Double-air-hole solid core polarization maintaining optical fiber and preparation method thereof

Abstract

The invention relates to a double-air-hole solid core polarization maintaining optical fiber and a preparation method thereof, the optical fiber comprises a quartz cladding (1), a circular fiber core (3) and two circular air holes (2), and the quartz cladding (1) is filled in the area between the circular fiber core (3) and the circular air holes (2); the circular fiber core (3) is located at the central position of the optical fiber, and the two circular air holes (2) are horizontally arranged relative to the circular fiber core (3); the circular fiber core (3) is made of quartz glass doped with GeO2. According to the double-air-hole solid core polarization maintaining optical fiber, the dependence degree on a stress rod is reduced, meanwhile, the good polarization maintaining performance is guaranteed, the bending loss is small, and the small-diameter polarization maintaining optical fiber is suitable for being applied to small-size precise devices. The method for preparing the double-air-hole solid core polarization maintaining optical fiber is feasible in process and easy to accurately control and draw.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fibers, and particularly relates to a polarization-maintaining optical fiber. Background Art

[0002] Polarization-maintaining optical fibers are optical fibers that have important applications in many fields such as optical fiber communication, optical fiber sensing, and precision optical instruments. Thin-diameter polarization-maintaining optical fibers can effectively save space and reduce load in fields such as aerospace and miniaturized navigation equipment that have extremely high requirements for space and weight; in high-density optical integration systems, thin-diameter polarization-maintaining optical fibers can be arranged more closely, increasing the number of integrated optical fibers, improving the integration and functional density of the system, and helping to achieve more channels or more complex optical functions. Generally, high birefringence in polarization-maintaining optical fibers can be achieved by introducing anisotropy in the stress or geometry of the fiber profile, effectively maintaining the polarization state of light. Common polarization-maintaining optical fibers include panda type, bow-tie type, and elliptical cladding type, which can achieve a birefringence of 10 -4 orders of magnitude by introducing symmetric stress rods in the cladding; another type of polarization-maintaining optical fiber is the relatively special photonic crystal polarization-maintaining optical fiber, which is formed by arranging periodic air holes. By destroying the circular symmetry of the fiber cross-section and regulating the number and spacing of air holes, a very high birefringence phenomenon is generated, which is one to two orders of magnitude higher than that of ordinary polarization-maintaining optical fibers. However, the design and manufacture of high-birefringence optical fibers not only require precise equipment but also suitable stress rod materials. Therefore, it is of great significance to design a polarization-maintaining optical fiber that simultaneously has high birefringence, simple manufacturing process, and reduced dependence on stress rods. Summary of the Invention

[0003] To solve the above problems, the present invention proposes a dual-air-hole solid-core polarization-maintaining optical fiber. By using two circular air holes that are close enough to the core to replace the stress rods, no additional stress rod material is required, reducing the dependence on stress rods. At the same time, using two air holes can not only ensure a large refractive index difference in the direction orthogonal to the core, but also has a simple structure, ensuring high enough process feasibility and accuracy.

[0004] To achieve the above object, the present invention is implemented by adopting the following technical solutions:

[0005] In a first aspect of the present invention, a dual-air-hole solid-core polarization-maintaining optical fiber is provided. The optical fiber includes a silica cladding 1, a circular core 3, and two circular air holes 2. The silica cladding 1 fills the region between the circular core 3 and the circular air holes 2; the circular core 3 is located at the center of the optical fiber, and the two circular air holes 2 are horizontally placed with respect to the circular core 3; the material of the circular core 3 is doped GeO 2 silica glass.

[0006] Preferably, the diameter D of the circular air hole 22 Satisfy 10μm ≤ D 2 ≤ 17μm.

[0007] Preferably, the diameter D of the circular core 3 3 Satisfy 2.5μm ≤ D 3 ≤ 3.5μm.

[0008] Preferably, the outer diameter D of the quartz cladding 1 1 Is 40μm.

[0009] Preferably, the distance L between the circular core 3 and the circular air hole 2 is ≤ 0.5μm.

