Tunable laser industrial waste gas online monitoring device and method based on reference compensation

Abstract

The present invention relates to a tunable laser exhaust gas online monitoring device based on the reference compensation principle, comprising a light source part, a reference path part having a Mach-Zehnder interferometer and a photoelectric detector, and a detection part having a photoelectric detector and a digital phase-locked amplifier; the light emitted by the light source part is divided into two paths, which enter the reference path and the detection part respectively; the detection part comprises a photoelectric detector and a phase-locked amplification signal processing unit, the photoelectric detector converts the optical signal passing through the exhaust gas into an electrical signal, and converts it into a digital signal through A / D; the digital signal is processed by the phase-locked amplification signal processing unit to obtain a gas absorption spectrum with a high signal-to-noise ratio. The present invention also provides a monitoring method implemented by using such a device.

Description

Technical Field

[0001] The invention relates to a device and method for measuring tunable laser absorption spectrum which can realize reference compensation and is applied to industrial waste gas detection. Background Art

[0002] At present, my country is still in a high-incidence stage of pollution incidents, especially air pollution incidents, which are the most frequent and the most serious. Online monitoring and analysis instruments for air pollution sources are further strengthened to improve monitoring efforts and reduce pollution incidents. Industrial waste gas is an important factor causing air pollution. Various waste gases are generated during chemical reactions in industrial processes such as energy, electricity, chemical industry, metallurgy, textiles, pharmaceuticals, and waste incineration. These waste gases contain a large amount of substances that are harmful to the human body, and their emissions must be strictly controlled.

[0003] Tunable laser absorption spectroscopy has the advantages of high measurement accuracy, wide application range, and fast response speed. It has good application prospects in the field of online monitoring of polluted gases, especially industrial waste gas. Its single-line spectral analysis technology has many unique advantages, which can eliminate background gas interference and improve detection sensitivity. With the continuous improvement of environmental protection and industrial waste gas emission requirements, tunable laser absorption spectroscopy is increasingly used in the field of trace gas detection, and its detection limit can reach the ppb level, eliminating the complex sampling and pretreatment system, with a simple structure and reduced maintenance. It is more representative than extractive sampling, has a faster response speed, and is more in line with the needs of industrial process automation control.

[0004] The key to tunable laser absorption spectroscopy technology lies in the laser tuning accuracy and the absorption spectrum line measurement accuracy. In applications, the spectral characteristics such as the continuously adjustable range and tuning rate, line width, power, and side mode suppression ratio of the laser optical device used in the dynamic tuning process of tunable laser absorption spectroscopy have a very large impact on the performance of the overall system. In tunable laser absorption spectroscopy, the location of the extreme point of the absorption peak and the use of line shape to detect temperature and pressure are all affected by the instantaneous wavelength, and the instantaneous line width directly affects the line shape and amplitude of the second harmonic signal. During the tuning process, the change in laser power will also have a certain impact on the intensity of the absorption spectrum line. Therefore, in the process of processing the second harmonic of the absorption spectrum line, it is also necessary to consider the laser power change and background subtraction to improve the measurement accuracy of the absorption spectrum.

[0005] In recent years, the detection technology of laser transient tuning characteristics has been increasingly widely used. T. Okoshi et al. first proposed the dual-beam heterodyne method to measure the steady-state linewidth of the laser, with a resolution of up to 50kHz. Jay W. Daeson improved the delayed self-heterodyne interference method in 2000, further improving the measurement resolution. S. Camatel compared the commonly used linewidth measurement methods and pointed out the influence of long fiber delay lines on linewidth measurement; Ju Youlun measured the short-term instability of the output laser frequency of a 2μm single longitudinal mode laser; Jia Yudong et al. analyzed and corrected the error of the delayed self-heterodyne method in measuring the laser linewidth. With the expansion of laser application occasions, the requirements for the measurement accuracy of its transient output characteristic parameters are becoming higher and higher. In the research on the measurement of semiconductor transient output characteristic parameters based on heterodyne coherent detection method, heterodyne coherent detection system and signal processing analysis method are the key to ensure measurement accuracy and reliability. Cao Chunyan et al. used an unbalanced fiber interferometer and frequency noise power spectrum to measure the linewidth of a narrow linewidth laser; Ma Mingxiang et al. conducted a detection study on the mode hopping of an ultra-narrow linewidth laser; Peng Xuefeng et al. analyzed the influence of the beat frequency line shape of two independent lasers on the linewidth measurement.

