CN116184679A - Terahertz imaging light path, adjustment system and method, and imaging light path construction method - Google Patents
Terahertz imaging light path, adjustment system and method, and imaging light path construction method Download PDFInfo
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- CN116184679A CN116184679A CN202211736898.XA CN202211736898A CN116184679A CN 116184679 A CN116184679 A CN 116184679A CN 202211736898 A CN202211736898 A CN 202211736898A CN 116184679 A CN116184679 A CN 116184679A
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
- G02B27/0938—Using specific optical elements
- G02B27/0977—Reflective elements
- G02B27/0983—Reflective elements being curved
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/59—Transmissivity
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B17/00—Systems with reflecting surfaces, with or without refracting elements
- G02B17/02—Catoptric systems, e.g. image erecting and reversing system
- G02B17/06—Catoptric systems, e.g. image erecting and reversing system using mirrors only, i.e. having only one curved mirror
- G02B17/0626—Catoptric systems, e.g. image erecting and reversing system using mirrors only, i.e. having only one curved mirror using three curved mirrors
- G02B17/0642—Catoptric systems, e.g. image erecting and reversing system using mirrors only, i.e. having only one curved mirror using three curved mirrors off-axis or unobscured systems in which not all of the mirrors share a common axis of rotational symmetry, e.g. at least one of the mirrors is warped, tilted or decentered with respect to the other elements
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
- G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
- G02B26/0816—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements
- G02B26/0833—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
- G02B27/0933—Systems for active beam shaping by rapid movement of an element
Abstract
The invention relates to the field of terahertz imaging, in particular to a terahertz transmission/reflection imaging light path based on an off-axis parabolic mirror, an adjustment system and method and an imaging light path construction method. The transmission type imaging and the reflection type imaging are innovatively combined in one set of imaging light path design scheme, and the conversion of two imaging modes can be completed only by increasing or decreasing the terahertz plane reflecting mirror element while other elements are unchanged. The method for adjusting the optical path is also described in detail, and the off-axis parabolic mirror is monitored and adjusted by using a high-resolution CCD camera under the assistance of visible light, so that the construction of the whole optical path is completed. The transmission imaging and reflection imaging rapid switching mode is used in the sampling stage of terahertz wave imaging, so that an imaging mode or experimental scheme which has high imaging sensitivity, does not damage the integrity of a sample and obtains sample information as much as possible is realized.
Description
Technical Field
The invention relates to the field of terahertz imaging, in particular to a terahertz transmission/reflection imaging light path based on an off-axis parabolic mirror, an adjustment system, an adjustment method and an imaging light path construction method.
Background
The frequency of the terahertz wave is between 0.1THz and 10THz, and the terahertz wave corresponds to electromagnetic waves with the wavelength of 3 mu m-3 mm, and in the electromagnetic spectrum, the terahertz wave is between infrared light and millimeter waves. In recent years, terahertz band related technology has received extensive attention from researchers around the world.
Terahertz imaging technology is one of recent research hotspots because of its features of no marks, high sensitivity, high resolution, and the like. With the continuous development of terahertz imaging technology, various terahertz imaging optical path systems with different application backgrounds are generated. For long-distance large-target detection imaging, compared with millimeter wave bands commonly used by traditional radars, terahertz waves have wider frequency spectrum and higher resolution, so that the terahertz waves have important application value in the field of national defense security imaging. In the field of biological or security inspection imaging at a relatively short distance, compared with common X-rays, the terahertz wave band electromagnetic wave energy is relatively low, so that substances cannot be damaged in the detection process, and the terahertz wave has irreplaceable advantages in the field of biomedical related security monitoring.
Common terahertz wave imaging modes can be divided into three types of transmission, reflection and attenuated total reflection. The transmission terahertz imaging has the advantages of simple optical path device, easy construction and adjustment of an optical path, high imaging definition and the like. However, the terahertz wave is easily absorbed by polar molecules (such as water molecules) in the biological sample to be observed, so that the terahertz wave has weak sample penetrating capability, and the thickness of the sample to be measured is greatly limited. And the final imaging quality is susceptible to interference and diffraction effects, affecting the final imaging effect. Although the thickness of the sample to be detected is not required to be too great, the diffuse reflection phenomenon of the terahertz waves on the surface of the sample also weakens the intensity of the signal light, which is not beneficial to the acquisition of sample information, leads to the reduction of imaging quality and brings higher requirements to the sensitivity of the terahertz camera. Attenuated total reflection images using total reflection of terahertz waves that occur at the attenuated radial prism-sample interface. When the incidence angle of the terahertz wave is larger than the critical angle of total reflection, evanescent waves are generated on the surface on which the light is incident, and the evanescent waves interact with the sample to be detected so as to carry sample information. However, the resolution and the observable area of the imaging mode are limited, and still further improvement is needed.
Based on the analysis, three terahertz wave imaging technologies have advantages and disadvantages, and it is difficult to obtain a high-quality image carrying the complete information of the sample to be detected by imaging in a single mode.
Accordingly, it is desirable to provide a terahertz transmission/reflection imaging optical path in order to solve the above-described problems.
Disclosure of Invention
The invention aims to solve the technical problems:
in order to better solve the problem of obtaining a high-quality image carrying the complete information of a sample to be tested, the invention provides a terahertz transmission/reflection imaging optical path design scheme and an adjusting method based on an off-axis parabolic mirror. The scheme combines transmission imaging and reflection imaging innovatively in one set of imaging light path design scheme, and conversion of two imaging modes can be completed only by increasing or decreasing terahertz plane reflecting mirror elements while other elements remain unchanged. The invention also introduces the adjusting method of the light path in detail, and the off-axis parabolic mirror is monitored and adjusted by using the CCD camera with high resolution under the assistance of visible light, so as to complete the construction of the whole light path.
