CN112540055A - Terahertz laminated imaging method and system with Bessel beam as probe - Google Patents
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Abstract
The invention discloses a terahertz laminated imaging method and system with Bessel beams as probes, and belongs to the technical field of terahertz imaging. The method comprises the following steps: the Bessel beam is used as an illumination probe, relative movement is carried out between the illumination probe and a sample in a plane which is perpendicular to the propagation direction of the light beam in a diffraction-free area of the Bessel beam, the intensity distribution of far-field diffraction images corresponding to different sample positions is collected on a recording surface, image reconstruction is carried out on the basis of the detected intensity distribution, and the absorption coefficient and phase information of the sample and the complex amplitude distribution of the illumination probe on an object plane are obtained. The Bessel beam is generated by the single-frequency continuous terahertz beam and the beam transmission module, the Bessel beam is used for scanning the sample, and the sample is placed at any position within the diffraction-free distance of the Bessel beam, so that the distance between the sample and the beam transmission module and the distance between the detector and the sample are increased.
Description
Technical Field
The invention belongs to the technical field of terahertz imaging, and particularly relates to a terahertz laminated imaging method and system with Bessel beams as probes.
Background
Terahertz waves (THz) are electromagnetic waves which are located between infrared bands and microwave bands, have the frequency within the range of 0.1 to 10THz and the corresponding wavelength of 0.03 to 3mm, and have various important characteristics such as wide spectrum, high penetrability, low energy, water phobicity and the like, so that the Terahertz waves have profound influence on the fields of security inspection anti-terrorism, nondestructive testing, medical imaging and the like.
Terahertz laminated imaging is a lens-free coherent diffraction imaging technology for recovering complex amplitude distribution of a sample by collecting overlapped diffraction patterns. The imaging principle is as follows: the illumination light beam constrained by the small hole forms an illumination probe on an object plane, the illumination probe and the sample move relatively, the intensity information of diffraction patterns corresponding to different sample positions is collected on the recording surface, and the absorption coefficient and the phase information of the sample and the complex amplitude distribution of the illumination probe on the object plane are obtained through image reconstruction algorithm reconstruction based on a phase recovery algorithm. The method not only can retain the unique transmission characteristic of the terahertz wave, but also can fully exert the advantages of high-resolution and rapid imaging of the laminated imaging on large-size samples. Compared with other imaging methods, the continuous terahertz wave imaging method has the advantages of compact optical path structure, low requirement on light source coherence, no limitation on sample size and the like, and is a continuous terahertz wave imaging technology which can be realized by utilizing terahertz devices at the present stage and meets the urgent need of modern biomedical nondestructive visualization research.
However, using an illumination beam confined by a small aperture as a probe leads to two problems: first, the distance between the sample and the well is difficult to determine. Second, a circular aperture will lose the energy of the beam, which will cause the sample to be closer to the detector, reducing the system's utilization of the beam. The two disadvantages limit the sample placement and detection range of the stacked imaging system.
Disclosure of Invention
Aiming at the defects and improvement requirements of the prior art, the invention provides a terahertz laminated imaging method and system with a Bessel beam as a probe, and aims to abandon the imaging structure of the conventional pinhole-generated probe and increase the distance between a terahertz detector and a sample.
To achieve the above object, according to a first aspect of the present invention, there is provided a terahertz stacked imaging method using a Bessel beam as a probe, the method including:
the Bessel beam is used as an illumination probe, relative movement is carried out between the illumination probe and a sample in a plane which is perpendicular to the propagation direction of the beam in a diffraction-free area of the Bessel beam, the intensity distribution of far-field diffraction images corresponding to different sample positions is collected on a recording surface, image reconstruction is carried out on the basis of the detected intensity distribution, and the absorption coefficient and phase information of the sample and the complex amplitude distribution of the illumination probe on an object plane are obtained.
Preferably, the detected intensity distribution is used as an input of an image reconstruction stage, and the complex amplitude of the initial light field is formed together with the random phase distribution, and the image reconstruction is performed based on an extended stack iteration engine algorithm.
To achieve the above object, according to a second aspect of the present invention, there is provided a terahertz stacked imaging system using a Bessel beam as a probe, the system including:
the terahertz generation module is used for emitting a single-frequency continuous terahertz wave beam;
the beam transmission module is used for converting the terahertz beam emitted by the terahertz generation module into Bessel beam and then irradiating the Bessel beam to the surface of the sample;
a sample displacement module for moving the sample within a non-diffractive region of the Bessel beam in a plane perpendicular to the direction of propagation of the beam;
the terahertz detection module is used for detecting the intensity distribution behind the sample when the sample is at different positions;
and the laminated imaging module is used for carrying out image reconstruction on the basis of the intensity distribution obtained by detection to obtain the absorption coefficient and phase information of the sample and the complex amplitude distribution of the illumination probe on the object plane.
