CN110879215A - Tunable laser industrial waste gas online monitoring device and method based on reference compensation - Google Patents

Abstract

The invention relates to a tunable laser waste gas online monitoring device based on a reference compensation principle, which comprises a light source part, a reference circuit part with a Mach-Zehnder interferometer and a photoelectric detector, and a detection part with the photoelectric detector and a digital phase-locked amplifier; the emergent light of the light source part is divided into two paths which respectively enter the reference path and the detection part; the detection part comprises a photoelectric detector and a phase-locked amplification signal processing unit, wherein the photoelectric detector converts an optical signal passing through the waste gas into an electric signal and converts the electric signal into a digital signal through A/D (analog/digital) conversion; the digital signal is processed by a phase-locked amplification signal processing unit to obtain a gas absorption spectrum with high signal-to-noise ratio. The invention also provides a monitoring method realized by using the device.

Description

Tunable laser on-line monitoring device and method for industrial waste gas based on reference compensation

technical field

The invention relates to a device and method for tunable laser absorption spectrum measurement which can realize reference compensation and is applied to industrial waste gas detection.

Background technique

At present, my country is still in the stage of high incidence of pollution incidents, especially air pollution incidents are the most frequent and the most serious harm. On-line monitoring and analysis instruments for air pollution sources are further strengthened to improve monitoring and reduce pollution incidents. Industrial waste gas is an important factor causing air pollution. Various waste gases are produced during chemical reactions in industrial processes such as energy, power, chemical industry, metallurgy, textile, pharmaceutical, and waste incineration. These waste gases contain a large amount of substances that are harmful to the human body. Emissions are strictly controlled.

Tunable laser absorption spectroscopy technology has the advantages of high measurement accuracy, wide application and fast response, and has good application prospects in the field of online monitoring of polluted gases, especially industrial waste gas. Its single-line spectroscopic 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, the application of tunable laser absorption spectroscopy in the field of trace gas detection is also increasing, and its detection limit can also reach the ppb level, eliminating the need for complex sampling. And pretreatment system, simple structure, reduced maintenance, more representative than extraction sampling, faster response, more in line with the needs of industrial process automation control.

The key to tunable laser absorption spectroscopy technology lies in the laser tuning accuracy and absorption spectrum measurement accuracy. In applications, such as the continuous tunable range, tuning rate, and line width during the dynamic tuning process of the laser optical device used in the tunable laser absorption spectroscopy measurement , power, side mode suppression ratio and other spectral characteristics have a great impact on the performance of the overall system. In the tunable laser absorption spectrum, the location of the extreme point of the absorption peak and the detection of temperature and pressure by the line shape 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 of laser power will also have a certain impact on the intensity of the absorption spectrum. Therefore, in the process of processing the second harmonic of the absorption spectrum, it is also necessary to consider the change of the laser power and the background subtraction, so as to improve the measurement accuracy of the absorption spectrum. .

In recent years, the detection technology of laser transient tuning characteristics has been widely used. T. Okoshi et al. first proposed the double-beam heterodyne method to measure the steady-state linewidth of lasers, and the resolution can reach 50kHz. JayW.Daeson improved the time-delayed self-heterodyne interferometry to further improve 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 measuring laser linewidth by time-delayed self-heterodyne method. With the expansion of laser applications, the requirements for the measurement accuracy of its transient output characteristic parameters are getting higher and higher. In the study of semiconductor transient output characteristic parameter measurement based on the heterodyne coherent detection method, the heterodyne coherent detection system and signal The processing and analysis method is the key to ensure the measurement accuracy and reliability. Cao Chunyan et al. measured the linewidth of narrow linewidth lasers by using unbalanced fiber interferometer and frequency noise power spectrum; Ma Mingxiang et al. conducted detection and research on mode hopping of ultra-narrow linewidth lasers; Peng Xuefeng et al. analyzed the performance of two independent lasers. Influence of beat frequency linetype on linewidth measurement.

references:

(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.

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

(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.

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

(5) Jia Yudong, Ou Pan, Yang Yuanhong, et al. Measurement of narrow linewidth laser linewidth by short fiber delay self-heterodyne method[J]. Journal of Beihang University, 2008(05):85-88.

