Terahertz wide-spectrum active imaging optical system

Abstract

The invention provides a terahertz wide-spectrum active imaging optical system. The terahertz wide-spectrum active imaging optical system comprises a terahertz light source, an irradiation light path system, a receiving light path system and a detector, the terahertz light source is used for emitting terahertz waves; the irradiation light path system is used for receiving the terahertz waves and irradiating the terahertz waves on the surface of an object after collimation and beam expansion; and the receiving light path system is used for receiving the terahertz waves reflected by the surface of the object and focusing and imaging the terahertz waves on the detector. According to the invention, the problems of low imaging speed and small imaging range caused by diffraction limit and point-by-point scanning in the traditional terahertz imaging technology can be effectively solved.

Description

Technical Field

[0001] The invention belongs to the technical field of optical reflection imaging and relates to a terahertz wide-spectrum active imaging optical system. Background Art

[0002] Terahertz (THz) waves, located between microwaves and infrared light in the electromagnetic spectrum, are in the transition stage from macroscopic electronics to microscopic photonics. It is recognized that the THz wave range extends from about 100 GHz to 20 THz. Waves in this range have both radio frequency and optical properties. In particular, they can be sent and received by antennas and can be focused by optical systems such as silicon lenses and metal mirrors. Terahertz waves have unique properties such as high penetration, low energy, and fingerprint spectroscopy. These properties make THz imaging technology have potential application value in multidisciplinary frontier fields such as national security, intelligent communications, and biomedicine.

[0003] Traditional terahertz imaging technology is generally point-by-point scanning imaging, with slow imaging speed and limited by the diffraction limit, with only sub-millimeter spatial resolution; the current imaging technology that breaks through the diffraction limit mainly uses near-field scanning imaging technology, which mainly includes near-field hole scanning technology and near-field probe scanning technology.

[0004] However, near-field imaging technology also achieves imaging by scanning the target object pixel by pixel, which has the disadvantages of long imaging time and small imaging area. In addition, this technology also has high requirements for terahertz light sources and target objects, low imaging efficiency, low detection sensitivity, and certain limitations in adapting to scenarios.

[0005] Therefore, how to provide a terahertz wide spectrum active imaging optical system with high imaging efficiency is a problem that those skilled in the art need to solve urgently. Summary of the invention

[0006] In view of this, the present invention proposes a terahertz wide spectrum active imaging optical system to solve the problems of low resolution, slow imaging speed and low efficiency caused by the diffraction limit and point-by-point scanning of traditional terahertz imaging technology.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] The present invention discloses a terahertz wide spectrum active imaging optical system, comprising: a terahertz light source, an irradiation optical path system, a receiving optical path system and a detector; wherein:

[0009] The terahertz light source is used to emit terahertz waves;

[0010] The irradiation optical path system is used to receive the terahertz wave, and irradiate the terahertz wave onto the surface of the object to be imaged after collimating and expanding the terahertz wave;

[0011] The receiving optical path system is used to receive the terahertz wave reflected by the surface of the object to be imaged, and focus the image on the detector.

[0012] Preferably, the illumination optical path system comprises an off-axis ellipsoidal reflector M1 and an off-axis parabolic reflector M2 which are sequentially arranged along the propagation direction of the terahertz wave;

[0013] The terahertz light source is located at the first focus of the off-axis ellipsoidal reflector M1, and the second focus of the off-axis ellipsoidal reflector M1 coincides with the focus of the off-axis parabolic reflector M2; the output light of the off-axis parabolic reflector M2 is parallel light parallel to the main axis of the parabola.

[0014] Preferably, the parameters of the off-axis ellipsoidal reflector M1 are: a curvature radius of -43.6364 mm, a cone coefficient of -0.2066, a diameter of 40 mm, and an eccentricity of 20 mm.

[0015] Preferably, the parameters of the off-axis parabolic reflector M2 are: a curvature radius of -420 mm, a cone coefficient of -1, a diameter of 120 mm, and an eccentricity of 90 mm.

