A multi-core elliptical polarization-maintaining optical fiber and its fabrication method

Abstract

This invention relates to a multi-core elliptical polarization-maintaining optical fiber and its fabrication method. The optical fiber includes a common cladding and polarization-maintaining core regions distributed within the common cladding. The polarization-maintaining core regions are elliptical, and there are at least three elliptical polarization-maintaining core regions spaced apart circumferentially. This invention features a compact structure, low crosstalk, and facilitates the placement of more polarization-maintaining core regions within a limited fiber cross-section. This allows the elliptical multi-core polarization-maintaining optical fiber to achieve higher density and greater capacity, as well as better polarization-maintaining performance, through space-division multiplexing. The uniform circumferential spacing of each elliptical polarization-maintaining core region forms a corresponding co-orientation structure, which is beneficial for controlling the cladding roundness during fiber drawing and for controlling inter-core crosstalk. This invention simultaneously increases the effective length of the optical fiber and achieves polarization-maintaining performance, providing more transmission channels and stronger birefringence performance, and has broad application prospects in the miniaturization of fiber optic gyroscopes and devices.

Description

Technical Field

[0001] This invention relates to a multi-core 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-type, bow-tie-type, and elliptical-type fibers. Panda-type and bow-tie-type fibers achieve polarization maintenance through stress birefringence, while elliptical-type fibers achieve it through geometric birefringence. 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 to achieve higher density, greater capacity, and better polarization-maintaining performance. However, most existing multi-core fibers are panda-type polarization-maintaining fibers, which suffer from a relatively loose structure, large stress zone occupancy, and low density. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a multi-core elliptical polarization-maintaining fiber and its preparation method, which addresses the shortcomings of the prior art. It has a compact structure, low crosstalk, 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 a polarization-maintaining core region distributed in the common cladding. The polarization-maintaining core region is an elliptical polarization-maintaining core region, and there are at least 3 elliptical polarization-maintaining core regions, which are arranged at intervals along the circumference.

[0005] According to the above scheme, the elliptical polarization-maintaining core region includes a circular fiber core and an elliptical cladding surrounding the fiber core, which are evenly spaced along the circumference.

[0006] According to the above scheme, the distance between the center of the elliptical polarization-maintaining core region and the center of the common cladding is 12~35μm, the diameter of the common cladding is 40~125μm, and the common cladding is a pure silicon dioxide glass layer.

[0007] 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.

[0008] 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%.

[0009] 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 12 to 40 μm, the ellipticity 1-b / a of the elliptical cladding is 0.3 to 0.6, and b is the minor axis of the elliptical cladding.

[0010] According to the above scheme, there are 3 to 4 elliptical core regions, which are evenly distributed along the circumference, and the major or minor axis of the elliptical cladding points to the common cladding center.

[0011] According to the above scheme, the birefringence of the polarization-maintaining fiber is 5.1 × 10⁻⁶. -4 ~5.8×10 -4 .

[0012] 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.

[0013] The preparation method of this invention is as follows:

[0014] First, make a circular fiber core rod.

[0015] Then, an elliptical clad core rod is fabricated. In-tube doping deposition is performed using a pure silica glass tube of a certain thickness. After deposition, the core is sintered into a solid circular core rod. This circular core rod is heated until it is molten, and radial pressure is applied to form a clad core rod with an elliptical or near-elliptical radial cross-section. The elliptical cladding is then ground into a core rod with a circular outer periphery and an elliptical inner doped core.

[0016] To prepare an elliptical polarization-maintaining core region 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 then fused together to form a solid elliptical polarization-maintaining core region core rod.

[0017] To prepare a 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 core regions. Solid elliptical polarization-maintaining core rods are then inserted into these holes according to their orientation to form the multi-core elliptical polarization-maintaining fiber preform.

[0018] 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.

