A method for fabricating elliptical cladding polarization-maintaining optical fiber and the resulting product.

Abstract

This invention discloses a method for fabricating elliptical clad polarization-maintaining optical fiber and the resulting product. The method includes the following steps: (1) fabricating an elliptical clad solid rod; (2) fabricating an optical fiber preform; and (3) fabricating the elliptical clad polarization-maintaining optical fiber. The method for fabricating elliptical clad polarization-maintaining optical fiber provided by this invention uses a rod-insertion method to fabricate an elliptical clad optical fiber preform by combining an elliptical clad solid rod with a circular outer contour and a glass rod with a similar circular outer contour for forming the core layer. On the one hand, the circular rod-insertion technique is mature and has good concentricity; on the other hand, the core layer and elliptical cladding are formed separately, and the radial deformation process for fabricating the elliptical cladding does not affect the circular outer contour of the core layer glass rod, and the core layer non-circularity does not decrease significantly, which is significantly superior to existing elliptical clad optical fibers. Therefore, the elliptical clad polarization-maintaining optical fiber obtained by this invention has good concentricity and core layer non-circularity.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber technology, and more specifically, relates to a method for preparing elliptical cladding polarization-maintaining optical fiber and a product thereof. Background Technology

[0002] With the increasing demand for fiber optic gyroscopes in China, the requirements for polarization-maintaining fibers are becoming more stringent. Currently, the main polarization-maintaining fibers used can be categorized based on stress region differences: panda-type, bow-tie-type, elliptical-clad polarization-maintaining fibers, and elliptical-core polarization-maintaining fibers. Among these, panda-type, bow-tie-type, and elliptical-clad polarization-maintaining fibers are stress-birefringent polarization-maintaining fibers, while elliptical-core polarization-maintaining fibers are geometrically birefringent. However, due to the elliptical core, they are prone to higher splice losses, and their birefringence performance is lower than that of stress-birefringent fibers. Therefore, stress-birefringent fibers are commonly used in fiber optic gyroscopes.

[0003] To further improve accuracy, future fiber optic gyroscopes will require longer fiber lengths within the same volume, leading to smaller fiber outer diameters while maintaining high birefringence. Theoretical analysis shows that as the fiber outer diameter decreases, the stress region also shrinks, making it more difficult to achieve the required birefringence for bow-tie and panda-type polarization-maintaining fibers, thus hindering their performance in fiber optic gyroscopes. However, compared to elliptical cladding polarization-maintaining fibers, the smaller stress region area allows for the maintenance of birefringence through the elliptical cladding stress region structure. Furthermore, in coupler fabrication, bow-tie and panda-type polarization-maintaining fibers require tapering, which becomes extremely difficult and results in significant coupling loss when the fiber cladding diameter decreases from 125 μm to 40 μm. Elliptical cladding polarization-maintaining fibers, on the other hand, are relatively easier to taper and couple. In conclusion, elliptical cladding polarization-maintaining fibers are better suited to the miniaturization and high-performance development trends of fiber optic gyroscopes.

[0004] Unlike elliptical core polarization-maintaining fibers with elliptical cladding and elliptical core, elliptical cladding polarization-maintaining fibers with circular core and elliptical cladding are more difficult to fabricate. They are generally produced by melting or drawing non-circular core rods and circular ferrules under pressure control. For example, patent EP0381473B1 uses MCVD to prepare the core rod and then uses a polishing method to prepare the elliptical cladding. However, the drawn fiber parameters show a significant problem with the cladding non-circularity, which affects the fiber's subsequent splicing or tapering performance. Chinese patent CN102295407A provides a method for fabricating elliptical cladding polarization-maintaining fibers using a splicing method. This results in an irregular inner surface of the elliptical cladding used for core deposition, leading to high core non-circularity in the final elliptical cladding polarization-maintaining fiber. Furthermore, it requires repeated removal from the deposition furnace, resulting in low production efficiency. There are also methods to directly fabricate elliptical cladding polarization-maintaining fibers using negative pressure methods. However, because the core, elliptical cladding, and outer cladding are fabricated in one step, the final core roundness and cladding non-roundness are very large, which affects the downstream use. Summary of the Invention

[0005] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides a method and product for fabricating elliptical cladding polarization-maintaining fiber. The purpose is to employ a rod-joining method, coaxially fitting a solid rod with an elliptical cladding having a circular outer contour with a circular glass rod used to form the core layer, thereby fabricating a circular cladding fiber preform and drawing it into an elliptical cladding fiber. This solves the technical problems of high non-circularity of the outer cladding and core layer and poor core-cladding concentricity in existing elliptical cladding polarization-maintaining fibers.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for fabricating an elliptical cladding polarization-maintaining fiber is provided, comprising the following steps:

[0007] (1) Preparation of elliptical cladding solid rod: When the glass part used to form the stress cladding of polarization-maintaining fiber is in a molten state, positive or negative pressure is applied in the diameter direction to form a solid rod with an elliptical cross-section. After machining and grinding, an elliptical cladding solid rod with a circular outer contour of cross-section is obtained.

