CN110888239A - Quasi-optical system for terahertz wave beam shaping - Google Patents
Quasi-optical system for terahertz wave beam shaping Download PDFInfo
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- CN110888239A CN110888239A CN201911366099.6A CN201911366099A CN110888239A CN 110888239 A CN110888239 A CN 110888239A CN 201911366099 A CN201911366099 A CN 201911366099A CN 110888239 A CN110888239 A CN 110888239A
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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
- G02B27/095—Refractive optical elements
- G02B27/0955—Lenses
Abstract
The invention discloses a quasi-optical system for shaping terahertz wave beams, which comprises a flat semiconductor crystal, a hyper-hemispherical silicon lens and an aspheric collimating lens, wherein one surface of the hyper-hemispherical silicon lens is a hyper-hemispherical surface, the other surface of the hyper-hemispherical silicon lens is a plane, the flat semiconductor crystal is fixed on the plane side of the hyper-hemispherical silicon lens, the aspheric collimating lens is arranged on the hyper-hemispherical surface side of the hyper-hemispherical silicon lens, and the flat semiconductor crystal generates collimating terahertz waves under the excitation of femtosecond laser pulses and direct current bias voltage; the generated terahertz waves directly enter the super-hemispherical silicon lens and the aspheric collimating lens and output parallel terahertz beams. The terahertz wave focusing device can also comprise an aspheric focusing lens and a focal plane, and a parallel terahertz wave beam enters the aspheric focusing lens and reaches the position of the test focal plane to generate a focusing terahertz wave. The terahertz signal transmission device reduces the influence of water vapor absorption, improves the system resolution, reduces the proportion of total reflection beams, enables the maximum transmittance to reach 72%, reduces the optical path of an air optical path, and improves the quality of terahertz signals.
Description
The technical field is as follows:
the invention belongs to the technical field of terahertz spectrum and imaging technology, and particularly relates to a quasi-optical system for terahertz beam shaping.
Background art:
terahertz refers to an electromagnetic wave with the frequency of 0.1-10 THz, the spectrum is located between infrared and millimeter waves, the research on terahertz is in a blank stage for a long time, and the research on terahertz wave bands gradually starts with the development of ultrafast photoelectron technology and low-scale semiconductor technology since the 80 s of the 20 th century. As a new technology, the terahertz technology is valued by governments of various countries, and is evaluated as one of ten major technologies for changing the future world in the united states in 2004, and is listed as a ten major strategic target of national pillars in 2005, and national Shanshan scientific conference with the subject of Etherz is held in China in 2005.
Terahertz waves have many unique advantages in substance detection and imaging: (1) the frequency band of the terahertz spectrum corresponds to a plurality of macromolecule integral vibration modes and intermolecular vibration modes, and the vibration modes are more sensitive to the external environment, so that the terahertz spectrum has the advantages that other testing means cannot achieve in the aspect of researching the material characteristics; (2) terahertz belongs to a measuring mode of integrating spectra, and can simultaneously obtain information of images and frequency spectrums, so that multi-dimensional analysis is carried out on an object to be detected; (3) the terahertz system is insensitive to black body radiation (thermal background) and can achieve a higher signal-to-noise ratio; (5) terahertz photon energy is very low and only has the level of meV, so that ionization effect on biological tissues is not generated, and the terahertz photon energy is very safe to living organisms; (6) terahertz has unique penetrability, has strong penetrability to a plurality of nonpolar materials (such as plastics, rubber, fibers, foams and the like), and is very suitable for the field of nondestructive testing.
Based on such excellent characteristics of the terahertz technology, the terahertz technology has potential application value and wide application prospect in various fields such as military, security inspection, product quality detection, biology, medicine, chemistry, physics and the like.
