Multi-core elliptical polarization maintaining optical fiber and preparation method thereof

Abstract

The invention relates to a multi-core elliptical polarization-maintaining optical fiber and a preparation method thereof, the optical fiber comprises a common cladding and polarization-maintaining core areas distributed in the common cladding, the polarization-maintaining core areas are elliptical polarization-maintaining core areas, the number of the elliptical polarization-maintaining core areas is at least three, and the elliptical polarization-maintaining core areas are arranged at intervals along the circumferential direction. The elliptical multi-core polarization-maintaining optical fiber is compact in structure and small in crosstalk, more polarization-maintaining core areas can be arranged in a limited optical fiber section, and the elliptical multi-core polarization-maintaining optical fiber has the advantages of being higher in density, larger in capacity and better in polarization-maintaining performance before multi-core polarization-maintaining light can be achieved in a space division multiplexing mode. And the elliptical polarization maintaining core areas are uniformly distributed at intervals along the circumferential direction to form a corresponding same-orientation structure, so that the cladding roundness of the optical fiber during wire drawing is favorably controlled, and inter-core crosstalk control is also facilitated. According to the invention, the effective length of the optical fiber is increased, the polarization maintaining performance is realized, more transmission channels are provided, the birefringence performance is stronger, and the fiber optic gyroscope has a wide application prospect in the aspects of miniaturization of fiber optic gyroscopes and devices.

Description

Technical Field

[0001] The invention relates to a multi-core elliptical polarization-maintaining optical fiber and a preparation method thereof, and belongs to the technical field of optical communication and sensing. Background Art

[0002] Polarization-maintaining fibers are widely used in various sectors of the national economy, including aerospace, aviation, navigation, industrial manufacturing, and communications. In interferometric fiber sensors based on optical coherence detection, using polarization-maintaining fibers ensures a constant linear polarization direction, improves the coherent signal-to-noise ratio, and enables high-precision measurement of physical quantities. Common types of polarization-maintaining fibers currently include panda, bowtie, and elliptical. Panda and bowtie polarization-maintaining fibers achieve polarization maintenance through stress birefringence, while elliptical polarization-maintaining fibers achieve polarization maintenance through geometric birefringence. Most existing polarization-maintaining fibers are single-core, meaning they have only one transmission channel. Their transmission capacity is gradually failing to meet increasing transmission demands. Multi-core polarization-maintaining fibers utilize space-division multiplexing (SDM) to achieve higher density, greater capacity, and improved polarization-maintaining performance. However, most existing multi-core fibers are panda-type, which suffers from a relatively loose structure, large stress zones, 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 optical fiber and a preparation method thereof in view of the deficiencies in the above-mentioned prior art. The optical fiber has a compact structure and low crosstalk, and can achieve higher density and larger capacity of the polarization-maintaining optical fiber.

[0004] The technical solution adopted by the present invention to solve the above-mentioned problems is: it includes a common cladding and a polarization-maintaining core area distributed in the common cladding, wherein the polarization-maintaining core area is an elliptical polarization-maintaining core area, and the number of the elliptical polarization-maintaining core areas is at least 3 and is arranged at intervals along the circumferential direction.

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

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

[0007] According to the above solution, the relative refractive index difference Δ1 of the circular core is 0.5-1.3%, and the diameter d1 is 1-2.5 μm.

[0008] According to the above solution, a circular inner cladding is tightly coated on the outside of the circular 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 solution, there are 3 to 4 elliptical polarization-maintaining core regions, which are evenly spaced along the circumference, and the major axis or minor axis of the elliptical cladding points to the common cladding center.

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

[0012] According to the above solution, the macrobending loss of the polarization-maintaining optical fiber is ≤0.1 dB when it is wound 10 times with a bending diameter of 10 mm.

