CN112255190A - Method, system, medium and device for filtering reflected pulse interference during THz-TDS (terahertz-time domain reflectometry) test sample - Google Patents
Method, system, medium and device for filtering reflected pulse interference during THz-TDS (terahertz-time domain reflectometry) test sample Download PDFInfo
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Abstract
The invention provides a method, a system, a medium and a device for filtering reflected pulse interference during THz-TDS test samples, which comprise the following steps: acquiring a time domain signal T of a sample according to a conventional THz-TDS test methodSam(t) and sample and absorption spectrum signals a (f); for TSam(t) performing time domain analysis on the main pulse and the first-stage reflection pulse to obtain time domain delay delta t corresponding to the main pulse and the first-stage reflection pulse; performing N-point discrete Fourier transform on A (f) to obtain an absorption spectrum interference curve S (t) and an interference power spectral density curve I (t) | S (t)/N & gt with abscissa as optical path difference2(ii) a Combining delta t to carry out filtering iteration on I (t) to obtain a new interference power spectral density curve Inew(t); mixing I (t) and Inew(t) comparing to obtain amplitude attenuation coefficient a of corresponding filter segmenti＝[Inew(ti)/I(ti)]1/2Obtaining the discrete Fourier transform S after filtering correctionnew(t); to pairSnew(t) performing inverse discrete Fourier transform to obtain an absorption spectrum after interference is filtered: a. thenew(f)＝IFFT[Snew(t)]. The invention does not need to acquire the dispersion information of the sample or the accurate time domain position of the reflected pulse signal in a priori, and realizes the high-efficiency filtering of the interference fringes of the absorption spectrum reflection peak.
Description
Technical Field
The invention relates to the technical field of terahertz, in particular to a method, a system, a medium and a device for filtering reflected pulse interference during THz-TDS test samples.
Background
The terahertz time-domain spectroscopy (TDS) is a novel substance detection means, and the technology obtains wide-spectrum radiation covering a plurality of terahertz (THz) ranges on a frequency domain through generating a narrow pulse signal with duration of a plurality of picoseconds (ps) orders on the time domain and through Fast Fourier Transform (FFT) on the time domain and the frequency domain. The signal generated and detected in the mode has coherent measurement capability, and can simultaneously acquire the substance absorption spectrum with high sensitivity and the time-resolved phase information, thereby acquiring richer spectral data of the measured substance. The terahertz time-domain spectroscopy (THz-TDS) technology can be used for carrying out spectral measurement on substances with different forms, and the phonon frequency of condensed substances and the vibration spectrum of macromolecules have a plurality of characteristic peaks in a THz waveband, carriers in liquid also have very sensitive response to THz radiation, and in addition, the THz waveband has characteristic response to low-frequency collective vibration in test substance molecules, extension of weak interaction hydrogen bonds and Van der Waals force among molecules, bending of skeleton vibration configuration of macromolecules, rotation and vibration transition of dipoles, low-frequency absorption frequency of crystal lattices in crystals, phonon vibration modes and the like. A large number of organic macromolecular groups show obvious and unique absorption and dispersion characteristics in a terahertz wave band, corresponding molecular structure information, spectrum fingerprint absorption, chemical component content and the like can be obtained through the terahertz spectrum of a test substance, and the method becomes a powerful tool for researching the properties of biochemical macromolecular organic configuration, structure and the like.
The THz-TDS is used for detecting the terahertz characteristic spectrum of a substance, a transmission type measuring mode is generally adopted, a sample wafer to be detected is ground and pressed into a sheet to be prepared before measurement, the sample wafer with standard size and thickness is formed, scattering and reflection influences caused by particles are reduced by improving the smoothness of the surface of the sample wafer, the thickness of the sample wafer is restrained, and the sample wafer is ensured to be located in the confocal range of a focused terahertz wave beam. The input signal power of the measurement mode is large, the constraint size of the focusing light spot is small, and therefore the terahertz waveband absorption spectrum of the substance with high signal-to-noise ratio and high accuracy can be obtained. However, the terahertz wave beam forms multiple reflections between the two surfaces of the sample just because the surface of the sample is relatively flat, and is superimposed with the real main pulse signal of the transmitted light beam. The superimposed TDS signal will generate a reflected pulse signal in the time domain signal and generate an obvious interference fringe in the frequency domain and the absorption spectrum signal. If large interference exists in the absorption spectrum, not only can the misjudgment of the absorption peak be caused, but also the waveform of the real absorption peak can be changed, and even the weak absorption peak is submerged, so that the inaccuracy of the terahertz spectrum test of the substance can be caused.
Corresponding data processing optimization work is developed in the literature aiming at filtering out reflection pulses in TDS transmission signals of substances. However, these efforts focus on how to filter the reflected pulse signal in the time domain, and in order to ensure the accuracy of the filtering effect, the reflected pulse signal and the main pulse signal are required to have higher similarity, and the reflected pulse signal and the main pulse signal are required to be separated as much as possible in the time domain, so that the sample amount is required to be increased to increase the separation degree of the main pulse and the reflected pulse in the sample signal in the time domain. However, the measured substance has certain dispersion for the terahertz wave beam, which causes broadening of the terahertz time-domain pulse of the reflection peak, i.e. the shape of the terahertz time-domain pulse is deformed to a certain extent compared with the real transmission TDS signal; in addition, for a thin sample, the tails of the reflected pulse signal and the real main pulse signal are overlapped and are difficult to fit in a morphological way, so that the large number of methods need a plurality of prior parameters to be adjusted manually, and a universal method which does not need manual intervention on most samples cannot be formed.
It is therefore desirable to be able to solve the problem of how to provide a versatile method of achieving efficient filtering of absorption spectrum reflection peak interference fringes without human intervention.
Disclosure of Invention
In view of the above-mentioned shortcomings of the prior art, the present invention provides a method, a system, a medium and a device for interference filtering of reflected pulses in THz-TDS test samples, so as to solve the problem of how to provide a general method for realizing high-efficiency filtering of interference fringes of absorption spectrum reflection peaks without manual intervention in the prior art.