[0019] According to the above scheme, the circular fiber core rod is covered with an inner cladding layer.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] The beneficial effects of this invention are as follows: 1. By adopting an elliptical polarization-maintaining core structure, the polarization-maintaining effect is achieved through geometric birefringence. Compared with the panda-type polarization-maintaining core layer, its structure is more compact, with less crosstalk, which is beneficial for setting more polarization-maintaining core regions in a limited fiber cross-section. This allows elliptical multi-core polarization-maintaining fibers to achieve higher density and greater capacity, as well as better polarization-maintaining performance, through space division multiplexing. 2. The elliptical polarization-maintaining core regions are evenly distributed circumferentially, and the major or 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 for controlling inter-core crosstalk. 3. By designing multiple elliptical polarization-maintaining core regions, the effective length of the fiber and polarization-maintaining performance can be increased simultaneously, providing more transmission channels and stronger birefringence performance. This has broad application prospects in the miniaturization of fiber optic gyroscopes and devices. 4. The circular inner cladding tightly wraps around the circular fiber core, which helps reduce the drawing stress and distortion of the fiber core and further reduces inter-core crosstalk. Attached Figure Description

[0024] Figure 1This is a schematic diagram of the radial structure of an embodiment of the present invention.

[0025] Figure 2 This is a schematic diagram of the radial structure of the second embodiment of the present invention.

[0026] Figure 3 This is a schematic diagram of the radial structure of the third embodiment of the present invention.

[0027] Figure 4 This is a schematic diagram of the radial structure of the fourth embodiment of the present invention. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0029] First embodiment of the invention, for example Figure 1 As shown, it includes a common cladding 1 and three elliptical polarization-maintaining core regions distributed within the common cladding. The three elliptical polarization-maintaining core regions are evenly spaced circumferentially. Each elliptical polarization-maintaining core region has the same structure, including 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 covers the circular fiber core, with a diameter d2 of 4.5 μm and a relative refractive index Δ2 of -0.8–0%. An elliptical cladding 2 surrounds a circular inner cladding. The minor axis of each elliptical cladding points to the center of the common cladding. The relative refractive index difference Δ3 is -1.5 to -0.7%, the major axis 2a of the elliptical cladding is 24 μm, and the ellipticity of the elliptical cladding is between 0.3 and 0.6. The distance between each elliptical polarization-maintaining core region and the center of the common cladding is 23 μm. The diameter of the common cladding is 80 μm, and the common cladding is a pure silica glass layer. Multiple polarization-maintaining core regions are arranged in a 120° rotationally symmetrical position with respect to the center of the common cladding. This arrangement of multiple core layers helps control the roundness of the fiber cladding and also helps control inter-core crosstalk. The birefringence of the polarization-maintaining fiber in this embodiment is 5.1 × 10⁻⁶. -4 ~5.8×10 -4 When the optical fiber is wound 10 times with a bending diameter of 10mm, the macrobending loss is ≤0.1dB.

[0030] The preparation method in this embodiment is as follows:

[0031] (1) First, a circular fiber core rod is made. The circular fiber core rod is covered with an inner cladding layer. The fiber core layer and the inner cladding layer are prepared by PCVD (plasma chemical vapor deposition) process. The fiber core layer is prepared by SiCl4 (tetrachlorosilane), GeCl4 (germanium tetrachloride), and O2 (oxygen) as raw materials. The inner cladding layer is prepared by 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.

[0032] (2) 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. 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. 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.

[0033] (3) 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.

[0034] (4) Preparation of multi-core elliptical polarization-maintaining fiber preform: Drill holes in a solid pure silica glass rod according to the number of cores and spacing of the polarization-maintaining core area, and insert the solid elliptical polarization-maintaining core rod into the holes according to the orientation to make a multi-core elliptical polarization-maintaining fiber preform.

[0035] (5) 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.

[0036] Second embodiment of the present invention, for example Figure 2 As shown, its main difference from the first embodiment lies in the arrangement of four elliptical core regions within the common cladding. These four elliptical core regions are evenly spaced circumferentially, meaning they are positioned at a 90° rotational symmetry relative to the center of the common cladding. Each elliptical core region has the same structure, and the minor axis of each elliptical cladding points towards the center of the common cladding. The remaining structure is the same as in the first embodiment.

[0037] Third embodiment of the present invention Figure 3 As shown, its main difference from the first embodiment is that three elliptical eccentricity-maintaining core regions are arranged in the common cladding. The major axis of each elliptical cladding points to the center of the common cladding. The three elliptical eccentricity-maintaining core regions are evenly spaced along the circumference. The structures of each elliptical eccentricity-maintaining core region are the same, and the rest of the structures are the same as those in the first embodiment.

