A Method and System for Extracting Laser Absorption Spectrum by Suppressing Multiplicative Noise

Abstract

The present invention provides a method and system for extracting laser absorption spectrum by suppressing multiplicative noise. The system includes a test laser generation module, a non-absorbing laser generation module, an optical fiber coupler, an optical fiber splitter, an interferometer, a photodetector, a data acquisition module, an online compensation module and a computer. The test laser generated by the test laser generation module is split by a spectroscope. One beam is used by the interferometer to record the wavelength, and the other beam is combined with the sine-modulated laser generated by the non-absorbing laser generation module through the optical fiber coupler and then emitted by a collimating mirror. The combined beam passes through the gas to be measured and is detected by the photodetector. After being collected by the data acquisition module, the test laser and the non-absorbing laser are separated by the online compensation module, and the intensity change of the non-absorbing laser is used to compensate the intensity change of the test laser caused by the light deflection effect, so as to achieve accurate extraction of the absorption spectrum. The invention has a simple structure, can overcome the interference of the light deflection effect on the laser absorption spectrum, and has broad application prospects in the field of dynamic absorption spectrum testing such as combustion diagnosis.

Description

Technical Field

[0001] The present invention relates to a laser gas absorption spectroscopy measurement system and method, and particularly to a method and system for extracting anti-optical deflection absorption spectrum, belonging to the technical field of laser spectroscopy and gas parameter measurement. Background Art

[0002] Combustion is a relatively complex physico-chemical change process of multi-phase, multi-scale, and multi-component in which fuel combustion chemical reaction, turbulent flow, and heat transfer occur simultaneously. This process involves phenomena such as mixing, flow, ignition, and heat transfer, covering many interdisciplinary issues such as chemical kinetics, fluid mechanics, and thermodynamics. In combustion diagnosis research, temperature is a particularly critical physical quantity, which is closely related to the overall efficiency of the combustion process. The purpose of most combustion reactions in industrial sites is to obtain energy, and temperature directly characterizes the amount of thermal energy released and the efficiency during the combustion process. The combustion process is often accompanied by a violent chemical reaction process, and the monitoring of this process requires a measurement technique with high dynamic response.

[0003] Laser absorption spectroscopy is a non-contact measurement method with fast response speed and high sensitivity, which is widely used in fields such as combustion diagnosis, atmospheric monitoring, and industrial sites. Accurately obtaining the absorption spectrum is the key to achieving accurate measurement based on laser absorption spectroscopy. The direct absorption spectroscopy (DAS) directly fits the ratio of the transmitted light intensity to the incident light intensity to obtain the absorption rate line shape function of the gas. The obtained absorption rate line shape function includes information such as the temperature, concentration, and pressure of the gas to be measured. However, the direct absorption method is easily affected by factors such as particulate matter concentration, laser intensity fluctuation, and spectral line overlap under high pressure, which will increase the measurement error. The direct absorption method is more suitable for the condition of strong absorption. In 2016, in the paper "Detection of gas temperature using a distributed feed-back laser at O2 absorption wavelength 760nm" published by Zhanrong Zhang et al. on page 673 of Volume 83 of the Journal of Optical Technology, the direct absorption method was used to measure the gas temperature in a tube furnace in the temperature range of 300 - 900K, and its temperature measurement accuracy at low temperature is better than that at high temperature. The direct absorption method has the advantages of being intuitive and simple and being able to extract the complete absorption spectrum, but it has weak anti-noise ability and is not suitable for industrial measurements in harsh environments. Adam J Reid et al. proposed the wavelength modulation method in the paper "Second-harmonic detection with tunable diode lasers—comparison of experiment and theory" published in the Journal of Applied Physics, Volume 26, page 203 in 1981. High-frequency modulation was performed on the narrowband laser scanning process, and the high-order harmonics of the high-frequency signal were used for the analysis and calculation of the gas absorption rate and the extraction of gas parameters. This method effectively reduces the background interference in the measurement system and improves the measurement accuracy of TDLAS. However, simply using the high-order harmonic signal for the calculation of gas parameters requires calibration of known gases, which increases the complexity of the system.