To achieve the above and other related objects, the present invention provides a THz-TDS assayThe method for filtering the interference of the reflected pulse during the sample comprises the following steps: the reference time domain signal of THz-TDS without sample is TRef(T), testing to obtain the time domain signal T of THz-TDS when the sample existsSam(t); for TRef(t) performing FFT and calculating the spectrum amplitude to obtain a corresponding frequency domain signal F without a sampleRef(f) To TSam(t) FFT and spectral amplitude calculation, transform to obtain corresponding frequency domain signal F with sample timeSam(f) So as to obtain a terahertz frequency domain absorption spectrum signal A (F) ═ F of the sampleRef(f)-FSam(f) (ii) a Performing FFT in spatial domain on the absorption spectrum signal A (f) to obtain a corresponding discrete Fourier transform spectrum S (t), S (t) FFT [ A (f)](ii) a Obtaining an original interference power spectral density curve I (t) with different t as abscissa, wherein I (t) | S (t)/N2N is the number of points for FFT of the absorption spectrum A (f); filtering iteration is carried out on the original interference power spectral density curve I (t) to obtain a new interference power spectral density curve Inew(t); comparing the original interference power spectral density curve I (t) with the new interference power spectral density curve Inew(t) comparing to obtain amplitude attenuation coefficient a of corresponding filter segmenti＝[Inew(ti)/I(ti)]1/2(i ═ n-1, n, n + 1); and according to the property S (t) of FFT conjugate symmetryk)＝S(tN-k)*(k is 1 to N-1), and the FFT value of the corresponding frequency point in the discrete fourier transform spectrum S (t) is corrected to obtain the filter-corrected discrete fourier transform Snew(t); to the filtered modified discrete Fourier transform Snew(t) and performing an inverse discrete fourier transform to obtain an absorption spectrum after interference fringe filtering: a. thenew(f)＝IFFT[Snew(t)]。
In an embodiment of the invention, the filtering iteration is performed on the original interference power spectral density curve I (t) to obtain a new interference power spectral density curve Inew(t) comprises: taking a component t closest to the time delay delta t of the main pulse and the first-order reflected pulse of the sample signal in an original interference power spectral density curve I (t)nAs the center frequency of the filter segment, wherein N is the subscript number of the horizontal axis t in I (t), (N belongs to 0-N-1);the free spectral range of the interference fringes is FSR: FSR ═ Δ F ═ c/OPD ═ 1/Δ t, where c is the speed of light, OPD is the path length difference between the two beams, and OPD ═ 2nsamL, L is the sample thickness, nsamThe refractive index of the sample in the terahertz waveband is adopted; get t n1 frequency point on each side, and is marked as tn-1，tn，tn+1Forming a filter section; maximum I (t) corresponding to frequency of filter segmenti) The value (I ═ n-1, n, n +1), denoted max I (t)i) A value of ImaxIs shown bymaxIs assigned to the average value I of the values of two adjacent frequency points I (t)new(ii) a Repeating the maximum I (t) corresponding to the frequency of the filter segmenti) The value (I ═ n-1, n, n +1), denoted max I (t)i) A value of ImaxIs shown bymaxIs assigned to the average value I of the values of two adjacent frequency points I (t)new(ii) a Stopping until any of the following cycle stop conditions are reached: reaching the specified filtering times; or Imax<Ith，IthIs a set threshold value; or when Inew≥ImaxWhen the current is over; thereby obtaining a filtered interference power spectral density curve Inew(t), wherein t is tk,(k∈0～N-1)。
In an embodiment of the invention, the reflection pulse interference filtering method for the THz-TDS test sample is applied to the transmission type THz-TDS; putting a sample at the focusing position of the terahertz wave beam to obtain a time-domain signal T of the sample after the terahertz wave beam penetrates through the sampleSam(t)。
In one embodiment of the present invention, the property S (t) is symmetric according to FFT conjugatek)＝S(tN-k)*(k is 1 to N-1), and the FFT conversion value at the corresponding frequency point in the original S (t) is corrected to obtain the filter-corrected discrete fourier transform Snew(t) comprises:
Snew(ti)＝S(ti)/ai(i＝n-1,n,n+1)
Snew(tN-i)＝S(tN-i)/ai(i＝n-1,n,n+1)
Snew(tj)＝S(tj)(j≠n-1,n,n+1,N-n-1,N-n,N-n+1)。
to realizeIn order to achieve the above object, the present invention further provides a system for filtering interference of reflected pulses in a THz-TDS test sample, comprising: the device comprises a test module, a transformation module, a discrete transformation module, a filtering iteration module, a filtering correction module and an inverse transformation module; the testing module is used for testing that the reference time domain signal of the THz-TDS is T when no sample is obtainedRef(T), testing to obtain the time domain signal T of THz-TDS when the sample existsSam(t); the transformation module is used for pair TRef(t) performing FFT and calculating the spectrum amplitude to obtain a corresponding frequency domain signal F without a sampleRef(f) To TSam(t) performing FFT and calculating the spectrum amplitude to obtain a corresponding frequency domain signal F with sample timeSam(f) So as to obtain a terahertz frequency domain absorption spectrum signal A (F) ═ F of the sampleRef(f)-FSam(f) (ii) a The discrete transform module is used for performing FFT (fast Fourier transform) of a spatial domain on the absorption spectrum signal A (f) to obtain a corresponding discrete Fourier transform spectrum S (t), S (t) ═ FFT [ A (f)](ii) a Obtaining an original interference power spectral density curve I (t) with different t as abscissa, wherein I (t) | S (t)/N2N is the number of points for FFT of the absorption spectrum A (f); the filtering iteration module is used for carrying out filtering iteration on the original interference power spectral density curve I (t) to obtain a new interference power spectral density curve Inew(t); the filtering modification module is used for converting the original interference power spectral density curve I (t) and the new interference power spectral density curve Inew(t) comparing to obtain amplitude attenuation coefficient a of corresponding filter segmenti＝[Inew(ti)/I(ti)]1/2(i ═ n-1, n, n + 1); and according to the property S (t) of FFT conjugate symmetryk)＝S(tN-k)*(k is 1 to N-1), and the FFT value of the corresponding frequency point in the discrete fourier transform spectrum S (t) is corrected to obtain the filter-corrected discrete fourier transform Snew(t); the inverse transformation module is used for performing discrete Fourier transformation S after filtering modificationnew(t) and performing an inverse discrete fourier transform to obtain an absorption spectrum after interference fringe filtering: a. thenew(f)＝IFFT[Snew(t)]。