The technical scheme provided by the invention is as follows:
a terahertz imaging light path based on an off-axis parabolic mirror comprises a terahertz source, a first off-axis parabolic mirror, a second off-axis parabolic mirror, a third off-axis parabolic mirror and a terahertz camera which are sequentially arranged;
the terahertz source is positioned at the focus of the first off-axis parabolic mirror and emits terahertz waves;
the first off-axis parabolic mirror converts terahertz waves into plane waves;
the second off-axis parabolic mirror receives plane wave back reflection emergent rays parallel to normal incidence, and a sample to be measured is placed at a focus or an equivalent focus of the second off-axis parabolic mirror and is positioned on an emergent light path;
the focus or equivalent focus position of the third off-axis parabolic mirror is corresponding to the same as the focus or equivalent focus of the second off-axis parabolic mirror, and the third off-axis parabolic mirror is used for receiving waveform carrying sample image information and reflecting and emitting;
the terahertz camera is positioned on the emergent light path of the third off-axis parabolic mirror and is used for acquiring an imaging result of a sample to be detected.
The invention further adopts the technical scheme that: the imaging light path is a transmission imaging light path, the waveform emitted by the second off-axis parabolic mirror is transmitted through the sample to be detected, the sample to be detected is placed at the focus of the second off-axis parabolic mirror, the position of the focus of the third off-axis parabolic mirror is the same as that of the focus of the second off-axis parabolic mirror, and the terahertz camera acquires the transmission imaging result of the sample to be detected, so that terahertz transmission imaging of the sample to be detected is realized.
The invention further adopts the technical scheme that: the imaging light path is a reflection imaging light path and further comprises a first plane reflecting mirror and a second plane reflecting mirror, wherein the first plane reflecting mirror is positioned between the second off-axis parabolic mirror and the sample to be detected, and the second plane reflecting mirror is positioned between the sample to be detected and the third off-axis parabolic mirror; the terahertz wave focused after being reflected by the second off-axis parabolic mirror changes the propagation direction after being reflected by the first plane reflecting mirror, and is converged at a point deviating from the original direction light path and is marked as an equivalent focus of the second off-axis parabolic mirror, a sample to be measured is placed at the equivalent focus, and the equivalent focus position of the third off-axis parabolic mirror is the same as the equivalent focus of the second off-axis parabolic mirror.
The invention further adopts the technical scheme that: the waveform reflected and emitted by the second off-axis parabolic mirror is reflected by the first plane reflecting mirror and then converged on the surface of the sample to be reflected, and the waveform carrying the image information of the sample is received by the third off-axis parabolic mirror after being reflected by the second plane reflecting mirror; the terahertz camera acquires a reflection imaging result of the sample to be detected, and terahertz reflection imaging of the sample to be detected is achieved.
An off-axis parabolic mirror-based adjustment system comprises a visible light source, a diaphragm, an off-axis parabolic mirror, a multidimensional adjustment translation stage and a CCD camera; the off-axis parabolic mirror is fixedly arranged on the multidimensional adjusting translation stage and is arranged in a light path; the CCD camera is fixedly arranged on the one-dimensional adjusting frame, the visible light parallel emergent beam is thinned through the diaphragm, then converged through the off-axis parabolic mirror to form a converged focus, and the position of the focus is detected by the CCD camera.
The invention further adopts the technical scheme that: the multidimensional adjusting translation table comprises an off-axis angle adjusting knob, a pitching knob and an inclination knob which are respectively used for adjusting the off-axis angle, the pitching angle and the left-right inclination angle of the off-axis parabolic mirror.
The method for determining the focus of the off-axis parabolic mirror by using the adjustment system comprises the following steps:
step 1: preconditioning step
1.1: adjusting the visible light source to enable the emergent beam of the visible light to be parallel to an optical platform of the system;
1.2: measuring a converging focal spot by placing the CCD camera at a position slightly deviated from the focus center;
1.3: moving the multidimensional adjusting translation stage to enable the emergent light beam to irradiate at the center of the off-axis parabolic mirror;
1.4: loading a diaphragm in front of an off-axis parabolic mirror to make the emergent beam a beamlets;
1.5: the CCD camera is moved horizontally and the imaging point of the beamlets on the CCD camera is observed. Then adjusting the pitching knob to enable the beamlets to image at the equal height position of the CCD camera;
step 2: determining focus
2.1: an off-axis angle adjusting knob of the electric adjusting translation stage is adjusted to enable the off-axis parabolic mirror to rotate around a central axis of the off-axis parabolic mirror until the focal spot detected by the CCD camera is a flat light spot with a horizontal or vertical shape;
2.2: moving the CCD camera to enable the CCD camera to be positioned in front of and behind a focus, observing whether focal spots in front of and behind the focus are vertical to each other, and if the front of the focus is a horizontal flat light spot, the rear of the focus is a vertical flat light spot, and vice versa; if the requirements are met, the process is carried out according to the subsequent steps, if the requirements are not met, the process is carried out again by returning to the previous step for readjusting the off-axis angle adjusting knob of the electric adjusting translation stage until the requirements are met;
2.3: adjusting an inclination knob of the electric adjustment translation stage to enable focal spots before and after the focal point position detected by the CCD camera to be as close to a circle as possible;
2.4: moving the CCD camera back and forth near the position of the focal point 8 to find the position with the minimum focal spot, wherein the position is the focal point of the off-axis parabolic mirror, observing the size of the focal spot by using the CCD camera, and recording the size of the focal spot at the focal point;
2.5: and recording the position of the CCD camera, wherein the position is the focal position of the off-axis parabolic mirror 7, and replacing the CCD camera at the position with a terahertz source according to the light path requirement to finish the determination of the focal position of the off-axis parabolic mirror.
