CN115165781A - Gas parameter measuring method based on phase-lock-free double-optical-comb absorption spectrum - Google Patents
Gas parameter measuring method based on phase-lock-free double-optical-comb absorption spectrum Download PDFInfo
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Abstract
The invention provides a gas parameter measuring method based on phase-lock-free double-optical-comb absorption spectrum, and belongs to the technical field of laser absorption spectrum. The phase-lock-free double-optical comb is split by an optical fiber coupler, one path is a measuring optical path, and the measuring optical path is received by a photoelectric detector after passing through an optical band-pass filter and gas to be measured; one path is a reference light path and is coupled to the photoelectric detector after passing through the optical band-pass filter; interference signals generated by the phase-lock-free double-optical comb on the photoelectric detector are filtered by the low-pass filter and then are acquired by the data acquisition card; the method comprises the steps of extracting frequency spectrum by performing Fourier transform on interference signals of a measurement and reference light path, extracting frequency spectrum jitter by frequency spectrum cross-correlation, compensating the frequency spectrum jitter by frequency spectrum shifting, obtaining an original absorption spectrum by suppressing noise by multiple times of averaging, and finally calculating parameters of the gas to be measured based on a corrected multispectral fitting method. The invention simplifies the system complexity of the optical frequency comb required in the gas parameter measurement, can realize the wide spectrum measurement and has wide application prospect.
Description
Technical Field
The invention relates to a gas parameter measuring method based on phase-lock-free double-optical-comb absorption spectrum, belonging to the technical field of laser diagnosis.
Background
As a non-contact measurement method with quick response, high sensitivity and interference resistance, a gas parameter measurement technology based on laser absorption spectrum is rapidly developed in recent years, and is widely applied to the fields of combustion diagnosis, atmosphere monitoring, industrial fields and the like.
The conventional Laser Absorption Spectroscopy technology mainly uses Tunable Diode Laser Absorption Spectroscopy (TDLAS). The tunable diode laser used in TDLAS technology has a limited wavelength scanning range, generally covers only one to two characteristic absorption peaks of a gas to be measured, and is used for different components to be measured, such as H 2 O、CO、CO 2 And CH 4 The laser of a different wavelength needs to be replaced for measurement. TDLAS generally uses colorimetry temperature measurement, carries out accurate measurement to the temperature in order to use colorimetry, and generally selects to be no less than two characteristic absorption peaks according to the test temperature range, and its low energy should have great difference, generally uses a plurality of lasers to scan a plurality of absorption spectral lines in the in-service use. For example, a publication of Simultaneous measurement of multi-flow parameter characteristics of scramjet engines using tunable diode laser sensors (Simutaneous measurements of multiple flow parameters for scramjet recording using tubular diode-laser sensors) with a center wave number of 7185cm, published by Li Fei et al in 2011 in application Optics (Applied Optics) volume 50, pages 36, 6697-6707 -1 And 7444cm -1 The two Distributed Feedback (DFB) lasers tested the airflow velocity, temperature and H at three different locations of the scramjet engine 2 And O concentration. Multiple lasers may also be selected for use in order to measure the concentration of multiple components. In 2009, g.b. rieker et al published in Applied Optics, volume 48, page 29, 5546, paper "measuring gas temperature and concentration without calibrated wavelength modulation spectroscopy in harsh environment" (Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in hyper-burning ramjet exhaust), testing temperature, CO, of hyper-burning ramjet exhaust 2 、H 2 In the process of O concentration, six DFB lasers with different wave bands are used, and laser signals with different wave bands are received by a plurality of detectors after grating light splitting, so that the complexity of the system is increased.
