Gold-coated bow-tie dual-core photonic crystal fiber mid-infrared polarization beam splitter

Abstract

The application provides a gold-plated film bowtie-shaped dual-core photonic crystal fiber mid-infrared polarization beam splitter, which comprises a substrate, the material of the substrate is pure silicon, a first circular air hole is arranged at the center of the substrate, the inside of the first circular air hole is plated with a gold film, two cores are symmetrically arranged on the two sides of the first circular air hole, two elliptical air holes are symmetrically arranged on the outside of the two cores, 38 second circular air holes are arranged to form a bowtie-shaped arrangement structure around the first circular air hole, the two cores and the two elliptical air holes, 72 third circular air holes are arranged to form a bowtie-shaped arrangement structure around the second circular air holes, the aperture of the third circular air hole is larger than that of the second circular air hole, and the spacing between any two adjacent third circular air holes and any two adjacent second circular air holes is equal.

Description

Technical Field

[0001] This application relates to the field of infrared polarization devices in photonic crystal fibers, and more specifically, to an infrared polarization beam splitter in a gold-plated bowtie-type dual-core photonic crystal fiber. Background Technology

[0002] With the rise of all-fiber networks in the era of "Internet of Everything and digital twins," a large number of fiber polarization beamsplitters are needed as crucial supporting components to build global, integrated, and industrialized all-fiber network systems. The 2–5 μm band in the mid-infrared spectral region is an important transmission window in all-fiber network systems, covering many important molecular fingerprints. Therefore, fiber polarization beams operating in the 2–5 μm band have particularly important applications in military defense, environmental monitoring, industrial applications, and biomedicine.

[0003] Traditional commercial fiber polarization beams are limited by fiber structure and substrate materials, and can no longer meet the actual performance requirements of all-fiber network systems for spectral application range, splitting bandwidth, and splitting length. Photonic crystal fibers offer a high degree of design freedom, allowing for flexible control over their cladding and core microstructures as well as substrate materials. They also allow for various post-processing techniques such as coating and filling, providing new research directions for the performance design of fiber polarization beams and their expansion into the mid-infrared spectral region (2–5 μm band). Summary of the Invention

[0004] This application proposes a gold-plated bowtie-type dual-core photonic crystal fiber mid-infrared polarization beam splitter to solve the technical problem of its irreversibility. Specifically:

[0005] Some embodiments of this application provide a gold-plated bowtie-type dual-core photonic crystal fiber mid-infrared polarization beam splitter, including:

[0006] The substrate is made of pure silicon.

[0007] A first circular air hole is disposed at the center of the substrate, and a gold film is plated on the inner side of the first circular air hole;

[0008] Two fiber cores are symmetrically arranged on both sides of the first circular air hole;

[0009] Two elliptical air holes are symmetrically arranged on the outer sides of the two fiber cores;

[0010] Thirty-eight second circular air holes are arranged in a bow-tie pattern around the first circular air hole, two fiber cores, and two elliptical air holes.

[0011] 72 third circular air holes are arranged in a bow tie pattern around the second circular air holes;

[0012] The diameter of the third circular air hole is larger than that of the second circular air hole; the spacing between any two adjacent third circular air holes and any two adjacent second circular air holes is equal.

[0013] In some embodiments, the diameter d1 of the first circular air hole ranges from 0.9 μm to 1.1 μm.

[0014] In some embodiments, the thickness t of the gold-plated film inside the first circular air hole ranges from 35 nm to 55 nm.

[0015] In some embodiments, the minor axis diameter dx of the two elliptical air holes ranges from 1.3 μm to 1.5 μm, and the major axis diameter dy ranges from 1.6 μm to 1.8 μm.

[0016] In some embodiments, the diameter d2 of the second circular air hole ranges from 1.1 μm to 1.3 μm;

[0017] The diameter d3 of the third circular air hole ranges from 1.65 μm to 1.85 μm.

[0018] In some embodiments, the distance Λ between any two adjacent third circular air holes and any two adjacent second circular air holes ranges from 1.9 μm to 2.3 μm.

[0019] In some embodiments, both fiber cores are solid pure silicon substrate materials.

[0020] In some embodiments, the two fiber cores include a first fiber core and a second fiber core, and the material dispersion equation of the substrate can be expressed as:

[0021]

[0022] Where λ is the wavelength of the incident light, λ₁ = 1.971 μm, ε = 11.6858, and a = 0.939816 μm. 2 b = 8.10461 × 10 -3 .

