CN102012216A - Terahertz multi-wavelength phase imaging method - Google Patents
Terahertz multi-wavelength phase imaging method Download PDFInfo
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- CN102012216A CN102012216A CN 201010291936 CN201010291936A CN102012216A CN 102012216 A CN102012216 A CN 102012216A CN 201010291936 CN201010291936 CN 201010291936 CN 201010291936 A CN201010291936 A CN 201010291936A CN 102012216 A CN102012216 A CN 102012216A
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Abstract
The invention relates to a terahertz multi-wavelength phase imaging method. The method comprises the following steps of: (1) performing two-dimension spectral imaging on a sample to be imaged to acquire a terahertz time-domain spectral signal of each point on the sample to be imaged; (2) performing Fourier transform on the terahertz time-domain spectral signal of each point on the sample to be imaged to acquire a phase spectrum of each point on the sample to be imaged in an effective frequency range of a system; (3) selecting a phase image at a section of frequencies in the effective frequency range for linear fitting to acquire a fitted straight line; and (4) solving the slope of the fitted straight line, wherein the acquired slope is proportional to the thickness of the sample to be detected, so that the outline of the sample to be detected can be reconstructed.
Description
Technical field
The present invention relates to Terahertz multi-wavelength phase imaging method, specifically the present invention relates to a kind of coherence of THz wave and characteristics of broadband property utilized, choose the position phase images processing of the pairing two or more frequencies of two or more wavelength wherein, thereby reconstruct the formation method of the contour images of sample.
Background technology
Phase imaging all is considered to the accurately the most promising formation method of reconstruction of objects all the time, and the axial resolution of its image can reach the part of wavelength.But all there is a bottleneck in the phase imaging technology all the time, and when the optical path difference that causes when testee was greater than imaging wavelength exactly, the breakpoint of 2 π can appear in the position phase images.
Multi-wavelength digital hologram method utilizes two different wavelength of laser respectively same object to be carried out the digital hologram imaging, produces two width of cloth position phase images and this two width of cloth image is merged processing, can not had the image of pseudo-breakpoint.But owing to needing to change wavelength, there is ground unrest separately in the imaging of twice difference, and two width of cloth images can amplify noise when doing the merging processing of subtracting each other, and the picture quality that obtains is relatively poor.
The Terahertz wave spectrum can provide the very wide interior phase information of spectral range, and one-shot measurement can obtain the pairing position of several wavelength phase images, therefore, uses multi-wavelength phase imaging method and have advantageous advantage in the THz wave imaging technique.Owing to be one-shot measurement, several that an obtain phase images has identical ground unrest simultaneously.
The position phase images of two wavelength correspondences is subtracted each other processing, when eliminating pseudo-breakpoint, can eliminate hot ground unrest and reduce 1/f noise greatly; The position of monopulse THz wave is directly proportional with frequency, and the multi-wavelength position of being come by this peculiar property development fits to picture mutually, because the multi-point fitting of position phase reduces the uncertainty of single-point position phase greatly, so this method can suppress picture noise.
Summary of the invention
The invention provides a kind of Terahertz multi-wavelength phase imaging method, to solve the technical matters that exists in the above-mentioned background technology, the breakpoint of 2 π appears in the phase of ascending the throne imaging meeting, and noise is amplified in multi-wavelength digital hologram imaging meeting, and the picture quality that obtains is relatively poor.
For solving the problems of the technologies described above, a kind of Terahertz multi-wavelength phase imaging method of the present invention comprises the steps: (1) to being carried out the two-dimension spectrum imaging by the imaging sample, obtains described by every terahertz time-domain spectroscopy signal on the imaging sample; (2) described terahertz time-domain spectroscopy signal by each point on the imaging sample is carried out Fourier transform, obtain in system's effective frequency range described by the phase spectrum of each point on the imaging sample; (3) the position phase images of choosing the band frequency in the effective frequency range is carried out linear fit, obtains fitting a straight line; (4) try to achieve the slope of described fitting a straight line, the gained slope is directly proportional with the thickness of sample, thereby can reconstruct the profile of sample.
Wherein, the two-dimension spectrum in the step (1) is imaged as the transmission-type imaging.
Wherein, the two-dimension spectrum in the step (1) is imaged as reflective imaging.