The method for constructing the transmission imaging optical path by using the method for determining the focus comprises the following steps:
step 1: determining the focus of a second off-axis parabolic mirror by using the method for determining the focus of the off-axis parabolic mirror, and placing a sample to be tested at the focus position; adjusting the position and the posture of the second off-axis parabolic mirror so that the light beam incident to the second off-axis parabolic mirror is perpendicular to the connecting line of the second off-axis parabolic mirror and the sample to be measured;
step 2: placing a first off-axis parabolic mirror in the direction of a light beam incident on a second off-axis parabolic mirror, determining the focus of the first off-axis parabolic mirror by using the method for determining the focus of the off-axis parabolic mirror, and placing a terahertz source at the position of the focus; adjusting the position and the gesture of the first off-axis parabolic mirror so that the connecting line of the first off-axis parabolic mirror and the terahertz source is perpendicular to the direction of the light beam incident to the second off-axis parabolic mirror;
step 3: and after the focus of the third off-axis parabolic mirror is coincident with the focus of the second off-axis parabolic mirror, adjusting the position and the posture of the third off-axis parabolic mirror so that an emergent light beam from the third off-axis parabolic mirror is perpendicular to the connecting line of the third off-axis parabolic mirror and the sample to be measured.
Step 4: and placing a terahertz camera in the direction of the emergent light beam of the third off-axis parabolic mirror, and completing the construction of a terahertz transmission imaging light path based on the off-axis parabolic mirror.
The method for constructing the reflection imaging light path by using the method for determining the focus comprises the following steps:
step 1: symmetrically placing the first plane reflecting mirror and the second plane reflecting mirror on a terahertz light path between the second off-axis parabolic mirror and the third off-axis parabolic mirror, and adjusting the positions and the postures of the first plane reflecting mirror and the second plane reflecting mirror;
step 2: the method for determining the focal point of the off-axis parabolic mirror is utilized to respectively determine and enable the equivalent focal point of the second off-axis parabolic mirror to coincide with the equivalent focal point of the third off-axis parabolic mirror, and a sample to be measured is placed at the equivalent focal point position;
step 3: placing a first off-axis parabolic mirror in the direction of a light beam incident on a second off-axis parabolic mirror, determining the focus of the first off-axis parabolic mirror by using the method for determining the focus of the off-axis parabolic mirror, and placing a terahertz source at the position of the focus; the position and the posture of the first off-axis parabolic mirror are adjusted so that the connecting line of the first off-axis parabolic mirror and the terahertz source is perpendicular to the direction of the light beam incident on the second off-axis parabolic mirror.
Step 4: and placing a terahertz camera in the direction of the emergent light beam of the third off-axis parabolic mirror, and completing the construction of a terahertz reflection imaging light path based on the off-axis parabolic mirror.
Advantageous effects
Compared with the prior art, the invention has the beneficial effects that:
1. according to the terahertz transmission/reflection imaging optical path based on the off-axis parabolic mirror and the construction method thereof, provided by the invention, the imaging mode or experimental device which has high imaging sensitivity, does not damage the integrity of a sample and obtains sample information as much as possible is realized by using a mode of fast switching between transmission imaging and reflection imaging in the sampling stage of terahertz wave imaging.
2. Compared with a transmission imaging mode only, the terahertz transmission/reflection imaging optical path based on the off-axis parabolic mirror and the construction method thereof can compensate the problem that diffraction and interference possibly generated during imaging affect acquisition of information of a sample to be detected, and can acquire more information of a shallow surface layer of the sample to be detected; compared with a reflection imaging mode only, the method can compensate for the problem that diffuse reflection influences in imaging to acquire information of the sample to be detected, and acquire more information of the part below the shallow surface layer of the sample to be detected.
3. The terahertz transmission/reflection imaging optical path based on the off-axis parabolic mirror and the construction method thereof provided by the invention have the advantages that the optical path design accords with a transmission mode or a reflection mode, the parameters accord with a calculation formula of transmission and reflection theory, and the designed adjustment system and the method for determining the focus of the off-axis parabolic mirror are simple, low in cost and high in sensitivity.
Drawings
FIG. 1 is a terahertz transmission imaging light path diagram based on an off-axis parabolic mirror;
FIG. 2 is a terahertz reflection imaging light path diagram based on an off-axis parabolic mirror;
FIG. 3 is a graphical representation of determining the position of the focal point of an off-axis parabolic mirror.
Wherein reference numerals are denoted as:
11: first off-axis parabolic mirror, 12: second off-axis parabolic mirror, 13: a third off-axis parabolic mirror; 14: terahertz source, 15: a terahertz camera;
1: diaphragm, 2: emitting beamlets, 3: electric multidimensional adjustment translation stage, 4: off-axis angle adjust knob, 5: pitch knob, 6: tilting knob, 7: off-axis parabolic mirror to be measured, 8: off-axis parabolic mirror focus, 9: a CCD camera.
