CN110879215A - Tunable laser industrial waste gas online monitoring device and method based on reference compensation - Google Patents
Tunable laser industrial waste gas online monitoring device and method based on reference compensation Download PDFInfo
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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
Technical Field
The invention relates to a device and a method for measuring tunable laser absorption spectrum capable of realizing reference compensation, which are applied to industrial waste gas detection.
Background
At present, China is still in a high-incidence stage of pollution events, and particularly, atmospheric pollution events are the most frequent and the most serious in harm. The online monitoring and analyzing instrument for the atmospheric pollution source is further strengthened, the monitoring strength is improved, and pollution events are reduced. Industrial waste gas is an important factor causing air pollution, and various waste gases generated in chemical reactions in industrial processes such as energy, electricity, chemical industry, metallurgy, textile, pharmacy, waste incineration and the like contain a large amount of substances harmful to human bodies, and the emission amount of the waste gases must be strictly controlled.
The tunable laser absorption spectrum technology has the advantages of high measurement precision, wide application occasions, high response speed and the like, and has good application prospect in the field of online monitoring of polluted gases, particularly industrial waste gases. The single-line spectrum analysis technology has a plurality of unique advantages, can eliminate background gas interference and improve detection sensitivity. With the continuous improvement of the requirements on environmental protection and industrial waste gas emission, the tunable laser absorption spectrum is more and more applied in the field of trace gas detection, the detection limit can also reach the ppb magnitude, a complex sampling and pretreatment system is omitted, the structure is simple, the maintenance amount is reduced, the sampling is more representative than an extraction type sampling, the response speed is higher, and the requirement of industrial process automatic control is met better.
The key of the tunable laser absorption spectrum technology lies in the laser tuning precision and the absorption spectrum line measurement precision, and in application, such as the dynamic tuning process of a laser used in tunable laser absorption spectrum measurement, the continuous adjustable range, the tuning speed, the line width, the power, the side mode suppression ratio and other spectral characteristics have great influence on the performance of the whole system. In the tunable laser absorption spectrum, the positioning of the absorption peak extreme point, the linear detection temperature and pressure intensity and the like are all influenced by the instantaneous wavelength, and the instantaneous line width directly influences the line type and the amplitude of a second harmonic signal. In the tuning process, the laser power change also has a certain influence on the absorption spectrum line intensity, so that the laser power change and background subtraction need to be considered in the second harmonic processing process of the absorption spectrum line, and the absorption spectrum measurement accuracy is improved.
In recent years, the detection technology of the transient tuning characteristic of the laser is increasingly widely applied, T.Okoshi and the like firstly put forward a double-beam heterodyne method to measure the steady-state line width of the laser, and the resolution can reach 50 kHz. In JayW, Daeson, the time delay self-heterodyne interference method is improved, and the measurement resolution is further improved. Comparing the commonly used line width measurement method by using the Camatel, and indicating the influence of the long optical fiber delay line on the line width measurement; ju Yolun measured the short-term instability of the laser frequency output by the 2 μm single longitudinal mode laser; the Jiayudong et al analyzed and corrected the error of the laser line width measured by the delay self-heterodyne method. Along with the expansion of the application occasions of the laser, the requirement on the measurement accuracy of the transient output characteristic parameters of the laser is higher and higher, and a heterodyne coherent detection system and a signal processing and analyzing method are the key points for ensuring the measurement accuracy and the reliability in the measurement research of the semiconductor transient output characteristic parameters based on the heterodyne coherent detection method. The Caochenpyrad et al measures the linewidth of a narrow linewidth laser by using an unbalanced fiber interferometer and a frequency noise power spectrum; the detection research is carried out on the mode hopping of the ultra-narrow linewidth laser by Marmingxi and the like; penxuefeng et al analyzed the effect of the beat line type of two independent lasers on line width measurement.
Reference documents:
(1)Okoshi T,Kikuchi K,Nakayama A.Novel method for high resolutionmeasurement 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-heterodyneinterferometer for linewidth measurements[J].IEEE Photonics TechnologyLetters,1992,4(9):1063-1066.
(3)Camatel S,Ferrero V.Narrow linewidth CW laser phase noisecharacterization methods for coherent transmission system applications[J].Journal ofLightwave Technology,2008,26(17):3048-3055.
(4) ju, Ju Yong Gai, Wang Yi, et al.2 μm Single longitudinal mode laser frequency short-term instability measurement [ J ] optics report, 2008,28(11).
(5) Jiayudong, European climbing, Yangyuan, et al, short fiber delay self-heterodyne method for measuring narrow linewidth laser linewidth [ J ]. Beijing university of aerospace, 2008(05):85-88.
