Terahertz fiber filter based on resonant structure

Abstract

The present invention discloses a terahertz fiber filter based on a resonant structure, comprising: a side-polished terahertz fiber and a resonant structure, wherein the resonant structure is a micro-nano rectangular parallelepiped column integrated into the side-polished flat area of ​​the side-polished terahertz fiber. The micro-nano rectangular parallelepiped column acts as a resonant cavity-like effect, so that terahertz waves that meet the resonant frequency of the micro-nano rectangular parallelepiped column are effectively coupled into the resonant cavity and interact with the micro-nano rectangular parallelepiped column to produce electromagnetic resonance. The electromagnetic resonance frequency is also called the center frequency of the filter. The terahertz electromagnetic energy at the center frequency is used to maintain electromagnetic oscillation, thereby forming a filter stopband. As the size of the micro-nano rectangular parallelepiped column structure changes, the filter can achieve filtering with a higher Q value, a larger maximum stopband attenuation depth, and a smaller insertion loss within any specific narrowband within the range of 0.1 to 3 THz.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz wave applications, and in particular relates to a terahertz optical fiber filter based on a resonance structure. Background Art

[0002] Due to their unique spectral characteristics, terahertz waves have important cross-cutting applications in astronomical observation, aviation communications, explosion-proof detection, and other fields. Filters, as key components in terahertz detection and communication systems, can extract characteristic signals and suppress interfering frequencies, improving the system's target detection performance.

[0003] To more effectively utilize terahertz waves in future communications, terahertz filtering technology with excellent filtering performance is crucial. Terahertz filters can be categorized as high-pass, low-pass, band-reject, and band-pass. Terahertz band-reject filters can prevent interference within specific frequency bands during transmission, providing a new approach to filtering specific frequencies and possessing enormous potential for application.

[0004] A band-stop filter is a device that can select a desired frequency for filtering, thereby obtaining a filtered signal wave. Patent document CN113721376A discloses a light-controlled terahertz fiber modulator, comprising a terahertz fiber for transmitting terahertz waves. The terahertz fiber has a polished area on its side, and the end face of the terahertz fiber with the polished area is "D"-shaped. A metamaterial with a micro-nanostructure is disposed on the polished area, and graphene is disposed on the metamaterial. However, this micro-nanostructure is fabricated for a wide spectrum modulator and cannot achieve the selection of a specific narrowband for filtering. In addition, this micro-nanostructure requires high-precision calculation and fabrication of parameters such as structural dimensions and the duty cycle between columnar structures, resulting in high costs. Summary of the Invention

[0005] The purpose of the present invention is to address the deficiencies in the above-mentioned prior art and provide a terahertz fiber filter based on a resonant structure to solve the problems of low Q value, excessive insertion loss, low stopband attenuation, and the need for high-precision equipment to prepare micro-nanostructures such as resonant cavities in traditional terahertz filters, which has high preparation accuracy requirements and high costs, so as to adapt to the application of terahertz waves in the fields of communication, imaging, detection, etc.

[0006] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0007] A terahertz fiber filter based on a resonant structure includes a side-polished terahertz fiber for transmitting terahertz waves. The cross-section of the polished area is D-shaped, and the polished area is divided into transition areas at both ends and a flat area in the middle. It is characterized in that micro-nano rectangular parallelepiped columns are integrated in the flat area as a resonant structure, so that the evanescent field generates electromagnetic resonance on the micro-nano rectangular parallelepiped columns, forming a filtering stopband.

[0008] Preferably, the micro-nano cuboid column is located at the longitudinal midline of the flat area.

[0009] Preferably, the length of the micro-nano cuboid column is 10 to 10,000 um.

[0010] Preferably, the cross-section of the micro-nano cuboid column is rectangular and the length is 20 to 150 μm.

[0011] Preferably, the cross-section of the micro-nano rectangular parallelepiped column is square with a side length of 40 μm, and at a specific size, it produces a strong band-stop filtering effect on terahertz waves in a specific frequency band.

[0012] Preferably, the side-polished terahertz optical fiber is a polymer microstructure optical fiber, the refractive index of the polymer material in the terahertz band is 1.50-1.53, and the diameter of the polymer microstructure optical fiber is 800-3000 μm.