[0004] Although tunable diode laser absorption spectroscopy (TDLAS) is very suitable for combustion monitoring, as a measurement technique that utilizes the absorption effect of line-of-sight laser, when the concentration of combustion products in the combustion field is too high, or when the pressure environment in the burner changes, after the refractive index on the light path changes, the laser light no longer propagates in a straight line in an environment with different refractive indices. Due to the high-speed dynamic environment of the combustion field, this change in the refractive index field caused by changes in concentration and pressure leads to a change in the light propagation path, resulting in a change in the light intensity incident on the photodetector, which causes a decrease in the light intensity of non-absorptive nature. The measurement technique based on laser absorption spectroscopy itself also determines the concentration, temperature, pressure, etc. of gases according to the absorption of laser by gases. When the light intensity change caused by this non-absorptive effect is coupled into the light intensity change caused by the gas absorption effect, it will lead to the inability to accurately extract the absorption spectrum of the gas. The paper "In situ determination of molecular oxygen concentrations in full-scale fire-suppression tests using tunable diode laser absorption spectroscopy" by H. Eric Schlosser et al., published in the Proceedings of the Combustion Institute, Volume 29, Pages 353 - 360 in 2002, proposed a model for laser absorption spectroscopy in combustion applications. This model takes into account the spontaneous radiation of the flame that needs to be considered during flame monitoring, as well as the deflection of light caused by changes in the pressure and concentration of flame combustion products, which affects the light intensity change on the detector. In this model, the radiation noise of the flame is regarded as additive noise, and the noise caused by factors such as fiber deflection and particle scattering is regarded as multiplicative noise.The paper "Numerical investigation of hyperspectral tomography for simultaneous temperature and concentration imaging" published by Lin Ma et al. in the 21st issue of Volume 47, page 3751 of "Applied Optics" in 2007 added 0.01% noise when measuring the combustion field based on the TDLAS technology to represent the basic additive random noise, and used a 5.12% noise level to represent the multiplicative noise brought about by effects such as light deflection, indicating that in the test application of TDLAS, the noise from the light deflection effect is the main noise source.

[0005] Regarding the suppression and reduction of the light deflection effect, a large number of researchers have conducted extensive research. The noise term brought by light is directly regarded as a parameter to be solved and substituted into the laser absorption spectroscopy model, so as to use the collected data to uniformly fit the absorption spectrum model including the deflection effect. The paper "TDLAS-based in situ measurement of absolute acetylene concentrations in laminar 2D diffusion flames" by Steven Wagner et al. published in the Proceedings of the Combustion Institute, Volume 32, Pages 839 - 846 in 2009, during the process of measuring acetylene concentration using the direct absorption method, defined the light attenuation caused by light deflection and particle scattering as multiplicative noise on the absorption rate, and regarded it as random noise and substituted it into the fitting of the absorption spectrum line shape, resulting in a large error in the fitting result. The paper "In situ TDLAS measurement of absolute acetylene concentration profiles in a non-premixed laminar counter-flow flame" by S. Wagner et al. published in Applic Physic B, Volume 107, Pages 585 - 589 in 2012 discussed the error when the laser based on a Gaussian beam is deflected, reduced the influence brought by light deflection by increasing the beam diameter, and eliminated the influence through modeling the deviation and using non-linear fitting. In the paper "High-temperature field application of two-dimensional temperature measurement technology using CT tunable diode laser absorption spectroscopy" by T. Kamimoto et al. published in Flow Measurement and Instrumentation, Volume 46, Pages 51 - 57 in 2015, during the two-dimensional imaging monitoring of the moisture temperature and concentration changes in the ignition process of the engine using the TDLAS technology, the polynomial fitting technology was used to compensate for the light deflection effect.This method can alleviate the influence of the light deflection effect on the absorption spectrum solution to a certain extent, but it cannot solve the absorption spectrum recovery problem of the simultaneous weakening of light caused by the absorption effect and the deflection effect. Moreover, this method relies on the data volume of multiple scans, resulting in a decrease in the test frame rate and speed.

