A high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter

Abstract

This invention belongs to the field of optical device technology, specifically relating to a high extinction ratio dual-core photonic crystal fiber terahertz polarization beamsplitter. Its cladding contains two inner elliptical air holes, two outer large air holes, eight intermediate air holes, and ten outer large elliptical air holes. The two inner elliptical air holes are centrally symmetrically arranged along the y-axis, the two outer large air holes are centrally symmetrically arranged along the x-axis, the eight intermediate air holes are arranged in groups of four symmetrically along both the y and x axes, and the ten outer large elliptical air holes are arranged in groups of five symmetrically along both the y and x axes. In each group of five outer large elliptical air holes, two are located on the outermost layer. The structure is flexible and compact, with excellent optical performance, simultaneously meeting the three key requirements of ultra-short length, high extinction ratio, and wide bandwidth. It overcomes the shortcomings of traditional polarization beams, such as long length, narrow operating bandwidth, and difficulty in meeting the requirements of integrated and miniaturized applications.

Description

Technical Field

[0001] This utility model belongs to the field of optical device technology, specifically relating to a high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter based on TOPAS material. Background Technology

[0002] Terahertz polarization beamsplitters, as a novel and rapidly developing optical device, are being studied in many fields. A polarization beam splitter decomposes an incident beam of light into two mutually orthogonal linearly polarized beams propagating along different paths, and is widely used in magneto-optical data storage, optical communication, optical sensing, and imaging. However, most polarization beamsplitters operate in the near-infrared band, and research on those operating in the terahertz band is relatively limited.

[0003] The terahertz band refers to electromagnetic waves with frequencies between 0.1 THz and 10 THz, falling between microwaves and infrared. Many high-performance optical devices have been commercialized, and devices capable of operating in different terahertz bands, such as those for terahertz time-domain spectroscopy identification, terahertz communication, and terahertz imaging, have a significant demand for terahertz polarization beamsplitters. This makes the development of efficient polarization beamsplitters extremely urgent.

[0004] Polarization beam splitters can split light into different paths based on its polarization state to meet the polarization control requirements of various devices. As devices strive for miniaturization, existing polarization beam splitters suffer from problems such as low effective refractive index difference between the two polarized light modes, resulting in long beam splitter lengths and complex manufacturing processes, which no longer meet miniaturization demands. Photonic crystal fibers, on the other hand, are widely studied due to their advantages such as flexible and tunable structure, low bending loss, and large mode field. Photonic crystal fiber polarization beam splitters are one of the important optical devices in network communication, and their performance directly affects the development trend of network communication. Furthermore, they offer advantages such as low loss and low dispersion in the terahertz band, making them particularly suitable for integrated and miniaturized applications.

[0005] The device length, bandwidth, and extinction ratio of a polarization beamsplitter are all indicators of its performance. Device length is a crucial indicator of whether the beamsplitter can completely separate two different polarization states; the shorter the device length, the better the beamsplitter's performance. Bandwidth is the wavelength range within which the polarization beamsplitter operates; a wider bandwidth means a larger operating range and better beamsplitter performance. The extinction ratio describes the degree of separation between two orthogonal polarization components at the beamsplitter's output. Only when the absolute value of the extinction ratio is greater than 20 dB can complete separation of the two different polarization states be guaranteed; a higher extinction ratio indicates better separation and better beamsplitter performance. Current polarization beams are relatively complex to manufacture, and they often have long lengths, low extinction ratios, and narrow bandwidths, making it difficult to meet the requirements of high-performance optical devices in the terahertz band. Utility Model Content

[0006] To address the aforementioned technical problems, this invention provides a high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter, which effectively solves the problems of long length, low extinction ratio, and narrow bandwidth found in existing polarization beam splitters.

[0007] The technical solution adopted in this utility model is as follows: a high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter, wherein the cladding is provided with 2 inner elliptical air holes, 2 outer large air holes, 8 intermediate air holes, and 10 outer large elliptical air holes; the 2 inner elliptical air holes are centrally symmetrically arranged on the y-axis, the 2 outer large air holes are centrally symmetrically arranged on the x-axis, the 8 intermediate air holes are arranged in groups of 4 symmetrically in the y-axis and x-axis directions, and the 10 outer large elliptical air holes are arranged in groups of 5 symmetrically in the y-axis and x-axis directions, and two of the 5 outer large elliptical air holes in each group are on the outer layer; the substrate material of the high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter is a cyclic olefin copolymer (COC), which has low material dispersion and relatively low material loss in the 0.1-1.5 terahertz range, and the refractive index of its core is higher than that of the cladding.