An Optical Frequency Comb (OFC), as a novel ultrashort pulse laser light source, is spectrally represented by a series of Frequency components with uniform intervals and coherent stable phase relationship, the Frequency intervals are laser pulse repetition frequencies, and the spectral range can cover tens of nanometers to hundreds of nanometers. Due to the advantages of short single pulse time, wide spectral range, capability of realizing mapping from optical frequency to radio frequency and the like, the optical frequency comb technology is rapidly developed in the last decade and is successfully applied to the fields of absolute distance measurement, ultrafast imaging, wide spectrum spectroscopy, atmospheric science and the like. At present, the optical frequency comb has achieved important application in the aspect of gas concentration measurement. In 2018, S.Coburn et al published in optics (Optica) volume 5, page 4, 320, a study of in-zone trace gas attribution using a field-deployed Dual-Frequency Comb spectrometer (Dual Frequency Comb, DFC) system to open areas of CH over 1km in a field-by-field trace gas attribution study 4 The leak rate is monitored. The optical frequency comb technology is also primarily applied to the field of high-temperature testing. C.A. Alrahman et al, 2014, published in the optical Rapid report (Optics Express) volume 22, page 11, 13889, the Cavity-enhanced optical frequency comb spectrum of water vapor in flames (Cavity-enhanced optical frequency comb spectrum of high-temperature H) 2 O in a flame) is tested by using a single optical frequency comb, a Fourier transform spectrometer is used as a spectral analysis means in the experiment to obtain a fine spectral structure, and the absorption spectrum of high-temperature water vapor is given. Although the paper does not give a calculation method and a result of the temperature and the concentration of the water vapor, the paper preliminarily shows the potential application value of the optical frequency comb in the field of high-temperature testing. The fourier transform spectrometer used in the paper is tested based on the michelson interferometer principle, and in order to obtain a wider wavelength test range, the fourier transform spectrometer needs to be additionally provided withThe scanning length of the mechanical arm enables the stability of the system to be reduced, and meanwhile, the mechanical scanning causes long equipment testing time and is not suitable for occasions needing rapid measurement. Schroeder et al, P.J. 2017, published in the journal of the society of Combustion (Proceedings of the Combustion Institute) volume 36, no. 3, 4565-4573, paper double-photon comb absorption Spectroscopy for 16MW gas turbine exhaust (Dual frequency comb laser absorption spectroscopy in a 16MW gas turbine exhaust) have conducted long-term monitoring of gas turbine exhaust temperature and carbon dioxide, water vapor concentrations. The used double-optical comb system covers hundreds of absorption spectral lines of water vapor and dozens of absorption spectral lines of carbon dioxide with the spectral resolution of 1.4pm within the range of 1435.5-1445.1 nm, a baseline is obtained in a high-order polynomial fitting mode, a spectral absorption curve is further obtained, and then the average temperature and the gas concentration on a laser path are obtained in a multispectral fitting mode. The mode of fitting the baseline by using the high-order polynomial is poor in fitting effect when the spectrum shape of the optical frequency comb is not flat enough or serious distortion exists, and in addition, when the absorption intensity is high, weak absorption existing at two wings of an absorption peak is not favorable for fitting the baseline. And the double-optical comb used in the paper tightly locks the repetition frequency and the carrier envelope phase shift frequency, the extraction of the carrier envelope phase shift frequency needs the optical frequency comb output spectrum to cover an octave, the extraction is carried out through f-2f interference, the complicated laser spectrum expansion, optical frequency multiplication and interference light path design, the high-bandwidth electrical feedback and the actuator design greatly increase the complexity and the design cost of the system, and the double-optical comb is not suitable for large-scale use. In 2020, yankangwen et al, published in "Chinese OPTICS LETTERS works" volume 18, no. 5, paper "Temperature measurement based on adaptive sampling double optical comb spectroscopy" (Temperature measurement on adaptive dual-absorption spectroscopy) measured the Temperature of high Temperature steam. In the thesis, the waveform and sampling rate of a dual-optical comb interference signal are adaptively adjusted by using a beat signal of a narrow linewidth laser and the dual-optical comb, so that the complex feedback design in the process of tightly locking the optical comb is avoided. However, the method adds two additional narrow linewidth lasers and beats the frequency signal at the same timeThe detection and the generation of the adaptive signal still need complex circuit design. Meanwhile, the bilinear method is used for calculating the water vapor temperature in the paper, so that the wide spectrum characteristic of the double optical comb cannot be effectively utilized.
Based on the background, the invention provides a gas parameter measuring method based on a phase-lock-free double-optical comb. The method comprises the steps of using a phase-lock-free double-optical comb as a light source, building a reference light path and a measuring light path to collect interference signals, carrying out Fourier transform after the interference signals are grouped, extracting and correcting frequency spectrum jitter caused by the fact that repetition frequency and carrier envelope phase shift frequency are not tightly locked by using a frequency spectrum cross-correlation method, and then calculating gas parameters by using a corrected multispectral fitting method. The method has the advantages that the double-optical comb is used as the light source, a wider spectrum range can be covered, and the measurement of the temperature of the gas to be measured and the concentration of various components can be realized; the double optical combs are phase-locked-free, the carrier envelope phase shift frequency does not need to be locked, and the light source design of the optical frequency combs is greatly simplified; by using the corrected multispectral fitting method, baseline fitting in absorption spectrum calculation is avoided, and intensity difference between the reference light path and the measurement light path along with wavelength change can be effectively solved.