[0006] From an optical perspective, the solution of using a condenser lens to reduce the light deflection effect has also been widely studied. The use of a condenser lens to converge lasers from different directions and positions into a stable light spot, thereby avoiding the non-absorption error caused by the light deflection effect, has been widely studied and used. In the paper "Ultra-high-precision mid-IR spectrometer II: system description and spectroscopic performance" published by P. weibring et al. in Volume 85, pages 207-218 of "Applied Physics B" (Applic Physic B) in 2006, when testing with an ultrafast mid-IR laser, due to the influence of light deflection, the signal was misaligned. An achromatic lens similar to that used in a camera was used to achieve optical convergence, improving the resistance of the optical system to the influence of light jitter. In the paper "Tunable diode laser absorption sensor for the simultaneous measurement of water film thickness, liquid- and vapor-phase temperature" published by H.Yang. et al. in Volume 104, pages 21-27 of "Applied Physics B" (Applic Physic B) in 2011, during the test of water film thickness, when using a collimating mirror to align and receive light, due to the change in the film thickness, the light was deflected. A small light-collecting lens was added in front of the collimating mirror to reduce the influence of fiber deflection jitter on light intensity collection.In the paper "CO concentration and temperature measurements in a shock tube for Martian mixtures by coupling OES and TDLAS" published by X. Lin et al. in Volume 110, pages 401 - 409 of "Applied Physics B" in 2012, the spectrum of the spontaneous emission of the flame was combined with the test method of tunable laser absorption spectroscopy to measure the temperature and concentration of high-temperature and high-pressure carbon monoxide in the shock tube. There are two parts of optical alignment involved in this system. One is to collect and test the laser for laser absorption spectroscopy emitted by the laser, and the other is to collect the self-luminescence of the flame. Both methods use the lens scheme for collection. Among them, in the collection of the self-luminescence of the flame, the noise jitter in the complex environment can be well suppressed. However, in the directional laser required for the test of laser absorption spectroscopy, the directional laser signal collected by the lens is severely interfered. Through a series of noise reduction fitting algorithms, the recovery error of the absorption spectrum is about 1%. This lens scheme can, to a certain extent, reduce the direction deflection of the laser propagating along a straight line. However, due to the complexity of the test environment, it is difficult to effectively arrange the complex optical lens combination, and the space for further improving the accuracy is limited.

[0007] In addition to adding lenses, expanding the spot diameter of the test laser is also a common technique used by researchers to overcome the light deflection effect. The paper "Development of a fan-beam TDLAS-based tomographic sensor for rapid imaging of temperature and gas concentration" published by Chang Liu et al. in Optic Express in 2015 introduced a method using a fan-shaped laser beam to solve the light deflection effect in the transverse plane, but it still has not been effectively solved longitudinally. In the results of concentration and temperature, 100 sets of data were averaged to overcome the deviation effect caused by the light deflection effect. The paper "Herriott cell enhanced SMF-coupled multi-scalar combustion diagnostics in a rapid compression expansion machine by supercontinuum laser" published by Peter Fendt et al. in the 25th issue, volume 29 of Optic Express in 2021 analyzed that the light deflection effect is mainly caused by the uneven density distribution and refractive index distribution in the combustion field, and mentioned using a single-mode fiber to couple spatial light to eliminate the light jitter effect. However, this method will result in a smaller signal coupled into the fiber, causing an increase in signal noise and a larger uncertainty in the test. These techniques using large beams or fiber coupling methods both require sacrificing the energy concentration of the laser, resulting in a decrease in the light intensity signal that the detector can respond to and an increase in random noise in the signal. In actual operation, in order to reduce random noise, more test cycles are needed to improve accuracy.

