Middle infrared band microstructure optical fiber polarization beam splitter

Abstract

The invention discloses a middle infrared band microstructure optical fiber polarization beam splitter, the polarization beam splitter comprises a centrosymmetric cladding and two fiber cores, the cladding and the fiber cores are composed of a substrate material GaS and air holes, and the cladding is located on the outer side of the fiber cores. The cladding is composed of first air circular holes and second air circular holes, and the plurality of first air circular holes are periodically arranged in a triangular lattice structure. The fiber core is composed of a first elliptical air hole, a second elliptical air hole, a third air round hole and a fourth air round hole, and the fiber core is located between the first elliptical air hole and the second elliptical air hole and is a solid core filled with a GaS substrate material. The length of the micro-structure optical fiber polarization beam splitter of the middle-infrared band is only 40 microns, and the extinction ratio at the wavelength of 4.42 microns can reach-70 dB; and single polarization output can be realized in a range of 4.29 [mu] m to 4.59 [mu] m. The device has the advantages of small size, high extinction ratio, large bandwidth and the like, adopts a single material, effectively reduces the manufacturing difficulty of the device, is easy to integrate and miniaturize, and has potential application value in the fields of mid-infrared optical fiber sensing, mid-infrared band communication, mid-infrared band environment detection, optical nonlinearity and the like.

Description

Technical Field

[0001] This invention relates to the field of microstructure fiber polarization beam splitter technology, and particularly to a microstructure fiber polarization beam splitter in the mid-infrared band. Background Technology

[0002] A polarization beamsplitter is an important optical component that separates incident light into two beams with orthogonal polarization states, allowing them to propagate in different directions. This device is widely used in optical communication, optical storage, and optical sensing. In recent years, due to the unique wavelength characteristics of mid-infrared electromagnetic waves, the mid-infrared band has received increasing attention in both civilian and military applications. Previously, research on polarization beamsplitters mainly focused on the near-infrared band, with relatively little research on mid-infrared polarization beamsplitters. Therefore, from a practical application perspective, designing a mid-infrared polarization beamsplitter will provide an effective solution for the development of mid-infrared fiber optic sensing, communication, and environmental monitoring technologies. Summary of the Invention

[0003] The purpose of this invention is to provide a microstructured fiber polarization beamsplitter in the mid-infrared band, capable of achieving ultra-short coupling length, high extinction ratio, and ultra-large bandwidth polarization beam splitting in the mid-infrared band. The specific solution is as follows:

[0004] This invention provides a microstructured fiber polarization beamsplitter in the mid-infrared band. The polarization beamsplitter includes a cladding and two fiber cores. Both the fiber cores and the cladding are made of GaS substrate and have a centrosymmetric structure. The cladding is located outside the fiber cores.

[0005] The cladding is composed of a plurality of first air holes and second air holes arranged in a triangular lattice structure, which are periodically arranged into two centrally symmetrical trapezoidal structures, and the first hole spacing between two adjacent first air holes is the same; the second air holes are centrally symmetrically distributed in the middle of the two trapezoidal structures, and the second hole spacing between adjacent second air holes and the first air holes is the same.

[0006] The fiber core is composed of a first elliptical air hole, a second elliptical air hole, a third air hole, and a fourth air hole, with the centers of the first elliptical air hole, the second elliptical air hole, and the fiber core all located in the same x-axis direction. The first elliptical air hole is located at the exact center of the polarization beam splitter, and its major axis is along the y-direction. Two second elliptical air holes are symmetrically distributed on either side of the first elliptical air hole, with their major axes along the x-direction, and the third aperture distance between them is the same as that between the second and first elliptical air holes. The two third air holes are tangent to the two vertices of the major axis of the third elliptical air hole. The fourth air hole is located near the third air hole. The fiber core is located between the first and second elliptical air holes.

[0007] The major axes of the first elliptical air hole and the second elliptical air hole are perpendicular to each other.

[0008] The first elliptical air hole is located at the center of the cladding.

[0009] Optionally, the centers of the first elliptical air hole, the second elliptical air hole, and the second air circular hole are all located in the same x-axis direction and in the middle of the trapezoidal structure in the cladding, wherein the second air circular hole is located outside the second elliptical air hole.

[0010] Optionally, the fourth air hole is centrally symmetrically distributed between the trapezoidal structure and the third air hole.

[0011] Optionally, the first elliptical air hole and the second elliptical air hole have different structural dimensions.

