CN109977349A - The method and apparatus for filtering out water vapor absorption peak in terahertz signal - Google Patents
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Abstract
The invention discloses a kind of method and apparatus for filtering out water vapor absorption peak in terahertz signal, are related to laser technology field.Wherein, this method comprises: Fourier transformation is carried out to terahertz signal to be processed, to obtain the frequency spectrum of the terahertz signal；The frequency response models of frequency spectrum and vapor based on the terahertz signal construct residual spectra, and using the full variation value of the residual spectra as objective function；The objective function is optimized, to determine the intensity of water vapor absorption line and the optimal estimation value of width, the frequency response estimated value of vapor is then determined according to the optimal estimation value of the intensity of the water vapor absorption line and width；The water vapor absorption peak in the terahertz signal is filtered out according to the frequency response estimated value of the vapor.By above step, interference of the vapor to terahertz signal can be effectively removed, the accuracy of subsequent Spectroscopic analysis results is helped to improve.
Description
Technical Field
The invention relates to the technical field of laser, in particular to a method and a device for filtering a water vapor absorption peak in a terahertz signal.
Background
Due to molecular vibration or rotational energy level transition, many materials show unique spectral characteristics in the terahertz waveband, which makes it possible to detect and identify various chemical substances (such as explosives, drugs, biomolecules, etc.) by utilizing terahertz radiation, and thus terahertz spectral analysis has attracted great research interest. In recent years, the THz-TDS (terahertz time domain spectroscopy) technology has been widely used. However, in practical circumstances, absorption of terahertz radiation by atmospheric water vapor will limit the application of THz-TDS technology.
Theories and experiments show that water vapor in the atmosphere has a strong absorption effect in a terahertz frequency range, a signal of hundreds of picosecond-order rapid oscillation appears behind a terahertz main pulse in a time domain, and a sharp absorption line appears at certain frequency positions on a frequency spectrum in a frequency domain. Due to the fact that the water vapor in the atmosphere has a strong absorption effect in the terahertz frequency range, on one hand, energy of terahertz radiation is attenuated after the terahertz radiation is transmitted in the air for a certain distance, on the other hand, the spectral signal to noise ratio at the position of an absorption line is reduced, the effect can distort the extracted sample spectrum, and further the spectral analysis result is inaccurate.
In the prior art, the conventional method for reducing the influence of water vapor is to fill a terahertz optical path part in the THz-TDS device with dry nitrogen. However, this approach obviously increases the complexity of the system, and is only suitable for use in a laboratory environment, and is not generally applicable in an industrial environment. Therefore, it is desirable to employ a signal post-processing method to solve the problem of spectral interference introduced by water vapor.
In the prior art, various methods exist for filtering out a water vapor absorption peak in a terahertz signal. For example, american scholars propose a method for filtering out a water vapor absorption peak in a terahertz signal using a neural network model. In this method, since the trained neural network depends on specific atmospheric conditions, the neural network needs to be retrained as the environment changes, and thus the method has a complicated process and great limitations. As another example, australian scholars propose signal processing methods based on theoretical modeling of the water vapor frequency response. However, this method needs to manually set optimization parameters, and can only process the case with a large time domain window width, so the method is less versatile.
Disclosure of Invention
Technical problem to be solved
The technical problem to be solved by the invention is to solve the technical problems that the existing method for filtering out the water vapor absorption peak in the terahertz signal is complex in algorithm processing process and poor in universality due to the fact that the method depends on atmospheric conditions, a neural network needs to be retrained along with the change of the environment, or only the condition that the time domain window is wide can be processed.
(II) technical scheme
In order to solve the technical problem, in one aspect, the invention provides a method for filtering a water vapor absorption peak in a terahertz signal.
The method for filtering out the water vapor absorption peak in the terahertz signal comprises the following steps: carrying out Fourier transform on a terahertz signal to be processed to obtain a frequency spectrum of the terahertz signal; constructing a residual spectrum based on the frequency spectrum of the terahertz signal and a frequency response model of water vapor, and taking the full variation value of the residual spectrum as a target function; wherein the parameters to be optimized in the objective function include the intensity and width of the water vapor absorption line; performing optimization solution on the objective function to determine optimal estimated values of the intensity and the width of the water vapor absorption line, and then determining a frequency response estimated value of the water vapor according to the optimal estimated values of the intensity and the width of the water vapor absorption line; and filtering out a water vapor absorption peak in the terahertz signal according to the estimated value of the frequency response of the water vapor.