(6) Cao Chunyan, Yao Qiong, Rao Wei, et al. Unbalanced fiber interferometer measurement method of narrow linewidth laser linewidth [J]. China Laser, 2011, 38(5).

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

(8) Peng Xuefeng, Ma Xiurong, Zhang Shuanggen, et al. Influence of beat frequency line pattern of two independent lasers on linewidth measurement [J]. China Laser, 2011, 38(4).

SUMMARY OF THE INVENTION

The invention mainly aims at the shortcomings of the prior art, and provides a tunable laser absorption spectrum gas on-line measuring device and method based on reference compensation. By using the device and method of the present invention, the dynamic tuning precision of the tunable laser absorption spectrum and the gas measurement precision and reliability can be improved. The technical solution is as follows:

A tunable laser exhaust gas online monitoring device based on the principle of reference compensation, comprising a light source part, a reference circuit part with a Mach-Zehnder interferometer and a photoelectric detector, and a detection part with a photoelectric detector 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 an internal temperature control part of the laser and an external semiconductor temperature control part; the light emitted from the light source part is divided into two paths, which enter the reference path and the detection part;

The reference path includes a Mach-Zehnder interferometer. The light emitted from the light source enters the Mach-Zehnder interferometer in the reference path, and is divided into two paths by a fiber optic beam splitter. One path passes through the fiber delay line and generates a certain optical path with the other path. The difference is that after the two paths of light enter the fiber combiner at the same time, they become electrical signals through the photodetector of the reference path, and the laser coherent heterodyne signal is obtained;

The detection part includes a photoelectric detector and a lock-in amplifying signal processing unit. The photoelectric detector converts the light 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 lock-in amplifying signal processing unit to obtain Gas absorption spectrum with high signal-to-noise ratio;

The realization of the reference measurement is based on the laser heterodyne coherent signal obtained by the reference circuit. 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 signal module. The current tuning coefficient is used to improve the tuning accuracy of the tunable laser; on the other hand, it is used to resample the gas absorption spectrum to correct the gas absorption spectrum.

Further, the external temperature control device includes a semiconductor refrigeration device. The external temperature control device may further include a controller, the temperature is preset by the controller, and the controller realizes closed-loop control of the semiconductor refrigeration device according to the temperature difference. The tunable lasers belong to semiconductor lasers.

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

The lock-in 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. Anti-aliasing filter and high-pass filter, the other one goes through a waveform shaper and a phase shifter; the processed digital signals are input to the multiplier, and phase-sensitive detection is performed on the multiplication result to distinguish the same frequency and the same phase of the two signals Then use a low-pass filter to filter out the noise to obtain a gas absorption spectrum with a high signal-to-noise ratio.

The present invention simultaneously provides the tunable laser waste gas on-line monitoring method realized by the device, comprising the following steps:

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

(2) Obtain the laser heterodyne coherent signal through the reference path;

(3) Using the laser heterodyne coherent signal to obtain the transient tuning characteristics of the laser, the method is as follows: calculate the autocorrelation matrix of the laser heterodyne coherent signal, and then decompose the eigenmatrix of the autocorrelation matrix to obtain signals representing different frequency and phase components, And convolve with the laser heterodyne coherent signal to obtain the laser transient frequency and phase;

(4) The laser transient frequency and phase are used as negative feedback input parameters, and the current tuning coefficient of the current tuning signal module is adjusted to improve the tuning accuracy of the tunable laser;

(5) The laser heterodyne coherent signal is processed to obtain a resampling signal for correcting the gas absorption spectrum. The processing process is as follows: first, the laser heterodyne coherent signal is processed, and the odd half cycle is used to directly determine the instantaneous relative amplitude of the cosine signal. Instantaneous phase, and in even half-cycles, invert the signal and then find its inverse cosine function to determine the instantaneous phase, and on this basis superimpose the initial phase of each half-cycle to obtain the instantaneous phase value after unwrapping, and the entire tuning cycle The obtained heterodyne coherent signal is processed and then subjected to moving average to obtain a square wave signal, which is used to correct the gas absorption spectrum.

