Compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling

Abstract

The application discloses a compact double-core photonic crystal fiber polarization beam splitter based on liquid crystal filling, and particularly relates to the technical field of optical communication, and solves the technical problem that the existing double-core photonic crystal fiber polarization beam splitter is difficult to simultaneously meet the requirements of short device length, high extinction ratio and long working bandwidth. The technical scheme is as follows: in the core region and the cladding region on the base material, the core region comprises a fourth circular air hole filled with liquid crystal and two cores, the two cores are formed by adjusting the distribution positions of the elliptical air holes, and the cladding region comprises a plurality of first circular air holes, two second circular air holes with gold films attached to the inner walls, two third circular air holes and four elliptical air holes. The application has the advantages of short device length, high extinction ratio and long working bandwidth, can completely cover the S+C+L optical communication waveband, and meets the requirements of high performance, miniaturization and high integration of optical devices for all-optical networks.

Description

Technical Field

[0001] This invention relates to the field of optical communication technology, and specifically to a compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling. Background Technology

[0002] Driven by emerging technologies such as the Internet, big data, and artificial intelligence, the ever-increasing demand for data transmission is propelling communication systems towards all-optical networks with ultra-high capacity, ultra-high speed, and ultra-long-distance transmission. Polarization beam splitters (PBS), as passive devices, can split a beam of light into two beams with orthogonal polarization directions, playing a crucial role in optical communication and optical sensing. Traditional single-mode fiber-based polarization beam splitters, due to their large size, low extinction ratio, and narrow operating bandwidth, struggle to meet the demands of all-optical networks for high-performance, miniaturized, and highly integrated optical devices. Photonic crystal fiber (PCF), as a novel waveguide, with its flexible structural design and excellent optical properties (such as cutoff-free single-mode transmission, ultra-low loss, and high birefringence), has become an ideal carrier for new PBSs. PCF-based PBSs not only offer significantly improved performance but also can be directly interfaced with fiber optic networks, thus experiencing rapid development. Among them, dual-core (DC) photonic crystal fiber (PBS) is the main research direction at present. Specifically, it induces birefringence by introducing asymmetric cores, elliptical air holes or circular air holes of different sizes, thereby achieving complete separation of two polarized lights.

[0003] Advances in materials science and filling technology have enabled the application of various functional materials (such as metals, oils, liquid crystals, magnetohydrodynamics, and gases) in PCF-PBS. By filling or coating the air pores of PCF with functional materials, light transmission characteristics can be effectively controlled, thereby achieving high-performance PBS. Liquid crystals, as a highly birefringent material, possess both the ordered molecular orientation of crystals and the fluidity and continuity of liquids. Their inclusion endows PCFs with diverse light transmission characteristics and real-time tuning capabilities. Furthermore, liquid crystals exhibit rapid response to changes in external fields (such as electric fields, temperature, and pressure), providing PCF-PBS with electric field and temperature tuning capabilities. Therefore, combining liquid crystal materials with novel DC-PCF structural designs has become one of the current hot research directions in PBS. To enhance the coupling characteristics of the DC-PCF-PBS dual core, metal films are often coated in the PCF air pores to excite the surface plasmon resonance (SPR) effect. The SPR effect is an optical phenomenon that occurs at the metal-dielectric interface. Under the excitation of incident light, surface plasmon polaritons (SPPs) modes can be formed on the metal surface. When the core mode and the SPP mode are phase-matched, coupling resonance will occur, thereby achieving effective control of the fiber mode field. SPR technology can significantly shorten the coupling length of the PCF, providing an effective way for the miniaturization design of PBS.

