CN103616347A - Method and device for realizing optimal modulation factor of multi-gas spectral line detection - Google Patents
Method and device for realizing optimal modulation factor of multi-gas spectral line detection Download PDFInfo
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Abstract
The invention belongs to the technical field of gas concentration, temperature, pressure or flow viscosity, and particularly relates to a method and a device which are capable of meeting multi-gas spectral line detection and enables each spectral line to realize optimal detection. The detection sensitivity of concentrations, pressures, temperatures or flow velocities of multiple gases of a TDLAS (Tunable Diode Laser Absorption Spectroscopy) system can be improved. The method and the device are suitable for online, in situ or offline detection or monitoring application of trace gases. Thus, according to the technical scheme of the invention, the method for realizing an optimal modulation factor of multi-gas spectral line detection comprises the steps: detecting absorption spectral lines of gases to laser, and measuring parameters of the gases, including concentrations, pressures, temperatures and flow velocities, wherein in a scanning period of laser wavelength, a wavelength modulation coefficient of the laser is variable, namely, two or more sine wave amplitudes are adopted in the one scanning period. The method and device are mainly applied to gas detection.
Description
Technical field
The invention belongs to the field of measuring technique of gas concentration, temperature, pressure or flow velocity, relate to that many gas compositions detect simultaneously or detect to many absorption lines of pure gas time, best by the index of modulation to all spectral lines to be measured, realize the method and apparatus that many gas spectral line detection sensitivity improves, specifically, relate to best index of modulation implementation method and the device that many gas spectral line detects.
Background technology
Tunable diode laser absorption spectroscopy is analyzed (Tunable diode laser absorption spectroscopy, TDLAS) be the gas detection technology of a kind of high sensitivity, high resolving power and quick response, be widely used in the fields such as gas monitor, ambient atmosphere detection and scientific research in industrial flow.It utilizes the wavelength tuning characteristic of diode laser,, by changing temperature or the Injection Current of diode laser, changes the output wavelength of laser instrument.In wavelength tuning range, detect the characteristic absorption spectrum of gas to be measured, by the linear analysis to absorption spectrum, concentration, temperature, pressure or flow velocity that can measurement gas.
For high concentration or the larger gas of absorption coefficient, conventionally adopt direct absorption spectrum detection method.Directly absorption spectrum is to detect by light intensity after tested gas with the variation of wavelength, by the intensive analysis to absorption signal, is calculated the concentration of tested gas by Bill Lang Baite (Beer-Lambert) law.This measuring method is affected by light-intensity variation, detecting device and amplifier noise, the external interference etc. of light source, its measuring error is large, sensitivity and precision lower.
In order to improve detection sensitivity, conventionally adopt Wavelength modulation spectroscopy (Wavelength modulation spectroscopy, WMS) technology.WMS technology is done wavelength-modulated to laser, produces length scanning and high frequency modulated, and the laser after gas absorption is received by photodetector, utilizes the high frequency modulated harmonic signal of phase-sensitive detection (conventionally using lock-in amplifier) technology for detection gas absorption spectra.WMS can suppress the impact of the various noises such as laser intensity fluctuating, system 1/f noise, drift, improves the signal to noise ratio (S/N ratio) of signal, and detection sensitivity is higher two more than the order of magnitude than direct absorption spectroanalysis technology.Length scanning in WMS technology and high frequency modulated, apply sawtooth wave the sinusoidal realization of stack by signal generator to diode laser, and sawtooth wave is corresponding to length scanning, and its sweep limit at least covers a complete gas absorption spectrum line; The wavelength-modulated amplitude that high frequency sinusoidal signal is corresponding and gas absorption spectrum line half width (half width at half-maximum, HWHM) ratio is defined as the index of modulation, the index of modulation is relevant with amplitude and the signal to noise ratio (S/N ratio) of harmonic wave detection signal, for 2 subharmonic, it is generally acknowledged that the best index of modulation is 2.2, the Signal-to-Noise now obtaining is high, and corresponding detection sensitivity is the highest.