(6) Canada, yaoqiong, baohiwei, et al, unbalanced fiber interferometer measurement of narrow linewidth laser linewidth [ J ]. china laser, 2011,38(5).
(7) Mamingxian ultra-narrow linewidth fiber ring cavity laser mode hopping research [ D ]. national defense science and technology university, 2010.
(8) Peng xue peak, maxiong, zhang dou, et al, influence of beat frequency line type of two independent lasers on line width measurement [ J ]. china laser, 2011,38(4).
Disclosure of Invention
The invention mainly aims at the defects in the prior art and provides a tunable laser absorption spectrum gas online measurement device and method based on reference compensation. The device and the method of the invention can improve the dynamic tuning precision of the tunable laser absorption spectrum and the gas measurement precision and reliability. The technical scheme is as follows:
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, wherein,
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.
Further, the external temperature control device comprises a semiconductor refrigeration device. The external temperature control device can also comprise 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. The tunable laser belongs to a semiconductor laser.
And the current tuning signal module is used for tuning the output wavelength of the tunable laser.
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, wherein the digital signal is divided into two paths, one path of the digital signal passes through the anti-aliasing filter and the high-pass filter, and the other path of the digital signal 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.
The invention also provides a tunable laser waste gas online monitoring method realized by adopting the device, which 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.
The process of correcting the gas absorption spectrum using the square wave signal is as follows: 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….λnExtracting 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….λnCorresponding absorption strength.
Due to the adoption of the technical scheme, the invention has the following advantages:
in the invention, a reference measurement mode is adopted to obtain heterodyne coherent signals of tunable laser, the transient tuning characteristic of the tunable laser is obtained by adopting a time-frequency analysis method and is used as a feedback input parameter to control tuning parameters of the laser, thereby improving the tuning precision of the tunable laser.
In the invention, the output wavelength of the tunable laser is controlled by adopting a tuning mode of combining temperature and current, so that the tuning range and the tuning precision of the tunable laser are improved, wherein the temperature tuning adopts a second-order temperature control tuning method, so that the temperature stability and the tuning speed are improved.
3, in the invention, the heterodyne coherent signal obtained by the reference path is subjected to phase extraction processing to obtain a square wave signal with the frequency in linear relation with the output wavelength of the tunable laser, and the square wave signal is used for correcting the gas absorption spectrum, thereby improving the wavelength measurement precision of the absorption spectrum.
4, in the invention, the first harmonic component of the gas absorption spectrum is extracted by adopting an S-G curve fitting method and is used as a reference, so that the gas absorption spectrum correction is realized, and the gas absorption spectrum intensity measurement precision is improved.
Drawings
FIG. 1 is a schematic structural diagram of a tunable laser industrial waste gas on-line monitoring device based on reference compensation
FIG. 2 is a schematic view of a light source part
FIG. 3 is a schematic diagram of a reference path
FIG. 4 is a flow chart of laser dynamic tuning parameter calculation based on heterodyne coherence signals
FIG. 5 is a schematic diagram of the principle and composition of the detecting part
FIG. 6 is a flowchart of a resampling and spectral wavelength correction method based on heterodyne coherent signals
FIG. 7 is a flow chart of a background subtraction method based on first harmonic signal components
FIG. 8 shows the measurement results of tunable laser absorption spectrum of ammonia gas with different concentrations in industrial waste gas
FIG. 9 is a schematic diagram of the correction of gas absorption spectrum by square wave signal
The specific implementation mode is as follows:
the tunable laser gas online measuring device based on reference compensation is described with reference to the accompanying drawings and embodiments.
According to the tunable laser characteristic, a laser tuning device and a laser tuning method based on second-order temperature tuning and current tuning are developed, a reference measuring device based on a heterodyne coherence principle is developed, a time-frequency analysis method is utilized to extract the transient tuning characteristic of the tunable laser, a dynamic tuning compensation method is designed, an absorption spectrum resampling correction method based on a heterodyne coherence signal is designed, and a tunable laser absorption spectrum first harmonic extraction and absorption spectrum correction method based on S-G fitting is designed.
The tunable laser spectrum measuring device system based on the reference compensation principle of the invention is shown in figure 1 and mainly 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, wherein,
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 an internal temperature control part and an external semiconductor temperature control part in the laser, wherein the external semiconductor temperature control part adopts an external temperature control device (mainly comprising a semiconductor refrigerating device), a temperature sensor and a temperature acquisition and feedback circuit to set control temperature for the semiconductor refrigerating device, the temperature sensor is used for acquiring 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 temperature, and the closed-loop control on the semiconductor refrigerating device is realized according to the temperature difference; and the current tuning signal module injects sawtooth wave current with certain frequency into the semiconductor laser by adopting a digital signal frequency synthesis technology so that the output wavelength of the tunable laser scans the absorption spectral line of the gas to be measured. The emergent light of the light source part is divided into two paths which respectively enter the reference path and the detection part.