Wherein, the two-dimension spectrum in the step (1) is imaged as two-dimentional focal plane imaging.
Wherein, the two-dimension spectrum in the step (1) is imaged as two-dimentional point by point scanning imaging.
Wherein, a band frequency of choosing in the step (3) is a band frequency of avoiding the water vapor absorption peak.
Wherein, a described band frequency is that 0.5THz is to 1.5THz.
Wherein, the method that obtains the fitting a straight line slope in the step (4) is for to described fitting a straight line differentiate.
The present invention handles the position phase images of the pairing two or more frequencies of two or more wavelength by technique scheme, reconstructs the contour images of sample.Occur the shortcoming of pseudo-breakpoint thereby overcome single wavelength phase imaging during greater than wavelength in optical path difference, remove ground unrest simultaneously, improve picture quality, reached beneficial technical effects.
Description of drawings
Fig. 1 is the mutually reflective imaging system synoptic diagram in Terahertz multi-wavelength position;
Fig. 2 is a Terahertz multi-wavelength position phase transmission-type imaging system synoptic diagram;
Fig. 3 is the optical photograph of metal washer;
Fig. 4 is traditional Terahertz image of metal washer;
Fig. 5 is the dual wavelength position phase images of the 0.9THz and the 1.1THz of metal washer;
Fig. 6 is the 0.5THz-1.5THz multi-wavelength position phase fitted figure picture of metal washer.
Description of reference numerals:
M1-M6: catoptron; L1-L2: concavees lens; 1: the femtosecond laser amplifier; 2: beam splitter; 3: frequency divider; 4: chopper; 5: the Terahertz emitter; 6: throw the face mirror; 7: by the imaging sample; 7 ': level crossing; 8,13: polaroid; 9,14: convex lens; 10: the tygon lens; 11: electro-conductive glass; 12: crystal detection; The 15:CCD camera; I: pump light; II: survey light.
Embodiment
For shape of the present invention, structure and characteristics can be understood better, below will enumerate preferred embodiment and also be elaborated in conjunction with the accompanying drawings.
Fig. 1 is the mutually reflective focal plane imaging system synoptic diagram in Terahertz multi-wavelength position, as shown in the figure, the employed LASER Light Source of this imaging system is the femtosecond laser amplifier (SpectraPhysics Hurricane Amplifier) that Spectra-Physics makes, the repetition frequency 1KHz of the laser pulse that produces, pulse width 75fs, centre wavelength 795nm, output power 650mW.
The femtosecond laser that femtosecond laser amplifier 1 produces is divided into two-beam through beam splitter 2: a branch of light of transmission is pretended and is pump light I, incide on the Terahertz emitter 5 through a series of mirror M 3-M6 and concavees lens L1, produce THz wave by the optical rectification effect, this Terahertz emitter 5 is the ZnTe crystal.THz wave is collimated into the parallel beam that diameter is 25mm through throwing face mirror 6.
Thz beam shines by on the imaging sample 7 with 15 ° incident angle.Focused on through tygon lens 10 by the THz wave of imaging sample 7 reflections, and by after 11 reflections of ITO electro-conductive glass, be imaged onto on the crystal detection 12, this crystal detection 12 is large-sized ZnTe crystal.
A branch of light of reflection is as surveying light II, and it is through mirror M 1, and M2 and concavees lens L2 are collimated into the light beam that diameter is 25mm by convex lens 9 then, sees through electro-conductive glass 11 and thz beam conllinear by crystal detection 12.Be provided with polaroid 8 between concavees lens L2 and the convex lens 9, crystal detection 12 rears are provided with another polaroid 13 with the polarization direction quadrature of polaroid 8.
When the polarization direction of surveying light II did not change, the detection light II light intensity of two polaroids by quadrature should be zero.Because in crystal detection 12, the Terahertz electric field changes the index ellipsoid of crystal, thereby the polarization state of the detection light II of outgoing is changed, so can measure the size of surveying light II polarization state change amount by two polaroids 8 and 13 of quadrature, and then obtain the time dependent time-domain spectroscopy of THz wave indirectly.Converge to CCD camera 15 from the detection light II of polaroid 13 outgoing by convex lens 14.This catches Princeton (Princeton) the CCD camera that image is used, and the time shutter is 32ms.