Yet, for many gas spectral lines while detection case, due to these line width [full width at half maximum, FWHM or half high half-breadth, HWHM] may be different, in order to reach high signal to noise ratio (S/N ratio), need to set respectively the different indexes of modulation for the parameter of each spectral line, adopt different high-frequency modulation signal amplitudes.And existing signal generator and TDLAS system constructive method, that sawtooth wave and the sine-wave superimposed producing respectively by signal generator realizes, adopted certain fixing sinusoidal wave amplitude, a sawtooch sweep in the cycle, can not take into account all line parameters, thereby the sensitivity and the precision that cause many gas spectral line to detect are lower.
Summary of the invention
The present invention is intended to overcome the deficiencies in the prior art, provides a kind of and can meet many gas spectral line and detect and make each spectral line all can realize the method and apparatus of optimum detection.Based on method and apparatus of the present invention, can improve the detection sensitivity of the many gas concentrations of TDLAS system, pressure, temperature or flow velocity, the method and device be applicable to trace gas online, original position or offline inspection or monitoring and measuring application.
For achieving the above object, technical scheme of the present invention is: the best index of modulation implementation method that many gas spectral line detects, detect gas to the absorption line of laser to comprising the measurement of parameter of the gas of concentration, pressure, temperature and flow velocity, it is characterized in that, within a scan period of optical maser wavelength, the wavelength-modulated coefficient of laser changes, and within a scan period, adopts two or more sinusoidal wave amplitudes.
Sinusoidal wave amplitude V
mby the modulating characteristic of diode laser, modulation voltage-current conversion coefficient of laser driver, line parameters to be measured, jointly determined:
δ=V
mαβ （1）
Wherein, δ is the wavelength-modulated degree of depth corresponding to high frequency sinusoidal signal, unit: nm; γ
mhalf high half-breadth HWHM of spectral line to be measured, unit: nm; α is the Voltage-current conversion coefficient of laser driver, unit: mA/V; β is the electric current tuning speed of diode laser, unit: nm/mA, and in the scan period, the width W of every corresponding modulated sinusoid of gas spectral line
s=k γ
m, k is constant, 4≤k≤8.
The change of the optical maser wavelength index of modulation is to realize by changing the amplitude of the high frequency sinusoidal signal on sawtooth wave that is added in laser instrument Injection Current, near every gas absorption spectrum line, the index of modulation according to the width setting laser wavelength of this spectral line, between different modulating coefficient, seamlessly transit, no matter have several absorption lines within a scan period, all make the index of modulation of all absorption lines reach best.
The best index of modulation implement device that many gas spectral line detects, comprise: laser driver, diode laser, signal generator, photodetector, prime amplifier, lock-in amplifier, AD converter, computing machine or flush bonding processor, optical alignment lens, gas cell, the optical alignment lens projects that laser driver drives the laser process gas cell of diode laser generation and is arranged on gas cell two ends is to photodetector, the output signal of photodetector is through prime amplifier, lock-in amplifier, AD converter outputs to computing machine or flush bonding processor, signal generator is added in the Injection Current of diode laser for generation of the modulation signal of two or more sinusoidal wave amplitudes, lock-in amplifier also output feedback signal to signal generator.
Signal generator comprises microcontroller, program storage, logical circuit, D-A converting circuit, data-carrier store, modulation signal output interface, synchronous demodulation signal output interface, microcontroller produces synchronous demodulation signal according to the program operation control logic circuit in program storage and outputs to synchronous demodulation signal output interface, logical circuit also produces modulation signal by D-A converting circuit and outputs to modulation signal output interface, modulation signal is by sawtooth wave, two or more sinusoidal wave range signals are formed by stacking, synchronizing signal is the pulse signal of synchronizeing with sawtooth wave.
Technical characterstic of the present invention and effect:
Can all accomplish the best index of modulation to all absorption lines in sweep limit, reduce the response to the various noises of spectral line periphery, realize optimal spectrum input.Accordingly, improve the precision of spectral analysis, the present invention can be for the detection of single absorption line, also can be for the detection of a plurality of absorption lines.Especially larger to the precision raising effect of multiline modulated spectrum analysis (as multiline temperature survey).