[0006] References:

[0007] (1) Okoshi T, Kikuchi K, Nakayama A. Novel method for high resolution measurement of laser output spectrum [J]. Electronics letters, 1980, 16(16): 630-631.

[0008] (2)Dawson JW, Park N, Vahala K J. An improved delayed self-heterodyne interferometer for linewidth measurements [J]. IEEE Photonics Technology Letters, 1992, 4(9): 1063-1066.

[0009] (3) Camatel S, Ferrero V. Narrow linewidth CW laser phase noise characterization methods for coherent transmission system applications [J]. Journal of Lightwave Technology, 2008, 26(17): 3048-3055.

[0010] (4) Ju Youlun, Wang Zhenguo, Wang Lei, et al. Measurement of short-term frequency instability of 2μm single longitudinal mode laser[J]. Acta Optica Sinica, 2008, 28(11).

[0011] (5) Jia Yudong, Ou Pan, Yang Yuanhong, et al. Short fiber delay self-heterodyne method for measuring narrow linewidth laser linewidth[J]. Journal of Beijing University of Aeronautics and Astronautics, 2008(05):85-88.

[0012] (6) Cao Chunyan, Yao Qiong, Rao Wei, et al. Unbalanced fiber interferometer measurement of narrow linewidth laser linewidth[J]. Chinese Journal of Lasers, 2011, 38(5).

[0013] (7) Ma Mingxiang. Study on mode hopping of ultra-narrow linewidth fiber ring cavity laser[D]. National University of Defense Technology, 2010.

[0014] (8) Peng Xuefeng, Ma Xiurong, Zhang Shuanggen, et al. Effect of the beat frequency profile of two independent lasers on linewidth measurement[J]. Chinese Journal of Lasers, 2011, 38(4). Summary of the invention

[0015] The present invention mainly aims at the deficiencies of the prior art and provides a tunable laser absorption spectroscopy gas online measurement device and method based on reference compensation. The device and method of the present invention can improve the dynamic tuning accuracy of the tunable laser absorption spectroscopy and the gas measurement accuracy and reliability. The technical solution is as follows:

[0016] A tunable laser exhaust gas online monitoring device based on the reference compensation principle includes a light source part, a reference path part having a Mach-Zehnder interferometer and a photodetector, and a detection part having a photodetector and a digital lock-in amplifier, wherein:

[0017] The light source part includes a tunable laser, a temperature control and feedback module and a current tuning signal module; the temperature control and feedback module includes the temperature control inside the laser and the temperature control part of the external semiconductor; the light source part emits light in two paths, which enter the reference path and the detection part respectively;

[0018] The reference path includes a Mach-Zehnder interferometer. The light emitted by the light source enters the Mach-Zehnder interferometer in the reference path and is divided into two paths by a fiber beam splitter. One path generates a certain optical path difference with the other path after passing through a fiber delay line. The two paths of light enter the fiber beam combiner at the same time and are converted into electrical signals by a photoelectric detector in the reference path, thereby obtaining a laser coherent heterodyne signal.

[0019] The detection part includes a photoelectric detector and a phase-locked amplification signal processing unit. The photoelectric detector converts the optical signal passing through the exhaust gas into an electrical signal, and then converts it into a digital signal through A / D. The digital signal is processed by the phase-locked amplification signal processing unit to obtain a gas absorption spectrum with a high signal-to-noise ratio.

[0020] The reference measurement is realized based on the laser heterodyne coherent signal obtained by the reference path. On the one hand, the transient tuning characteristics of the laser are obtained to obtain the transient frequency and phase of the laser. The transient frequency and phase of the laser are used as negative feedback input parameters to adjust the current tuning coefficient of the current tuning signal module to improve the tuning accuracy of the tunable laser; on the other hand, it is used to resample the gas absorption spectrum, thereby correcting the gas absorption spectrum.

[0021] Furthermore, the external temperature control device includes a semiconductor refrigeration device. The external temperature control device may also include a controller, through which the temperature is preset, and the controller implements closed-loop control of the semiconductor refrigeration device according to the temperature difference. The tunable laser is a semiconductor laser.

[0022] The current tuning signal module is used to tune the output wavelength of the tunable laser.