[0023] In some embodiments, the relative permittivity ε of the gold film m The following relationship must be satisfied:

[0024]

[0025] Among them, ω, ω D and γ D Let Δε and ε represent the angular frequency of the incident light wave, the plasma frequency, and the damping frequency, respectively. ∞Ω represents the weighting coefficient and the high-frequency dielectric constant, respectively. L and Γ L represents the frequency and bandwidth of the Lorentz oscillator, respectively, and j represents the imaginary unit.

[0026] Compared with related technologies, the above-described solutions of this application have at least the following beneficial effects:

[0027] (1) The gold-plated pure silicon substrate bow-tie dual-core photonic crystal fiber mid-infrared polarization beamsplitter of the present invention achieves ultra-wide beam-splitting bandwidth for both cores simultaneously in the mid-infrared spectral region (2-5 μm band). The beam-splitting bandwidth of core A corresponds to a wavelength range of 3.17-3.90 μm (bandwidth 730 nm), and the beam-splitting bandwidth of core B corresponds to a wavelength range of 3.19-3.76 μm (bandwidth 570 nm), which can play a special and important role in military defense, environmental monitoring, industrial applications, and biomedicine.

[0028] (2) The gold-plated pure silicon substrate bow-tie dual-core photonic crystal fiber mid-infrared polarization beam splitter of the present invention has an ultra-short beam splitting length of only 194μm, which is beneficial to reduce the volume of the polarization beam splitter and improve the compactness and integration of the all-fiber system.

[0029] (3) The gold-plated pure silicon substrate bow-tie dual-core photonic crystal fiber mid-infrared polarization beam splitter of the present invention has ultra-low insertion loss, with X-polarized light insertion loss of less than 0.2dB and Y-polarized light insertion loss of less than 0.04dB, which is beneficial to improving optical signal transmission efficiency and meeting the requirements of high-precision measurement application scenarios. Attached Figure Description

[0030] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application. It is obvious that the drawings described below are merely some embodiments of this application, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. In the drawings:

[0031] Figure 1 This is a cross-sectional view of the infrared polarization beam splitter in a pure silicon substrate bowtie-type dual-core photonic crystal fiber with gold-plated film provided in an embodiment of the present invention.

[0032] Figure 2 This is a graph showing the relationship between the real part of the effective refractive index of the odd and even supermodes in the X-polarization direction and the real part of the effective refractive index of the second-order surface plasmon polariton mode in the X-polarization direction of the infrared polarization beam splitter on a pure silicon substrate with a gold-plated pure silicon substrate, as well as the wavelength.

[0033] Figure 3 This is a graph showing the relationship between the real part of the effective refractive index of the odd and even supermodes in the Y-polarization direction and the real part of the effective refractive index of the second-order surface plasmon polariton mode in the Y-polarization direction of the infrared polarization beam splitter on a pure silicon substrate with a gold-plated pure silicon substrate, as well as the wavelength.

[0034] Figure 4 The graphs show the relationship between the coupling lengths in the X and Y polarization directions of the infrared polarization beam splitter in the pure silicon substrate bowtie dual-core photonic crystal fiber with gold-plated film provided in this embodiment of the invention, and the relationship between the ratio of the coupling lengths in the X and Y polarization directions and the wavelength.

[0035] Figure 5 This is a graph showing the relationship between the normalized output power and transmission length of core A in the infrared polarization beam splitter of the pure silicon substrate bowtie dual-core photonic crystal fiber with gold-plated film provided in this embodiment of the invention.

[0036] Figure 6 This is a graph showing the relationship between the normalized output power and transmission length of core B in the infrared polarization beam splitter of the pure silicon substrate bowtie dual-core photonic crystal fiber with gold-plated film provided in this embodiment of the invention.

[0037] Figure 7 This is a graph showing the relationship between the extinction ratio and wavelength of the gold-plated pure silicon substrate bow-tie dual-core photonic crystal fiber mid-infrared polarization beamsplitter provided in this embodiment of the invention when the beam splitting length is 194 μm.

[0038] Figure 8 This is a graph showing the relationship between the extinction ratio and wavelength of the gold-plated pure silicon substrate bow-tie dual-core photonic crystal fiber mid-infrared polarization beamsplitter provided in this embodiment of the invention when the beam splitting length is 194 μm.