[0008] Furthermore, the radius R1 of the outer large air hole is larger than the radius R of the middle air hole.

[0009] Furthermore, the radius R1 is 0.178 mm ~ 0.202 mm, and the radius R is 0.148 mm ~ 0.17 mm.

[0010] Furthermore, the radius R1 is 0.1927 mm, and the radius R is 0.162 mm.

[0011] Furthermore, the major axis a of the inner elliptical air hole is 0.161mm ~ 0.171mm, its minor axis b is 0.035mm ~ 0.045mm, and the hole distance d between the two inner elliptical air holes is 0.108mm ~ 0.12mm.

[0012] Furthermore, the major axis a of the inner elliptical air hole is 0.161 mm, its minor axis b is 0.035 mm, and the hole distance d between the two inner elliptical air holes is 0.12 mm.

[0013] Furthermore, the major axis a1 of the outer large elliptical air hole is 0.166mm ~ 0.176mm, and its minor axis b1 is 0.09mm ~ 0.108mm.

[0014] Furthermore, the major axis a1 of the outer large elliptical air hole is 0.176 mm, and its minor axis b1 is 0.108 mm.

[0015] Furthermore, the hole spacing Λ between two adjacent intermediate air holes and the hole spacing Λ between adjacent intermediate air holes and outer large air holes are 0.373mm ~ 0.395mm.

[0016] Furthermore, the hole spacing Λ between two adjacent intermediate air holes and the hole spacing Λ between adjacent intermediate air holes and outer large air holes are 0.383 mm.

[0017] Furthermore, the high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter is composed of a substrate material and periodically arranged air holes. Two inner elliptical air holes are located in the center of the fiber core, positioned above and below the core center along the y-axis; two outer large air holes are located to the left and right of the inner elliptical air holes along the x-axis; eight intermediate air holes are arranged horizontally axially symmetrically above and below the inner elliptical air holes, forming a dual-core structure with the two inner elliptical air holes and the two outer large air holes; of the ten outer large elliptical air holes, five are located on the upper side along the y-axis, and the other group is located on the lower side along the y-axis, exhibiting an axially symmetrical structure in the horizontal direction. The inner elliptical air holes, intermediate air holes, outer large air holes, and outer large elliptical air holes are arranged from the inside out within the cladding.

[0018] Furthermore, the high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter can be applied in the terahertz band.

[0019] The beneficial effects of this invention are as follows: This invention provides a high extinction ratio dual-core photonic crystal fiber terahertz polarization beamsplitter. Based on photonic crystal fiber, this polarization beamsplitter achieves a length of only 29.50688 mm by optimizing parameters such as the structure and spacing of air holes in the cladding. At a frequency of 0.54 THz, the beamsplitter achieves an ultra-high extinction ratio of -324.257 dB, with an operating bandwidth ranging from 0.47 THz to 0.66 THz. Its significant improvements in length, extinction ratio, and bandwidth make it particularly suitable for integrated and miniaturized applications, and it holds significant application potential in terahertz band optical communication, optical sensing, and Internet information technology. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the cross-section of a high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter.

[0021] Figure 2 This is a three-dimensional schematic diagram of a high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter.

[0022] Figure 3 This is a schematic diagram showing the relationship between the effective refractive index and frequency of four supermodes in a high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter.

[0023] Figure 4 This is a schematic diagram showing the relationship between coupling length and frequency of a high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter.

[0024] Figure 5 This is a schematic diagram showing the relationship between the normalized output power of the x-polarization and y-polarization states of core A of a high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter and the fiber length.

[0025] Figure 6 This is a schematic diagram showing the relationship between the normalized output power of the x-polarization and y-polarization states of core B of a high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter and the fiber length.

[0026] Figure 7 This is a schematic diagram showing the functional relationship between the extinction ratio and frequency of a high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter. Detailed Implementation Example

[0027] like Figure 1 As shown, the cladding 5 of the high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter contains two inner elliptical air holes 1, two outer large air holes 2, eight intermediate air holes 3, and ten outer large elliptical air holes 4. The two inner elliptical air holes are centrally symmetrically arranged on the y-axis, the two outer large air holes are centrally symmetrically arranged on the x-axis, the eight intermediate air holes are arranged in groups of four symmetrically in the y- and x-axis directions, and the ten outer large elliptical air holes are arranged in groups of five symmetrically in the y- and x-axis directions. Two of the five outer large elliptical air holes in each group are located on the outer layer, exhibiting an axially symmetrical structure in the x- and y-axis directions. The substrate material of the high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter is a cyclic olefin copolymer (COC). COC exhibits low material dispersion and relatively low material loss in the 0.1-1.5 terahertz range, and its core refractive index is higher than that of the cladding.

