CN110297418B - Terahertz wave digital holographic imaging method based on terahertz diffraction pattern decomposition - Google Patents
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Abstract
The invention discloses a terahertz wave digital holographic imaging method based on terahertz diffraction pattern decomposition, which comprises terahertz hologram recording, hologram pre-propagation, pre-propagation diffraction surface complex amplitude image decomposition and multilayer complex amplitude image reconstruction and synthesis processes. Wherein: (1) the terahertz hologram recording process comprises the following steps: and acquiring and processing the terahertz hologram in a terahertz digital holographic imaging system to obtain a large-aperture normalized terahertz hologram. (2) Hologram pre-propagation process: the free propagation is performed a specific distance to obtain a complex amplitude image of the pre-propagating diffraction plane. (3) And (3) a pre-propagation diffraction surface complex amplitude image decomposition process: and calculating the foreground surface and background surface thresholds of the pre-propagation diffraction surface complex amplitude image, manufacturing a binary mask image, and obtaining a sub complex amplitude image of the pre-propagation diffraction surface. (4) Multi-slice complex amplitude image reconstruction and synthesis process: and reconstructing the sub-complex amplitude images of all object planes, and synthesizing to obtain the terahertz wave digital holographic multi-layer imaging result.
Description
Technical Field
The invention relates to the technical field of terahertz wave digital holographic multi-layer imaging methods, in particular to a terahertz wave digital holographic imaging method based on terahertz diffraction pattern decomposition.
Background
The terahertz wave band has the characteristics of both the infrared wave band and the microwave wave band due to the fact that the terahertz wave band is in the transition wave band from infrared to microwave, so that the terahertz wave band has wide application prospect in the imaging field, and the digital holographic technology has unique advantages for imaging a plurality of focusing layers of a specific target sample due to the simple imaging structure and the flexible data processing mode. In the traditional imaging process, the problem that the required multi-focusing-layer detection result cannot be obtained due to the fact that samples are distributed at a plurality of focusing distances exists, the terahertz digital holographic technology can give full play to the advantages and the characteristics of the samples, and the terahertz digital holographic technology becomes a multi-focusing-layer imaging technology for realizing terahertz wave bands.
The invention aims to provide a terahertz wave digital holographic layered imaging method based on diffraction pattern decomposition. The method is a novel imaging technology in a terahertz waveband, and the amplitude and phase information of a sample containing focusing surfaces in different depth directions can be obtained by recording the terahertz hologram of the sample through single imaging by means of the digital processing process of a terahertz hologram by a computer. Compared with the traditional terahertz wave imaging means, the multi-layer complex amplitude detection imaging method can realize multi-layer complex amplitude detection imaging of the sample without mechanically adjusting the position of a focus plane and carrying out multiple exposure measurements. The terahertz wave imaging technology is an imaging technology which can penetrate through a non-metal nonpolar mask and can realize different depth focal plane information by single recording exposure by combining the selective penetrability and the nondestructive safety of the terahertz wave, and can be widely applied to the fields of nondestructive detection and material characterization.
Disclosure of Invention
The invention aims to provide a terahertz wave digital holographic imaging method based on terahertz diffraction pattern decomposition, which aims to solve the defects of the prior art in the background art, obtain complex amplitude information of focusing surfaces of target samples at different depths and realize multi-layer complex amplitude imaging of the samples.
In order to achieve the purpose, the invention provides the following technical scheme: a terahertz wave digital holographic imaging method based on terahertz diffraction pattern decomposition comprises the following steps: the method comprises the following steps of terahertz hologram recording process, hologram pre-propagation process, pre-propagation diffraction surface complex amplitude image decomposition process and multilayer surface complex amplitude image reconstruction and synthesis process:
(1) and (3) a terahertz hologram recording process. A sample containing multi-layer information in the depth direction is placed in a terahertz digital holographic imaging system, terahertz waves are modulated by the sample, carry the multi-layer information of the sample, and interfere with terahertz reference waves which are not modulated by the sample to form a terahertz hologram. In order to improve the imaging resolution of the system, a two-dimensional translation table carrying a terahertz detector performs two-dimensional movement in a recording plane perpendicular to the illumination direction of terahertz waves according to a given scanning sequence, and terahertz sub-holograms at different positions are collected. After the sample is removed, the two-dimensional translation table carrying the terahertz detector records the terahertz sub-background image according to the same scanning sequence in the terahertz sub-hologram recording process. And dividing the corresponding terahertz sub-hologram acquired at the same position by the terahertz sub-background image to obtain a sub-normalized hologram, and splicing the sub-normalized hologram images according to the scanning sequence to obtain the large-aperture normalized terahertz hologram.