In an embodiment of the invention, the filtering iteration module is used for comparing the original data with the original dataFiltering iteration is carried out on the initial interference power spectral density curve I (t) to obtain a new interference power spectral density curve Inew(t) comprises: taking a component t closest to the time delay delta t of the main pulse and the first-order reflected pulse of the sample signal in an original interference power spectral density curve I (t)nAs the center frequency of the filter segment, wherein N is the subscript number of the horizontal axis t in I (t), (N belongs to 0-N-1); the free spectral range of the interference fringes is FSR: FSR ═ Δ F ═ c/OPD ═ 1/Δ t, where c is the speed of light, OPD is the path length difference between the two beams, and OPD ═ 2nsamL, L is the sample thickness, nsamThe refractive index of the sample in the terahertz waveband is adopted; get t n1 frequency point on each side, and is marked as tn-1，tn，tn+1Forming a filter section; maximum I (t) corresponding to frequency of filter segmenti) The value (I ═ n-1, n, n +1), denoted max I (t)i) A value of ImaxIs shown bymaxIs assigned to the average value I of the values of two adjacent frequency points I (t)new(ii) a Repeating the maximum I (t) corresponding to the frequency of the filter segmenti) The value (I ═ n-1, n, n +1), denoted max I (t)i) A value of ImaxIs shown bymaxIs assigned to the average value I of the values of two adjacent frequency points I (t)new(ii) a Stopping until any of the following cycle stop conditions are reached: reaching the specified filtering times; or Imax<Ith，IthIs a set threshold value; or when Inew≥ImaxWhen the current is over; thereby obtaining a filtered interference power spectral density curve Inew(t), wherein t is tk,(k∈0～N-1)。
In an embodiment of the invention, the reflection pulse interference filtering method for the THz-TDS test sample is applied to the transmission type THz-TDS; putting a sample at the focusing position of the terahertz wave beam to obtain a time-domain signal T of the sample after the terahertz wave beam penetrates through the sampleSam(t)。
In one embodiment of the present invention, the property S (t) is symmetric according to FFT conjugatek)＝S(tN-k)*(k is 1 to N-1), and the FFT conversion value at the corresponding frequency point in the original S (t) is corrected to obtain the filter-corrected discrete fourier transform Snew(t) comprises:
Snew(ti)＝S(ti)/ai(i＝n-1,n,n+1)
Snew(tN-i)＝S(tN-i)/ai(i＝n-1,n,n+1)
Snew(tj)＝S(tj)(j≠n-1,n,n+1,N-n-1,N-n,N-n+1)。
to achieve the above object, the present invention further provides a computer readable storage medium, on which a computer program is stored, the computer program, when being executed by a processor, implementing any one of the above methods for interference filtering of reflected pulses when testing a THz-TDS test sample.
In order to achieve the above object, the present invention further provides a device for interference filtering of reflected pulses when testing a THz-TDS sample, comprising: a processor and a memory; the memory is used for storing a computer program; the processor is connected with the memory and is used for executing the computer program stored in the memory so as to enable the THz-TDS test sample time reflection pulse interference filtering device to execute any one of the above THz-TDS test sample time reflection pulse interference filtering methods.
Finally, the invention also provides a system for filtering the reflected pulse interference during the THz-TDS test sample, which comprises: the THz-TDS testing device comprises a reflected pulse interference filtering device and a communication signal transmitting device when testing the sample; the communication signal transmitting device is used for transmitting N times of transmitting signals with the number of M.
As mentioned above, the method, the system, the medium and the device for filtering the reflected pulse interference during the THz-TDS test sample have the following beneficial effects: the dispersion information of the sample or the time domain position where the reflected pulse signal appears is not required to be obtained in a priori, and the high-efficiency filtering of the interference fringes of the absorption spectrum reflection peak is realized.
Drawings
FIG. 1a is a flow chart of a method for interference filtering of reflected pulses in a THz-TDS test sample according to an embodiment of the present invention;
FIG. 1b is a diagram of a terahertz beam propagation model in an embodiment of the method for interference filtering of reflected pulses in the THz-TDS test sample according to the present invention;
FIG. 1c shows the present inventionIn one embodiment of the method for filtering the interference of the reflected pulse when the THz-TDS test sample is not used, the time domain signal of the THz-TDS when the sample is not used is TRef(T) time domain signal of THz-TDS with sample is TSam(t) schematic drawing;
FIG. 1d is a schematic diagram of an absorption spectrum of an embodiment of the method for interference filtering of reflected pulses in THz-TDS test samples according to the present invention;
FIG. 1e is a graph showing the original interference power spectral density curve I (t) of the THz-TDS test sample reflected pulse interference rejection method of the present invention in one embodiment;
FIG. 1f shows the reflection pulse interference filtering method for THz-TDS test sample according to the present invention, wherein the time domain signal of THz-TDS without sample is TRef(T) time domain signal of THz-TDS with sample is TSam(t) schematic drawing;
FIG. 1g is a schematic diagram of an absorption spectrum of a THz-TDS test sample according to another embodiment of the present invention;
FIG. 1h is a diagram showing a new interference power spectral density curve of the reflected pulse interference filtering method of the THz-TDS test sample according to one embodiment of the present invention;
FIG. 1i is a schematic diagram showing an absorption spectrum of a THz-TDS test sample after interference fringes are filtered by the reflection pulse interference filtering method in one embodiment of the present invention;
FIG. 2 is a schematic diagram of a reflection pulse interference rejection system for a THz-TDS test sample according to an embodiment of the present invention;
FIG. 3 is a schematic structural diagram of an embodiment of a reflected pulse interference rejection apparatus for THz-TDS test samples according to the present invention.
Description of the element reference numerals
21 test module
22 transformation module
23 discrete transform module
24 filtering iteration module
25 filtering correction module
26 inverse transform module
31 processor
32 memory
Detailed Description
The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It is to be noted that the features in the following embodiments and examples may be combined with each other without conflict.
It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present invention, so that the components related to the present invention are only shown in the drawings rather than drawn according to the number, shape and size of the components in actual implementation, the type, quantity and proportion of the components in actual implementation can be changed freely, and the layout of the components can be more complicated.
According to the method, the system, the medium and the device for filtering the reflected pulse interference during the THz-TDS sample testing, the dispersion information of the sample or the time domain position where the reflected pulse signal appears is not required to be acquired in advance, and the high-efficiency filtering of the absorption spectrum reflection peak interference fringes is realized.