Detailed Description
In order to more clearly illustrate the technical solution of the implementation of the present invention, the following will briefly introduce each module required in the description of the embodiments. It is obvious that the drawings in the following description are merely flow charts of the present invention, and that it is possible for a person skilled in the art to extend from this drawing without inventive effort. The embodiments described below by referring to the drawings are illustrative and intended to explain the present invention and should not be construed as limiting the invention.
Example 1
Referring to fig. 1, the terahertz transmission imaging optical path based on the off-axis parabolic mirror comprises a terahertz source 14, three first off-axis parabolic mirrors 11, second off-axis parabolic mirrors 12, third off-axis parabolic mirrors 13 and a terahertz camera 15 which are sequentially arranged;
a terahertz source 14, located at the focal point of the first off-axis parabolic mirror 11, emitting terahertz waves;
a first off-axis parabolic mirror 11 that converts terahertz waves into plane waves;
the second off-axis parabolic mirror 12 receives the plane wave back reflection emergent light parallel to normal incidence, and a sample to be measured is placed at the focus of the second off-axis parabolic mirror 12 and is positioned on an emergent light path;
the third off-axis parabolic mirror 13 has the same focal point position as that of the second off-axis parabolic mirror 12, and is used for receiving the waveform carrying the image information of the sample and reflecting and emitting;
and the terahertz camera 15 is positioned on the emergent light path of the third off-axis parabolic mirror 13 and is used for acquiring an imaging result of the sample to be detected. The waveform emitted by the second off-axis parabolic mirror 12 is transmitted through the sample to be detected, and the terahertz camera 15 acquires the transmission imaging result of the sample to be detected, so that terahertz transmission imaging of the sample to be detected is realized.
Example 2
Referring to fig. 2, the terahertz reflection imaging optical path based on the off-axis parabolic mirror includes: the terahertz wave detection device comprises a terahertz source 14, three first off-axis parabolic mirrors 11, a second off-axis parabolic mirror 12 and a third off-axis parabolic mirror 13 which are the same in model, and a terahertz camera 15, wherein the terahertz camera is used for terahertz wave detection, and the terahertz source comprises two terahertz wave first plane reflectors and a second terahertz wave first plane reflector. The first off-axis parabolic mirror 11, the second off-axis parabolic mirror 12, two first plane mirrors and two second plane mirrors which are symmetrically arranged, and the third off-axis parabolic mirror 13 are sequentially arranged on the emergent light path of the terahertz wave.
Specifically, a first planar mirror is located between the second off-axis parabolic mirror 12 and the sample to be measured, and a second planar mirror is located between the sample to be measured and a third off-axis parabolic mirror 13; the terahertz wave focused after being reflected by the second off-axis parabolic mirror 12 changes the propagation direction after being reflected by the first plane reflecting mirror, and is converged at a point deviating from the original direction light path and is marked as an equivalent focus of the second off-axis parabolic mirror 12, the sample to be measured is placed at the equivalent focus, and the equivalent focus position of the third off-axis parabolic mirror 13 is the same as the equivalent focus of the second off-axis parabolic mirror 12.
The waveform emitted by the second off-axis parabolic mirror 12 is reflected by the first plane reflecting mirror, then is converged on the surface of the sample to be reflected, and the waveform carrying the image information of the sample is received by the third off-axis parabolic mirror 13 after being reflected by the second plane reflecting mirror; the terahertz camera 15 obtains a reflection imaging result of the sample to be measured, and terahertz reflection imaging of the sample to be measured is achieved.
Example 3
As shown in fig. 3, an off-axis parabolic mirror-based adjustment system comprises a visible light source, a diaphragm 1, an off-axis parabolic mirror, a multidimensional adjustment translation stage 3 and a CCD camera 9; the visible light source emits parallel light beams 2, and the off-axis parabolic mirror 7 to be measured is fixedly arranged on the multi-dimensional adjusting translation table 3 and is arranged in a light path; the CCD camera 9 is fixedly arranged on the one-dimensional adjusting frame, the visible light parallel emergent beam 2 is focused by the off-axis parabolic mirror 7 to be measured to form a focused focus after the beam 2 is thinned by the diaphragm 1, and the position of the focus is detected by the CCD camera 9.
The multidimensional adjusting translation table 3 is an electric adjusting translation table and comprises an off-axis angle adjusting knob 4, a pitching knob 5 and a tilting knob 6 which are respectively used for adjusting off-axis angles, pitch angles and left-right dip angles of the off-axis parabolic mirror 7 to be measured.
Example 4
The core of the construction light path is to find the focus position of each off-axis parabolic mirror. Taking the first off-axis parabolic mirror 11 as an example of the off-axis parabolic mirror 7 to be measured, the focal position of the first off-axis parabolic mirror 11, that is, the placement position of the terahertz source 14 is determined.
As shown in fig. 3, an off-axis parabolic mirror 7 to be measured is fixed on an electric adjustment translation stage 3, the off-axis parabolic mirror 7 to be measured can be any one of the first, second and third off-axis parabolic mirrors described in fig. 1 and fig. 2, and the electric multi-dimensional adjustment translation stage 3 comprises three knobs, namely an off-axis angle adjustment knob 4, a pitching knob 5 and an inclining knob 6, wherein the off-axis parabolic mirror can be rotated by adjusting the off-axis angle adjustment knob 4, so that the off-axis angle of the off-axis parabolic mirror can be adjusted; the pitch angle of the off-axis parabolic mirror can be adjusted by adjusting the pitch knob 5; by adjusting the tilt knob 6, an adjustment of the left and right tilt angles of the off-axis parabolic mirror can be achieved.