Accompanying drawing explanation
Fig. 1 is diode laser modulation signal waveform of the present invention (embodiment 1)
Near a.CO2, CO, C2H2 absorption line 1582nm
B. high frequency sinusoidal modulation signal
C. sawtooth wave harmonic ringing.
Fig. 2 is modulated signal producing circuit block diagram of the present invention.
Microcontroller
Program storage
Logical circuit
D-A converting circuit
Data-carrier store
Modulation signal output interface
Synchronous demodulation signal output interface.
The TDLAS system chart that Fig. 3 (embodiment 1), Fig. 5 (embodiment 2) are application the inventive method and device carries out gas detection
8 laser drivers
9 diode lasers
10 signal generators
11 photodetectors
12 prime amplifiers
13 lock-in amplifiers
14 AD converter
15 computing machines (flush bonding processor)
16,17 optical alignment lens
18 gas cells.
Fig. 4 is diode laser modulation signal waveform of the present invention (embodiment 2)
Near the absorption line of a.H2O 1395nm
B. high frequency sinusoidal modulation signal
C. sawtooth wave harmonic ringing.
Embodiment
Many gas spectral line: refer to the absorption line of multiple gases component or many absorption lines of gas.
The present invention is a kind of does to a plurality of gas absorption spectrum lines the method and apparatus that the best index of modulation detects, within a scan period of optical maser wavelength, Voltage-current conversion coefficient based on gas line parameters to be measured, laser tuning characteristic and laser driver, a plurality of modulation signal amplitudes are set flexibly, to guarantee that the detection of each spectral line to be measured reaches the best index of modulation.More modulation signal amplitude at a length scanning in the cycle is that the signal generator based on stored digital mode is realized, and signal generator is the circuit (module) that programmable logic device (PLD) (FPGA and/or CPLD) forms.When the present invention is mainly used in many absorption lines, analyze, detect, can be for the measurement of gas concentration, temperature, pressure or flow velocity.
Technical scheme of the present invention is:
The wavelength-modulated absorption spectrum method that the index of modulation is variable, within a scan period of optical maser wavelength, the wavelength-modulated coefficient of laser changes, and within a scan period, adopts two or more sinusoidal wave amplitudes.Sinusoidal wave amplitude Vm is determined jointly by the modulating characteristic of diode laser, modulation voltage-current conversion coefficient of laser driver, line parameters to be measured.
δ=V
mαβ （1）
Wherein, δ is the wavelength-modulated degree of depth (unit: nm) corresponding to high frequency sinusoidal signal; γ
mit is half high half-breadth [HWHM(unit: nm)] of spectral line to be measured; α is the Voltage-current conversion coefficient (unit: mA/V) of laser driver; β is the electric current tuning speed (unit: nm/mA) of diode laser.
The wavelength-modulated absorption spectrum method that the described index of modulation is variable, the change of the optical maser wavelength index of modulation is to realize by changing the amplitude of the high frequency sinusoidal signal on sawtooth wave that is added in laser instrument Injection Current.Near every gas absorption spectrum line, the index of modulation according to the width setting laser wavelength of this spectral line, seamlessly transits between different modulating coefficient.No matter have several absorption lines within a scan period, all make the index of modulation of all absorption lines reach best.The index of modulation when departing from gas absorption spectra reduces (below 1/10th of the conventional index of modulation), with this, can eliminate near interference spectral line.
The wavelength-modulated absorption spectrum device that the described index of modulation is variable, its concrete methods of realizing is to produce modulation (and/or tuning) signal by signal generator or custom-designed signal generating circuit.Can use signal generator and software editing random waveform signal thereof, by signal generating module, export specific modulation signal; Can also adopt custom-designed signal generating circuit plate, based on stored digital mode, the circuit (module) being formed by programmable logic device (PLD) (FPGA and/or CPLD), its output signal comprises that am signals is for the modulation of laser instrument Injection Current, also comprise that restituted signal output is for the needed reference signal of phase-locked detection, also comprise that synchronizing signal output is for data acquisition or processing.
