High-birefringence multi-core elliptical polarization-maintaining optical fiber and preparation method thereof

Abstract

This invention relates to a high birefringence multi-core elliptical polarization-maintaining fiber and its fabrication method. The fiber includes a common cladding and stress regions and polarization-maintaining core regions distributed within the common cladding. There are at least three stress regions, evenly spaced circumferentially. The center lines connecting adjacent stress regions form a regular polygon. An elliptical polarization-maintaining core is positioned at the midpoint of each side of the regular polygon. The major axis of each elliptical polarization-maintaining core is aligned with the corresponding side of the regular polygon. Each elliptical polarization-maintaining core is configured with two adjacent stress regions to form a polarization-maintaining core region. This invention, by designing multiple elliptical cores and adding stress bars, provides more transmission channels, exhibits stronger birefringence performance, and reduces fabrication process requirements. The arrangement of shared stress regions forms multiple independent channels with a compact structure and low crosstalk, achieving higher density and greater capacity in multi-core polarization-maintaining fibers, and has broad application prospects in the miniaturization of fiber optic gyroscopes and devices.

Description

Technical Field

[0001] This invention relates to a high birefringence multicore elliptical polarization-maintaining optical fiber and its fabrication method, belonging to the field of optical communication and sensing technology. Background Technology

[0002] Polarization-maintaining fibers are widely used in various sectors of the national economy, including aerospace, aviation, marine, industrial manufacturing, and communications. In interferometric fiber optic sensors based on optical coherence detection, polarization-maintaining fibers ensure the linear polarization direction remains unchanged, improving the coherence signal-to-noise ratio and enabling high-precision measurement of physical quantities. Common polarization-maintaining fibers include panda, bowtie, and elliptical types. Panda and bowtie fibers achieve polarization maintenance through stress birefringence, while elliptical fibers achieve it through geometric birefringence. Elliptical polarization-maintaining fibers have a more compact structure compared to other types. Most existing polarization-maintaining fibers are single-core structures, meaning they have only one transmission channel, and their transmission capacity is gradually failing to meet increasing transmission demands. Multi-core polarization-maintaining fibers utilize space division multiplexing, achieving higher density, greater capacity, and better polarization-maintaining performance. While multi-core elliptical polarization-maintaining fibers offer compact structure and high density, they also present challenges such as high manufacturing precision requirements and difficulties in guaranteeing polarization performance and inter-core crosstalk. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a high birefringence multi-core elliptical polarization-maintaining fiber and its preparation method, which addresses the shortcomings of the prior art. It has a compact structure, stronger birefringence performance, and enables the polarization-maintaining fiber to achieve higher density and greater capacity.

[0004] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows: it includes a common cladding and stress zones and eccentricity-maintaining core zones distributed in the common cladding. There are at least 3 stress zones, which are evenly distributed along the circumference. The center lines of each adjacent stress zone form a regular polygon. An elliptical eccentricity-maintaining core is arranged at the midpoint of each side of the regular polygon. The major axis of the elliptical eccentricity-maintaining core is flush with the corresponding side of the regular polygon. Each elliptical eccentricity-maintaining core and its two adjacent stress zones are configured to form an eccentricity-maintaining core zone.

[0005] According to the above scheme, the radial distance (radius) between the stress zone and the center of the optical fiber is 12~35μm.

[0006] According to the above scheme, the diameter of the stress zone is 8~30μm, and the stress zone is a boron-doped quartz glass layer with a boron molar percentage of 15~25%.

[0007] According to the above scheme, the elliptical polarization maintaining core includes a circular fiber core and an elliptical cladding surrounding the fiber core, which are evenly spaced along the circumference.

[0008] According to the above scheme, the diameter of the common cladding layer is 40-125 μm, and the common cladding layer is a pure silicon dioxide glass layer.

[0009] According to the above scheme, the relative refractive index difference Δ1 of the circular fiber core is 0.5 to 1.3%, and the diameter d1 is 1 to 2.5 μm.