[0016] Preferably, the receiving optical path system is an off-axis three-reflector system; the off-axis three-reflector system comprises a primary reflector M3, a secondary reflector M4 and a third reflector M5 which are sequentially arranged along the propagation direction of the terahertz wave;

[0017] The primary reflector M3 and the secondary reflector M4 have positive optical focal lengths, and the tertiary reflector M5 has negative optical focal lengths; the secondary reflector M4 is located at the focal planes of the primary reflector M3 and the tertiary reflector M5 at the same time;

[0018] The position setting of the off-axis three-mirror system satisfies that the main light of the terahertz light source propagates along the path formed by the line connecting the surface vertices of the primary reflector M3, the secondary reflector M4 and the third reflector M5.

[0019] Preferably, the vertex curvature radius of the primary reflector M3, the secondary reflector M4 and the tertiary reflector M5 satisfies the Pittsburgh condition:

[0020]

[0021] Among them, R1, R2 and R3 are the radii of the primary reflector M3, the secondary reflector M4 and the tertiary reflector M5 respectively.

[0022] Preferably, the reflector surface equations of the secondary reflector M4 and the tertiary reflector M5 are respectively obtained by adding even-order polynomial correction terms to the corresponding standard surface equations.

[0023] Preferably, the parameters of the main reflector M3 are: a curvature radius of -613.8618 mm, a cone coefficient of 0.2133, a diameter of 100 mm, and an eccentricity of 88 mm.

[0024] Preferably, the parameters of the secondary reflector M4 are: the radius of curvature is 176.3965 mm, the cone coefficient is 4.8874, the 4th order term of the even-order polynomial is -1.4587E-08, the 6th order term of the even-order polynomial is 7.7865E-12, the 8th order term of the even-order polynomial is 4.8097E-16, and the diameter is 61 mm.

[0025] Preferably, the parameters of the three reflectors M5 are: curvature radius of -177.1562 mm, cone coefficient of 0.2675, 4th order term of even-order polynomial of -2.8704E-09, 6th order term of even-order polynomial of 1.1727E-13, 8th order term of even-order polynomial of -5.2644E-18, diameter of 120 mm, and eccentricity of -78 mm.

[0026] Preferably, the propagation light paths among the primary reflector M3, the secondary reflector M4 and the tertiary reflector M5 are not blocked by each other.

[0027] Preferably, it also includes a sample stage for placing the object to be imaged and making the object to be imaged be located within the irradiation range of the light emitted by the irradiation light path system, and having a displacement adjustment structure so that the object to be imaged can change its posture.

[0028] Preferably, the terahertz wide spectrum active imaging optical system is used to achieve active imaging in an area ranging from 100*100 mm.

[0029] It can be seen from the above technical solution that, compared with the prior art, the beneficial effects of the present invention include:

[0030] 1. The collimated light source using a total reflection element in the present invention has significant advantages. Specifically, since no transmission element is included, the total reflection element will not produce transmission loss during the light energy transmission process. This feature greatly improves the light energy utilization rate. At the same time, the total reflection element can also achieve a more uniform light intensity distribution, avoiding the phenomenon of uneven light spots or alternating light and dark, thereby ensuring the high uniformity of the light source.

[0031] 2. The receiving system has the advanced ability to achieve large-scale imaging, which enables it to significantly surpass the limitations of the currently widely used point imaging or line imaging technology. The receiving optical path system can capture and integrate image information over a larger range to generate a more complete, detailed and detailed image. This breakthrough not only broadens the application field of imaging systems, but also provides more powerful and flexible imaging solutions for scientific research, industrial inspection, medical imaging and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below. Obviously, the drawings in the following description are only embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on the provided drawings without creative work.