[0004] As can be seen from the published patents, although DC-PCF-PBS has achieved a breakthrough in a single performance indicator, it is difficult to simultaneously meet the performance requirements of short device length, high extinction ratio and long operating bandwidth. For example, patent CN114935791B can achieve multi-window communication, but the bandwidth of each communication window is relatively narrow (1.31μm / 23nm, 1.55μm / 20nm, 1.75μm / 19nm, 1.84μm / 11nm), and the length of the beam splitter is 16.1mm; patent CN109254348B can achieve a relatively short beam splitting length of 83.9μm, but the bandwidth less than -10dB is only 32.1nm; patent CN103091770A can achieve a bandwidth of 146nm, but the minimum extinction ratio is -39.4dB, and the length of the beam splitter is 63mm; patent CN108415121B can achieve a high extinction ratio of -76.24dB and a beam splitting length of 156μm, but the bandwidth is only 60nm. Summary of the Invention

[0005] Therefore, this invention solves the technical problem that existing DC-PCF-PBS cannot simultaneously meet the requirements of short device length, high extinction ratio, and long operating bandwidth. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling provided by this invention has the advantages of short device length, high extinction ratio, and long operating bandwidth. It has a length of only 14.43 μm, a maximum extinction ratio of -89.04 dB, and an operating bandwidth of 180 nm. It can completely cover the S+C+L optical communication band and meet the requirements of all-optical networks for high performance, miniaturization, and high integration of optical devices.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling, comprising a substrate material and a core region and a cladding region disposed on the substrate material; the core region is provided with a central fourth circular air hole filled with liquid crystal and two fiber cores, the two fiber cores being formed by adjusting the distribution position of the elliptical air hole, and the two fiber cores being distributed on both sides of the central fourth circular air hole along the x-axis direction;

[0007] The cladding region includes multiple first circular air holes, two second circular air holes with gold films attached to their inner walls, two third circular air holes, and four elliptical air holes; with the central fourth circular air hole of the liquid crystal filling as the center, the multiple first circular air holes, the two second circular air holes with gold films attached to their inner walls, the four elliptical air holes, and the two third circular air holes surround the central fourth circular air hole of the liquid crystal filling in a multi-layer structure, and each layer of the multi-layer structure is a hexagonal structure except for the two trapezoidal structures in the fiber core region;

[0008] Two of the third circular air holes are distributed on both sides of the fiber core region along the x-axis, and four elliptical air holes are distributed on both sides of the fiber core region along the y-axis, with two elliptical air holes on each side. The major axis of the elliptical air holes is parallel to the x-axis. The two second circular air holes with gold film attached to their inner walls are distributed on both sides of the central fourth circular air hole along the y-axis. A plurality of first circular air holes are distributed on the outer sides of the second circular air holes, third circular air holes, and elliptical air holes.

[0009] Furthermore, the fourth circular air hole at the center of the liquid crystal filling is located at the structural center of the photonic crystal fiber polarization beam splitter.

[0010] Furthermore, the hexagonal structure is a regular hexagonal structure.

[0011] Furthermore, the plurality of first circular air holes are arranged in an equilateral triangle periodicity and are symmetrically distributed along the center; the two third circular air holes are symmetrically distributed on both sides of the fiber core region along the x-axis; the four elliptical air holes are symmetrically distributed on both sides of the fiber core region along the y-axis; and the two second circular air holes with gold film attached to their inner walls are symmetrically distributed on both sides of the central fourth circular air hole along the y-axis.

[0012] Furthermore, the photonic crystal fiber polarization beam splitter also includes a perfectly matched layer disposed on the outermost periphery of the substrate material, the thickness of which is 2 μm.

[0013] Furthermore, the substrate material is SF57 glass.

[0014] Furthermore, the liquid crystal is an E7 type nematic liquid crystal, and the diameter of the central fourth circular air hole is 0.40 μm.

[0015] Furthermore, the diameter of the second circular air hole is 0.71 μm, and the thickness of the gold film is 50 nm.

[0016] Furthermore, the major axis of the elliptical air hole is 0.90 μm and the minor axis is 0.60 μm.

[0017] Further, the diameter of the first circular air hole is 0.71 μm; the diameter of the third circular air hole is 0.85 μm; the center-to-center distance between two adjacent first circular air holes, the center-to-center distance between two adjacent first circular air holes and the second circular air hole, the center-to-center distance between two adjacent first circular air holes and the third circular air hole, and the center-to-center distance between two adjacent second circular air holes and the fourth circular air hole are the same, and the center-to-center distance is 0.95 μm; the center-to-center distance between adjacent first circular air holes and the elliptical air hole is the same, and the center-to-center distance is 0.76 μm.