Disclosure of Invention
Aiming at the problem of extracting the temperature, concentration and pressure parameters of the gas to be measured, the invention discloses a gas parameter measuring method based on a phase-lock-free double-optical comb.
The measuring system mainly comprises two optical frequency combs with slightly different repetition frequencies, a 2 multiplied by 2 optical fiber coupler, two optical band-pass filters, a collimator, two low-pass filters, two photoelectric detectors, a data acquisition card and a computer. Because the spectrum coverage range of the optical frequency comb can reach hundreds of nanometers, only optical filters with different wave bands and bandwidths need to be replaced according to different to-be-detected gases and wave band requirements.
Firstly, a measuring system is set up based on a phase-lock-free double-optical-comb light source, interference signals of a measuring light path and a reference light path are collected, and then Fourier spectrum transformation is carried out to obtain a spectrum with absorption information and a spectrum without the absorption information; secondly, extracting and correcting the frequency spectrum jitter of the interference signal by a frequency spectrum cross correlation method, and mapping the radio frequency spectrum to a spectrum; finally, calculating the gas parameters by a corrected multispectral fitting method.
Step one, building a light path and collecting interference signals:
two optical frequency combs each having a repetition frequency f r1 、f r2 (f r1 >f r2 ) Difference in repetition frequency f rep ＝f r1 -f r2 The carrier envelope phase shift frequencies of the two optical frequency combs are not locked, the two optical frequency combs are coupled through a 2 x 2 optical fiber coupler and then divided into a light beam A and a light beam B, the light beam A penetrates through a reference light path, and the light beam B penetrates through a measurement light path; in the reference optical path, the light beam a is directly coupled to the photodetector 151 through the optical band-pass filter 141, and the electrical signal output by the photodetector 151 is filtered by the low-pass filter 161 to obtain the interference signal S without absorption spectrum information ref ，S ref Repetition frequency f rep (ii) a In the measurement optical path, the light beam B is collimated by the collimator 121, passes through the gas 131 to be measured, is coupled to the photodetector 152 through the optical band-pass filter 142, and the electrical signal output by the photodetector 152 is filtered by the low-pass filter 162 to obtain the interference signal S containing the absorption spectrum information meas ，S meas Repetition frequency f rep (ii) a The double-optical comb light source maps the optical frequency signals difficult to detect to radio frequency in a multi-heterodyne interference mode, and in order to ensure the one-to-one correspondence between the radio frequency components and the optical frequency components of the interference signals, the bandwidths of the optical band-pass filter 141 and the optical band-pass filter 142 cannot exceed
The bandwidth of the low pass filter 161 and the low pass filter 162 cannot exceed
Finally, synchronously acquiring interference signals S by using a data acquisition card 171 ref 、S meas The signal sampling rate isf r1 The collected signals are uploaded to a computer 181 for post-processing;
the optical band-pass filter 141 and the optical band-pass filter 142 have the same center wavelength λ and bandwidth Δ λ, and the low-pass filter 161 and the low-pass filter 162 have the same cutoff frequency;
step two, extracting and compensating interference signal frequency spectrum jitter:
longitudinal mode frequency f of longitudinal mode number p included in optical frequency comb 101 p Can be expressed as:
f p ＝pf r1 +f ceo1 , (1)
wherein f is r1 Is the repetition frequency, f, of the optical frequency comb 101 ceo1 Is the carrier envelope offset frequency of the optical-frequency comb 102;
longitudinal-mode frequency f of longitudinal-mode number q included in optical-frequency comb 102 q Can be expressed as:
f q ＝qf r2 +f ceo2 , (2)
wherein f is r2 Is the repetition frequency, f, of the optical frequency comb 102 ceo2 Is the carrier envelope offset frequency of the optical-frequency comb 102;
the optical frequency comb 101 and the optical frequency comb 102 generate radio frequency sub-combs through multi-heterodyne interference, the frequency range of interference signals is limited through the optical band- pass filters 141 and 142, so that the comb tooth pairs of the optical frequency comb 101 and the optical frequency comb 102 are uniquely corresponding to the comb teeth of the radio frequency sub-combs, and the longitudinal mode number of the optical frequency comb 101 corresponding to the first comb tooth pair generating beat frequencies in the range of the optical band-pass filters is recorded as n s1 The number of longitudinal modes of the optical-frequency comb 102 is denoted as n s2 The beat frequency generated by the comb is denoted as f o It can be expressed as:
f o ＝n s1 f r1 -n s2 (f r1 -f rep )+f ceo1 -f ceo2 . (3)
then the radio frequency sub-combs generated by multi-heterodyne interference by optical-frequency combs 101 and 102 may be represented as
f nRF ＝n RF f rep +f o , (4)
Wherein, the radio frequency isThe repetition frequency of the comb is equal to the difference f between the repetition frequencies of the optical-frequency comb 101 and the optical-frequency comb 102 req ；n RF Has a value range of
