CN115494024A - Optical path device and focusing method for terahertz continuous wave tomography - Google Patents

Abstract

The invention aims to provide an optical path device and a focusing method for terahertz continuous wave tomography, and belongs to the technical field of terahertz imaging. The optical path device can directly focus the emission beam emitted by the terahertz continuous wave generator to a sample by only adopting two off-axis ellipsoidal reflectors, and then directly focus the focused beam after transmitting the sample to the terahertz continuous wave receiving device; two lenses or reflecting surfaces required in the collimation process are omitted, the simplification of the light path diagram of the traditional THz-CT-CW imaging system is realized, and the difficulty in adjusting the positions of the lenses is reduced.

Description

Optical path device and focusing method for terahertz continuous wave tomography

technical field

The invention belongs to the technical field of terahertz imaging, and in particular relates to an optical path device and a focusing method for terahertz continuous wave tomography.

Background technique

The operating frequency of the terahertz segment is 100 GHz to 10 THz, and the corresponding wavelength is 3 mm to 30 μm. This frequency band is higher than the microwave and millimeter wave frequency bands, and lower than the infrared and visible light frequency bands; and because the electromagnetic waves in this frequency band are located in the transition region from macroelectronics to microphotonics, they have some unique characteristics. In the field of imaging, compared with microwave and millimeter-wave imaging, the resolution of terahertz imaging is much higher than that of microwave and millimeter-wave frequency bands; compared with infrared and visible light imaging, terahertz waves have a stronger ability to penetrate non-polar substances; Compared with X-rays, terahertz imaging has higher imaging contrast for materials with low dielectric constant, and its photon energy is much lower than that of X-rays, so it will not ionize materials and organisms, so it is safer. At present, terahertz imaging is mainly used in non-destructive testing, biomedicine and other fields.

At present, in the terahertz three-dimensional imaging technology, the relatively mature technology is terahertz computer-aided tomography (CT). In terahertz CT imaging, it is divided into continuous wave (CW) and pulse (TDS) imaging methods. Among them, THz-CW imaging system has been widely used because of its simple system, low cost, and fast imaging speed. . The quality of THz-CW imaging results largely depends on the design of the optical path device, so the optical path device of THz continuous wave tomography has become one of the research focuses. In order to meet the requirements of imaging resolution, the beam emitted by the terahertz continuous wave generator needs to be focused on the sample, and the beam focused on the sample is a thinner Gaussian beam, that is, the beam waist radius at the sample is smaller. However, so far, the optical path devices of most traditional THz-CT-CW imaging systems use no less than 4 mirrors or lenses, or a combination of the two. For example, the THz-CT-CW system in the document "Theoretical and Experimental Research on Terahertz Tomography" uses four lenses, and the schematic diagram of the optical path device structure is shown in Figure 1: firstly, the beam emitted by the terahertz continuous wave generator Collimate, then focus on the sample, collimate the beam after it passes through the sample, and finally focus on the terahertz continuous wave receiving device; the THz-CT-CW system proposed in the document "Investigation on reconstruction methods applied to 3D terahertz computedtomography" Two parabolic reflectors and two lenses are used, and the structural diagram of the optical path device is shown in Figure 2: the beam emitted by the terahertz continuous wave generator is collimated after being reflected by the parabolic reflector, and then focused on the sample through the lens, transmitted The beam after the sample is collimated by the lens, and finally focused to the terahertz continuous wave receiving device by the parabolic mirror. It can be seen from this that the optical path device of the THz-CT-CW system proposed in the prior art uses no less than four optical path elements. At this time, the realization of the optical path is relatively complicated, and when adjusting the position between the lenses will face great difficulties.

Therefore, how to design the optical path device so that it can achieve the optical path performance required by the THz-CT-CW system with fewer optical path components has become a research focus.