In one embodiment, as shown in fig. 1a, the method for interference filtering of reflected pulses in THz-TDS test samples of the present invention comprises the following steps:
step S11, testing to obtain a reference time domain signal T of THz-TDS when no sample existsRef(T), testing to obtain the time domain signal T of THz-TDS when the sample existsSam(t)。
Specifically, the THz-TDS test sample reflection pulse interference filtering method is applied to transmission type THz-TDS; putting a sample at the focusing position of the terahertz wave beam to obtain a time-domain signal T of the sample after the terahertz wave beam penetrates through the sampleSam(t) of (d). The time domain signal of THz-TDS is T when no sample existsRef(t) is the reference time domain signal。
Specifically, the time domain and frequency domain joint analysis is carried out on a sample signal received during the transmission measurement of the THz-TDS system, so as to eliminate the interference caused by the reflection of the upper surface and the lower surface of the sample. The model of terahertz beam propagation in the case of transmissive measurement of a sample in the THz-TDS system is shown in fig. 1b, where 11 is an incident terahertz beam, 12 is a main pulse signal transmission path, 13 is a first-order reflected pulse signal transmission path, and 14 is a transmissive measurement sample thickness L. The application mode of the system is that firstly, the time domain signal of THz-TDS is tested to be T when no sample existsRef(T), then, putting a sample at the focusing position of the terahertz wave beam, and testing the time domain signal T of the THz-TDS after the sample is penetratedSam(T) the time domain signal of THz-TDS without sample is TRef(T) time domain signal of THz-TDS with sample is TSam(T) generally as shown in FIG. 1c, TSamIn the (t) signal, there is a significant reflected pulse signal introduced by the reflection at the upper and lower surfaces of the sample, in addition to the main pulse signal transmitted through the sample. Wherein 15 is a main time domain signal pulse of THz-TDS without a sample (i.e. a main time domain signal pulse of THz-TDS), 16 is a main time domain signal pulse of THz-TDS with a sample, 17 is a primary reflected time domain signal pulse of THz-TDS with a sample, and 18 is a delay delta t between the main time domain signal pulse of THz-TDS with a sample and the primary reflected time domain signal pulse.
Step S12, for TRef(t) performing FFT and calculating the spectrum amplitude to obtain a corresponding frequency domain signal F without a sampleRef(f) To TSam(t) FFT and spectral amplitude calculation, transform to obtain corresponding frequency domain signal F with sample timeSam(f) So as to obtain a terahertz frequency domain absorption spectrum signal A (F) ═ F of the sampleRef(f)-FSam(f)。
Specifically, the amplitude of the TDS time domain signal is proportional to the electric field intensity of the terahertz wave beam, the terahertz wave beam is reflected twice on the upper and lower surfaces of the sample according to the fresnel reflection law, and the phase of the reflected pulse signal is the same as that of the main pulse signal no matter the polarization direction of the incident wave beam is s-polarization or t-polarization, that is, in the time domain signal, the polarity of the reflected pulse is consistent with that of the main pulse (amplitude is consistent with that of the main pulse) (amplitude isThe same positive or the same negative) so that the time domain delay delta t between the reflected pulse and the main pulse can be obtained by searching an extreme point from the time domain signal; it should be noted that, because the sample to be measured has dispersion on the terahertz signal, and some samples have a certain tail in the main pulse of the terahertz signal, which will cause deformation such as pulse broadening in the time domain between the reflected peak signal and the main pulse, Δ t is not the precise position where the reflected pulse appears, and only can roughly reflect the approximate delay relationship between the main pulse and the reflected pulse signal. According to Fresnel reflection law, when the first-stage reflection pulse is reflected twice by the interface, the electric field intensity ratio of the reflected signal to the incident signal is (n)sam-nair)2/(nsam+nair)2Wherein n isairIs the refractive index (n) of airair＝1),nsamIs the refractive index of the terahertz wave band of the sample (n of the sample in general)sam>1.3), the electric field amplitude of the multi-stage reflected pulse is negligible with respect to the electric field amplitude of the main pulse, i.e. in the time domain signal of the sample, only the influence of the first stage reflected pulse needs to be considered. For TRef(t) performing FFT (fast Fourier transform) to obtain corresponding non-sample time-frequency domain signal FRef(f) To TSam(t) FFT to obtain corresponding sampled time-frequency domain signal FSam(f) From the sampled time-frequency domain signal FSam(f) Deduction of the non-sample time-frequency domain signal FRef(f) So as to obtain a terahertz frequency domain absorption spectrum signal A (F) ═ F of the sampleRef(f)-FSam(f) (the ordinate of the frequency domain signal of the terahertz frequency domain absorption spectrum signal a (f) is a logarithmic coordinate). Namely the method for acquiring the terahertz waveband absorption spectrum of the conventional THz-TDS sample. If the reflected pulse signal is not filtered out, then a significant interference fringe appears in the absorption spectrum. When the contrast of the interference fringe is large, not only a false absorption peak is introduced, but also the true absorption peak information can be overwhelmed when the contrast is large, thereby causing a serious error in the absorption spectrum test, as shown in fig. 1 d. The interference fringe 19 in the absorption spectrum and the interference fringe FSR 110 (which can be considered as the period of the interference fringe).
Step S13, for absorption spectrum signal A (f)FFT transform in the line space domain to obtain a corresponding discrete Fourier transform spectrum S (t), S (t) FFT [ A (f)](ii) a Obtaining an original interference power spectral density curve I (t) with different t as abscissa, wherein I (t) | S (t)/N2N is the number of points for FFT of the absorption spectrum a (f).
Specifically, N is the number of points of FFT of the absorption spectrum a (f), and the number of data points of a (f), i.e., i (t) | s (t)/N |, is2Where t is tkAnd (k is from 0 to N-1) is a set of discrete points. The physical meaning of I (t) is that for the absorption spectrum signal A (f), the absorption spectrum signal A (f) can be decomposed into superposition of a plurality of sinusoidal interference components, and each interference component corresponds to different delay amounts t of the main pulse signal and the first-stage reflection pulse signal; in i (t), except for the dc component and the linear drift (usually caused by scattering), the power of the interference signal corresponding to Δ t is much stronger than that of the rest interference components, and the Δ t component, the dc component and the linear drift component are far apart from each other on the horizontal axis, so that interference and misjudgment are not introduced. Wherein, the component t closest to delta t in the original interference power spectral density curve I (t) is takennAs the center frequency of the filter segment, wherein N is the subscript number of the horizontal axis t in I (t), (N belongs to 0-N-1); the free spectral range of the interference fringes is FSR: FSR ═ Δ F ═ c/OPD ═ 1/Δ t, where c is the speed of light, OPD is the path length difference between the two beams, and OPD ═ 2nsamL, L is the sample thickness, nsamThe refractive index of the sample in the terahertz waveband is shown. Thus, a frequency domain interference period delta F of the absorption spectrum interference fringe and a time domain signal T of THz-TDS when a sample exists are establishedSam(t) the relationship between the main pulse and the first stage reflected pulse delay Δ t. As shown by FSR Δ F c/OPD 1/Δ t, when the sample is used in a larger amount, the thickness L of the sample may be increased accordingly, so as to increase the OPD of the main beam and the first-order reflected beam and the separation Δ t of the main beam and the first-order reflected beam in the time-domain signal, which is a prerequisite for conventional time-domain filtering.