Optionally, the method further comprises: and constructing a complex refractive index model of the water vapor according to a preset spectral line type, and then constructing a frequency response model of the water vapor according to the complex refractive index model of the water vapor.
Optionally, the constructing a complex refractive index model of water vapor according to a preset spectral line type includes: and extracting a spectral parameter value of a water vapor absorption line in a terahertz signal frequency band from an HITRAN database according to the environmental parameter when the terahertz signal is measured, then determining an angular frequency value of the water vapor absorption line according to the spectral parameter value, and then constructing a complex refractive index model of the water vapor according to the angular frequency value of the water vapor absorption line and a preset spectral line type.
Optionally, the preset spectral profile comprises a lorentzian spectral profile, and the lorentzian spectral profile satisfies:
wherein f isijOmega and gij(ω) represents a spectral linear function, ω represents angular frequency, ωijRepresenting the angular frequency, Γ, of the water vapour absorption lineijIndicating the width of the water vapor absorption line.
Optionally, the frequency response model of the water vapor satisfies:
wherein H (ω) represents the frequency response of water vapor,denotes the complex refractive index of water vapor, ω denotes the angular frequency, L denotes the propagation distance of the terahertz signal, c denotes the speed of light, KijDenotes the water vapor absorption line intensity, n (ω) denotes the real part of the complex refractive index of water vapor,representing the imaginary part of the complex refractive index of water vapour, fijOmega and gij(ω) represents a spectral line function.
Optionally, the objective function satisfies:
wherein,expressing optimization of K and Γ such that the objective function TV [ R (ω, K, Γ)]Take the minimum value, TV R (ω,K,Γ)]representing the full variation value of the remaining spectra,denotes the residual spectrum, K denotes the water vapor absorption line intensity KijR represents the water vapor absorption line width rijSet of (1), the constraint condition when the objective function is optimized and solved is KijIs not less than 0 and gammaijMore than or equal to 0, m represents an angular frequency foot mark of a water vapor absorption line in a terahertz signal frequency band,represents the spectrum of the terahertz signal, H (ω) represents the frequency response of water vapor,representing a window function over a set frequency domain.
Optionally, the step of filtering out a water vapor absorption peak in the terahertz signal according to the estimated value of the frequency response of the water vapor comprises: carrying out deconvolution operation on the frequency spectrum of the terahertz signal according to the frequency response estimation value of the water vapor to obtain a signal frequency spectrum with a water vapor absorption peak removed; and carrying out Fourier transform processing on the signal frequency spectrum of which the water vapor absorption peak is filtered out to obtain a terahertz signal of which the water vapor absorption peak is filtered out.
In order to solve the technical problem, on the other hand, the invention further provides a device for filtering out a water vapor absorption peak in the terahertz signal.
The device for filtering out the water vapor absorption peak in the terahertz signal comprises: the terahertz signal processing device comprises a transformation module, a processing module and a processing module, wherein the transformation module is used for carrying out Fourier transformation on a terahertz signal to be processed so as to obtain a frequency spectrum of the terahertz signal; the building module is used for building a residual spectrum based on the frequency spectrum of the terahertz signal and a frequency response model of water vapor, and taking the full variation value of the residual spectrum as an objective function; wherein the parameters to be optimized in the objective function include the intensity and width of the water vapor absorption line; the optimization solving module is used for carrying out optimization solving on the objective function so as to determine the optimal estimated values of the intensity and the width of the water vapor absorption line, and then determining the frequency response estimated value of the water vapor according to the optimal estimated values of the intensity and the width of the water vapor absorption line; and the filtering module is used for filtering a water vapor absorption peak in the terahertz signal according to the frequency response estimation value of the water vapor.
Optionally, the building module is further configured to build a complex refractive index model of the water vapor according to a preset spectral line type, and then build a frequency response model of the water vapor according to the complex refractive index model of the water vapor.