Contents of the invention

In view of the problems existing in the background technology, the purpose of the present invention is to provide an optical path device and a focusing method for terahertz continuous wave tomography. The optical path device only uses two off-axis ellipsoid mirrors, which can directly focus the emission beam emitted by the terahertz continuous wave generator to the sample, and then directly focus the focused beam transmitted through the sample to the terahertz continuous wave generator. Wave receiving device; two lenses or reflective surfaces required for the collimation process are omitted, the simplification of the optical path diagram of the traditional THz-CT-CW imaging system is realized, and the difficulty of adjusting the lens position is reduced.

To achieve the above object, the technical scheme of the present invention is as follows:

An optical path device for terahertz continuous wave tomography, including a three-dimensional translation stage 30, a rotary stage 31, and a left optical path and a right optical path arranged vertically symmetrically with respect to the rotary stage 31, and the axis of symmetry is the vertical center of the rotary stage 31 line, where the left optical path includes a terahertz continuous wave generator 1, a rectangular waveguide 2, a horn antenna 3, and the first off-axis ellipsoidal mirror 12, and the right optical path includes a second off-axis ellipsoidal mirror 12 and a terahertz continuous wave The receiving device 11; the rotating platform 31 is fixedly arranged on the three-dimensional translation platform 30, and the sample to be tested is placed in the center of the rotating platform 31;

The terahertz continuous wave generator 1 is used to generate the transmitting beam 5, and the transmitting beam 5 is transmitted to the reflecting surface 19 of the first off-axis ellipsoid mirror 12 after passing through the rectangular waveguide 2 and the horn antenna 3, and the first focused The beam I (7) is incident on the surface of the sample to be tested, and the first focused beam I (7) passing through the sample to be tested is reflected by the reflection surface 19 of the second off-axis ellipsoid mirror 12, and the second focused beam I (7) obtained after reflection is The beam II (10) is received by the terahertz continuous wave receiving device 11;

The minimum beam waist radius of the transmitting beam 5 and the second focused beam II (10) is w1, the minimum beam waist radius of the first focused beam I (7) w2, the minimum beam waist radius of the transmitting beam 5 is located to the first distance The distance of the reflection point 6 of the axis ellipsoid reflector 12 is d1, and the distance from the plane where the minimum beam waist radius of the first focused beam I (7) is to the first off-axis ellipsoid reflector 12 reflection point 6 is d2; The included angle between the major axis 15 of the ellipsoid of the axial ellipsoid reflector and the incident direction 4 of the emission beam 5, and the angle between the major axis 15 of the ellipsoid of the second off-axis ellipsoid reflector and the second focused beam II (10) The angle between the propagation directions is α; the angle between the propagation direction of the first focused beam I (7) and the incident direction 4 of the emission beam 5, and the propagation direction of the first focused beam I (7) and the first The included angles between the propagation directions of the two focused beams II (10) are both β; the phase center of the horn antenna 3 is located at the first focal point 22 of the ellipse 25 where the first off-axis ellipsoid reflector 12 is located, and the terahertz continuous wave The phase center of the receiving device 11 is located at the first focal point 22 of the ellipse 25 where the second off-axis ellipsoid reflector 12 is located; the phase center of the first focused beam I (7) is located at the ellipse where the first off-axis ellipsoid reflector 12 is located 25 at the second focal point 23; the depth of the reflection surface 19 of the two off-axis ellipsoid mirrors 12 is d3, the width is W, and the length L, and the ellipse 25 first focus 22 of the off-axis ellipsoid mirror 12 is reflected to the reflector The distance from the point 6 is R1, and the distance from the second focal point 23 to the reflection point 6 of the mirror is R2.

Further, the side lobe level of the horn antenna 3 is lower than the main lobe level by more than 20dB.

Further, the horn antenna 3 is a conical dual-mode horn antenna or a corrugated horn antenna, which ensures that the transmit beam 5 radiated by the horn antenna 3 is a Gaussian beam of higher quality.

Further, the off-axis ellipsoid reflector 12 is made of aluminum alloy, and the reflective surface 19 should be polished so that the first focused beam I (7) and the second focused beam II (10) are high-quality Gaussian beams .