[0010] Preferably, the working wavelength of the optical fiber is 820nm, and the birefringence coefficient can reach 10 -4 Magnitude.

[0011] In the second aspect of the present invention, a method for preparing a double air-hole solid-core polarization-maintaining optical fiber is provided, including the following steps: First, design appropriate optical fiber preform parameters by scaling up according to the parameters of the optical fiber; Second, arrange a quartz glass rod doped with GeO 2 , a thin-walled hollow tube, and a quartz capillary rod in the outer casing according to the designed structure by using the stacking method, and then form an optical fiber preform through high-temperature treatment; Third, put the optical fiber preform into an optical fiber drawing tower and heat it to 1400 - 2000°C for high-temperature melting drawing to obtain the optical fiber as described in claim 1.

[0012] Preferably, the outer diameter of the outer casing is 15mm.

[0013] Preferably, the duty ratio of the thin-walled hollow core tube is 34 / 38, and the diameter is 1 - 1.4mm.

[0014] Preferably, the diameter of the quartz capillary rod is 0.2 - 4.5mm.

[0015] The advantages and positive effects of the present invention are as follows:

[0016] In the double air-hole solid-core polarization-maintaining optical fiber of the present invention, the dependence on the stress rod is reduced. At the same time, good polarization-maintaining performance is ensured and it has small bending loss. This thin-diameter polarization-maintaining optical fiber is suitable for small-size precision device applications. The method for preparing the double air-hole solid-core polarization-maintaining optical fiber of the present invention is feasible in process, easy to accurately control and draw. Description of the Drawings

[0017] Figure 1 Is a cross-sectional schematic diagram of a double air-hole polarization-maintaining solid-core optical fiber.

[0018] Figure 2 Is Figure 1The birefringence of a polarization-maintaining solid-core fiber with double air holes of the shown structure varies with the diameter D of the circular air hole 2 Schematic diagram of the variation

[0019] Figure 3 is Figure 1 The birefringence of a polarization-maintaining solid-core fiber with double air holes of the shown structure varies with the diameter D of the circular core 3 Schematic diagram of the variation

[0020] Figure 4 is Figure 1 Schematic diagram of the birefringence of a polarization-maintaining solid-core fiber with double air holes of the shown structure varying with the spacing L between the circular core and the circular air hole

[0021] Among them, 1 is the quartz cladding region, 2 is the circular air hole, and 3 is the circular core Specific implementation manner

[0022] To better understand the technical solution of the present invention, the content of the present invention includes but is not limited to the specific implementation manners below. Similar technologies and methods should be regarded as within the scope of protection of the present invention. To make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the following will be described in detail with reference to the accompanying drawings and specific embodiments

[0023] The polarization-maintaining solid-core fiber with double air holes in this embodiment includes a quartz cladding 1, a circular core 3, and two circular air holes 2. The quartz cladding 1 fills the region between the circular core 3 and the circular air holes 2; the circular core 3 is located at the central position of the fiber, and the two circular air holes 2 are horizontally placed with respect to the circular core 3; the material of the circular core 3 is quartz glass doped with GeO 2 and the working wavelength of the fiber is 820 nm

[0024] The outer diameter of the cladding of existing commercial thin-diameter polarization-maintaining fibers generally needs to be ≤80 μm. In order for the fiber of the present invention to meet a more compact and precise system, the outer diameter D of the quartz cladding 1 1 is 40 μm, and it is benchmarked against existing commercial thin-diameter polarization-maintaining fibers in the market and can be replaced and used plug and play. When the outer diameter of the quartz cladding 1 is small, the surface stress generated by bending is greatly reduced, which not only significantly improves the mechanical properties of the fiber, ensures the service life under the bending state, but also reduces the influence on optical properties such as birefringence, and can meet the use requirements of a small bending diameter. In addition, the small outer diameter of the cladding also significantly reduces the volume of the fiber, which is quite important in the application of some small-sized precision devices, such as the aerospace field

[0025] In the dual-air-hole solid-core polarization-maintaining fiber of the present invention, two air holes that are close enough to the core are used to adjust the refractive index of the core in a single axial direction, so that the core has significantly different refractive indices in two orthogonal directions perpendicular to each other, thereby achieving the polarization-maintaining effect. Using two air holes can not only ensure a large refractive index difference in the orthogonal directions of the core, but also does not require an additional stress rod material like traditional panda fibers. Moreover, the simple structure of the two air holes can also ensure high enough process feasibility and accuracy.