Preferably, the terahertz generation module includes: the terahertz generating device comprises a terahertz generating source and a chopper, wherein the chopper modulates continuous terahertz waves emitted by the terahertz generating source to generate a time-domain rectangular wave signal.
Preferably, the beam transmission module includes: the terahertz wave beam detector comprises a collimating lens and an axicon lens, wherein the collimating lens collimates a terahertz wave beam into a Gaussian wave beam, and the axicon lens refracts the Gaussian wave beam into a Bessel wave beam.
Preferably, the refractive index n of the material is determined according to the vertex angle tau of the axicon lens0And a refractive index n of the surrounding medium, according to the formula θ ═ arcsin (n/n)0cos (tau/2)) + (tau-pi)/2, and solving the half-vertex angle theta of the pyramid covered by the refracted wave beam;
then according to the formula Zmax＝w0W is selected from/tan theta0Such that the maximum diffraction-free distance can be obtained when the axicon lens is placed there, wherein ZmaxDenotes the undiffracted distance, w, of the Bessel beam0Representing the waist radius of the gaussian beam.
Preferably, the diameter of the axicon lens is larger than the diameter of the gaussian beam.
Preferably, the terahertz detection module includes: the terahertz detector is used for detecting the square value of the electric field intensity of the terahertz wave beam on the diffraction field and converting the value into a current signal; the phase-locked amplifier is used for converting the obtained current signal into a voltage signal and carrying out amplification processing.
Preferably, the stacked imaging module takes the detected intensity distribution as an input of an image reconstruction stage, and forms the complex amplitude of the initial light field together with the random phase distribution, and performs image reconstruction based on an extended stacked iterative engine algorithm.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
(1) the Bessel beam is generated by the single-frequency continuous terahertz beam and the beam transmission module, the Bessel beam is used for scanning a sample, and the sample can be placed at any position within the non-diffraction distance of the Bessel beam, so that the distance between the sample and the beam transmission module and the distance between the detector and the sample are increased.
(2) The invention uses the measured intensity distribution as the intensity distribution of the initial probe, and the actually measured intensity is used as the intensity distribution of the initial probe wave beam, thereby not only conforming to the actual situation, but also accelerating the convergence speed of the algorithm.
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Fig. 1 is a structural diagram of a terahertz stacked imaging system using a Bessel beam as a probe according to the present invention;
FIG. 2 is a schematic diagram of a scanning process of a terahertz stacked imaging system provided by the present invention;
FIG. 3 is a simulation result of a reconstructed sample, wherein (a) is an input original pattern; (b) the iteration times are set to 10 times, and the initial light intensity is a reconstruction result of random distribution; (c) the iteration number is set to 10, and the initial light intensity is the reconstruction result of the measured value.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
The invention provides a terahertz laminated imaging method taking Bessel wave beams as probes, which comprises the following steps: the Bessel beam is used as an illumination probe, relative movement is carried out between the illumination probe and a sample in a plane which is perpendicular to the propagation direction of the light beam in a diffraction-free area of the Bessel beam, the intensity distribution of far-field diffraction images corresponding to different sample positions is collected on a recording surface, image reconstruction is carried out on the basis of the detected intensity distribution, and the absorption coefficient and phase information of the sample and the complex amplitude distribution of the illumination probe on an object plane are obtained.
The conventional means for generating a probe beam is to use a small hole to block a terahertz beam emitted from a terahertz source, and to control the field distribution of the probe beam by the size and shape of the small hole. The terahertz wave beam is shielded, so that the energy of the terahertz wave beam is reduced, the distance between the detector and the small hole has a certain range, and after the distance exceeds the range, the noise of the detector for receiving the terahertz wave signal is increased, and the result is influenced. In this range, the question of which position the sample is located in, which gives the best imaging, becomes difficult to answer. The prior art uses a simulation method to provide the best data, which has certain complexity.
The present invention replaces the conventional approach with a Bessel beam, which is used as a probe beam to irradiate the sample after generating the Bessel beam using a lens. In doing so, the sample can be placed within the non-diffractive region of the Bessel beam, as long as the sample is within the non-diffractive region of the Bessel beam, and the imaging quality is not affected by the distance between the sample and the lens. Meanwhile, the lens does not cut down the energy of the terahertz wave beam, so that the distance from the detector to the imaging sample can be further increased.