Wherein c is the speed of light;
when the repetition frequency of the optical frequency comb and the carrier envelope phase shift frequency are not tightly locked, the comb teeth frequency of the radio frequency sub-comb will also correspondingly shake, which can be expressed as
δf nRF ＝n RF δf rep +δf o . (5)
Consider n in (3) s1 、n s2 Is much higher than n in (4) RF Thus | δ fo |＞＞|n RF δf req Such that the comb tooth frequency of the radio frequency sub-comb can be approximately expressed as
δf nRF ＝n RF δf rep +δf o ≈δf o . (6)
Thus, the jitter between the radio spectrum envelopes of the interference signals is approximately δ fo By calculating delta fo The frequency spectrums of the interference signals in different time can be shifted, so that the frequency spectrums of the interference signals in different time can be aligned, and subsequent averaging is facilitated.
According to the repetition frequency f of the interference signal rep And a sampling rate f r1 Interference signal S obtained by sampling data acquisition card ref Dividing the period into a number of data points per period
Get N number of data points as
The interference signal of (2) is denoted as S ref (i) I =1,2, \ 8230;, N; similarly, for the interference signal S meas The number of the data points of N groups is obtained by carrying out period division
The interference signal of (2) is denoted as S meas (i) I =1,2, \ 8230, N, i denotes the number of interference signals;
are respectively paired with S ref (i) And S meas (i) Fourier transform is carried out to obtain the frequency spectrum I ref (i) And I meas (i)；
Extracting measuring light path interference signal frequency spectrum I by frequency spectrum cross-correlation method meas (i) Relative to I meas (1) Is calculated by spectral cross-correlation, i (i), i =1,2, \ 8230, N, the spectral jitter f (i) is calculated by spectral cross-correlation: respectively calculate I meas (i) And I meas (1) The corresponding independent variable when the cross-correlation function obtains the maximum value is the frequency spectrum jitter f (i), i =1,2, \8230;
will I meas (i) Moving f (I) to realize I meas (i) The frequency spectrum of interference signal of the measuring light path after alignment is marked as I meas,align (i) (ii) a Similarly, will I ref (i) Moving f (I) to realize I ref (i) Aligning on the frequency domain, and recording the frequency spectrum of the reference light path interference signal after aligning as I ref,align (i)；
Averaging the aligned interference signal spectrums to suppress intensity noise and calculate an original absorption spectrum:
averaging the interference signal spectra of the aligned measurement optical path and reference optical path to suppress intensity noise, which can be expressed as:
the absorption spectrum of the gas to be measured in the radio frequency domain is calculated through logarithm operation, and can be represented as:
selecting the absorption spectrum of the gas emission domain to be measuredA certain absorption peak above, the corresponding frequency of which is denoted as f tag Searching the spectrum transition frequency corresponding to the absorption peak through the spectrum database of the molecule to be detected and recording the frequency as v tag Then the absorption spectrum of the radio frequency domain can be mapped to a spectrum by:
thereby obtaining the absorption spectrum alpha of the gas to be measured meas (v)；
Correcting the absorption spectrum of the gas to be measured, and eliminating the intensity difference of the reference light path and the measurement light path along with the wavelength change:
in actual optical configuration, the light intensities of the reference light path and the measurement light path are different inevitably, and the light intensity ratio between the two paths changes along with the wavelength, so that the compensation coefficient is used for compensating the calculation error caused by the light intensity difference of the reference light path and the measurement light path along with the wavelength in the calculation process of the absorption spectrum. The true absorption spectrum alpha (v) and the measured absorption spectrum alpha meas (v) Can be expressed as:
α(v)＝α meas (v)+k 0 +k(v), (11)
wherein k is 0 A constant part representing a compensation coefficient, k (v) being a part of the compensation coefficient varying with wavelength;
considering that the compensation coefficient changes slowly with the wavelength, a difference strategy can be adopted to restrain the change of the compensation coefficient, and at a fixed frequency point v ref Here, the following approximate expression can be obtained:
α(v)-α(v ref )＝α meas (v)+k(v)-α meas (v ref )-k(v ref )≈α meas (v)-α meas (v ref ). (12)
selecting a series of reference frequency points in the whole spectrum to form a set { v } ref -to extend (13) the applicability of the selected set of frequency points { v } ref Is located in the slowly changing part of the absorption spectrum, away from the strong transition of the gas molecules to be measured. Selection procedure at reference frequency pointSimulating the absorption spectrum of the gas to be measured by using a spectrum database; and extracting a frequency point corresponding to the local minimum value of the simulated absorption spectrum, and taking the starting frequency point and the ending frequency point of the simulated absorption spectrum as reference frequency points.