[0023] The phase-locked amplification signal processing unit includes an anti-aliasing filter, a high-pass filter, a waveform shaper, a phase shifter, a multiplier, a phase-sensitive detector and a low-pass filter. The digital signal is divided into two paths, one path passes through the anti-aliasing filter and the high-pass filter, and the other path passes through the waveform shaper and the phase shifter; the two processed digital signals are input into the multiplier, and the multiplication result is subjected to phase-sensitive detection to identify the same frequency and same phase components of the two signals; and then the low-pass filter is used to filter out noise to obtain a gas absorption spectrum with a high signal-to-noise ratio.

[0024] The present invention also provides a method for online monitoring of tunable laser exhaust gas using the device, which comprises the following steps:

[0025] (1) Using the detection part to obtain a gas absorption spectrum with a high signal-to-noise ratio;

[0026] (2) Obtaining a laser heterodyne coherent signal through a reference path;

[0027] (3) using the laser heterodyne coherent signal to obtain the transient tuning characteristics of the laser, the method is as follows: calculating the autocorrelation matrix of the laser heterodyne coherent signal, and then performing characteristic matrix decomposition on the autocorrelation matrix to obtain signals representing different frequency and phase components, and convolving them with the laser heterodyne coherent signal to obtain the transient frequency and phase of the laser;

[0028] (4) Using the laser transient frequency and phase as negative feedback input parameters to adjust the current tuning coefficient of the current tuning signal module to improve the tuning accuracy of the tunable laser;

[0029] (5) The laser heterodyne coherent signal is processed to obtain a resampled signal for correcting the gas absorption spectrum. The processing process is as follows: first, the laser heterodyne coherent signal is processed. The instantaneous phase is determined directly by the instantaneous relative amplitude of the cosine signal in the odd half-cycle. In the even half-cycle, the signal is flipped and its inverse cosine function is used to determine the instantaneous phase. On this basis, the initial phase of each half-cycle is superimposed to obtain the instantaneous phase value after unwrapping. The heterodyne coherent signal obtained in the entire tuning cycle is processed and then sliding averaged to obtain a square wave signal, which is used to correct the gas absorption spectrum.

[0030] The process of correcting the gas absorption spectrum using the square wave signal is as follows: align the gas absorption spectrum with the square wave signal according to the number of sampling points, select the zero crossing point of the square wave, which corresponds to the wavelength λ1, λ2…λ scanned during the laser tuning process n According to the sampling point corresponding to the zero-crossing position of the square wave, the corresponding sampling point position in the gas absorption spectrum is extracted to obtain the wavelengths λ1, λ2…λ in the gas absorption spectrum. n The corresponding absorption intensity.

[0031] The present invention adopts the above technical solution, which has the following advantages:

[0032] 1. In the present invention, a reference measurement method is used to obtain a heterodyne coherent signal of a tunable laser, and a time-frequency analysis method is used to obtain a transient tuning characteristic of the tunable laser, which is used as a feedback input parameter to control the laser tuning parameters, thereby improving the tuning accuracy of the tunable laser.

[0033] 2. In the present invention, a tuning method combining temperature and current is used to control the output wavelength of the tunable laser, thereby improving the tuning range and accuracy of the tunable laser. The temperature tuning adopts a second-order temperature control tuning method, thereby improving the temperature stability and tuning speed.

[0034] 3. In the present invention, the heterodyne coherent signal obtained by the reference path is subjected to phase extraction processing to obtain a square wave signal whose frequency is linearly related to the output wavelength of the tunable laser. The gas absorption spectrum is corrected using the square wave signal, thereby improving the measurement accuracy of the absorption spectrum wavelength.

[0035] 4. In the present invention, the SG curve fitting method is used to extract the first harmonic component of the gas absorption spectrum, and it is used as a reference to realize gas absorption spectrum correction, thereby improving the measurement accuracy of the gas absorption spectrum intensity. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Schematic diagram of the structure of the tunable laser industrial waste gas online monitoring device based on reference compensation

[0037] Figure 2 Schematic diagram of the light source

[0038] Figure 3 Schematic diagram of reference road composition

[0039] Figure 4 Flowchart for calculating dynamic tuning parameters of a laser based on heterodyne coherence signals

[0040] Figure 5 Schematic diagram of the detection principle and composition

[0041] Figure 6 Flowchart of the resampling and spectral wavelength correction method based on heterodyne coherent signals

[0042] Figure 7 Flow chart of the background subtraction method based on the first harmonic signal component

[0043] Figure 8 The results of tunable laser absorption spectroscopy measurement of different concentrations of ammonia in industrial waste gas

[0044] Fig. 9 Schematic diagram of correcting gas absorption spectrum using square wave signal DETAILED DESCRIPTION

[0045] The tunable laser gas online measurement device based on reference compensation of the present invention is described in conjunction with the accompanying drawings and embodiments.