In the practical application of the terahertz system, a very key point is that terahertz waves are collected and shaped, so that the terahertz waves can accurately penetrate through an object to be measured to obtain effective information. At present, a commonly used method is that a specially prepared semiconductor crystal is excited by femtosecond laser to emit terahertz waves, the divergence angle of the terahertz waves is restrained by a silicon lens, terahertz wave beams are further focused by a pair of plano-convex lenses, and the focal point is positioned on the surface of an object to be measured. Fig. 1 shows a conventional terahertz beam shaping system, which is composed of a terahertz photoconductive transmitting antenna (1), a collimating plano-convex lens (2), and a focusing plano-convex lens (3), wherein a semiconductor crystal and a silicon lens for transmitting terahertz waves are integrated in the terahertz photoconductive transmitting antenna (1), the collimating plano-convex lens (2) and the focusing plano-convex lens (3) are both spherical, and the materials are HDPE or TPX.
However, the current application has the following problems: (1) the terahertz transmitting antenna is independently provided by a manufacturer (such as menlosystem, toptica and the like), and cannot be well matched with a subsequent lens system; (2) the silicon lens and the plano-convex lens are standard spherical surfaces and have larger spherical aberration, so that terahertz wave beams cannot be well focused; (3) the refractive index of the silicon lens is large, and the critical angle of total reflection of the silicon lens is small, so that a large part of terahertz waves are trapped inside the silicon lens; (4) the double plano-convex lens is made of High Density Polyethylene (HDPE) or poly-4-methyl-1-pentene (TPX) which is a commonly used material, but the HDPE has an obvious absorption peak at a 2.2THz position, the transmittance at a high frequency band is not high, the transmittance of the TPX at a frequency higher than 6THz is also low, and the two materials can cause large attenuation to a high-frequency part of a terahertz signal to influence the measurement effect; (5) the transmission optical path of the terahertz wave beam in the air is too long, the signal is weakened due to the problem of water vapor absorption, and the signal distortion is also caused by the water vapor absorption peak.
The invention content is as follows:
the invention aims to overcome the defects in the prior art, and seeks to design a quasi-optical system for terahertz beam shaping, and a mechanical structure is designed by combining a specific optical system optical path structure. Overcomes the defects of low matching degree, large spherical aberration, low transmittance, signal attenuation and the like of the prior general terahertz wave beam transmission light path lens,
in order to achieve the purpose, the quasi-optical system for terahertz wave beam shaping comprises a flat semiconductor crystal, a hyper-hemispherical silicon lens and a collimating lens, wherein one surface of the hyper-hemispherical silicon lens is a hyper-hemispherical surface, the other surface of the hyper-hemispherical silicon lens is a plane, the flat semiconductor crystal is fixed on the plane side of the hyper-hemispherical silicon lens, the collimating lens is arranged on the hyper-hemispherical surface side of the hyper-hemispherical silicon lens, and the optical axes of the flat semiconductor crystal, the hyper-hemispherical silicon lens and the collimating lens are overlapped.
Further, the collimating lens is an aspheric collimating lens, and the convex surface of the aspheric collimating lens is aspheric.
The invention relates to a quasi-optical system for terahertz wave beam shaping, which further comprises a focusing lens and a focal plane, wherein the focusing lens is arranged on one side of the collimating lens, the collimating lens is close to the convex surface of the focusing lens, the focal plane is arranged at the focal position of the focusing lens, and the optical axes of a flat semiconductor crystal, a hyper-hemispherical silicon lens, the collimating lens and the focusing lens are coincided.
Furthermore, the collimating lens is an aspheric collimating lens, the focusing lens is an aspheric focusing lens, and convex surfaces of the aspheric collimating lens and the aspheric focusing lens are aspheric surfaces.
Further, the distance of the optical path of air between the aspheric collimating lens and the aspheric focusing lens is L1, L1 is generally 5-50mm, and the distance L2 between the aspheric focusing lens and the focal plane is generally not more than 100 mm.
Furthermore, the hyper-hemispherical surface of the hyper-hemispherical silicon lens is fixed in a groove which is arranged on the plane side of the collimating lens and corresponds to the hyper-hemispherical surface of the hyper-hemispherical silicon lens.
Further, a terahertz antireflection film is plated on the surface of the hyper-hemispherical silicon lens.
Furthermore, both the aspheric focusing lens and the aspheric collimating lens adopt a novel cyclic olefin copolymer (TopasCOC).