[0008] (2) Preparation of optical fiber preform: Drill a hole in the center of the elliptical cladding solid rod prepared in step (1), and insert a circular glass rod for forming the polarization-maintaining fiber core layer to obtain the preform of the polarization-maintaining fiber.

[0009] (3) Preparation of the elliptical cladding polarization-maintaining fiber: The preform of the polarization-maintaining fiber prepared in step (2) is drawn into a fiber to obtain the elliptical cladding polarization-maintaining fiber.

[0010] Preferably, in the method for preparing the elliptical cladding polarization-maintaining fiber, step (1) involves the outer circular outline of the elliptical cladding solid rod sharing a central axis with the elliptical-like solid rod.

[0011] Preferably, in the method for preparing the elliptical cladding polarization-maintaining fiber, step (1) involves fusing cylindrical connectors together at both ends of a solid rod that is elliptical in shape, clamping the cylindrical connectors at both ends onto three-jaw connectors at both ends of a lathe, so that the central axis of the solid rod coincides with the spindle of the lathe for rotary machining.

[0012] Preferably, in the method for preparing the elliptical clad polarization-maintaining fiber, step (1) involves applying positive pressure in the diameter direction using a flat mold in the molten state, i.e., molding to form a solid rod with a cross-section of an ellipse and parallel sides; clamping the parallel sides of the solid rod with a cross-section of an ellipse and fusing it with the cylindrical connector, preferably vertically.

[0013] Preferably, in the method for preparing the elliptical clad polarization-maintaining fiber, the cross-sectional diameter of the cylindrical connector is less than or equal to the minor axis of the elliptical solid rod and is comparable to it.

[0014] Preferably, in the method for preparing the elliptical cladding polarization-maintaining fiber, the glass component used to form the polarization-maintaining fiber cladding is prepared by in-tube deposition in a circular liner.

[0015] Preferably, in the method for preparing the elliptical clad polarization-maintaining fiber, the glass component used to form the stress cladding of the polarization-maintaining fiber has multiple layers, forming corresponding layers of a solid rod in the shape of an ellipse, wherein at least one layer is a boron-doped stress cladding, and a pure quartz or fluorine-doped inner cladding is concentrically nested in the boron-doped layer.

[0016] Preferably, in the method for preparing the elliptical clad polarization-maintaining fiber, the diameter of the central hole in step (2) does not exceed the short axis of the boron-doped stress cladding, and more preferably does not exceed the short axis of the pure quartz core.

[0017] Preferably, in the method for fabricating the elliptical cladding polarization-maintaining fiber, the relative refractive index of the elliptical cladding is between -1.6 and -0.7%, the ratio of the major axis of the elliptical cladding to the diameter of the outer cladding is ≤0.7; the ellipticity of the elliptical cladding is between 0.3 and 0.7, and the ellipticity is the ratio of the difference to the sum of the lengths of the major and minor axes of the ellipse; the ratio of the shortest distance from the elliptical cladding to the core in the slow axis direction to the diameter of the core is 0.1 to 1.2.

[0018] Preferably, in the method for preparing the elliptical clad polarization-maintaining fiber, the circular glass rod used to form the core layer of the polarization-maintaining fiber includes a core layer, wherein the relative refractive index difference Δ1% of the core layer is between 0.5% and 1.3%.

[0019] Preferably, in the method for fabricating the elliptical clad polarization-maintaining fiber, the circular glass rod has a multi-layered nested refractive index profile structure, specifically, the core layer of the circular glass rod has adjacent cladding layers on its outer side, and the relative refractive index Δ2% of the adjacent cladding layers is between -0.8% and 0%.

[0020] According to another aspect of the present invention, an elliptical cladding polarization-maintaining optical fiber is provided, having a coaxial circular outer cladding, an elliptical cladding, and a circular core.

[0021] The concentricity between the elliptical cladding and the circular core layer is ≤0.5μm, preferably ≤0.3μm, and the cladding non-circularity is ≤1, preferably ≤0.5.