The process of calibrating the gas absorption spectrum with the square wave signal is as follows: align the gas absorption spectrum with the square wave signal according to the number of sampling points, and select the zero-crossing point of the square wave, which corresponds to the wavelengths λ ₁ , λ ₂ .... scanned during the laser tuning process. λ _n , according to the sampling point corresponding to the zero-crossing point position of the square wave, extract the corresponding sampling point position in the gas absorption spectrum, so as to obtain the absorption intensity corresponding to the wavelengths λ ₁ , λ ₂ ....λ _n in the gas absorption spectrum.

The present invention has the following advantages due to taking the above technical solutions:

1. In the present invention, a reference measurement method is used to obtain the heterodyne coherent signal of the tunable laser, and the transient tuning characteristic of the tunable laser obtained by the time-frequency analysis method is used as a feedback input parameter to control the laser tuning parameter, Thus, the tuning accuracy of the tunable laser is improved.

2. In the present invention, a tuning method combining temperature and current is used to control the output wavelength of the tunable laser, which improves the tuning range and precision of the tunable laser, wherein the temperature tuning adopts a second-order temperature-controlled tuning method, which improves the temperature stability and tuning. speed.

3. In the present invention, the heterodyne coherent signal obtained by the reference path is processed by phase extraction to obtain a square wave signal whose frequency is linearly related to the output wavelength of the tunable laser, and the gas absorption spectrum is corrected by using the square wave signal to improve The measurement accuracy of absorption spectrum wavelength is improved.

4. In the present invention, the S-G curve fitting method is used to extract the first harmonic component of the gas absorption spectrum and use it as a reference to realize the correction of the gas absorption spectrum and improve the measurement accuracy of the gas absorption spectrum intensity.

Description of drawings

Figure 1 is a schematic structural diagram of a tunable laser industrial waste gas online monitoring device based on reference compensation

Figure 2 is a schematic diagram of the composition of the light source part

Figure 3 is a schematic diagram of the composition of the reference circuit

Fig. 4 is the flow chart of the calculation of the dynamic tuning parameters of the laser based on the heterodyne coherence signal

Figure 5 is a schematic diagram of the principle and composition of the detection part

Fig. 6 is a flow chart of resampling and spectral wavelength correction method based on heterodyne coherent signal

Fig. 7 is a flow chart of the background subtraction method based on the first harmonic signal component

Figure 8 shows the measurement results of tunable laser absorption spectrum of ammonia gas with different concentrations of industrial waste gas

Fig. 9 Schematic diagram of calibration of gas absorption spectrum using square wave signal

Detailed ways:

The tunable laser gas on-line measurement device based on reference compensation of the present invention will be described with reference to the accompanying drawings and embodiments.

According to the characteristics of the tunable laser, the 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 principle of heterodyne coherence, and uses a time-frequency analysis method to extract the transient tuning characteristics of the tunable laser. , a dynamic tuning compensation method is designed, a resampling correction method for absorption spectrum based on heterodyne coherent signal is designed, and a first harmonic extraction and absorption spectrum correction method for tunable laser absorption spectrum based on S-G fitting is designed.

The tunable laser spectrum measurement device system based on the reference compensation principle of the present invention is shown in FIG. 1 . It mainly includes a light source part, mainly including a light source part, a reference path part with a Mach-Zehnder interferometer and a photoelectric detector, and a photoelectric detector and a photoelectric detector. The detection part of the digital lock-in amplifier, where,

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 an internal temperature control part of the laser and an external semiconductor temperature control part, wherein the external semiconductor temperature control part includes an external temperature control device ( Mainly including semiconductor refrigeration device), temperature sensor and temperature acquisition and feedback circuit, set the control temperature for the semiconductor refrigeration device, use the temperature sensor to collect the temperature of the tunable laser, and compare the temperature between the tunable laser temperature and the set temperature with the temperature acquisition and feedback circuit Temperature difference, realizes closed-loop control of semiconductor refrigeration devices according to temperature difference; current tuning signal module, using digital signal frequency synthesis technology, injects a sawtooth current of a certain frequency into the semiconductor laser, so that the output wavelength of the tunable laser scans the absorption spectrum of the gas to be measured . The light emitted from the light source part is divided into two paths, which enter the reference path and the detection part respectively.