[0026] The diameter D of the circular air hole 2 has a value range of 10 μm ≤ D 2 ≤ 17 μm. As Figure 2 shown, within this value range, the birefringence coefficient of the fiber at the working wavelength is of the order of 10 -4 ; the diameter D of the circular core 3 3 has a value range of 2.5 μm ≤ D 3 ≤ 3.5 μm. As Figure 3 shown, within this value range, the birefringence coefficient of the fiber at the working wavelength is of the order of 10 -4 .

[0027] In a preferred embodiment of the present invention, the diameter D of the circular core 3 3 is 3 μm, and the diameter D of the circular air hole 2 2 is 10 μm. Since the core is a germanium-doped high-refractive-index core, when light enters the fiber, the light is mainly transmitted through the principle of total internal reflection in the core. Therefore, the light field is basically restricted near the core region. For the air holes to achieve the effect of adjusting the refractive index of the core, they need to be close enough to the core and ensure process feasibility. As Figure 4 shown, when the distance L between the circular core 3 and the circular air hole 2 is L ≤ 0.5 μm, the effect of adjusting the refractive index can be ensured, and the polarization-maintaining performance can be guaranteed. When L is 0.1 and 0.2 μm, the birefringence of the fiber at the working wavelength can both reach above 2×10 -4 .

[0028] The preparation method of the dual-air-hole solid-core polarization-maintaining fiber includes the following steps: First, design appropriate fiber preform parameters by proportionally enlarging according to the parameters of the fiber; Second, arrange a germanium-doped GeO 2 quartz glass rod, a thin-walled hollow tube, and a quartz capillary rod in the outer sleeve according to the designed structure using the stacking method, and then form a fiber preform through high-temperature treatment; Third, place the fiber preform in a fiber drawing tower and heat it to 1400 - 2000 °C for high-temperature melting drawing to obtain the fiber as described in claim 1.

[0029] Specifically, in the first step, the cross-section of the double-air-hole solid-core polarization-maintaining fiber is enlarged proportionally. According to the diameter of the circular core 3 of the fiber, the diameter of the circular air hole 2, and the outer diameter of the quartz cladding 1, quartz glass rods doped with GeO of corresponding sizes are selected. 2 Quartz glass rods, thin-walled hollow tubes, quartz capillary rods, and outer tubes. Among them, the quartz glass rod doped with GeO 2 becomes the circular core after being drawn into a fiber, the thin-walled hollow tube becomes the circular air hole after being drawn into a fiber, and multiple quartz capillary rods with different diameters are used to fill the area between the circular core and the circular air hole in the outer tube.

[0030] In a preferred embodiment of the present application, the quartz glass rod doped with GeO 2 is polished, buffed, and thinned so that the diameter of the quartz glass rod doped with GeO 2 is 0.8 mm.

[0031] Fiber drawing is essentially a process of proportionally reducing the high-temperature melting of the fiber preform and then drawing it. When drawing a thin-diameter fiber with an outer diameter of 40 μm, a smaller-sized outer tube needs to be selected to reduce the proportionally reducing multiple and better control the fiber structure. According to actual preparation experience, an outer tube with an inner diameter of 12 mm, an outer diameter of 15 mm, and a duty ratio of 12 / 15 is selected. The outer tube is made of high-purity quartz material. At this time, the structure can be accurately stacked and formed, and it meets the requirements for drawing thin-diameter fibers.