[0013] Preferably, the side polished thickness of the side-polished terahertz optical fiber varies with the diameter of the optical fiber, and the side polished thickness is 200 to 1500 μm.

[0014] Preferably, the micro-nano cuboid column material has a carrier concentration of not less than 10 15 / cm 3 , carrier migration is not less than 100cm 2 / (V·s), P-type doped semiconductor materials such as silicon, germanium or gallium arsenide with a resistivity of not less than 500Ω / cm.

[0015] The working principle of the present invention is as follows:

[0016] When a terahertz wave is injected into the input port of a side-polished terahertz fiber, it leaks out of the polished area, forming an evanescent field. This evanescent field interacts with the micro-nano rectangular pillars to produce electromagnetic resonance. The terahertz electromagnetic energy at the center frequency is used to sustain the electromagnetic oscillation, thus forming a filter stopband.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The technical means of combining a side-polished terahertz fiber structure with a micro-nano rectangular column provides a new technical solution for selecting a specific narrowband for filtering for the structure disclosed in patent document CN113721376A. Compared with existing literature and technologies, it has improved parameter indicators such as enhancing stopband attenuation, reducing insertion loss, and increasing the Q value of the terahertz filter. In addition, the preparation technology of the micro-nano rectangular resonant structure is mature, simple and inexpensive, which effectively reduces the difficulty and cost of preparation. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the structure of a terahertz filter according to an embodiment of the present invention;

[0020] Figure 2 This is a schematic diagram of the end face structure of a terahertz optical fiber according to an embodiment of the present invention;

[0021] Figure 3 Schematic diagram of the cross-sectional structure of the side-polished flat area of ​​a side-polished terahertz optical fiber according to an embodiment of the present invention;

[0022] Figure 4 Schematic diagram of the micro-nano cuboid column structure according to an embodiment of the present invention;

[0023] Figure 5 This is a working principle diagram of a terahertz filter according to an embodiment of the present invention;

[0024] Figure 6 This is a transmission spectrum of a terahertz filter according to an embodiment of the present invention;

[0025] Figure 7 This is a diagram showing how the Q value of a terahertz filter according to an embodiment of the present invention is affected by changes in structural dimensions;

[0026] Figure 8 This is a diagram showing the influence of structural size changes on the maximum stopband attenuation depth of the terahertz filter according to an embodiment of the present invention;

[0027] Figure 9 This is a diagram showing the effect of structural size changes on the insertion loss of a terahertz filter according to an embodiment of the present invention;

[0028] Icons: 1-side-polished terahertz fiber; 2-micro-nano rectangular column; 3-side-polished flat area; 4-air hole; 5-micro-nano rectangular column cross section. DETAILED DESCRIPTION

[0029] The accompanying drawings are for illustrative purposes only and are not to be construed as limiting the present invention. To better illustrate the following embodiments, some components in the accompanying drawings may be omitted, enlarged, or reduced in size, and do not represent actual product dimensions. Those skilled in the art will appreciate that some well-known structures and their descriptions may be omitted from the accompanying drawings.

[0030] like Figure 1 As shown, an embodiment of the present invention provides a terahertz filter, comprising: an upper layer of micro-nano rectangular columns 2, and a lower layer of substrate which is a side-polished terahertz optical fiber 1; the device integrates the micro-nano rectangular columns 2 on a flat area 3 of the side-polished terahertz optical fiber 1. When the terahertz wave is coupled into the terahertz optical fiber 1, the terahertz wave of a specific frequency band is localized in the micro-nano rectangular columns 2 of the upper layer and is filtered by the structure during transmission, while the remaining frequency bands are output to the terahertz optical fiber 1 for filtering.

[0031] The polymer microstructured optical fiber used in the embodiment of the present invention is an optical fiber developed by the Xi'an Institute of Optics and Precision Mechanics of the Chinese Academy of Sciences for transmitting terahertz waves. Figure 2 As shown, the fiber material is COP polymer, the middle area is an air hole 4, the fiber diameter is 1mm, and it is suitable for transmitting terahertz waves with a frequency of 0.1-3THz. The polymer microstructure fiber used in the embodiment of the present invention is made of COP polymer. The absorption coefficient of this fiber to terahertz waves is lower than 3cm -1 The optical fiber side is polished by a fiber wheel side polishing machine. The side polishing removes the cladding layer with a length of 0.5 cm and a depth of 0.4 mm, resulting in a special optical fiber structure with a "D" shape in the flat area.