Further, the reflection surface 19 of the off-axis ellipsoidal reflector 12 is a part of the spheroidal surface, and the overlapping part of the spheroidal surface and the off-axis ellipsoidal reflector 12 with a smaller area is selected as the off-axis ellipsoidal reflector The reflective surface 19 of 12; wherein, the ellipsoid of revolution is obtained by rotating the ellipse 25 around the major axis 15 of the ellipsoid for one revolution.

The present invention also provides a focusing method for the reflecting surface 19 of the off-axis ellipsoidal mirror 12, comprising the following steps:

S1. Determine the basic parameters of the optical path device according to actual needs, the basic parameters of the optical path device include: the minimum beam waist radius w1 (13) of the emission beam 5, the minimum beam waist radius w2 (16) of the first focused beam I (7), The distance d2 from the plane where the minimum beam waist radius of the first focused beam I is to the reflection point 6 of the first off-axis ellipsoid mirror, and the distance between the propagation direction of the first focused beam I (7) and the incident direction 4 of the transmitted beam 5 The included angle β;

S2. calculate the focal length f and the distance d1 (14) of the reflection surface 19 of any off-axis ellipsoid reflector 12, and the specific calculation formula is:

Wherein, λ ₀ is the vacuum wavelength of the terahertz wave;

S3. Calculate the ellipse 25 parameters of the off-axis ellipsoid mirror 12, and the specific calculation formula is:

a=(R ₁ +R ₂ )/2

Wherein i=1,2, a is the semi-major axis of the ellipse 25, b is the semi-minor axis of the ellipse 25, c is the semi-focal length of the ellipse 25, and α is the ellipse major axis 15 of the first off-axis ellipsoid reflector 12 and The included angle between the incident direction 4 of the transmitting beam 5, or the included angle between the long axis 15 of the ellipsoidal surface of the second off-axis ellipsoidal reflector 12 and the propagation direction of the second focused beam II (10); R1 (20 ) is the distance from the phase center of the horn antenna 3 to the reflection point 6 of the first off-axis ellipsoid mirror 12, or the distance from the phase center of the terahertz continuous wave receiving device 11 to the reflection point 6 of the second off-axis ellipsoid mirror 12 , R2(21) is the distance from the phase center of the first focused beam I(7) to the reflection point 6 of the first off-axis ellipsoid mirror 12.

Further, in step S1, in order to ensure that the transmit beam 5 radiated by the horn antenna 3 and the first focused beam I(7) focused on the center of the rotating table 31 have higher directivity, w1(13) and w2 (16) are greater than λ ₀ ; d2(17) needs to select a suitable value to ensure that the rotary table 31 and the sample can be placed between two off-axis ellipsoid mirrors 12; w2(16) needs to select a suitable value to ensure that The maximum radius R ₀ of any height section of the sample to be tested satisfies:

Further, in step S3, in order to ensure that most of the energy of the transmission beam 5 emitted by the horn antenna 3 is reflected by the reflection surface 19 of the off-axis ellipsoid reflector 12, the size of the reflection surface 19 of the off-axis ellipsoid reflector 12 needs to be appropriate Select, the coverage angle θ of its reflecting surface 19 (i.e. the angle at which the reflecting surface 19 covers the horn antenna) (26) should satisfy,

where A and B are constants.

Further, the value range of the included angle α(18) is preferably 35°-55°; the selected range of the included angle β is preferably 70°-110°, so that the reflection performance of the off-axis ellipsoid mirror 12 is better.

In summary, owing to adopting above-mentioned technical scheme, the beneficial effect of the present invention is:

The terahertz continuous wave three-dimensional tomographic optical path device provided by the present invention realizes direct focusing of the transmission beam emitted by the horn antenna to the sample, and then directly focuses the focused beam transmitted through the sample to the terahertz continuous wave receiving device; The device saves two lenses or reflective surfaces required in the collimation process of the traditional optical path device, thereby realizing the simplification of the optical path diagram of the traditional THz-CT-CW imaging system and reducing the difficulty of adjusting the lens position.