[0012] Optionally, the diameter of the third air hole is much smaller than the diameter of the first air hole.

[0013] Optionally, the diameter of the fourth air hole is much smaller than the diameter of the first air hole.

[0014] Optionally, the second hole spacing is greater than the first hole spacing; the third hole spacing is greater than or equal to the first hole spacing.

[0015] Optionally, the major axis of the first elliptical air hole is a1, which ranges from 0.5 to 1 μm, the minor axis is b1, which ranges from 0.1 to 0.4 μm, and the ellipticity η1 = a1 / b1, wherein η1 is greater than or equal to 2.5.

[0016] Optionally, the major axis of the second elliptical air hole is a2, which ranges from 0.5 to 0.9 μm, the minor axis is b1, which ranges from 0.4 to 0.6 μm, and the ellipticity η2 = a2 / b2, where η2 > 1.25.

[0017] Optionally, the first air diameter ranges from 1.0 to 1.4 μm.

[0018] Optionally, the diameter of the second air hole ranges from 1.0 to 1.4 μm.

[0019] Optionally, the diameter of the third air hole ranges from 0.05 to 0.3 μm.

[0020] Optionally, the diameter of the fourth circular air hole ranges from 0.05 to 0.3 μm.

[0021] Optionally, the first aperture spacing is in the range of 1.1 to 1.5 μm.

[0022] Optionally, the second aperture spacing ranges from 1.2 to 1.6 μm.

[0023] Optionally, the ratio of the third hole spacing to the first hole spacing is in the range of 1 to 2.

[0024] Both fiber cores are solid cores filled with GaS substrate material.

[0025] Compared with the prior art, the above-described solutions of the present invention have at least the following technical effects:

[0026] (1) The present invention provides a microstructure fiber polarization beam splitter in the mid-infrared band. The device uses a single GaS material as the substrate material and achieves polarization beam splitting in the mid-infrared band by optimizing the structure and arrangement of the air holes.

[0027] (2) This invention overcomes the shortcomings of traditional polarization beam splitters, such as large size, low extinction ratio and narrow bandwidth. It adopts a microstructure fiber core design to achieve a fiber length of only 40μm for polarization beam splitting. The extinction ratio at a wavelength of 4.44μm can reach -70dB, and the bandwidth at a wavelength of 4.51μm with an extinction ratio of less than -20dB can reach 300nm.

[0028] (3) This invention has the advantages of small size, high extinction ratio and large bandwidth. It uses a single material, which will effectively reduce the manufacturing difficulty of the device and make it easy to integrate and miniaturize. It is one of the effective solutions for mid-infrared applications such as mid-infrared fiber optic sensing, communication and environmental detection. Attached Figure Description

[0029] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. In the drawings:

[0030] Figure 1 This is a cross-sectional view of a microstructured fiber polarization beam splitter in the mid-infrared band provided in an embodiment of the present invention;

[0031] Figure 2 This is a mode field distribution diagram of a microstructured fiber polarization beam splitter in the mid-infrared band provided in an embodiment of the present invention;

[0032] Figure 3 This is a graph showing the relationship between the real part of the effective refractive index of each polarization mode and the wavelength of the microstructure fiber polarization beam splitter in the mid-infrared band provided in this embodiment of the invention.

[0033] Figure 4 This is a graph showing the relationship between the coupling length and coupling length ratio of the microstructure fiber polarization beam splitter in the mid-infrared band provided in this embodiment of the invention as a function of wavelength.

[0034] Figure 5 This is a graph showing the relationship between the normalized output power of the microstructure fiber polarization beam splitter in the mid-infrared band and the propagation length, provided in an embodiment of the present invention.

[0035] Figure 6 This is a graph showing the relationship between the extinction ratio and wavelength of the microstructured fiber polarization beam splitter in the mid-infrared band provided in this embodiment of the invention.

[0036] Explanation of reference numerals in the attached figures:

[0037] 1. GaS substrate material;

[0038] A. First fiber core;

[0039] B. Second fiber core;

[0040] 2. First circular air hole;

[0041] 3. Second circular air hole;

[0042] 4. Third circular air hole;

[0043] 5. Fourth circular air hole;

[0044] 6. First elliptical air hole

[0045] 7. Second elliptical air hole

[0046] Λ1, Spacing of the first hole

[0047] Λ2, Spacing of the second hole

[0048] Λ3, Spacing of the third hole

[0049] a1, the length of the major axis of the first elliptical air hole

[0050] b1, the minor axis length of the first elliptical air hole

[0051] a2, the length of the major axis of the second elliptical air hole

[0052] b2. The minor axis of the second elliptical air hole Detailed Implementation

[0053] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. The invention will be further described below with reference to the accompanying drawings.