Because Real Time Image System can not use lock-in amplifier to remove noise, so the signal to noise ratio (S/N ratio) of system is generally lower.In order to reduce macrocyclic smooth ground unrest, system has adopted dynamic subtraction technique.Femtosecond laser amplifier 1 produces the signal that size equals the repetition frequency 1KHz of laser pulse, is divided into two synchronizing signals that frequency is 1/32Hz and 1/64Hz by frequency divider 3, is used for triggering optical chopper 4 and CCD camera respectively.Like this, catch two frame continuous images when the CCD camera, wherein a frame is a terahertz signal, and another frame then is that terahertz signal has been blocked by chopper 4, and signal just can extract.Because image is alternately to catch in the time of millisecond magnitude, if two two field pictures are subtracted each other, be about to signal pattern and background image and subtract each other, macrocyclic background drift will reduce greatly.
Adopt two-dimentional focal plane imaging in the present embodiment, imaging mode among the present invention also can be two-dimentional point by point scanning imaging mode, the system of two dimension point by point scanning imaging is on the system-based of a some terahertz time-domain signal in existing test sample product, sample is fixed on the two-dimension translational platform, this two-dimension translational platform can two dimension move, whenever move once this time-domain spectroscopy signal of a point measurement, till each point on the sample is all measured.So just can obtain the terahertz time-domain spectroscopy signal of each point on the sample, finish two-dimentional point by point scanning.
In the above-described embodiments, to adopted the mode of reflective focal plane imaging by imaging sample 7.In another embodiment of the present invention, also can adopt the mode of transmission-type focal plane imaging, to being carried out imaging by imaging sample 7.
Reflection and transmission are corresponding different materials, and the material Terahertz that has can see through, so general selective transmission formula is measured, the material Terahertz that has can not see through, still can reflected terahertz now, such as metal, generally adopt reflective measurement.In addition, when needs are specialized in the reflection of material or transmissison characteristic, also need to use different imaging modes.
As shown in Figure 2, specifically adopt the system of transmission-type focal plane imaging as follows:
The femtosecond laser that femtosecond laser amplifier 1 produces is divided into two-beam through beam splitter 2: a branch of light of transmission is pretended and is pump light I, incide on the Terahertz emitter 5 through a series of mirror M 3-M6 and concavees lens L1, produce THz wave by the optical rectification effect, this Terahertz emitter 5 is the ZnTe crystal.THz wave is collimated into the parallel beam that diameter is 25mm through throwing face mirror 6.
Thz beam shines on the level crossing 7 ' with 15 ° incident angle.Be located at level crossing 7 ' by imaging sample 7 and throw between the face mirror 6 or be located between level crossing 7 ' and the tygon lens 10, make THz wave from by after 7 transmissions of imaging sample, focus on through tygon lens 10, reflexed on the crystal detection 12 by ITO electro-conductive glass 11, this crystal detection 12 is large-sized ZnTe crystal.
A branch of light of reflection is as surveying light II, and it is through mirror M 1, and M2 and concavees lens L2 are collimated into the light beam that diameter is 25mm by convex lens 9 then, sees through electro-conductive glass 11 and thz beam conllinear by crystal detection 12.Be provided with polaroid 8 between concavees lens L2 and the convex lens 9, crystal detection 12 rears are provided with another polaroid 13 with the polarization direction quadrature of polaroid 8.
When the polarization direction of surveying light II did not change, the detection light II light intensity of two polaroids by quadrature should be zero.Because in crystal detection 12, the Terahertz electric field changes the index ellipsoid of crystal, thereby the polarization state of the detection light II of outgoing is changed, so can measure the size of surveying light II polarization state change amount by two polaroids 8 and 13 of quadrature, and then obtain the time dependent time-domain spectroscopy of THz wave indirectly.Converge to CCD camera 15 from the detection light II of polaroid 13 outgoing by convex lens 14.This catches Princeton (Princeton) the CCD camera that image is used, and the time shutter is 32ms.