Step S14, filtering iteration is carried out on the original interference power spectral density curve I (t) to obtain a new interference power spectral density curve Inew(t)。
Due to various reasons such as sample dispersion, noise, scattering, uneven sample interface and the like, and due to the fence effect and frequency domain signal leakage in the FFT conversion process, the interference fringes in the obtained sample absorption spectrum are not an ideal periodic signal with 1/Δ t as a period, i.e., in i (t), there is usually no impulse function δ (Δ t) with the abscissa as Δ t, but a power spectrum envelope with Δ t as the center, and the power of interference components corresponding to different t values in the corresponding interval is gradually reduced by filtering the power spectrum envelope. The following method can remove the influence caused by the barrier effect and the frequency domain signal leakage in the FFT process.
In particular, as shown in fig. 1e, the original interference power spectral density curve i (t), where 111 is a maximum t corresponding to the original interference power spectral density curvenLocation. Filtering iteration is carried out on the original interference power spectral density curve I (t) to obtain a new interference power spectral density curve Inew(t) comprises: taking a component t closest to the time delay delta t of the main pulse and the first-order reflected pulse of the sample signal in an original interference power spectral density curve I (t)nAs the center frequency of the filter segment, wherein N is the subscript number of the horizontal axis t in I (t), (N belongs to 0-N-1); the free spectral range of the interference fringes is FSR: FSR ═ Δ F ═ c/OPD ═ 1/Δ t, where c is the speed of light, OPD is the path length difference between the two beams, and OPD ═ 2nsamL, L is the sample thickness, nsamThe refractive index of the sample in the terahertz waveband is adopted; the free spectral range of the interference fringes is FSR (i.e., the frequency separation between two adjacent minima or maxima in the interference fringes). Get t n1 frequency point on each side, and is marked as tn-1，tn，tn+1Forming a filter section; maximum I (t) corresponding to frequency of filter segmenti) The value (I ═ n-1, n, n +1), denoted max I (t)i) A value of ImaxIs shown bymaxIs assigned to the average value I of the values of two adjacent frequency points I (t)new(ii) a Note that if ImaxThe corresponding frequency point is not tnBut is tn-1Or tn+1Then there is an adjacent frequency point whose sequence number is not in the filter segment. Repeating the maximum I (t) corresponding to the frequency of the filter segmenti) The value (I ═ n-1, n, n +1), denoted max I (t)i) A value of ImaxIs shown bymaxIs assigned to the average value of the values of two adjacent frequency points I (t)Inew(ii) a Stopping until any of the following cycle stop conditions are reached: reaching the specified filtering times; or Imax<Ith，IthIs a set threshold value; or when Inew≥ImaxWhen the current is over; thereby obtaining a filtered interference power spectral density curve Inew(t), wherein t is tk(k is from 0 to N-1). Wherein, S (t)k)＝S(tN-k)*(k is 1 to N-1) and Inew(t)，t＝tkAnd k belongs to the same k (k belongs to 0-N-1), but the value range is different.
Step S15, the original interference power spectral density curve I (t) and the new interference power spectral density curve Inew(t) comparing to obtain amplitude attenuation coefficient a of corresponding filter segmenti＝[Inew(ti)/I(ti)]1/2(i ═ n-1, n, n + 1); and according to the property S (t) of FFT conjugate symmetryk)＝S(tN-k)*(k is 1 to N-1), and the FFT value of the corresponding frequency point in the discrete fourier transform spectrum S (t) is corrected to obtain the filter-corrected discrete fourier transform Snew(t)。
In particular, said property of symmetry according to FFT conjugates S (t)k)＝S(tN-k)*(k is 1 to N-1), and the FFT conversion value at the corresponding frequency point in the original S (t) is corrected to obtain the filter-corrected discrete fourier transform Snew(t) comprises: snew(ti)＝S(ti)/ai(i＝n-1,n,n+1)；Snew(tN-i)＝S(tN-i)/ai(i＝n-1,n,n+1)；Snew(tj)＝S(tj)(j≠n-1,n,n+1,N-n-1,N-n,N-n+1)。
Step S16, discrete Fourier transform S after filter correctionnew(t) and performing an inverse discrete fourier transform to obtain an absorption spectrum after interference fringe filtering: a. thenew(f)＝IFFT[Snew(t)]。
In particular, the filtered absorption spectrum A is completednew(f) Compared with the original absorption spectrum A (f), the absorption spectrum interference fringe introduced by the first-stage reflection pulse of the sample can be effectively reduced, and meanwhile, the absorption spectrum interference fringe can be effectively keptThe morphology of the true absorption peak of the sample in the spectrum. By the method, under the condition that the dispersion information of the sample is not required to be acquired in a priori, the accurate time domain position of the reflected pulse signal is not required to be known, high-efficiency filtering of the interference fringes of the reflection peak of the absorption spectrum is realized, and the wave shape of the real absorption peak is not seriously influenced. Meanwhile, the method does not need the precondition that the reflection pulse and the main pulse are completely separated in the time domain, which is required by the conventional time domain reflection peak filtering method, so that for some thin samples or samples with longer trailing main pulses, when the reflection pulse and the main pulse are overlapped, the common time domain filtering method fails, and the method can still effectively filter the interference fringes in the absorption spectrum, thereby widening the application range of the THz-TDS.