[0039] Figure 9 This is a graph showing the relationship between the insertion loss and wavelength of X and Y polarized light in the gold-plated pure silicon substrate bowtie dual-core photonic crystal fiber mid-infrared polarization beam splitter provided in this embodiment of the invention when the beam splitting length is 194μm.

[0040] Explanation of reference numerals in the attached figures:

[0041] 1. First circular air hole 1, gold film 2, elliptical air hole 3, second circular air hole 4, third circular air hole 5, spacing between any two adjacent air holes 6, substrate 7, first fiber core 8, second fiber core 9. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0043] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to limit the application. The singular forms “a,” “said,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms, and “multiple” generally includes at least two unless the context clearly indicates otherwise.

[0044] It should be understood that the term "and / or" used in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this article generally indicates that the preceding and following related objects have an "or" relationship.

[0045] It should be understood that although the terms first, second, third, etc., may be used to describe embodiments of this application, these terms should not be used in isolation. These terms are only used to distinguish between different terms. For example, first may also be referred to as second without departing from the scope of embodiments of this application, and similarly, second may also be referred to as first.

[0046] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that an article or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or device. Without further limitation, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the article or device that includes said element.

[0047] In related technologies, researchers mainly design dual-core photonic crystal fiber polarization beamsplitters by selectively filling the air holes of dual-core photonic crystal fibers with materials such as liquid crystals or liquids, or by coating the inner walls of the air holes with materials such as graphene or metal oxide nanofilms. For example, a liquid-filled dual-core photonic crystal fiber polarization beamsplitter with two elliptical air holes has a beam splitting length of 78 μm and a beam splitting bandwidth of 44 nm near core A at approximately 1.55 μm. Another dual-core photonic crystal fiber polarization beamsplitter with air holes filled with gold nanorods on a Ge20Sb15Se65 glass substrate has three different sizes of air holes and two elliptical air holes, with a beam splitting length of 230 μm and a beam splitting bandwidth of 358 nm (1.367–1.725 μm) for core A. A liquid crystal-filled dual-core photonic crystal fiber polarization beamsplitter with diamond-shaped air holes has four different sizes of air holes, a beam splitting length of 58 μm, and a beam splitting bandwidth of 335 nm (1.306–1.641 μm) for core A. An infrared polarization beamsplitter in a pure silicon substrate dual-core photonic crystal fiber with two gold-coated layers has one large elliptical air hole, a beam splitting length of 660 μm, and beam splitting bandwidths of 450 nm and 140 nm for cores A and B near 3.3 μm, respectively, with a maximum insertion loss of 0.15 dB.

[0048] In summary, by adding elliptical air holes, using irregularly arranged air hole cladding, and introducing novel functional materials into dual-core photonic crystal fibers, other effects can be excited within the fiber while disrupting its structural symmetry. This allows dual-core photonic crystal fiber polarization beamsplitters to achieve wider splitting bandwidth, shorter splitting length, or lower insertion loss. However, filling multiple air holes with metal rods or metal films increases material costs; while filling them with fluid materials such as liquids or liquid crystals increases the difficulty of integrating the device into an all-fiber system. Furthermore, the splitting bandwidth of one fiber core wastes the bandwidth resources of another.

[0049] Conventional dual-core photonic crystal fiber polarization beamsplitters are mainly used in conventional communication bands (1.26–1.675 μm), and are difficult to apply to mid-infrared polarization beamsplitters in dual-core photonic crystal fibers in the 2–5 μm band. Furthermore, the significant difference in splitting bandwidth between the two cores makes it difficult to achieve ultra-wide splitting bandwidth for both cores simultaneously in the 2–5 μm band.

[0050] This application provides a gold-plated bowtie-type dual-core photonic crystal fiber mid-infrared polarization beam splitter, comprising: a substrate 7 made of pure silicon; a first circular air hole 1 disposed at the center of the substrate 7, with a gold-plated film 2 deposited on the inner side of the first circular air hole 1; two fiber cores, including a first fiber core A and a second fiber core B, symmetrically disposed on both sides of the first circular air hole 1; two elliptical air holes 3 symmetrically disposed on the outer sides of the two fiber cores; 38 second circular air holes 4 forming a bowtie-type arrangement around the first circular air hole 1, the first fiber core A and the second fiber core B, and the two elliptical air holes 3; and 72 third circular air holes 5 forming a bowtie-type arrangement around the second circular air holes 4; wherein the diameter of the third circular air holes 5 is larger than the diameter of the second circular air holes 4; and the spacing between any two adjacent third circular air holes 5 and any two adjacent second circular air holes 4 is equal.

