阅读说明：本技术 一种大景深太赫兹成像光学系统 (Large-field-depth terahertz imaging optical system ) 是由 王玉建 朱新勇 刘永利 张朝惠 郭永玲 于 2021-08-13 设计创作，主要内容包括：本发明公开了一种大景深太赫兹成像光学系统,其包括太赫兹源、准直透镜、楔形硅片、第一聚焦透镜、被测样品、第二聚焦透镜和太赫兹探测器,沿着太赫兹源发出的太赫兹波传播方向依次设置准直透镜、楔形硅片、第一聚焦透镜和被测样品,楔形硅片下表面与太赫兹源光轴夹角为45°,被测样品反射的太赫兹波沿原路返回至楔形硅片处在下表面发生一次反射,其传播方向旋转90°与太赫兹源的光轴垂直,第二聚焦透镜和太赫兹探测器沿一次反射的太赫兹波的传播方向依次设置。与传统斜入式光学系统相比,光学系统景深增加,可以达到±5～10mm；且可以有效的排除硅片的二次反射峰,从而避免二次反射峰对于测试结果的影响。(The invention discloses a large-field-depth terahertz imaging optical system which comprises a terahertz source, a collimating lens, a wedge-shaped silicon wafer, a first focusing lens, a tested sample, a second focusing lens and a terahertz detector, wherein the collimating lens, the wedge-shaped silicon wafer, the first focusing lens and the tested sample are sequentially arranged along the propagation direction of terahertz waves emitted by the terahertz source, the included angle between the lower surface of the wedge-shaped silicon wafer and the optical axis of the terahertz source is 45 degrees, terahertz waves reflected by the tested sample return to the wedge-shaped silicon wafer along the original path and are subjected to primary reflection, the propagation direction of the terahertz waves is rotated by 90 degrees and is vertical to the optical axis of the terahertz source, and the second focusing lens and the terahertz detector are sequentially arranged along the propagation direction of the terahertz waves subjected to the primary reflection. Compared with the traditional inclined-in optical system, the depth of field of the optical system is increased and can reach +/-5-10 mm; and the secondary reflection peak of the silicon chip can be effectively eliminated, so that the influence of the secondary reflection peak on the test result is avoided.)

1. A terahertz imaging optical system with large depth of field is characterized by comprising a terahertz source, a collimating lens, a wedge-shaped silicon wafer, a first focusing lens, a tested sample, a second focusing lens and a terahertz detector, wherein the terahertz source is used for emitting terahertz waves with a certain divergence angle, the collimating lens, the wedge-shaped silicon wafer, the first focusing lens and the tested sample are sequentially arranged along the terahertz wave propagation direction, the collimating lens is used for collimating the terahertz waves, the first focusing lens is used for focusing the terahertz waves transmitted by the wedge-shaped silicon wafer on the surface of the tested sample, the included angle between the lower surface of the wedge-shaped silicon wafer and the optical axis of the terahertz source is 45 degrees and used for changing the propagation direction of the terahertz waves reflected by the surface of the sample, the terahertz waves reflected by the tested sample return to the wedge-shaped silicon wafer along the original path to be secondarily reflected, the terahertz waves are primarily reflected on the lower surface, and the propagation direction of the terahertz waves is rotated by 90 degrees and is perpendicular to the optical axis of the terahertz source, the secondary reflection occurs on the upper surface, the transmission direction of the secondary reflection is rotated to be not equal to 90 degrees, the second focusing lens and the terahertz detector are sequentially arranged along the transmission direction of the once-reflected terahertz wave, the second focusing lens is used for focusing the once-reflected terahertz wave, the terahertz detector is positioned at the focal point of the second focusing lens and used for receiving the once-reflected terahertz wave, and the twice-reflected terahertz wave exceeds the receiving range of the terahertz detector.

2. The large-depth-of-field terahertz imaging optical system according to claim 1, wherein a light transmission aperture of the wedge-shaped silicon wafer 8 is not less than 1.4 times of a light transmission aperture of the lens, and a wedge-shaped included angle between an upper surface and a lower surface is generally selected to be 0.1-0.5 °.

3. The large-depth-of-field terahertz imaging optical system according to claim 1, wherein the collimating lens is a plano-convex lens with a certain focal length, the focal point is located on the surface of the sample to be measured, and the optical axis of the collimating lens coincides with the optical axis of the terahertz wave source.

4. The large-depth-of-field terahertz imaging optical system according to claim 1, wherein the first focusing lens is a plano-convex lens with a certain focal length, the focal point is located on the surface of the sample to be measured, the optical axis of the first focusing lens coincides with the optical axis of the terahertz wave source after the wedge-shaped silicon wafer is offset, the optical axis of the terahertz wave source after the wedge-shaped silicon wafer is offset is parallel to the optical axis of the terahertz wave source and has a certain offset, and the offset is determined by the thickness of the wedge-shaped silicon wafer.