[0018] The technical effects and advantages provided by the present invention in the above technical solution are as follows:

[0019] 1. The compact dual-core photonic crystal fiber polarization beamsplitter based on liquid crystal filling provided by this invention enhances the structural asymmetry by employing circular and elliptical air holes of different sizes. Simultaneously, the introduction of gold film and liquid crystal material further improves the birefringence effect, resulting in a shorter device length and better beam splitting performance. This polarization beamsplitter has a length of only 14.43 μm and achieves a maximum extinction ratio of -89.04 dB at a working wavelength of 1550 nm.

[0020] 2. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling provided by the present invention has a wider communication band, with a bandwidth of 180nm, which can completely cover the S+C+L optical communication band.

[0021] 3. The compact dual-core photonic crystal fiber polarization beamsplitter based on liquid crystal filling provided by the present invention has excellent characteristics of short device length, high extinction ratio and long operating bandwidth, which meets the requirements of all-optical networks for high performance, miniaturization and high integration of optical devices. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0023] Figure 1 This is a schematic diagram of the radial cross-sectional structure of Embodiment 1 of the present invention;

[0024] Figure 2 (a) is a field distribution diagram of the x-polarized state even mode in Embodiment 1 of the present invention;

[0025] Figure 2 (b) is a field distribution diagram of the odd mode of the x-polarization state in Embodiment 1 of the present invention;

[0026] Figure 2 (c) is a field distribution diagram of the y-polarized state even mode in Embodiment 1 of the present invention;

[0027] Figure 2 (d) is a field distribution diagram of the odd mode of the y-polarization state in Embodiment 1 of the present invention;

[0028] Figure 3 This is a graph showing the relationship between the coupling length ratio of Embodiment 1, Comparative Examples 1 and 2 of the present invention and the wavelength.

[0029] Figure 4 This is a graph showing the relationship between the coupling length ratio of Embodiment 1, Comparative Examples 3 and 4 of the present invention and the wavelength.

[0030] Figure 5 This is a graph showing the relationship between the coupling length ratio of Embodiment 1, Comparative Examples 5 and 6 of the present invention and the wavelength.

[0031] Figure 6 This is a graph showing the relationship between the coupling length ratio of Embodiment 1, Comparative Examples 7 and 8 of the present invention and the wavelength.

[0032] Figure 7This is a graph showing the relationship between the coupling length ratio of Embodiment 1, Comparative Examples 9 and 10 of the present invention and the wavelength.

[0033] Figure 8 This is a graph showing the relationship between the coupling length ratio of Embodiment 1 of the present invention and Comparative Examples 11 and 12 as a function of wavelength;

[0034] Figure 9 This is a graph showing the relationship between the coupling length ratio of Embodiment 1 and Comparative Examples 13, 14 and 15 of the present invention and the wavelength.

[0035] Figure 10 This is a graph showing the normalized power of the output terminals of fiber cores A and B in the x and y polarization directions as a function of transmission distance when the incident wavelength is 1.55 μm, according to Embodiment 1 of the present invention.

[0036] Figure 11 This is a graph showing the extinction ratio of fiber cores A and B as a function of wavelength when the beam splitting length is 14.43 μm, according to Embodiment 1 of the present invention.

[0037] Explanation of reference numerals in the attached figures:

[0038] 1. Substrate material; 2. First circular air hole; 3. Second circular air hole; 4. Gold film; 5. Elliptical air hole; 6. Third circular air hole; 7. Fourth circular air hole; 8. Liquid crystal; 9. Perfect matching layer. Detailed Implementation

[0039] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0040] The substrate material of the photonic crystal fiber polarization beam splitter of this invention is SF57 glass, which belongs to soft glass materials. Compared with traditional silica glass fiber, soft glass fiber has significant advantages in optical performance. It not only has higher linear and nonlinear refractive indices, but also exhibits wider transmission characteristics. Therefore, soft glass materials, represented by SF57 glass, are a new type of substrate material that is currently widely used. Its dispersion relation, the Sellmeier equation, can be given by equation (1):

[0041]

[0042] In the formula, λ is the incident light wavelength, in μm, A i The fitting coefficients are denoted as A0 = 3.24748 and A1 = -0.00954782 μm. -2 A2 = 0.0493626 μm 2 A3 = 0.00294294 μm 4 A4 = -1.48144 × 10-4 μm 6 A5 = 2.78427 × 10 -5 μm 8 .