Optionally, the constructing module constructs a complex refractive index model of water vapor according to a preset spectral line type, including: the building module extracts a spectral parameter value of a water vapor absorption line in a terahertz signal frequency band from a database according to an environmental parameter when the terahertz signal is measured, then determines an angular frequency value of the water vapor absorption line according to the spectral parameter value, and then builds a complex refractive index model of the water vapor according to the angular frequency value of the water vapor absorption line and a preset spectral line type.
(III) advantageous effects
The technical scheme of the invention has the following advantages: in the embodiment of the invention, the interference of the water vapor on the terahertz signal can be effectively eliminated by the steps of constructing the residual spectrum by using the frequency response model of the terahertz signal spectrum and the water vapor obtained based on Fourier transform, performing optimization solution by using the total variation value of the residual spectrum as an objective function, determining the frequency response estimation value of the water vapor according to the optimal estimation value of the intensity and the width of the water vapor absorption line obtained by the optimization solution, and filtering the water vapor absorption peak in the terahertz signal according to the frequency response estimation value of the water vapor, thereby being beneficial to improving the accuracy of the subsequent spectral analysis result. Compared with the prior art, the method provided by the embodiment of the invention has the advantages of no dependence on atmospheric conditions, capability of directly and automatically processing in a frequency domain, strong algorithm universality and the like.
Drawings
Fig. 1 is a schematic flow chart of a method for filtering out a water vapor absorption peak in a terahertz signal according to a first embodiment of the present invention;
FIG. 2 is a schematic flow chart of a method for filtering out a water vapor absorption peak in a terahertz signal according to a second embodiment of the present invention;
fig. 3 is a schematic block diagram of a device for filtering out a water vapor absorption peak in a terahertz signal according to a third embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.
Example one
Fig. 1 is a schematic flow chart of a method for filtering a water vapor absorption peak in a terahertz signal according to a first embodiment of the present invention. As shown in fig. 1, the method for filtering out a water vapor absorption peak in a terahertz signal provided by an embodiment of the present invention includes:
step S101, carrying out Fourier transform on a terahertz signal to be processed to obtain a frequency spectrum of the terahertz signal.
The terahertz signal to be processed can be a windowed signal measured in an air environment, and can also be an original signal measured in the air environment. The windowed signal measured in the air environment is equal toOriginal signal Eair(t) product of the window function W (t). Illustratively, the window function w (t) may be a rectangular window function, whose expression is: when T is 0 ≦ T, w (T) 1; when T ≧ T, W (T) is 0. Where T is the length of the rectangular window function.
Step S102, constructing a residual spectrum based on the frequency spectrum of the terahertz signal and a frequency response model of water vapor, and taking the full variation value of the residual spectrum as an objective function. Wherein the parameters to be optimized in the objective function comprise the intensity and width of the water vapor absorption line.
For example, when the terahertz signal to be processed is a windowed signal measured in an air environment, the constructed residual spectrum can be represented as:
wherein R (ω, K, Γ) represents the residual spectrum,represents the spectrum of the terahertz signal, H (ω) represents the frequency response of water vapor,representing a window function over a set frequency domain.
In general, the full variation value of the function u (x) is equivalent to the area integral of the first derivative of the function u (x), which can be defined as:
TV[u(x)]＝∫|▽u|dx
in the present invention, the objective function constructed based on the residual spectrum R (ω, K, Γ) can be expressed as:
wherein R (ω, K, Γ) represents the residual spectrum, TV [ R (ω, K, Γ) ] represents the full variation value of the residual spectrum, and m represents the angular frequency subscript of the water vapor absorption line within the terahertz signal frequency band.
And S103, carrying out optimization solution on the objective function to determine the optimal estimated values of the intensity and the width of the water vapor absorption line, and then determining the frequency response estimated value of the water vapor according to the optimal estimated values of the intensity and the width of the water vapor absorption line.
In specific implementation, the objective function can be optimized and solved by adopting a particle swarm optimization algorithm or other optimization algorithms. When the optimization solution is carried out, an objective function constructed based on the residual spectrum R (omega, K, gamma) satisfies the following conditions:
wherein,expressing optimization of K and Γ such that the objective function TV [ R (ω, K, Γ)]Get the minimum value, TV [ R (ω, K, Γ)]Denotes the total variation value of the residual spectrum, K denotes the water vapor absorption line intensity KijR represents the water vapor absorption line width rijSet of (1), the constraint condition when the objective function is optimized and solved is KijIs not less than 0 and gammaij≥0。
And step S104, filtering out a water vapor absorption peak in the terahertz signal according to the frequency response estimation value of the water vapor.