[0013] The preparation method technical solution of the present invention is: First make a round fiber core rod, Then, an elliptical cladding core rod is made, and a pure silica glass tube of a certain thickness is used for doping deposition inside the tube. After the deposition is completed, it is fired and melted into a circular core rod with a solid doped core layer. The circular core rod with the doped core layer is heated so that it is in a molten state and radial pressure is applied to the circular core rod to form a doped core layer core rod with an elliptical or quasi-elliptical radial cross section. The elliptical cladding is then ground into a core rod with a circular outer circumference and an elliptical doped core layer inside. Prepare an elliptical polarization-maintaining core rod, punch a center hole in the core rod containing the elliptical doped core layer, insert the round fiber core rod and melt it into a solid elliptical polarization-maintaining core rod. Prepare a multi-core elliptical polarization-maintaining optical fiber preform rod, punch holes on a solid pure silica glass rod according to the number and spacing of cores in the polarization-maintaining core area, and insert the solid elliptical polarization-maintaining core rod into the hole according to the orientation to make a multi-core elliptical polarization-maintaining optical fiber preform rod. Drawing into fiber, the multi-core elliptical polarization-maintaining optical fiber preform is clamped into the optical fiber drawing furnace and drawn into a multi-core elliptical polarization-maintaining optical fiber.

[0014] According to the above solution, the circular core rod is covered with an inner cladding.

[0015] According to the above scheme, the elliptical cladding core rod is deposited by PCVD (plasma chemical vapor deposition) process, using SiCl4 (tetrachlorosilane), BCl3 (boron trichloride), O2 (oxygen), and C2F6 (hexafluoroethane) as deposition raw materials. By adjusting the flow rate and proportion of the gases, a core rod with the required diameter and refractive index is prepared. During the deposition process, the SiCl4 flow rate is 500-2000sccm (sccm is the unit of volume flow rate, referring to standard milliliters per minute), the BCl3 flow rate is 250-800sccm, the O2 flow rate is 1500-5000sccm, and the C2F6 flow rate is 20-300sccm.

[0016] According to the above scheme, the circular fiber core rod is deposited by PCVD process, using SiCl4 (tetrachlorosilane), GeCl4 (germanium tetrachloride) and O2 (oxygen) as deposition raw materials. By adjusting the flow rate and proportion of the gas, a core rod with the required diameter and refractive index is prepared. During the deposition process, the SiCl4 flow rate is 500-800sccm, the GeCl4 flow rate is 20-50sccm, and the O2 flow rate is 1000-1500sccm.

[0017] According to the above scheme, the inner cladding is deposited by PCVD process, using SiCl4 (tetrachlorosilane), C2F6 (hexafluoroethane) and O2 (oxygen) as deposition raw materials. During the deposition process, the SiCl4 flow rate is 800-1200sccm, the C2F6 flow rate is 50-150sccm, and the O2 flow rate is 2000-3000sccm, and the cladding is deposited before the core layer.

[0018] The beneficial effects of the present invention are as follows: 1. The elliptical polarization-maintaining core structure is adopted to achieve the polarization-maintaining effect through geometric birefringence. Compared with the panda-shaped polarization-maintaining core layer, its structure is more compact and has less crosstalk, which is conducive to setting more polarization-maintaining cores in a limited optical fiber cross-section, so that the elliptical multi-core polarization-maintaining optical fiber can achieve higher density and larger capacity before the multi-core polarization-maintaining light by using space division multiplexing, as well as the advantages of better polarization-maintaining performance. 2. Each elliptical polarization-maintaining core is evenly spaced along the circumference, and the major axis or minor axis of the elliptical cladding points to the center of the common cladding, forming a corresponding isotropic structure, which is conducive to controlling the cladding roundness of the optical fiber during drawing and is also conducive to controlling crosstalk between cores. 3. By designing multiple elliptical polarization-maintaining cores, the effective length of the optical fiber can be increased and the polarization-maintaining performance can be achieved at the same time, providing more transmission channels and stronger birefringence performance, which has broad application prospects in the miniaturization of optical fiber gyroscopes and devices. 4. A circular inner cladding is tightly coated on the outside of the circular core, which is conducive to reducing the drawing stress and distortion of the core and further reducing crosstalk between cores. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1It is a schematic diagram of the radial structure of an embodiment of the present invention.

[0020] Figure 2 It is a schematic diagram of the radial structure of the second embodiment of the present invention.

[0021] Figure 3 It is a schematic diagram of the radial structure of the third embodiment of the present invention.

[0022] Figure 4 It is a schematic diagram of the radial structure of the fourth embodiment of the present invention. DETAILED DESCRIPTION

[0023] The present invention is further described in detail below with reference to the embodiments and accompanying drawings.