Specifically, as shown in fig. 1f, in an embodiment, 60mg of pure pyrazinamide is ground and tabletted to form a test sample piece with a diameter of 13mm and a thickness L of 0.4mm, and a transmission type THz-TDS system is used to respectively obtain a time domain signal T of THz-TDS without sampleRef(T)112, testing the time domain signal T of THz-TDS when the sample existsSam(t)113, as shown in FIG. 1f, the time-domain signal of THz-TDS with the sample (sample transmission time-domain signal 113) can obtain the delay Δ t between the first-stage reflected signal and the main pulse as 8.16 ps. The time domain signal T of the THz-TDS without the sample is measuredRef(T)112 and the time domain signal of THz-TDS with sample is TSam(t)113, performing FFT to obtain respective frequency domain signals, and subtracting the frequency domain signal of the sample from the reference frequency domain signal to obtain an absorption spectrum signal of the tested pyrazinamide sample sheet, as shown in fig. 1g, it can be seen from the figure that when the reflection peak in the time domain signal of THz-TDS with the sample is not processed, very obvious interference fringes exist in the absorption spectrum, and serious interference exists on the true absorption peak of the sample. Interference power spectrum analysis is performed on the absorption spectrum shown in fig. 1g, specifically, FFT conversion is performed on the absorption spectrum signal a (f) in the spatial domain to obtain a corresponding discrete fourier transform spectrum s (t), s (t) FFT [ a (f)](ii) a Obtaining an original interference power spectral density curve I (t) with different t as abscissa, wherein I (t) | S (t)/N2N is toAnd (f) collecting the points of the spectrum A (A) subjected to FFT (fast Fourier transform) to obtain the corresponding I (t). Then, the central point t of the filter segment corresponding to the delta t is found out from the I (t)nFiltering iteration is carried out on the original interference power spectral density curve I (t) to obtain a new interference power spectral density curve Inew(t), as shown in FIG. 1 h. In this example, the iteration termination condition is adopted as Inew≥ImaxAnd then, the filtering iteration is stopped, and in the example, the filtering is finished after 3 times of iteration, so that the speed is high. For the new interference power spectral density curve I after the filtering is finishednew(t), performing inverse discrete Fourier transform to obtain an absorption spectrum with interference fringes removed, as shown in FIG. 1 i. Comparing fig. 1g and 1i, it can be seen that the interference fringes are greatly suppressed, and the true absorption peak of pyrazinamide can still be maintained, for example, in fig. 1i, there are three distinct absorption peaks in the filtered pyrazinamide absorption spectrum, which are respectively located at 0.50, 0.72, and 1.47THz, which better correspond to the literature (zhangqi, squaraine, ash dan, etc. 'THz time domain spectroscopy qualitative and quantitative analysis of pyrazinamide and isoniazid in antituberculosis drugs', "journal of drug analysis", vol. 36, No. 6, 1082-1088, 2016); in the example, the total amount of the used substances is only 60mg, the thickness is only 0.4mm, and compared with 280mg and a test sample sheet with the thickness of 1.5mm used in the literature, the sample amount and the sample sheet thickness can be greatly reduced, so that the testing capability of the THz-TDS system on the sample with less substance content is improved.
As shown in fig. 2, in an embodiment, the THz-TDS test sample time reflection pulse interference filtering system of the present invention includes a test module 21, a transform module 22, a discrete transform module 23, a filter iteration module 24, a filter modification module 25, and an inverse transform module 26; the testing module is used for testing that the reference time domain signal of the THz-TDS is T when no sample is obtainedRef(T), testing to obtain the time domain signal T of THz-TDS when the sample existsSam(t); the transformation module is used for pair TRef(t) performing FFT and calculating the spectrum amplitude to obtain a corresponding frequency domain signal F without a sampleRef(f) To TSam(t) performing FFT and calculating the spectrum amplitude to obtain a corresponding frequency domain signal F with sample timeSam(f) So as to obtain a terahertz frequency domain absorption spectrum signal A (F) ═ F of the sampleRef(f)-FSam(f) (ii) a (ii) a The discrete transform module is used for performing FFT (fast Fourier transform) of a spatial domain on the absorption spectrum signal A (f) to obtain a corresponding discrete Fourier transform spectrum S (t), S (t) ═ FFT [ A (f)](ii) a Obtaining an original interference power spectral density curve I (t) with different t as abscissa, wherein I (t) | S (t)/N2N is the number of points for FFT of the absorption spectrum A (f); the filtering iteration module is used for carrying out filtering iteration on the original interference power spectral density curve I (t) to obtain a new interference power spectral density curve Inew(t); the filtering modification module is used for converting the original interference power spectral density curve I (t) and the new interference power spectral density curve Inew(t) comparing to obtain amplitude attenuation coefficient a of corresponding filter segmenti＝[Inew(ti)/I(ti)]1/2(i ═ n-1, n, n + 1); and according to the property S (t) of FFT conjugate symmetryk)＝S(tN-k)*(k is 1 to N-1), and the FFT value of the corresponding frequency point in the discrete fourier transform spectrum S (t) is corrected to obtain the filter-corrected discrete fourier transform Snew(t); the inverse transformation module is used for performing discrete Fourier transformation S after filtering modificationnew(t) and performing an inverse discrete fourier transform to obtain an absorption spectrum after interference fringe filtering: a. thenew(f)＝IFFT[Snew(t)]。
The filtering iteration module is used for carrying out filtering iteration on the original interference power spectral density curve I (t) to obtain a new interference power spectral density curve Inew(t) comprises: component t of delay delta t of main pulse and first-stage reflection pulse of sample signalnAs the center frequency of the filter segment, wherein N is the subscript number of the horizontal axis t in I (t), (N belongs to 0-N-1); the free spectral range of the interference fringes is FSR: FSR ═ Δ F ═ c/OPD ═ 1/Δ t, where c is the speed of light, OPD is the path length difference between the two beams, and OPD ═ 2nsamL, L is the sample thickness, nsamThe refractive index of the sample in the terahertz waveband is adopted; get tn1 frequency point on each side, and is marked as tn-1，tn，tn+1Forming a filter section; maximum I (t) corresponding to frequency of filter segmenti) The value (I ═ n-1, n, n +1), denoted max I (t)i) A value of ImaxIs shown bymaxIs assigned to the average value I of the values of two adjacent frequency points I (t)new(ii) a Repeating the maximum I (t) corresponding to the frequency of the filter segmenti) The value (I ═ n-1, n, n +1), denoted max I (t)i) A value of ImaxIs shown bymaxIs assigned to the average value I of the values of two adjacent frequency points I (t)new(ii) a Stopping until any of the following cycle stop conditions are reached: reaching the specified filtering times; or Imax<Ith，IthIs a set threshold value; or when Inew≥ImaxWhen the current is over; thereby obtaining a filtered interference power spectral density curve Inew(t), wherein t is tk,(k∈0～N-1)。
In an embodiment of the invention, the reflection pulse interference filtering method for the THz-TDS test sample is applied to the transmission type THz-TDS; putting a sample at the focusing position of the terahertz wave beam to obtain a time-domain signal T of the sample after the terahertz wave beam penetrates through the sampleSam(t)。
In one embodiment of the present invention, the property S (t) is symmetric according to FFT conjugatek)＝S(tN-k)*(k is 1 to N-1), and the FFT conversion value at the corresponding frequency point in the original S (t) is corrected to obtain the filter-corrected discrete fourier transform Snew(t) comprises:
Snew(ti)＝S(ti)/ai(i＝n-1,n,n+1)
Snew(tN-i)＝S(tN-i)/ai(i＝n-1,n,n+1)
Snew(tj)＝S(tj)(j≠n-1,n,n+1,N-n-1,N-n,N-n+1)。
it should be noted that the structures and principles of the test module 21, the transform module 22, the discrete transform module 23, the filtering iteration module 24, the filtering correction module 25, and the inverse transform module 26 correspond to the steps in the THz-TDS test sample reflection pulse interference filtering method one to one, and therefore, the description thereof is omitted.