The Bessel beam is used as a probe, the sample is placed at the position where the Bessel beam light spot is minimum, the position of the sample is easy to determine, the energy of the terahertz source is not lost through a small hole, and the detection distance is longer.
Preferably, the detected intensity distribution is used as an input of an image reconstruction stage, and a complex amplitude of an initial light field is formed together with a random phase distribution, and image reconstruction is performed based on an extended Ptychographic Iterative Engine (ePIE).
As shown in fig. 1, the present invention provides a terahertz stacked imaging system using a Bessel beam as a probe, which includes:
the terahertz generation module is used for emitting a single-frequency continuous terahertz wave beam.
Preferably, the terahertz generation module includes: the terahertz generating device comprises a terahertz generating source 1 and a chopper 2, wherein the chopper modulates continuous terahertz waves emitted by the terahertz generating source to generate a time-domain rectangular wave signal. In the embodiment, the terahertz generating source generates a terahertz wave beam with the frequency of 100GHz, the power is 3mW, and the divergence angle is about 5.7 degrees.
And the beam transmission module is used for converting the terahertz beam emitted by the terahertz generation module into Bessel beam and then irradiating the Bessel beam to the surface of the sample.
Preferably, the beam transmission module includes: the terahertz wave beam detector comprises a collimating lens 3 and an axicon lens 4, wherein the collimating lens collimates a terahertz wave beam into a Gaussian wave beam, and the axicon lens refracts the Gaussian wave beam into a Bessel wave beam. In this embodiment, the terahertz emission source 1 maintains a fixed height, and the centers of the collimating lens 3 and the axicon lens 4 and the center of the terahertz emission source 1 are maintained on the same straight line. In the embodiment, the collimating lens 3 is made of HDPE, the refractive index of the collimating lens is 1.61, the focal length of the collimating lens is 100mm, the collimating lens 3 is placed 94mm behind the terahertz emission source 1, and divergent terahertz beams emitted by the terahertz emission source 1 are collimated into Gaussian beams; the base angle of the axicon lens 4 is 10 deg., the material is LY1101 material for 3D printing, and the refractive index is 1.54.
Preferably, the refractive index n of the material is determined according to the vertex angle tau of the axicon lens0And a refractive index n of the surrounding medium, according to the formula θ ═ arcsin (n/n)0cos (tau/2)) + (tau-pi)/2, and solving the half-vertex angle theta of the pyramid covered by the refracted wave beam;
then according to the formula Zmax＝w0W is selected from/tan theta0Such that the maximum diffraction-free distance can be obtained when the axicon lens is placed there, wherein ZmaxDenotes the undiffracted distance, w, of the Bessel beam0Representing the waist radius of the gaussian beam.
Preferably, the diameter of the axicon lens is larger than the diameter of the gaussian beam.
In this embodiment, the distance between the axicon lens 4 and the collimating lens 3 is 113mm, the diameter of the gaussian beam incident on the front surface of the axicon is 44mm, the diameter of the axicon 4 used is 4 inches, i.e. 101.6mm, the vertex angle of the axicon lens 4 is 160 °, the refractive index of the material is 1.54, and θ is arcsin (n/n) according to the formula0cos (τ/2)) + (τ - π)/2 gives a refracted beam covering the half-apex angle of the axicon of 39.5 °, according to the formula Zmax＝w0The non-diffraction distance of the Bessel beam obtained by/tan θ was 26.7 mm.
In the embodiment, the chopper 2 is spaced from the terahertz emission source 1 by 2 mm; the distance between the collimating lens 3 and the terahertz emission source 1 is 86mm, and the distance between the collimating lens 3 and the axicon lens 4 is 113 mm; the terahertz detector 7 is spaced 225mm from the axicon lens 4.
And the sample displacement module is used for moving the sample in a plane which is perpendicular to the propagation direction of the light beam in the diffraction-free area of the Bessel beam.
The two-dimensional electric displacement platform 6 in the embodiment is composed of two stepping motors, the moving range of the two stepping motors is 100mm, the two stepping motors are perpendicular to each other, a sample clamp is used for clamping the sample, and the movement of the sample is controlled.
The terahertz detection module is used for detecting the intensity distribution behind the sample when the sample is at different positions.
