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

Abstract

The invention discloses a compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling, particularly relates to the technical field of optical communication, and solves the technical problem that an existing dual-core photonic crystal fiber polarization beam splitter is difficult to meet the requirements of short device length, high extinction ratio and long working bandwidth at the same time. According to the technical scheme, in a fiber core area and a cladding area on a substrate material, the fiber core area comprises a fourth circular air hole filled with liquid crystal and two fiber cores, and the two fiber cores are formed after the distribution positions of oval air holes are adjusted; the cladding area 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 oval air holes. The optical device has the advantages of being short in device length, high in extinction ratio and long in working bandwidth, can completely cover S + C + L optical communication wave bands, and meets the requirements of an all-optical network for high performance, miniaturization and high integration level of the optical device.

Description

Technical Field

[0001] The invention relates to the technical field of optical communication, and in particular to a compact double-core photonic crystal optical fiber polarization beam splitter based on liquid crystal filling. Background Art

[0002] Driven by emerging technologies such as the Internet, big data, and artificial intelligence, the growing demand for data transmission is driving the rapid development of communication systems towards all-optical networks with ultra-large capacity, ultra-high speed, and ultra-long-distance transmission. As a passive device, the polarization beam splitter (PBS) can decompose a beam of light into two beams of light in orthogonal polarization directions, and plays an important role in the fields of optical communication and optical sensing. Traditional single-mode fiber-based polarization beam splitters are difficult to meet the requirements of all-optical networks for high performance, miniaturization, and high integration of optical devices due to defects such as large device size, low extinction ratio, and narrow working bandwidth. Photonic crystal fiber (PCF), as a new type of waveguide, has become an ideal carrier for the new PBS with its flexible structural design and excellent optical properties (such as non-stop single-mode transmission, ultra-low loss, high birefringence, etc.). The PCF-based PBS has not only significantly improved performance, but also can be directly connected to the optical fiber network, so it has been rapidly developed. Among them, dual-core (DC) photonic crystal fiber PBS is the main research direction at present. It specifically introduces asymmetric cores, elliptical air holes or circular air holes of different sizes to induce birefringence effect, thereby achieving complete separation of two polarized lights.

[0003] The development of materials science and filling technologies has enabled the application of various functional materials (such as metals, oils, liquid crystals, magnetic fluids, gases, etc.) in PCF-PBS. By filling or coating functional materials in the air holes of PCF, the optical transmission characteristics can be effectively regulated, thereby achieving excellent-performance PBS. As a highly birefringent material, liquid crystal has both the molecular orientation order of crystals and the fluidity and continuity of liquids. Its addition endows PCF with diverse optical transmission characteristics and real-time tuning capabilities. In addition, liquid crystal has the characteristic of quickly responding to changes in external fields (such as electric fields, temperatures, pressures, etc.), which provides the electric field and temperature tuning functions for PCF-PBS. Therefore, the combination of liquid crystal materials and the design of new DC-PCF structures has become one of the hot research directions for PBS currently. To enhance the coupling characteristics of the dual cores of DC-PCF-PBS, a metal film is often coated in the air holes of PCF 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 effectively regulating the fiber mode field. The SPR technology can significantly shorten the coupling length of PCF, providing an effective way for the miniaturization design of PBS.

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

[0005] Therefore, the present invention solves the technical problem that the existing DC-PCF-PBS is difficult to simultaneously meet the requirements of short device length, high extinction ratio and long working bandwidth; the compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling provided by the present invention has the advantages of short device length, high extinction ratio and long working bandwidth. Its length is only 14.43μm, the maximum extinction ratio is -89.04dB, and the operating bandwidth reaches 180nm, which can completely cover the S+C+L optical communication bands and meet the requirements of all-optical networks for high performance, miniaturization and high integration of optical devices.

[0006] To achieve the above object, the present invention is realized through the following technical solutions: 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 provided on the substrate material; a central fourth circular air hole filled with liquid crystal and two cores are provided on the core region. The two cores are formed by adjusting the distribution positions of elliptical air holes, and the two cores are distributed on both sides of the central fourth circular air hole along the x-axis direction.

[0007] The cladding region includes a plurality of 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 filled with liquid crystal as the center, the plurality of 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 filled with liquid crystal to form a multi-layer structure. Except for the two trapezoidal structures where the core region is located, each of the remaining layers is a hexagonal structure.