Because Real Time Image System can not use lock-in amplifier to remove noise, so the signal to noise ratio (S/N ratio) of system is generally lower.In order to reduce macrocyclic smooth ground unrest, system has adopted dynamic subtraction technique.Femtosecond laser amplifier 1 produces the signal that size equals the repetition frequency 1KHz of laser pulse, is divided into two synchronizing signals that frequency is 1/32Hz and 1/64Hz by frequency divider 3, is used for triggering optical chopper 4 and CCD camera respectively.Like this, catch two frame continuous images when the CCD camera, wherein a frame is a terahertz signal, and another frame then is that terahertz signal has been blocked by chopper 4, and signal just can extract.Because image is alternately to catch in the time of millisecond magnitude, if two two field pictures are subtracted each other, be about to signal pattern and background image and subtract each other, macrocyclic background drift will reduce greatly.
Terahertz multi-wavelength phase imaging method of the present invention adopts above-mentioned Terahertz multi-wavelength phase imaging system to finish, and comprises the steps:
1, will be fixing by imaging sample 7, to being carried out the two-dimension spectrum imaging, obtain by every terahertz time-domain spectroscopy signal on the imaging sample 7 by imaging sample 7.This two-dimension spectrum imaging can be the reflective imaging to being undertaken by imaging sample 7, also can be the transmission-type imaging.
2,, obtain in system's effective frequency range by the phase spectrum of each point on the imaging sample 7 to being carried out Fourier transform by the terahertz time-domain spectroscopy signal of each point on the imaging sample 7.
3, fit to picture mutually for the multi-wavelength position, the position phase images of the band frequency in the selecting system effective frequency range is carried out linear fit.Preferably, can select to avoid a band frequency of water vapor absorption peak, carry out linear fit to the position phase images of 1.5THz, avoiding, and influence the accuracy of measurement result because of airborne water vapour absorbs as 0.5THz.
4, because the slope of the fitting a straight line that obtains is directly proportional with the thickness of sample, therefore try to achieve the slope information of fitting a straight line, can go out contour of object by computer reconstruction.
This method that obtains the fitting a straight line slope can be to utilize the fitting a straight line of position phase differentiate obtains to frequency.
Especially, the dual wavelength phase imaging is a special case of multi-wavelength phase imaging, and it comprises the steps:
1, will be fixing by imaging sample 7, to being carried out the two-dimension spectrum imaging, obtain by every terahertz time-domain spectroscopy signal on the imaging sample 7 by imaging sample 7.This two-dimension spectrum imaging can be the reflective imaging to being undertaken by imaging sample 7, also can be the transmission-type imaging.
2, to being carried out Fourier transform by the terahertz time-domain spectroscopy signal of each point on the imaging sample 7, in system's effective frequency range, choose wherein two frequencies, obtain two position phase images
Know
Selecting frequency γ in effective frequency range for example
1=0.9THz and γ
2=1.1THz, promptly corresponding two discrete wavelength λ
1=0.33mm and λ
2=0.27mm.Extraction is corresponding to λ
1And λ
2The position phase images
Know
3, two width of cloth position phase images are subtracted each other the phase images that puts in place
And
2 π are mended at the place, can produce the new position phase images of a width of cloth like this
Can in very large range there be breakpoint in it.Image subtract each other processing, ground unrest identical in two width of cloth Terahertz list wavelength position phasors can be removed, improve picture quality greatly.
Because the position phase images that the multi-wavelength phase imaging obtains is through being straight line behind the linear fit, so it is a special case of multi-wavelength match differentiate that dual wavelength position phase images is subtracted each other, the slope information of phase images that can put in place equally, thus the profile information of sample obtained.
Fig. 3-6 is respectively the optical photograph of metal washer, traditional Terahertz image of metal washer, the dual wavelength position phase images of the 0.9THz of metal washer and 1.1THz, and the 0.5THz-1.5THz multi-wavelength position phase fitted figure picture of metal washer.Traditional Terahertz image of the metal washer among Fig. 4 adopts single-frequency, and imaging obtains to metal washer.
As can be seen, for the metal washer in the present embodiment, the image that adopts traditional single-frequency formation method to obtain can not accurately reflect the profile information of metal washer, has the breakpoint of 2 π from Fig. 3-6, and picture quality is relatively poor.And adopting phase imaging method of the present invention, picture quality obviously to be better than the picture quality that classic method obtains, the clear-cut of metal washer and accurate need not manually to eliminate pseudo-breakpoint.