Preferably, the terahertz detection module includes: the terahertz detector 7 is used for detecting the square value of the electric field intensity of the terahertz wave beam on the diffraction field and converting the value into a current signal; the phase-locked amplifier is used for converting the obtained current signal into a voltage signal and carrying out amplification processing.
And the laminated imaging module is used for carrying out image reconstruction on the basis of the intensity distribution obtained by detection to obtain the absorption coefficient and phase information of the sample and the complex amplitude distribution of the illumination probe on the object plane.
In the imaging system provided by the embodiment, the centers of all the lenses, the center of the terahertz emission source and the center of the detector are on the same horizontal line. Wherein, the two-dimensional electric displacement platform 6 and the detector stepping motor 8 are both connected with a stepping motor controller 10.
As shown in fig. 2, the scanning phase of the stacked imaging system is as follows:
(1) modulating terahertz waves emitted by a terahertz generating source into time-domain rectangular wave signals by using a chopper;
(2) collimating the time domain rectangular wave signals, and collimating the modulated time domain rectangular wave signals into Gaussian beams by adopting a collimating lens;
(3) generating a Bessel probe beam, using an axicon lens to pass a Gaussian beam through the axicon lens to form the Bessel probe beam capable of detecting a sample, and irradiating the sample by using the beam;
(4) detecting the distribution of the diffraction field behind the sample, and converting the electric field intensity into a current signal;
(5) converting the current signal into a voltage signal and amplifying the voltage signal;
(6) the amplified voltage signal is the diffraction field distribution formed by the probe passing through the sample, and the diffraction field distribution is stored and marked as Ik(u, v); wherein k is the movement of the sample to the kth position; u and v are the coordinates at the detector plane;
(7) and (3) controlling the sample to translate in the horizontal and vertical directions, stopping moving the sample when the sample moves to a certain fixed position, repeating the steps (1) to (6), detecting the diffraction field distribution formed behind the sample by using a detector, and finally obtaining a three-dimensional diffraction field distribution matrix I (u, v, k).
Preferably, in the stacked imaging scanning mode, the scanning frequency of the terahertz detector is continuously adjustable between 500Hz and 5000 Hz. The lower the selected frequency is, the slower the corresponding speed is during scanning, the more the number of points sampled is, and the longer the scanning time is.
Preferably, the stacked imaging module 9 takes the detected intensity distribution as an input of an image reconstruction stage, forms a complex amplitude of an initial light field together with a random phase distribution, and performs image reconstruction based on an extended stacked iterative engine algorithm, which specifically includes the following steps:
(1) random guess of complex amplitude transmittance o of the sample to be measuredk(x, y) and phase of probe beam incident on sample surface
Wherein k is the movement of the sample to the kth position; x and y are coordinates of the plane where the sample to be detected is located;
(2) constructing complex amplitude of probe beam using recorded electric field intensity of probe beam
Wherein, A (x, y) is the amplitude distribution of the recorded probe beam;
(3) when the probe is applied to the sample (x)k,yk) When in treatment, "the exit field distribution" behind the sample and close to the surface of the sample is: psik(x,y)＝pk(x-xk,y-yk)ok(x, y); when the condition t is far smaller than Rw/lambda is satisfied, the approximation of the above formula can be established, where t is the thickness of the sample, R is the resolution of the imaging system, w is the size of the emergent light spot, and lambda is the wavelength of the terahertz wave.
(4) Transmitting the emergent field distribution to the plane of the detector by using an angular spectrum transmission theory,
wherein the content of the first and second substances,
and
respectively Fourier transform and inverse Fourier transform, fxAnd fyIs the spatial frequency, d is the distance between the probe plane and the sample plane;
(5) replacing the recorded field intensity distribution with the recorded intensity I (u, v) of the diffraction field
While preserving the originally calculated phase while preserving the field intensity distribution in (1),
wherein, the angle refers to the phase;
(6) transmitting the complex amplitude distribution of the wave field to the plane of the sample by using the angular spectrum transmission theory again
(7) The estimates for the sample and probe are updated according to the following formula:
wherein represents a complex conjugate;
(8) and (4) repeating the steps (3) to (7) until the probe traverses the whole sample, completing one iteration at the moment, inputting the complex amplitude transmittance of the newly obtained sample into the algorithm as sample distribution, performing the next iteration, and finally stopping the iteration until the error of the sample is smaller than the preset value or the iteration number reaches the preset value, and outputting the complex amplitude transmittance of the sample and the complex amplitude distribution of the probe.