[0022] Preferably, in the elliptical cladding polarization-maintaining fiber, the relative refractive index of the elliptical cladding is between -1.6 and -0.7%, the ratio of the major axis of the elliptical cladding to the diameter of the outer cladding is ≤0.7, the ellipticity of the elliptical cladding is between 0.3 and 0.7, and the ellipticity is the ratio of the difference to the sum of the lengths of the major and minor axes of the ellipse; the ratio of the shortest distance from the elliptical cladding to the core in the slow axis direction to the diameter of the circular core is 0.1 to 1.2.

[0023] Preferably, in the elliptical cladding polarization-maintaining fiber, the relative refractive index difference of the circular core layer is between 0.5% and 1.3%.

[0024] Preferably, the elliptical cladding polarization-maintaining fiber has an adjacent cladding outside the circular core layer, and the relative refractive index of the adjacent cladding is between -0.8% and 0%.

[0025] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0026] The method for fabricating elliptical cladding polarization-maintaining fiber provided by this invention involves using a nested rod method to fabricate an elliptical cladding fiber preform by combining a solid elliptical cladding rod with a circular outer contour and a glass rod with the same circular outer contour for forming the core layer. This method benefits from two advantages: firstly, the circular nested rod technique is mature and provides excellent concentricity; secondly, the core layer and elliptical cladding are formed separately, and the radial deformation process for fabricating the elliptical cladding does not affect the circular outer contour of the core layer glass rod, thus preventing a significant decrease in core layer non-circularity, which is significantly superior to existing elliptical cladding fibers. Therefore, the elliptical cladding polarization-maintaining fiber fabricated by this invention exhibits excellent concentricity and core layer non-circularity.

[0027] Meanwhile, the radial pressure is used to form an elliptical cladding, which is significantly more regular than the elliptical cladding formed by stacking, and the process is simplified. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the fabrication method of the elliptical cladding polarization-maintaining fiber provided by the present invention;

[0029] Figure 2 This is a schematic diagram of the end face structure of the elliptical cladding polarization-maintaining fiber provided in an embodiment of the present invention;

[0030] Figure 3A schematic diagram of the fast axis refractive index profile of an elliptical cladding polarization-maintaining fiber provided in an embodiment of the present invention;

[0031] Figure 4 This is a schematic diagram of a solid bar with an elliptical cladding and a round bar concentrically joined.

[0032] In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein: 1 is the core layer, 2 is the adjacent cladding, 3 is the elliptical cladding, 4 is the outer cladding, 5 is the inner coating, 6 is the outer coating, 7 is the lathe, 8 is the heat lamp, 9 is the three-jaw joint, 10 is the flat straight head, 11 is the round bar, 12 is the elliptical solid bar, and 13 is the dial indicator;

[0033] D1 is the core diameter, D2 is the diameter of the adjacent cladding, D3 is the major axis diameter of the elliptical cladding, D4 is the outer cladding diameter, D5 is the inner coating diameter, D6 is the outer coating diameter, D7 is the shortest distance between the elliptical cladding and the core, and D8 is the minor axis diameter of the elliptical cladding; Δ1% is the relative refractive index of the core, Δ2% is the relative refractive index of the adjacent cladding, Δ3% is the relative refractive index difference of the elliptical cladding, and Δ4% is the relative refractive index difference of the outer cladding. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0035] In this invention, the relative refractive index difference is calculated as Δ=[(n i 2 -n0 2 ) / (2n i 2 )]*100%≈[(n i -n0) / n i ]*100%, where ni is the refractive index of each corresponding part, and n0 is the refractive index of pure silica quartz glass.

[0036] The method for fabricating elliptical cladding polarization-maintaining optical fiber provided by this invention, such as... Figure 1 As shown, it includes the following steps:

[0037] (1) Preparation of elliptical cladding solid rod: When the glass part used to form the stress cladding of polarization-maintaining fiber is in a molten state, positive or negative pressure is applied in the diameter direction to form a solid rod with an elliptical cross-section. After machining and grinding, an elliptical cladding solid rod with a circular outer contour of cross-section is obtained.

[0038] The glass component used to form the polarization-maintaining fiber cladding is prepared by in-tube deposition in a circular liner, and then sintered and shrunk to obtain the glass component used to form the stress cladding of the polarization-maintaining fiber. During shrunking, the glass component is in a molten state, and a positive or negative pressure can be directly applied in the diameter direction to form a solid rod with an elliptical cross-section. The in-tube deposition method can easily form a multilayer structure of glass components with different dopants. In this ordered scheme, the glass component used to form the stress cladding of the polarization-maintaining fiber has multiple layers, with corresponding layers forming an elliptical solid rod. At least one layer is a boron-doped stress cladding, and a pure quartz layer is concentrically nested in the boron-doped layer. The embedded pure quartz core layer has good processing performance and is less likely to cause the glass component to crack during the drilling process, thus improving the yield. Therefore, the diameter of the central hole does not exceed the minor axis of the boron-doped stress cladding, and preferably does not exceed the minor axis of the pure quartz core layer.