[0051] This invention discloses a gold-plated pure silicon substrate bow-tie dual-core photonic crystal fiber mid-infrared polarization beamsplitter. In the mid-infrared spectral region (2–5 μm band), both fiber cores simultaneously achieve ultra-wide beam-splitting bandwidths. The beam-splitting bandwidth of fiber core A corresponds to a wavelength range of 3.17–3.90 μm (bandwidth 730 nm), and the beam-splitting bandwidth of fiber core B corresponds to a wavelength range of 3.19–3.76 μm (bandwidth 570 nm). It also features an ultra-short beam-splitting length of only 194 μm, which helps reduce the size of the polarization beamsplitter and improves the compactness and integration of the all-fiber system. Furthermore, this invention's gold-plated pure silicon substrate bow-tie dual-core photonic crystal fiber mid-infrared polarization beamsplitter exhibits ultra-low insertion loss, with X-polarized light insertion loss below 0.2 dB and Y-polarized light insertion loss below 0.04 dB, which helps improve optical signal transmission efficiency and meets the requirements of high-precision measurement applications.

[0052] The embodiments of this application will be further described below with reference to the accompanying drawings.

[0053] This invention provides a mid-infrared polarization beam splitter for a pure silicon substrate with a gold-plated bowtie-type dual-core photonic crystal fiber, the cross-sectional structure of which is shown in the figure below. Figure 1As shown. The specific structure includes: a substrate 7, the material of which is pure silicon; a first circular air hole 1 is formed at the center of the substrate 7, and a gold film 2 is plated inside the first circular air hole 1; two fiber cores, such as first fiber core A and second fiber core B, are symmetrically formed on both sides of the first circular air hole 1, and the first fiber core A and second fiber core B are also made of pure silicon; two elliptical air holes 3 are symmetrically arranged on the outside of the two fiber cores; a plurality of second circular air holes 4, such as 38, are arranged to form a bow-tie-shaped arrangement around the first circular air hole 1, the first fiber core A and the second fiber core B, and the two elliptical air holes 3; a plurality of third circular air holes 5, such as 72, are arranged to form a bow-tie-shaped arrangement around the second circular air holes 4; wherein the diameter of the third circular air holes 5 is larger than the diameter of the second circular air holes 4; the spacing between any two adjacent third circular air holes 5 is equal, and the spacing between any two adjacent second circular air holes 4 is equal. The bow-tie-shaped arrangement generally has an outline with rhombuses symmetrically arranged on both sides and a roughly rectangular shape in the middle.

[0054] The polarization beam splitter has a gold film 2 deposited inside the first circular air hole 1. The outer diameter of the first circular air hole 1 is d1, and the thickness of the gold film is t. When light waves propagate along the surface of the gold film, surface plasmon polariton modes are excited on the surface. Under certain conditions, the surface plasmon polariton modes and the core mode will resonate and couple, thereby significantly changing the coupling length of the X-polarized and Y-polarized modes and promoting the separation of X-polarized and Y-polarized light. Because gold films have high chemical stability and corrosion resistance, and the preparation process is relatively mature, they are more suitable than other metals for exciting surface plasmon polariton modes.

[0055] The polarization beam splitter has two elliptical air holes 3 on its left and right sides, one for the first fiber core A and the other for the second fiber core B, with a minor axis diameter of d. x The major axis diameter is d y This helps to squeeze the fiber core polarization mode toward the circular air holes of the gold-plated film, making it easier for the fiber core polarization mode and the surface plasmon polariton mode to couple.

[0056] The polarization beam splitter is surrounded by an inner bowtie cladding around the two fiber cores and two elliptical air holes 3. The inner bowtie cladding consists of 38 small-diameter second circular air holes 4, each with a diameter of d2. The second circular air holes 4 widen the leakage transmission channel of the fiber core mode, which is beneficial to enhancing the periodic energy exchange between the first fiber core A and the second fiber core B.

[0057] The polarization beam splitter is surrounded by an outer bowtie cladding with a small-diameter second circular air hole 4 in the inner bowtie cladding. The outer bowtie cladding consists of 72 large-diameter third circular air holes 5, each with a diameter of d3. The third circular air holes 5 enhance the confinement of polarized light transmitted in the first fiber core A and the second fiber core B, which helps to reduce the insertion loss of the device.