(2) Hologram pre-propagation process. Pre-propagating the large-aperture normalized terahertz hologram in the step (1) to a diffraction plane P along the object plane direction in a computer through a free space propagation algorithm to obtain the amplitude and phase distribution of a pre-propagated diffraction plane, namely a pre-propagated diffraction plane complex amplitude image U (m, n). The propagation distance of the diffraction surface is dp. Defining the maximum distance and the minimum distance between different layering planes of the sample and the recording plane of the terahertz detector as dmaxAnd dminThen d ispIs defined as
(3) And (3) a pre-propagation diffraction surface complex amplitude image decomposition process. Calculating and extracting amplitude and phase images of a pre-propagation diffraction surface complex amplitude image U (m, n), judging and calculating a maximum variance threshold value T of a foreground surface and a background surface by adopting a maximum inter-class variance method, and performing binarization operation on the pre-propagation diffraction surface complex amplitude image based on the obtained threshold value T to obtain a binarization mask image K (m, n), which can be expressed as
Edge mean fuzzification processing is carried out on the binary mask image K (m, n) to obtain a mask image K ' (m, n) of a pre-propagation diffraction surface, x isolated non-zero value region images in the mask image K ' (m, n) are extracted one by one through non-zero value region identification, and zero values are filled into pixel sizes of the original mask image K ' (m, n)Size, resulting in x sub-mask images K 'containing only a single isolated non-zero value image region'i(i ═ 1, 2.., x). The obtained x sub-mask images K'iRespectively multiplying the complex amplitude images U of the pre-propagation diffraction surfaces to obtain sub complex amplitude images U of x pre-propagation diffraction surfacesi(i＝1，2，...，x)。
Ui＝U×K′i，i＝1，2，...，x
(4) And (3) reconstructing and synthesizing the multi-slice complex amplitude image. Calculating each sub-complex amplitude image U of the pre-propagation diffraction surface by an automatic focusing algorithmiThe propagation distance is focused, the propagation is carried out through a free space propagation algorithm, and a complex amplitude image O of each object plane is obtained through reconstructioni(i ═ 1, 2.., x). The complex amplitude image O of each object planeiRespectively corresponding to the sub-mask images K'iMultiplying to obtain an image O'iCalculating the complex amplitude image O of each object planeiAverage value of (2)
And is associated with the corresponding sub-mask image (1-K ') to be inverted'i) Multiplying to obtain an image O ″)iWill be image O'iAnd image O ″)iThe multi-slice complex amplitude imaging results O of the target sample are additively combined, which can be expressed as
Preferably, terahertz waves are output from one side of the avalanche diode terahertz wave source, a sample is arranged on one side of the avalanche diode terahertz wave source facing the terahertz waves, the terahertz wave pyroelectric detector is mounted at the tail end of the terahertz waves, and a detector two-dimensional translation table is fixed below the terahertz wave pyroelectric detector.
Preferably, the structure of the avalanche diode terahertz wave source, the sample and the terahertz wave pyroelectric detector is a terahertz coaxial imaging system.
Preferably, in the pre-propagation diffraction surface complex amplitude image decomposition process, an amplitude distribution image of the pre-propagation diffraction surface is extracted for an amplitude type sample, and a phase distribution image of the pre-propagation diffraction surface is extracted for a phase type sample.
The invention provides a terahertz wave digital holographic imaging method based on terahertz diffraction pattern decomposition, which has the following beneficial effects compared with the prior art:
the invention provides a terahertz wave digital holographic layered imaging method based on diffraction pattern decomposition.
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FIG. 1 is a flow chart of a terahertz wave digital holographic imaging method based on terahertz diffractogram decomposition according to the invention;
FIG. 2 is a schematic diagram of a terahertz wave digital holographic layered imaging system based on terahertz diffractogram decomposition.