[0054] The terminology used in the embodiments of this invention is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms “a,” “the,” and “the” as used in the embodiments of this invention 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.

[0055] 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.

[0056] It should be understood that although the terms first, second, third, etc. may be used to describe the invention in the embodiments, the invention should not be limited to these terms.

[0057] 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.

[0058] The invention will now be described in detail with reference to an optional embodiment.

[0059] This invention provides a microstructure fiber polarization beam splitter in the mid-infrared band. Figure 1 This is a cross-sectional schematic diagram of a microstructured fiber polarization beam splitter in the mid-infrared band provided according to an embodiment of the present invention. Figure 1 The polarization beam splitter shown includes a centrally symmetrical cladding and two fiber cores, wherein the substrate material 1 of both the fiber cores and the cladding is GaS, and the cladding is located outside the fiber core region.

[0060] The cladding is composed of a plurality of first air holes 2 and second air holes 3 arranged in a triangular lattice structure, which are periodically arranged into two centrally symmetrical trapezoidal structures, and the first hole spacing Λ1 between two adjacent first air holes 2 is the same; the second air holes 3 are centrally symmetrically distributed in the middle of the two trapezoidal structures in the cladding, and the second hole spacing Λ2 between adjacent second air holes 3 and first air holes 2 is the same.

[0061] The fiber core is composed of a first elliptical air hole 6, a second elliptical air hole 7, fiber cores A and B, a third air hole 4, and a fourth air hole 5, with the centers of the first elliptical air hole 6, the second elliptical air hole 7, and fiber cores A and B all located along the same x-axis. Specifically, the first elliptical air hole 6 is located at the exact center of the polarization beam splitter, and its major axis is along the y-direction; two second elliptical air holes 7 are symmetrically distributed on either side of the first elliptical air hole 6, with their major axes along the x-direction, and the third aperture distance between them is the same as that between the second elliptical air holes 7 and the first elliptical air hole 6; two third air holes 4 are tangent to the two vertices of the major axis of the third elliptical air hole 6; the fourth air hole 5 is located near the third air hole 4; and fiber cores A and B are located between the first elliptical air hole 6 and the second elliptical air hole 7.

[0062] The major axes of the first elliptical air hole 6 and the second elliptical air hole 7 are perpendicular to each other; the first elliptical air hole 6 is located at the center of the cladding; the centers of the first elliptical air hole 6, the second elliptical air hole, and the second air hole 3 are all located in the same x-axis direction and in the middle of the trapezoidal structure of the cladding, wherein the second air hole 3 is located outside the second elliptical air hole 7; the fourth air hole 5 is centrally symmetrically distributed between the trapezoidal structure and the third air hole 4 in the cladding.

[0063] The first elliptical air hole 6 and the second elliptical air hole 7 have different structural dimensions.

[0064] Optionally, the major axis of the first elliptical air hole 6 is a1, ranging from 0.5 to 1 μm, and the minor axis is b1, ranging from 0.1 to 0.4 μm. The ellipticity η1 = a1 / b1, where η1 is greater than or equal to 2.5. The major axis of the second elliptical air hole 7 is a2, ranging from 0.5 to 0.9 μm, and the minor axis is b1, ranging from 0.4 to 0.6 μm. The ellipticity η2 = a2 / b2, where η2 > 1.25. The diameter d3 of the third circular air hole 4 ranges from 0.05 to 0.3 μm. The diameter d4 of the fourth circular air hole 5 ranges from 0.05 to 0.3 μm.

[0065] The diameters d3 and d4 of the third air hole 4 and the fourth air hole 5 are both much smaller than the diameter d1 of the first air hole 2.

[0066] Optionally, the diameter d1 of the first air hole 2 ranges from 1.0 to 1.4 μm; the diameter d2 of the second air hole 3 ranges from 0.9 to 1.5 μm; the diameter d3 of the third air hole 4 ranges from 0.05 to 0.3 μm; and the diameter d4 of the fourth air hole 5 ranges from 0.05 to 0.3 μm.

[0067] The second hole spacing Λ2 is greater than the first hole spacing Λ1; the third hole spacing Λ3 is greater than or equal to the first hole spacing Λ1.