In this step, deconvolution operation may be performed on the frequency spectrum of the terahertz signal according to the frequency response estimation value of the water vapor to obtain a signal frequency spectrum with a water vapor absorption peak removed; and then carrying out Fourier transform processing on the signal frequency spectrum of which the water vapor absorption peak is removed to obtain a terahertz signal of which the water vapor absorption peak is removed.
In the embodiment of the invention, the interference of the water vapor on the terahertz signal can be effectively eliminated by the steps of constructing the residual spectrum by using the frequency response model of the terahertz signal spectrum and the water vapor obtained based on Fourier transform, performing optimization solution by using the total variation value of the residual spectrum as an objective function, determining the frequency response estimation value of the water vapor according to the optimal estimation value of the intensity and the width of the water vapor absorption line obtained by the optimization solution, and filtering the water vapor absorption peak in the terahertz signal according to the frequency response estimation value of the water vapor, thereby being beneficial to improving the accuracy of the subsequent spectral analysis result. Compared with the prior art, the method provided by the embodiment of the invention has the advantages of no dependence on atmospheric conditions, capability of directly and automatically processing in a frequency domain, strong algorithm universality and the like.
Example two
Fig. 2 is a schematic flow chart of a method for filtering out a water vapor absorption peak in a terahertz signal according to a second embodiment of the present invention. As shown in fig. 2, the method for filtering out a water vapor absorption peak in a terahertz signal according to an embodiment of the present invention includes:
step S201, carrying out Fourier transform on the terahertz signal to be processed to obtain the frequency spectrum of the terahertz signal.
The terahertz signal to be processed can be a windowed signal measured in an air environment, and can also be an original signal measured in the air environment. The windowed signal measured in the air environment is equal to the original signal Eair(t) product of the window function W (t). Illustratively, the window function w (t) may be a rectangular window function, whose expression is: when T is 0 ≦ T, w (T) 1; when T ≧ T, W (T) is 0. Where T is the length of the rectangular window function.
Step S202, extracting spectral parameter values of water vapor absorption lines in the terahertz signal frequency band from an HITRAN database according to the environmental parameters when the terahertz signal is measured.
Wherein the measuring the environmental parameter in the terahertz signal comprises: and measuring the ambient temperature, humidity and pressure of the terahertz signal. During specific implementation, the environmental humidity can be measured through a hygrometer, the environmental temperature is measured through a thermometer, and the environmental pressure is measured through a barometer.
The HITRAN database is a molecular absorption spectrum database with international standard, and comprises spectrum data of various molecules (including common molecules in the atmosphere) and isotopes thereof, wherein the spectrum data contains water vapor of 0-25233 cm-1Spectral data of absorption lines within the range. In this step, the spectral parameters extracted from the HITRAN database according to the environmental parameters when the terahertz signals are measured may include: absorption line frequency vijAbsorption line intensity S, air broadening absorption line width gammaairSelf-broadening absorption line width gammaself、γairTemperature dependent index nairAir pressure induced absorption line offset deltaair。
Step S203, determining an angular frequency value of a water vapor absorption line according to the spectral parameter value, and then constructing a complex refractive index model of the water vapor according to the angular frequency value of the water vapor absorption line and a preset spectral line type.
Illustratively, after extracting spectral parameter values from the HITRAN database, the angular frequency values of the water vapor absorption lines may be determined according to the following formula:
ν′ij(P)＝νij+δair(Pref)P；
ωij＝2πcv′ij(P)
wherein, ω isijIs the angular frequency value of the water vapor absorption line, c is the speed of light, and P is the ambient pressure at which the terahertz signal is measured.
Illustratively, the predetermined spectral profile may be a Lorentzian (Lorentzian) spectral profile or a VanVleck-Weisskopf spectral profile. Wherein the Lorentz spectrum line type satisfies:
wherein f isijOmega and gij(ω) represents a spectral linear function, ω represents angular frequency, ωijRepresenting the angular frequency, Γ, of the water vapour absorption lineijIndicating the width of the water vapor absorption line.