[0008] In recent years, combined with wavelength modulation technology, some anti-deflection methods by modulating laser signals have also been developed. The paper "Demonstration of non-absorbing interference rejection using wavelength modulation spectroscopy in high-pressure shock tubes" by WeiWei et al., published in "Applic Physic B", Volume 125, Page 9 in 2019, suppressed the non-absorbing light intensity changes caused by the high-pressure environment in the laser tube and the easily contaminated optical observation window. The method of wavelength modulation absorption spectroscopy was adopted, and the results showed that the wavelength modulation absorption spectroscopy method had limited ability to suppress this light deflection. The paper "In-Cylinder Temperature Measurements in a Motored IC Engine using TDLAS" by Sebastian Burkle et al., published in "Flow Turbulence Combust", Volume 101, Pages 139 - 159 in 2018, found that during the monitoring of the combustion process in the engine cylinder using laser absorption spectroscopy, the turbulence at the combustion chamber outlet was severe. When using laser light to measure absorption, the severe airflow interference caused serious light deflection, which affected the recovery of the absorption spectrum. It was found in the study that this jitter phenomenon was related to the engine speed. Therefore, in its outlook, it was mentioned that the light deflection effect on the measurement of the absorption spectrum could be avoided by increasing the scanning speed. The paper "Simultaneous Detection of CO and CO2 at Elevated Temperatures Using Tunable Diode Laser Absorption Spectroscopy Near 1570nm" by Guangzhen Gao et al., published in "Optics and Spectroscopy", Volume 114, Pages 340 - 346 in 2013, adopted the test technology of wavelength modulation absorption spectrum and used the fundamental harmonic signal extracted from the absorption signal of wavelength modulation absorption spectroscopy to normalize the extracted second harmonic signal when testing the concentrations of carbon monoxide and carbon dioxide in a high-temperature environment using laser absorption spectroscopy, thus achieving immunity to the deflection effect.However, during the experiment, the temperature measurement was achieved through a static temperature gas cell, and an optical lens was also used at the outlet for restoration. Although the wavelength modulation method can limit the solution bandwidth of the absorption spectrum to a narrow band through modulation and demodulation calculations, when the frequency range of the multiplicative noise caused by this light deflection and jitter is close to the frequency range of the absorption spectrum, this narrow-band filtering method cannot separate the noise from the extraction process of the absorption spectrum.

[0009] A beam of non-absorbing reference light is used, which is collinear with the laser for laser absorption spectroscopy measurement. Then, the jitters of the two beams of light are separated through spectroscopic techniques, thereby compensating for the light intensity change caused by fiber deflection, which is also reflected in recent literature. Chinese Patent CN 107505063 A, "A Laser Ray Deflection Correction Device and Method Based on High-Frequency Sinusoidal Calibration Light" (Patent No.: 201710568850.5), introduces a laser ray deflection correction device and method based on high-frequency sinusoidal calibration light, including a visible light laser light source, an infrared tunable laser light source, a fiber optic coupler, a collimator, a photodetector, etc. The visible light laser light source and the infrared tunable laser light source are controlled to output visible light signals and infrared laser signals in the form of high-frequency sine waves and scanning sawtooth waves respectively. Among them, the laser in the visible light band emitted by the visible light laser is used as the calibration light, and the infrared band laser emitted by the infrared tunable laser is used as the detection light; the calibration light and the detection light are coupled through a fiber optic coupler and then incident on the measured temperature region after passing through the collimator. At the receiving end, a broadband photodetector is used to simultaneously detect the transmitted calibration light and the detection light signals; based on the above process, the characteristics of light ray deflection and laser absorption of the calibration light and the detection light signals are used to eliminate the influence of light ray deflection. This scheme requires the frequency of the high-frequency sine wave to be at least 100 times higher than the sawtooth wave scanning frequency to ensure the detailed description of the light ray deflection effect. Moreover, during the demodulation process, the number of points within a certain period of time needs to be measured. In addition, this method uses visible light as the reference light and requires a detector that can detect both visible light and infrared light at the same time. The large-bandwidth detector will be affected by the visible light in the environment during use, bringing a lot of noise to the measurement. Chinese Patent CN 107560738 A, "A Laser Ray Deflection Correction Device and Method Based on Dichroic Beam Splitting" (Patent No.: 201710567248.X), introduces a beam splitting scheme for visible light light source and infrared laser light source using a dichroic mirror. The infrared laser is split into two beams by a fiber optic beam splitter, one beam is used as a reference, and the other beam and the near-infrared laser are coupled through a fiber optic coupler and then emitted through the collimator. The emitted light passes through the measured flame and then irradiates on the dichroic beam splitting plate. The infrared laser detector is placed on the reflected light path, and the near-infrared laser detector is placed on the transmitted light path. The absorption and deflection relationships between the three laser signals are used to eliminate the influence of laser deflection. Although this method ensures that the propagation paths of the lasers before reaching the dichroic beam splitter are the same, the light paths after the dichroic mirror cannot be controlled.Chinese Patent CN 107560754 A, "A Laser Beam Deflection Correction Device and Method Based on a Modulated Sawtooth Wave Signal" (Patent No.: 201710564892.1), introduces a scheme for splitting a visible light source and an infrared laser source using a dichroic mirror. The visible light laser source and the infrared tunable laser source are controlled to emit visible light laser and infrared band laser in the form of a sawtooth wave scanning signal modulated by a high-frequency sine wave with the same amplitude and phase. The visible light laser serves as the calibration light, and the infrared band laser serves as the detection light. The calibration light and the detection light are coupled by an optical fiber coupler and collimated by a collimator and then incident on the temperature region to be measured. The receiving end uses a dichroic beam splitter to separate the calibration light and the detection light and respectively incident them on visible light and near-infrared photodetectors for detection. This technical solution can solve the problem of light deflection to a certain extent, but still requires solving it using the modulation signal within a certain period of time.