Modulation signal is controlled the Injection Current of diode laser by laser driver, realization is to the scanning of optical maser wavelength and high frequency modulated, laser is received by photoelectric detector and lock-in amplifier demodulation through gas absorption, by computing machine, data are done to further processing again, obtain the parameters such as concentration, temperature, pressure and flow velocity of gas.
Below in conjunction with drawings and Examples, describe the specific embodiment of the present invention in detail:
Near embodiment 1:(1580nm, measure three kinds of gas concentrations)
In 1580.5mn～1581.0nm wavelength coverage, can measure CO
2, CO, C
2h
2three kinds of gases, the parameter of absorption line can be consulted HITRAN database and be obtained.Use Distributed Feedback Laser, a length scanning cycle, can cover, measure the concentration of these three kinds of gases.But three spectral lines to be measured have different line parameters, as shown in table 1.
Table 1.CO
2, CO, C
2h
2the parameter of absorption line (T=296K, P=1atm, x near 1580nm
cO2=x
cO=x
c2H2=1%, 97% air)
| Sequence number | Gas molecule | Line wavelength/nm | Absorption Line is strong/cm/molecule | Line width/nm |
| 1. | CO 2 | 1580.993 | 1.13E-23 | 0.0359 |
| 2. | CO | 1580.828 | 1.57E-23 | 0.0319 |
| 3. | C 2H 2 | 1581.129 | 9.08E-25 | 0.0385 |
Use Distributed Feedback Laser in the cycle, to cover this three spectral lines at a length scanning, with current existing method, the high frequency Sine Modulated amplitude of laser instrument is fixed, therefore, identical to the wavelength-modulated amplitude of these three spectral lines, but the index of modulation is different.Cannot take into account and make them can both reach optimum detection.
The spectroscopic analysis methods that multiline of the present invention detects, by a plurality of different modulation signal amplitudes are set in the cycle at a length scanning, can realize the optimum detection (having three spectral lines in the present embodiment) to all spectral lines.Three absorption lines of gas to be measured, propose the high-frequency modulation signal put, sawtooch sweep signal as shown in Figure 1.What wherein Fig. 1 a provided is the absorption line of three kinds of gases to be measured, and Fig. 1 b is that high frequency sinusoidal signal, Fig. 1 c are sawtooth signals, and the stack of waveform shown in Fig. 1 b and Fig. 1 c forms the modulation signal of diode laser.
The spectral analysis device of detection multiline of the present invention, comprise the parts such as diode laser 9, laser driver 8, signal generator 10, photodetector 11, prime amplifier 12, lock-in amplifier 13, gas cell 18, computing machine (or flush bonding processor) 15, optical alignment lens 16/17, gas cell 18, its principle of compositionality as shown in Figure 3.Wherein diode laser, lock-in amplifier, gas cell, computing machine can adopt commercial instrument or module.Signal generator can adopt the arbitrary-function generator of supporting programming mode, or adopts custom-designed signal generating circuit (module), and what in the present embodiment, use is custom-designed signal generating circuit module, and its schematic block circuit diagram as shown in Figure 2.Wherein the line width of sinusoidal wave amplitude based on table 1, calculates according to formula (1), is respectively:
A
1=79mV
A
2=70mV
A
3=85mV
A
min=7mV
Between the sinusoidal signal of different amplitudes, seamlessly transit, form needed modulation signal, and with the modulation signal of the superimposed formation laser driver of sawtooth wave, be written to the data-carrier store (as shown in Figure 2) of signal generating circuit, this modulation signal outputs to the modulation signal input end (as shown in Figure 3) of laser driver.
The sinusoidal signal frequency f of signal generating circuit module output can be for getting 10kHz in several kHz-hundreds of kHz(the present embodiment), the restituted signal that offers lock-in amplifier is that frequency is to get 20kHz in 2f(the present embodiment) square-wave signal.Signal generating circuit module also produces synchronizing signal, for the acquisition process of data.