Set of frequency points { v } ref α corresponding to meas (v ref) Is interpolated to obtain alpha meas,ref (v) (13) can be rewritten as
α(v)-α ref (v)≈α meas (v)-α meas,ref (v), (13)
Wherein alpha is ref (v) From a selected set of reference frequencies v ref Corresponding true absorption rate α (v) ref ) And interpolating values in the measured spectral range.
The modified absorption spectrum becomes
α meas,fit (ν)＝α meas (ν)-α meas,ref (ν), (14)
Corresponding modified absorption spectrum alpha of the spectral database database,fit (v) Calculated according to the difference method
α database,fit (ν)＝α database (ν)-α database,ref (ν), (15)
Wherein alpha is database (v) Is the absorption spectrum, alpha, of the gas molecule to be measured calculated in the database database,ref (v) Is alpha database (v ref ) Interpolation in the measured spectral range.
Step five, calculating gas parameters by the corrected multispectral fitting algorithm:
the optical frequency comb is a wide-spectrum laser, covers a plurality of transition spectral lines of a plurality of gas components, and therefore gas parameters based on multispectral fitting are possible. In the multispectral fitting, the temperature, concentration and pressure of the gas to be measured are selected as fitting parameters for the optimization problem
minh(T,X,P)＝min||α fit,ref (ν)-α databse,ref (ν,T,X,P)|| 1 , (16)
Wherein the operator | · | non-woven 1 Representing the 1 norm of the vector.
By solving the minimization problem of (17), the temperature, concentration and pressure parameters of the gas to be measured can be obtained.
In the actual parameter fitting process, fitting constraints can be added according to specific test conditions so as to simplify the optimization problem; for example, in an atmospheric environment, the gas pressure can be taken as a known constant; for a gas-tight chamber, the pressure and temperature are linear.
In view of the wide spectral range of the optical frequency comb, the optical band-pass filter with a certain spectral range is selected for the optical frequency comb to avoid the occurrence of frequency spectrum aliasing in the system, so that the optical filtering is carried out on the optical frequency comb, the spectral information loss is caused to a certain degree, and different types of gas molecules such as H are required to be subjected to optical filtering if the optical frequency comb is required to be subjected to optical filtering 2 O、CO 2 、CH 4 、C 2 H 2 And meanwhile, measurement is carried out, and the selected spectral line exceeds the spectral range of a single optical band-pass filter, so that the gas to be measured can be analyzed and measured by using the combination of the optical band-pass filters with different spectral bands, and at the moment, part of devices (the optical band-pass filter and the photoelectric detector) in the attached drawing 1 need to be expanded.
Drawings
FIG. 1 is a typical structure diagram of the measurement of gas parameters of a phase-lock-free double-optical comb, which is composed of the following parts: the system comprises an optical frequency comb 101, an optical frequency comb 102, a 2 x 2 optical fiber coupler 111, a collimator 121, a gas to be measured 131, an optical band-pass filter 141, an optical band-pass filter 142, a photoelectric detector 151, a photoelectric detector 152, a low-pass filter 161, a low-pass filter 162, a data acquisition card 171 and a computer 181.
FIG. 2 is a flow chart of a gas parameter measurement method based on phase-lock-free double optical combs.
Detailed Description
The present invention is further illustrated by the following examples.
This example is for C 2 H 2 The temperature concentration of the gas pool was measured and its spectrum was obtained in the 1530 + -3.75 nm range, the spectral jitter was corrected by the proposed spectral cross-correlation, and C was calculated by multi-spectral fitting 2 H 2 Temperature and concentration.