The present invention handles the position phase images of the pairing two or more frequencies of two or more wavelength, reconstructs the contour images of sample.Occur the shortcoming of pseudo-breakpoint thereby overcome single wavelength phase imaging during greater than wavelength in optical path difference, remove ground unrest simultaneously, improve picture quality, reached beneficial technical effects.
The above description of this invention is illustrative, and nonrestrictive, and those skilled in the art is understood, and can carry out many modifications, variation or equivalence to it within spirit that claim limits and scope, but they will fall within the scope of protection of the present invention all.
Claims (8)
1. a Terahertz multi-wavelength phase imaging method is characterized in that, comprises the steps:
(1), obtain described by every terahertz time-domain spectroscopy signal on the imaging sample to being carried out the two-dimension spectrum imaging by the imaging sample;
(2) described terahertz time-domain spectroscopy signal by each point on the imaging sample is carried out Fourier transform, obtain in system's effective frequency range described by the phase spectrum of each point on the imaging sample;
(3) the position phase images of choosing the band frequency in the effective frequency range is carried out linear fit, obtains fitting a straight line;
(4) try to achieve the slope of described fitting a straight line, the gained slope is directly proportional with the thickness of sample, thereby can reconstruct the profile of sample.
2. Terahertz multi-wavelength phase imaging method as claimed in claim 1 is characterized in that the two-dimension spectrum in the step (1) is imaged as the transmission-type imaging.
3. Terahertz multi-wavelength phase imaging method as claimed in claim 1 is characterized in that the two-dimension spectrum in the step (1) is imaged as reflective imaging.
4. Terahertz multi-wavelength phase imaging method as claimed in claim 1 is characterized in that the two-dimension spectrum in the step (1) is imaged as two-dimentional focal plane imaging.
5. Terahertz multi-wavelength phase imaging method as claimed in claim 1 is characterized in that the two-dimension spectrum in the step (1) is imaged as two-dimentional point by point scanning imaging.
6. Terahertz multi-wavelength phase imaging method as claimed in claim 1 is characterized in that a band frequency of choosing in the step (3) is a band frequency of avoiding the water vapor absorption peak.
7. Terahertz multi-wavelength phase imaging method as claimed in claim 6 is characterized in that a described band frequency is that 0.5THz is to 1.5THz.
8. Terahertz multi-wavelength phase imaging method as claimed in claim 1 is characterized in that, the method that obtains the fitting a straight line slope in the step (4) is to described fitting a straight line differentiate.
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN109116708A (en) * | 2018-10-19 | 2019-01-01 | 许之敏 | A kind of dual wavelength reflective digital holographic microscope |
| WO2019237650A1 (en) * | 2018-06-12 | 2019-12-19 | 清华大学 | Data reconstruction method and apparatus for terahertz spectral imaging, and device and storage medium |
| WO2023109713A1 (en) * | 2021-12-15 | 2023-06-22 | 深圳先进技术研究院 | Terahertz imaging method, system and device based on empirical wavelet transform |
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| CN101377406A (en) * | 2007-08-31 | 2009-03-04 | 佳能株式会社 | Apparatus and method for obtaining information related to terahertz waves |
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| CN101377406A (en) * | 2007-08-31 | 2009-03-04 | 佳能株式会社 | Apparatus and method for obtaining information related to terahertz waves |
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| 《OPTICS LETTERS》 20061215 Liangliang Zhang er.al Terahertz multiwavelength phase imaging without 2pi ambiguity 3668-3670 1-8 第31卷, 第24期 2 * |
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2019237650A1 (en) * | 2018-06-12 | 2019-12-19 | 清华大学 | Data reconstruction method and apparatus for terahertz spectral imaging, and device and storage medium |
| US11293860B2 (en) | 2018-06-12 | 2022-04-05 | Tsinghua University | Terahertz spectral imaging data reconstruction method, apparatus, device, and storage medium |
| CN109116708A (en) * | 2018-10-19 | 2019-01-01 | 许之敏 | A kind of dual wavelength reflective digital holographic microscope |
| WO2023109713A1 (en) * | 2021-12-15 | 2023-06-22 | 深圳先进技术研究院 | Terahertz imaging method, system and device based on empirical wavelet transform |
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Application publication date: 20110413 |