[0008] Among them, the two third circular air holes are distributed on both sides of the core region along the x-axis direction, the four elliptical air holes are distributed on both sides of the core region along the y-axis direction, with two elliptical air holes distributed on each side. The long axis of the elliptical air holes is parallel to the x-axis, and the two second circular air holes with gold films attached to their inner walls are distributed on both sides of the central fourth circular air hole along the y-axis direction; the plurality of first circular air holes are distributed outside the second circular air holes, the third circular air holes, and the elliptical air holes.

[0009] Furthermore, the central fourth circular air hole filled with liquid crystal is distributed at the structural center of the photonic crystal fiber polarization beam splitter.

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

[0011] Further, multiple said first circular air holes are periodically distributed in an equilateral triangle and symmetrically distributed about the center; two said third circular air holes are symmetrically distributed about the center on both sides of the core region along the x-axis direction; four said elliptical air holes are symmetrically distributed about the center on both sides of the core region along the y-axis direction; two said second circular air holes with gold films attached to the inner walls are symmetrically distributed about the center on both sides of the fourth circular air hole along the y-axis direction.

[0012] Further, the photonic crystal fiber polarization beam splitter further includes a perfectly matched layer provided on the outermost periphery of the substrate material, and the thickness of the perfectly matched layer is 2 μm.

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

[0014] Further, the liquid crystal is E7-type nematic liquid crystal, and the diameter of the fourth circular air hole in the center is 0.40 μm.

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

[0016] Further, the major axis length of the elliptical air hole is 0.90 μm, and the minor axis length 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 adjacent two first circular air holes, the center-to-center distance between adjacent two first circular air holes and the second circular air hole, the center-to-center distance between adjacent two first circular air holes and the third circular air hole, and the center-to-center distance between adjacent two 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] In the above technical solution, the technical effects and advantages provided by the present invention are:

[0019] 1. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling provided by the present invention uses circular air holes and elliptical air holes with different sizes to enhance the asymmetry of the structure. At the same time, the introduction of the gold film and liquid crystal material further improves the birefringence effect, making the device length shorter and the beam splitting effect better. The length of this polarization beam splitter is only 14.43 μm, and the maximum extinction ratio of -89.04 dB is obtained at the 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 180 nm, which can fully cover the S+C+L optical communication bands.

[0021] 3. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling provided by the present invention has excellent characteristics of short device length, high extinction ratio, and long working bandwidth, meeting the requirements of all-optical networks for high performance, miniaturization, and high integration of optical devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings.

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

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

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

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

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

[0028] Figure 3 It is a graph showing the relationship between the coupling length ratio and wavelength of Embodiment 1 of the present invention and Comparative Example 1 and Comparative Example 2;

[0029] Figure 4 It is a graph showing the relationship between the coupling length ratio and wavelength of Embodiment 1 of the present invention and Comparative Example 3 and Comparative Example 4;

[0030] Figure 5 It is a graph showing the relationship between the coupling length ratio and wavelength of Embodiment 1 of the present invention and Comparative Example 5 and Comparative Example 6;

[0031] Figure 6 It is a graph showing the relationship between the coupling length ratio and wavelength of Embodiment 1 of the present invention and Comparative Example 7 and Comparative Example 8;

[0032] Figure 7It is a graph showing the variation of the coupling length ratio of Example 1 of the present invention with respect to Comparative Example 9 and Comparative Example 10 as a function of wavelength;

[0033] Figure 8 It is a graph showing the variation of the coupling length ratio of Example 1 of the present invention with respect to Comparative Example 11 and Comparative Example 12 as a function of wavelength;

[0034] Figure 9 It is a graph showing the variation of the coupling length ratio of Example 1 of the present invention with respect to Comparative Example 13, Comparative Example 14 and Comparative Example 15 as a function of wavelength;

[0035] Figure 10 It is a graph showing the variation of the normalized power in the x and y polarization directions at the output ends of cores A and B as a function of the transmission distance when the incident wavelength of Example 1 of the present invention is 1.55 μm;

[0036] Figure 11 It is a graph showing the variation of the extinction ratio of cores A and B as a function of wavelength when the beam splitting length of Example 1 of the present invention is 14.43 μm.