[0010] According to the above scheme, a circular inner cladding is tightly wrapped around the circular fiber core. The diameter d2 of the circular inner cladding is 2 to 6.2 μm, and the relative refractive index difference Δ2 is -0.8 to 0%.

[0011] According to the above scheme, the relative refractive index difference Δ3 of the elliptical cladding is -1.5 to -0.7%, the major axis 2a of the elliptical cladding is 7 to 25 μm, the ellipticity 1-b / a of the elliptical cladding is 0.1 to 0.5, and b is the minor axis of the elliptical cladding.

[0012] According to the above scheme, there are 3 to 4 elliptical core regions that are evenly spaced along the circumference.

[0013] According to the above scheme, the birefringence of the polarization-maintaining fiber is 6.0 × 10⁻⁶. -4 ~6.5×10 -4 .

[0014] According to the above scheme, the macrobending loss of the polarization-maintaining fiber is ≤0.1dB when it is wound 10 times with a bending diameter of 10mm.

[0015] The preparation method of this invention is as follows:

[0016] Fabrication of circular fiber core rods and stress rods

[0017] Fabrication of elliptical clad core rods: In-tube doping deposition is performed using a pure silica glass tube of a certain thickness. After deposition, a circular core rod with a solid doped core layer is sintered and shrunk. This circular core rod is then heated to a molten state, and radial pressure is applied to form a doped core rod with an elliptical or near-elliptical radial cross-section and a clad core. The elliptical cladding is then ground into a core rod with a circular outer periphery and an elliptical inner doped core layer.

[0018] Preparation of elliptical polarization-maintaining core rod: A core rod containing an elliptical doped core layer is drilled with a central hole, and a circular fiber core rod is inserted and fused together to form a solid elliptical polarization-maintaining core rod.

[0019] Fabrication of multi-core elliptical polarization-maintaining fiber preform: Holes are drilled in a solid pure silica glass rod according to the number and spacing of the polarization-maintaining cores and the number and spacing of the stress rods. The solid elliptical polarization-maintaining core rods and stress rods are inserted into the holes according to their orientation to form a multi-core elliptical polarization-maintaining fiber preform.

[0020] Multi-core elliptical polarization-maintaining fiber preforms are clamped into fiber drawing furnaces and drawn into multi-core elliptical polarization-maintaining fibers.

[0021] According to the above scheme, the circular fiber core rod is covered with an inner cladding layer.

[0022] According to the above scheme, the elliptical cladding mandrel is deposited using PCVD (plasma chemical vapor deposition) technology, with SiCl4 (tetrachlorosilane), BCl3 (boron trichloride), O2 (oxygen), and C2F6 (hexafluoroethane) as deposition raw materials. By adjusting the flow rate and ratio of these gases, a mandrel with the required diameter and refractive index is prepared. During the deposition process, the flow rate of SiCl4 is 500–2000 sccm (sccm is a volumetric flow rate unit, referring to standard milliliters per minute), the flow rate of BCl3 is 250–800 sccm, the flow rate of O2 is 1500–5000 sccm, and the flow rate of C2F6 is 20–300 sccm.

[0023] According to the above scheme, the circular fiber core rod is deposited using PCVD process, with SiCl4 (tetrachlorosilane), GeCl4 (germanium tetrachloride), and O2 (oxygen) as deposition raw materials. By adjusting the flow rate and ratio of the gases, a core rod with the required diameter and refractive index is prepared. During the deposition process, the flow rate of SiCl4 is 500-800 sccm, the flow rate of GeCl4 is 20-50 sccm, and the flow rate of O2 is 1000-1500 sccm. After deposition, the core rod is sintered and shrunk into a solid core rod.

[0024] According to the above scheme, the inner cladding is deposited using PCVD process, with SiCl4 (tetrachlorosilane), C2F6 (hexafluoroethane), and O2 (oxygen) as deposition raw materials. During the deposition process, the flow rate of SiCl4 is 800-1200 sccm, the flow rate of C2F6 is 50-150 sccm, and the flow rate of O2 is 2000-3000 sccm. The cladding is deposited before the fiber core layer.