Because the terahertz waves emitted in parallel are difficult to directly obtain, the terahertz waves are emitted in parallel instead of visible light, and the high-resolution CCD camera 9 is used for assisting in light path adjustment. The visible light parallel emergent beam 2 is converged by the off-axis parabolic mirror 7 to be detected to form a converging focus, and the specific position of the focus is determined by means of detection by the CCD camera 9 with high resolution; the CCD camera 9 is placed on a one-dimensional adjusting frame movable in the horizontal direction, and the camera is moved to be near the focal point.
1. The system prepares for the adjustment step:
(1) Adjusting the front-end optical element of the visible light system to enable the visible light emergent beam 2 to be parallel to the optical platform of the system;
(2) The CCD camera 9 is placed at a position slightly deviated from the focus center, and a convergent focal spot is measured, wherein the deviation degree is based on the focal spot measured by the CCD camera 9, and the shape of the focal spot can be obviously reflected in size;
(3) Moving the electric adjustment translation stage 3 to make the emergent light beam 2 irradiate on the center of the off-axis parabolic mirror as much as possible;
(4) Loading an aperture diaphragm 1 in front of an off-axis parabolic mirror to make an outgoing beam 2 a beamlets 2;
(5) The CCD camera 9 is moved a large distance in the horizontal direction, and the imaging point of the beamlets 2 on the CCD camera 9 is observed. The pitch knob 5 of the off-axis parabolic mirror is then adjusted to image the beamlets 2 at the same height as the CCD camera 9.
2. Determining a specific focal point position:
(1) An off-axis angle adjusting knob 4 of the electric adjusting translation stage 3 is adjusted to enable the off-axis parabolic mirror to rotate around a central axis of the off-axis parabolic mirror until the focal spot detected by the CCD camera 9 is a flat spot with a horizontal or vertical shape;
(2) The CCD camera 9 on the one-dimensional adjusting frame movable in the horizontal direction is moved so as to be positioned in front of and behind the focal point. Observing whether the focal spots before and after the focal spot are perpendicular to each other, i.e.: if the front of the focal spot is a horizontal flat spot, the back of the focal spot should be a vertical flat spot, and vice versa. If the requirements are met, the method is carried out according to the follow-up steps, if the requirements are not met, the method is required to return to the previous step to readjust the off-axis angle adjusting knob 4 of the electric adjusting translation stage 3 until the requirements are met.
(3) Adjusting the tilt knob 6 of the electric adjustment translation stage 3 to enable focal spots before and after the focal position detected by the CCD camera 9 to be as close to a circle as possible;
(4) The CCD camera 9 is moved back and forth, the position with the minimum focal spot is found, the position is the focal point of the off-axis parabolic mirror, the size of the focal spot is observed by the CCD camera 9, and the size of the focal spot at the focal point is recorded.
(5) And recording the position of the CCD camera 9, namely the focal position of the off-axis parabolic mirror, and replacing the CCD camera 9 at the position with the terahertz source 14 according to the light path requirement to finish the focal position determination work of the first off-axis parabolic mirror 11.
Example 5
With reference to fig. 1, a terahertz transmission imaging optical path based on an off-axis parabolic mirror is constructed by using the method for determining the position of the focal point 8 of the off-axis parabolic mirror described in example 4.
When the terahertz transmission imaging light path is actually built, the sample to be measured should be started from the position. The position of the sample to be measured is the focal positions of the second off-axis parabolic mirror 12 and the third off-axis parabolic mirror 13. Firstly, constructing a light path before the light enters the sample to be measured. The position and posture of the second off-axis parabolic mirror 12 are adjusted so that the light beam 2 incident on the second off-axis parabolic mirror 12 is perpendicular to the line connecting the second off-axis parabolic mirror 12 with the sample to be measured, and the first off-axis parabolic mirror 11 is placed at a suitable distance in the direction of the light beam 2 incident on the second off-axis parabolic mirror 12. The position and attitude of the first off-axis parabolic mirror 11 are adjusted so that the line connecting the first off-axis parabolic mirror 11 and the terahertz source 14 is perpendicular to the direction of the light beam 2 incident on the second off-axis parabolic mirror 12. A terahertz source 14 is placed at the focal position of the first off-axis parabolic mirror 11. And then setting up a light path after the light is transmitted through the sample to be measured. The position and attitude of the third off-axis parabolic mirror 13 are adjusted such that the outgoing light beam 2 from the third off-axis parabolic mirror 13 is perpendicular to the line connecting the third off-axis parabolic mirror 13 with the sample. The terahertz camera 15 is placed at a suitable distance in the direction of the outgoing light beam 2 of the third off-axis parabolic mirror 13. Finally, the construction of the terahertz transmission imaging light path based on the off-axis parabolic mirror is completed.
The terahertz source 14 is located at the focal point of the first off-axis parabolic mirror 11, and the terahertz wave emitted by the terahertz source is converted into a plane wave after being reflected by the first off-axis parabolic mirror 11, and is incident on the second off-axis parabolic mirror 12 in parallel. The sample to be measured is placed at the focal point of the second off-axis parabolic mirror 12 and this position is also the focal point position of the third off-axis parabolic mirror 13. The terahertz wave is transmitted through the sample to be detected, the image information of the sample is carried, and the terahertz wave is reflected by the third off-axis parabolic mirror 13 and then is emitted to the terahertz detector in parallel, so that the terahertz imaging of the sample to be detected can be realized through the whole light path.