[0033] Figure 1 A schematic diagram of the optical path of a terahertz wide spectrum active imaging optical system provided by an embodiment of the present invention;

[0034] Figure 2 A structural diagram of a terahertz wide spectrum active imaging optical system provided by an embodiment of the present invention;

[0035] Figure 3 Cross sections of emitted light at different positions of the irradiation optical path system of the terahertz imaging system provided in an embodiment of the present invention Figure 1 (Use the horizontal bromine tungsten lamp filament as the point light source);

[0036] Figure 4 Cross sections of emitted light at different positions of the irradiation optical path system of the terahertz imaging system provided in an embodiment of the present invention Figure 2 (Use the longitudinal bromine tungsten lamp filament as the point light source);

[0037] Figure 5 Field curvature / distortion curve diagram of the coaxial three-mirror system provided by the embodiment of the present invention

[0038] Figure 6 A modulation transfer function curve diagram of a receiving optical path system of a terahertz imaging system provided by an embodiment of the present invention at an image plane;

[0039] Figure 7 A point diagram of each field of view of a receiving optical path system of a terahertz imaging system provided by an embodiment of the present invention on an image plane;

[0040] Figure 8 A wavefront diagram of the central field of view of the receiving optical path system of the terahertz imaging system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0041] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] like Figure 1 As shown, an embodiment of the present invention provides a terahertz wide-spectrum active imaging optical system, comprising: a terahertz light source 1, an irradiation optical path system, a receiving optical path system and a detector 3; the terahertz light source 1 is used to emit terahertz waves; the irradiation optical path system is used to receive the terahertz waves, and collimate and expand the terahertz waves and then irradiate them on the surface of the object to be imaged 2; the receiving optical path system is used to receive the terahertz waves reflected by the surface of the object to be imaged 2, and focus the images on the detector 3.

[0043] like Figure 2 As shown, in the illumination optical path system, L1 is the distance between the light source and M1, and L2 is the distance between M1 and M2; in the receiving optical path system, L3 is the distance between the lower edge light of M3 and the upper edge light of M4 in the direction perpendicular to the optical axis when avoiding obstruction, L4 is the distance between the lower edge light of M3 and the upper edge light of M5 in the direction perpendicular to the optical axis when avoiding obstruction, and L5 is the distance between the lower edge light of M4 and the upper edge light of M5 in the direction perpendicular to the optical axis when avoiding obstruction. The focal length of the illumination optical path system is -78.75mm, which can collimate a point light source with a divergence angle of no more than 30° into a parallel beam with a diameter of 100mm, that is, the terahertz wide spectrum active imaging optical system is used to realize active imaging of a 100*100mm area.

[0044] In one embodiment, the illumination optical path system includes an off-axis ellipsoidal reflector M1 and an off-axis parabolic reflector M2 which are sequentially arranged along the propagation direction of the terahertz wave;

[0045] The terahertz light source 1 is located at the first focus of the off-axis ellipsoidal reflector M1, and the second focus of the off-axis ellipsoidal reflector M1 coincides with the focus of the off-axis parabolic reflector M2; the outgoing light of the off-axis parabolic reflector M2 is parallel light parallel to the main axis of the parabola.

[0046] The light emitted by the terahertz light source 1 is focused on the focus of the off-axis parabolic reflector M2 through the off-axis ellipsoidal reflector M1, and the off-axis parabolic reflector M2 collimates the light passing through its own focus into parallel light for emission.

[0047] In this embodiment, the parameters of the off-axis ellipsoidal reflector M1 are: a curvature radius of -43.6364 mm, a cone coefficient of -0.2066, a diameter of 40 mm, and an eccentricity of 20 mm.

[0048] In this embodiment, the parameters of the off-axis parabolic reflector M2 are: a curvature radius of -420 mm, a cone coefficient of -1, a diameter of 120 mm, and an eccentricity of 90 mm.

[0049] In this embodiment, the illumination optical path system is a total reflection system, which reduces the loss of light energy. In order to avoid uneven illumination caused by eccentric elements, a combination of two eccentric elements is selected to collimate the light source. The system is off-axis to avoid obstruction; the optical path of the system is that the main light propagates along the line connecting the vertices of each element, the aperture is on the M1 off-axis ellipsoidal reflector, the light source is placed at one focus of M1, and the other focus coincides with the focus of the M2 off-axis parabolic reflector. After reflection, it will be emitted in a direction parallel to the main axis of the parabola. The two reflectors are optimized and designed by optical design software, and the specific structural parameters and polynomial coefficients of the system are listed in Table 1. The system structure size is approximately 315mm (z) × 209mm (y) × 120mm (x).