[0025] According to the above scheme, the stress bar is prepared by depositing a boron-doped layer using PCVD technology, with SiCl4 (tetrachlorosilane), BCl3 (boron trichloride), and O2 (oxygen) as deposition raw materials. By adjusting the flow rate and ratio of these gases, stress bars with the required diameter and refractive index are prepared. Specifically, during the boron-doped layer preparation process, the flow rate of SiCl4 is 500–700 sccm, the flow rate of BCl3 is 500–700 sccm, and the flow rate of O2 is 1500–3000 sccm. After deposition, the rod is sintered and shrunk into a solid core.

[0026] The beneficial effects of this invention are as follows: 1. By designing multiple elliptical fiber cores and adding stress bars, the effective length of the optical fiber and polarization-maintaining performance can be increased simultaneously, providing more transmission channels, stronger birefringence performance, and reducing inter-core crosstalk. Furthermore, the addition of stress bars not only increases birefringence performance but also reduces the requirements for the ellipticity of the elliptical cores, lowering the manufacturing process requirements. 2. The arrangement of shared stress zones allows for the positioning of multiple cores for polarization maintenance, forming multi-core polarization-maintaining optical fibers with multiple independent channels transmitting simultaneously, thereby enhancing spectral efficiency. Its compact structure and low crosstalk facilitate the setting of more polarization-maintaining core regions within a limited optical fiber cross-section. This enables elliptical multi-core polarization-maintaining optical fibers to achieve higher density and greater capacity, as well as better polarization-maintaining performance, through space-division multiplexing, showing broad application prospects in the miniaturization of fiber optic gyroscopes and devices. 3. The elliptical polarization-maintaining core regions are evenly spaced circumferentially, and the minor axis of the elliptical cladding points to the common cladding center, forming a corresponding iso-orientation structure. This is beneficial for controlling the cladding roundness during fiber drawing and also for controlling inter-core crosstalk. 4. A circular inner cladding is tightly wrapped around the circular fiber core, which helps to reduce the fiber core's drawing stress and distortion, and can further reduce inter-core crosstalk. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the radial structure of an embodiment of the present invention.

[0028] Figure 2 This is a schematic diagram of the radial structure of the second embodiment of the present invention. Detailed Implementation

[0029] The present invention will now be described in further detail with reference to the embodiments and accompanying drawings.

[0030] One embodiment of the present invention is as follows: Figure 1As shown, the structure includes a common cladding 1 and stress zones 2 and polarization-maintaining core zones distributed within the common cladding. The common cladding has a diameter of 80 μm and is a pure silica glass layer. There are three stress zones, evenly distributed circumferentially. The radial distance (radius) between each stress zone and the fiber center is 20 μm. The diameter of each stress zone is 19 μm, and each stress zone is a boron-doped silica glass layer with a boron molar percentage of 15-25%. The birefringence effect provided by the stress zones effectively reduces the core ellipticity requirement. The centers of adjacent stress zones form an equilateral triangle. An elliptical polarization-maintaining core is positioned at the midpoint of each side of the equilateral triangle. The major axis of each elliptical polarization-maintaining core is aligned with the corresponding side of the equilateral triangle, and the minor axis points towards the center of the common cladding. Each elliptical polarization-maintaining core is configured with its two adjacent stress zones to form a polarization-maintaining core zone. Each of the elliptical polarization-maintaining core regions has the same structure. Each elliptical polarization-maintaining core includes a circular fiber core 3 with a relative refractive index difference Δ1 of 0.5–1.3% and a diameter d1 of 2 μm. A circular inner cladding 4 tightly surrounds the circular fiber core, with a diameter d2 of 4 μm and a relative refractive index Δ2 of -0.8–0%. An elliptical cladding 5 surrounds the inner cladding, with a relative refractive index difference Δ3 of -1.5–-0.7%, a major axis 2a of 17 μm, and an ellipticity 1-b / a of 0.1–0.5. The birefringence of the polarization-maintaining fiber in this embodiment is 6.0 × 10⁻⁶. -4 ~6.5×10 -4 When the optical fiber is wound 10 times with a bending diameter of 10mm, the macrobending loss is ≤0.1dB.