5. The large-depth-of-field terahertz imaging optical system according to claim 1, wherein the first focusing lens focal length is generally not less than 100 mm.

6. The large-depth-of-field terahertz imaging optical system according to claim 1, wherein the second focusing lens is a plano-convex lens with a certain focal length, the focal point is located at the photoconductive crystal of the terahertz detector 7, and the optical axis coincides with the optical axis of the primary reflected detection light.

7. The large-depth-of-field terahertz imaging optical system according to claim 1, wherein the terahertz source divergence angle and the terahertz detector aperture angle are both 12.5 °, the focal length of the collimating lens and the second focusing lens is 38.1mm, the clear aperture is 35mm, the focal length of the first focusing lens is 100mm, the clear aperture is 35mm, the clear aperture of the wedge-shaped silicon wafer is 50.8mm, and the wedge angle is 0.1 °.

The technical field is as follows:

the invention belongs to the technical field of terahertz application, and particularly relates to a large-field-depth terahertz imaging optical system.

Background art:

terahertz waves are a generic term for a specific electromagnetic radiation, between microwaves and infrared rays, in the field of electronics, also known as millimeter waves and submillimeter waves, and in the field of spectroscopy, also known as far infrared rays. The frequency of the terahertz wave is between 0.1THz and 10THz, and the wavelength is between 0.03mm and 3 mm. Terahertz waves have good penetrability on dielectric materials such as plastics, ceramics and semiconductors and nonpolar materials, so that the internal structure and tissue of an object can be detected by utilizing the terahertz waves, and nondestructive testing is realized. Compared with X-ray or ultrasonic imaging, the terahertz wave photon energy is lower than that of X-ray, electromagnetic radiation cannot be caused to an operator or an object to be detected, and meanwhile, the terahertz wave length is shorter than that of ultrasonic waves, so that higher imaging resolution can be realized.

At present, a relatively mature terahertz wave generation and detection method adopts a photoelectric conduction principle, the terahertz wave generated by the method belongs to a pulse signal with transient change, and the typical terahertz pulse width is only 1ps, so that time-of-flight tomography can be performed by utilizing terahertz pulses. Especially for the multilayer structural body with opaque visible light wave band, the unique penetrability of terahertz waves can be utilized to carry out nondestructive thickness detection, the longitudinal resolution can reach dozens of micrometers, and meanwhile, the internal defects can also be detected.

The terahertz source and the detector based on the photoconductive principle are respectively a single-point terahertz source and a single-point terahertz detector, a beam shaping optical system of the terahertz source and the detector in time-of-flight tomography adopts a reflection optical path, the size of a light spot focused on a sample can represent the transverse resolution of imaging, and the smaller the size of the light spot, the higher the resolution. Generally, when time-of-flight tomography is performed, a sample to be measured has a plurality of reflecting surfaces with different depths, and therefore, it is necessary to ensure that the plurality of reflecting surfaces are all within the depth of field range of the optical system, that is, the larger the depth of field is, the more stable the imaging quality is.

Currently, an oblique incidence optical system is commonly used, and as shown in fig. 1, the system includes a terahertz source 1, a first collimating lens 2, a first focusing lens 3, a sample to be measured 4, a second collimating lens 5, a second focusing lens 6, and a terahertz detector 7. A terahertz wave beam with a certain divergence angle emitted by a terahertz source 1 is collimated by a first collimating lens 2, focused by a first focusing lens 3 and converged to a point on a detected sample 4, namely a detection point, is emitted by the surface of the detected sample, is collected by a second collimating lens 5 and a second focusing lens 6 in sequence, is finally converged to the center of a terahertz detector 7, and is induced and converted into a detectable electric signal. The depth of field range of the optical path is small and is only 1-2 mm, and after the depth of field range is exceeded, the position and the shape of a focused light spot generate large deviation, so that imaging distortion is caused.

The invention content is as follows:

the invention aims to provide a large-field-depth terahertz imaging optical system, which solves the defect that the field depth range of the existing oblique incidence type optical path is small.