[0058] The spacing between any two adjacent air holes in the polarization beam splitter is equal, that is, the spacing 6 between any two adjacent second circular air holes 4, between any two adjacent third circular air holes 5, and between any two adjacent second circular air holes 4 and third circular air holes 5 is Λ, which can reduce the difficulty of fabricating the polarization beam splitter.

[0059] The polarization beam splitter substrate material is pure silicon. Pure silicon has excellent thermal and chemical stability, is suitable for special environments such as high temperature and high pressure, has very low absorption loss in the mid-infrared spectral region (2-5μm band), and has a mature fabrication process. Therefore, it is more suitable as the substrate material for mid-infrared polarization beam splitters than other substrate materials.

[0060] The polarization beam splitter involves three materials: pure silicon, air, and gold. There are no other materials. Except for the gold-plated first circular air hole 1 in the center, the other air holes are filled with air.

[0061] In some embodiments, the outer diameter of the first circular air hole 1 in the central gold-plated film is d1, which ranges from 0.90 μm to 1.1 μm. The thickness of the gold film 2 of the first circular air hole 1 in the central gold-plated film is t, which ranges from 35 nm to 55 nm. The minor axis diameter of the two elliptical air holes 3 is d. x Its range is 1.3μm to 1.5μm, and its major axis diameter is d. y The diameter of each of the 38 small-diameter second circular air holes 4 in the inner bowtie-type cladding is d2, ranging from 1.1μm to 1.3μm. The diameter of each of the 72 large-diameter third circular air holes 5 in the outer bowtie-type cladding is d3, ranging from 1.65μm to 1.85μm. The spacing between any two adjacent air holes is Λ, ranging from 1.9μm to 2.3μm.

[0062] like Figure 1As shown, 8 and 9 represent the first fiber core A and the second fiber core B, respectively. Both the first and second fiber cores A and B are made of pure silicon substrate material, and they serve as two transmission channels for light waves. Since the refractive index of air is lower than that of pure silicon, the light guiding mechanism of both the first and second fiber cores A and B is total internal reflection, enabling light waves to propagate within them.

[0063] Both the first core A and the second core B are made of pure silicon substrate material and are solid cores. The material dispersion equation (refractive index) for pure silicon substrate material can be expressed as:

[0064]

[0065] Where λ is the wavelength of the incident light, λ₁ = 1.971 μm, ε = 11.6858, and a = 0.939816 μm. 2 b = 8.10461 × 10 -3 .

[0066] The inner wall of the first circular air hole 1 is coated with a gold film 2, the relative permittivity of which is (ε) m It can be expressed by the following relation:

[0067]

[0068] Among them, ω, ω D and γ D Let Δε and ε represent the angular frequency of the incident light wave, the plasma frequency, and the damping frequency, respectively. ∞ Ω represents the weighting coefficient and the high-frequency dielectric constant, respectively. L and Γ L Let represent the frequency and bandwidth of the Lorentz oscillator, respectively, and j represent the imaginary unit. The specific parameters involved in this expression are shown in Table 1.

[0069] Table 1 ε of Gold material m Detailed parameters in expression (2)

[0070]

[0071] According to the mode coupling theory of dual-core photonic crystal fibers, due to the evanescent field, energy is exchanged between two closely spaced but separated cores. Assuming light enters from the first core A, under certain conditions, the light energy will periodically couple from the first core A to the second core B, and then gradually return to the first core A, as the light wave propagation length increases. The coupling length is defined as the length through which all the transmission power of the dual-core photonic crystal fiber is coupled from the first core A to the second core B. Since the two cores of a dual-core photonic crystal fiber are generally structurally symmetrical and have identical dielectric material distributions, the modes of a dual-core photonic crystal fiber can generally be considered as a supermode formed by the superposition of four modes: even supermodes and odd supermodes in the X and Y polarization directions. The propagation constants and effective refractive indices corresponding to these four modes are each defined as follows: and Therefore, the formula for the coupling length of a dual-core photonic crystal fiber is:

[0072]

[0073] Among them CL X CL represents the coupling length from the first fiber core A to the second fiber core B, where X-polarized light is completely coupled. Y This represents the coupling length from the first fiber core A to the second fiber core B.