In the figure: 1. an avalanche diode terahertz wave source; 1a, terahertz waves; 2. a sample; 3. a detector two-dimensional translation stage; 4. terahertz wave pyroelectric detector.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to data in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments.
Referring to fig. 1-2, the present invention provides a technical solution: a terahertz wave digital holographic imaging method based on terahertz diffraction pattern decomposition comprises the following steps:
(1) and (3) a terahertz hologram recording process. A sample containing multi-layer information in the depth direction is placed in a terahertz digital holographic imaging system, terahertz waves are modulated by the sample, carry the multi-layer information of the sample, and interfere with terahertz reference waves which are not modulated by the sample to form a terahertz hologram. In order to improve the imaging resolution of the system, a two-dimensional translation table carrying a terahertz detector performs two-dimensional movement in a recording plane perpendicular to the illumination direction of terahertz waves according to a given scanning sequence, and terahertz sub-holograms at different positions are collected. After the sample is removed, the two-dimensional translation table carrying the terahertz detector records the terahertz sub-background image according to the same scanning sequence in the terahertz sub-hologram recording process. And dividing the corresponding terahertz sub-hologram acquired at the same position by the terahertz sub-background image to obtain a sub-normalized hologram, and splicing the sub-normalized hologram images according to the scanning sequence to obtain the large-aperture normalized terahertz hologram.
(2) Hologram pre-propagation process. Pre-propagating the large-aperture normalized terahertz hologram in the step (1) to a diffraction plane P along the object plane direction in a computer through a free space propagation algorithm to obtain the amplitude and phase distribution of a pre-propagated diffraction plane, namely a pre-propagated diffraction plane complex amplitude image U (m, n). The propagation distance of the diffraction surface is dp. Defining the maximum distance and the minimum distance between different layering planes of the sample and the recording plane of the terahertz detector as dmaxAnd dminThen d ispIs defined as
(3) And (3) a pre-propagation diffraction surface complex amplitude image decomposition process. An amplitude distribution image of the pre-propagation diffraction surface is extracted for the amplitude type sample, and a phase distribution image of the pre-propagation diffraction surface is extracted for the phase type sample. Calculating and extracting amplitude and phase images of a pre-propagation diffraction surface complex amplitude image U (m, n), judging and calculating a maximum variance threshold value T of a foreground surface and a background surface by adopting a maximum inter-class variance method, and performing binarization operation on the pre-propagation diffraction surface complex amplitude image based on the obtained threshold value T to obtain a binarization mask image K (m, n), which can be expressed as
For two valuesThe mask image K (m, n) is changed to complete edge mean fuzzification processing, so that a mask image K ' (m, n) of a pre-propagation diffraction surface is obtained, x isolated non-zero value region images in the mask image K ' (m, n) are extracted one by one through non-zero value region identification, zero values of the x isolated non-zero value region images are filled into the pixel size of the original mask image K ' (m, n), and x sub-mask images K ' only containing a single isolated non-zero value image region are obtained 'i(i ═ 1, 2.., x). The obtained x sub-mask images K'iRespectively multiplying the complex amplitude images U of the pre-propagation diffraction surfaces to obtain sub complex amplitude images U of x pre-propagation diffraction surfacesi(i＝1，2，...，x)。
Ui＝U×K′i，i＝1，2，...，x
(4) And (3) reconstructing and synthesizing the multi-slice complex amplitude image. Calculating each sub-complex amplitude image U of the pre-propagation diffraction surface by an automatic focusing algorithmiThe propagation distance is focused, the propagation is carried out through a free space propagation algorithm, and a complex amplitude image O of each object plane is obtained through reconstructioni(i ═ 1, 2.., x). The complex amplitude image O of each object planeiRespectively corresponding to the sub-mask images K'iMultiplying to obtain an image O'iCalculating the complex amplitude image O of each object planeiAverage value of (2)
And is associated with the corresponding sub-mask image (1-K ') to be inverted'i) Multiplying to obtain an image O ″)iWill be image O'iAnd image O ″)iThe multi-slice complex amplitude imaging results O of the target sample are additively combined, which can be expressed as
A terahertz wave digital holographic imaging method based on terahertz diffraction pattern decomposition comprises an avalanche diode terahertz wave source 1, a terahertz wave 1a, a detector two-dimensional translation table 3 and a terahertz wave pyroelectric detector 4, wherein the terahertz wave 1a is output from one side of the avalanche diode terahertz wave source 1, a sample 2 is arranged on one side, facing the terahertz wave 1a, of the avalanche diode terahertz wave source 1, the terahertz wave pyroelectric detector 4 is installed at the tail end of the terahertz wave 1a, the detector two-dimensional translation table 3 is fixed below the terahertz wave pyroelectric detector 4, the avalanche diode terahertz wave source 1, the sample 2 and the terahertz wave pyroelectric detector 4 are structurally a terahertz coaxial imaging system, the avalanche diode terahertz wave source 1 is used for generating and outputting the terahertz wave 1a, the detector two-dimensional translation table 3 is used for placing the terahertz wave pyroelectric detector 4, and the terahertz wave pyroelectric detector 4 is used for collecting terahertz holograms and background images.