[0046] According to the characteristics of tunable lasers, the present invention develops a laser tuning device and method based on second-order temperature tuning and current tuning, develops a reference measurement device based on the heterodyne coherence principle, utilizes a time-frequency analysis method to extract the transient tuning characteristics of the tunable laser, designs a dynamic tuning compensation method, designs an absorption spectrum resampling correction method based on heterodyne coherent signals, and designs a tunable laser absorption spectrum first harmonic extraction and absorption spectrum correction method based on SG fitting.

[0047] The tunable laser spectrum measuring device system based on the reference compensation principle of the present invention is as follows Figure 1 As shown, it mainly includes a light source part, a reference path part with a Mach-Zehnder interferometer and a photodetector, and a detection part with a photodetector and a digital lock-in amplifier, wherein:

[0048] The light source part includes a tunable laser, a temperature control and feedback module, and a current tuning signal module; the temperature control and feedback module includes the temperature control inside the laser and the external semiconductor temperature control part, wherein the external semiconductor temperature control part includes the use of an external temperature control device (mainly including a semiconductor refrigeration device), a temperature sensor, and a temperature acquisition and feedback circuit to set the control temperature of the semiconductor refrigeration device, and the temperature sensor is used to collect the temperature of the tunable laser. The temperature acquisition and feedback circuit compares the temperature difference between the tunable laser temperature and the set temperature, and realizes closed-loop control of the semiconductor refrigeration device according to the temperature difference; the current tuning signal module uses digital signal frequency synthesis technology to inject a certain frequency of sawtooth wave current into the semiconductor laser so that the tunable laser output wavelength scans the absorption spectrum of the gas to be measured. The light source part emits light in two ways, which enter the reference path and the detection part respectively.

[0049] In this embodiment, the circuit part of the light source is implemented by analog circuit. A better implementation method is to add a control chip to control the working state of the semiconductor cooling device, thereby providing a stable temperature environment for the tunable laser.

[0050] The main component of the reference path is the Mach-Zehnder interferometer. The light emitted by the light source enters the Mach-Zehnder interferometer in the reference path and is divided into two paths through the fiber beam splitter. After one path passes through the fiber delay line, a certain optical path difference is generated with the other path. After the two paths of light enter the fiber beam combiner at the same time, they are converted into electrical signals through the photoelectric detector of the reference path to obtain laser coherent heterodyne signals.

[0051] The detection part includes a photoelectric detector and a phase-locked amplifier signal processing unit. The photoelectric detector converts the optical signal passing through the exhaust gas into an electrical signal, and converts it into a digital signal through A / D. The digital signal is processed by the phase-locked amplifier signal processing unit. The phase-locked amplifier signal processing unit includes an anti-aliasing filter, a high-pass filter, a waveform shaper, a phase shifter, a multiplier, a phase-sensitive detector and a low-pass filter. The digital signal is divided into two paths, one path passes through the anti-aliasing filter and the high-pass filter, and the other path passes through the waveform shaper and the phase shifter. The two processed digital signals are input into the multiplier, and the multiplication result is subjected to phase-sensitive detection to identify the same frequency and same phase components of the two signals. Then the low-pass filter is used to filter out the noise to obtain a gas absorption spectrum with a high signal-to-noise ratio.

[0052] The realization of reference measurement is based on the processing of laser heterodyne coherent signals obtained by the reference path. On the one hand, it can obtain the transient tuning characteristics of the laser, and on the other hand, it can be used to resample the gas absorption spectrum.

[0053] The method of obtaining the transient tuning characteristics of the laser using the laser heterodyne coherent signal is to calculate the autocorrelation matrix of the laser heterodyne coherent signal, then perform characteristic matrix decomposition on the autocorrelation matrix to obtain signals representing different frequency and phase components, and convolve them with the laser heterodyne coherent signal to obtain the transient frequency and phase of the laser. The transient frequency and phase of the laser are used as negative feedback input parameters to adjust the current tuning coefficient of the current tuning signal module to improve the tuning accuracy of the tunable laser.