[0031] The preparation method in this embodiment is as follows:

[0032] (1) Fabrication of circular core rods: The circular core rods are covered with an inner cladding layer. The core layer and the inner cladding layer are prepared by PCVD (plasma chemical vapor deposition) process. The core layer is prepared using SiCl4 (tetrachlorosilane), GeCl4 (germanium tetrachloride), and O2 (oxygen) as raw materials. The inner cladding layer is prepared using SiCl4 (tetrachlorosilane), C2F6 (hexafluoroethane), and O2 (oxygen) as raw materials. By adjusting the flow rate and ratio of the gas, a core rod with the required diameter and refractive index is prepared under appropriate deposition rate conditions. In the core layer preparation process, the SiCl4 flow rate is 600–700 sccm (sccm is a volumetric flow rate unit, referring to standard milliliters per minute); the GeCl4 flow rate is 30–40 sccm, and the O2 flow rate is 1200–1400 sccm; in the inner cladding layer preparation process, the SiCl4 flow rate is 900–1100 sccm, the C2F6 flow rate is 70–90 sccm, and the O2 flow rate is 2200–2800 sccm.

[0033] (2) Fabrication of circular stress rods: The stress rods are fabricated using PCVD technology to deposit a boron-doped layer. SiCl4 (tetrachlorosilane), BCl3 (boron trichloride), and O2 (oxygen) are used as deposition raw materials. By adjusting the flow rate and ratio of these gases, stress rods with the required diameter and refractive index are prepared. Specifically, during the boron-doped layer fabrication process, the flow rate of SiCl4 is 500–700 sccm, the flow rate of BCl3 is 500–700 sccm, and the flow rate of O2 is 2000–2800 sccm. After deposition, the rods are sintered and shrunk into solid core rods.

[0034] (3) Then, an elliptical cladding core rod is made: an in-tube doping deposition is performed using a pure silica glass tube of a certain thickness, and an elliptical cladding is prepared using the PCVD process; specifically, an elliptical cladding is prepared using SiCl4 (tetrachlorosilane), BCl3 (boron trichloride), O2 (oxygen), and C2F6 (hexafluoroethane) as raw materials, and a core rod with the required diameter and refractive index is prepared under appropriate deposition rate conditions by adjusting the flow rate and ratio of the gas. During the preparation process, the SiCl4 flow rate is 900–1500 sccm (sccm is a volumetric flow rate unit, referring to standard milliliters per minute); the BCl3 flow rate is 300–500 sccm; the O2 flow rate is 2000–3500 sccm; and the C2F6 flow rate is 80–250 sccm. After deposition, a circular core rod of doped core layer is sintered and shrunk into a solid core. When the circular core rod of doped core layer is heated to a molten state, radial pressure is applied to make the circular core rod form a doped core layer core rod with an elliptical or quasi-elliptical radial cross section and a cladding. Then, the elliptical cladding is ground into a core rod with a circular outer periphery and an elliptical inner core layer.

[0035] (4) Preparation of elliptical polarization-maintaining core rod: The above-mentioned core rod containing elliptical doped core layer is drilled with a central hole, and the circular fiber core rod is inserted into the central hole and then melted and shrunk into a solid elliptical polarization-maintaining core rod.

[0036] (5) Preparation of multi-core elliptical polarization-maintaining fiber preform: Drill holes in a solid pure silica glass rod according to the number and spacing of the polarization-maintaining cores and the number and spacing of the stress rods. Insert the solid elliptical polarization-maintaining core rods and stress rods into the holes according to their orientation to make a multi-core elliptical polarization-maintaining fiber preform.