It should be noted that the division of the modules of the above system is only a logical division, and the actual implementation may be wholly or partially integrated into one physical entity, or may be physically separated. And these modules can be realized in the form of software called by processing element; or may be implemented entirely in hardware; and part of the modules can be realized in the form of calling software by the processing element, and part of the modules can be realized in the form of hardware. For example, the x module may be a processing element that is set up separately, or may be implemented by being integrated in a chip of the apparatus, or may be stored in a memory of the apparatus in the form of program code, and the function of the x module may be called and executed by a processing element of the apparatus. Other modules are implemented similarly. In addition, all or part of the modules can be integrated together or can be independently realized. The processing element described herein may be an integrated circuit having signal processing capabilities. In implementation, each step of the above method or each module above may be implemented by an integrated logic circuit of hardware in a processor element or an instruction in the form of software.
For example, the above modules may be one or more integrated circuits configured to implement the above methods, such as: one or more Specific Integrated circuits (ASICs), or one or more Microprocessors (MPUs), or one or more Field Programmable Gate Arrays (FPGAs), etc. For another example, when one of the above modules is implemented in the form of a Processing element scheduler code, the Processing element may be a general-purpose processor, such as a Central Processing Unit (CPU) or other processor capable of calling program code. For another example, these modules may be integrated together and implemented in the form of a system-on-a-chip (SOC).
In an embodiment of the present invention, the present invention further includes a computer readable storage medium, on which a computer program is stored, wherein the computer program, when executed by a processor, implements any one of the above methods for interference filtering of reflected pulses when testing THz-TDS samples.
Those of ordinary skill in the art will understand that: all or part of the steps for implementing the above method embodiments may be performed by hardware associated with a computer program. The aforementioned computer program may be stored in a computer readable storage medium. When executed, the program performs steps comprising the method embodiments described above; and the aforementioned storage medium includes: various media that can store program codes, such as ROM, RAM, magnetic or optical disks.
As shown in FIG. 3, in one embodiment, the apparatus for filtering interference of reflected pulses in THz-TDS test sample of the present invention comprises: a processor 31 and a memory 32; the memory 32 is for storing a computer program; the processor 31 is connected to the memory 32 and is configured to execute a computer program stored in the memory 32, so that the THz-TDS test sample time reflected pulse interference filtering apparatus executes any one of the THz-TDS test sample time reflected pulse interference filtering methods.
Specifically, the memory 32 includes: various media that can store program codes, such as ROM, RAM, magnetic disk, U-disk, memory card, or optical disk.
Preferably, the Processor 31 may be a general-purpose Processor, including a Central Processing Unit (CPU), a Network Processor (NP), and the like; the Integrated Circuit may also be a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other Programmable logic device, discrete Gate or transistor logic device, or discrete hardware components.
In summary, the method, the system, the medium and the device for filtering the reflected pulse interference during the THz-TDS sample testing do not need to acquire the dispersion information of the sample or the time domain position of the reflected pulse signal in a priori manner, and realize the high-efficiency filtering of the interference fringes of the absorption spectrum reflection peak. Therefore, the invention effectively overcomes various defects in the prior art and has high industrial utilization value.
The foregoing embodiments are merely illustrative of the principles and utilities of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or change the above-mentioned embodiments without departing from the spirit and scope of the present invention. Accordingly, it is intended that all equivalent modifications or changes which can be made by those skilled in the art without departing from the spirit and technical spirit of the present invention be covered by the claims of the present invention.
Claims (10)
1. A method for filtering reflected pulse interference during THz-TDS test sample is characterized by comprising the following steps:
the reference time domain signal of THz-TDS without sample is TRef(T), testing to obtain the time domain signal T of THz-TDS when the sample existsSam(t)；
For TRef(t) performing FFT and calculating the spectrum amplitude to obtain a corresponding frequency domain signal F without a sampleRef(f) To TSam(t) FFT and spectral amplitude calculation, transform to obtain corresponding frequency domain signal F with sample timeSam(f) So as to obtain a terahertz frequency domain absorption spectrum signal A (F) ═ F of the sampleRef(f)-FSam(f)；
Performing FFT in spatial domain on the absorption spectrum signal A (f) to obtain a corresponding discrete Fourier transform spectrum S (t), S (t) FFT [ A (f)](ii) a Obtaining an original interference power spectral density curve I (t) with different t as abscissa, wherein I (t) | S (t)/N2N is the number of points for FFT of the absorption spectrum A (f);
filtering iteration is carried out on the original interference power spectral density curve I (t) to obtain a new interference power spectral density curve Inew(t)；
Comparing the original interference power spectral density curve I (t) with the new interference power spectral density curve Inew(t) comparing to obtain amplitude attenuation coefficient a of corresponding filter segmenti＝[Inew(ti)/I(ti)]1/2(i ═ n-1, n, n + 1); and according to the property S (t) of FFT conjugate symmetryk)＝S(tN-k)*(k is 1 to N-1), and the FFT value of the corresponding frequency point in the discrete fourier transform spectrum S (t) is corrected to obtain the filter-corrected discrete fourier transform Snew(t)；
To the filtered modified discrete Fourier transform Snew(t) and performing an inverse discrete fourier transform to obtain an absorption spectrum after interference fringe filtering: a. thenew(f)＝IFFT[Snew(t)]。
2. The method for interference filtering of reflected pulses when testing THz-TDS samples as claimed in claim 1, wherein the filtering iteration is performed on the original interference power spectral density curve I (t) to obtain a new interference power spectral density curve I (t)new(t) comprises:
taking a component t closest to the time delay delta t of the main pulse and the first-order reflected pulse of the sample signal in an original interference power spectral density curve I (t)nAs the center frequency of the filter segment, wherein N is the subscript number of the horizontal axis t in I (t), (N belongs to 0-N-1); the free spectral range of the interference fringes is FSR: FSR ═ Δ F ═ c/OPD ═ 1/Δ t, where c is the speed of light, OPD is the path length difference between the two beams, and OPD ═ 2nsamL, L is the sample thickness, nsamThe refractive index of the sample in the terahertz waveband is adopted;