In order to achieve the purpose, the large-depth-of-field terahertz imaging optical system comprises a terahertz source, a collimating lens, a wedge-shaped silicon wafer, a first focusing lens, a tested sample, a second focusing lens and a terahertz detector, wherein the terahertz source is used for emitting terahertz waves with a certain divergence angle, the collimating lens, the wedge-shaped silicon wafer, the first focusing lens and the tested sample are sequentially arranged along the terahertz wave propagation direction, the collimating lens is used for collimating the terahertz waves, the first focusing lens is used for focusing the terahertz waves transmitted by the wedge-shaped silicon wafer on the surface of the tested sample, the included angle between the lower surface of the wedge-shaped silicon wafer and the optical axis of the terahertz source is 45 degrees and used for changing the propagation direction of the terahertz waves reflected by the surface of the sample, the terahertz waves reflected by the tested sample return to the wedge-shaped silicon wafer along the original path to generate secondary reflection, the terahertz waves generate primary reflection on the lower surface, and the propagation direction of the terahertz waves rotates 90 degrees and is perpendicular to the optical axis of the terahertz source, the secondary reflection occurs on the upper surface, the transmission direction of the secondary reflection is rotated to be not equal to 90 degrees, the second focusing lens and the terahertz detector are sequentially arranged along the transmission direction of the once-reflected terahertz wave, the second focusing lens is used for focusing the once-reflected terahertz wave, the terahertz detector is positioned at the focal point of the second focusing lens and used for receiving the once-reflected terahertz wave, and the twice-reflected terahertz wave exceeds the receiving range of the terahertz detector.

Preferably, the clear aperture of the wedge-shaped silicon wafer 8 is not less than 1.4 times of the clear aperture of the lens, and the wedge-shaped included angle between the upper surface and the lower surface is generally selected to be 0.1-0.5 degrees.

Specifically, the collimating lens is a plano-convex lens with a certain focal length, the focal point is located on the surface of the measured sample, and the optical axis of the collimating lens coincides with the optical axis of the terahertz wave source.

Specifically, the first focusing lens is a plano-convex lens with a certain focal length, the focal point is located on the surface of the tested sample, the optical axis of the first focusing lens coincides with the optical axis of the terahertz wave source after the wedge-shaped silicon wafer is offset, the optical axis of the terahertz wave source after the wedge-shaped silicon wafer is offset is parallel to the optical axis of the terahertz wave source and has a certain offset, and the offset is determined by the thickness of the wedge-shaped silicon wafer.

To further increase the depth of field, the first focusing lens focal length is typically no less than 100 mm.

Specifically, the second focusing lens is a plano-convex lens having a certain focal length, the focal point is located at the photoconductive crystal of the terahertz detector 7, and the optical axis coincides with the optical axis of the primarily reflected detection light.

The divergence angle of the terahertz source and the aperture angle of the terahertz detector are both 12.5 degrees, the focal length of the collimating lens and the second focusing lens is 38.1mm, the light-passing aperture is 35mm, the focal length of the first focusing lens is 100mm, the light-passing aperture is 35mm, the light-passing aperture of the wedge-shaped silicon wafer is 50.8mm, and the wedge angle is 0.1 degree.

Compared with the prior art, the invention has the following beneficial effects:

1) the depth of field of the optical system is increased and can reach +/-5-10 mm;

2) whether the focus light spot is within or beyond the field depth range, the focus light spot is always positioned at the center of the optical axis, and the deviation of the position and the shape cannot be generated, so that the image distortion is avoided;

3) the wedge-shaped silicon chip is used as a light splitting device, so that the secondary reflection peak of the silicon chip can be effectively eliminated, and the influence of the secondary reflection peak on the test result is avoided.

4) The optical system is more compact, and is favorable for the design of an integrated and miniaturized terahertz lens.

Description of the drawings:

fig. 1 is a schematic structural diagram of a conventional oblique incidence optical system.

Fig. 2 is a schematic structural principle diagram of a large-depth-of-field terahertz imaging optical system according to the present invention.

Fig. 3 is a comparison graph of focusing spots on the surface of a sample under different defocusing amounts between the conventional oblique incidence optical system in fig. 1 and the large-depth-of-field terahertz imaging optical system in fig. 2.

FIG. 4 is a comparison of a parallel silicon wafer and a wedge silicon wafer.

The specific implementation mode is as follows:

the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments, and all other embodiments obtained by a person of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the protection scope of the present invention.

Example 1:

as shown in fig. 2, the large-depth-of-field terahertz imaging optical system according to the present invention includes a terahertz source 1, a collimating lens 2, a wedge-shaped silicon wafer 8, a first focusing lens 3, a sample to be measured 4, a second focusing lens 6, and a terahertz detector 7.