Description of drawings

Fig. 1 is a schematic diagram of a traditional terahertz continuous wave three-dimensional tomography optical path device, including four lenses.

Fig. 2 is a schematic diagram of a traditional terahertz continuous wave three-dimensional tomography optical path device, including 2 mirrors and 2 lenses.

Fig. 3 is a schematic diagram of an optical path device for terahertz continuous wave tomography of the present invention.

Fig. 4 is a schematic diagram of the left optical path device of the present invention.

Fig. 5 is a schematic diagram of the right optical path device of the present invention.

Fig. 6 is a schematic diagram of an ellipsoid corresponding to an off-axis ellipsoid reflector in the optical path device of the present invention.

Fig. 7 is a schematic diagram of an off-axis ellipsoidal reflector in the optical path device of the present invention.

Fig. 8 is a front view of the off-axis ellipsoid reflector in the optical path device of the present invention.

Fig. 9 is a top view of the off-axis ellipsoid reflector in the optical path device of the present invention.

Fig. 10 is a left side view of the off-axis ellipsoid mirror in the optical path device of the present invention.

Fig. 11 is a diagram of FEKO simulation results of the optical path device for terahertz continuous wave tomography of the present invention.

Fig. 12 is a diagram of the imaging target and results of the THz-CT-CW system based on the optical path device provided by the present invention.

In the figure, 1 is the terahertz continuous wave generator, 2 is the rectangular waveguide, 3 is the horn antenna, 4 is the incident direction, 5 is the transmitting beam, 6 is the reflection point, 7 is the focused beam I, 8 is the reflection direction I, 9 is the reflection direction II, 10 is the focused beam II, 11 is the terahertz continuous wave receiving device, 12 is the off-axis ellipsoid mirror, 13 is the minimum beam waist radius w1 of the transmitted beam 5 or the focused beam II (10), and 14 is The distance d1 from the plane where w1(13) is located to the reflection point 6, 15 is the long axis of the ellipsoid, 16 is the minimum beam waist radius w2 of the focused beam I(7), and 17 is the distance from the plane where w2(16) is located to the reflection point 6 d2, 18 is the angle α between the long axis 15 of the ellipsoid and the incident direction 4 or the reflection direction II (9), 19 is the reflection surface of the off-axis ellipsoid mirror 12, and 20 is the focus 22 of the ellipse 25 to the reflection point The distance R1 of 6, 21 is the distance R2 from the focal point 23 of the ellipse 25 to the reflection point 6, 22 and 23 are the focal points of the ellipse 25, 24 is the minor axis of the ellipse 25, 25 is the ellipse, and 26 is the coverage angle θ of the reflective surface 19 , 27 is the length L of the off-axis ellipsoidal mirror 12, 28 is the depth d3 of the reflecting surface 19, 29 is the width W of the off-axis ellipsoidal mirror 12, 30 is a three-dimensional translation stage, 31 is a rotating stage, and 32 is a focusing Lens, 33 is a parabolic reflector, and 34 is a tangential plane of an ellipsoid at reflection point 6.

detailed description

In order to make the purpose, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the implementation methods and accompanying drawings.

An optical path device for terahertz continuous wave tomography, its structural schematic diagram is shown in Figure 3, including a three-dimensional translation stage 30, a rotating stage 31, and a left optical path and a right optical path arranged vertically symmetrically with respect to the rotating stage 31, wherein , the left optical path includes a terahertz continuous wave generator 1, a rectangular waveguide 2, a horn antenna 3, and a first off-axis ellipsoidal reflector 12, and the right optical path includes a second off-axis ellipsoidal reflector 12 and a terahertz continuous wave receiving device 11 The rotary table 31 is fixedly arranged on the three-dimensional translation table 30, and the sample to be tested is placed in the center of the rotary table 31;