[0038] Fourth embodiment of the present invention Figure 4 As shown, its main difference from the second embodiment lies in the presence of four elliptical polarization-maintaining core regions within the common cladding. These four elliptical polarization-maintaining core regions are evenly spaced circumferentially, with the major axis of each elliptical cladding pointing towards the center of the common cladding. The structures of each elliptical polarization-maintaining core region are identical, while the remaining structures are the same as in the first embodiment.

Claims

1. A multi-core elliptical polarization-maintaining optical fiber, comprising a common cladding and polarization-maintaining core regions distributed within the common cladding, characterized in that... The polarization-maintaining core region is an elliptical polarization-maintaining core region, and there are at least three elliptical polarization-maintaining core regions arranged at intervals along the circumference; each elliptical polarization-maintaining core region includes a circular fiber core and an elliptical cladding surrounding the fiber core, evenly spaced along the circumference; the relative refractive index difference of the circular fiber core... The value of 1 is 0.5% to 1.3%, and the diameter d1 is 1 to 2.5 μm.

2. The multi-core elliptical polarization-maintaining optical fiber according to claim 1, characterized in that... The distance between the center of the elliptical polarization-maintaining core region and the center of the common cladding is 12~35μm, the diameter of the common cladding is 40~125μm, and the common cladding is a pure silicon dioxide glass layer.

3. The multi-core elliptical polarization-maintaining optical fiber according to claim 1 or 2, characterized in that... A circular inner cladding is tightly wrapped around the circular fiber core. The diameter d2 of the circular inner cladding is 2–6.2 μm, and the relative refractive index difference is [not specified]. 2 ranges from -0.8% to 0%.

4. The multi-core elliptical polarization-maintaining optical fiber according to claim 1 or 2, characterized in that... The relative refractive index difference of the elliptical cladding The 3% ranges from -1.5 to -0.7%, the major axis 2a of the elliptical cladding is 12 to 40 μm, the ellipticity 1-b / a of the elliptical cladding is 0.3 to 0.6, and b is half the minor axis of the elliptical cladding.

5. The multi-core elliptical polarization-maintaining optical fiber according to claim 1 or 2, characterized in that... The elliptical core regions are 3 to 4 in number, evenly spaced along the circumference, and the major or minor axis of the elliptical cladding points to the common cladding center.

6. The multi-core elliptical polarization-maintaining optical fiber according to claim 1 or 2, characterized in that... The birefringence of the polarization-maintaining fiber is 5.1×10⁻⁴ to 5.8×10⁻⁴.

7. The 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.

8. A method for fabricating a multi-core elliptical polarization-maintaining optical fiber as described in any one of claims 1-7, characterized in that: First, a circular fiber core rod is fabricated; then, an elliptical clad core rod is fabricated. In-tube doping deposition is performed using a pure silica glass tube of a certain thickness. After deposition, the doped core layer is sintered and shrunk into a solid circular core rod. This circular core rod is heated until it is in 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 clad core is then ground into a core rod with a circular outer periphery and an elliptical inner doped core layer. This process is used to prepare an elliptical core rod. The process involves:

1. A circular polarization-maintaining core rod is formed by drilling a central hole in a core rod containing an elliptical doped core layer, inserting the circular core rod, and then fusing it to form a solid elliptical polarization-maintaining core rod.

2. A multi-core elliptical polarization-maintaining fiber preform is prepared by drilling holes in a solid pure silica glass rod according to the number and spacing of the polarization-maintaining core regions, inserting the solid elliptical polarization-maintaining core rod into the holes according to the orientation.

3. A fiber is drawn by clamping the multi-core elliptical polarization-maintaining fiber preform into a fiber drawing furnace to form a multi-core elliptical polarization-maintaining fiber.

9. The method for fabricating a multi-core elliptical polarization-maintaining optical fiber according to claim 8, characterized in that... The circular fiber core rod is covered with an inner cladding layer.

10. The method for fabricating a multi-core elliptical polarization-maintaining optical fiber according to claim 8 or 9, 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.

11. The method for fabricating a multi-core elliptical polarization-maintaining optical fiber according to claim 8 or 9, 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.

12. The method for fabricating a multi-core elliptical polarization-maintaining optical fiber according to claim 9, 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.

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