[0074] When X-polarized light and Y-polarized light have been transmitted for a certain length, and the X-polarized light is fully coupled to the first fiber core A for the first time, while the Y-polarized light is fully coupled to the second fiber core B, the polarization beam splitting function is considered to have been achieved. That is, the X-polarized light exists only in the first fiber core A, and the Y-polarized light exists only in the second fiber core B. This transmission length is called the beam splitting length.

[0075] The beam splitting length is often CL. X and CL Y To obtain the shortest beam length, a multiple of CL is defined. Y and CL X The ratio, CLR, of a dual-core photonic crystal fiber polarization beam splitter can be expressed as:

[0076]

[0077] When the transmission length L satisfies L = pCL X =qCL Y When p and q are positive integers (odd and even), X-polarized light and Y-polarized light are located in two separate fiber cores, thus achieving polarization beam splitting. From this rule, it can be seen that when CLR = 2 / 1 or 1 / 2, the splitting length of the dual-core photonic crystal fiber polarization beam splitter is the shortest.

[0078] Furthermore, when light is incident from the first fiber core A, according to the surface plasmon resonance effect, surface plasmon polariton modes of different orders are excited on the surface of the gold film 2, and each order of surface plasmon polariton mode also has two polarization directions, X and Y. When the wavelength of the incident light is at a certain wavelength position, the real part of the effective refractive index of the even or odd supermode in a certain polarization direction and the surface plasmon polariton mode of a certain order on the gold film surface will undergo a significant abrupt change. That is, the even or odd supermode in that polarization direction will undergo significant surface plasmon resonance coupling with the surface plasmon polariton mode of a certain order on the gold film surface at that wavelength. This wavelength is called the resonance wavelength, which greatly changes the effective refractive index difference between the even and odd supermodes in that polarization direction, thereby achieving the purpose of changing the coupling length in that polarization direction, and finally obtaining the required coupling length ratio. Within the given parameter range, second-order surface plasmon polariton modes will be excited on the surface of the gold film 2 material. Only the odd supermode and the second-order surface plasmon polariton mode in the X-polarization direction, as well as the odd supermode and the second-order surface plasmon polariton mode in the Y-polarization direction, will exhibit significant resonant coupling at two different wavelengths, while the even supermode in the X-polarization direction and the even supermode in the Y-polarization direction will not change. Therefore, the coupling lengths in the X-polarization and Y-polarization directions and the ratio of the coupling lengths between them will vary considerably.

[0079] The key performance indicator for polarization beam splitters is the extinction ratio (ER). The extinction ratios of fiber core A and fiber core B can be expressed as:

[0080]

[0081] in, and These represent the normalized output power of the X-polarized light and the Y-polarized light in the first fiber core A at the optical output port, respectively. and These represent the normalized output power of the X-polarized and Y-polarized light in the second fiber core B at the optical output port, respectively. The beam-splitting bandwidth of a polarization beamsplitter can be expressed as the wavelength width corresponding to an extinction ratio (ER) greater than 20 dB. The wider the beam-splitting bandwidth, the better the performance of the polarization beamsplitter.

[0082] The insertion loss (IL) of X-polarized and Y-polarized light is another key indicator for evaluating the performance of polarization beam splitters, and can be expressed as:

[0083]

[0084] Because of this, and These represent the normalized output power of the X-polarized and Y-polarized light at the optical output port, respectively, while Pin represents the input optical power of the polarization beam splitter. A lower insertion loss (IL) indicates better performance of the polarization beam splitter.

[0085] Below, the infrared polarization beam splitter of the pure silicon substrate bowtie-type dual-core photonic crystal fiber with gold-plated film of the present invention is demonstrated at d1 = 1.05 μm, t = 45 nm, d x =1.35μm, d y Taking the following parameters as examples, d1 = 1.75 μm, d2 = 1.15 μm, d3 = 1.70 μm, and Λ = 2.2 μm, the performance of this polarization beam splitter will be explained.

[0086] Figure 2 This is a graph showing the relationship between the effective real part of the refractive index and wavelength for even and odd supermodes in the X-polarization direction, as well as for second-order surface plasmon polariton modes in the X-polarization direction. Figure 2 As shown, in the wavelength range of 2.5–4.2 μm, the real part of the effective refractive index of the even supermode in the X-polarization direction gradually decreases with increasing wavelength, without any abrupt change; while the real part of the effective refractive index of the odd supermode in the X-polarization direction and the second-order surface plasmon polariton mode in the X-polarization direction shows a significant abrupt change at 2.994 μm, that is, the two modes undergo surface plasmon resonance coupling at the resonance wavelength of 2.994 μm.