[0037] (6) The multi-core elliptical polarization-maintaining fiber preform is clamped into the fiber drawing furnace and drawn into a multi-core elliptical polarization-maintaining fiber.

[0038] Second embodiment of the present invention, for example Figure 2As shown, its main difference from the first embodiment lies in the arrangement of four stress zones 2 and four elliptical polarization-maintaining core zones within the common cladding 1. The four stress zones are evenly spaced circumferentially, with a radial distance (radius) of 19 μm between each stress zone and the fiber center. The diameter of each stress zone is 15 μm. The centers of adjacent stress zones form a regular quadrilateral (square). An elliptical polarization-maintaining core is positioned at the midpoint of each side of this square. The major axis of each elliptical polarization-maintaining core is aligned with the corresponding side of the square, while its minor axis points towards the center of the common cladding. Each elliptical polarization-maintaining core is configured with its two adjacent stress zones to form a polarization-maintaining core zone. All elliptical polarization-maintaining core zones have the same structure. The elliptical polarization-maintaining core includes a circular core 3 with a relative refractive index difference Δ1 of 0.5–1.3% and a diameter d1 of 1.8 μm. A circular inner cladding 4 is tightly wrapped around the circular core, with a diameter d2 of 4 μm and a relative refractive index Δ2 of -0.8–0%. An elliptical cladding 5 surrounds the inner cladding, with a relative refractive index difference Δ3 of -1.5–-0.7%, a major axis 2a of 15 μm, and an ellipticity 1-b / a of 0.1–0.5. The birefringence of the polarization-maintaining fiber in this embodiment is 6.0 × 10⁻⁶. -4 ~6.5×10 -4 When the optical fiber is wound 10 times with a bending diameter of 10mm, the macrobending loss is ≤0.1dB. The rest of the structure is the same as the first embodiment.

Claims

1. A high birefringence multi-core elliptical polarization-maintaining optical fiber, comprising a common cladding and stress regions and polarization-maintaining core regions distributed within the common cladding, characterized in that... The stress zones are at least three, evenly distributed along the circumference. The center lines of adjacent stress zones form a regular polygon. An elliptical eccentric core is placed at the midpoint of each side of the regular polygon. The major axis of the elliptical eccentric core is flush with the corresponding side of the regular polygon. Each elliptical eccentric core and its two adjacent stress zones are configured to form an eccentric core area.

2. The high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 1, characterized in that... The radial distance between the stress zone and the center of the optical fiber is 12~35μm.

3. The high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 1 or 2, characterized in that... The diameter of the stress zone is 8~30μm, and the stress zone is a boron-doped quartz glass layer with boron accounting for 15~25% of the molar percentage.

4. The high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 1 or 2, characterized in that... The elliptical polarization-maintaining core includes a circular fiber core and an elliptical cladding surrounding the fiber core, which are evenly spaced along the circumference.

5. The high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 1 or 2, characterized in that... The diameter of the common cladding layer is 40-125 μm, and the common cladding layer is a pure silicon dioxide glass layer.

6. The high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 4, characterized in that... The relative refractive index difference Δ1 of the circular fiber core is 0.5 to 1.3%, and the diameter d1 is 1 to 2.5 μm.

7. The high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 6, characterized in that... A circular inner cladding is tightly wrapped around the circular fiber core. The diameter d2 of the circular inner cladding is 2 to 6.2 μm, and the relative refractive index difference Δ2 is -0.8 to 0%.

8. The high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 4, characterized in that... The relative refractive index difference Δ3 of the elliptical cladding is -1.5 to -0.7%, the major axis 2a of the elliptical cladding is 7 to 25 μm, the ellipticity 1-b / a of the elliptical cladding is 0.1 to 0.5, and b is the minor axis of the elliptical cladding.

9. The high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 1 or 2, characterized in that... The elliptical polarization-maintaining core regions are 3 to 4 in number and are evenly spaced along the circumference.