[0043] In this invention, gold is chosen as the material for exciting surface plasmons. Compared to silver and aluminum, gold has the following advantages: First, gold has strong corrosion resistance and can remain stable under various environmental conditions; second, gold exhibits excellent oxidation resistance and can maintain its performance for a long time without being damaged by oxidation; third, gold has good ductility, which facilitates the preparation of high-quality nanofilm structures. Therefore, this invention selects gold as the material for exciting surface plasmons. The relative permittivity of gold can be given by the Drude-Lorentz model using equation (2):

[0044]

[0045] Where: ε m ε is the relative permittivity of gold. ∞ ω is the dielectric constant of the metal at high frequencies, and ω is the incident light frequency. D and γ D These are the plasma frequency and damping frequency of gold, respectively, Ω L and Г L Here, ε represents the intensity and spectral width of the Lorentz oscillator, respectively; Δε is the weighting factor for the Lorentz term; and j denotes the sign of the imaginary part of the complex number. The numerical values ​​of the above parameters are ε. ∞ =5.9673, ω D = 2π × 2113.6 THz, γ D = 2π × 15.92 THz, Ω L = 2π × 650.07 THz, Г L =2Π×104.86THz, Δε=1.09.

[0046] This invention utilizes E7-type nematic liquid crystal (NLC) filled within the central fourth circular air hole 7 to further enhance the birefringence effect and reduce the fiber splitting length. The refractive index of E7-type NLC can be classified as ordinary refractive index n. o and special refractive index n e Specifically, this can be given by the Cauchy model, as shown in equations (3) and (4) below:

[0047]

[0048] In the formula, A i B i C i As temperature-dependent coefficients, the values ​​of each temperature coefficient at 25℃ are: A o=1.4994, B o =0.0070μm 2 C o =0.0004μm 4 A e =1.6933, B e =0.0078μm 2 C e =0.0028μm 4 .

[0049] The relative permittivity tensor of a liquid crystal can be given by equation (5):

[0050]

[0051] In the formula, α is the rotation angle of the NLC molecule, which is defined as the angle between the long axis direction of the wheel-shaped NLC molecule and the x-axis direction. In this invention, α is set to 0°, that is, the long axis of the E7 type NLC material molecule is parallel to the x-axis.

[0052] Coupling length (CL) refers to the minimum distance required for optical power in one fiber core to be completely transferred to another. Coupling occurs between two mode fields with the same polarization direction and similar effective refractive index. In DC-PCF, the coupling length in the x and y polarization directions can be given by equation (6):

[0053]

[0054] In the formula: λ is the incident light wavelength, β i even β i odd n i even n i odd These represent the propagation constants and effective refractive indices of the even and odd modes in the x or y polarization directions, respectively.

[0055] The coupling length ratio (CLR) refers to the ratio of the coupling lengths in the x and y polarization directions, which can be given by equation (7):

[0056]

[0057] In the formula, CL x CL is the coupling length in the x-polarization direction. y Where m is the coupling length in the y-polarization direction, and m and n are positive integers with opposite parity. When the CLR is 2 or 1 / 2, the PBS can achieve a smaller device size and better beam splitting performance.

[0058] For a DC-PCF, when light is incident on the fiber core, it propagates in the form of sine and cosine trigonometric functions. Therefore, in fiber cores A and B, the normalized output power (NOP) can be expressed by equations (8) and (9):

[0059]

[0060] In the formula, These represent the output power of fiber cores A and B, respectively, where i represents the x or y polarization direction, and P... in denoted by z, which represents the incident light power in the fiber core, and z represents the distance the incident light travels within the fiber.

[0061] Extinction ratio (ER) describes the ability of different polarization states to be separated at an output port after light waves have traveled a certain distance in the fiber core. The extinction ratios of fiber cores A and B can be expressed by equations (10) and (11):

[0062]

[0063] In the formula, These represent the output power of fiber cores A and B in the x or y polarization direction, respectively. When ER is less than -20dB (i.e., the optical power of one polarized light is 100 times that of the other polarized light), the two polarized lights can be completely separated. At this time, the wavelength range covered is the bandwidth of PBS.