The terahertz continuous wave generator 1 is used to generate the transmitting beam 5, and the transmitting beam 5 is transmitted to the reflecting surface 19 of the first off-axis ellipsoid mirror 12 after passing through the rectangular waveguide 2 and the horn antenna 3, and the first focused The beam I (7) is incident on the surface of the sample to be tested, and the first focused beam I (7) passing through the sample to be tested is reflected by the reflection surface 19 of the second off-axis ellipsoid mirror 12, and the second focused beam I (7) obtained after reflection is The beam II (10) is received by the terahertz continuous wave receiving device 11;

The minimum beam waist radius of the transmitting beam 5 and the second focused beam II (10) is w1, the minimum beam waist radius of the first focused beam I (7) w2, the minimum beam waist radius of the transmitting beam 5 is located to the first distance The distance of the reflection point 6 of the axial ellipsoidal reflector is d1, and the distance from the plane where the minimum beam waist radius of the first focused beam I (7) is to the first off-axis ellipsoidal reflector reflection point 6 is d2; the first off-axis ellipsoidal reflector Angle between the major axis 15 of the ellipsoid of the spherical reflector and the incident direction 4 of the launch beam 5, and the major axis 15 of the ellipsoid of the second off-axis ellipsoid reflector and the direction of propagation of the second focused beam II (10) The included angles between are all α; the included angle between the propagation direction of the first focused beam I(7) and the incident direction 4 of the emission beam 5, and the propagation direction of the first focused beam I(7) and the second focused The included angles between the propagation directions of the beam II (10) are both β; the phase center of the horn antenna 3 is located at the first focal point 22 of the ellipse where the first off-axis ellipsoid mirror 12 is located, and the terahertz continuous wave receiving device The phase center of 11 is located at the first focal point 22 of the ellipse where the second off-axis ellipsoidal mirror 12 is located, and the phase center of the first focused beam I (7) is located at the 25th ellipse where the first off-axis ellipsoidal mirror 12 is located Two focal points 23 places; The center of described rotary table 31 is positioned on the first reflection direction I (8, promptly the propagating direction of the first focused beam), and the distance from the reflection point 6 of two off-axis ellipsoid mirrors 12 is equal It is d2(17); the reflection surface depth of two off-axis ellipsoid reflectors is d3, the width is W, the length L, the distance from the first focal point 22 of the ellipse 25 to the reflection point 6 is R1, and the distance from the second focal point 23 of the ellipse 25 to The distance of the reflection point 6 is R2.

Fig. 4 and Fig. 5 are respectively schematic diagrams of the left optical path device and the right optical path device of the optical path device of the present invention. As shown in Figures 4 and 5, the major axis 15 of the ellipsoidal surface of the off-axis ellipsoidal reflector 12 forms an included angle α (15) with the incident direction 4 of the emission beam 5 and the reflection direction II (9); the reflection direction I (8) β with the incident direction 4 and the reflection direction II(9). Fig. 6 is a schematic diagram of an ellipsoid corresponding to an off-axis ellipsoid reflector in the optical path device of the present invention. As shown in Figure 6, the phase center of the horn antenna 3 is located at the ellipsoid focal point 22 of the first off-axis ellipsoid reflector 12, and the plane where the minimum beam waist radius w1 of the emission beam 5 is reflected to the first off-axis ellipsoid reflector The distance d1 of point 6 is not equal to the distance R1 from the phase center of horn antenna 3 to the reflection point 6 of the first off-axis ellipsoid mirror; the plane where the minimum beam waist radius w2 of the first focused beam I (7) is to the first The distance d2 of the reflection point 6 of an off-axis ellipsoid mirror is not equal to the distance R2 from the second focus 23 of the ellipse 25 where the first off-axis ellipsoid mirror is located to the reflection point 6 .