In this embodiment, the circuit part of the light source is realized by an analog circuit. A better embodiment is to add a control chip to control the working state of the semiconductor refrigeration device, so as to provide a stable temperature environment for the tunable laser.

The main component of the reference path is the Mach-Zehnder interferometer. The light emitted from the light source enters the Mach-Zehnder interferometer in the reference path, and is divided into two paths through the fiber optic beam splitter. With a certain optical path difference, after the two paths of light enter the fiber combiner at the same time, they become electrical signals through the photodetector of the reference path, and the laser coherent heterodyne signal is obtained.

The detection part includes a photodetector and a lock-in amplifying signal processing unit. The photodetector converts the light 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 lock-in amplifying signal processing unit. The lock-in 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 aliasing filter and the high-pass filter, the other one passes through the waveform shaper and the phase shifter; the processed digital signals are input to 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 use a low-pass filter to filter out the noise to obtain a gas absorption spectrum with a high signal-to-noise ratio.

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

The method of using laser heterodyne coherent signal to obtain the transient tuning characteristics of laser is to calculate the autocorrelation matrix of the laser heterodyne coherent signal, and then decompose the autocorrelation matrix to the eigenmatrix to obtain signals representing different frequency and phase components, which are combined with the laser The heterodyne coherent signal is convoluted 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.

Processing the laser heterodyne coherent signal of the reference path can obtain the resampling 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 relative amplitude of the cosine signal. Phase, and in even half-cycles, invert the signal and then find its inverse cosine function to determine the instantaneous phase, and superimpose the initial phase of each half-cycle on this basis to obtain the instantaneous phase value after unwrapping. The obtained heterodyne coherent signal is processed and then subjected to moving average to obtain a square wave signal, which can be used to correct the gas absorption spectrum. The specific process is shown in Figure 9: the gas absorption spectrum measured by the detection unit is compared with the square wave signal The wave signal is aligned according to the number of sampling points, and the zero-crossing point of the square wave is selected, which corresponds to the wavelengths λ ₁ , λ ₂ . . . λ _n scanned during the laser tuning process. According to the sampling point corresponding to the zero-crossing point position of the square wave, the corresponding sampling point position in the gas absorption spectrum is extracted, so as to obtain the absorption intensity corresponding to the wavelengths λ ₁ , λ ₂ . . . λ _n in the gas absorption spectrum.

The operation process can be divided into the following steps:

(1) The tunable laser works. The schematic diagram of the light source part is shown in Figure 2. The laser is a tunable narrow linewidth laser, and the output wavelength and power of the laser can be tuned by changing the temperature and current.

The parameters of the tunable laser are set according to the measurement gas, and 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 working process of the external temperature control 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, and the temperature acquisition and feedback circuit compares the temperature of the tunable laser with that of the tunable laser. Set the temperature difference between temperatures and achieve closed-loop control through semiconductor refrigeration devices. When the external temperature control is stable, use the internal temperature controller of the laser to scan the temperature to achieve second-order temperature tuning. The current tuning signal module adopts the digital signal frequency synthesis technology, and injects a sawtooth wave 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.

The advantage of second-order temperature tuning is that the external control ensures the stable working temperature of the laser, reduces the tuning temperature difference, and the response speed of the internal temperature control of the laser is relatively fast.

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

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

In the process of laser dynamic tuning, the signal generated by heterodyne coherence appears as a non-stationary signal from the time-frequency distribution, and its autocorrelation function and its Fourier transform both appear as time-varying signals. Time-frequency analysis is used to map the signal. onto a two-dimensional plane of time-frequency in order to reveal the frequency characteristics contained in the signal and how it changes over time. In the invention, the flow of the signal time-frequency decomposition algorithm is shown in Figure 4.