[0010] Aiming at the problems existing in the above laser absorption spectroscopy measurement technology, the present invention proposes a method and system for extracting laser absorption spectra to suppress multiplicative noise. By testing two modules, a laser generation module and a non-absorbing laser generation module, which respectively emit a narrowband swept laser with absorption for the gas to be measured and a sinusoidally modulated laser signal without absorption near the absorption spectrum band, the intensity change of the non-absorbing laser is used to record the non-absorbing light intensity change caused by the light deflection effect as the intensity calibration for the narrowband swept laser with absorption for the gas to be measured. During the synchronous demodulation of the intensities of the non-absorbing laser and the test laser signal, a local demodulation method is adopted. According to the jitter frequency range of the signal, a sufficient number of points are selected to demodulate the scanning signal and the sinusoidal modulation signal, realizing the anti-deflection extraction of the absorption spectrum. The structure of the present invention is simple and the cost is low, providing a new means for anti-deflection testing of laser absorption spectroscopy technology in combustion field testing. Summary of the Invention

[0011] (1) Technical problems to be solved

[0012] The purpose of the present invention is to provide a method for extracting absorption spectra resistant to optical deflection, and to build an online compensation system combined with an online demodulation method to solve the problem that the light deflection effect in the actual test environment affects the extraction of the gas absorption spectrum.

[0013] (2) Technical solutions

[0014] The main implementation process of the present invention, a method and system for extracting laser absorption spectra to suppress multiplicative noise, includes the following steps:

[0015] Step 1: The sawtooth wave modulated current generated by the 102 test laser generation module drives the tunable laser in this module to emit laser light that scans near the absorption spectral band of the gas to be measured. The 101 non-absorption laser generation module generates a high-frequency sine modulated current to drive the tunable laser in this module to emit a laser signal modulated in the spectral band near the 102 test laser generation module. Among them, the test laser emitted by the test laser generation module is divided into two beams by the 103 optical fiber splitter. One beam forms an interference signal through the 104 interferometer and records the wavelength according to the interference signal. The other beam and the non-absorption sine modulated laser emitted by the 101 non-absorption laser generation module are simultaneously input into the 105 variable gain fiber coupler to form a mixed laser signal, which can be expressed as:

[0016] I0(t) = G1·I S (t) + G2·I M (t) (1)

[0017] Among them, G1 and G2 are the gain adjustments of the variable gain fiber coupler for the two laser beams. I S is the test laser signal, and its relationship with time t can be expressed as follows:

[0018] I S (t) = A0 + A1t + A2t 2 + A3t 3 (2)

[0019] Among them, A0, A1, A2, and A3 respectively represent the terms of different orders caused by the non-linearity of the laser. In this modulation mode, the tunable laser controlled by the test laser generation module emits a central wavelength at the center of the absorption peak of the gas to be measured, which is λ1. The central emission wavelength of the tunable laser controlled by the non-absorption laser generation module is controlled at a non-absorption position near the absorption peak λ1 of the gas to be measured, which can be expressed as λ1 + Δλ. The gas to be measured has no absorption at the position of λ1 + Δλ. Among them, the change in the emission intensity I M (t) of the tunable laser driven by the sine current in the non-absorption laser generation module with time t can be expressed as follows:

[0020]

[0021] Among them, I H represents the amplitude of the sine drive, f H is the sine modulation frequency, is the initial phase of the sine modulation signal.

[0022] Step 2: The 105 variable gain fiber coupler transmits the mixed laser of the non-absorbing laser and the test laser into the 106 collimating mirror. The laser emits from the 106 collimating mirror, passes through the gas to be measured, and the laser signal is received by the 107 photodetector. Among them, the light intensity signal I t (t) after optical absorption and light deflection can be expressed as:

[0023] I t (t) = (G1·I S (t)·τ(t) + G2·I M (t))·P(t) (4)

[0024] Among them, τ(t) represents the gas absorption rate, and P(t) represents the light intensity change caused by light jitter.