7 is a schematic diagram of the off-axis ellipsoid reflector in the optical path device of the present invention. It can be seen from the figure that the reflection surface 19 of the off-axis ellipsoid reflector 12 and the coverage of the terahertz continuous wave generator 1 . In the figure, the propagation direction of the electromagnetic wave radiated by the horn antenna is the axis x+, and the energy is the strongest at the x-axis, and the farther away from the x-axis, the weaker the energy is. The coverage angle of the reflecting surface 19 of the spherical mirror 12 is θ. The phase centers of the horn antenna 3 and the terahertz continuous wave receiving device 11 are respectively located at the focal points 22 of the two off-axis ellipsoidal mirrors 12, and the center of the rotating table 31 is located in the reflection direction I(8), and is away from the two off-axis ellipsoidal mirrors 12. The distance of the reflection point 6 of the apoxipheric mirror 12 is d2(17).

Fig. 8, Fig. 9 and Fig. 10 are respectively the front view, top view and left side view of the off-axis ellipsoid mirror in the optical path device of the present invention. The reflective surface 19 of the off-axis ellipsoid reflector 12 is a part of a spheroid; The reflection surface 19 of the off-axis ellipsoid mirror 12 is obtained by cutting the plane of the tangent plane 34 at the reflection point 6 to the spheroid with a smaller area.

Example 1

The focusing method of the reflective surface 19 of the off-axis ellipsoid mirror 12 in the above-mentioned optical path device comprises the following steps:

S1. Determine the basic parameters of the optical path device according to actual needs. The basic parameters of the optical path device include: the minimum beam waist radius w1 (13) of the emission beam 5, the minimum beam I (7) focused on the center of the rotary table 31 The beam waist radius w2(16), the distance d2(17) from the plane where the minimum beam waist radius of the first focused beam I is to the reflection point 6 of the first off-axis ellipsoid mirror, and the propagation of the first focused beam I(7) The angle β between the direction (reflection direction I(8)) and the propagation direction of the second focused beam II(10) (reflection direction II(9)); in this example, the working of the THz continuous wave generator 1 The frequency is 110GHz, the corresponding wavelength λ ₀ =2.73mm, set w1(13)=7mm, w2(16)=6mm, d2(17)=170mm, β=90°, and the maximum radius of the imaging target is 40mm;

Then the maximum radius R0 of the cross-section at any height of the sample (the cross-section is parallel to _xoy ) should be:

The sample of this size is within the Rayleigh range, and the focused beam I(7) is approximately a plane wave, which satisfies the basic conditions of CT imaging;

S2. According to the following formula, find the distance d1(14) from the focal length f and w1(13) of the reflection surface 19 to the reflection point 6:

In the present embodiment f=97.47, d1=196.19mm;

S3. Determine the parameters of the ellipse 25: the phase center of the horn antenna 3 or the terahertz continuous wave receiving device 11 is located at the focus 22 of the ellipse 25, and the phase center of the focused beam I (7) is located at the focus 23 of the ellipse 25; therefore, the ellipse 25 The distance R1 (20) between the focal point 22 and the reflection point 6, the distance R2 (21) between the focus 23 of the ellipse 25 and the reflection point 6, the semi-major axis a of the ellipse 25, the semi-minor axis b, the semi-focal length c and the length of the ellipsoid The angle α between the axis 15 and the incident direction 4 or the reflection direction II(9) can be expressed by the relational formula:

a=(R ₁ +R ₂ )/2(5)

where i=1,2;

It can be obtained that R1=212.43mm, R2=180.12mm, α=40.3°, a=196.27mm, b=138.31mm, c=139.26mm.

Thus, the ellipsoid can be obtained by rotating the ellipse 25 along the long axis 15 of the ellipsoid for one revolution. After determining the parameters of the ellipsoidal surface, intercept the smaller overlapping area of the ellipsoidal surface and the off-axis ellipsoidal reflector as the reflective surface 19 of the off-axis ellipsoidal reflector 12 . As shown in Figures 7, 8, 9 and 10, in the present embodiment, in order to ensure that most of the energy of the transmitted beam 5 emitted by the horn antenna 3 is reflected by the reflection surface 19 of the off-axis ellipsoid reflector 12, the off-axis ellipsoid The length L (27)=210mm of reflecting mirror 12, the width W (29)=130mm of off-axis ellipsoid reflecting mirror 12, the depth d3(28)=17mm of reflecting surface 19; Then the coverage angle θ (26mm) of reflecting surface 19 )≈16.4°, take A=2,

at this time,

Therefore, the coverage angle θ(26) of the reflective surface 19 meets the requirements.