[0024] The first embodiment of the present invention is Figure 1 As shown, the fiber optic fiber comprises 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 along the circumference. Each of the elliptical polarization-maintaining core regions has the same structure, including a circular core 3 with a relative refractive index difference Δ1 of 0.5 to 1.3% and a diameter d1 of 2 μm. The circular core is tightly coated with a circular inner cladding 4 with a diameter d2 of 4.5 μm and a relative refractive index Δ2 of -0.8 to 0%. An elliptical cladding 2 is wrapped around the circular inner cladding, with the short axis of each elliptical cladding pointing to the center of the common cladding, the relative refractive index difference Δ3 is -1.5 to -0.7%, the long 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 at positions that are 120° rotationally symmetrical with respect to the center of the common cladding. By configuring multiple core layers in this way, it is beneficial to control the roundness of the optical fiber cladding and also to control the crosstalk between cores. The birefringence coefficient of the polarization-maintaining optical fiber described in this embodiment is 5.1×10 -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.

[0025] The preparation method of this embodiment is as follows: (1) First, a circular fiber core rod is prepared. The circular fiber core rod is coated with an inner cladding. The core layer and the inner cladding are prepared separately by PCVD (plasma chemical vapor deposition) process. The core layer is prepared by SiCl4 (tetrachlorosilane), GeCl4 (germanium tetrachloride), and O2 (oxygen) as raw materials, and the inner cladding 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 speed conditions. Among them, during the core layer preparation process, the SiCl4 flow rate is 600-700sccm (sccm is the unit of volume flow rate, referring to standard milliliters per minute); the GeCl4 flow rate is 30-40sccm, and the O2 flow rate is 1200-1400sccm; during the inner cladding preparation process, the SiCl4 flow rate is 900-1100sccm, the C2F6 flow rate is 70-90sccm, and the O2 flow rate is 2200-2800sccm.

[0026] (2) Then, an elliptical cladding core rod is made: a pure silica glass tube of a certain thickness is used for in-tube doping deposition, and an elliptical cladding is prepared by PCVD process; specifically, SiCl4 (tetrachlorosilane), BCl3 (boron trichloride), O2 (oxygen), and C2F6 (hexafluoroethane) are used as raw materials to prepare the elliptical cladding. 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 speed conditions. During the preparation process, the SiCl4 flow rate is 900-1500sccm (sccm is a unit of volume flow rate, referring to standard milliliters per minute); the BCl3 flow rate is 300-500sccm, the O2 flow rate is 2000-3500sccm, and the C2F6 flow rate is 80-250sccm; after the deposition is completed, the circular core rod of the doped core layer is fired and melted to form a solid core, and radial pressure is applied to the circular core rod of the doped core layer when it is in a molten state so that the circular core rod forms a doped core layer core rod with an elliptical or quasi-elliptical radial cross-section and a cladding, and then the elliptical cladding is ground into a core rod with a circular outer circumference and an elliptical doped core layer inside through processing.

[0027] (3) Preparation of an elliptical polarization-maintaining core rod: Punch a center hole in the core rod containing the elliptical doped core layer, insert a circular fiber core rod into the center hole, and then melt and shrink it into a solid elliptical polarization-maintaining core rod.

[0028] (4) Preparation of multi-core elliptical polarization-maintaining optical fiber preform: Drill holes on a solid pure silica glass rod according to the number and spacing of cores in 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 optical fiber preform.

[0029] (5) The multi-core elliptical polarization-maintaining optical fiber preform is clamped into an optical fiber drawing furnace and drawn into a multi-core elliptical polarization-maintaining optical fiber.

[0030] The second embodiment of the present invention is Figure 2 As shown, the main difference from the first embodiment lies in the deployment of four elliptical polarization-maintaining cores within the common cladding. These cores are evenly spaced circumferentially, positioned 90° rotationally symmetrically about the center of the common cladding. Each elliptical polarization-maintaining core has the same structure, and the minor axis of each elliptical cladding points toward the center of the common cladding. The remaining structure is the same as the first embodiment.