The principle of work of detection multiline spectral analysis device of the present invention is, laser driver is controlled temperature and the Injection Current of diode laser, Injection Current is subject to the control of input modulating signal, produce the laser of wavelength tuning, after the gas absorption in gas cell, photodetector picks up spectral signal and send into lock-in amplifier after prime amplifier amplifies, and signal generator provides the modulation signal of laser driver needs and the reference signal that lock-in amplifier needs.The simulating signal of the output of lock-in amplifier is converted to digital signal through AD converter and is sent to computing machine (or flush bonding processor) and processes, analyze, and is calculated the concentration of gas by Lambert-beer law.
Embodiment 2:(1.395um measures temperature, open light path)
By measuring many absorption lines of hydrone, measure temperature, near H 1395.6nm
2the absorption line parameter of O can be consulted HITRAN database and be obtained, as shown in table 2.Use Distributed Feedback Laser, a length scanning cycle can cover the absorption line of two H2O.Yet because two spectral lines to be measured have different line width, they are respectively:
Table 2.H
2parameter (T=296K, P=1atm, the x of near O absorption line 1395.6nm
h2O=10%, 90% air)
| Sequence number | Line wavelength/nm | Wave number/cm -1 | Absorption Line is strong/cm/molecule | Line width/ |
| 1. | 1395.693 | 7164.90 | 1.47E-22 | 0.0202 |
| 2. | 1395.514 | 7165.82 | 5.93E-21 | 0.0505 |
Use Distributed Feedback Laser in the cycle, to cover this two spectral lines at a length scanning, with current existing method, the high frequency Sine Modulated amplitude of laser instrument is fixed, therefore, identical to the wavelength-modulated amplitude of these two spectral lines, but the index of modulation is different.Cannot take into account and make them can both reach optimum detection.
The spectroscopic analysis methods that multiline of the present invention detects, by a plurality of different modulation signal amplitudes are set in the cycle at a length scanning, can realize the optimum detection (having two spectral lines in the present embodiment) to all spectral lines.Two absorption lines of gas to be measured, propose the high-frequency modulation signal put, sawtooch sweep signal as shown in Figure 4.That wherein Fig. 4 a provides is H in scanning wave band
2the absorption line of O, Fig. 4 b is that high frequency sinusoidal signal, Fig. 4 c are sawtooth signals, the stack of waveform shown in Fig. 4 b and Fig. 4 c forms the modulation signal of diode laser.
The spectral analysis device of detection multiline of the present invention, comprises the parts such as diode laser, laser driver, signal generator, lock-in amplifier, gas cell, computing machine, and its principle of compositionality as shown in Figure 5.Wherein diode laser, lock-in amplifier, gas cell, computing machine can adopt commercial instrument or module.Signal generator can adopt the arbitrary-function generator of supporting programming mode, or adopts custom-designed signal generating circuit (module), and what in the present embodiment, use is custom-designed signal generating circuit module, and its schematic block circuit diagram as shown in Figure 2.Wherein the line width of sinusoidal wave amplitude based on table 2, calculates according to formula (1), is respectively:
A
1=45mV
A
2=111mV
A
min=5mV
Between the sinusoidal signal of different amplitudes, seamlessly transit, form needed modulation signal, and with the modulation signal of the superimposed formation laser driver of sawtooth wave, be written to the data-carrier store (as shown in Figure 2) of signal generating circuit, this modulation signal outputs to the modulation signal input end (as shown in Figure 5) of laser driver.
The sinusoidal signal frequency f of signal generating circuit module output can be for getting 50kHz in several kHz-hundreds of kHz(the present embodiment), the restituted signal that offers lock-in amplifier is that frequency is to get 100kHz in 2f(the present embodiment) square-wave signal.Signal generating circuit module also produces synchronizing signal, for the acquisition process of data.