[0039] To ensure that the core layer of the fabricated polarization-maintaining cladding fiber is centered within the fiber stress cladding, the outer circular outline of the elliptical cladding solid rod shares a central axis with the elliptical-shaped solid rod. Figure 4 As shown, this ensures that the circular glass rod used to form the core layer is centered on the elliptical cladding solid rod during the subsequent drilling and cladding processes.

[0040] However, when machining and rounding a quasi-elliptical solid bar, the clamping accuracy and stability on the lathe are inferior to those on a cylindrical solid bar. To ensure the machining accuracy, especially the accuracy of the outer contour circle of the elliptical cladding solid bar sharing a central axis with the quasi-elliptical solid bar, it is preferable to weld cylindrical joints to both ends of the quasi-elliptical solid bar along a central axis. These cylindrical joints are then clamped onto three-jaw joints at both ends of the lathe, ensuring that the central axis of the solid bar coincides with the lathe spindle for rotational machining, thus improving the concentricity of each layer. Preferably, the cylindrical joints are made of pure quartz glass, which not only ensures stable clamping of the quasi-elliptical solid bar but also allows for effective checking of concentricity and runout using a dial indicator, facilitating calibration.

[0041] As can be seen from the above, stable clamping of a solid rod with a near-elliptical cross-section is crucial in machining processes. When compression molding is used, i.e., when the glass component used to form the stress cladding of polarization-maintaining optical fiber is in a molten state and positive pressure is applied in the diametrical direction, a flat mold can be used to form a solid rod with a near-elliptical cross-section and parallel sides, which provides more stable clamping. Preferably, the parallel sides of the solid rod with a near-elliptical cross-section are clamped so that they are vertically welded to the cylindrical connector, preventing misalignment of the weld between the cylindrical connector and the near-elliptical solid rod due to flow at the molten end face caused by gravity.

[0042] The cross-sectional diameter of a cylindrical joint should generally not exceed the minor axis of the elliptical solid rod, but should be as large as possible under this premise. The preferred design is that the diameter is approximately equal to the minor axis of the elliptical solid rod.

[0043] The relative refractive index Δ3% of the elliptical cladding is between -1.6 and -0.7%, the ratio of the major axis D3 of the elliptical cladding to the diameter D4 of the outer cladding is D3 / D4≤0.7, the ellipticity (D3-D8) / (D3+D8) of the elliptical cladding is between 0.3 and 0.7, and the ratio of the shortest distance D7 from the elliptical cladding to the core in the slow axis direction to the core diameter D7 / D1 is 0.1 to 1.2.

[0044] (2) Preparation of optical fiber preform: Drill a hole in the center of the elliptical cladding solid rod prepared in step (1), and insert a circular glass rod for forming the polarization-maintaining fiber core layer to obtain the preform of the polarization-maintaining fiber.

[0045] The circular glass rod used to form the polarization-maintaining fiber core includes a core layer, wherein the relative refractive index difference of the core layer is between 0.5% and 1.3%.

[0046] Preferably, the circular glass rod has a multi-layered nested refractive index profile structure. Specifically, the outer side of the core layer of the circular glass rod has adjacent cladding layers, and the relative refractive index of the adjacent cladding layers is between -0.8% and 0%.

[0047] (3) Preparation of the elliptical cladding polarization-maintaining fiber: The preform of the polarization-maintaining fiber prepared in step (2) is drawn into a fiber to obtain the elliptical cladding polarization-maintaining fiber.

[0048] The elliptical cladding polarization-maintaining fiber prepared according to the method provided by this invention typically includes a circular outer cladding, an elliptical boron-doped cladding, and a circular core. Generally, an elliptical pure silica layer is present between the elliptical boron-doped cladding and the circular core. Preferably, an adjacent cladding is present outside the circular core. The core-cladding concentricity between the elliptical cladding and the circular core is ≤0.5 μm, preferably ≤0.3 μm, and the cladding non-circularity is ≤1, preferably ≤0.5. Currently, the core-cladding concentricity of elliptical cladding is generally around 1 μm. The elliptical cladding polarization-maintaining fiber provided by this invention improves the core-cladding concentricity and cladding non-circularity (i.e., the non-circularity of the bare fiber), while the core non-circularity is undoubtedly comparable to existing panda core polarization-maintaining fibers and bow-tie type polarization-maintaining fibers, showing a significant advantage over existing elliptical cladding polarization-maintaining fibers.