[0037] Explanation of reference numerals:

[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, perfectly matched layer. Detailed implementation manners

[0039] In order 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 the present invention is SF57 glass, which belongs to soft glass materials. Compared with traditional silica glass fibers, soft glass fibers have significant advantages in optical properties. They not only have higher linear and nonlinear refractive indices but also exhibit wider transmission characteristics. Therefore, soft glass materials represented by SF57 glass are a new type of substrate material that is currently used more frequently. Its Sellmeier equation of the dispersion relationship can be given by Equation (1):

[0041]

[0042] In the formula, λ is the wavelength of the incident light, in μm as the unit, A i is a fitting coefficient, where A 0 = 3.24748, A 1 = -0.00954782 μm -2 and A 2 = 0.0493626 μm 2 and A 3= 0.00294294 μm 4 , A 4 = -1.48144×10 -4 μm 6 , A 5 = 2.78427×10 -5 μm 8 .

[0043] In the present invention, the material for exciting surface plasmons is selected as gold. Compared with 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 antioxidant performance and can maintain its performance without being oxidized and damaged for a long time; Third, gold has good ductility, which is convenient for preparing high-quality nano-thin film structures. Therefore, the present invention selects gold as the material for exciting surface plasmons. The relative dielectric constant Drude-Lorentz model of gold can be given by Equation (2):

[0044]

[0045] In the formula: ε m is the relative dielectric constant of gold, ε ∞ is the dielectric constant of the metal at high frequencies, ω is the incident light frequency, ω D and γ D are the plasma frequency and damping frequency of gold respectively, Ω L and Г L are the strength and spectral width of the Lorentz oscillator respectively, Δε is the weighting factor of the Lorentz term, and j represents the symbol of the imaginary part of a 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×104.86 THz, Δε = 1.09.

[0046] In the design of the present invention, by filling the E7-type nematic liquid crystal (NLC) in the central fourth circular air hole 7, the birefringence effect can be further enhanced and the beam splitting length of the optical fiber can be reduced. The refractive indices of the E7-type NLC can be divided into the ordinary refractive index n o and the extraordinary refractive index n e , and can be specifically given by the Cauchy model as follows in Equations (3) and (4):

[0047]

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

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

[0050]

[0051] In the formula, α is the rotation angle of the NLC molecule, defined as the angle between the long axis direction of the wheel-shaped NLC molecule and the x-axis direction. In the present 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] The coupling length (Coupling Length, CL) refers to the minimum distance required for the optical power in one core to be completely transferred to another core. Coupling occurs between two mode fields with the same polarization direction and similar effective refractive indices. In the DC-PCF, the coupling lengths 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 respectively represent the propagation constants and effective refractive indices of the even and odd modes in the x or y polarization directions.

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

[0056]

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

[0058] For the DC - PCF, when light is incident on the core, it propagates in the form of sine and cosine trigonometric functions. Then, in cores A and B, the Normalized Output Power (NOP) can be expressed by Eqs. (8) and (9):

[0059]

[0060] In the equations, respectively represent the output powers of cores A and B, i represents the x - or y - polarization direction, and P in represents the incident optical power in the core, and z represents the distance that the incident light travels in the optical fiber.

[0061] The extinction ratio (ER) is used to describe the ability to separate different polarization states at one output port after the optical wave travels a certain distance in the core. The extinction ratios of cores A and B can be expressed by Eqs. (10) and (11):

[0062]

[0063] In the equations, respectively represent the output powers of cores A and B in the x - or y - polarization direction. When the ER is less than - 20 dB (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 covered wavelength range is the bandwidth of the PBS.

[0064] Embodiment 1

[0065] The photonic crystal fiber polarization beam splitter in this embodiment, as Figure 1 shown, includes a substrate material 1 and a core region and a cladding region provided on the substrate material 1; the core region includes a central fourth circular air hole 7 filled with liquid crystal 8 and two cores; the central fourth circular air hole 7 filled with liquid crystal 8 is distributed at the center of the structure of the photonic crystal fiber polarization beam splitter, and the two cores are formed by adjusting the distribution positions 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. The left - hand side is core A, and the right - hand side is core B.