Example 6
Referring to fig. 2, the terahertz reflection imaging optical path of the off-axis parabolic mirror is based on the aforementioned transmission optical path, the parameters, the placement position and the pose of the optical device of the imaging device are not changed, the sample to be measured is moved from the original position, and two plane reflectors are symmetrically placed on the terahertz optical path between the second off-axis parabolic mirror 12 and the third off-axis parabolic mirror 13, so that the second off-axis parabolic mirror 12 and the third off-axis parabolic mirror 13 are focused at the same point, and the point is the equivalent focus of the two off-axis parabolic mirrors. In adjusting the positions and attitudes of the two plane mirrors, the position of the equivalent focus can be determined by means of visible light and the CCD camera 9 in a similar manner to that in the embodiment 1. After the plane reflector is adjusted, placing a sample to be tested at the equivalent focus of the two off-axis parabolic mirrors, and completing the construction of the terahertz reflection imaging light path based on the off-axis parabolic mirrors.
The terahertz source 14 is located at the focal point of the first off-axis parabolic mirror 11, and the terahertz wave emitted by the terahertz source is converted into a plane wave after being reflected by the first off-axis parabolic mirror 11, and is incident on the second off-axis parabolic mirror 12 in parallel. The terahertz waves reflected by the second off-axis parabolic mirror 12 are reflected by the plane mirror and then converged at the place where the sample to be measured is placed, and the position is the equivalent focal position of the second off-axis parabolic mirror 12 and the equivalent focal position of the third off-axis parabolic mirror 13. After being reflected by the surface of the sample to be detected, the terahertz wave carries the image information of the sample and is reflected by the third off-axis parabolic mirror 13 and then is emitted to the terahertz detector in parallel, and the terahertz imaging of the sample to be detected can be realized through the whole light path.
In summary, the embodiment of the invention uses the mode of fast switching between transmission imaging and reflection imaging in the sampling stage of terahertz wave imaging, so as to realize an imaging mode or experimental device which has high imaging sensitivity, does not damage the integrity of a sample and obtains sample information as much as possible.
The model of the device in the embodiment of the invention is not limited, and the device can complete the functions.
Example 7
Important specification parameters of the off-axis parabolic mirror are: caliber, focal length and off-axis angle. Off-axis parabolic mirrors are typically metallic mirrors, the aperture describing their physical dimensions. An off-axis mirror of 1 inch (25.4 mm) was machined from a cylindrical metal having a diameter of 1 inch, and an off-axis mirror of 2 inches (50.4 mm) was machined from a cylindrical metal having a diameter of 2 inches. The focal length of an off-axis parabolic mirror is typically referred to by default as its reflective focal length, and the focal length of a standard off-axis parabolic mirror is typically 1 inch, 2 inches, 4 inches and 6 inches. Off-axis angle is an indicator of the direction of propagation of a light beam through an off-axis parabolic mirror, which is typically 90 °. For an off-axis parabolic mirror with an off-axis angle of 90 deg., the direction of propagation of the collimated beam should be perpendicular to the off-axis parabolic mirror base to achieve beam focusing. Other off-axis angles may also be set if a particular design is performed.
By defining three specification parameters above the off-axis parabolic mirror and combining the specific conditions of the optical path, the distance between the lens and the sample in the imaging optical path and the size of the incident angle in the reflecting optical path can be calculated, so that the optical path is suitable for imaging samples with different sizes.
The geometry of the imaging optical path provided by this embodiment can be described simply in a cartesian coordinate system. The first off-axis parabolic mirror 11 collimates the light beam, the second off-axis parabolic mirror 12 focuses the collimated light beam on the target, the third off-axis parabolic mirror 13 re-collimates the beam reflected or refracted by the sample, and the re-collimated beam is focused on the detector aperture. The device position is expressed in a Cartesian coordinate system as follows: the focal axis of the first off-axis parabolic mirror 11 is in the positive z-axis direction (i.e., the terahertz source 14 emits a beam that propagates in the negative z-axis direction). The two collimated beams 2 in the two paths are collinear with the x-axis. The second off-axis parabolic mirror 12 is of the same size as the third off-axis parabolic mirror 13 with the focal axis lying in the y-z plane. The detector aperture plane (i.e., the imaging plane) is coplanar with the x-y plane. In the reflected light path, the magnitude θ of the incident angle of the beam onto the sample can be adjusted by adjusting the aperture r and focal length parameter f of the second and third off-axis parabolic mirrors 13:
the size and imaging resolution of the observation range of the sample to be tested based on the terahertz transmission/reflection imaging optical path method of the off-axis parabolic mirror are tested.
The specification parameters of the second and third off-axis parabolic mirrors 13 affect the size of the observation range at the sample to be measured. Taking 2.5THz terahertz as an example, the sizes of the observation ranges of the samples to be tested of the second off-axis parabolic mirror 13 and the third off-axis parabolic mirror 13 with different focal lengths are tested, and the observation ranges of the samples are determined by the sizes of the focused light spots at the positions of the samples to be tested. The total trend of the spot size changes with the change of the measuring position along the propagation direction of the light beam 2, which is marked at the position of the sample to be measured along the propagation direction of the light beam 2, is firstly reduced and then increased. The experimental results of spot size measured at different positions in the propagation direction of the terahertz beam 2 after focusing the beam 2 by an off-axis parabolic mirror having a caliber of 1 inch and a focal length of 1 or 2 inches are shown in table 1. It can be seen that the spot diameters of the two different lenses are respectively 0.33mm and 0.56mm, namely, the off-axis parabolic mirrors with two different focal lengths are suitable for the observation ranges of the samples to be detected with different sizes.