[0064] Example 1

[0065] This embodiment uses a photonic crystal fiber polarization beam splitter, such as... Figure 1 As shown, it includes a substrate material 1 and a core region and a cladding region disposed on the substrate material 1; the core region includes a central fourth circular air hole 7 of a liquid crystal 8 and two cores; the central fourth circular air hole 7 of the liquid crystal 8 is distributed at the structural center of the photonic crystal fiber polarization beam splitter, and the two cores are formed by adjusting the distribution position of the elliptical air holes 5. The two cores are distributed on both sides of the central fourth circular air hole 7 along the x-axis direction, with core A on the left and core B on the right.

[0066] The cladding region includes fifty-two first circular air holes 2, two second circular air holes 3 with gold film attached to their inner walls, two third circular air holes 6, and four elliptical air holes 5; with the fourth circular air hole 7 at the center of the filling liquid crystal 8 as the center, the fifty-two first circular air holes 2, the two second circular air holes 3 with gold film attached to their inner walls, the four elliptical air holes 5, and the two third circular air holes 6 surround the fourth circular air hole 7 at the center of the filling liquid crystal 8 in a multi-layer structure, and each layer of the multi-layer structure is a regular hexagonal structure except for the two trapezoidal structures in the fiber core region;

[0067] Two third circular air holes 6 are centrally symmetrically distributed on both sides of the fiber core region along the x-axis; four elliptical air holes 5 are centrally symmetrically distributed on both sides of the fiber core region along the y-axis, with two elliptical air holes 5 on each side, and the major axis of the elliptical air holes 5 is parallel to the x-axis; two second circular air holes 3 with gold film 4 attached to their inner walls are centrally symmetrically distributed on both sides of the central fourth circular air hole 7 along the y-axis; and fifty-two first circular air holes 2 are centrally symmetrically distributed on the outer sides of the second circular air holes 3, the third circular air holes 6, and the elliptical air holes 5.

[0068] The photonic crystal fiber polarization beam splitter also includes a perfectly matched layer 9 disposed on the outermost layer of the substrate material 1, with a thickness of 2 μm. The substrate material 1 is SF57 glass. The liquid crystal 8 is an E7 type nematic liquid crystal, with a central fourth circular air hole 7 having a diameter of 0.40 μm. The second circular air hole 3 has a diameter of 0.71 μm, and the gold film 4 has a thickness of 50 nm. The elliptical air hole 5 has a major axis length of 0.90 μm and a minor axis length of 0.60 μm. The first circular air hole 2 has a diameter of 0.71 μm. The diameter of the third circular air hole 6 is 0.85 μm; the center-to-center distance between two adjacent first circular air holes 2, between two adjacent first circular air holes 2 and the second circular air hole 3, between two adjacent first circular air holes 2 and the third circular air hole 6, and between two adjacent second circular air holes 3 and the fourth circular air hole 7 are the same, with a center-to-center distance of 0.95 μm; the center-to-center distance between adjacent first circular air holes 2 and the elliptical air hole 5 is the same, with a center-to-center distance of 0.76 μm.

[0069] Figure 1 This is a schematic diagram of the radial cross-sectional structure of Embodiment 1 of the present invention, with spatial coordinates xyz; d1 is the diameter of the first circular air hole 2, d2 is the diameter of the fourth circular air hole 7, d3 is the diameter of the third circular air hole 6, d4 is the diameter of the second circular air hole 3, a is the length of the major axis of the elliptical air hole 5, b is the length of the minor axis of the elliptical air hole 5, t is the thickness of the gold film 4, and Λ is the distance between the centers.

[0070] Figure 2 (a) is a field distribution diagram of the x-polarized state even mode in Embodiment 1 of the present invention. Figure 2 (b) is a field distribution diagram of the x-polarization odd mode of Embodiment 1 of the present invention. Figure 2 (c) is a field distribution diagram of the y-polarized state even mode in Embodiment 1 of the present invention. Figure 2 (d) is a diagram showing the odd-mode field distribution of the y-polarized state in Embodiment 1 of the present invention. It can be seen that the coupled light energy is mainly concentrated at the fiber core, but some of the mode field energy in the x-polarized direction is dispersed into the air holes of the liquid crystal, and some of the mode field energy in the y-polarized direction is dispersed on the surface of the gold thin film.