Claims (5)
1. the best index of modulation implementation method that gas spectral line more than a kind detects, it is characterized in that, detect gas to the absorption line of laser to comprising the measurement of parameter of the gas of concentration, pressure, temperature and flow velocity, it is characterized in that, within a scan period of optical maser wavelength, the wavelength-modulated coefficient of laser changes, and within a scan period, adopts two or more sinusoidal wave amplitudes.
2. the best index of modulation implementation method that many gas spectral line as claimed in claim 1 detects, is characterized in that, sinusoidal wave amplitude V
mby the modulating characteristic of diode laser, modulation voltage-current conversion coefficient of laser driver, line parameters to be measured, jointly determined:
δ=V
mαβ （1）
Wherein, δ is the wavelength-modulated degree of depth corresponding to high frequency sinusoidal signal, unit: nm; γ
mhalf high half-breadth HWHM of spectral line to be measured, unit: nm; α is the Voltage-current conversion coefficient of laser driver, unit: mA/V; β is the electric current tuning speed of diode laser, unit: nm/mA, and in the scan period, the width W of every corresponding modulated sinusoid of gas spectral line
s=k γ
m, k is constant, 4≤k≤8.
3. the best index of modulation implementation method that many gas spectral line as claimed in claim 1 detects, it is characterized in that, the change of the optical maser wavelength index of modulation is to realize by changing the amplitude of the high frequency sinusoidal signal on sawtooth wave that is added in laser instrument Injection Current, near every gas absorption spectrum line, the index of modulation according to the width setting laser wavelength of this spectral line, between different modulating coefficient, seamlessly transit, no matter have several absorption lines and spectral line not overlapping within a scan period, all make the index of modulation of all absorption lines reach best.
4. the best index of modulation implement device that gas spectral line more than a kind detects, it is characterized in that, comprise: laser driver, diode laser, signal generator, photodetector, prime amplifier, lock-in amplifier, AD converter, computing machine or flush bonding processor, optical alignment lens, gas cell, the optical alignment lens projects that laser driver drives the laser process gas cell of diode laser generation and is arranged on gas cell two ends is to photodetector, the output signal of photodetector is through prime amplifier, lock-in amplifier, AD converter outputs to computing machine or flush bonding processor, signal generator is added in the Injection Current of diode laser for generation of the modulation signal of two or more sinusoidal wave amplitudes, lock-in amplifier also output feedback signal to signal generator.
5. the best index of modulation implement device that many gas spectral line as claimed in claim 4 detects, it is characterized in that, signal generator comprises microcontroller, program storage, logical circuit, D-A converting circuit, data-carrier store, modulation signal output interface, synchronous demodulation signal output interface, microcontroller produces synchronous demodulation signal according to the program operation control logic circuit in program storage and outputs to synchronous demodulation signal output interface, logical circuit also produces modulation signal by D-A converting circuit and outputs to modulation signal output interface, modulation signal is by sawtooth wave, two or more sinusoidal wave range signals are formed by stacking, synchronizing signal is the pulse signal of synchronizeing with sawtooth wave.
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| CN107367571A (en) * | 2017-08-15 | 2017-11-21 | 江苏师范大学 | A kind of combustion of fossil fuel efficiency detection device and detection method |
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| CN110567914A (en) * | 2019-10-12 | 2019-12-13 | 海隆石油工业集团有限公司 | Multi-beam laser spectrum gas detection device and system based on optical beam splitter |
| CN111044485A (en) * | 2019-12-04 | 2020-04-21 | 清华大学合肥公共安全研究院 | Tunable laser absorption harmonic demodulation circuit and method based on FPGA (field programmable Gate array) synchronous signals |
| CN115275772A (en) * | 2022-09-26 | 2022-11-01 | 南京旭奥科技有限公司 | TDLAS technology-based specific time laser wavelength control method and device |
| CN117405627A (en) * | 2023-12-14 | 2024-01-16 | 北京中科智易科技股份有限公司 | Gas quality laser analysis system and analysis method |
| CN117405627B (en) * | 2023-12-14 | 2024-02-20 | 北京中科智易科技股份有限公司 | Gas quality laser analysis system and analysis method |
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