And S204, constructing a frequency response model of the water vapor according to the complex refractive index model of the water vapor.
Illustratively, the frequency response model of water vapor constructed by this step satisfies:
wherein H (ω) represents the frequency response of water vapor,denotes the complex refractive index of water vapor, ω denotes the angular frequency, L denotes the propagation distance of the terahertz signal, c denotes the speed of light, KijDenotes the water vapor absorption line intensity, n (ω) denotes the real part of the complex refractive index of water vapor,representing the imaginary part of the complex refractive index of water vapour, fijOmega and gij(ω) represents a spectral line function.
Step S205, constructing a residual spectrum based on the frequency spectrum of the terahertz signal and the frequency response model of the water vapor, and taking the full variation value of the residual spectrum as an objective function.
For example, when the terahertz signal to be processed is a windowed signal measured in an air environment, the constructed residual spectrum can be represented as:
wherein R (ω, K, Γ) represents the residual spectrum,represents the spectrum of the terahertz signal, H (ω) represents the frequency response of water vapor,representing a window function over a set frequency domain.
In general, the full variation value of the function u (x) is equivalent to the area integral of the first derivative of the function u (x), which can be defined as:
TV[u(x)]＝∫|▽u|dx；
in the embodiment of the present invention, the objective function constructed based on the residual spectrum R (ω, K, Γ) can be expressed as:
wherein R (ω, K, Γ) represents the residual spectrum, TV [ R (ω, K, Γ) ] represents the full variation value of the residual spectrum, and m represents the angular frequency subscript of the water vapor absorption line within the terahertz signal frequency band.
And S206, carrying out optimization solution on the objective function to determine the optimal estimated values of the intensity and the width of the water vapor absorption line.
In specific implementation, the objective function can be optimized and solved by adopting a particle swarm optimization algorithm or other optimization algorithms. When the optimization solution is carried out, an objective function constructed based on the residual spectrum R (omega, K, gamma) satisfies the following conditions:
wherein,expressing optimization of K and Γ such that the objective function TV [ R (ω, K, Γ)]Get the minimum value, TV [ R (ω, K, Γ)]Denotes the total variation value of the residual spectrum, and K denotes the water vapor absorption line intensity KijR represents the water vapor absorption line width rijSet of (1), the constraint condition when the objective function is optimized and solved is KijIs not less than 0 and gammaij≥0。
And step S207, determining the frequency response estimation value of the water vapor according to the optimal estimation values of the intensity and the width of the water vapor absorption line.
In this step, the optimal estimated values of the intensity and width of the water vapor absorption line may be substituted into the frequency response model of the water vapor constructed in step S204 to obtain an estimated value of the frequency response of the water vapor.
And S208, filtering out a water vapor absorption peak in the terahertz signal according to the frequency response estimation value of the water vapor.
In this step, deconvolution operation may be performed on the frequency spectrum of the terahertz signal according to the frequency response estimation value of the water vapor to obtain a signal frequency spectrum with a water vapor absorption peak removed. In specific implementation, the deconvolution operation can be performed according to the following formula:
wherein,a signal spectrum showing the filtered water vapor absorption peak,represents the spectrum of the terahertz signal, H (ω) represents the frequency response of water vapor,representing a window function over a set frequency domain.
Then, Fourier transform processing is carried out on the signal frequency spectrum with the water vapor absorption peak removed, so that a terahertz signal with the water vapor absorption peak removed is obtained.
In the embodiment of the invention, the interference of water vapor on the terahertz signal can be effectively eliminated through the steps, so that the accuracy of the subsequent spectral analysis result is improved. Compared with the prior art, the method provided by the embodiment of the invention has the advantages of no dependence on atmospheric conditions, capability of directly and automatically processing in a frequency domain, strong algorithm universality and the like.
EXAMPLE III
Fig. 3 is a schematic block diagram of a device for filtering out a water vapor absorption peak in a terahertz signal according to a third embodiment of the present invention. As shown in fig. 3, an apparatus 300 for measuring a complex refractive index of a sample based on a terahertz frequency band according to an embodiment of the present invention includes: the system comprises a transformation module 301, a construction module 302, an optimization solving module 303 and a filtering module 304.
The conversion module 301 is configured to perform fourier transform on a terahertz signal to be processed to obtain a frequency spectrum of the terahertz signal.