[0071] Comparative Example 1

[0072] Similar to Example 1, except that the diameter d1 of the first circular air hole is 0.66 μm.

[0073] Comparative Example 2

[0074] Similar to Example 1, except that the diameter d1 of the first circular air hole is 0.76 μm.

[0075] Comparative Example 3

[0076] Similar to Example 1, except that the diameter d2 of the fourth circular air hole is 0.35 μm.

[0077] Comparative Example 4

[0078] Similar to Example 1, except that the diameter d2 of the fourth circular air hole is 0.45 μm.

[0079] Comparative Example 5

[0080] Similar to Example 1, except that the diameter d3 of the third circular air hole is 0.80 μm.

[0081] Comparative Example 6

[0082] Similar to Example 1, except that the diameter d3 of the third circular air hole is 0.90 μm.

[0083] Comparative Example 7

[0084] Same as in Example 1, except that the distance Λ between the air holes is 0.90 μm.

[0085] Comparative Example 8

[0086] Same as in Example 1, except that the distance Λ between the air holes is 1.00 μm.

[0087] Comparative Example 9

[0088] Similar to Example 1, except that the major axis length a of the elliptical air hole is 0.80 μm.

[0089] Comparative Example 10

[0090] Same as in Example 1, except that the major axis length a of the elliptical air hole is 1.00 μm.

[0091] Comparative Example 11

[0092] Same as in Example 1, except that the minor axis length b of the elliptical air hole is 0.50 μm.

[0093] Comparative Example 12

[0094] Similar to Example 1, except that the minor axis length b of the elliptical air hole is 0.70 μm.

[0095] Comparative Example 13

[0096] Same as in Example 1, except that the thickness t of the gold film is 40 nm.

[0097] Comparative Example 14

[0098] Same as in Example 1, except that the thickness t of the gold film is 60 nm.

[0099] Comparative Example 15

[0100] Same as in Example 1, except for the thickness t of the gold film, where t is 0 nm.

[0101] The relationship between the coupling length ratio and wavelength in Example 1 and Comparative Examples 1-15 is as follows: Figures 3-9 As shown in the figure. Example 1 shows the curves of normalized power in the x and y polarization directions at the output ends of fiber cores A and B as a function of transmission distance when the incident wavelength is 1.55 μm, and the curves of extinction ratio of fiber cores A and B as a function of wavelength when the beam splitting length is 14.43 μm, respectively. Figure 10 and Figure 11 As shown.

[0102] Compared with Comparative Examples 1 and 2, Example 1 is as follows: Figure 3 As shown, at the same wavelength, the coupling length ratio decreases as the diameter d1 of the first circular air hole increases. Increasing the diameter d1 confines more light within the fiber core, thus enhancing the coupling strength of the dual cores. The coupling length ratio curve for d1 = 0.71 μm is smoother, and the coupling length ratio is closest to 2 at the 1.55 μm communication window. Therefore, this invention selects d1 = 0.71 μm as the diameter of the first circular air hole.

[0103] Compared with Comparative Examples 3 and 4, Example 1 is as follows: Figure 4 As shown, the three curves exhibit very small differences in variation. From the magnified view, it can be seen that within the wavelength range of 1.52 μm to 1.58 μm, the coupling length ratio decreases as the diameter d2 of the fourth circular air hole increases. Increasing the diameter d2 of the fourth circular air hole leads to more liquid crystal filling within the air hole, resulting in a greater influence from the liquid crystal and weakening the coupling strength of the x-polarized mode. d2 plays a fine-tuning role in the structural optimization process. After fully considering the performance and fabrication tolerance of the polarization beam splitter, this invention selects a fourth circular air hole diameter of d2 = 0.40 μm.