Applying the above-mentioned optical path device to the THz-CT-CW imaging system, the FEKO simulation result of the optical path is shown in Figure 11. It can be seen that the optical path shown in Figure 3 is successfully realized, and the number of optical elements used is less than that in Figure 1 and Figure 2 Optical components used. The sample to be tested is imaged using the traditional THz-CT-CW imaging method. Figure 12(a) is a schematic diagram of the sample to be tested, and Figure 12(b) is the result of reconstructing the collected cross-sectional data using the filtered back projection algorithm From the figure, it can be seen that based on the optical path device of the present invention, the THz-CT-CW imaging system can reconstruct the contours of the edge of the cup and the screw.

The above is only a specific embodiment of the present invention. Any feature disclosed in this specification, unless specifically stated, can be replaced by other equivalent or alternative features with similar purposes; all the disclosed features, or All method or process steps may be combined in any way, except for mutually exclusive features and/or steps.

Claims (9)

1. The light path device for terahertz continuous wave tomography is characterized by comprising a three-dimensional translation table, a rotary table, a left light path and a right light path, wherein the left light path and the right light path are vertically and symmetrically arranged relative to the rotary table, a symmetry axis is a vertical central line of the rotary table, the left light path comprises a terahertz continuous wave generator, a rectangular waveguide, a horn antenna and a first off-axis ellipsoidal reflector, and the right light path comprises a second off-axis ellipsoidal reflector and a terahertz continuous wave receiving device; the rotating table is fixedly arranged on the three-dimensional translation table, and a sample to be detected is placed in the center of the rotating table;

the terahertz continuous wave generator is used for generating a transmitting wave beam, the transmitting wave beam is transmitted to the reflecting surface of the first off-axis ellipsoidal reflector after passing through the rectangular waveguide and the horn antenna, a first focusing wave beam obtained after reflection is incident to the surface of a sample to be detected, the first focusing wave beam penetrating through the sample to be detected is reflected by the reflecting surface of the second off-axis ellipsoidal reflector, and a second focusing wave beam Jiao Boshu obtained after reflection is received by the terahertz continuous wave receiving device;

the minimum beam waist radius of the transmitting beam and the minimum beam waist radius of the second focusing Jiao Boshu are both w1, the minimum beam waist radius w2 of the first focusing beam, the distance from the plane where the minimum beam waist radius of the transmitting beam is located to the reflection point of the first off-axis ellipsoidal mirror is d1, and the distance from the plane where the minimum beam waist radius of the first focusing beam is located to the reflection point of the first off-axis ellipsoidal mirror is d2; the included angle between the ellipsoidal long axis of the first off-axis ellipsoidal reflector and the incident direction of the emission beam and the included angle between the ellipsoidal long axis of the second off-axis ellipsoidal reflector and the propagation direction of the second focusing Jiao Boshu are both alpha; the included angle between the propagation direction of the first focused beam and the incident direction of the transmitted beam, and the included angle between the propagation direction of the first focused beam and the propagation direction of the second focused beam Jiao Boshu are both beta; the phase center of the horn antenna is located at a first focus of an ellipse where the first off-axis ellipsoidal reflector is located, and the phase center of the terahertz continuous wave receiving device is located at a first focus of an ellipse where the second off-axis ellipsoidal reflector is located; the phase center of the first focusing beam is positioned at the second focus of the ellipse where the first off-axis ellipsoidal reflector is positioned; the depth of the reflecting surfaces of the two off-axis ellipsoidal reflectors is d3, the width is W, the length is L, the distance from the first focus of the ellipse where the off-axis ellipsoidal reflector is located to the reflecting point of the reflector is R1, and the distance from the second focus to the reflecting point of the reflector is R2.