TABLE 1 results of spot size measurements of terahertz beams focused by off-axis parabolic mirrors
The specification parameters of the second and third off-axis parabolic mirrors 13 affect the imaging resolution of the sample to be measured. Taking 2.5THz terahertz as an example, the imaging resolution sizes of the second and third off-axis parabolic mirrors 13 of different focal lengths were tested.
And testing the resolution of a terahertz transmission imaging light path based on the off-axis parabolic mirror, and manufacturing 5 slits with widths sequentially increased from 2mm to 4mm by using an FR-4 plate, wherein terahertz waves are not transmitted by an FR-4 material. The measured value of the slit width was compared with the true value as shown in table 2. It can be seen that the measured values are smaller than the true values, and that there is no significant difference in the resolution of the measurements of the off-axis parabolic mirrors for the two focal lengths.
TABLE 2 comparison between actual and measured values of slit widths of Plastic plates
And testing the resolution of the terahertz transmission reflection optical path based on the off-axis parabolic mirror, and taking five iron wires with sequentially increased diameters as samples to be tested. The measured value of the wire width was compared with the true value as shown in table 3. It can be found that in the transmission light path, the deviation of the measurement result after focusing the off-axis parabolic mirror with the focal length of 1 inch and the true value is smaller, the error is smaller than 0.05mm, and the deviation of the measurement value after focusing the off-axis parabolic mirror with the focal length of 2 inches and the true value is larger, but the deviation is gradually reduced along with the increase of the diameter of the iron wire.
TABLE 3 comparison between actual and measured wire diameter
As can be seen by analysis, the focal length of the specification parameters of the second and third off-axis parabolic mirrors 13 should be determined in combination with the sample size to be measured and the imaging resolution requirements. The sample observation range of the off-axis parabolic mirror with large focal length is larger, and the off-axis parabolic mirror with small focal length can obtain higher imaging resolution.
Although embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and not to be construed as limiting the invention, and that variations, modifications, alternatives, and variations may be made in the above embodiments by those skilled in the art without departing from the spirit and principles of the invention.
Claims (9)
1. A terahertz imaging light path based on an off-axis parabolic mirror is characterized in that: the terahertz imaging device comprises a terahertz source, a first off-axis parabolic mirror, a second off-axis parabolic mirror, a third off-axis parabolic mirror and a terahertz camera which are sequentially arranged, wherein the types of the first off-axis parabolic mirror, the second off-axis parabolic mirror, the third off-axis parabolic mirror and the terahertz camera are the same;
the terahertz source is positioned at the focus of the first off-axis parabolic mirror and emits terahertz waves;
the first off-axis parabolic mirror converts terahertz waves into plane waves;
the second off-axis parabolic mirror receives plane wave back reflection emergent rays parallel to normal incidence, and a sample to be measured is placed at a focus or an equivalent focus of the second off-axis parabolic mirror and is positioned on an emergent light path;
the focus or equivalent focus position of the third off-axis parabolic mirror is corresponding to the same as the focus or equivalent focus of the second off-axis parabolic mirror, and the third off-axis parabolic mirror is used for receiving waveform carrying sample image information and reflecting and emitting;
the terahertz camera is positioned on the emergent light path of the third off-axis parabolic mirror and is used for acquiring an imaging result of a sample to be detected.
2. The terahertz imaging optical path according to claim 1, wherein: the imaging light path is a transmission imaging light path, the waveform emitted by the second off-axis parabolic mirror is transmitted through the sample to be detected, and the terahertz camera obtains a transmission imaging result of the sample to be detected, so that terahertz transmission imaging of the sample to be detected is realized.
3. The terahertz imaging optical path according to claim 1, wherein: the imaging light path is a reflection imaging light path and further comprises a first plane reflecting mirror and a second plane reflecting mirror, wherein the first plane reflecting mirror is positioned between the second off-axis parabolic mirror and the sample to be detected, and the second plane reflecting mirror is positioned between the sample to be detected and the third off-axis parabolic mirror; the terahertz wave focused after being reflected by the second off-axis parabolic mirror changes the propagation direction after being reflected by the first plane reflecting mirror, and is converged at a point deviating from the original direction light path and is marked as an equivalent focus of the second off-axis parabolic mirror, a sample to be measured is placed at the equivalent focus, and the equivalent focus position of the third off-axis parabolic mirror is the same as the equivalent focus of the second off-axis parabolic mirror.
4. The terahertz imaging optical path according to claim 3, wherein: the waveform reflected and emitted by the second off-axis parabolic mirror is reflected by the first plane reflecting mirror and then converged on the surface of the sample to be reflected, and the waveform carrying the image information of the sample is received by the third off-axis parabolic mirror after being reflected by the second plane reflecting mirror; the terahertz camera acquires a reflection imaging result of the sample to be detected, and terahertz reflection imaging of the sample to be detected is achieved.
5. An off-axis parabolic mirror based adjustment system is characterized in that: the device comprises a visible light source, a diaphragm, an off-axis parabolic mirror, a multidimensional adjusting translation stage and a CCD camera; the off-axis parabolic mirror is fixedly arranged on the multidimensional adjusting translation stage and is arranged in a light path; the CCD camera is fixedly arranged on the one-dimensional adjusting frame, the visible light parallel emergent beam is thinned through the diaphragm, then converged through the off-axis parabolic mirror to form a converged focus, and the position of the focus is detected by the CCD camera.