In this embodiment, the circuit portion of the light source is implemented by an analog circuit. In a better implementation mode, a control chip is added for controlling the working state of the semiconductor refrigeration device, so that a stable temperature environment is provided for the tunable laser.
The main component of the reference path is a Mach-Zehnder interferometer, the emergent light of the 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, 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, the phase-locked amplification 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 of the digital signal passes through the anti-aliasing filter and the high-pass filter, and the other path of the digital signal 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.
The implementation of the reference measurement is based on the processing of the laser heterodyne coherent signal obtained by the reference path, so that on one hand, the transient tuning characteristic of the laser can be obtained, and on the other hand, the method can be used for resampling the gas absorption spectrum.
The method for obtaining the transient tuning characteristic of the laser by using the laser heterodyne coherent signal comprises the steps of 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, performing convolution on the signals and the laser heterodyne coherent signal to obtain the transient frequency and the phase of the laser, using the transient frequency and the phase of the laser as negative feedback input parameters, and adjusting a current tuning coefficient of a current tuning signal module to improve the tuning precision of the tunable laser.
The process of processing the reference path laser heterodyne coherent signal to obtain a resampling signal for correcting the gas absorption spectrum includes the steps of processing the laser heterodyne coherent signal, determining an instantaneous phase by using an odd number half period and directly determining an instantaneous phase through an instantaneous relative amplitude of a cosine signal, turning the signal in an even number half period and determining an instantaneous phase by solving an inverse cosine function of the signal, superposing an initial phase of each half period on the basis of the instantaneous phase, thus obtaining an instantaneous phase value after unwrapping, processing the heterodyne coherent signal obtained in the whole tuning period and then performing sliding average to obtain a square wave signal, and correcting the gas absorption spectrum by using the square wave signal, wherein the specific process is shown in fig. 9: aligning the gas absorption spectrum measured by the detection unit with the square wave signal according to the number of sampling points, and selecting the zero crossing point of the square wave corresponding to the scanned wavelength lambda in the tuning process of the laser1，λ2….λn. Extracting 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….λnCorresponding absorption strength.
The operation flow can be divided into the following steps:
(1) the tunable laser works, wherein the schematic structural diagram of a light source part is shown in fig. 2, the laser is a tunable narrow linewidth laser, and the tuning of the output wavelength and the power of the laser can be realized by changing the temperature and the current.
And setting parameters of the tunable laser according to the measured gas, wherein the temperature tuning adopts a second-order temperature control tuning method and is divided into an external semiconductor temperature control part and a laser internal temperature control part. The external temperature control working process comprises the steps of setting temperature for an external temperature control device (the external temperature control device selected in the embodiment is a semiconductor refrigeration device), collecting external temperature control temperature by a temperature sensor, comparing the temperature difference between the temperature of the tunable laser and the set temperature by a temperature collection and feedback circuit, and realizing closed-loop control through the semiconductor refrigeration device. And when the external temperature control is stable, the temperature is scanned by using the temperature controller in the laser, so that second-order temperature tuning is realized. The current tuning signal module adopts a digital signal frequency synthesis technology, and sawtooth wave current with certain frequency is injected into the semiconductor laser, so that the tunable laser can output wavelength to scan the absorption spectral line of the gas to be measured.
The second-order temperature tuning has the advantages that external control ensures stable working temperature of the laser, tuning temperature difference is reduced, response speed of internal temperature control of the laser is high, and meanwhile, temperature tuning precision can be improved by adopting a negative feedback mode according to a reference measurement result.
The current tuning signal form is composed of a low-frequency sawtooth wave signal and a high-frequency square wave signal with certain bias, and the amplitude of the square wave signal is modulated by the sawtooth wave signal, so that a quasi-continuous wavelength modulation driving signal is obtained. The tuning of the output wavelength of the laser is realized by the low-frequency sawtooth wave current signal, and a tuning range covers a complete gas absorption line; the quasi-continuous current signal (here, square wave) realizes the quasi-continuous driving of the laser, reduces the self-heating effect of the laser, and improves the tuning precision of the laser.
(2) Reference compensation signal obtaining and calculating, the reference path structure is shown in fig. 3, heterodyne coherent signals with different optical path differences are obtained by adopting a mach-zehnder interferometer and an ultra-short time delay method, incident light is divided into two paths through an optical fiber beam splitter, 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 the optical fiber beam combiner to generate coherent heterodyne models, and are converted into electric signals through a photoelectric detector for further processing.