[0028] The radius R1 of the outer large air hole is larger than the radius R of the middle air hole. Radius R1 is 0.1927 mm, and radius R is 0.162 mm. The major axis a of the inner elliptical air hole is 0.161 mm, and its minor axis b is 0.035 mm. The distance d between the two inner elliptical air holes is 0.12 mm. The major axis a1 of the outer large elliptical air hole is 0.176 mm, and its minor axis b1 is 0.108 mm. The distance Λ between two adjacent middle air holes and the distance Λ between adjacent middle air holes and the outer large air hole is 0.383 mm.

[0029] like Figure 2As shown, the polarization beam splitter confines most of the optical field energy within the fiber core, completely separating the two different polarization states. Dual-core photonic crystal fibers exhibit four modes: odd mode in the x-direction, even mode in the x-direction, odd mode in the y-direction, and even mode in the y-direction.

[0030] like Figure 3 As shown, due to the asymmetry of the photonic crystal fiber polarization beam splitter structure, the effective refractive index of the four supermodes increases with increasing frequency. It can be seen that the effective refractive indices of the four supermodes are different, with the y-mode (odd-even) being larger than the x-mode (odd-even), and they tend to converge as the frequency increases.

[0031] The change of coupling length with frequency is as follows Figure 4 As shown, as the frequency increases from 0.42 THz to 0.66 THz, the coupling lengths (lx, ly) in both the x and y directions show an upward trend, with the increase in the x direction being greater than that in the y direction. This is because the higher the frequency, the smaller the effective refractive index difference between the odd and even modes.

[0032] Figure 5 and Figure 6 The relationship between the normalized power variations of the x-polarized and y-polarized modes of the dual-core polarization beam splitter and the transmission distance is shown. It can be observed that the normalized power between x-polarization and y-polarization varies periodically with the transmission length; however, due to birefringence caused by structural asymmetry, the normalized power between x-polarization and y-polarization is not synchronized. Figure 5 As shown, when L = 29.50688 mm = 2lx = ly, the two polarization modes are almost completely separated, while Figure 6 The results show that core B, like core A, leads to the same conclusion: L = 29.50688 mm is the shortest length to satisfy the beam splitting condition. These two polarization modes are periodically separated, with the period length matching the minimum operating length of the designed polarization beam splitter.

[0033] Figure 7 The relationship between extinction ratio and frequency is shown when the polarization beamsplitter length is 29.50688 mm. The results show that at a frequency of 0.54 THz, the extinction ratio reaches as high as -324.257 dB. At this point, the polarized light in the two directions is almost completely separated, and the output power of the y-polarized light is much smaller than that of the x-polarized light. When the absolute value of the extinction ratio is ≥20 dB, the bandwidth reaches 0.19 THz.

[0034] Based on the above analysis, the high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter has an fiber length of 29.50688 mm, a maximum extinction ratio of -324.257 dB, and a bandwidth of 0.19 THz.

[0035] A high extinction ratio dual-core photonic crystal fiber terahertz polarization beamsplitter uses cyclic olefin copolymer (COC) as the substrate material and introduces a central air hole to achieve a dual-core structure. The influence of structural parameters on the performance of the polarization beamsplitter was analyzed using the finite element method. Simultaneously, at a frequency of 0.54 THz, the two polarization modes were almost completely separated. The shortest length is 29.50688 mm, the maximum extinction ratio reaches -324.257 dB, and the bandwidth is 0.19 THz (0.47 THz ~ 0.66 THz). With its short fiber length and high extinction ratio, the results indicate that the terahertz photonic crystal fiber polarization beamsplitter has great application potential in terahertz band optical communication, optical sensing, photoelectric detection, and imaging. Example

[0036] Based on the technical solution of Embodiment 1 above, the radius R1 is 0.178 mm, and the radius R is 0.148 mm. The major axis a of the inner elliptical air hole is 0.166 mm, its minor axis b is 0.04 mm, and the hole distance d between the two inner elliptical air holes is 0.114 mm. The major axis a1 of the outer large elliptical air hole is 0.171 mm, and its minor axis b1 is 0.099 mm. The hole distance Λ between two adjacent intermediate air holes and the hole distance Λ between adjacent intermediate air holes and the outer large air hole is 0.373 mm. Example