The terahertz signal to be processed can be a windowed signal measured in an air environment, and can also be an original signal measured in the air environment. The windowed signal measured in the air environment is equal to the original signal Eair(t) product of the window function W (t). Illustratively, the window function w (t) may be a rectangular window function, whose expression is: when T is 0 ≦ T, w (T) 1; when T ≧ T, W (T) is 0. Where T is the length of the rectangular window function.
A building module 302, configured to build a residual spectrum based on the frequency spectrum of the terahertz signal and a frequency response model of water vapor, and take a full variation value of the residual spectrum as an objective function. Wherein the parameters to be optimized in the objective function comprise the intensity and width of the water vapor absorption line.
For example, when the terahertz signal to be processed is a windowed signal measured in an air environment, the residual spectrum constructed by the construction module 302 can be represented as:
wherein R (ω, K, Γ) represents the residual spectrum,represents the spectrum of the terahertz signal, H (ω) represents the frequency response of water vapor,representing a window function over a set frequency domain.
In general, the full variation value of the function u (x) is equivalent to the area integral of the first derivative of the function u (x), which can be defined as:
TV[u(x)]＝∫|▽u|dx；
in the present invention, the objective function constructed by the construction module 302 based on the residual spectrum R (ω, K, Γ) can be expressed as:
wherein R (omega, K, gamma) represents a residual spectrum, TV [ R (omega, K, gamma) ] represents a fully variable value of the residual spectrum, and m represents an angular frequency subscript of a water vapor absorption line in a terahertz signal frequency band.
Further, the building module 302 is further configured to build a complex refractive index model of the water vapor according to a preset spectral line type, and then build a frequency response model of the water vapor according to the complex refractive index model of the water vapor.
In an alternative embodiment, the constructing module 302 constructing the complex refractive index model of water vapor according to a preset spectral line type includes: the building module 302 extracts a spectral parameter value of a water vapor absorption line in a terahertz signal frequency band from a database according to an environmental parameter when the terahertz signal is measured, then the building module 302 determines an angular frequency value of the water vapor absorption line according to the spectral parameter value, and then a complex refractive index model of the water vapor is built according to the angular frequency value of the water vapor absorption line and a preset spectral line type.
In this alternative embodiment, after extracting the spectral parameter values from the HITRAN database, the construction module 302 may determine the angular frequency values of the water vapor absorption lines according to the following formula:
ν′ij(P)＝νij+δair(Pref)P；
ωij＝2πcv′ij(P)；
wherein, ω isijIs the angular frequency value of the water vapor absorption line, c is the speed of light, and P is the ambient pressure at which the terahertz signal is measured.
In this alternative embodiment, the predetermined spectral profile may be a Lorentzian (Lorentzian) spectral profile or a Van Vleck-Weisskopf spectral profile. Wherein the Lorentz spectrum line type satisfies:
wherein f isijOmega and gij(ω) represents a spectral linear function, ω represents angular frequency, ωijRepresenting the angular frequency, Γ, of the water vapour absorption lineijIndicating the width of the water vapor absorption line.
And S204, constructing a frequency response model of the water vapor according to the complex refractive index model of the water vapor.
Illustratively, the frequency response model of water vapor constructed by this step satisfies:
wherein H (ω) represents the frequency response of water vapor,indicating birefringence of water vapourThe ratio, ω, denotes the angular frequency, L denotes the propagation distance of the terahertz signal, c denotes the speed of light, KijDenotes the water vapor absorption line intensity, n (ω) denotes the real part of the complex refractive index of water vapor,representing the imaginary part of the complex refractive index of water vapour, fijOmega and gij(ω) represents a spectral line function.
Step S205, constructing a residual spectrum based on the frequency spectrum of the terahertz signal and the frequency response model of the water vapor, and taking the full variation value of the residual spectrum as an objective function.
For example, when the terahertz signal to be processed is a windowed signal measured in an air environment, the constructed residual spectrum can be represented as:
wherein R (ω, K, Γ) represents the residual spectrum,represents the spectrum of the terahertz signal, H (ω) represents the frequency response of water vapor,representing a window function over a set frequency domain.