[0050] Table 1 System structure parameters and coefficients

[0051] parameter M1 M2 Radius(mm) -43.6364 -420 Interval(mm) 290 100 Semi-diameter (mm) 20 60 Tilt (°) 40 0 Eccentricity(mm) 20 90 Cone coefficient -0.2066 -1

[0052] Through simulation and observation of the outgoing beam, a bromine tungsten lamp is used as the terahertz source. Considering that the filament has two types, horizontal and vertical, it needs to be set separately. The z coordinate of the position of the filament light source is defined as 0, and the surface detector 3 is set at 0mm, 100mm, 500mm, 1000mm, 2000mm, and 2500mm of the outgoing beam.

[0053] like Figure 3 As shown, (a)-(f) are the cross-sectional diagrams of the emitted light when the z coordinate value is 0mm, 100mm, 500mm, 1000mm, 2000mm, and 2500mm respectively when the transverse bromine tungsten lamp filament is used as the point light source and the light beam is collimated by M2. At 100mm, the RMSX radius of the beam is 26.5374mm, the RMS Y radius is 26.8624, and at 2500mm, the RMS X radius is 35.1582mm, and the RMS Y radius is 38.2222mm. Calculation shows that the divergence angle in the x direction is 0.206°, and the divergence angle in the y direction is 0.279°.

[0054] like Figure 4As shown, (a)-(f) are the cross-sectional diagrams of the emitted light when the z coordinate value is 0mm, 100mm, 500mm, 1000mm, 2000mm, and 2500mm respectively when the longitudinal bromine tungsten lamp filament is used as the point light source and the light beam is collimated by M2. At 100mm, the RMSX radius of the beam is 27.0417mm, the RMS Y radius is 26.3866mm, at 2500mm, the RMS X radius is 38.2433mm, and the RMSY radius is 34.8420mm. Calculation shows that the divergence angle in the x direction is 0.267°, and the divergence angle in the y direction is 0.202°.

[0055] In one embodiment, the receiving optical path system is an off-axis three-mirror system; the off-axis three-mirror system includes a primary mirror M3, a secondary mirror M4 and a third mirror M5 which are sequentially arranged along the propagation direction of the terahertz wave;

[0056] The primary reflector M3 and the secondary reflector M4 have positive optical focal powers, and the tertiary reflector M5 has negative optical focal powers, that is, the optical focal powers of the primary reflector M3, the secondary reflector M4 and the tertiary reflector M5 are concave, convex and concave respectively; the secondary reflector M4 is located at the focal plane of the primary reflector M3 and the tertiary reflector M5 at the same time;

[0057] The position setting of the off-axis three-mirror system satisfies that the main light of the terahertz light source 1 propagates along the path formed by the line connecting the surface vertices of the primary reflector M3, the secondary reflector M4 and the third reflector M5.

[0058] In this embodiment, the aperture is placed on the secondary reflector M4 , and a polarizing plate 4 is arranged on the optical path between the tertiary reflector M5 and the detector 3 .

[0059] In this embodiment, the F number of the receiving system is 1.22, the focal length is 122.4 mm, and the field of view is 1.909°×2.545°.

[0060] In this embodiment, the vertex curvature radius of the primary reflector M3, the secondary reflector M4 and the tertiary reflector M5 meets the Pittsburgh condition:

[0061]

[0062] Among them, R1, R2 and R3 are the radii of the primary reflector M3, the secondary reflector M4 and the tertiary reflector M5 respectively, and the error range of the approximate Petzman condition is usually ±10 -4 (1 / mm).

[0063] In this embodiment, the reflector surface equations of the secondary reflector M4 and the tertiary reflector M5 are respectively obtained by adding even-order polynomial correction terms to the corresponding standard surface equations.