The specific steps are:

The laser heterodyne coherent signal can be expressed as

可调谐激光器瞬时频率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

After the eigenmatrix decomposition, in order to obtain different frequency and phase signal decomposition, first calculate the autocorrelation function of the heterodyne coherent signal

R _c =I*I' (2)

The autocorrelation matrix R _c is obtained, and then Rc is decomposed into the characteristic matrix to obtain the following matrix:

Rc=H*V* ^HT (3)

Among them, H and V represent different frequency and phase components, respectively. The frequency f and phase are obtained by convolving H and V with I(t) respectively.

f=H*I(t) (4)

Further, the obtained transient characteristics of the laser can include the instantaneous wavelength and the instantaneous phase, which 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.

(3) The detection part adopts the digital phase-locking method to realize the detection of gas absorption spectrum signals. The core device includes a photoelectric detector and a phase-locked amplifying signal processing unit. Among them, the lock-in amplification adopts the digital signal processing method. The processing steps are: first, the photodetector 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 channels, and one channel is subjected to anti-aliasing filtering With high-pass filtering, waveform shaping and phase shifting are carried out all the way; then, the two-way 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, the narrow-band low-pass filter is used to filter the signal. By removing the noise, an amplified signal with a high signal-to-noise ratio is obtained. The digital phase locking method in the present invention reduces the volume of the analog system, improves the system integration and stability, simplifies the hardware circuit of the system, and reduces the volume of the instrument.

(4) The gas absorption spectrum is resampled by the heterodyne coherent signal to realize the wavelength correction of the absorption spectrum. The resampling process is as follows: First, the laser heterodyne coherent signal is processed, and the instantaneous phase is directly determined by the instantaneous relative amplitude of the cosine signal with the odd half cycle, and the instantaneous phase is determined by inverting the signal and then finding its inverse cosine function in the even half cycle. On this basis, the initial phase of each half-cycle is superimposed, thereby obtaining the instantaneous phase value after unwrapping. According to the above algorithm, the heterodyne coherent signal obtained in the whole tuning period is processed and then the moving average is performed to obtain a square wave signal, in which the frequency of the square wave signal has a linear relationship with the wavelength. The square wave signal can be used to measure the gas absorption spectrum. correction for improved wavelength accuracy

(5) The S-G curve fitting method is used to extract the first harmonic component of the gas absorption spectrum, and use it as a reference to realize the calibration of the gas absorption spectrum. The method flow is shown in Figure 7:

The S-G curve fitting formula can be expressed as

y _k+j =a ₀ +a ₁ j+a ₂ j ² +…+a _p j ^p (6)

where a _p is the fitting coefficient, j is the absorption spectrum value at different wavelength points, through the SG polynomial smoothing method, the equally spaced data points in the window with a width of 2p+1 are fitted to the p-order polynomial, so as to obtain the first harmonic weight.

The gas measurement results obtained after processing are shown in Figure 8.

Claims (10)

1. A tunable laser waste gas on-line monitoring device based on the reference compensation principle comprises a light source part, a reference circuit part with a Mach-Zehnder interferometer and a photoelectric detector, and a detection part with the photoelectric detector and a digital phase-locked amplifier, and is characterized in that,

the light source part comprises a tunable laser, a temperature control and feedback module and a current tuning signal module; the temperature control and feedback module comprises a temperature control part inside the laser and an external semiconductor temperature control part; the emergent light of the light source part is divided into two paths which respectively enter the reference path and the detection part;

the reference path comprises a Mach-Zehnder interferometer, emergent light of a light source enters the Mach-Zehnder interferometer in the reference path and is divided into two paths through an optical fiber beam splitter, wherein one path generates a certain optical path difference with the other path after passing through an optical fiber delay line, and the two paths of light simultaneously enter an optical fiber beam combiner and are converted into electric signals through a photoelectric detector of the reference path to obtain laser coherent heterodyne signals;

the detection part comprises a photoelectric detector and a phase-locked amplification signal processing unit, wherein the photoelectric detector converts an optical signal passing through the waste gas into an electric signal and converts the electric signal into a digital signal through A/D (analog/digital) conversion; the digital signal is processed by a phase-locked amplification signal processing unit to obtain a gas absorption spectrum with high signal-to-noise ratio.