[0068] Optionally, the first aperture spacing Λ1 ranges from 1.1 to 1.5 μm; the second aperture spacing Λ2 ranges from 1.2 to 1.6 μm; and the ratio of the third aperture spacing Λ3 to the first aperture spacing Λ1 ranges from 1 to 2.

[0069] Within the aforementioned size range, the two different polarization directions of the dual core exhibit strong asymmetry, thereby enabling a high birefringence effect.

[0070] Both fiber cores are solid cores filled with GaS substrate material. GaS substrate material is used as the base material in this embodiment to meet the requirements of the polarization beam splitter for application in the mid-infrared band.

[0071] Furthermore, in this embodiment, 70 first air holes 1 form two centrally symmetrical trapezoidal structures. Each trapezoidal structure consists of 5 layers and 35 first air holes 2, with the number of first air holes 2 in each layer being 5, 6, 7, 8, and 9 respectively. There are 4 second air holes, which are centrally symmetrically distributed in the middle of the two trapezoidal structures in groups of two. There are 2 third air holes 5, which are tangent to the two ends of the apex of the major axis of the first elliptical air hole 6. There are 4 fourth air holes 5, which are centrally symmetrically distributed in groups of two between the two trapezoidal structures in the cladding and the third air holes 4, and the hole spacing between adjacent fourth air holes 5 and third air holes 4 is equidistant. There are 2 second elliptical air holes, which are centrally symmetrically distributed in the middle of the two trapezoidal structures in the cladding. Wherein, d1 = 1.1 μm, d2 = 1.1 μm, d3 = 0.2 μm, d1 = 0.4 μm; a1 = 0.7 μm, b1 = 0.2 μm, η1 = 3.5; a2 = 0.8 μm, b2 = 0.57 μm, η2 = 1.4; Λ1 = 1.15 μm, Λ2 = 1.3 μm, Λ3 = 2.07 μm, and the hole distance between the fourth air hole 5 and the third air hole 4 is 0.78 μm.

[0072] like Figure 2 As shown, the energy is mainly concentrated in the dual cores, with cores A and B influencing each other to form mode coupling. The arrows in the figure represent the electric field directions. The microstructured fiber exhibits four modes: odd mode in the X direction, even mode in the X direction, odd mode in the Y direction, and even mode in the Y direction. When the operating wavelength is 4.51 μm, the mode field distribution in the microstructured fiber is as follows: Figure 2 As shown in (a), 2(b), 2(c), and 2(d), the refractive index of each polarization mode in this embodiment varies with wavelength as follows: Figure 3 As shown.

[0073] In this embodiment, the relationship between the coupling length and the coupling length ratio and the wavelength is as follows: Figure 4 As shown. Figure 2 The dual-core photonic crystal fiber shown contains four supermodes: an odd mode in the X direction, an even mode in the X direction, an odd mode in the Y direction, and an even mode in the Y direction. According to mode coupling theory, odd and even modes with the same polarization state couple along the propagation direction of the fiber, allowing polarized light energy to transfer from one core to another. When the light energy of a certain polarization state in the incident core is 0, the corresponding propagation distance is called the coupling length; the ratio of the coupling lengths in the two polarization directions is called the coupling length ratio. Figure 4It can be seen that the X and Y polarization coupling lengths and coupling length ratios both decrease with increasing wavelength, reaching a minimum of 2 at a wavelength of 4.51 μm. The coupling length and coupling length ratio determine the transmission distance for light coupling in the optical fiber, and thus determine the size of the polarization beam splitter, making them one of the most crucial performance indicators of fiber polarization beam splitters.

[0074] The relationship between the normalized output power of the fiber polarization beam splitter and the propagation length in this embodiment is shown in the figure below. Figure 5 As shown, when the fiber length is 40μm, the X-polarized light energy reaches its maximum value in a fiber core at a wavelength of 4.51μm, while the Y-polarized light energy reaches its minimum value at the same time, thus achieving complete separation of X and Y polarized light.

[0075] In this embodiment, the extinction ratio of the polarization beam splitter varies with wavelength as follows: Figure 6 As shown in the figure, the extinction ratio at a wavelength of 4.42 μm can reach -70 dB, and the extinction ratio in the wavelength range of 4.29 μm to 4.59 μm is less than -20 dB. The operating bandwidth can reach 300 nm, thus exhibiting a large extinction ratio and wide bandwidth.