[0064] In this embodiment, the parameters of the main reflector M3 are: a curvature radius of -613.8618 mm, a cone coefficient of 0.2133, a diameter of 100 mm, and an eccentricity of 88 mm.

[0065] In this embodiment, the parameters of the secondary reflector M4 are: the radius of curvature is 176.3965 mm, the cone coefficient is 4.8874, the 4th order term of the even-order polynomial is -1.4587E-08, the 6th order term of the even-order polynomial is 7.7865E-12, the 8th order term of the even-order polynomial is 4.8097E-16, and the diameter is 61 mm.

[0066] In this embodiment, the parameters of the three reflectors M5 are: the radius of curvature is -177.1562 mm, the cone coefficient is 0.2675, the 4th order term of the even-order polynomial is -2.8704E-09, the 6th order term of the even-order polynomial is 1.1727E-13, the 8th order term of the even-order polynomial is -5.2644E-18, the diameter is 120 mm, and the eccentricity is -78 mm.

[0067] In this embodiment, the distance from the image plane to the secondary reflector M4 (rear working distance) is 60mm, and the system structure size is about 143mm(z)×273mm(y)×120mm(x). L3, L4, and L5 are 9mm, 56mm, and 24mm respectively. The radius of the primary mirror, the secondary mirror, and the third mirror The surface shape of the three reflectors is optimized by optical design software, and the specific structural parameters and polynomial coefficients of the obtained system are listed in Table 2.

[0068] Table 2 System structure parameters and coefficients

[0069]

[0070]

[0071] Figure 5 This is a curve diagram of the field curvature and distortion of the optical system of this embodiment. It can be seen from the figure that the field curvature and distortion of the system are very small, wherein the distortion value is 0.0473%, which will have a slight impact on the image quality. However, in the optical system, when the distortion value is less than 2%, the image quality is generally considered to be good. In addition, the distortion existing in the system can also be corrected through subsequent image processing.

[0072] In this embodiment, the propagation light paths among the primary reflector M3, the secondary reflector M4 and the tertiary reflector M5 are not blocked by each other.

[0073] In one embodiment, a sample stage is also included for placing the object 2 to be imaged and making the object 2 to be imaged be located within the irradiation range of the light emitted by the irradiation light path system, and having a displacement adjustment structure to change the posture of the object to be imaged.

[0074] Figure 6 is the modulation transfer function curve of the optical system of this embodiment at the image plane. Since the element is a free-form surface, multiple viewing points should be selected for evaluation. Figure 6 It can be seen that the MTF curves of each field of view are close to the diffraction limit curve of the system, and the imaging quality is close to the diffraction limit. Since the terahertz wavelength is too large, the pixel size of some terahertz imaging detectors is too small, resulting in the inability to meet the calculated Nyquist frequency, and Rayleigh judgment can also be used.

[0075] Figure 7 This is the point diagram of each field of view of the optical system of this embodiment on the image plane. The circle in the figure represents the Airy disk radius of 50.81μm. The maximum RMS value of the full field point diagram is 41.30μm, which is much smaller than the Airy disk radius. The imaging quality of the system is close to the diffraction limit.

[0076] Figure 8 is the wavefront diagram of the optical system of this embodiment at the image plane. The maximum wavefront aberration between the actual wavefront and the ideal wavefront does not exceed λ / 4, and the optical system has no aberration according to Rayleigh judgment. The wavefront aberration of each field of view is shown in the following table.

[0077] Table 3 Wave aberration of each field of view on the image plane

[0078] Field of view (mm) Wave aberration (-47，35) 0.0436 (0，50) 0.0466 (47，35) 0.0436 (-67，0) 0.0393 (0，0) 0.0293 (67，0) 0.0393 (-47，-35) 0.0501 (0，50) 0.0615 (47，-23) 0.0501

[0079] The terahertz wide spectrum active imaging optical system provided by the present invention is introduced in detail above. In this embodiment, specific examples are applied to illustrate the principle and implementation mode of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation mode and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

[0080] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined in the present embodiments may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown in the present embodiment, but will conform to the widest range consistent with the principles and novel features disclosed in the present embodiment.