[0066] The cladding region includes fifty-two first circular air holes 2, two second circular air holes 3 with gold films 4 attached to their inner walls, two third circular air holes 6, and four elliptical air holes 5; centered around the central fourth circular air hole 7 filled with liquid crystal 8, the fifty-two first circular air holes 2, two second circular air holes 3 with gold films 4 attached to their inner walls, four elliptical air holes 5, and two third circular air holes 6 surround the central fourth circular air hole 7 filled with liquid crystal 8 to form a multi-layer structure. Except for the two trapezoidal structures where the core region is located, each of the remaining layers is a regular hexagonal structure;

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

[0068] The photonic crystal fiber polarization beam splitter further includes a perfectly matched layer 9 provided on the outermost periphery of the substrate material 1, and the thickness of the perfectly matched layer is 2 μm. The substrate material 1 is SF57 glass. The liquid crystal 8 is E7 type nematic liquid crystal, and 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. The major axis length of the elliptical air hole 5 is 0.90 μm, and the minor axis length is 0.60 μm. 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 adjacent two first circular air holes 2, the center-to-center distance between adjacent two first circular air holes 2 and the second circular air hole 3, the center-to-center distance between adjacent two first circular air holes 2 and the third circular air hole 6, and the center-to-center distance between adjacent two 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 elliptical air holes 5 is the same, and the center-to-center distance is 0.76 μm.

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

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

[0071] Comparative Example 1

[0072] Same as Embodiment 1, the difference is that the diameter d 1 , d 1 is 0.66 μm.

[0073] Comparative Example 2

[0074] Same as Embodiment 1, the difference is that the diameter d 1 , d 1 is 0.76 μm.

[0075] Comparative Example 3

[0076] Same as Embodiment 1, the difference is that the diameter d 2 , d 2 is 0.35 μm.

[0077] Comparative Example 4

[0078] Same as Embodiment 1, the difference is that the diameter d 2 , d 2 is 0.45 μm.

[0079] Comparative Example 5

[0080] Same as Embodiment 1, the difference is that the diameter d 3 , d 3 is 0.80 μm.

[0081] Comparative Example 6

[0082] Same as Embodiment 1, the difference is that the diameter d 3 , d 3 is 0.90 μm.

[0083] Comparative Example 7

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

[0085] Comparative Example 8

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

[0087] Comparative Example 9

[0088] Same as 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 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 Example 1, except that the minor axis length b of the elliptical air hole is 0.50 μm.

[0093] Comparative Example 12

[0094] Same as 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 Example 1, except that the thickness t of the gold film is 40 nm.

[0097] Comparative Example 14

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

[0099] Comparative Example 15

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

[0101] The relationship between the coupling length ratios of Example 1 and Comparative Examples 1 to 15 varying with wavelength is as Figures 3 - 9 shown. For Example 1, when the incident wavelength is 1.55 μm, the curves of the normalized power of the output ends x and y polarization directions of cores A and B varying with the transmission distance, and when the beam splitting length is 14.43 μm, the curves of the extinction ratio of cores A and B varying with wavelength are respectively as Figure 10 and Figure 11 shown.

[0102] When comparing Example 1 with Comparative Example 1 and Comparative Example 2, asFigure 3 As shown, at the same wavelength, the coupling length ratio decreases as the diameter d of the first circular air hole 1 increases. The increase in the diameter d of the first circular air hole 1 causes more light to be confined in the core, thereby enhancing the coupling strength of the two cores. The change curve of the coupling length ratio with d 1 = 0.71 μm is smoother, and the coupling length ratio at the 1.55 μm communication window is closest to 2. Therefore, the present invention selects d 1 = 0.71 μm as the diameter of the first circular air hole.

[0103] Compared with Comparative Example 3 and Comparative Example 4 in Example 1, as Figure 4 shown, the differences in the changes of the three curves are very small. It can be seen from the local enlarged view that in the wavelength change range of 1.52 μm to 1.58 μm, the coupling length ratio decreases as the diameter d of the fourth circular air hole 2 increases. The increase in the diameter d of the fourth circular air hole 2 results in more liquid crystal filled in the air hole, and the influence of the liquid crystal becomes larger, resulting in a weakening of the coupling strength of the x-polarized mode. d 2 plays a fine-tuning role in the structural optimization process. After fully considering the performance and preparation tolerance of the polarization beam splitter, the present invention selects d 2 = 0.40 μm as the diameter of the fourth circular air hole.

[0104] Compared with Comparative Example 5 and Comparative Example 6 in Example 1, as Figure 5 shown, the differences in the changes of the three curves gradually increase starting from 1.6 μm and exhibit a blue shift as the diameter d of the third circular air hole 3 increases. It can be seen from the local enlarged view that at 1.55 μm, the coupling length ratio decreases as the diameter d of the third circular air hole 3 increases. The increase in the diameter d of the third circular air hole 3 results in a decrease in the core area and a decrease in the binding ability of the core to the polarization mode, thus making the coupling effect more likely to occur. To obtain a higher-performance polarization beam splitter, the present invention selects d 3 = 0.85 μm as the diameter of the third circular air hole.