[0087] Figure 3 This is a graph showing the relationship between the real part of the effective refractive index and wavelength for even and odd supermodes in the Y-polarization direction, as well as for second-order surface plasmon polariton modes in the Y-polarization direction. Figure 3 As shown, in the wavelength range of 2.5–4.2 μm, the real part of the effective refractive index of the even supermode in the Y-polarization direction gradually decreases with increasing wavelength, without any abrupt change; while the real part of the effective refractive index of the odd supermode in the Y-polarization direction and the second-order surface plasmon polariton mode in the Y-polarization direction exhibits a significant abrupt change at 2.911 μm, indicating that the two modes undergo surface plasmon resonance coupling at the resonance wavelength of 2.911 μm.

[0088] Figure 4 This is a graph showing the relationship between the coupling lengths in the X and Y polarization directions, and the ratio of their coupling lengths, as a function of wavelength. (Example:) Figure 4As shown, within the wavelength range of 2.5–4.2 μm, the coupling length in the X-polarization direction initially decreases rapidly with increasing wavelength, exhibiting a significant abrupt change at the resonant wavelength of 2.994 μm. After 2.994 μm, it shows a trend of first increasing and then decreasing, but the overall rate of change tends to level off. Similarly, the coupling length in the Y-polarization direction initially decreases rapidly with increasing wavelength, exhibiting a significant abrupt change at the resonant wavelength of 2.911 μm. After 2.911 μm, it shows a trend of first increasing and then decreasing, but the overall rate of change tends to level off. The coupling length ratio gradually decreases between 2.5 and 2.911 μm, experiences a rapid increase between 2.911 and 2.994 μm, and then gradually stabilizes around 2 after 2.994 μm. At a wavelength of 3.28 μm, the coupling length in the X-polarization direction is 97 μm, and the coupling length in the Y-polarization direction is 194 μm, with a coupling length ratio exactly equal to 2. Therefore, this polarization beam splitter can achieve complete splitting of X-polarized and Y-polarized light at 194 μm.

[0089] Figure 5 This is a graph showing the relationship between the normalized output power and transmission length of fiber core A. (Example) Figure 5 As shown, assuming light is incident from the first fiber core A, the normalized output power of the X-polarized light in the first fiber core A reaches a minimum of 0 at 97 μm and a maximum at 194 μm as the transmission length increases. Meanwhile, the normalized output power of the Y-polarized light in the first fiber core A also reaches a minimum of 0 at 194 μm as the transmission length increases. Therefore, at a transmission length of 194 μm, only X-polarized light exists in the first fiber core A, and Y-polarized light is absent.

[0090] Figure 6 This is a graph showing the relationship between the normalized output power and transmission length of the second fiber core B. (Example:) Figure 6 As shown, assuming light is incident from the first fiber core A, the normalized output power of the X-polarized light in the second fiber core B reaches its maximum value at 97 μm and its minimum value of 0 at 194 μm as the transmission length increases. Meanwhile, the normalized output power of the Y-polarized light in the second fiber core B reaches its maximum value at 194 μm as the transmission length increases. Therefore, at a transmission length of 194 μm, only Y-polarized light exists in the second fiber core B, and no X-polarized light exists. This further demonstrates that the polarization beam splitter completely separates the X-polarized light into the first fiber core A and completely separates the Y-polarized light into the second fiber core B at 194 μm, achieving complete beam splitting of X-polarized and Y-polarized light.

[0091] Figure 7 This is a graph showing the relationship between the extinction ratio and wavelength of the first fiber core A when the beam splitting length is 194 μm. Figure 7As shown, with increasing wavelength, the extinction ratio of the first fiber core A has two maximum peaks. Within the range of 3.17–3.90 μm, the extinction ratio of the first fiber core A is greater than 20 dB. Therefore, the beam-splitting bandwidth of the first fiber core A of this polarization beam splitter reaches 730 nm, achieving an ultra-wide beam-splitting bandwidth in the mid-infrared spectral region.

[0092] Figure 8 This is a graph showing the relationship between the extinction ratio and wavelength of the second fiber core B when the beam splitting length is 194 μm. Figure 8 As shown, with increasing wavelength, the extinction ratio of the second fiber core B exhibits two maximum peaks. Within the range of 3.19–3.76 μm, the extinction ratio of the second fiber core B is greater than 20 dB. Therefore, the beam-splitting bandwidth of the second fiber core B in this polarization beam splitter reaches 570 nm, achieving an ultra-wide beam-splitting bandwidth in the mid-infrared spectral region.