[0054] The reference laser heterodyne coherent signal can be processed to obtain a resampled signal for correcting the gas absorption spectrum. The processing process is to first process the laser heterodyne coherent signal, and use the odd half-cycle to directly determine the instantaneous phase through the instantaneous relative amplitude of the cosine signal. In the even half-cycle, the signal is flipped and then its arccosine function is used to determine the instantaneous phase. On this basis, the initial phase of each half-cycle is superimposed to obtain the instantaneous phase value after unwrapping. The heterodyne coherent signal obtained in the entire tuning cycle is processed and then sliding averaged to obtain a square wave signal. The square wave signal can be used to correct the gas absorption spectrum. The specific process is as follows: Fig. 9 As shown: the gas absorption spectrum measured by the detection unit is aligned with the square wave signal according to the number of sampling points, and the zero crossing point of the square wave is selected, which corresponds to the wavelengths λ1, λ2…λ scanned during the laser tuning process. n According to the sampling point corresponding to the zero-crossing position of the square wave, the corresponding sampling point position in the gas absorption spectrum is extracted to obtain the wavelengths λ1, λ2…λ in the gas absorption spectrum. n The corresponding absorption intensity.

[0055] The operation process can be divided into the following steps:

[0056] (1) Tunable laser operation, where the light source structure is shown in the figure below. Figure 2 As shown, the laser is a tunable narrow linewidth laser, and the laser output wavelength and power can be tuned by changing the temperature and current.

[0057] The tunable laser parameters are set according to the measured gas, wherein the temperature tuning adopts the second-order temperature control tuning method, which is divided into two parts: external semiconductor temperature control and laser internal temperature control. The external temperature control working process is to set the temperature of the external temperature control device (the external temperature control device selected in this embodiment is a semiconductor refrigeration device), the temperature sensor collects the external temperature control temperature, the temperature collection and feedback circuit compares the temperature difference between the tunable laser temperature and the set temperature, and realizes closed-loop control through the semiconductor refrigeration device. When the external temperature control is stable, the temperature is scanned by the internal temperature controller of the laser to realize the second-order temperature tuning. The current tuning signal module adopts the digital signal frequency synthesis technology to inject a sawtooth current of a certain frequency into the semiconductor laser so that the output wavelength of the tunable laser can scan the absorption spectrum of the gas to be measured.

[0058] The advantages of second-order temperature tuning are that external control ensures the stability of the laser operating temperature, reduces the tuning temperature difference, and the internal temperature control of the laser responds quickly. At the same time, based on the reference measurement results, the negative feedback method is used to improve the temperature tuning accuracy.

[0059] The current tuning signal is composed of a low-frequency sawtooth signal with a certain bias and a high-frequency square wave signal. The amplitude of the square wave signal is modulated by the sawtooth signal, thereby obtaining a quasi-continuous wavelength modulation drive signal. Among them, the low-frequency sawtooth current signal realizes the tuning of the laser output wavelength and makes the tuning range cover a complete gas absorption line; the quasi-continuous current signal (here is a square wave) realizes the quasi-continuous drive of the laser, reduces the self-heating effect of the laser, and improves the laser tuning accuracy.

[0060] (2) Acquisition and calculation of reference compensation signal, the reference circuit structure is as follows Figure 3 As shown, the Mach-Zehnder interferometer and the ultra-short time delay method are used to obtain heterodyne coherent signals with different optical path differences. The incident light is divided into two paths by the fiber beam splitter. After one path passes through the fiber delay line, a certain optical path difference is generated with the other path. After the two paths of light enter the fiber combiner at the same time, a coherent heterodyne signal is generated, and after passing through the photoelectric detector, it is converted into an electrical signal for further processing.

[0061] During the dynamic tuning of the laser, the signal generated after heterodyne coherence is a non-stationary signal in terms of time-frequency distribution. Its autocorrelation function and its Fourier transform are both time-varying signals. Time-frequency analysis is used to map the signal to a two-dimensional plane of time and frequency in order to reveal the frequency characteristics contained in the signal and its changes over time. The signal time-frequency decomposition algorithm in the invention is as follows: Figure 4 .

[0062] The specific steps are:

[0063] The laser heterodyne coherent signal can be expressed as

[0064]

[0065] Where I(t) is the output of the laser at time t, f i is the instantaneous wavelength of the laser output, t is the time, Tunable laser instantaneous frequency

[0066] After the characteristic matrix is ​​decomposed, the signal decomposition of different frequencies and phases can be obtained. First, the autocorrelation function of the heterodyne coherent signal is calculated.