[0025] Step 3: After the 107 photodetector converts the laser light intensity signal into a voltage signal, the 108 data acquisition module realizes the digital conversion of the analog signal at the digital sampling rate of f s , and transmits it to the 109 online compensation module for local recursive demodulation; the 201 local data extractor packs the i-th sampling point to the (i + M)-th sampling point transmitted from the 108 data acquisition module to the 109 online compensation module and sends it to the 203 configurable demodulator for online demodulation, where the value range of i is from 1 to T·f s -M; the 203 configurable digital demodulator in the 109 online compensation module can be expressed as R(k) according to the modulation waveform configuration form designed by the 202 core controller:

[0026]

[0027] Among them, the value range of k is from 1 to M; the 203 configurable digital demodulator performs the following calculation on the local data group Y i received from the 201 local data extractor and R:

[0028]

[0029] Calculate the laser intensity vector X i at the i-th sampling point:

[0030] X i = F i (M) -1 J i (M) (7)

[0031] The 204 absorption spectrum compensator calculates the test laser light intensity at the i-th sampling point according to the laser intensity vector X i as:

[0032]

[0033] After all the i points are traversed and calculated, the 205 data upload module uploads I c and the corresponding wavelength change signal v in terms of time from the 109 online compensation module to which the 205 data upload module belongs to the 110 computer for further calculation.

[0034] Step 4: The 110 computer selects the data points at the non-absorbing positions, fits the baseline I ref , and calculates the absorbance τ as:

[0035]

[0036] Corresponding τ and the wavelength change signal v recorded and resolved from the interferometer in terms of time, and the absorption spectrum α of the gas to be measured can be recovered.

[0037] (III) Beneficial effects

[0038] The beneficial effect of the present invention is to propose a method and system for extracting laser absorption spectrum to suppress multiplicative noise. Specifically, a test laser with an absorption band for the gas to be measured and a non-absorbing laser near this band are simultaneously emitted; the local demodulation method is used to synchronously demodulate the two mixed lasers, and the changes in the intensity of the non-absorbing laser and the intensity of the absorption laser are densely separated and extracted; the influence of the light ray deflection effect on the light intensity change is calibrated by the non-absorbing laser, so as to correct the intensity of the laser in the absorption band; since the spectral bands of the two lasers are similar, in the actual operation process, the differences in the transmission loss and the responsivity of the optical fiber and the photodetector to the two lasers can be ignored, improving the calibration accuracy of the absorption laser with respect to the light ray deflection effect by the non-absorbing laser; the local recursive demodulation method is used to separate the intensity of the non-absorbing laser and the intensity of the absorption laser, and the range of local demodulation can be selected according to the actual noise situation, improving the extraction density of the absorption laser intensity points and the resolution of the absorption spectrum. The invention has a simple structure, a practical and feasible scheme, an accurate and reliable method, and provides a reliable new scheme for obtaining the absorption spectrum resistant to light ray deflection in the combustion field. Description of the drawings

[0039] Appendix Figure 1 : A system diagram for extracting anti-optical deflection absorption spectrum by local demodulation

[0040] Appendix Figure 2 : Structure diagram of the online compensation module

[0041] Appendix Figure 3 : Mixed signal of the test laser and the non-absorbing laser

[0042] Appendix Figure 4 : Light intensity change coupling the light ray deflection effect and the absorption effect

[0043] Appendix Figure 5: Compensated absorption spectrum Detailed implementation method

[0044] Reference appendix Figure 1 It is a system diagram for extracting a locally demodulated anti-optical deflection absorption spectrum. Refer to the appendix Figure 2 It is a structural diagram of the online compensation module. The operation steps in combination with an example are as follows:

[0045] Step 1: The sawtooth wave modulation current generated by the 102 test laser generation module drives the tunable laser in this module to emit laser light that scans near the absorption spectral band of gas water molecules at 7185.58 cm -1 The high-frequency sine modulation current generated by the 101 non-absorption laser generation module drives the tunable laser in this module to emit non-absorption sine modulation laser light near 7186.0 cm -1 Among them, the test laser emitted by the test laser generation module is divided into two beams by the 103 optical fiber splitter. One beam forms an interference signal through the 104 interferometer and records the wavelength according to the interference signal. The other beam and the non-absorption sine modulation laser light emitted by the 101 non-absorption laser generation module are simultaneously input into the 105 variable gain fiber coupler to form a mixed laser signal, which can be expressed as:

[0046] I0(t) = G1·I S (t) + G2·I M (t) (1)

[0047] Among them, G1 and G2 are the gain adjustments of the 105 variable gain fiber coupler for the two beams of laser light, which are respectively set to 1. I S is the test laser signal. The relationship between the current driving its emission and the time t within a scanning period T = 1 ms can be expressed as follows:

[0048] i S (t) = a0 + a1t (2)

[0049] Among them, a0 = 5 mA, a1 = 40 mA / ms; the intensity change I of the tunable laser driven by the sine current of the non-absorption laser generation module M , the relationship between the current driving its emission and the time t within a scanning period T = 1 ms can be expressed as follows:

[0050]

[0051] Among them, i H represents that the amplitude of the sine drive is set to 5 mA, f H is the sine modulation frequency set to 500 kHz, is the initial phase of the sine modulation signal set to 0; a mixed laser signal is formed as shown in the appendixFigure 3 As shown. Step 2: The 105 variable gain fiber optic coupler transmits the mixed laser of the non-absorbing laser and the test laser into the 106 collimating mirror. The laser emits from the 106 collimating mirror, passes through the gas to be measured, and the laser signal is received by the 107 photodetector. Among them, the light intensity signal I t (t) can be expressed as:

[0052] I t (t) = (G1·I S (t)·τ(t) + G2·I M (t))·P(t) (4)

[0053] Among them, τ(t) represents the gas absorption rate, and P(t) represents the change in light intensity caused by light jitter. Let G1 = 1 and G2 = 1 as shown in the appendix Figure 4 as shown.

[0054] Step 3: After the 107 photodetector converts the laser light intensity signal into a voltage signal, the 108 data acquisition module realizes the digital conversion of the analog signal at a digital sampling rate of 40 MHz and transmits it to the 109 online compensation module for local recursive demodulation; the 201 local data extractor packs the i-th sampling point to the i + 160-th sampling point transmitted by the 108 data acquisition module to the 109 online compensation module and sends it to the 203 configurable demodulator for online demodulation, where the value of i is an arithmetic sequence with a difference of 80 between 1 and 39840; the 203 configurable digital demodulator in the 109 online compensation module can be expressed as R(k) according to the modulation waveform configuration form designed by the 202 core controller:

[0055]

[0056] The 203 configurable digital demodulator makes the following calculation on the local data group Y i received from the 201 local data extractor and R:

[0057]

[0058] Calculate the laser intensity vector X i at the i-th sampling point:

[0059] X i = F i (M) -1 J i (M) (7)

[0060] The 204 absorption spectrum compensator calculates the test laser light intensity at the i-th sampling point according to the laser intensity vector X i as:

[0061]

[0062] After all sampling points are traversed and calculated, the 205 data upload module uploads I c and the wavelength change signal v corresponding to it in time from the 109 online compensation module to which the 205 data upload module belongs to the 110 computer for further calculation.

[0063] Step Four: The 110 computer selects the data points at the non-absorbing positions, fits the baseline I ref , and calculates the absorbance τ as:

[0064]

[0065] Corresponding τ and the wavelength change signal v recorded and resolved from the interferometer in time, and the absorption spectrum α of the gas to be measured that can be restored is as shown in the appendix Figure 5 .

[0066] The above description of the present invention and its implementation manners is not limited thereto. What is shown in the drawings is only one of the implementation manners of the present invention. Without departing from the purpose of the present invention, any structure or embodiment designed without creativity similar to this technical solution belongs to the protection scope of the present invention.