In the dynamic tuning process of the laser, signals generated after heterodyne coherence are represented as non-stationary signals from time-frequency distribution, autocorrelation functions and Fourier transformation of the signals are represented as time-varying signals, and the signals are mapped onto a two-dimensional plane of time frequency by adopting time-frequency analysis so as to reveal frequency characteristics contained in the signals and the change condition of the frequency characteristics along with the time. The signal time-frequency decomposition algorithm flow in the invention is shown in figure 4.
The method comprises the following specific steps:
the laser heterodyne coherent signal may be represented as
Where I (t) is the output of the laser at time t, fiFor the instantaneous wavelength of the laser output, t is the time,
tunable laser instantaneous frequency
After the characteristic matrix decomposition, in order to obtain the decomposition of signals with different frequencies and phases, firstly, calculating the self-correlation function of the heterodyne coherent signal
Rc＝I*I′ (2)
Obtaining an autocorrelation matrix RcThen, the Rc is subjected to characteristic matrix decomposition to obtain the following form matrix:
Rc＝H*V*HT(3)
whereinH and V respectively represent different frequency and phase components, and H and V are respectively convolved with I (t) to respectively obtain frequency f and phase
f＝H*I(t) (4)
Furthermore, the obtained transient characteristic reference of the laser comprises the instantaneous wavelength and the instantaneous phase, the transient characteristic reference is used as a negative feedback input parameter, the current tuning coefficient is adjusted, and the temperature tuning coefficient improves the tuning precision of the laser.
(3) The detection part adopts a digital phase-locking method to realize the detection of gas absorption spectrum signals, and the core device comprises a photoelectric detector and a phase-locking amplification signal processing unit. The phase-locked amplification adopts a digital signal processing method, and the processing steps are as follows: firstly, converting an optical signal into an electric signal by a photoelectric detector, and converting the electric signal into a digital signal by A/D (analog/digital) conversion; then dividing the signal into two paths, one path of the signal is subjected to anti-aliasing filtering and high-pass filtering, and the other path of the signal is subjected to waveform shaping and phase shifting; then, the two paths of signals are input into a multiplier, and the multiplication result is subjected to phase-sensitive detection to identify the same-frequency and same-phase components of the two paths of signals; finally, a narrow-band low-pass filter is used for filtering noise, and an amplified signal with a high signal-to-noise ratio is obtained. The digital phase locking method reduces the volume of an analog system, improves the integration level and stability of the system, simplifies a hardware circuit of the system and reduces the volume of an instrument.
(4) And resampling the gas collection spectrum by utilizing the heterodyne coherent signal to realize wavelength correction of the absorption spectrum. The resampling process is that firstly, the laser heterodyne coherent signal is processed, the instantaneous phase is determined by the instantaneous relative amplitude of the cosine signal directly by using the odd half period, and in the even half period, the signal is inverted and then the inverse cosine function is solved to determine the instantaneous phase, and the initial phase of each half period is superposed on the instantaneous phase, thereby obtaining the instantaneous phase value after unwrapping. Processing heterodyne coherent signals obtained in the whole tuning period according to the algorithm and then carrying out sliding average to obtain square wave signals, wherein the frequency of the square wave signals is in a linear relation with the wavelength, and the square wave signals can be used for correcting the measured gas absorption spectrum, so that the wavelength precision is improved
(5) The method comprises the following steps of extracting a first harmonic component of a gas absorption spectrum by adopting an S-G curve fitting method, taking the first harmonic component as a reference, and realizing gas absorption spectrum correction, wherein the flow of the method is shown in figure 7:
wherein the S-G curve fitting formula can be expressed as
yk+j＝a0+a1j+a2j2+…+apjp(6)
Wherein a ispAnd j is an absorption spectrum value of different wavelength points as a fitting coefficient, and the equidistant data points in a window with the width of 2p +1 are fitted into a p-order polynomial in an S-G polynomial smoothing method, so that the first harmonic component is obtained.
The results of the gas measurements obtained after the treatment are shown in fig. 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….λnExtracting 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….λnCorresponding absorption strength.
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| CN114062312A (en) * | 2021-10-24 | 2022-02-18 | 中南大学 | Phase-locked amplification method and system in TDLAS gas detection |
| CN114447760A (en) * | 2021-12-07 | 2022-05-06 | 金雷 | Temperature control optical module and working method thereof |
| CN115356086A (en) * | 2022-03-31 | 2022-11-18 | 西北核技术研究所 | Method for measuring dynamic line width of wavelength scanning laser based on absorption spectrum |
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| CN114062312A (en) * | 2021-10-24 | 2022-02-18 | 中南大学 | Phase-locked amplification method and system in TDLAS gas detection |
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