[0105] Compared with Comparative Example 7 and Comparative Example 8 in Example 1, as Figure 6As shown, at the same wavelength, the coupling length ratio increases with the increase of the distance Λ between air holes. The increase of the distance Λ between air holes reduces the air hole filling ratio in the cladding region, resulting in a decrease in the refractive index difference between the core and the cladding region, and more optical energy leaks into the cladding region, thus weakening the coupling strength between the cores. The curve with Λ = 0.90μm has jumps and unevenness, and at the 1.55μm communication window, only the coupling length ratio with Λ = 0.95μm is close to 2. Therefore, the present invention selects Λ = 0.95μm as the distance between air holes.

[0106] Comparing Example 1 with Comparative Example 9 and Comparative Example 10, as Figure 7 shown, at short wavelengths, the coupling length ratio is proportional to the major axis length a of the elliptical air holes, and at long wavelengths, the coupling length ratio is inversely proportional to the major axis length a of the elliptical air holes. The increase of the major axis length a of the elliptical air holes increases the asymmetry between the dual-core structures, enhances the coupling strength of the x-polarized mode, and the coupling strength of the y-polarized mode first weakens and then strengthens. At the 1.55μm communication window, the coupling length ratios in the three cases are 2.063, 2.008, and 2.041 respectively. Obviously, the coupling length ratio when a = 0.90μm is closer to 2. Therefore, the present invention selects a = 0.90μm as the major axis length of the elliptical air holes.

[0107] Comparing Example 1 with Comparative Example 11 and Comparative Example 12, as Figure 8 shown, the three curves change relatively uniformly and at the same wavelength, the coupling length ratio decreases with the increase of the minor axis length b of the elliptical air holes. The increase of the minor axis length b of the elliptical air holes compresses the core in the vertical direction, enhancing the coupling effect in the y-polarization direction. At the 1.55μm communication window, only the coupling length ratio when b = 0.60μm is close to 2. Therefore, the present invention selects b = 0.60μm as the minor axis length of the elliptical air holes.

[0108] Comparing Example 1 with Comparative Example 13, Comparative Example 14 and Comparative Example 15, as Figure 9 shown, the differences in the changes of the three curves with gold films gradually decrease starting from 1.37μm, and the coupling length ratio when t = 0nm always varies in the range of 1.0 - 1.2. It can be seen from the local enlarged view that in the wavelength range of 1.52μm - 1.58μm, the coupling length ratio increases with the increase of the gold film thickness t. The increase of the gold film thickness t weakens the coupling strength between the two cores, resulting in an increase in the coupling length. Since the changes in the coupling length ratios under the three gold film thicknesses are not significant, and the coupling length ratio when t = 0nm does not meet the conditions of 2 or 1 / 2, after fully considering the manufacturing cost and excellent performance of the polarization beam splitter, the present invention selects t = 50nm as the gold film thickness.

[0109] AsFigure 10 As shown, when the incident wavelength of the photonic crystal fiber polarization beam splitter in Embodiment 1 is 1.55 μm, the normalized powers of the x and y polarization directions at the output ends of cores A and B propagate in the form of sine and cosine functions. When the transmission distance is 14.43 μm, the normalized power of the x polarization direction at the output end of core A and the normalized power of the y polarization direction at the output end of core B reach the maximum value, while the normalized power of the y polarization direction at the output end of core A and the normalized power of the x polarization direction at the output end of core B reach the minimum value. If light is incident from the core A end, the light output from core A is y-polarized light, and the light output from core B is x-polarized light, that is, the x and y polarized lights are completely separated in the two cores. Therefore, the shortest length of this beam splitter is 14.43 μm.

[0110] As Figure 11 As shown, when the beam splitting length of the photonic crystal fiber polarization beam splitter in Embodiment 1 is 14.43 μm, the extinction ratio of core A reaches a peak of -89.04 dB at 1.55 μm, and within the wavelength range of 1.46 μm to 1.64 μm, the extinction ratio is always less than -20 dB, that is, the bandwidth is 180 nm, which can completely cover the S+C+L optical communication band; the extinction ratio of core B reaches peaks of -80.03 dB and -58.62 dB at 1.55 μm and 1.65 μm respectively, and within the wavelength range of 1.45 μm to 1.69 μm, the extinction ratio is always less than -20 dB, that is, the bandwidth is 240 nm, which can completely cover the S+C+L+U optical communication band.