[0104] Compared with Comparative Examples 5 and 6, Example 1 is as follows: Figure 5 As shown, the difference in the variation of the three curves gradually increases from 1.6 μm and exhibits a blue shift as the diameter d3 of the third circular air hole increases. From the magnified view, it can be seen that at 1.55 μm, the coupling length ratio decreases as the diameter d3 of the third circular air hole increases. Increasing the diameter d3 of the third circular air hole reduces the fiber core area, decreasing the fiber core's ability to confine the polarization mode, thus making the coupling effect more likely to occur. To obtain a higher-performance polarization beam splitter, this invention selects a third circular air hole diameter of d3 = 0.85 μm.

[0105] Compared with Comparative Examples 7 and 8, Example 1 is as follows: Figure 6 As shown, at the same wavelength, the coupling length ratio increases with the increase of the distance Λ between the air holes. The increase in the distance Λ between the air holes reduces the air hole filling ratio in the cladding region, leading to a decrease in the refractive index difference between the core and cladding regions. More light energy leaks into the cladding region, thus weakening the coupling strength between the cores. The curve for Λ = 0.90 μm exhibits abrupt changes and is not smooth. Furthermore, at the 1.55 μm communication window, only the coupling length ratio for Λ = 0.95 μm is close to 2. Therefore, this invention selects Λ = 0.95 μm as the distance between the air holes.

[0106] Compared with Comparative Examples 9 and 10, Example 1 is as follows: Figure 7 As shown, at short wavelengths, the coupling length ratio is directly proportional to the major axis length *a* of the elliptical air aperture, while at long wavelengths, it is inversely proportional to *a*. Increasing the major axis length *a* of the elliptical air aperture increases the asymmetry between the dual-core structures, resulting in enhanced coupling strength of the x-polarized mode and a weakening followed by a strengthening of the y-polarized mode coupling strength. At a communication window of 1.55 μm, the coupling length ratios for the three cases are 2.063, 2.008, and 2.041, respectively. Clearly, the coupling length ratio at *a* = 0.90 μm is closer to 2; therefore, this invention selects *a* = 0.90 μm as the major axis length of the elliptical air aperture.

[0107] Compared with Comparative Examples 11 and 12, Example 1 is as follows: Figure 8 As shown, the three curves change relatively uniformly and at the same wavelength, the coupling length ratio decreases as the minor axis length b of the elliptical air hole increases. Increasing the minor axis length b of the elliptical air hole compresses the fiber core in the vertical direction, enhancing the coupling effect in the y-polarization direction. At the 1.55 μm communication window, only when b = 0.60 μm is the coupling length ratio close to 2; therefore, this invention selects a minor axis length of 0.60 μm for the elliptical air hole.

[0108] Compared with Comparative Examples 13, 14 and 15, Example 1 is as follows: Figure 9 As shown, the difference in the three curves with the gold film gradually decreases from 1.37 μm, and the coupling length ratio at t = 0 nm remains within the range of 1.0 to 1.2. From the magnified view, it can be seen that in the wavelength range of 1.52 μm to 1.58 μm, the coupling length ratio increases with the increase of the gold film thickness t. Increasing the gold film thickness t weakens the coupling strength between the two fiber cores, leading to a larger coupling length. Since the coupling length ratio changes little under the three gold film thicknesses, and the coupling length ratio at t = 0 nm cannot meet the condition of 2 or 1 / 2, after fully considering the manufacturing cost and excellent performance of the polarization beam splitter, this invention selects t = 50 nm as the gold film thickness.

[0109] like Figure 10 As shown, in Example 1, the photonic crystal fiber polarization beamsplitter, when the incident wavelength is 1.55 μm, exhibits normalized power in the x and y polarization directions at the output ends of fiber cores A and B, propagating in the form of sine and cosine functions. At a transmission distance of 14.43 μm, the normalized power in the x polarization direction at the output end of fiber core A and the y polarization direction at the output end of fiber core B reaches its maximum value, while the normalized power in the y polarization direction at the output end of fiber core A and the x polarization direction at the output end of fiber core B reaches its minimum value. If light is incident from the end of fiber core A, then fiber core A outputs y-polarized light, and fiber core B outputs x-polarized light; that is, x and y polarized light are completely separated in the two fiber cores. Therefore, the shortest length of this beamsplitter is 14.43 μm.