[0093] Figure 9 This graph shows the relationship between insertion loss and wavelength for X- and Y-polarized light when the beam splitting length is 194 μm. Figure 9 As shown, within the wavelength range corresponding to the beam splitting bandwidth, the insertion loss of X-polarized light is less than 0.2 dB, and the insertion loss of Y-polarized light is less than 0.04 dB. Therefore, this polarization beam splitter achieves ultra-low insertion loss for both X-polarized and Y-polarized light within the beam splitting bandwidth.

[0094] Example 1: The outer diameter of the first circular air hole 1 in the center of the gold-plated film is d1 = 0.9 μm. The thickness of the gold film 2 in the center of the first circular air hole is t = 35 nm. The minor axis diameter d of the two elliptical air holes 3 is... x = 1.3μm, major axis diameter d y =1.6μm. The inner bowtie cladding has 38 small-diameter second circular air holes (d2 = 1.1μm). The outer bowtie cladding has 72 large-diameter third circular air holes (d3 = 1.65μm). The distance between any two adjacent air holes is Λ = 1.9μm.

[0095] Example 2: The outer diameter of the first circular air hole 1 in the center of the gold-plated film is d1 = 1.1 μm. The thickness of the gold film 2 surrounding the first circular air hole 1 in the center is t = 55 nm. The minor axis diameter d of the two elliptical air holes 3 is... x =1.5μm, major axis diameter d y =1.8μm. The inner bowtie cladding has 38 small-diameter second circular air holes (4), with a diameter d2 = 1.3μm. The outer bowtie cladding has 72 large-diameter third circular air holes (5), with a diameter d3 = 1.85μm. The distance between any two adjacent air holes Λ = 2.3μm.

[0096] Finally, it should be noted that the various embodiments in this specification are described by way of example, and each embodiment focuses on the differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the systems or apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple, and relevant parts can be referred to the method section.

[0097] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A gold-plated bowtie-type dual-core photonic crystal fiber mid-infrared polarization beam splitter, characterized in that... ,include: The substrate is made of pure silicon. A first circular air hole is disposed at the center of the substrate, and a gold film is plated on the inner side of the first circular air hole; Two fiber cores are symmetrically arranged on both sides of the first circular air hole; Two elliptical air holes are symmetrically arranged on the outer sides of the two fiber cores; Thirty-eight second circular air holes are arranged in a bow-tie pattern around the first circular air hole, two fiber cores, and two elliptical air holes. 72 third circular air holes are arranged in a bow tie pattern around the second circular air holes; The diameter of the third circular air hole is larger than that of the second circular air hole; the spacing between any two adjacent third circular air holes and any two adjacent second circular air holes is equal.

2. The polarization beam splitter according to claim 1, characterized in that... , The diameter d1 of the first circular air hole ranges from 0.9 μm to 1.1 μm.

3. The polarization beam splitter according to claim 1, characterized in that... , The thickness t of the gold-plated film inside the first circular air hole ranges from 35nm to 55nm.

4. The polarization beam splitter according to claim 1, characterized in that... , The minor axis diameter dx of the two elliptical air holes ranges from 1.3μm to 1.5μm, and the major axis diameter dy ranges from 1.6μm to 1.8μm.

5. The polarization beam splitter according to claim 1, characterized in that... , The diameter d2 of the second circular air hole ranges from 1.1 μm to 1.3 μm; The diameter d3 of the third circular air hole ranges from 1.65 μm to 1.85 μm.

6. The polarization beam splitter according to claim 1, characterized in that... , The distance α between any two adjacent third circular air holes and any two adjacent second circular air holes ranges from 1.9 μm to 2.3 μm.

7. The polarization beam splitter according to claim 1, characterized in that... , Both fiber cores are made of solid pure silicon substrate material.

8. The polarization beam splitter according to claim 1, characterized in that... , The relative permittivity εm of the gold film satisfies the following relationship: in, ω , ω D and γ D Δ represents the angular frequency of the incident light wave, the plasma frequency, and the damping frequency, respectively. ε and ε ∞ Ω represents the weighting coefficient and the high-frequency dielectric constant, respectively. L and Γ L represents the frequency and bandwidth of the Lorentz oscillator, respectively, and j represents the imaginary unit.

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