Compared with the prior art, the invention has the following beneficial effects: the optical system adopts an integrated design from a semiconductor crystal for transmitting the terahertz waves to a sample to be detected, so that the utilization efficiency of the optical system is ensured to the greatest extent, the optical path structure is optimized, the size of the optical system is shortened, and the influence of water vapor absorption in the transmission process of the terahertz waves is reduced; by adopting the aspheric collimating lens and the focusing lens, the aberration of an optical system is greatly reduced, and the resolution of the system is improved; the semiconductor crystal, the hyper-hemispherical lens and the collimating lens are all glued, so that the proportion of total reflection beams is reduced, the maximum transmittance is 72%, the optical path of an air optical path is reduced, and the influence of water vapor absorption in the transmission process of terahertz waves is reduced; the aspheric collimating lens and the aspheric focusing lens adopt novel cyclic olefin copolymer (Topas COC), so that the stability and reliability of the lens in the using process are improved, chromatic aberration is effectively controlled, the absorption of a high frequency band is reduced, and the quality of terahertz signals is improved.
Description of the drawings:
fig. 1 is a schematic structural diagram of a conventional terahertz beam shaping system.
Fig. 2 is a schematic view of the structural principle of a quasi-optical system for terahertz beam shaping according to embodiment 1.
FIG. 3 is a diagram of wavefront analysis of an emergent wave of example 1
Fig. 4 is a schematic structural diagram of a quasi-optical system for terahertz beam shaping according to embodiment 2.
Fig. 5 is a diagram of wavefront analysis of example 2.
Fig. 6 is an MTF graph of example 2.
Fig. 7 is an image plane spot diagram of example 2.
The specific implementation mode is as follows:
the invention is further illustrated by the following specific examples in combination with the accompanying drawings.
Example 1:
the quasi-optical system for terahertz wave beam shaping comprises a flat semiconductor crystal 1, a hyper-hemispherical silicon lens 2 and a collimating lens 3, wherein one surface of the hyper-hemispherical silicon lens 2 is a hyper-hemispherical surface, the other surface of the hyper-hemispherical silicon lens is a plane, the flat semiconductor crystal 1 is glued to the plane side of the hyper-hemispherical silicon lens 2, the collimating lens 3 is arranged on the hyper-hemispherical surface side of the hyper-hemispherical silicon lens 2, and the optical axes of the flat semiconductor crystal 1, the hyper-hemispherical silicon lens 2 and the collimating lens 3 are overlapped. The hyper-hemispherical silicon lens 2 is made of high-resistance silicon materials, the refractive index of semiconductor crystal materials is generally 3.4-3.6, the critical angle of total reflection in the air is 16.13-17.10 degrees, high-resistance silicon with the refractive index close to that of the semiconductor crystal materials is used as the materials of the hyper-hemispherical silicon lens, the hyper-hemispherical silicon lens and the hyper-hemispherical silicon lens are installed in a laminating mode, excessive total reflection is avoided, and the maximum transmittance of terahertz waves emitted by the semiconductor crystal can reach 72 percent.
Furthermore, the hyper-hemispherical surface of the hyper-hemispherical silicon lens 2 is clamped into a groove which is formed on the plane side of the collimating lens 3 and corresponds to the hyper-hemispherical surface of the hyper-hemispherical silicon lens 2, the connection part is glued and fixed, the hyper-hemispherical silicon lens and the hyper-hemispherical silicon lens are directly glued and fixedly connected, the air light path is shortened, and the water vapor absorption is reduced.
Furthermore, the surface of the hyper-hemispherical silicon lens 2 is plated with a terahertz antireflection film so as to improve the transmittance.
Further, the collimating lens 3 is an aspheric collimating lens 3, and a convex surface of the aspheric collimating lens 3 is an aspheric surface. By adopting the aspheric surface design, the spherical aberration of the system can be corrected to the maximum extent, the diameter of a light spot is reduced, the transverse resolution of the system is improved, and the minimum diameter of the light spot can reach 3 mu m.