[0032] like Figure 3 As shown, a side-polished terahertz fiber 1 with a polished area length of approximately 3 mm is obtained. The flat polished surface of the side-polished fiber forms a "leakage window" for the transmitted light in the fiber core. The evanescent field energy easily leaks out of the polished area. The micro-nano rectangular pillars 2 then act as a resonant cavity, effectively coupling terahertz waves that match the resonant frequency of the micro-nano rectangular pillars 2 into the resonant cavity. The interaction with the micro-nano rectangular pillars 2 produces electromagnetic resonance. The terahertz electromagnetic energy at the center frequency is used to maintain the electromagnetic oscillation, thereby forming a filter stopband.

[0033] The upper layer of the embodiment of the present invention has a carrier concentration of not less than 10 15 / cm 3 , carrier migration is not less than 100cm 2 / (V·s), the resistivity is not less than 500Ω / cm, the micro-nano rectangular column 2 is made of silicon material, and the rectangular structure 2 is integrated in Figure 4The longitudinal midline of the flat region 3 of the side-cast optical fiber is shown. The micro-nano rectangular pillar 2 is fabricated using high-precision physical cutting technology. A high-precision mechanical cutter is used to cut a 50cm diameter, round, smooth silicon substrate into 45x45µm square cross-sections 5 with a length of 1mm. The micro-nano rectangular pillar 2 possesses many novel properties in the optical field. At this structural size, it can effectively localize and filter terahertz waves with a center frequency of 1.15322THz and a half-wavewidth of 0.00129THz, achieving a maximum stopband attenuation of 9.32dB and a Q value of 893.9.

[0034] like Figure 5 The figure shows the working principle of the embodiment. The two ends of the terahertz optical fiber are connected to the terahertz time-domain spectrometer emission end and the terahertz time-domain spectrometer detection port respectively. After the terahertz wave is coupled into the terahertz optical fiber 1, filtering can be achieved. After the terahertz wave is input, the evanescent field leaks out from the side-throwing port. On the one hand, the micro-nano rectangular column 2 has an excellent Q value and performs filtering in an extremely narrow half-wave width, laying the foundation for accurate terahertz wave filtering. In addition, the micro-nano rectangular column 2 material designed by the Drude model and integrated in the longitudinal center line of the flat area 3 has a high carrier concentration (≥10 15 / cm 3 ), that is, the high carrier concentration can effectively localize and filter the terahertz waves localized in the micro-nano cuboid pillars 2, enhancing filtering capability and, in turn, effectively increasing the filter's maximum stopband attenuation. Furthermore, the side-thrown fiber 1 structure, with its sufficient side-throw length and depth, fully increases the contact length between the terahertz wave and the metamaterial surface, effectively improving the utilization efficiency of doped carriers and thus achieving better maximum stopband attenuation for terahertz waves.

[0035] like Figure 6 The transmission diagram of the embodiment with different cross-section sizes is shown. The terahertz filter is simulated using the finite element method. Figure 6 It can be seen that the structure with a size of 40*40*1000um filters the terahertz wave with a center frequency of 1.232THz and a half-wave width of 0.001187THz. The structure with a size of 45*45*1000um filters the terahertz wave with a center frequency of 1.15322THz and a half-wave width of 0.00129THz. The structure with a size of 50*50*1000um filters the terahertz wave with a center frequency of 1.0757THz and a half-wave width of 0.00125THz. Taking silicon material as an example, the change of the center frequency conforms to the change law of the Lorentz function and satisfies the formula: f0=0.72418[THz]+(2*A / π)*(w / (4*(x-xc) 2 +w 2))[THz] where A, w, and xc are the Lorentz constants: A = 42.58436, W = 47.09044, and xc = 31.25958; x represents the structure size. As the structure size increases, the center frequency of the resonant peak redshifts. This is because as the structure size increases, the wavelength of the terahertz wave that enters the structure increases, and the frequency decreases, causing the center frequency to redshift.