[0049] The relative refractive index Δ3% of the elliptical cladding is between -1.6 and -0.7%, the ratio of the major axis D3 of the elliptical cladding to the diameter D4 of the outer cladding is D3 / D4≤0.7, the ellipticity (D3-D8) / (D3+D8) of the elliptical cladding is between 0.3 and 0.7, and the ratio of the shortest distance D7 from the elliptical cladding to the core layer in the slow axis direction to the diameter of the circular core layer, D7 / D1, is 0.1 to 1.2.

[0050] The relative refractive index difference of the circular core layer is between 0.5% and 1.3%; preferably, the outer side of the circular core layer has an adjacent cladding layer, and the relative refractive index of the adjacent cladding layer is between -0.8% and 0%.

[0051] The following is an example:

[0052] In the example:

[0053] Core / cladding concentricity error: tested according to IEC 60793-1-20 standard;

[0054] Cladding non-circularity: tested according to IEC 60793-1-20 standard;

[0055] Birefringence, B = λ / beatlength, where λ is the wavelength and beatlength is the beat length, which is tested according to IEC 60793-1-60 standard;

[0056] Example 1

[0057] The method for fabricating elliptical cladding polarization-maintaining fiber provided by the present invention includes the following steps:

[0058] (1) Preparation of elliptical cladding solid rod: The glass part used to form the stress cladding of polarization-maintaining fiber is in a molten state. Positive pressure is applied in the diameter direction to form a solid rod with an elliptical cross section. After machining and rounding, an elliptical cladding solid rod with a circular outer contour of cross section is obtained.

[0059] (1-1) Preparation of glass components for forming polarization-maintaining fiber cladding by in-tube deposition method:

[0060] Boron-doped layers and pure silicon core layers or fluorine-doped core layers are prepared using PCVD (plasma chemical vapor deposition) technology, respectively. Specifically, in this application, the boron-doped layer is prepared using SiCl4 (tetrachlorosilane), BCl3 (boron trichloride), and O2 (oxygen) as raw materials, and the pure silicon core layer or fluorine-doped 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.

[0061] In the boron doped layer preparation process, the SiCl4 flow rate is 500–2000 sccm (sccm is a volumetric flow rate unit, referring to standard milliliters per minute); the BCl3 flow rate is 250–800 sccm; and the O2 flow rate is 1500–5000 sccm. In the core layer preparation process, the SiCl4 flow rate is 500–2000 sccm; the C2F6 flow rate is 0–300 sccm; and the O2 flow rate is 1500–5000 sccm.

[0062] (1-2) Heating, shrinking, and shaping:

[0063] After deposition, the pipe is placed on a shrinking lathe and heated to 1950°C. The surface tension generated by the high temperature shrinks the pipe into a solid bar.

[0064] A solid rod is heated to 2000℃ using a heating device, and pressure is applied through a flat mold to produce a certain deformation distance before stopping, thus forming a quasi-elliptical clad solid rod.

[0065] In Examples 1-5, the outer diameter of the original rod is 40.5 mm, and the hot pressing distances are 5 mm, 7 mm, 9 mm, 11 mm, and 13 mm, respectively.

[0066] (1-3) Machining:

[0067] First, the elliptical clad solid bar is clamped with a straight clamp, and the round bar is clamped with a three-jaw joint. After being heated and melted together by flame, the three-jaw joint is released, and the concentricity is effectively checked by using a dial indicator to control the runout ≤0.1mm.

[0068] After the rod cools down, the clamping points are switched. The round part of the elliptical clad solid rod is clamped by a three-jaw joint, and the other end is clamped by a three-jaw joint. After the rod is melted and joined by flame heating, the three-jaw chuck of the other end of the original rod is released. The runout of the unclamped original rod is effectively checked by using a dial indicator. Once the runout is controlled to be ≤0.1mm, the rod calibration is stopped.

[0069] Finally, the cylindrical ends are clamped onto the three-jaw joint of the lathe, and the elliptical solid bar is ground round. The outer diameter of the round bar and the grinding outer diameter in Examples 1-5 are 35mm, 33mm, 31mm, 29mm and 27mm respectively.

[0070] (2) Preparation of optical fiber preform: Drill a hole in the center of the elliptical cladding solid rod prepared in step (1), and insert a circular glass rod for forming the polarization-maintaining fiber core layer to obtain the preform of the polarization-maintaining fiber.

[0071] (2-1) Preparation of a circular glass rod:

[0072] The core layer and adjacent cladding layers are prepared using PCVD (plasma chemical vapor deposition) process. Specifically, in this application, the core layer is prepared using SiCl4 (tetrachlorosilane), GeCl4 (germanium tetrachloride), and O2 (oxygen) as raw materials, and the adjacent cladding layers are prepared using SiCl4 (tetrachlorosilane), C2F6 (hexafluoroethane), and O2 (oxygen) as raw materials. By adjusting the flow rate and ratio of the gases, a core rod with the required diameter and refractive index is prepared under appropriate deposition rate conditions.