Furthermore, the aspheric collimating lens adopts a novel cyclic olefin copolymer (Topas COC), the COC has the advantages of low density, high refractive index (the terahertz waveband is 1.5258), high transmittance, large Abbe number, low birefringence, extremely low water absorption (only 1/10 of PMMA and PC), good heat resistance, low thermal expansion coefficient, stable chemical performance, excellent mechanical performance and the like, the forming is convenient, the aspheric collimating lens is more stable and reliable, the large Abbe number ensures that the refractive index change of the aspheric collimating lens in the terahertz waveband is very small, chromatic aberration can be effectively controlled, the high transmittance ensures low absorption of a high frequency band, the high frequency absorption of terahertz wave beams is reduced, and the transmittance is increased.
The quasi-optical system for terahertz beam shaping described in this embodiment is used to output a collimated terahertz beam. The method specifically comprises the following steps: a flat semiconductor crystal 1 (generally LT-GaAs, InGaAs) generates terahertz waves under the excitation of femtosecond laser pulses and direct-current bias voltage; the generated terahertz waves directly enter the super-hemispherical silicon lens 2 and the aspheric collimating lens 3, and parallel terahertz wave beams are output.
Fig. 3 is a diagram of wavefront analysis of the emergent wave in example 1, and it can be seen from the diagram that both the wavefront PV value and the RMS value of the emergent wave are 0, which indicates that the output is a plane wave, and the collimation effect meets the design requirement.
Example 2:
the quasi-optical system for terahertz wave beam shaping, which is related by the embodiment, comprises a flat semiconductor crystal 1, a hyper-hemispherical silicon lens 2, a collimating lens 3, a focusing lens 4 and a focal plane 5, wherein one surface of the hyper-hemispherical silicon lens 2 is a hyper-hemispherical surface, the other surface is a plane, the flat semiconductor crystal 1 is glued to the plane side of the hyper-hemispherical silicon lens 2, the collimating lens 3 is arranged on the hyper-hemispherical surface side of the hyper-hemispherical silicon lens 2, the focusing lens 4 is arranged on one side of the collimating lens 3, the collimating lens 3 is close to the convex surface of the focusing lens 4, the focal plane 5 is arranged at the focal position of the focusing lens 4, and the optical axes of the flat semiconductor crystal 1, the hyper-hemispherical silicon lens 2, the.
Furthermore, the hyper-hemispherical surface of the hyper-hemispherical silicon lens 2 is clamped into a groove which is formed on the plane side of the collimating lens 3 and corresponds to the hyper-hemispherical surface of the hyper-hemispherical silicon lens 2, the connection part is glued and fixed, the hyper-hemispherical silicon lens and the hyper-hemispherical silicon lens are directly glued and fixedly connected, the air light path is shortened, and the water vapor absorption is reduced.
Furthermore, the surface of the hyper-hemispherical silicon lens 2 is plated with a terahertz antireflection film so as to improve the transmittance.
Further, the collimating lens 3 is an aspheric collimating lens 3, the focusing lens 4 is an aspheric focusing lens 4, and the convex surfaces of the aspheric collimating lens 3 and the aspheric focusing lens 4 are aspheric surfaces.
Further, the aspherical focusing lens and the aspherical collimating lens 3 employ a novel cyclic olefin copolymer (TopasCOC).
Further, the distance of the optical path of the air between the aspheric collimating lens 3 and the aspheric focusing lens 4 is L1, the L1 is required to be as small as possible on the premise of ensuring the assembling property so as to reduce the influence of water vapor absorption, the assembling property and the high transmittance are considered comprehensively, and the L1 is generally selected from 5 to 50 mm. The distance L2 between the aspheric focusing lens 4 and the test sample/focal plane 5 can be designed according to the actual application scenario, and should not be too large, generally not larger than 100mm, so as to avoid excessive water vapor absorption loss.
The quasi-optical system for terahertz beam shaping described in this embodiment is used for outputting a focused terahertz beam. The method specifically comprises the following steps: a flat semiconductor crystal 1 (generally LT-GaAs, InGaAs) generates terahertz waves under the excitation of femtosecond laser pulses and direct-current bias voltage; the generated terahertz waves directly enter the super-hemispherical silicon lens 2 and the aspheric collimating lens 3 and output parallel terahertz wave beams; the parallel terahertz wave beam enters the aspheric focusing lens 4 after passing through an air optical path L1, and reaches the position of the test focal plane 5 after passing through an air optical path L2.