In general, the full variation value of the function u (x) is equivalent to the area integral of the first derivative of the function u (x), which can be defined as:
TV[u(x)]＝∫|▽u|dx；
in the embodiment of the present invention, the objective function constructed based on the residual spectrum R (ω, K, Γ) can be expressed as:
wherein, fijOmega and gij(ω) represents a spectral linear function, ω represents angular frequency, ωijRepresenting the angular frequency, Γ, of the water vapour absorption lineijIndicating the width of the water vapor absorption line.
Further, based on this alternative embodiment, the building module 302 may build a frequency response model of the water vapor as shown in the following equation:
wherein H (ω) represents the frequency response of water vapor,denotes the complex refractive index of water vapor, ω denotes the angular frequency, L denotes the propagation distance of the terahertz signal, which can be measured by a distance meter, c denotes the speed of light, KijRepresenting the intensity of the absorption line of water vapor, n (omega) representing the real part of the complex refractive index of water vapor, the imaginary part of the complex refractive index of water vapor, fijOmega and gij(ω) represents a spectral line function.
And the optimization solving module 303 is used for performing optimization solving on the objective function to determine optimal estimated values of the intensity and the width of the water vapor absorption line, and then determining an estimated value of the frequency response of the water vapor according to the optimal estimated values of the intensity and the width of the water vapor absorption line.
In specific implementation, the optimization solving module 303 may adopt a particle swarm optimization algorithm or other optimization algorithms to perform optimization solving on the objective function. When the optimization solution is carried out, an objective function constructed based on the residual spectrum R (omega, K, gamma) satisfies the following conditions:
wherein,expressing optimization of K and Γ such that the objective function TV [ R (ω, K, Γ)]Get the minimum value, TV [ R (ω, K, Γ)]Denotes the total variation value of the residual spectrum, K denotes the water vapor absorption line intensity KijR represents the water vapor absorption line width rijSet of (1), the constraint condition when the objective function is optimized and solved is KijIs not less than 0 and gammaij≥0。
And a filtering module 304, configured to filter a water vapor absorption peak in the terahertz signal according to the estimated value of the frequency response of the water vapor.
Specifically, the filtering module 304 may perform deconvolution operation on the frequency spectrum of the terahertz signal according to the frequency response estimation value of the water vapor to obtain a signal frequency spectrum with a water vapor absorption peak filtered; then, the filtering module 304 performs fourier transform processing on the signal spectrum with the water vapor absorption peak removed, so as to obtain a terahertz signal with the water vapor absorption peak removed.
In the device provided by the embodiment of the invention, the building module is used for building the residual spectrum based on the frequency spectrum of the terahertz signal obtained by Fourier transform and the frequency response model of the water vapor, the full variation value of the residual spectrum is used as an objective function, the optimization solving module is used for carrying out optimization solving on the objective function, the frequency response estimation value of the water vapor is determined according to the optimal estimation value of the intensity and the width of the water vapor absorption line obtained by the optimization solving, and the filtering module is used for filtering the water vapor absorption peak in the terahertz signal according to the frequency response estimation value of the water vapor, so that the interference of the water vapor on the terahertz signal can be effectively eliminated, and the accuracy of the subsequent spectral analysis result is improved. Compared with the prior art, the device provided by the embodiment of the invention has the advantages of no dependence on atmospheric conditions, capability of directly and automatically processing in a frequency domain, strong algorithm universality and the like.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. A method for filtering out a water vapor absorption peak in a terahertz signal is characterized by comprising the following steps:
carrying out Fourier transform on a terahertz signal to be processed to obtain a frequency spectrum of the terahertz signal;
constructing a residual spectrum based on the frequency spectrum of the terahertz signal and a frequency response model of water vapor, and taking the full variation value of the residual spectrum as a target function; wherein the parameters to be optimized in the objective function include the intensity and width of the water vapor absorption line;
performing optimization solution on the objective function to determine optimal estimated values of the intensity and the width of the water vapor absorption line, and then determining a frequency response estimated value of the water vapor according to the optimal estimated values of the intensity and the width of the water vapor absorption line;
and filtering out a water vapor absorption peak in the terahertz signal according to the estimated value of the frequency response of the water vapor.
2. The method of claim 1, further comprising:
and constructing a complex refractive index model of the water vapor according to a preset spectral line type, and then constructing a frequency response model of the water vapor according to the complex refractive index model of the water vapor.