[0076] The above analysis shows that the microstructured fiber polarization beamsplitter in the mid-infrared band provided by this invention achieves excellent beam splitting performance in the mid-infrared band by introducing GaS substrate material and optimizing the air hole microstructure array. The fiber length of the polarization beamsplitter is only 40 μm, and the extinction ratio at a wavelength of 4.42 μm can reach -70 dB. Single polarization output can be achieved in the range of 4.29 μm to 4.59 μm. This invention has the advantages of small size, high extinction ratio, and large bandwidth. Moreover, by using a single material, it effectively reduces the fabrication difficulty of the device, making it easy to integrate and miniaturize. It has potential application value in mid-infrared fiber optic sensing, mid-infrared band communication, mid-infrared band environmental detection, and optical nonlinearity.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention 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; and these 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 the present invention.

Claims

1. A microstructure fiber polarization beam splitter in the mid-infrared band, characterized in that: include: The fiber has a centrally symmetrical cladding and two cores, wherein the substrate material of both the cladding and the cores is GaS, and the cladding is located outside the cores. The cladding is composed of a plurality of first air holes and second air holes arranged in a triangular lattice structure, which are periodically arranged into two centrally symmetrical trapezoidal structures, and the first hole spacing between two adjacent first air holes is the same; the second air holes are centrally symmetrically distributed in the middle of the two trapezoidal structures, and the second hole spacing between adjacent second air holes and the first air holes is the same. The fiber core is composed of a first elliptical air hole, a second elliptical air hole, a third air hole, and a fourth air hole, with the centers of the first elliptical air hole, the second elliptical air hole, and the fiber core all located in the same x-axis direction. The first elliptical air hole is located at the exact center of the polarization beam splitter, and its major axis is along the y-direction. Two second elliptical air holes are symmetrically distributed on either side of the first elliptical air hole, with their major axes along the x-direction, and the third aperture distance between them is the same as that between the second and first elliptical air holes. The two third air holes are tangent to the two vertices of the major axis of the third elliptical air hole. The fourth air hole is located near the third air hole. The fiber core is located between the first and second elliptical air holes.

2. The microstructure fiber polarization beam splitter in the mid-infrared band according to claim 1, characterized in that: The major axes of the first elliptical air hole and the second elliptical air hole are perpendicular to each other; the first elliptical air hole is located at the center of the cladding; the centers of the first elliptical air hole, the second elliptical air hole, and the second air hole are all located in the same x-axis direction and in the middle of the trapezoidal structure of the cladding, wherein the second air hole is located outside the second elliptical air hole; the fourth air hole is centrally symmetrically distributed between the trapezoidal structure of the cladding and the third air hole.

3. The microstructure fiber polarization beam splitter in the mid-infrared band according to claim 1, characterized in that: The first elliptical air hole and the second elliptical air hole have different structural dimensions; the diameters of the third air hole and the fourth air hole are both much smaller than the diameter of the first air hole.

4. The microstructure fiber polarization beam splitter in the mid-infrared band according to claim 1, characterized in that: The second hole spacing is greater than the first hole spacing; the third hole spacing is greater than or equal to the first hole spacing.

5. The microstructure fiber polarization beam splitter in the mid-infrared band according to claim 3, characterized in that: The first elliptical air hole has a major axis length of a1, ranging from 0.5 to 1 μm, a minor axis length of b1, ranging from 0.1 to 0.4 μm, and an ellipticity η1 = a1 / b1, where η1 is greater than or equal to 2.5; the second elliptical air hole has a major axis length of a2, ranging from 0.5 to 0.9 μm, a minor axis length of b1, ranging from 0.4 to 0.6 μm, and an ellipticity η2 = a2 / b2, where η2 > 1.

25.

6. The microstructure fiber polarization beam splitter in the mid-infrared band according to claim 3, characterized in that: The diameter of the first air hole ranges from 1.0 to 1.4 μm; the diameter of the second air hole ranges from 0.9 to 1.5 μm; the diameter of the third air hole ranges from 0.05 to 0.3 μm; and the diameter of the fourth air hole ranges from 0.05 to 0.3 μm.

7. The microstructure fiber polarization beam splitter in the mid-infrared band according to claim 4, characterized in that: The first hole spacing ranges from 1.1 to 1.5 μm, the second hole spacing ranges from 1.2 to 1.6 μm, and the ratio of the third hole spacing to the first hole spacing ranges from 1 to 2.

8. The microstructure fiber polarization beam splitter in the mid-infrared band according to claim 1, characterized in that: Both fiber cores are solid cores filled with GaS substrate material.

Images