2. The optical circuit device according to claim 1, wherein the side lobe level of the horn antenna is smaller than the main lobe level by more than 20 dB.

3. The optical circuit apparatus as claimed in claim 1, wherein the horn antenna is a conical dual mode horn antenna or a corrugated horn antenna.

4. The optical circuit apparatus of claim 1, wherein the off-axis ellipsoidal mirror is fabricated from aluminum alloy and the reflective surface is polished such that the first focused beam and the second focused beam Jiao Boshu are high quality gaussian beams.

5. The optical circuit device according to claim 1, wherein the reflecting surface of the off-axis ellipsoidal mirror is a part of a rotating ellipsoid, and an overlapped part with a smaller area of the rotating ellipsoid and the off-axis ellipsoidal mirror is selected as the reflecting surface of the off-axis ellipsoidal mirror; the rotating ellipsoid is obtained by rotating an ellipse around the major axis of the ellipsoid for a circle.

6. The method for focusing the reflecting surface of an off-axis ellipsoidal mirror in an optical path device according to claim 1, comprising the steps of:

s1, determining basic parameters of a light path device according to actual needs, wherein the basic parameters of the light path device comprise: the minimum beam waist radius w1 of the transmitted beam, the minimum beam waist radius w2 of the first focused beam, the distance d2 from the plane where the minimum beam waist radius of the first focused beam is located to the reflection point of the first off-axis ellipsoidal mirror, and the included angle beta between the propagation direction of the first focused beam and the incident direction of the transmitted beam;

s2, calculating to obtain the focal length f and the distance d1 of the reflecting surface of any off-axis ellipsoidal reflector, wherein the specific calculation formula is as follows:

wherein λ is ₀ Is the vacuum wavelength of the terahertz wave;

s3, calculating to obtain the ellipse parameters of the off-axis ellipsoidal reflector, wherein the specific calculation formula is as follows:

a＝(R ₁ +R ₂ )/2

wherein i =1,2, a is the semi-major axis of the ellipse, b is the semi-minor axis of the ellipse, c is the semi-focal length of the ellipse, and α is the angle between the major axis of the ellipsoid of the first off-axis ellipsoidal mirror and the incident direction of the transmit beam, or the angle between the major axis of the ellipsoid of the second off-axis ellipsoidal mirror and the propagation direction of the second focus Jiao Boshu; r1 is the distance from the phase center of the horn antenna to the reflection point of the first off-axis ellipsoidal reflector, or the distance from the phase center of the terahertz continuous wave receiving device to the reflection point 6 of the second off-axis ellipsoidal reflector, and R2 is the distance from the phase center of the first focused beam to the reflection point of the first off-axis ellipsoidal reflector.

7. The focusing method of the reflection surface according to claim 6, wherein in step S1, in order to ensure that the radiation beam radiated from the horn antenna and the first focused beam focused at the center of the turntable have high directivity, w1 and w2 are both larger than λ ₀ (ii) a d2 needs to be chosen to be of a suitable value to ensure that the turntable and the sample can be placed between the two off-axis ellipsoidal mirrors; w2 needs to be selected to be a proper value so as to ensure the maximum radius R of any height section of the sample to be measured ₀ Satisfies the following conditions:

8. the focusing method of the reflective surface of claim 6, wherein in step S3, in order to ensure that most of the energy of the transmitting beam emitted from the horn antenna is reflected by the reflective surface of the off-axis ellipsoidal mirror, the size of the reflective surface of the off-axis ellipsoidal mirror is properly selected, and the coverage angle θ of the reflective surface is satisfied

Wherein A and B are constants.

9. The focusing method of a reflecting surface according to claim 6, wherein the included angle α is in the range of 35 ° to 55 °; the selection range of the included angle beta is 70-110 degrees.

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