Step one, building a light path and collecting interference signals:
two optical frequency combs each having a repetition frequency f r1 ＝60.0008MHz、f r2 =60.000MHz, repetition frequency difference f rep ＝f r1 -f r2 =800Hz, and is divided into two beams after being coupled by a 2 × 2 fiber coupler, the two beams are divided into a light beam a and a light beam B, the light beam a passes through the reference light path, and the light beam B passes through the measurement light path; in the reference optical path, the light beam a is directly coupled to the photodetector 151 through the optical band-pass filter 141 (pass band range 1530 ± 3.75 nm), and the electrical signal output by the photodetector 151 is filtered by the low-pass filter 161 to obtain the interference signal S without absorption spectrum information ref The repetition frequency is 800Hz; in the measuring light path, the light beam B is collimated by the collimator 121 and passes through the gas to be measured (in this example, C is selected) 2 H 2 The internal parameters of the gas pool to be measured are uniform, the pressure is 1atm, the temperature is 295K 2 H 2 The concentration is 45 percent, and the rest component is N 2 The length of the gas cell is 9 cm), the signal is coupled to the photodetector 152 through the optical band-pass filter 142 (the pass band range 1530 +/-3.75 nm), and the electric signal output by the photodetector 152 is filtered by the low-pass filter 162 to obtain the interference signal S containing the absorption spectrum information meas The repetition frequency is 800Hz; the bandwidth of the low-pass filter 161 and the low-pass filter 162 is 30MHz; finally, synchronously acquiring interference signals S by using a data acquisition card ref 、S meas The sampling rate of the signal is 60.008MHz, and the collected signal is uploaded to a computer for post-processing;
step two, extracting and compensating interference signal frequency spectrum jitter:
according to the repetition frequency f of the interference signal rep And a sampling rate f r1 Interference signal S obtained by sampling the data acquisition card in the step one ref Carrying out periodic division to obtain N groups of interference signals, wherein the number of data points of each group of interference signals is
Is recorded as S ref (i) I =1,2, \ 8230;, N; similarly, for the interference signal S meas Carrying out periodic division to obtain N groups of interference signals, wherein the number of data points of each group of interference signals is
Is recorded as S meas (i) I represents a group number of the interference signal;
are respectively paired with S ref (i) And S meas (i) Fourier transform is carried out to obtain the frequency spectrum I ref (i) And I meas (i)；
Extracting measuring light path interference signal frequency spectrum I by frequency spectrum cross-correlation method meas (i) Relative to I meas (1) Is provided, I =1,2, \ 8230n, N, respectively, I is given by meas (i) Moving f (I) to realize I meas (i) The frequency spectrum of interference signal of the measuring light path after alignment is marked as I meas,align (i)；I ref (i) Moving f (I) to realize I ref (i) Alignment in the frequency domain; the frequency spectrum of the reference light path interference signal after alignment is recorded as I ref,align (i)；
Averaging the aligned interference signal spectrums to suppress intensity noise and calculate an original absorption spectrum:
averaging interference signal frequency spectrums of the aligned measuring light path and the reference light path obtained in the step two respectively to suppress intensity noise to obtain I meas,ave And I ref,ave From I meas,ave And I ref,ave Calculating the absorption spectrum alpha of the gas to be measured in the radio frequency domain meas (f)：
Calculating to obtain an original absorption spectrum alpha according to the mapping relation between the radio frequency and the optical frequency meas (v)；
Correcting the absorption spectrum of the gas to be detected and calculating gas parameters:
in the spectral range 6521.7-6550.9cm -1 A series of reference frequency points {6521.71015cm ] are selected in -1 、6522.09349cm -1 、6525.66225cm -1 、6528.00795cm -1 、6530.84653cm -1 、6532.71762cm -1 、6535.77525cm -1 、6538.02968cm -1 、6540.69484cm -1 、6542.79411cm -1 、6545.92475cm -1 、6548.08791cm -1 、6550.28758cm -1 、6550.90823cm -1 Constitute a set { v } ref H, a selected set of frequency points { v } ref The site of the slow change in the acetylene absorption spectrum is far from the strong transition line of acetylene. Set of frequency points { v ref α corresponding to meas (v ref ) Is interpolated to obtain alpha meas,ref (v) Obtaining a corrected absorption spectrum alpha corresponding to the original absorption spectrum meas,fit (v)
α meas,fit (ν)＝α meas (v)-α meas,ref (v). (18)
Set of frequency points { v ref Absorption spectrum alpha calculated from HITRAN spectrum database database (v ref ) Interpolation is carried out to obtain alpha database,ref (v) Obtaining a corresponding corrected fitting spectrum alpha of the spectrum database database,fit (v)：
α database,fit (ν)＝α database (ν)-α database,ref (ν), (19)
In the modified multispectral fit, temperature, acetylene concentration, pressure were chosen as the fitting parameters:
minh(T,X,P)＝min||α fit,ref (ν)-α databse,ref (ν,T,X,P)|| 1 , (20)
wherein the operator | · | non-woven 1 Representing the 1 norm of the vector.