6. The system of claim 5, wherein the system comprises: the multidimensional adjusting translation table comprises an off-axis angle adjusting knob, a pitching knob and an inclination knob which are respectively used for adjusting the off-axis angle, the pitching angle and the left-right inclination angle of the off-axis parabolic mirror.
7. A method of determining the focus of an off-axis parabolic mirror using an adjustment system according to any one of claims 5 to 6, comprising the steps of:
step 1: preconditioning step
1.1: adjusting the visible light source to enable the emergent beam of the visible light to be parallel to an optical platform of the system;
1.2: measuring a converging focal spot by placing the CCD camera at a position slightly deviated from the focus center;
1.3: moving the multidimensional adjusting translation stage to enable the emergent light beam to irradiate at the center of the off-axis parabolic mirror;
1.4: loading a diaphragm in front of an off-axis parabolic mirror to make the emergent beam a beamlets;
1.5: moving the CCD camera horizontally, observing an imaging point of the beamlets on the CCD camera, and then adjusting a pitching knob to enable the beamlets to be imaged at the same height position of the CCD camera;
step 2: determining focus
2.1: an off-axis angle adjusting knob of the electric adjusting translation stage is adjusted to enable the off-axis parabolic mirror to rotate around a central axis of the off-axis parabolic mirror until the focal spot detected by the CCD camera is a flat light spot with a horizontal or vertical shape;
2.2: moving the CCD camera to be positioned in front of and behind the focal point 8, observing whether the focal spots in front of and behind the focal point are mutually vertical, and if the focal point is a horizontal flat light spot, the focal point is a vertical flat light spot, and vice versa; if the requirements are met, the process is carried out according to the subsequent steps, if the requirements are not met, the process is carried out again by returning to the previous step for readjusting the off-axis angle adjusting knob of the electric adjusting translation stage until the requirements are met;
2.3: adjusting an inclination knob of the electric adjustment translation stage to enable focal spots before and after the focal point position detected by the CCD camera to be as close to a circle as possible;
2.4: moving the CCD camera back and forth near the focal point position to find the minimum focal spot position, wherein the minimum focal spot position is the focal point position of the off-axis parabolic mirror, observing the size of the focal spot by using the CCD camera, and recording the focal spot size at the focal point position;
2.5: and recording the position of the CCD camera, wherein the position is the focal position of the off-axis parabolic mirror, and replacing the CCD camera at the position with a terahertz source according to the light path requirement to finish the determination of the focal position of the off-axis parabolic mirror.
8. A method of constructing the transmission imaging optical path of any one of claims 1-2 using the method of claim 7, comprising the steps of:
step 1: determining the focus of a second off-axis parabolic mirror by using the method for determining the focus of the off-axis parabolic mirror, and placing a sample to be tested at the focus position; adjusting the position and the posture of the second off-axis parabolic mirror so that the light beam incident to the second off-axis parabolic mirror is perpendicular to the connecting line of the second off-axis parabolic mirror and the sample to be measured;
step 2: placing a first off-axis parabolic mirror in the direction of a light beam incident on a second off-axis parabolic mirror, determining the focus of the first off-axis parabolic mirror by using the method for determining the focus of the off-axis parabolic mirror, and placing a terahertz source at the position of the focus; adjusting the position and the gesture of the first off-axis parabolic mirror so that the connecting line of the first off-axis parabolic mirror and the terahertz source is perpendicular to the direction of the light beam incident to the second off-axis parabolic mirror;
step 3: after the focus of the third off-axis parabolic mirror is ensured to coincide with the focus of the second off-axis parabolic mirror, the position and the posture of the third off-axis parabolic mirror are regulated so that an emergent light beam from the third off-axis parabolic mirror is perpendicular to the connecting line of the third off-axis parabolic mirror and a sample to be detected;
step 4: and placing a terahertz camera in the direction of the emergent light beam of the third off-axis parabolic mirror, and completing the construction of a terahertz transmission imaging light path based on the off-axis parabolic mirror.
9. A method of constructing the reflection imaging optical path of any one of claims 3-4 using the method of claim 7, characterized by: the method comprises the following steps:
step 1: symmetrically placing the first plane reflecting mirror and the second plane reflecting mirror on a terahertz light path between the second off-axis parabolic mirror and the third off-axis parabolic mirror, and adjusting the positions and the postures of the first plane reflecting mirror and the second plane reflecting mirror;
step 2: the method for determining the focal point of the off-axis parabolic mirror is utilized to respectively determine and enable the equivalent focal point of the second off-axis parabolic mirror to coincide with the equivalent focal point of the third off-axis parabolic mirror, and a sample to be measured is placed at the equivalent focal point position;
step 3: placing a first off-axis parabolic mirror in the direction of a light beam incident on a second off-axis parabolic mirror, determining the focus of the first off-axis parabolic mirror by using the method for determining the focus of the off-axis parabolic mirror, and placing a terahertz source at the position of the focus; adjusting the position and the gesture of the first off-axis parabolic mirror so that the connecting line of the first off-axis parabolic mirror and the terahertz source is perpendicular to the direction of the light beam incident to the second off-axis parabolic mirror;
step 4: and placing a terahertz camera in the direction of the emergent light beam of the third off-axis parabolic mirror, and completing the construction of a terahertz reflection imaging light path based on the off-axis parabolic mirror.
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