[0037] Based on the technical solution of Embodiment 1 above, the radius R1 is 0.202 mm, and the radius R is 0.17 mm. The major axis a of the inner elliptical air hole is 0.171 mm, its minor axis b is 0.045 mm, and the hole distance d between the two inner elliptical air holes is 0.108 mm. The major axis a1 of the outer large elliptical air hole is 0.166 mm, and its minor axis b1 is 0.09 mm. The hole distance Λ between two adjacent intermediate air holes and the hole distance Λ between adjacent intermediate air holes and the outer large air hole is 0.373 mm.

[0038] The high extinction ratio dual-core photonic crystal fiber terahertz polarization beamsplitter increases structural asymmetry by introducing inner and outer elliptical air holes, thereby enhancing birefringence; simultaneously, it increases the coupling length difference between the x-polarization and y-polarization states. Effective separation of polarized light can be achieved through reasonable fiber length design. The fiber terahertz polarization beamsplitter features a simple air hole arrangement, a reasonable structural design, and excellent optical performance, not only shortening fiber length but also providing a wide bandwidth and a high extinction ratio. The air holes in the cladding are arranged in a rhomboid pattern. By periodically arranging the inner elliptical air holes, outer large air holes, intermediate air holes, and outer large elliptical air holes, the polarization beamsplitter forms a dual-core structure, namely core A and core B. This structure increases asymmetry, thereby improving the birefringence effect.

Claims

1. A high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter, characterized in that: The cladding (5) of the high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter is provided with 2 inner elliptical air holes (1), 2 outer large air holes (2), 8 intermediate air holes (3) and 10 outer large elliptical air holes (4); the 2 inner elliptical air holes (1) are centrally symmetrically arranged on the y-axis, the 2 outer large air holes (2) are centrally symmetrically arranged on the x-axis, the 8 intermediate air holes (3) are arranged symmetrically in groups of 4 in the y-axis and x-axis directions, and the 10 outer large elliptical air holes (4) are arranged symmetrically in groups of 5 in the y-axis and x-axis directions, and two of the 5 outer large elliptical air holes (4) in each group are on the outer layer; The substrate material of the high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter is a cyclic olefin copolymer; The refractive index of the core of the high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter is higher than that of the cladding.

2. The high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter according to claim 1, characterized in that: The radius R1 of the outer large air hole (2) is greater than the radius R of the middle air hole (3).

3. The high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter according to claim 2, characterized in that: The radius R1 is 0.178mm ~ 0.202mm, and the radius R is 0.148mm ~ 0.17mm.

4. The high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter according to claim 3, characterized in that: The radius R1 is 0.1927 mm, and the radius R is 0.162 mm.

5. The high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter according to claim 1, characterized in that: The major axis a of the inner elliptical air hole (1) is 0.161mm ~ 0.171mm, the minor axis b is 0.035mm ~ 0.045mm, and the hole distance d between the two inner elliptical air holes (1) is 0.108mm ~ 0.12mm.

6. The high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter according to claim 5, characterized in that: The major axis a of the inner elliptical air hole (1) is 0.161 mm, its minor axis b is 0.035 mm, and the hole spacing d of the two inner elliptical air holes (1) is 0.12 mm.

7. The high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter according to claim 1, characterized in that: The major axis a1 of the outer large elliptical air hole (4) is 0.166mm ~ 0.176mm, and its minor axis b1 is 0.09mm ~ 0.108mm.

8. The high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter according to claim 7, characterized in that: The major axis a1 of the outer large elliptical air hole (4) is 0.176 mm, and its minor axis b1 is 0.108 mm.

9. The high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter according to claim 1, characterized in that: The hole spacing Λ between two adjacent intermediate air holes (3) and the hole spacing Λ between adjacent intermediate air holes (3) and outer large air holes (2) are 0.373mm ~ 0.395mm.

10. The high extinction ratio dual-core photonic crystal fiber terahertz polarization beam splitter according to claim 9, characterized in that: The hole spacing Λ between two adjacent intermediate air holes (3) and the hole spacing Λ between adjacent intermediate air holes (3) and outer large air holes (2) is 0.383 mm.

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