[0031] The third embodiment of the present invention is Figure 3 As shown, the main difference between it and the first embodiment is that three elliptical polarization-maintaining core regions are arranged in the common cladding, the major axis of each elliptical cladding points to the center of the common cladding, and the three elliptical polarization-maintaining core regions are evenly spaced along the circumferential direction. The structures of the elliptical polarization-maintaining core regions are the same, and the rest of the structure is the same as that of the first embodiment.

[0032] The fourth embodiment of the present invention is Figure 4 As shown, the main difference between this embodiment and the second embodiment lies in the deployment of four elliptical polarization-maintaining cores within the common cladding. These four cores are evenly spaced circumferentially, with the major axis of each elliptical cladding pointing toward the center of the common cladding. The elliptical polarization-maintaining cores have the same structure, and the remaining structure is the same as the first embodiment.

Claims

1. A multi-core elliptical polarization-maintaining optical fiber comprising a common cladding and a polarization-maintaining core region distributed in 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, which are arranged at intervals along the circumferential direction.

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

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

4. The multi-core elliptical polarization-maintaining optical fiber according to claim 2, characterized in that The relative refractive index difference Δ1 of the circular fiber core is 0.5-1.3%, and the diameter d1 is 1-2.5 μm.

5. The multi-core elliptical polarization-maintaining optical fiber according to claim 2 or 4, characterized in that A circular inner cladding is tightly coated on the outside of the circular 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%.

6. The multi-core elliptical polarization-maintaining optical fiber according to claim 2 or 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 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.

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

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

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

10. A method for preparing a multi-core elliptical polarization-maintaining optical fiber according to any one of claims 1 to 9, characterized in that: First, a circular core rod is made; then an elliptical cladding core rod is made, and a pure silica glass tube of a certain thickness is used for doping deposition inside the tube. After the deposition is completed, the tube is fired and melted to form a circular core rod with a solid doped core layer. The circular core rod with the doped core layer is heated so that it is in a molten state and radial pressure is applied to the circular core rod to form a doped core layer core rod with an elliptical or quasi-elliptical radial cross section, and then the elliptical cladding is ground into a core rod with a circular outer circumference and an elliptical doped core layer inside by processing; the elliptical core rod is prepared. To prepare a circular polarization-maintaining core rod, a center hole is punched into a core rod containing an elliptical doped core layer, and the circular core rod is inserted and then melted into a solid elliptical polarization-maintaining core rod; to prepare a multi-core elliptical polarization-maintaining optical fiber preform, holes are punched on a solid pure silica glass rod according to the number and spacing of cores in the polarization-maintaining core region, and the solid elliptical polarization-maintaining core rod is inserted into the holes according to the orientation to form a multi-core elliptical polarization-maintaining optical fiber preform; to draw into fiber, the multi-core elliptical polarization-maintaining optical fiber preform is clamped into an optical fiber drawing furnace and drawn into a multi-core elliptical polarization-maintaining optical fiber.

11. The method for preparing a multi-core elliptical polarization-maintaining optical fiber according to claim 10, characterized in that The circular fiber core rod is covered with an inner cladding.

12. The method for preparing a multi-core elliptical polarization-maintaining optical fiber according to claim 10 or 11, characterized in that The elliptical cladding core rod is deposited using a PCVD process, using SiCl4, BCl3, O2, and C2F6 as deposition raw materials. By adjusting the flow rates and proportions of the gases, a core rod with the required diameter and refractive index is prepared. During the deposition process, the SiCl4 flow rate is 500-2000sccm, the BCl3 flow rate is 250-800sccm, the O2 flow rate is 1500-5000sccm, and the C2F6 flow rate is 20-300sccm.

13. The method for preparing a multi-core elliptical polarization-maintaining optical fiber according to claim 10 or 11, characterized in that The circular fiber core rod is deposited using the PCVD process, 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 SiCl4 flow rate is 500-800sccm, the GeCl4 flow rate is 20-50sccm, and the O2 flow rate is 1000-1500sccm.

14. The method for preparing a multi-core elliptical polarization-maintaining optical fiber according to claim 11, characterized in that The inner cladding is deposited by PCVD process, using SiCl4, C2F6 and O2 as deposition raw materials. During the deposition process, the SiCl4 flow rate is 800-1200sccm, the C2F6 flow rate is 50-150sccm, and the O2 flow rate is 2000-3000sccm, and the cladding is deposited before the core layer.

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