was methane, the methane gas concentration and temperature distribution in the measurement optical path were uniform, and the tunable diode laser 101 emitted a periodically tuned laser covering the methane at 6046.2 cm-1 Up to 6047.7cm -1 The absorption line at , the tuned laser is generated by superposition of 1KHz high frequency triangle wave and 2Hz low frequency sawtooth wave, the laser frequency amplitude modulated by 1KHz high frequency triangle wave is 0.0379cm -1 The laser frequency amplitude modulated by the 2Hz low-frequency sawtooth wave is 1.5cm -1 , including 50 frequency points in the free spectrum range, and the acquisition frequency of the acquisition module 106 is 40 MHz. The following are the operating steps of the embodiment.

[0039] Step 1: The tunable diode laser 101 emits continuous laser light. The scanning waveform of the laser light is controlled by the current set by the program. The step interval is the free spectrum range. The scanning band covers the methane at 6046.2 cm -1 Up to 6047.7cm -1 The absorption line at , the ring-down cavity length is fixed, and the free spectrum range is 0.0303cm -1 The laser is reflected multiple times in the ring-down cavity, and part of the light is transmitted each time it is reflected. The acquisition module 106 collects the transmitted light intensity signal and converts it into an electrical signal.

[0040] Step 2: After the wavelength is modulated, the tuned laser is incident into the ring-down cavity to achieve mode matching; the tunable diode laser 101 is controlled by adjusting the current parameter to generate a continuously tuned laser signal, and when the laser wavelength matches the longitudinal mode of the optical ring-down cavity 104, the transmitted light intensity is an exponential decay signal; since the cavity length of the optical ring-down cavity 104 is fixed, the laser wavelength that can achieve mode matching is also a fixed value, but due to the influence of environmental vibration factors, the cavity length will change slightly, resulting in a small gap between the actual mode-matched laser wavelength and the theoretical mode-matched laser wavelength, which in turn leads to a situation where the number of wavelength points is more than the theoretical distribution, and each laser wavelength that achieves mode matching needs to be corrected;

[0041] The current adjustment parameter signal of the laser and the light intensity signal transmitted by the optical ring-down cavity 104 are synchronously collected to correct the matching wavelength; the current adjustment parameter of the laser is controlled to have a one-to-one correspondence with the wavelength of the laser, which is calibrated in advance by an etalon, and the laser wavelength that achieves mode matching at this time is determined according to the current parameter result corresponding to the position where the light intensity signal rises rapidly;

[0042] Step 3: The laser of a certain wavelength within the tuning range realizes mode matching in the optical ring-down cavity 104 to generate an exponentially decaying light intensity signal; the tunable diode laser 101 emits laser according to the set laser scanning waveform and scanning frequency, and the laser realizes mode matching in the optical ring-down cavity 104. The time domain signal collected by the acquisition module 106 is an exponentially decaying light intensity signal I(t), which is expressed as:

[0043]

[0044] Among them, the decay time I0 is the initial light intensity of the ring-down signal, t is the time, c is the speed of light, R is the reflectivity of the reflector, L is the distance between the two reflectors of the optical ring-down cavity 104, and α(v) is the coefficient of the spectral absorption rate, which is expressed as:

[0045]

[0046] Among them, bias The influence on the absorption rate at each laser wavelength point is consistent. ext is the aerosol extinction rate of the open light path;

[0047] A linear model of the exponential decay signal is established; the acquisition module 106 converts the exponential decay signal from an optical signal into a discrete electrical signal, and performs data processing in the parameter extraction module 107. At this time, the received time-domain discrete light intensity sequence is expressed as:

[0048]

[0049] Where x(n) is the sampling data time series of the exponential decay signal, t s is the sampling time, β is the exponential decay coefficient, satisfying β = 1 / τ, A0 is the bias, and the logarithm of formula (3) is converted into a linearized model, which is expressed as follows:

[0050]

[0051] The decay time is iteratively solved by the decay signal linearization model; the discrete Fourier transform of the time domain discrete sequence x(n) is performed, which is expressed as:

[0052]

[0053] Where X(k) is the discrete Fourier transform result, T is the total signal time, N is the total number of data points, and when k = 0, δ k The value is 1, when k>0, δ k The value is 0, and the ratio of the real part to the imaginary part of the frequency domain signal is expressed as:

[0054]

[0055] Select the discrete Fourier transform coefficient when k = 1, and calculate the exponential decay coefficient expression as follows:

[0056]

[0057] When calculating the initial light intensity, take the real part of the discrete Fourier transform coefficient when k=1. At this time, the coefficient of bias A0 is 0, and the initial light intensity is obtained:

[0058]

[0059] Take k = 0, then the discrete Fourier transform result is a real number, the imaginary part is 0, and the bias A0 is:

[0060]

[0061] Substitute the initial light intensity and bias data calculated by discrete Fourier transform into the linearization model, fit the linearization model to obtain the exponential decay coefficient, use the original light intensity signal to make a difference with the fitting result, calibrate the light intensity data, first correct the value of x(n) at n=1, obtain the calibration light intensity data x(n) after the first iteration, update the frequency domain signal X(K) according to discrete Fourier transform, recalculate the exponential decay coefficient according to formula (7), update the initial light intensity according to formula (8), update the bias according to formula (9), substitute it into the linearization model for iterative processing, correct the value of x(n) at n=2, and then correct the corresponding light intensity data point in each iteration, stop after reaching the set number of iterations, and obtain the exponential decay time;

[0062] Step 4: Fit the ring-down cavity absorption spectrum to solve the gas concentration; process all exponential decay time results according to formula (2), that is, obtain the absorbance of a single wavelength point with the same offset. Due to the extinction effect of aerosol on the laser in the open optical path, it is necessary to process multiple absorbance results corresponding to multiple mode matchings of a single wavelength point to remove the aerosol extinction rate α. ext The influence on the absorption rate; the corrected laser wavelength is used as the horizontal coordinate, and the calculated absorption rate of a single wavelength point with the same offset is used as the vertical coordinate, that is, the absorption spectrum with offset d0 is obtained, and the two parameters of gas concentration and offset d0 are set as the unknown quantities to be determined, and the obtained absorption spectrum is fitted to finally obtain the gas concentration.

Claims

1. A method for measuring gas concentration based on wavelength-corrected open ring-down cavity absorption spectrum, characterized in that: The measurement system includes a tunable diode laser 101, an acousto-optic modulator 102, a collimator 103, an optical ring-down cavity 104, a photodetector 105, an acquisition module 106, a parameter extraction module 107, and a threshold detection feedback circuit 108. The tunable diode laser 101 outputs a continuous laser whose wavelength changes with a time period. The acousto-optic modulator 102 performs switching control on the input laser. The collimator 103 converts the fiber light into spatial light. The laser is incident on the optical ring-down cavity 104. The gas absorbs the light. The laser is reflected multiple times between the two reflectors. The photodetector 105 converts the transmitted light signal The acquisition module 106 synchronously acquires the current control signal and the light intensity signal of the laser, and uploads the acquired data to the parameter extraction module 107 for processing. After the threshold detection feedback circuit 108 detects the effective exponential decay signal, it controls the acousto-optic modulator 102 to turn off the continuous laser input, and corrects the matching wavelength according to the corresponding relationship between the current control signal and the laser wavelength. The acquired exponential decay light intensity signal is processed to obtain the decay time. According to the decay time data obtained under the open optical path, the influence of the aerosol is removed to obtain the absorption spectrum, and then the gas concentration is extracted. Specifically, the following steps are included: Step 1: After the wavelength is modulated, the tuned laser is incident into the ring-down cavity to achieve mode matching; the tunable diode laser 101 is controlled by adjusting the current parameter to generate a continuously tuned laser signal, and when the laser wavelength matches the longitudinal mode of the optical ring-down cavity 104, the transmitted light intensity is an exponential decay signal; since the cavity length of the optical ring-down cavity 104 is fixed, the laser wavelength that can achieve mode matching is also a fixed value, but due to the influence of environmental vibration factors, the cavity length will change slightly, resulting in a small gap between the actual mode-matched laser wavelength and the theoretical mode-matched laser wavelength, which in turn leads to a situation where the number of wavelength points is more than the theoretical distribution, and each laser wavelength that achieves mode matching needs to be corrected; Step 2: synchronously collect the current adjustment parameter signal of the laser and the light intensity signal transmitted by the optical ring-down cavity 104 to correct the matching wavelength; the current adjustment parameter of the laser is controlled to have a one-to-one correspondence with the wavelength of the laser, which is calibrated in advance by an etalon, and the laser wavelength that achieves mode matching at this time is determined according to the current parameter result corresponding to the position where the light intensity signal rises rapidly; Step 3: Solve the ring-down cavity absorption spectrum and calculate the gas concentration; linearize the exponential decay signal generated by mode matching, iteratively solve the corresponding exponential decay time, obtain the absorption rate corresponding to the corrected laser wavelength, and obtain the gas concentration based on the absorption spectrum.