10. The high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 1 or 2, characterized in that... The polarization-maintaining fiber has a birefringence of 6.0 × 10⁻⁶. -4 ~6.5×10 -4 .

11. The high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 1 or 2, characterized in that... The macrobending loss of the polarization-maintaining fiber is ≤0.1dB when it is wound 10 times with a bending diameter of 10mm.

12. A method for fabricating a high birefringence multi-core elliptical polarization-maintaining optical fiber as described in any one of claims 1-11, characterized in that... Fabrication of circular fiber core rods and stress rods; fabrication of elliptical clad core rods: In-tube doping deposition is performed using a pure silica glass tube of a certain thickness. After deposition, a circular core rod with a solid doped core layer is sintered and shrunk. Radial pressure is applied while heating the circular core rod to a molten state, causing it to form a doped core rod with an elliptical or near-elliptical radial cross-section and a clad layer. The elliptical clad layer is then ground into a rod with a circular outer periphery and an elliptical inner doped core layer. Preparation of elliptical polarimetric maintaining cores. Core rod preparation: A core rod containing an elliptical doped core layer is drilled with a central hole, and a circular fiber core rod is inserted and fused together to form a solid elliptical polarization-maintaining core rod. Preparation of multi-core elliptical polarization-maintaining fiber preform: Holes are drilled on a solid pure silica glass rod according to the number and spacing of the polarization-maintaining cores and the number and spacing of the stress rods. The solid elliptical polarization-maintaining core rod and the stress rods are inserted into the holes according to their orientation to form a multi-core elliptical polarization-maintaining fiber preform. The multi-core elliptical polarization-maintaining fiber preform is clamped in an optical fiber drawing furnace and drawn into a multi-core elliptical polarization-maintaining fiber.

13. The method for fabricating a high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 12, characterized in that... The circular fiber core rod is covered with an inner cladding layer.

14. The method for fabricating a high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 12, characterized in that... The elliptical cladding mandrel is deposited using PCVD technology, with SiCl4, BCl3, O2, and C2F6 as deposition raw materials. By adjusting the flow rate and ratio of the gases, a mandrel with the required diameter and refractive index is prepared. During the deposition process, the flow rate of SiCl4 is 500–2000 sccm, the flow rate of BCl3 is 250–800 sccm, the flow rate of O2 is 1500–5000 sccm, and the flow rate of C2F6 is 20–300 sccm.

15. The method for fabricating a high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 12, characterized in that... The circular fiber core rod is deposited using PCVD technology, with SiCl4, GeCl4, and O2 as deposition raw materials. By adjusting the flow rate and ratio of the gases, a core rod with the required diameter and refractive index is prepared. During the deposition process, the flow rate of SiCl4 is 500-800 sccm, the flow rate of GeCl4 is 20-50 sccm, and the flow rate of O2 is 1000-1500 sccm. After deposition, the core rod is sintered and shrunk into a solid core rod.

16. The method for fabricating a high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 13, characterized in that... The inner cladding is deposited using PCVD process, with SiCl4, C2F6 and O2 as deposition raw materials. During the deposition process, the flow rate of SiCl4 is 800-1200 sccm, the flow rate of C2F6 is 50-150 sccm, and the flow rate of O2 is 2000-3000 sccm. The cladding is deposited before the fiber core layer.

17. The method for fabricating a high birefringence multi-core elliptical polarization-maintaining optical fiber according to claim 12, characterized in that... The stress bar is prepared by depositing a boron-doped layer using PCVD technology, with SiCl4, BCl3, and O2 as deposition raw materials. By adjusting the flow rate and ratio of the gases, a stress bar with the required diameter and refractive index is prepared. In the boron-doped layer preparation process, the flow rate of SiCl4 is 500-700 sccm, the flow rate of BCl3 is 500-700 sccm, and the flow rate of O2 is 1500-3000 sccm. After deposition, it is sintered and shrunk into a solid rod.

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