The existing imaging methods randomly estimate the amplitude distribution of an incident light field when a sample is restored, so that the algorithm can be converged after more iterations. Fig. 3 (a) is an image of an original amplitude type sample, and the amplitude type sample is subjected to lamination imaging, the number of iterations is set to 10, and the pattern shown in fig. 3 (b) is obtained by reduction, that is, the shape of the sample cannot be effectively reduced. By using the method of the present invention, the number of iterations is also set to 10, and the reconstructed image shown in (c) in fig. 3 can be obtained.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (9)
1. A terahertz laminated imaging method taking Bessel beams as probes is characterized by comprising the following steps:
the Bessel beam is used as an illumination probe, relative movement is carried out between the illumination probe and a sample in a plane which is perpendicular to the propagation direction of the beam in a diffraction-free area of the Bessel beam, the intensity distribution of far-field diffraction images corresponding to different sample positions is collected on a recording surface, image reconstruction is carried out on the basis of the detected intensity distribution, and the absorption coefficient and phase information of the sample and the complex amplitude distribution of the illumination probe on an object plane are obtained.
2. The method of claim 1, wherein the detected intensity distribution is used as input to an image reconstruction stage, together with a random phase distribution to form the complex amplitude of the initial light field, and the image reconstruction is performed based on an extended stacked iterative engine algorithm.
3. A terahertz stacked imaging system using Bessel beams as probes, the system comprising:
the terahertz generation module is used for emitting a single-frequency continuous terahertz wave beam;
the beam transmission module is used for converting the terahertz beam emitted by the terahertz generation module into Bessel beam and then irradiating the Bessel beam to the surface of the sample;
a sample displacement module for moving the sample within a non-diffractive region of the Bessel beam in a plane perpendicular to the direction of propagation of the beam;
the terahertz detection module is used for detecting the intensity distribution behind the sample when the sample is at different positions;
and the laminated imaging module is used for carrying out image reconstruction on the basis of the intensity distribution obtained by detection to obtain the absorption coefficient and phase information of the sample and the complex amplitude distribution of the illumination probe on the object plane.
4. The system of claim 3, wherein the terahertz generating module comprises: the terahertz generating device comprises a terahertz generating source and a chopper, wherein the chopper modulates continuous terahertz waves emitted by the terahertz generating source to generate a time-domain rectangular wave signal.
5. The system of claim 3, wherein the beam transmission module comprises: the terahertz wave beam detector comprises a collimating lens and an axicon lens, wherein the collimating lens collimates a terahertz wave beam into a Gaussian wave beam, and the axicon lens refracts the Gaussian wave beam into a Bessel wave beam.
6. The system of claim 5, wherein the refractive index n of the material is determined according to the vertex angle τ of the axicon lens and the refractive index of the material0And a refractive index n of the surrounding medium, according to the formula θ ═ arcsin (n/n)0cos (tau/2)) + (tau-pi)/2, and solving the half-vertex angle theta of the pyramid covered by the refracted wave beam;
then according to the formula Zmax＝w0W is selected from/tan theta0Such that the maximum diffraction-free distance can be obtained when the axicon lens is placed there, wherein ZmaxDenotes the undiffracted distance, w, of the Bessel beam0Representing the waist radius of the gaussian beam.
7. The system of claim 6, wherein the axicon lens has a diameter greater than a diameter of the gaussian beam.
8. The system of claim 3, wherein the terahertz detection module comprises: the terahertz detector is used for detecting the square value of the electric field intensity of the terahertz wave beam on the diffraction field and converting the value into a current signal; the phase-locked amplifier is used for converting the obtained current signal into a voltage signal and carrying out amplification processing.
9. The system of any one of claims 3 to 8, wherein the stacked imaging module takes the detected intensity distribution as input to an image reconstruction stage, which together with a random phase distribution constitutes the complex amplitude of the initial light field, and performs image reconstruction based on an extended stacked iterative engine algorithm.
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN109374580A (en) * | 2018-09-30 | 2019-02-22 | 北京工业大学 | A kind of Terahertz lamination image probe positional error correction method |
| CN113091644A (en) * | 2021-06-09 | 2021-07-09 | 中国工程物理研究院激光聚变研究中心 | Large-aperture optical element surface shape detection method based on stacked coherent diffraction imaging |
| CN113866992A (en) * | 2021-09-10 | 2021-12-31 | 华中科技大学 | Spherical harmonic cone lens for generating non-diffraction light beams in terahertz wave band |
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