[0036] like Figure 7 The figure shows the Q value changes of different cross-section sizes of the embodiment. The finite element method is used to simulate the terahertz filter. Figure 7 It can be seen that in Figure 5 The Q value of section 5 with a side length of 40*40um can reach 1037.91, and the Q value of more than 700 can be achieved in the structural size between 40 and 55um.

[0037] like Figure 8 The maximum stopband attenuation depth changes of different cross-section sizes of the embodiment are shown. The finite element method is used to simulate the terahertz filter. Figure 8 It can be seen that in Figure 4 The maximum stopband attenuation of -9.32dB can be achieved at the section 5 with a side length of 45*45um, but the stopband attenuation of the structure size between 40 and 55um exceeds 3dB.

[0038] like Figure 9 The insertion loss changes of different cross-section sizes of the embodiment are shown. The finite element method is used to simulate the terahertz filter. Figure 9 It can be seen that in Figure 5 The insertion loss at section 5 with a side length of 45*45um is only 4.82dB, and the stopband attenuation of the structure size between 40 and 55um is less than 5dB.

[0039] As can be seen from the above, the present invention can effectively improve the low Q value, low stopband attenuation, and high cost of micro-nanostructures such as resonant cavities, which require high-precision equipment for fabrication. It can also appropriately adjust parameters such as the size of the micro-nano rectangular column 2 to obtain filtering characteristics in appropriate frequency bands, providing an important solution for the development of terahertz filters.

[0040] Although the embodiments disclosed in the present invention are as described above, the contents thereof are merely embodiments adopted to facilitate understanding of the technical solutions of the present invention and are not intended to limit the present invention. Any person skilled in the art of the present invention may make any modifications and changes in the form and details of the implementation without departing from the core technical solutions disclosed in the present invention. However, the scope of protection defined by the present invention shall still be subject to the scope defined in the appended claims.

Claims

1. A terahertz fiber filter based on a resonant structure, comprising a side-polished terahertz fiber for transmitting terahertz waves, wherein the cross section of the polished area is D-shaped, and the polished area is divided into transition areas at both ends and a flat area in the middle, characterized in that: In the flat area, micro-nano rectangular parallelepiped columns are integrated as resonant structures. The evanescent field interacts with the micro-nano rectangular parallelepiped columns to generate electromagnetic resonance. Forming a filter stop band; The structural dimension x and the center frequency f0 of the micro-nano rectangular parallelepiped column satisfy the formula: f0 = 0.72418 [THz] + (2*A / π)*(w / (4*(x-xc) 2 +w 2 ))[THz], where A, w, xc is the Lorentz constant, A = 42.58436, W=47.09044, xc=31.25958.

2. The terahertz optical fiber filter based on a resonant structure according to claim 1, characterized in that: The micro-nano cuboid column is located at the longitudinal midline of the flat area.

3. The terahertz optical fiber filter based on a resonant structure according to claim 1 or 2, characterized in that: The length of the micro-nano rectangular parallelepiped column is 10 to 10,000 μm.

4. The terahertz optical fiber filter based on a resonant structure according to claim 3, characterized in that: The cross section of the micro-nano cuboid column is rectangular and the length is 20 to 150 μm.

5. The terahertz optical fiber filter based on a resonant structure according to claim 3, characterized in that: The cross section of the micro-nano cuboid column is a square with a side length of 40 μm.

6. The terahertz optical fiber filter based on a resonant structure according to any one of claims 1 to 5, characterized in that: The side-polished terahertz optical fiber is a polymer microstructure optical fiber, and the diameter of the optical fiber is 800-3000 μm.

7. The terahertz optical fiber filter based on a resonant structure according to claim 6, characterized in that: The side polishing thickness of the side-polished terahertz optical fiber varies with the diameter of the optical fiber, and the side polishing thickness is 200 to 1500 μm.

8. The terahertz optical fiber filter based on a resonant structure according to any one of claims 1 to 5, characterized in that: The material of the micro-nano rectangular parallelepiped column has a carrier concentration of not less than 10 15 / cm 3 , carrier migration is not less than 100cm 2 / (V·s), P-type doped silicon material with a resistivity of not less than 500Ω / ㎝.

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