The above description of the invention and its embodiments is not intended to be limiting, and the illustrations in the drawings are intended to represent only one embodiment of the invention. Without departing from the spirit of the invention, it is within the scope of the invention to design structures or examples similar to the technical solutions without creation.
Claims (3)
1. A gas parameter measurement method based on phase-lock-free double-optical-comb absorption spectrum is characterized in that a phase-lock-free double-optical comb is used as a light source for gas parameter measurement, and the measurement method comprises the following steps:
step one, building a light path and collecting interference signals:
the repetition frequencies of the optical frequency combs 101 and 102 are respectively f r1 、f r2 (f r1 ＞f r2 ) Difference in repetition frequency f rep ＝f r1 -f r2 Carrier envelope phase shift frequency f of optical-frequency combs 101 and 102 ceo1 、f ceo2 When the two optical frequency combs are unlocked, the two optical frequency combs are coupled through a 2 x 2 optical fiber coupler and then divided into a light beam A and a light beam B, wherein the light beam A passes through a reference optical path, and the light beam B passes through a measuring optical path; in the reference optical path, the light beam a is directly coupled to the photodetector 151 through the optical band-pass filter 141, and the electrical signal output by the photodetector 151 is filtered by the low-pass filter 161 to obtain the interference signal S without absorption spectrum information ref ，S ref Repetition frequency f rep (ii) a In the measurement light path, the light beam B is collimated by the collimator 121, passes through the gas 131 to be measured, is coupled to the photodetector 152 through the optical band-pass filter 142, and the electrical signal output by the photodetector 152 is filtered by the low-pass filter 162 to obtain the interference signal S containing the absorption spectrum information meas ，S meas Repetition frequency f rep (ii) a The double-optical comb light source maps the optical frequency signals difficult to detect to radio frequency in a multi-heterodyne interference mode, and in order to ensure that the frequency components of the interference signals correspond to the optical frequency components one to one, the bandwidths of the optical band-pass filter 141 and the optical band-pass filter 142 cannot exceed
The bandwidth of the low pass filter 161 and the low pass filter 162 cannot exceed
The optical band-pass filter 141 and the optical band-pass filter 142 have the same center wavelength λ and bandwidth Δ λ, and the low-pass filter 161 and the low-pass filter 162 have the same cutoff frequency; synchronous acquisition of interference signals S using a data acquisition card 171 ref 、S meas Signal sampling rate of f r1 And the collected signals are uploaded to a computer 181 for post-processing:
step two, extracting and compensating the interference signal frequency spectrum jitter:
interference signal S meas 、S ref According to repetition frequency f rep And a sampling rate f r1 Into N groups S meas (i) And S ref (i) I =1,2,. N; each group of data points is
Calculating the interference signal frequency spectrum I of the measuring light path meas (i) And reference optical path interference signal spectrum I ref (i) Extracting the frequency spectrum jitter and compensating to obtain the compensated measuring light path interference signal frequency spectrum I meas，align (i) And reference optical path interference signal spectrum I ref，align (i)，i＝1，2，...，N；
Step three, averaging the aligned interference signal frequency spectrums to suppress intensity noise, and measuring the light path to obtain an average frequency spectrum I meas，ave The reference optical path obtains an average frequency spectrum I ref，ave And calculating the original absorption spectrum by logarithmic operation
Selecting the absorption spectrum alpha of the gas emission domain to be measured meas (f) Searching the spectrum transition frequency corresponding to the absorption peak through the spectrum database of the molecule to be detected, and mapping the absorption spectrum of the radio frequency domain to the spectrum alpha meas (v)；
Correcting the absorption spectrum of the gas to be measured, and eliminating the intensity difference of the reference light path and the measuring light path along with the wavelength change;
and fifthly, calculating gas parameters by using the corrected multispectral fitting algorithm.