3. The method of claim 2, wherein constructing the complex refractive index model of water vapor according to a predetermined spectral profile comprises:
and extracting a spectral parameter value of a water vapor absorption line in a terahertz signal frequency band from an HITRAN database according to the environmental parameter when the terahertz signal is measured, then determining an angular frequency value of the water vapor absorption line according to the spectral parameter value, and then constructing a complex refractive index model of the water vapor according to the angular frequency value of the water vapor absorption line and a preset spectral line type.
4. The method of claim 2, wherein the predetermined spectral profile comprises a lorentzian spectral profile satisfying:
wherein f isijOmega and gij(ω) represents a spectral linear function, and ω represents an angular frequencyRate, ωijRepresenting the angular frequency, Γ, of the water vapour absorption lineijIndicating the width of the water vapor absorption line.
5. The method of claim 4, wherein the frequency response model of the water vapor satisfies:
wherein H (ω) represents the frequency response of water vapor,denotes the complex refractive index of water vapor, ω denotes the angular frequency, L denotes the propagation distance of the terahertz signal, c denotes the speed of light, KijDenotes the water vapor absorption line intensity, n (ω) denotes the real part of the complex refractive index of water vapor,representing the imaginary part of the complex refractive index of water vapour, fijOmega and gij(ω) represents a spectral line function.
6. The method of claim 1, wherein the objective function satisfies:
wherein,expressing optimization of K and Γ such that the objective function TV [ R (ω, K, Γ)]Get the minimum value, TV [ R (ω, K, Γ)]Representing the full variation value of the remaining spectra,denotes the residual spectrum, K denotes the water vapor absorption line intensity KijR represents the water vapor absorption line width rijSet of (1), the constraint condition when the objective function is optimized and solved is KijIs not less than 0 and gammaijMore than or equal to 0, m represents an angular frequency foot mark of a water vapor absorption line in a terahertz signal frequency band,represents the spectrum of the terahertz signal, H (ω) represents the frequency response of water vapor,representing a window function over a set frequency domain.
7. The method of claim 1, wherein the step of filtering out a water vapor absorption peak in the terahertz signal according to the estimated frequency response of the water vapor comprises:
carrying out deconvolution operation on the frequency spectrum of the terahertz signal according to the frequency response estimation value of the water vapor to obtain a signal frequency spectrum with a water vapor absorption peak removed; and carrying out inverse Fourier transform processing on the signal frequency spectrum of which the water vapor absorption peak is filtered out to obtain a terahertz signal of which the water vapor absorption peak is filtered out.
8. An apparatus based on terahertz frequency band measurement, characterized in that the apparatus comprises:
the terahertz signal processing device comprises a transformation module, a processing module and a processing module, wherein the transformation module is used for carrying out Fourier transformation on a terahertz signal to be processed so as to obtain a frequency spectrum of the terahertz signal;
the building module is used for building a residual spectrum based on the frequency spectrum of the terahertz signal and a frequency response model of water vapor, and taking the full variation value of the residual spectrum as an objective function; wherein the parameters to be optimized in the objective function include the intensity and width of the water vapor absorption line;
the optimization solving module is used for carrying out optimization solving on the objective function so as to determine the optimal estimated values of the intensity and the width of the water vapor absorption line, and then determining the frequency response estimated value of the water vapor according to the optimal estimated values of the intensity and the width of the water vapor absorption line;
and the filtering module is used for filtering a water vapor absorption peak in the terahertz signal according to the frequency response estimation value of the water vapor.
9. The apparatus of claim 8, wherein the construction module is further configured to construct a complex refractive index model of the water vapor according to a preset spectral line type, and then construct a frequency response model of the water vapor according to the complex refractive index model of the water vapor.
10. The apparatus of claim 8, wherein the construction module constructs a complex refractive index model of water vapor according to a predetermined spectral profile comprising:
the building module extracts a spectral parameter value of a water vapor absorption line in a terahertz signal frequency band from a database according to an environmental parameter when the terahertz signal is measured, then determines an angular frequency value of the water vapor absorption line according to the spectral parameter value, and then builds a complex refractive index model of the water vapor according to the angular frequency value of the water vapor absorption line and a preset spectral line type.
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