2. The method for measuring gas concentration based on wavelength-corrected open ring-down cavity absorption spectrum according to claim 1 is characterized in that: According to the light intensity signal collected by the acquisition module 106, the exponential decay signal generated by each mode matching is linearized, the initial light intensity and the initial value of the bias are obtained according to the discrete Fourier transform calculation, the exponential decay time is obtained by the linearization model iterative solution method, and the vertical coordinate data of the absorption spectrum is obtained, which specifically includes the following steps: Step 1: A laser of a certain wavelength within the tuning range realizes mode matching in the optical ring-down cavity 104 to generate an exponentially decaying light intensity signal; the tunable diode laser 101 emits laser according to the set laser scanning waveform and scanning frequency, and the laser realizes mode matching in the optical ring-down cavity 104. The time domain signal collected by the acquisition module 106 is an exponentially decaying light intensity signal I(t), which is expressed as: Among them, the decay time I0 is the initial light intensity of the ring-down signal, t is the time, c is the speed of light, R is the reflectivity of the reflector, L is the distance between the two reflectors of the optical ring-down cavity 104, and α(v) is the coefficient of the spectral absorption rate, which is expressed as: Among them, bias The influence on the absorption rate at each laser wavelength point is consistent. ext is the aerosol extinction rate of the open light path; Step 2: Establish a linear model of the exponential decay signal; the acquisition module 106 converts the exponential decay signal from an optical signal into a discrete electrical signal, and performs data processing in the parameter extraction module 107. At this time, the received time-domain discrete light intensity sequence is expressed as: Where x(n) is the sampling data time series of the exponential decay signal, t s is the sampling time, β is the exponential decay coefficient, satisfying β = 1 / τ, A0 is the bias, and the logarithm of formula (3) is converted into a linearized model, which is expressed as follows: Step 3: Iterate the decay time by linearizing the decay signal model; perform discrete Fourier transform on the discrete sequence x(n) in the time domain, expressed as: Where X(k) is the discrete Fourier transform result, T is the total signal time, N is the total number of data points, and when k = 0, δ k The value is 1, when k>0, δ k The value is 0, and the ratio of the real part to the imaginary part of the frequency domain signal is expressed as: Select the discrete Fourier transform coefficient when k = 1, and calculate the exponential decay coefficient expression as follows: When calculating the initial light intensity, take the real part of the discrete Fourier transform coefficient when k=1. At this time, the coefficient of bias A0 is 0, and the initial light intensity is obtained: Take k = 0, then the discrete Fourier transform result is a real number, the imaginary part is 0, and the bias A0 is: Substitute the initial light intensity and bias data calculated by discrete Fourier transform into the linearization model, fit the linearization model to obtain the exponential decay coefficient, use the original light intensity signal to make a difference with the fitting result, calibrate the light intensity data, first correct the value of x(n) at n=1, obtain the calibration light intensity data x(n) after the first iteration, update the frequency domain signal X(K) according to discrete Fourier transform, recalculate the exponential decay coefficient according to formula (7), update the initial light intensity according to formula (8), update the bias according to formula (9), substitute it into the linearization model for iterative processing, correct the value of x(n) at n=2, and then correct the corresponding light intensity data point in each iteration, stop after reaching the set number of iterations, and obtain the exponential decay time; Step 4: Fit the ring-down cavity absorption spectrum to solve the gas concentration; process all exponential decay time results according to formula (2), that is, obtain the absorbance of a single wavelength point with the same offset. Due to the extinction effect of aerosol on the laser in the open optical path, it is necessary to process multiple absorbance results corresponding to multiple mode matchings of a single wavelength point to remove the aerosol extinction rate α. ext The influence on the absorption rate; the corrected laser wavelength is used as the horizontal coordinate, and the calculated absorption rate of a single wavelength point with the same offset is used as the vertical coordinate, that is, the absorption spectrum with offset d0 is obtained, and the two parameters of gas concentration and offset d0 are set as the unknown quantities to be determined, and the obtained absorption spectrum is fitted to finally obtain the gas concentration.

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