[0073] In the core layer preparation process, the SiCl4 flow rate is 500–2000 sccm (sccm is a volumetric flow rate unit, referring to standard milliliters per minute); the GeCl4 flow rate is 5–100 sccm; the O2 flow rate is 1500–5000 sccm; the C2F6 flow rate is 0–300 sccm; and the O2 flow rate is 1500–5000 sccm.

[0074] After deposition, the pipe is placed on a shrinking lathe and heated to 1950°C. The surface tension generated by the high temperature shrinks the pipe into a solid bar.

[0075] The outer diameter of the mandrel is etched to 9mm to facilitate subsequent mandrel insertion operations.

[0076] (2-2) Drilling holes:

[0077] Using a drilling lathe, the center of the elliptical clad solid bar is drilled to 10mm, and then degreased, cleaned, and dried.

[0078] (2-3) Sleeve of bars:

[0079] The RIT method is used to insert a circular glass rod into a perforated elliptical clad solid rod.

[0080] (3) Preparation of the elliptical cladding polarization-maintaining fiber: The preform of the polarization-maintaining fiber prepared in step (2) is drawn into a fiber to obtain the elliptical cladding polarization-maintaining fiber.

[0081] The drawing furnace temperature is 1900~2100℃, the drawing speed is 200~600m / min, and the drawing tension is 20~90g.

[0082] Fiber end face structure such as Figure 2 As shown, from the inside out, the fiber comprises: a circular core, an adjacent cladding, an elliptical cladding, an outer cladding, an inner coating, and an outer coating. The diameter of the circular core is D1, the diameter of the adjacent cladding is D2, the major axis diameter of the elliptical cladding is D3, the diameter of the outer cladding is D4, the diameter of the inner coating is D5, the diameter of the outer coating is D6, the shortest distance between the elliptical cladding and the core is D7, and the minor axis diameter of the elliptical cladding is D8. The refractive index profile is shown below. Figure 3 As shown, its parameters are as follows:

[0083]

[0084]

[0085] Example 2

[0086] The method for fabricating elliptical cladding polarization-maintaining fiber provided by the present invention includes the following steps:

[0087] (1) Preparation of elliptical cladding solid rod: The glass part used to form the stress cladding of polarization-maintaining fiber is in a molten state. By applying negative pressure during melting and shrinking, a solid rod with an elliptical cross-section can be formed. After machining and grinding, an elliptical cladding solid rod with a circular outer contour of cross-section is obtained.

[0088] (1-1) Preparation of glass components for forming polarization-maintaining fiber cladding by in-tube deposition method:

[0089] Boron-doped layers and pure silicon core layers or fluorine-doped core layers are prepared using PCVD (plasma chemical vapor deposition) technology, respectively. Specifically, in this application, the boron-doped layer is prepared using SiCl4 (tetrachlorosilane), BCl3 (boron trichloride), and O2 (oxygen) as raw materials, and the pure silicon core layer or fluorine-doped 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.

[0090] In the boron doped layer preparation process, the SiCl4 flow rate is 500–2000 sccm (sccm is a volumetric flow rate unit, referring to standard milliliters per minute); the BCl3 flow rate is 250–800 sccm; and the O2 flow rate is 1500–5000 sccm. In the core layer preparation process, the SiCl4 flow rate is 500–2000 sccm; the C2F6 flow rate is 0–300 sccm; and the O2 flow rate is 1500–5000 sccm.

[0091] (1-2) Heating, shrinking, and shaping:

[0092] The deposited tubing is placed on a shrinking lathe and heated to 1950℃. The high temperature causes surface tension to induce shrinkage, which is then compacted under negative pressure to produce a near-elliptical clad solid rod. Its minor axis outer diameters are 34mm, 32mm, 30mm, 28mm, and 26mm.

[0093] (1-3) Machining:

[0094] First, the elliptical clad solid bar is clamped with a straight clamp, and the round bar is clamped with a three-jaw joint. After being heated and melted together by flame, the three-jaw joint is released, and the concentricity is effectively checked by using a dial indicator to control the runout ≤0.1mm.

[0095] After the rod cools down, the clamping points are switched. The round part of the elliptical clad solid rod is clamped by a three-jaw joint, and the other end is clamped by a three-jaw joint. After the rod is melted and joined by flame heating, the three-jaw chuck of the other end of the original rod is released. The runout of the unclamped original rod is effectively checked by using a dial indicator. Once the runout is controlled to be ≤0.1mm, the rod calibration is stopped.