[0067] R c =I*I′ (2)

[0068] Get the autocorrelation matrix R c , and then perform characteristic matrix decomposition on Rc to obtain the following matrix:

[0069] Rc=H*V*H T (3)

[0070] Where H and V represent different frequency and phase components respectively. Convolution operation is performed on H and V with I(t) to obtain frequency f and phase

[0071] f=H*I(t) (4)

[0072]

[0073] Furthermore, the obtained laser transient characteristic references including instantaneous wavelength and instantaneous phase can be used as negative feedback input parameters to adjust the current tuning coefficient and the temperature tuning coefficient to improve the laser tuning accuracy.

[0074] (3) The detection part adopts a digital phase-locked method to realize gas absorption spectrum signal detection. The core device includes a photoelectric detector and a phase-locked amplification signal processing unit. The phase-locked amplification adopts a digital signal processing method, and the processing steps are: first, the photoelectric detector converts the optical signal into an electrical signal, and then converts it into a digital signal through A / D; then the signal is divided into two paths, one for anti-aliasing filtering and high-pass filtering, and the other for waveform shaping and phase shifting; then, the two signals are input into the multiplier, and the multiplication result is subjected to phase-sensitive detection to identify the same frequency and same phase components of the two signals; finally, a narrowband low-pass filter is used to filter out noise to obtain an amplified signal with a high signal-to-noise ratio. The digital phase-locked method in the present invention reduces the volume of the analog system, improves the system integration and stability, simplifies the system hardware circuit, and reduces the volume of the instrument.

[0075] (4) Use heterodyne coherent signals to resample the gas absorption spectrum to achieve wavelength correction of the absorption spectrum. The resampling process is to first process the laser heterodyne coherent signal, and use the odd half-cycle to directly determine the instantaneous phase through the instantaneous relative amplitude of the cosine signal. In the even half-cycle, the signal is flipped and its inverse cosine function is used to determine the instantaneous phase. On this basis, the initial phase of each half-cycle is superimposed to obtain the instantaneous phase value after unwrapping. According to the above algorithm, the heterodyne coherent signal obtained in the entire tuning cycle is processed and then sliding averaged to obtain a square wave signal, where the frequency of the square wave signal is linearly related to the wavelength. The square wave signal can be used to correct the measured gas absorption spectrum, thereby improving the wavelength accuracy.

[0076] (5) The SG curve fitting method is used to extract the first harmonic component of the gas absorption spectrum and use it as a reference to achieve gas absorption spectrum correction. The method flow is as follows: Figure 7 As shown:

[0077] The SG curve fitting formula can be expressed as

[0078] y k+j =a0+a1j+a2j 2 +…+a p j p (6)

[0079] where a p is the fitting coefficient, j is the absorption spectrum value at different wavelengths, and the equally spaced data points in a window with a width of 2p+1 are fitted to a p-order polynomial through the SG polynomial smoothing method to obtain the first harmonic component.

[0080] The gas measurement results obtained after processing are as follows Figure 8 shown.