The following table is a table of design parameters of the quasi-optical system for terahertz beam shaping in examples 1 and 2
FIG. 5 is a diagram of wave front analysis of example 2, and it can be seen that the PV value of the image surface wave front is 0.0009 λ, the RMS value is 0.0002 λ, and the aberration of the optical system has been reduced to a very small level, which is far enough to meet the requirement of use.
FIG. 6 is an MTF curve of embodiment 2, which can be used to comprehensively evaluate various aberrations of an optical system, and it can be seen that the MTF curve approaches the diffraction limit of an ideal optical system, and meets the use requirements.
Fig. 7 is an image plane spot diagram of embodiment 2, and it can be seen that the spot diameter is 2.811 μm, which significantly improves the lateral resolution of the system.
The quasi-optical system of the invention can be used symmetrically, and can be used for receiving terahertz wave beams by adjusting the circuit structure of the flat semiconductor crystal.
Claims (9)
1. The quasi-optical system for terahertz wave beam shaping is characterized by comprising a flat semiconductor crystal, a hyper-hemispherical silicon lens and a collimating lens, wherein one surface of the hyper-hemispherical silicon lens is a hyper-hemispherical surface, the other surface of the hyper-hemispherical silicon lens is a plane, the flat semiconductor crystal is fixed on the plane side of the hyper-hemispherical silicon lens, the collimating lens is arranged on the hyper-hemispherical surface side of the hyper-hemispherical silicon lens, and the optical axes of the flat semiconductor crystal, the hyper-hemispherical silicon lens and the collimating lens are coincided.
2. The quasi-optical system for terahertz beam shaping as claimed in claim 1, wherein the collimating lens is an aspheric collimating lens, and the aspheric collimating lens is aspheric.
3. The quasi-optical system for terahertz beam shaping of claim 2, wherein the aspheric collimating lens employs a Topas COC.
4. The quasi-optical system for terahertz beam shaping according to claim 1, further comprising a focusing lens and a focal plane, wherein the focusing lens is disposed on one side of the collimating lens, the collimating lens and the focusing lens are adjacent to each other in convex surface, the focal plane is disposed at the focal position of the focusing lens, and the optical axes of the flat semiconductor crystal, the hyper-hemispherical silicon lens, the collimating lens and the focusing lens coincide.
5. The quasi-optical system for terahertz beam shaping as claimed in claim 4, wherein the collimating lens is an aspheric collimating lens, the focusing lens is an aspheric focusing lens, and the aspheric collimating lens and the aspheric focusing lens are aspheric surfaces.
6. The quasi-optical system for terahertz beam shaping as claimed in claim 5, wherein an air optical path distance L1 between the aspheric collimating lens and the aspheric focusing lens is 5-50mm, typically 5-50mm, and the aspheric focusing lens is not more than 100mm away from the focal plane directly at a distance L2.
7. The quasi-optical system for terahertz beam shaping of claim 6, wherein both the aspheric focusing lens and the aspheric collimating lens employ Topas COC.
8. The quasi-optical system for terahertz beam shaping as claimed in any one of claims 1 to 7, wherein the hyper-hemispherical silicon lens hyper-hemispherical surface is fixed in a groove corresponding to the hyper-hemispherical surface of the hyper-hemispherical silicon lens formed on the plane side of the collimating lens.
9. The quasi-optical system for terahertz beam shaping of claim 8, wherein the hyper-hemispherical silicon lens is coated with a terahertz antireflection film on the surface.
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| CN201911366099.6A CN110888239A (en) | 2019-12-26 | 2019-12-26 | Quasi-optical system for terahertz wave beam shaping |
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN111323926A (en) * | 2020-04-07 | 2020-06-23 | 中国科学院半导体研究所 | Gaussian beam shaping mirror, optical system and application thereof |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN111323926A (en) * | 2020-04-07 | 2020-06-23 | 中国科学院半导体研究所 | Gaussian beam shaping mirror, optical system and application thereof |
| CN111323926B (en) * | 2020-04-07 | 2021-05-11 | 中国科学院半导体研究所 | Gaussian beam shaping mirror, optical system and application thereof |
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