[0096] Finally, the cylindrical ends are clamped on the three-jaw connector of the lathe, and the elliptical solid rod is ground round. The diameters of the round rod and the elliptical solid rod in Examples 1-5 after grinding are 34mm, 32mm, 30mm, 28mm and 26mm respectively. (2) Preparation of optical fiber preform: Drill a hole in the center of the elliptical cladding solid rod prepared in step (1), and insert a circular glass rod for forming the polarization-maintaining fiber core layer to obtain the preform of the polarization-maintaining fiber;

[0097] (2-1) Preparation of a circular glass rod:

[0098] The core layer and adjacent cladding layers are prepared using PCVD (plasma chemical vapor deposition) process. Specifically, in this application, the core layer is prepared using SiCl4 (tetrachlorosilane), GeCl4 (germanium tetrachloride), and O2 (oxygen) as raw materials, and the adjacent cladding layers are prepared using SiCl4 (tetrachlorosilane), C2F6 (hexafluoroethane), and O2 (oxygen) as raw materials. By adjusting the flow rate and ratio of the gases, a core rod with the required diameter and refractive index is prepared under appropriate deposition rate conditions.

[0099] In the core layer preparation process, the SiCl4 flow rate is 500–2000 sccm (sccm is a volumetric flow rate unit, referring to standard milliliters per minute); the GeCl4 flow rate is 5–100 sccm; the O2 flow rate is 1500–5000 sccm; the C2F6 flow rate is 0–300 sccm; and the O2 flow rate is 1500–5000 sccm.

[0100] After deposition, the pipe is placed on a shrinking lathe and heated to 1950°C. The surface tension generated by the high temperature shrinks the pipe into a solid bar.

[0101] The outer diameter of the mandrel is etched to 9mm to facilitate subsequent mandrel insertion operations.

[0102] (2-2) Drilling holes:

[0103] Using a drilling lathe, a 10mm hole is drilled in the center of the elliptical cladding solid bar, followed by degreasing, cleaning, and drying.

[0104] (2-3) Sleeve of bars:

[0105] The RIT method is used to insert a circular glass rod into a perforated elliptical clad solid rod.

[0106] (3) Preparation of the elliptical cladding polarization-maintaining fiber: The preform of the polarization-maintaining fiber prepared in step (2) is drawn into a fiber to obtain the elliptical cladding polarization-maintaining fiber.

[0107] The drawing furnace temperature is 1900~2100℃, the drawing speed is 200~600m / min, and the drawing tension is 20~90g.

[0108] Fiber end face structure such as Figure 2 As shown, from the inside out, the fiber comprises: a circular core, an adjacent cladding, an elliptical cladding, an outer cladding, an inner coating, and an outer coating. The diameter of the circular core is D1, the diameter of the adjacent cladding is D2, the major axis diameter of the elliptical cladding is D3, the diameter of the outer cladding is D4, the diameter of the inner coating is D5, the diameter of the outer coating is D6, the shortest distance between the elliptical cladding and the core is D7, and the minor axis diameter of the elliptical cladding is D8. The refractive index profile is shown below. Figure 3 As shown, the fiber optic parameters are as follows:

[0109]

[0110]

[0111] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for fabricating an elliptical cladding polarization-maintaining optical fiber, characterized in that, Includes the following steps: (1) Preparation of elliptical cladding solid rod: When the glass part used to form the stress cladding of polarization-maintaining fiber is in a molten state, positive or negative pressure is applied in the diameter direction to form a solid rod with an elliptical cross-section. After machining and grinding, an elliptical cladding solid rod with a circular outer contour of cross-section is obtained. (2) Preparation of optical fiber preform: Drill a hole in the center of the elliptical cladding solid rod prepared in step (1), and insert a circular glass rod for forming the polarization-maintaining fiber core layer to obtain the preform of the polarization-maintaining fiber. (3) Preparation of the elliptical cladding polarization-maintaining fiber: The preform of the polarization-maintaining fiber prepared in step (2) is drawn into a fiber to obtain the elliptical cladding polarization-maintaining fiber.

2. The method for fabricating elliptical cladding polarization-maintaining fiber as described in claim 1, characterized in that, Step (1) The outer contour circle of the elliptical cladding solid rod shares the same central axis as the elliptical solid rod.

3. The method for fabricating elliptical cladding polarization-maintaining optical fiber as described in claim 2, characterized in that, Step (1) Weld cylindrical connectors together at both ends of a solid bar that is elliptical in shape. Clamp the cylindrical connectors at both ends on the three-jaw connectors at both ends of the lathe so that the central axis of the solid bar coincides with the spindle of the lathe for rotary machining.