Claims

1. A laser absorption spectroscopy extraction system for suppressing multiplicative noise, the system composition includes a test laser generation module, a non-absorbing laser generation module, an optical fiber coupler, an interferometer, a photodetector, a data acquisition module and an online compensation module; the test laser generation module emits a test laser signal including the absorption spectral band of the gas to be measured in a sawtooth wave modulation mode. After the test laser generated by the test laser generation module is split by a beam splitter, one beam of the test laser signal has its wavelength recorded by the interferometer, and the other beam of the test laser signal is combined with the sinusoidally modulated laser generated by the non-absorbing laser generation module through the optical fiber coupler and then emitted by a collimating mirror, passes through the gas to be measured and is detected by the photodetector. After being collected by the data acquisition module, the online compensation module uses the method of local recursive demodulation to separate the test laser from the non-absorbing laser, and uses the intensity change of the non-absorbing laser to compensate for the intensity change brought to the test laser by the light deflection effect, so as to achieve accurate absorption spectrum extraction; The test laser generation module uses a tunable near-infrared laser, and a sawtooth current is used to modulate the tunable laser to emit a scanning signal near the absorption spectrum of the gas to be measured; the non-absorbing laser generation module used uses a tunable near-infrared laser with a central wavelength band similar to that of the near-infrared laser selected for the test laser generation module. In the non-absorbing region near the central wavelength band of the test laser emitted by the test laser generation module, a high-frequency sine current is used to drive the tunable laser to emit sinusoidally modulated infrared laser; after the two laser beams are combined by a variable-gain fiber coupler, they are emitted by a collimating mirror; among them, The luminous intensity change I of the tunable laser driven by the sawtooth wave current adopted by the test laser generation module S (t) varying with time t within a scanning period T can be expressed as follows: I S y(t) = A0 + A1t + A2t 2 + A3t 3 (1) Among them, A0, A1, A2, and A3 respectively represent terms of different orders caused by the nonlinearity of the laser; in this modulation mode, the tunable laser controlled by the test laser generation module emits a center wavelength at the center of the absorption peak of the gas to be measured, denoted as λ1; the center emission wavelength of the tunable laser controlled by the non-absorption laser generation module is controlled at a non-absorption position near the absorption peak λ1 of the gas to be measured, which can be expressed as λ1 + Δλ, and there is no absorption of the gas to be measured at the position of λ1 + Δλ; among them, the light intensity change I M (t) varying with time t can be expressed as follows: Among them, I H represents the amplitude of the sine drive, and f H is the sine modulation frequency, is the initial phase of the sine modulation signal; after the test laser generated by the test laser generation module is split by a beam splitter, one test laser signal has its wavelength recorded by an interferometer, and the other test laser signal is combined with the laser modulated by the non-absorption laser generation module through a fiber optic coupler with variable gain and then emitted by a collimating mirror. The laser intensity emitted by the collimating mirror can be expressed as: I0(t) = G1I S (t) + G2I M (t) (3) wherein, G1 and G2 are variable-gain fiber couplers for adjusting the gain of the two laser beams; The online compensation module uses the method of local recursive demodulation to separate the test laser from the non-absorbing laser, and uses the intensity change of the non-absorbing laser to compensate for the intensity change brought to the test laser by the light deflection effect, so as to achieve accurate absorption spectrum extraction; wherein, the light intensity signals after optical absorption and light deflection can be expressed as: I t (t) = ((G1·I S (t)·τ(t)) + G2·I M (t))·P(t) (4) wherein, τ(t) represents the gas absorption rate, and P(t) represents the light intensity change caused by light jitter; The method of local recursive demodulation involved in the online compensation module is as follows: The analog-to-digital converter performs digital conversion on the analog signal at a digital sampling rate of f s The local data extractor packs the i-th sampling point to the (i + M)-th sampling point and sends it to the configurable demodulator for online demodulation, where the value range of i is from 1 to T·f s - M; The configurable digital demodulator in the online demodulation module can be expressed as R(k) according to the modulation waveform configuration form designed by the core controller: wherein, the value range of k is from 1 to M; the configurable digital demodulator performs the following calculation on the local data group Y received from the local data extractor i and R: Calculate the laser intensity vector X at the i-th sampling point i : X i = F i (M) -1 J i (M) (7) The absorption spectrum compensator is based on the laser intensity vector X i The measured laser light intensity at the i-th sampling point is calculated as follows: After all sampling points are traversed and calculated, select the data points at the non-absorbing positions and fit the baseline I ref , and calculate the absorbance τ as follows: By corresponding τ to the wavelength change signal v recorded and calculated from the interferometer according to time, the absorption spectrum α of the gas to be measured can be restored.

Images