In summary, the terahertz wave digital holographic imaging method based on terahertz diffraction pattern decomposition comprises the following steps:
(1) and (3) a terahertz hologram recording process. The target samples are metal needles and asparagus that contain multi-level information in the depth direction. The sample is placed in a terahertz digital holographic imaging system, terahertz waves are modulated by the sample, carry multi-layer information of the sample, and interfere with terahertz reference waves which are not modulated by the sample to form a terahertz hologram. In order to improve the imaging resolution of the system, a two-dimensional translation table carrying a terahertz detector performs two-dimensional movement in a recording plane perpendicular to the illumination direction of terahertz waves according to a given rectangular scanning sequence, and nine terahertz sub-holograms at different positions are collected. After the sample is removed, the two-dimensional translation stage carrying the terahertz detector records nine corresponding terahertz sub-background images according to the same scanning sequence in the terahertz sub-hologram recording process. And dividing the corresponding terahertz sub-hologram and the terahertz sub-background image collected at the same position to obtain nine sub-normalized holograms, and splicing the sub-normalized hologram images according to the scanning sequence to obtain the large-aperture normalized terahertz hologram of one experimental sample.
(2) Hologram pre-propagation process. Pre-propagating the large-aperture normalized terahertz hologram in the step (1) to a diffraction plane in a computer along the direction of a needle and an asparagus fern plane through a free space propagation algorithm to obtain the amplitude and phase distribution of the pre-propagated diffraction plane, namely pre-propagatingAnd (3) propagating the diffraction surface complex amplitude image U (m, n), wherein m is less than or equal to 320, and n is less than or equal to 320. The maximum distance and the minimum distance between different layering surfaces of the sample and the recording plane of the terahertz detector are respectively dmax65mm and dminWhen the diffraction surface has a propagation distance d of 45mmpTaking the average of the two, i.e. dp＝55mm。
(3) And (3) a pre-propagation diffraction surface complex amplitude image decomposition process. Calculating and extracting amplitude and phase images of a pre-propagation diffraction surface complex amplitude image U (m, n), judging and calculating a maximum variance threshold value of a foreground surface and a background surface by adopting a maximum inter-class variance method to obtain a threshold value of 0.8118, carrying out binarization operation on the pre-propagation diffraction surface complex amplitude image based on the obtained threshold value to obtain a binarization mask image K (m, n), which can be expressed as
Performing edge mean blurring processing on the binary mask image K (m, n) to obtain a mask image K ' (m, n) of a pre-propagation diffraction surface, extracting 2 isolated non-zero value area images in the mask image K ' (m, n), namely the area image where the asparagus fern is located and the area image where the metal needle is located one by one through non-zero value area identification, and filling zero values to the pixel size 320 × 320 of the original mask image K ' (m, n) to obtain a sub-mask image K ' only containing the asparagus fern area '1Sub-mask pattern K 'only containing metal needle region'2. Prepared from K'1、K′2Respectively multiplying the complex amplitude images U of the pre-propagation diffraction surfaces to obtain sub complex amplitude images U of the pre-propagation diffraction surfaces only containing the positions of the asparagus fern areas1And a sub-complex amplitude image U of the pre-propagating diffraction surface containing only the positions of the metal needle regions2。
(4) And (3) reconstructing and synthesizing the multi-slice complex amplitude image. Calculating to obtain a sub-complex amplitude image U of the pre-propagation diffraction surface by an automatic focusing algorithm1、U2The focus propagation distances of (a) are-5 mm and 5mm, respectively. Propagating through a free space propagation algorithm, and reconstructing to obtain a complex amplitude image O of each object plane1、O2. To smooth the objectSurface complex amplitude image O1、O2Respectively corresponding to the sub-mask images K'1、K′2Multiplying to obtain an image O'1、O′2Calculating the complex amplitude image O of each object plane1、O2Average value of (2)