The realization of reference measurement is based on a laser heterodyne coherent signal obtained by a reference path, on one hand, the transient tuning characteristic of the laser is obtained, the transient frequency and the phase of the laser are used as negative feedback input parameters, and the current tuning coefficient of a current tuning signal module is adjusted to improve the tuning precision of the tunable laser; and on the other hand, the method is used for resampling the gas absorption spectrum so as to correct the gas absorption spectrum.

2. The tunable laser online exhaust gas monitoring device according to claim 1, wherein the external semiconductor temperature control part comprises an external temperature control device, a temperature sensor and a temperature acquisition and feedback circuit, the external temperature control device is preset for temperature, the temperature sensor acquires the temperature of the tunable laser, the temperature acquisition and feedback circuit compares the temperature difference between the temperature of the tunable laser and the set control temperature, and the external temperature control device is controlled in a closed loop according to the temperature difference.

3. The tunable laser online exhaust gas monitoring device according to claim 1, wherein the external temperature control device is a device capable of reducing the temperature of the tunable laser.

4. The tunable laser online exhaust gas monitoring device according to claim 1, wherein the external temperature control device is a semiconductor refrigeration device.

5. The tunable laser online waste gas monitoring device according to claim 1, wherein the external temperature control device further comprises a controller, the temperature is preset through the controller, and the controller realizes closed-loop control on the semiconductor refrigeration device according to the temperature difference.

6. The on-line tunable laser exhaust gas monitoring device according to claim 1, wherein the tunable laser is a semiconductor laser.

7. The tunable laser online exhaust gas monitoring device according to claim 1, wherein the current tuning signal module is used for tuning the output wavelength of the tunable laser.

8. The tunable laser online waste gas monitoring device according to claim 1, wherein the phase-locked amplified signal processing unit comprises 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; inputting the two paths of processed digital signals into a multiplier, performing phase-sensitive detection on the multiplication result, and identifying the same-frequency and same-phase components of the two paths of signals; and then, a low-pass filter is used for filtering noise to obtain a gas absorption spectrum with a high signal-to-noise ratio.

9. The tunable laser online exhaust gas monitoring method realized by the device of claim 1 comprises the following steps:

(1) obtaining a gas absorption spectrum with a high signal-to-noise ratio by using the detection part;

(2) obtaining a laser heterodyne coherent signal through a reference path;

(3) the method for acquiring the transient tuning characteristic of the laser by using the laser heterodyne coherent signal comprises the following steps: calculating an autocorrelation matrix of the laser heterodyne coherent signal, then performing characteristic matrix decomposition on the autocorrelation matrix to obtain signals representing different frequency and phase components, and performing convolution on the signals and the laser heterodyne coherent signal to obtain the transient frequency and phase of the laser;

(4) the transient frequency and the phase of the laser are used as negative feedback input parameters, and the current tuning coefficient of the current tuning signal module is adjusted to improve the tuning precision of the tunable laser;

(5) processing the laser heterodyne coherent signal to obtain a resampling signal for correcting the gas absorption spectrum, wherein the processing process comprises the following steps: firstly, processing a laser heterodyne coherent signal, determining an instantaneous phase by using an odd half period and directly passing through an instantaneous relative amplitude of a cosine signal, turning the signal over and solving an inverse cosine function of the signal to determine the instantaneous phase in an even half period, superposing the initial phase of each half period on the basis to obtain an unwrapped instantaneous phase value, processing the heterodyne coherent signal obtained in the whole tuning period, then carrying out sliding average to obtain a square wave signal, and correcting a gas absorption spectrum by using the square wave signal.

10. The method of claim 4, wherein the correction of the gas absorption spectrum using the square wave signal is performed by: aligning the gas absorption spectrum with the square wave signal according to the number of sampling points, selecting the zero crossing point of the square wave, and correspondingly scanning the wavelength lambda in the laser tuning process₁，λ₂….λ_nExtracting the corresponding sampling point position in the gas absorption spectrum according to the sampling point corresponding to the zero crossing point position of the square wave, thereby obtaining the wavelength lambda in the gas absorption spectrum₁，λ₂….λ_nCorresponding absorption strength.

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