[0111] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention accordingly. Any equivalent structure or equivalent process transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied in other related technical fields, shall be included in the patent protection scope of the present invention by the same token.

Claims

1. A compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling, characterized in that: The invention comprises a base material (1), wherein a core region and a cladding region are arranged on the base material (1), wherein the cladding region is located outside the core region, and wherein the cladding region comprises a plurality of first circular air holes (2), two second circular air holes (3) whose inner walls are attached with a gold film (4), two third circular air holes (6), and four elliptical air holes (5); an area surrounded 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, wherein a central fourth circular air hole (7) filled with liquid crystal (8) is arranged at the center thereof, and a line connecting the two second circular air holes (3) along the Y-axis direction divides the core region into two cores, and the two cores are distributed on both sides of the central fourth circular air hole (7) along the x-axis direction.

2. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling according to claim 1, characterized in that: With the fourth circular air hole (7) filled with liquid crystal (8) as the center, a plurality of the first circular air holes (2), two second circular air holes (3) with gold films (4) attached to the inner walls, four elliptical air holes (5) and two third circular air holes (6) surround the fourth circular air hole (7) filled with liquid crystal (8) to form a multi-layer structure, and except for the core region having two trapezoidal structures, each of the other layers of the multi-layer structure is a hexagonal structure.

3. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling according to claim 2, characterized in that: The two third circular air holes (6) are distributed on both sides of the fiber core region along the x-axis direction, the four elliptical air holes (5) are distributed on both sides of the fiber core region along the y-axis direction, two elliptical air holes (5) are distributed on each side, the long axis of the elliptical air hole (5) is parallel to the x-axis, and two second circular air holes (3) with gold films (4) attached to the inner walls are distributed on both sides of the central fourth circular air hole (7) along the y-axis direction; and the plurality of the first circular air holes (2) are distributed on the outer sides of the second circular air hole (3), the third circular air hole (6) and the elliptical air hole (5).

4. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling according to claim 3, characterized in that: The central fourth circular air hole (7) filled with liquid crystal (8) is distributed at the structural center of the photonic crystal fiber polarization beam splitter; and the hexagonal structure is a regular hexagonal structure.

5. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling according to claim 4, characterized in that: The plurality of first circular air holes (2) are periodically distributed in the form of regular triangles and are symmetrically distributed along the center; the two third circular air holes (6) are symmetrically distributed along the x-axis direction on both sides of the fiber core region; the four elliptical air holes (5) are symmetrically distributed along the y-axis direction on both sides of the fiber core region; and the two second circular air holes (3) with gold films (4) attached to the inner walls are symmetrically distributed along the y-axis direction on both sides of the central fourth circular air hole (7).

6. The liquid crystal filled compact dual-core photonic crystal fiber polarization beam splitter according to claim 5, characterized in that: It also comprises a perfect matching layer (9) arranged at the outermost periphery of the base material (1), wherein the thickness of the perfect matching layer (9) is 2 μm.

7. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling according to claim 6, characterized in that: The substrate material (1) is SF57 glass.

8. The liquid crystal-filled compact dual-core photonic crystal fiber polarization beam splitter according to claim 7, characterized in that: The liquid crystal (8) is an E7 type nematic phase liquid crystal, the diameter of the fourth central 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.

9. The liquid crystal-filled compact dual-core photonic crystal fiber polarization beam splitter according to claim 8, characterized in that: The elliptical air hole (5) has a major axis length of 0.90 μm and a minor axis length of 0.60 μm.

10. The compact dual-core photonic crystal fiber polarization beam splitter based on liquid crystal filling according to claim 9, 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 spacing between two adjacent first circular air holes (2), the center spacing between two adjacent first circular air holes (2) and second circular air holes (3), the center spacing between two adjacent first circular air holes (2) and third circular air holes (6), and the center spacing between two adjacent second circular air holes (3) and fourth circular air holes (7) are the same, and the center spacings are all 0.95 μm; the center spacing between the adjacent first circular air holes (2) and the elliptical air holes (5) is the same, and the center spacing is 0.76 μm.

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