get tn1 frequency point on each side, and is marked as tn-1，tn，tn+1Forming a filter section;
finding the maximum I (t) corresponding to the frequency of the filter segmenti) The value (I ═ n-1, n, n +1), denoted max I (t)i) A value of ImaxIs shown bymaxIs assigned to the average value I of the values of two adjacent frequency points I (t)new；
Repeating the maximum I (t) corresponding to the frequency of the filter segmenti) The value (I ═ n-1, n, n +1), denoted max I (t)i) A value of ImaxIs shown bymaxIs assigned to the average value I of the values of two adjacent frequency points I (t)new(ii) a Stopping until any of the following cycle stop conditions are reached: reaching the specified filtering times; or Imax<Ith，IthIs a set threshold value; or when Inew≥ImaxWhen the current is over; thereby obtaining a filtered interference power spectral density curve Inew(t), wherein t is tk,(k∈0～N-1)。
3. The THz-TDS test sample time reflection pulse interference filtering method according to claim 1, wherein the THz-TDS test sample time reflection pulse interference filtering method is applied to a transmissive THz-TDS; putting a sample at the focusing position of the terahertz wave beam to obtain a time-domain signal T of the sample after the terahertz wave beam penetrates through the sampleSam(t)。
4. The THz-TDS test sample time reflected pulse interference filtering method of claim 1, wherein the property S (t) of conjugate symmetry according to FFT is characterized byk)＝S(tN-k)*(k is 1 to N-1), and the FFT conversion value at the corresponding frequency point in the original S (t) is corrected to obtain the filter-corrected discrete fourier transform Snew(t) comprises:
Snew(ti)＝S(ti)/ai(i＝n-1,n,n+1)
Snew(tN-i)＝S(tN-i)/ai(i＝n-1,n,n+1)
Snew(tj)＝S(tj)(j≠n-1,n,n+1,N-n-1,N-n,N-n+1)。
5. a THz-TDS sample time reflected pulse interference rejection system, comprising: the device comprises a test module, a transformation module, a discrete transformation module, a filtering iteration module, a filtering correction module and an inverse transformation module;
the testing module is used for testing that the reference time domain signal of the THz-TDS is T when no sample is obtainedRef(T), testing to obtain the time domain signal T of THz-TDS when the sample existsSam(t)；
The transformation module is used for pair TRef(t) performing FFT and calculating the spectrum amplitude to obtain a corresponding frequency domain signal F without a sampleRef(f) To TSam(t) performing FFT and calculating the spectrum amplitude to obtain a corresponding frequency domain signal F with sample timeSam(f) So as to obtain a terahertz frequency domain absorption spectrum signal A (F) ═ F of the sampleRef(f)-FSam(f)；
Said dispersionThe transformation module is used for carrying out FFT (fast Fourier transform) on the absorption spectrum signal A (f) in a spatial domain to obtain a corresponding discrete Fourier transform spectrum S (t), S (t) ═ FFT [ A (f)](ii) a Obtaining an original interference power spectral density curve I (t) with different t as abscissa, wherein I (t) | S (t)/N2N is the number of points for FFT of the absorption spectrum A (f);
the filtering iteration module is used for carrying out filtering iteration on the original interference power spectral density curve I (t) to obtain a new interference power spectral density curve Inew(t)；
The filtering modification module is used for converting the original interference power spectral density curve I (t) and the new interference power spectral density curve Inew(t) comparing to obtain amplitude attenuation coefficient a of corresponding filter segmenti＝[Inew(ti)/I(ti)]1/2(i ═ n-1, n, n + 1); and according to the property S (t) of FFT conjugate symmetryk)＝S(tN-k)*(k is 1 to N-1), and the FFT value of the corresponding frequency point in the discrete fourier transform spectrum S (t) is corrected to obtain the filter-corrected discrete fourier transform Snew(t)；
The inverse transformation module is used for performing discrete Fourier transformation S after filtering modificationnew(t) and performing an inverse discrete fourier transform to obtain an absorption spectrum after interference fringe filtering: a. thenew(f)＝IFFT[Snew(t)]。
6. The THz-TDS test sample time reflection pulse interference filtering system of claim 5, wherein the filter iteration module is used for performing filter iteration on the original interference power spectral density curve I (t) to obtain a new interference power spectral density curve I (t)new(t) comprises:
taking a component t closest to the time delay delta t of the main pulse and the first-order reflected pulse of the sample signal in an original interference power spectral density curve I (t)nAs the center frequency of the filter segment, wherein N is the subscript number of the horizontal axis t in I (t), (N belongs to 0-N-1); the free spectral range of the interference fringes is FSR: FSR ═ Δ F ═ c/OPD ═ 1/Δ t, where c is the speed of light, OPD is the path length difference between the two beams, and OPD ═ 2nsamL, L is the sample thickness, nsamThe refractive index of the sample in the terahertz waveband is adopted;
get tn1 frequency point on each side, and is marked as tn-1，tn，tn+1Forming a filter section;
maximum I (t) corresponding to frequency of filter segmenti) The value (I ═ n-1, n, n +1), denoted max I (t)i) A value of ImaxIs shown bymaxIs assigned to the average value I of the values of two adjacent frequency points I (t)new；
Repeating the maximum I (t) corresponding to the frequency of the filter segmenti) The value (I ═ n-1, n, n +1), denoted max I (t)i) A value of ImaxIs shown bymaxIs assigned to the average value I of the values of two adjacent frequency points I (t)new(ii) a Stopping until any of the following cycle stop conditions are reached: reaching the specified filtering times; or Imax<Ith，IthIs a set threshold value; or when Inew≥ImaxWhen the current is over; thereby obtaining a filtered interference power spectral density curve Inew(t), wherein t is tk,(k∈0～N-1)。
7. The THz-TDS test sample time reflection pulse interference rejection system of claim 5, wherein the THz-TDS test sample time reflection pulse interference rejection method is applied to transmissive THz-TDS; putting a sample at the focusing position of the terahertz wave beam to obtain a time-domain signal T of the sample after the terahertz wave beam penetrates through the sampleSam(t)。
8. The THz-TDS test sample time reflected pulse interference rejection system of claim 5, wherein said property S (t) is symmetric according to FFT conjugatek)＝S(tN-k)*(k is 1 to N-1), and the FFT conversion value at the corresponding frequency point in the original S (t) is corrected to obtain the filter-corrected discrete fourier transform Snew(t) comprises:
Snew(ti)＝S(ti)/ai(i＝n-1,n,n+1)
Snew(tN-i)＝S(tN-i)/ai(i＝n-1,n,n+1)
Snew(tj)＝S(tj)(j≠n-1,n,n+1,N-n-1,N-n,N-n+1)。
9. a computer readable storage medium having a computer program stored thereon, wherein the computer program is executed by a processor to implement the method of reflected pulse interference rejection for THz-TDS test samples of any of claims 1 to 4.
10. A device for filtering interference of reflected pulses when THz-TDS (terahertz time-domain reflectometry) tests samples is characterized by comprising: a processor and a memory;
the memory is used for storing a computer program;
the processor is connected with the memory and is used for executing the computer program stored in the memory to enable the THz-TDS test sample time reflection pulse interference filtering device to execute the THz-TDS test sample time reflection pulse interference filtering method of any one of claims 1 to 4.
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