And is associated with the corresponding sub-mask image (1-K ') to be inverted'1)、(1-K′2) Multiplying to obtain an image O ″)1、O″2Will be image O'1、O′2And image O ″)1、O″2Adding and synthesizing a multi-layer complex amplitude imaging result O of the target sample, wherein the metal needle and the asparagus fern are clear images at the focusing propagation distance in the image O, and the step can be expressed as
The optical path composition of the imaging system is shown in figure 2, an avalanche diode terahertz wave source 1 is adopted to generate and output terahertz waves 1a, wherein the central wavelength of the wave source 1 is 300mm, and the frequency is 0.1 THz; the detector two-dimensional translation table 3 is used for placing a terahertz wave pyroelectric detector 4 to complete two-dimensional displacement in the scanning and recording processes of the detector; the pixel interval of the terahertz wave pyroelectric detector 4 is 80 μm, the pixel size is 160 pixels, and the terahertz wave pyroelectric detector is used for collecting terahertz holographic images and background images, so that the use process of the whole terahertz wave digital holographic imaging method based on terahertz diffraction pattern decomposition is completed.
Experimental results of typical embodiments of the invention prove that the terahertz wave digital holographic multi-layer imaging method can effectively realize terahertz wave digital holographic multi-layer imaging and obtain complex amplitude information of focusing surfaces of target samples at different depths. Although the present invention has been described in detail with reference to particular embodiments, the embodiments of the invention described herein are not intended to be exhaustive or limited to the precise forms disclosed. Rather, the embodiment chosen to illustrate the problem was chosen to enable one skilled in the art to practice the invention. Variations and modifications exist without departing from the true scope of the invention as described and defined in the claims.
Claims (4)
1. A terahertz wave digital holographic imaging method based on terahertz diffraction pattern decomposition comprises the following steps: the method comprises the following steps of terahertz hologram recording process, hologram pre-propagation process, pre-propagation diffraction surface complex amplitude image decomposition process and multilayer surface complex amplitude image reconstruction and synthesis process:
(1) in the terahertz hologram recording process, a sample containing multi-layer information in the depth direction is placed in a terahertz digital holographic imaging system, terahertz waves are modulated by the sample, carry the multi-layer information of the sample, and interfere with terahertz reference waves which are not modulated by the sample to form a terahertz hologram; in order to improve the imaging resolution of the system, a two-dimensional translation table carrying a terahertz detector performs two-dimensional movement in a recording plane vertical to the illumination direction of terahertz waves according to a given scanning sequence, and terahertz sub-holograms at different positions are collected; after a sample is removed, a two-dimensional translation table carrying a terahertz detector records terahertz sub-background images according to the same scanning sequence in the recording process of terahertz sub-holograms; dividing the corresponding terahertz sub-hologram and the terahertz sub-background image collected at the same position to obtain a sub-normalized hologram, and splicing the sub-normalized hologram images according to a scanning sequence to obtain a large-aperture normalized terahertz hologram;
(2) pre-propagating the large-aperture normalized terahertz hologram in the step (1) to a diffraction plane P in a computer by a free space propagation algorithm along the object plane direction to obtain the amplitude and phase distribution of a pre-propagated diffraction plane, namely a pre-propagated diffraction plane complex amplitude image U (m, n), wherein the propagation distance of the diffraction plane is dpDefining the maximum distance and the minimum distance between different layering planes of the sample and the recording plane of the terahertz detector as dmaxAnd dminThen d ispIs defined as