Claims

1. A terahertz wide spectrum active imaging optical system, characterized in that: include: Terahertz light source, irradiation optical path system, receiving optical path system and detector; wherein, The terahertz light source is used to emit terahertz waves; The irradiation optical path system is used to receive the terahertz wave, and collimate and expand the terahertz wave before irradiating it onto the surface of the object; The receiving optical path system is used to receive the terahertz wave reflected by the surface of the object and focus the image on the detector.

2. The terahertz wide spectrum active imaging optical system according to claim 1, characterized in that: The illumination optical path system comprises an off-axis ellipsoidal reflector M1 and an off-axis parabolic reflector M2 which are sequentially arranged along the propagation direction of the terahertz wave; The terahertz light source is located at the first focus of the off-axis ellipsoidal reflector M1, and the second focus of the off-axis ellipsoidal reflector M1 coincides with the focus of the off-axis parabolic reflector M2; the output light of the off-axis parabolic reflector M2 is parallel light parallel to the main axis of the parabola.

3. The terahertz wide spectrum active imaging optical system according to claim 2, characterized in that: The parameters of the off-axis ellipsoidal reflector M1 are: a curvature radius of -43.6364 mm, a cone coefficient of -0.2066, a diameter of 40 mm, and an eccentricity of 20 mm.

4. A terahertz wide spectrum active imaging optical system according to claim 2 or 3, characterized in that: The parameters of the off-axis parabolic reflector M2 are: a curvature radius of -420 mm, a cone coefficient of -1, a diameter of 120 mm, and an eccentricity of 90 mm.

5. The terahertz wide spectrum active imaging optical system according to claim 1, characterized in that: The receiving optical path system is an off-axis three-mirror system; the off-axis three-mirror system comprises a primary mirror M3, a secondary mirror M4 and a triple mirror M5 which are sequentially arranged along the propagation direction of the terahertz wave; The primary reflector M3 and the secondary reflector M4 have positive optical focal lengths, and the tertiary reflector M5 has negative optical focal lengths; the secondary reflector M4 is located at the focal planes of the primary reflector M3 and the tertiary reflector M5 at the same time; The position setting of the off-axis three-mirror system satisfies that the main light of the terahertz light source propagates along the path formed by the line connecting the surface vertices of the primary reflector M3, the secondary reflector M4 and the third reflector M5.

6. The terahertz wide spectrum active imaging optical system according to claim 5, characterized in that: The vertex curvature radius of the primary reflector M3, the secondary reflector M4 and the tertiary reflector M5 meets the Pittsburgh condition: Among them, R1, R2 and R3 are the radii of the primary reflector M3, the secondary reflector M4 and the tertiary reflector M5 respectively.

7. The terahertz wide spectrum active imaging optical system according to claim 5, characterized in that: The reflector surface equations of the secondary reflector M4 and the tertiary reflector M5 are respectively obtained by adding even-order polynomial correction terms to the corresponding standard surface equations.

8. The terahertz wide spectrum active imaging optical system according to claim 7, characterized in that: The parameters of the main reflector M3 are: curvature radius of -613.8618 mm, cone coefficient of 0.2133, diameter of 100 mm, and eccentricity of 88 mm.

9. The terahertz wide spectrum active imaging optical system according to claim 7, characterized in that: The parameters of the secondary reflector M4 are: the radius of curvature is 176.3965 mm, the cone coefficient is 4.8874, the 4th order term of the even-order polynomial is -1.4587E-08, the 6th order term of the even-order polynomial is 7.7865E-12, the 8th order term of the even-order polynomial is 4.8097E-16, and the diameter is 61 mm.

10. The terahertz wide spectrum active imaging optical system according to claim 7, characterized in that: The parameters of the three reflectors M5 are: curvature radius of -177.1562 mm, cone coefficient of 0.2675, 4th order term of even-order polynomial of -2.8704E-09, 6th order term of even-order polynomial of 1.1727E-13, 8th order term of even-order polynomial of -5.2644E-18, diameter of 120 mm, and eccentricity of -78 mm.

Images