Claims

1. A tunable laser exhaust gas online monitoring method based on the reference compensation principle, the monitoring device used includes a light source part, a reference path part with a Mach-Zehnder interferometer and a photodetector, and a detection part with a photodetector and a digital lock-in amplifier, wherein: The light source part includes a tunable laser, a temperature control and feedback module and a current tuning signal module; The temperature control and feedback module includes the temperature control inside the laser and the temperature control part of the external semiconductor. The light source part emits light in two ways, which enter the reference path and the detection part respectively. The reference path includes a Mach-Zehnder interferometer. The light emitted by the light source enters the Mach-Zehnder interferometer in the reference path and is divided into two paths through a fiber beam splitter. One path generates a certain optical path difference with the other path after passing through a fiber delay line. The two paths of light enter the fiber beam combiner at the same time and are converted into electrical signals through a photoelectric detector in the reference path, thereby obtaining a laser heterodyne coherent signal. The detection part includes a photoelectric detector and a phase-locked amplification signal processing unit. The photoelectric detector converts the optical signal passing through the exhaust gas into an electrical signal, and then converts it into a digital signal through A / D. The digital signal is processed by the phase-locked amplification signal processing unit to obtain a gas absorption spectrum with a high signal-to-noise ratio. The reference measurement is realized based on the laser heterodyne coherent signal obtained by the reference path. On the one hand, the transient tuning characteristics of the laser are obtained, and the transient frequency and phase of the laser are obtained. The transient frequency and phase of the laser are used as negative feedback input parameters to adjust the current tuning coefficient of the current tuning signal module to improve the tuning accuracy of the tunable laser; on the other hand, it is used to resample the gas absorption spectrum, thereby correcting the gas absorption spectrum; The tunable laser exhaust online monitoring method comprises the following steps: The gas absorption spectrum with high signal-to-noise ratio is obtained by using the detection part; A laser heterodyne coherent signal is obtained through a reference path; The laser heterodyne coherent signal is used to obtain the transient tuning characteristics of the laser. The method is as follows: the autocorrelation matrix of the laser heterodyne coherent signal is calculated, and then the autocorrelation matrix is ​​decomposed into characteristic matrices to obtain signals representing different frequency and phase components, and the signals are convolved with the laser heterodyne coherent signal to obtain the transient frequency and phase of the laser; The instantaneous frequency and phase of the laser are used as negative feedback input parameters to adjust the current tuning coefficient of the current tuning signal module to improve the tuning accuracy of the tunable laser. The laser heterodyne coherent signal is processed to obtain a resampled signal for correcting the gas absorption spectrum. The processing process is as follows: first, the laser heterodyne coherent signal is processed, and the instantaneous phase is determined directly by the instantaneous relative amplitude of the cosine signal in the odd half-cycle. In the even half-cycle, the signal is flipped and its arccosine function is used to determine the instantaneous phase. On this basis, the initial phase of each half-cycle is superimposed to obtain the instantaneous phase value after unwrapping. The heterodyne coherent signal obtained in the entire tuning period is processed and then sliding averaged to obtain a square wave signal, which is used to correct the gas absorption spectrum.

2. The tunable laser exhaust online monitoring method according to claim 1 is characterized in that: The external semiconductor temperature control part includes an external temperature control device, a temperature sensor and a temperature acquisition and feedback circuit. The temperature of the external temperature control device is preset, the temperature sensor acquires the temperature of the tunable laser, and the temperature acquisition and feedback circuit compares the temperature difference between the tunable laser temperature and the set control temperature, and realizes closed-loop control of the external temperature control device according to the temperature difference.

3. The tunable laser exhaust online monitoring method according to claim 2 is characterized in that: The external temperature control device is a device that can reduce the temperature of the tunable laser.

4. The tunable laser exhaust online monitoring method according to claim 2 is characterized in that: The external temperature control device is a semiconductor refrigeration device.

5. The tunable laser exhaust online monitoring method according to claim 2 is characterized in that: The external temperature control device also includes a controller, through which the temperature is preset, and the controller implements closed-loop control of the semiconductor refrigeration device according to the temperature difference.

6. The method for online monitoring of tunable laser exhaust gas according to claim 1, characterized in that: The tunable laser is a semiconductor laser.

7. The method for online monitoring of tunable laser exhaust gas according to claim 1, characterized in that: The current tuning signal module is used to tune the output wavelength of the tunable laser.

8. The method for online monitoring of tunable laser exhaust gas according to claim 1, characterized in that: The phase-locked amplification signal processing unit includes an anti-aliasing filter, a high-pass filter, a waveform shaper, a phase shifter, a multiplier, a phase-sensitive detector and a low-pass filter. The digital signal is divided into two paths, one path passes through the anti-aliasing filter and the high-pass filter, and the other path passes through the waveform shaper and the phase shifter; the two processed digital signals are input into the multiplier, and the multiplication result is subjected to phase-sensitive detection to identify the same frequency and same phase components of the two signals; and then the low-pass filter is used to filter out noise to obtain a gas absorption spectrum with a high signal-to-noise ratio.

9. The method for online monitoring of tunable laser exhaust gas according to claim 1, characterized in that: The process of correcting the gas absorption spectrum using the square wave signal is as follows: align the gas absorption spectrum with the square wave signal according to the number of sampling points, select the zero crossing point of the square wave, which corresponds to the wavelength λ1, λ2…λ scanned during the laser tuning process. n According to the sampling point corresponding to the zero-crossing position of the square wave, the corresponding sampling point position in the gas absorption spectrum is extracted to obtain the wavelengths λ1, λ2…λ in the gas absorption spectrum. n The corresponding absorption intensity.

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