(3) The method comprises the steps of decomposing a pre-propagation diffraction surface complex amplitude image, calculating and extracting amplitude and phase images of a pre-propagation diffraction surface complex amplitude image U (m, n), judging and calculating a maximum variance threshold T of a foreground surface and a background surface by adopting a maximum inter-class variance method, carrying out binarization operation on the pre-propagation diffraction surface complex amplitude image based on the obtained threshold T to obtain a binarization mask image K (m, n), which can be expressed as
Edge mean blurring processing is carried out on the binary mask image K (m, n) to obtain a mask image K ' (m, n) of a pre-propagation diffraction surface, x isolated non-zero value region images in the mask image K ' (m, n) are extracted one by one through non-zero value region identification, zero values are filled into pixel sizes of the original mask image K ' (m, n), and x sub-mask images K ' only containing a single isolated non-zero value image region are obtained 'i(i ═ 1, 2,. times, x); obtaining X sub-mask images K'iRespectively multiplying the complex amplitude images U of the pre-propagation diffraction surfaces to obtain sub complex amplitude images U of x pre-propagation diffraction surfacesi(i＝1，2，...，x)
Ui＝U×K′i，i＝1，2，...，x
(4) A multi-layer complex amplitude image reconstruction and synthesis process; calculating each sub-complex amplitude image U of the pre-propagation diffraction surface by an automatic focusing algorithmiThe propagation distance is focused, the propagation is carried out through a free space propagation algorithm, and a complex amplitude image O of each object plane is obtained through reconstructioni(i ═ 1, 2,. times, x); the complex amplitude image O of each object planeiRespectively corresponding to the sub-mask images K'iMultiplying to obtain an image O'iCalculating the complex amplitude image O of each object planeiAverage value of (2)
And is associated with the corresponding sub-mask image (1-K ') to be inverted'i) Multiplying to obtain an image O ″)iWill be image O'iAnd image O ″)iThe multi-slice complex amplitude imaging results O of the target sample are additively combined, which can be expressed as
2. The terahertz wave digital holographic imaging method based on terahertz diffractogram decomposition according to claim 1, wherein: one side of the avalanche diode terahertz wave source (1) outputs terahertz waves (1a), a sample (2) is arranged on one side, facing the terahertz waves (1a), of the avalanche diode terahertz wave source (1), a terahertz wave pyroelectric detector (4) is installed at the tail end of the terahertz waves (1a), and a detector two-dimensional translation table (3) is fixed below the terahertz wave pyroelectric detector (4).
3. The terahertz wave digital holographic imaging method based on terahertz diffraction pattern decomposition as claimed in claim 2, wherein: the structure of the avalanche diode terahertz wave source (1), the sample (2) and the terahertz wave pyroelectric detector (4) is a terahertz coaxial imaging system.
4. The terahertz wave digital holographic imaging method based on terahertz diffractogram decomposition according to claim 1, wherein: in the pre-propagation diffraction surface complex amplitude image decomposition process, an amplitude distribution image of a pre-propagation diffraction surface is extracted for an amplitude type sample, and a phase distribution image of the pre-propagation diffraction surface is extracted for a phase type sample.
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| CN106094487A (en) * | 2016-08-18 | 2016-11-09 | 中国工程物理研究院激光聚变研究中心 | Terahertz in-line holographic imaging systems based on multiple recording distances and formation method |
| CN107145052A (en) * | 2017-05-24 | 2017-09-08 | 上海交通大学 | Holographic microscopic imaging method based on digital interpolative and phase iteration |
| CN109188881A (en) * | 2018-10-12 | 2019-01-11 | 中国地质大学（北京） | A kind of THz wave digital hologram imaging method and system |
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| CN106094487A (en) * | 2016-08-18 | 2016-11-09 | 中国工程物理研究院激光聚变研究中心 | Terahertz in-line holographic imaging systems based on multiple recording distances and formation method |
| CN107145052A (en) * | 2017-05-24 | 2017-09-08 | 上海交通大学 | Holographic microscopic imaging method based on digital interpolative and phase iteration |
| CN109188881A (en) * | 2018-10-12 | 2019-01-11 | 中国地质大学（北京） | A kind of THz wave digital hologram imaging method and system |
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