[0110] like Figure 11 As shown, in Example 1, the photonic crystal fiber polarization beamsplitter, with a splitting length of 14.43 μm, exhibits an extinction ratio of -89.04 dB at 1.55 μm for fiber core A, and this extinction ratio remains less than -20 dB within the wavelength range of 1.46 μm to 1.64 μm, resulting in a bandwidth of 180 nm, which fully covers the S+C+L optical communication band. Similarly, the extinction ratio of fiber core B reaches peak values ​​of -80.03 dB and -58.62 dB at 1.55 μm and 1.65 μm, respectively, and this extinction ratio remains less than -20 dB within the wavelength range of 1.45 μm to 1.69 μm, resulting in a bandwidth of 240 nm, which also fully covers the S+C+L+U optical communication band.

[0111] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. Any equivalent structural or procedural transformations made based on the description and drawings of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.

Claims

1. A compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling, characterized in that, The substrate material (1) is provided with a core region and a cladding region. The cladding region is located outside the core region. The cladding region includes a plurality of first circular air holes (2), two second circular air holes (3) with gold film (4) attached to their inner walls, two third circular air holes (6), and four elliptical air holes (5). The area enclosed by the second circular air holes (3), the two third circular air holes (6), and the four elliptical air holes (5) is the core region. A central fourth circular air hole (7) filled with liquid crystal (8) is provided in the center. The line connecting the two second circular air holes (3) along the Y-axis divides the core region into two cores. The two cores are distributed on both sides of the central fourth circular air hole (7) along the x-axis. The fourth circular air hole (7) at the center of the liquid crystal filling (8) is distributed at the structural center of the photonic crystal fiber polarization beam splitter; a plurality of first circular air holes (2), two second circular air holes (3) with gold film (4) attached to the inner wall, four elliptical air holes (5) and two third circular air holes (6) surround the fourth circular air hole (7) at the center of the liquid crystal filling (8) in a multi-layer structure, and except for the fiber core area which is two trapezoidal structures, each of the other layers is a regular hexagonal structure; The four elliptical air holes (5) are distributed on both sides of the fiber core region along the y-axis, with two elliptical air holes (5) distributed on each side. The major axis of the elliptical air holes (5) is parallel to the x-axis. Multiple first circular air holes (2) are arranged in an equilateral triangle periodic pattern and are symmetrically distributed around the center on the outside of the second circular air holes (3), the third circular air holes (6), and the elliptical air holes (5); two third circular air holes (6) are symmetrically distributed around the center on both sides of the fiber core region along the x-axis; four elliptical air holes (5) are symmetrically distributed around the center on both sides of the fiber core region along the y-axis; two second circular air holes (3) with gold film (4) attached to their inner walls are symmetrically distributed around the center fourth circular air hole (7) on both sides along the y-axis. The beam splitter coupling length ratio is close to 2, covering the S+C+L+U optical communication bands.

2. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling according to claim 1, characterized in that, It also includes a perfect matching layer (9) disposed on the outermost periphery of the substrate material (1), the perfect matching layer (9) having a thickness of 2 μm.

3. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling according to claim 2, characterized in that, The substrate material (1) is SF57 glass.

4. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling according to claim 3, characterized in that, The liquid crystal (8) is an E7 type nematic liquid crystal, the diameter of the central fourth circular air hole (7) is 0.40 μm; the diameter of the second circular air hole (3) is 0.71 μm, and the thickness of the gold film (4) is 50 nm.

5. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling according to claim 4, characterized in that, The major axis of the elliptical air hole (5) is 0.90 μm and the minor axis is 0.60 μm.

6. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling according to claim 5, characterized in that, The diameter of the first circular air hole (2) is 0.71 μm; the diameter of the third circular air hole (6) is 0.85 μm; the center-to-center distance between two adjacent first circular air holes (2), the center-to-center distance between two adjacent first circular air holes (2) and the second circular air hole (3), the center-to-center distance between two adjacent first circular air holes (2) and the third circular air hole (6), and the center-to-center distance between two adjacent second circular air holes (3) and the fourth circular air hole (7) are the same, and the center-to-center distance is 0.95 μm; the center-to-center distance between adjacent first circular air holes (2) and the elliptical air hole (5) is the same, and the center-to-center distance is 0.76 μm.

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