2. The method for measuring the gas parameters based on the phase-lock-free double-optical-comb absorption spectrum as claimed in claim 1, wherein the method of spectrum cross-correlation is used for extracting the spectrum jitter of the interference signal and compensating the spectrum jitter by spectrum shifting:
according to the repetition frequency f of the interference signal rep And a sampling rate f r1 Interference signal S obtained by sampling data acquisition card ref Dividing the period into a number of data points per period
Get N number of data points as
The interference signal of (2) is denoted as S ref (i) I =1,2,. N; similarly, for the interference signal S meas The period division is carried out to obtain N groups of data points
Interference signal of (2), denoted as S meas (i) I =1, 2.., N, i denotes a sequence number of the interference signal for periodic division;
are respectively paired with S ref (i) I =1, 2., N and S meas (i) I =1, 2.. N, is fourier transformed to obtain its frequency spectrum I ref (i) I =1, 2.., N and I meas (i)，i＝1，2，...，N；
Extracting measuring light path interference signal frequency spectrum I by frequency spectrum cross-correlation method meas (i) Relative to I meas (1) I =1,2, N, I is calculated separately meas (i) And I meas (1) The independent variable corresponding to the cross-correlation function when the cross-correlation function obtains the maximum value is the frequency spectrum jitter f (i), i =1, 2.
Will I meas (i) Moving f (I) to obtain I meas，align (i) (ii) a Similarly, will I ref (i) Moving f (I) to obtain I ref，align (i)。
3. The method for measuring gas parameters based on the phase-lock-free dual-optical-comb absorption spectrum according to claim 1, wherein the original absorption spectrum is corrected and the gas parameters are calculated by a corrected multispectral fitting method:
in actual optical configuration, the light intensities of the reference light path and the measurement light path are different inevitably, and the two pathsThe light intensity ratio between the paths can change along with the wavelength change, so that a compensation coefficient is used for compensating the calculation error caused by the light intensity difference of the reference light path and the measuring light path along with the wavelength change in the calculation process of the absorption spectrum; the true absorption spectrum alpha (v) and the measured absorption spectrum alpha meas (v) The relationship of (c) can be expressed as:
α(v)＝α meas (v)+k 0 +k(v)， (1)
wherein k is 0 A constant part representing a compensation coefficient, k (v) being a part of the compensation coefficient varying with wavelength;
considering that the compensation coefficient changes slowly with the wavelength, a difference strategy can be adopted to restrain the change of the compensation coefficient, and at a fixed frequency point v ref Here, the following approximate expression can be obtained:
α(v)-α(v ref )＝α meas (v)+k(v)-α meas (v ref )-k(v ref )≈α meas (v)-α meas (v ref )， (2)
selecting a series of reference frequency points in the whole spectrum to form a set { v } ref To extend the applicability of (3), a selected set of frequency points { v } ref The absorption spectrum database is used for simulating the absorption spectrum of the gas to be detected in the process of selecting a reference frequency point, wherein the absorption spectrum database is positioned at a part with slowly changing absorption spectra and is far away from the strong transition of the gas molecules to be detected; extracting a frequency point corresponding to the local minimum value of the simulated absorption spectrum, and taking a starting frequency point and an ending frequency point of the simulated absorption spectrum as reference frequency points;
set of frequency points { v ref Theta corresponding to meas (v ref ) Is interpolated to obtain alpha meas，ref (v) And (3) can be rewritten as
α(v)-α ref (v)≈α meas (v)-α meas，ref (v)， (3)
Wherein alpha is ref (v) From a selected set of reference frequencies v ref The corresponding true absorption rate α (v) ref ) Interpolation is carried out in the measured spectrum range;
the modified absorption spectrum becomes
α meas，fit (v)＝α meas (v)-α meas，ref (v)， (4)
Corresponding modified absorption spectrum alpha of the spectral database database，fit (v) Calculated according to the difference method
α database，fit (v)＝α database (v)-α database，ref (v)， (5)
Wherein alpha is database (v) Is the absorption spectrum, alpha, of the gas molecule to be measured calculated in the database database，ref (v) Is alpha database (v ref ) Interpolation within the measured spectral range;
in the multispectral fitting, the temperature, concentration and pressure of the gas to be measured are selected as the fitting parameters for the optimization problem, namely
min h(T，X，P)＝min||α fit，ref (v)-α databse，ref (v，T，X，P)|| 1 ， (6)
Wherein the operator | · | non-woven 1 A 1 norm representing a vector;
and (4) solving the minimization problem of (7) to obtain the temperature, concentration and pressure parameters of the gas to be measured.
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