4. The method for fabricating elliptical cladding polarization-maintaining optical fiber as described in claim 3, characterized in that, Step (1) In the molten state, a straight mold is used to apply positive pressure in the diameter direction, that is, to form a solid rod with parallel sides and an elliptical cross-section; the parallel sides of the solid rod with an elliptical cross-section are clamped and fused with the cylindrical joint.

5. The method for fabricating an elliptical cladding polarization-maintaining optical fiber as described in claim 4, characterized in that, Preferably, the solid rod is vertically welded to the cylindrical joint.

6. The method for fabricating an elliptical cladding polarization-maintaining optical fiber as described in claim 3, characterized in that, The cross-sectional diameter of the cylindrical joint is less than or equal to the minor axis of the elliptical solid rod.

7. The method for fabricating an elliptical cladding polarization-maintaining optical fiber as described in claim 1, characterized in that, The glass component used to form the stress cladding of the polarization-maintaining optical fiber is prepared by in-tube deposition in a circular liner.

8. The method for fabricating an elliptical cladding polarization-maintaining optical fiber as described in claim 1, characterized in that, The glass element used to form the stress cladding of the polarization-maintaining fiber has multiple layers, forming corresponding layers of a solid rod in the shape of an ellipse, wherein at least one layer is a boron-doped stress cladding, and a pure quartz or fluorine-doped inner cladding is concentrically nested in the boron-doped layer.

9. The method for fabricating an elliptical cladding polarization-maintaining optical fiber as described in claim 8, characterized in that, In step (2), the diameter of the center hole should not exceed the short axis of the boron-doped stress cladding.

10. The method for fabricating an elliptical cladding polarization-maintaining optical fiber as described in claim 9, characterized in that, The diameter of the central hole does not exceed the short axis of the pure quartz core layer.

11. The method for fabricating an elliptical cladding polarization-maintaining optical fiber as described in claim 1, characterized in that, The relative refractive index of the elliptical cladding is between -1.6 and -0.7%, and the ratio of the major axis of the elliptical cladding to the diameter of the outer cladding is ≤0.7; the ellipticity of the elliptical cladding is between 0.3 and 0.7, and the ellipticity is the ratio of the difference and sum of the lengths of the major and minor axes of the ellipse; the ratio of the shortest distance from the elliptical cladding to the core layer along the slow axis to the diameter of the core layer is 0.1 to 1.

2.

12. The method for fabricating an elliptical cladding polarization-maintaining fiber as described in claim 1, characterized in that, The circular glass rod used to form the polarization-maintaining fiber core includes a core layer, wherein the relative refractive index difference Δ1% of the core layer is between 0.5% and 1.3%.

13. The method for fabricating an elliptical cladding polarization-maintaining optical fiber as described in claim 12, characterized in that, The circular glass rod has a multi-layered nested refractive index profile structure. Specifically, the core layer of the circular glass rod has adjacent cladding layers on the outside, and the relative refractive index Δ2% of the adjacent cladding layers is between -0.8% and 0%.

14. An elliptical cladding polarization-maintaining optical fiber prepared according to any one of claims 1 to 13, characterized in that, It has a coaxial circular outer cladding, an elliptical cladding, and a circular core layer; The concentricity between the elliptical cladding and the circular core is ≤0.5μm, and the out-of-roundness of the cladding is ≤1.

15. The elliptical cladding polarization-maintaining fiber as described in claim 14, characterized in that, The concentricity between the elliptical cladding and the circular core is ≤0.3μm, and the out-of-roundness of the cladding is ≤0.

5.

16. The elliptical cladding polarization-maintaining optical fiber as described in claim 14, characterized in that, The relative refractive index of the elliptical cladding is between -1.6 and -0.7%, and the ratio of the major axis of the elliptical cladding to the diameter of the outer cladding is ≤0.7; the ellipticity of the elliptical cladding is between 0.3 and 0.7, and the ellipticity is the ratio of the difference and sum of the lengths of the major and minor axes of the ellipse; the ratio of the shortest distance from the elliptical cladding to the core layer in the slow axis direction to the diameter of the circular core layer is 0.1 to 1.

2.

17. The elliptical cladding polarization-maintaining optical fiber as described in claim 14, characterized in that, The relative refractive index difference of the circular core layer is between 0.5% and 1.3%.

18. The elliptical cladding polarization-maintaining optical fiber as described in claim 14, characterized in that, The circular core layer has adjacent cladding layers on its outer side, and the relative refractive index of the adjacent cladding layers is between -0.8% and 0%.

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