The terahertz source 1 is used for emitting terahertz waves with a certain divergence angle, the collimating lens 2, the wedge-shaped silicon wafer 8, the first focusing lens 3 and the sample 4 to be detected are sequentially arranged along the terahertz wave propagation direction, the collimating lens 2 is used for collimating the terahertz waves, the first focusing lens 3 is used for focusing the terahertz waves transmitted by the wedge-shaped silicon wafer 8 on the surface of the sample 4 to be detected, the included angle between the lower surface of the wedge-shaped silicon wafer 8 and the optical axis of the terahertz source is 45 degrees and is used for changing the terahertz wave propagation direction reflected back from the surface of the sample, the terahertz waves reflected by the sample 4 to be detected return to the wedge-shaped silicon wafer 8 along the original path to generate secondary reflection, the terahertz waves are subjected to primary reflection on the lower surface, the terahertz waves rotate 90 degrees and are perpendicular to the optical axis of the terahertz source, the secondary reflection is performed on the upper surface, the rotation direction of the terahertz waves is not equal to 90 degrees, and the second focusing lens 6 and the terahertz detector 7 are sequentially arranged along the terahertz wave propagation direction of the primary reflection, the second focusing lens 6 is used for focusing the once-reflected terahertz waves, the terahertz detector 7 is located at the focus of the second focusing lens 6 and used for receiving the once-reflected terahertz waves, and the twice-reflected terahertz waves exceed the receiving range of the terahertz detector, so that the influence of twice-reflected signals on detection results is avoided.

Fig. 3 is a comparison diagram of focusing spots on the surface of a sample under different defocusing amounts of a traditional oblique incidence optical system and the large-depth-of-field terahertz imaging optical system related to the embodiment under the same terahertz source 1, terahertz detector and lens. As can be seen from the figure, in the large-depth-of-field terahertz imaging optical system, the position of a light spot focused on a sample does not change along with the defocusing amount, and the shape can be kept in a perfect circle; in the oblique incidence system, the spot position moves in a direction parallel to the incident surface with a change in defocus amount, and the shape is also distorted.

A terahertz wave beam with a certain divergence angle emitted by a terahertz source 1 passes through a collimating lens 2 and then is changed into a parallel light beam, a part of the parallel light beam is reflected after passing through a wedge-shaped silicon wafer 8, a part of the parallel light beam is continuously transmitted forwards and is converged to a point on a tested sample 4 through a first focusing lens 3, namely a detection point, the parallel light beam is reflected by the surface of the sample, the original path passes through the first focusing lens 3, a part of the transmission loss occurs on the lower surface of the wedge-shaped silicon wafer 8, a part of the reflection occurs, the transmission direction is 90 degrees of turning, the parallel light beam is converged to a photoconductive crystal at the center of a terahertz detector 7 after passing through a second focusing lens 6, and then the parallel light beam is converted into a detectable electric signal, namely a terahertz signal carrying sample information at the detection point.

The available terahertz echo is a primary reflection peak on the lower surface of the silicon wafer, but due to the fresnel law, a secondary reflection peak inevitably exists on the upper surface of the silicon wafer, as shown in fig. 4, when the silicon wafer is a parallel silicon wafer, the secondary reflection peak is also collected by the detector, so that the detection accuracy is influenced; therefore, the wedge-shaped silicon wafer 8 is adopted in the embodiment, so that the secondary reflection peak and the primary reflection peak have a certain included angle, and the secondary reflection peak and the primary reflection peak can exceed the collection range of the terahertz detector 7 after being transmitted for a certain distance, thereby avoiding the influence of the secondary reflection peak on the detection accuracy.

Preferably, the clear aperture of the wedge-shaped silicon wafer 8 is not less than 1.4 times of the clear aperture of the lens, the wedge-shaped included angle between the upper surface and the lower surface is generally selected to be 0.1-0.5 degrees, if the wedge-shaped included angle is too large, the light path can be seriously deformed, and if the wedge-shaped included angle is too small, the effect of eliminating secondary reflection peaks cannot be achieved.

Specifically, the collimating lens 2 is a plano-convex lens with a certain focal length, the focal point is located on the surface of the sample 4 to be measured, and the optical axis of the collimating lens 2 coincides with the optical axis of the terahertz wave source.

Specifically, the first focusing lens 3 is a plano-convex lens with a certain focal length, the focal point is located on the surface of the sample 4 to be measured, the optical axis coincides with the optical axis of the terahertz wave source after the offset of the wedge-shaped silicon wafer 8, the optical axis of the terahertz wave source after the offset of the wedge-shaped silicon wafer 8 is parallel to the optical axis of the terahertz wave source and has a certain offset, and the offset is determined by the thickness of the wedge-shaped silicon wafer 8. To further increase the depth of field, the first focusing lens 3 is typically no less than 100mm in focal length.

Specifically, the second focusing lens 6 is a plano-convex lens having a certain focal length, the focal point is located at the photoconductive crystal of the terahertz detector 7, and the optical axis coincides with the optical axis of the primarily reflected detection light.

In the embodiment, the divergence angle of the terahertz source 1 and the aperture angle of the terahertz detector 7 are both 12.5 degrees, the focal length of the collimating lens 2 and the focal length of the second focusing lens 6 are 38.1mm, the clear aperture of the collimating lens is 35mm, the focal length of the first focusing lens 3 is 100mm, the clear aperture of the first focusing lens is 35mm, the clear aperture of the wedge-shaped silicon wafer is 50.8mm, and the wedge angle is 0.1 degree.