[0032] Since only when the circular air hole 2 is close enough to the circular core can the refractive index adjustment effect be ensured and the polarization-maintaining performance be guaranteed. Therefore, a thin-walled hollow tube is selected when making the fiber preform. Considering the need to meet the polarization-maintaining requirement and avoid extrusion and breakage during high-temperature drawing, a thin-walled hollow tube with a duty ratio of 34 / 38 is selected. The diameter of the thin-walled air tube is 1 - 1.4 mm, and preferably, the diameter of the thin-walled air tube is 1 mm.

[0033] Quartz capillary rods are used to fill the area between the circular core and the circular air hole in the outer tube. The sizes of the quartz capillary rods used for filling are 4.5 mm, 3.3 mm, 2.0 mm, 1.54 mm, 1 mm, 0.8 mm, 0.5 mm, 0.3 mm, and 0.2 mm in diameter.

[0034] In the second step, GeO-doped 2The quartz glass rod, thin-walled hollow tube, and quartz capillary rod are arranged in the outer casing according to the designed structure using the stacking method. The stacking method is to stack multiple components made of quartz or doped materials with specific structures and properties in a certain order and manner, and then through a high-temperature treatment process, fuse these components into an integral optical fiber preform. Its core principle is to utilize the optical properties of different materials, and by precisely controlling the number of stacked layers, thickness, and material distribution, etc., to achieve precise regulation of the optical properties such as the refractive index distribution of the optical fiber preform, so as to meet the performance requirements of different types of optical fibers. Preferably, in the second step of the optical fiber in the present invention, a doped GeO quartz glass rod with a diameter of 0.8 mm 2 The quartz glass rod, with a duty cycle of 34 / 38, the thin-walled hollow tube with a diameter of 1 mm, and quartz capillary rods with different diameters are filled in different gaps according to the arrangement requirements, and are arranged in the outer casing with an inner diameter of 12 mm and an outer diameter of 15 mm according to the designed structure using the stacking method to form an optical fiber preform.

[0035] In the third step, the optical fiber preform is placed in an optical fiber drawing tower and heated to 1400 - 2000 °C for high-temperature melting and drawing. Specifically, the optical fiber preform is installed on the rod feeding system of the optical fiber drawing tower, and the optical fiber preform is heated and drawn using a high-temperature graphite furnace, heated to 1400 - 2000 °C, and a low-refractive-index glue is coated during the drawing process and cured using ultraviolet light, finally obtaining the dual-air-hole solid-core polarization-maintaining optical fiber of the present invention.

[0036] In the dual-air-hole solid-core polarization-maintaining optical fiber of the present invention, two circular air holes that are close enough to the core are used to replace the stress rod, reducing the dependence on the stress rod. At the same time, using two air holes can not only ensure a large refractive index difference in the direction orthogonal to the core, with a birefringence coefficient of 10 -4 in the working wavelength order of magnitude, ensuring the polarization-maintaining performance and having a small bending loss. The overall outer diameter of the optical fiber is 40 μm, and this thin-diameter polarization-maintaining optical fiber is suitable for applications in small-sized precision devices. The method for preparing the dual-air-hole solid-core polarization-maintaining optical fiber of the present invention is feasible in technology and is easy to precisely control and draw.

[0037] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Those of ordinary skill in the art can modify the technical solutions described in the present invention, or perform equivalent replacements for some of the technical features, and these modifications or replacements do not cause the corresponding technical solutions to deviate from the scope described in the present invention.

Claims

1. A double air hole solid core polarization maintaining optical fiber, characterized in that: The optical fiber comprises a quartz cladding (1), a circular fiber core (3) and two circular air holes (2); the quartz cladding (1) fills the area between the circular fiber core (3) and the circular air holes (2); the circular fiber core (3) is located at the center of the optical fiber, and the two circular air holes (2) are placed horizontally with respect to the circular fiber core (3); the material of the circular fiber core (3) is quartz glass doped with GeO2.

2. A double air hole solid core polarization maintaining optical fiber according to claim 1, characterized in that: The diameter D2 of the circular air hole (2) satisfies 10 μm≤D2≤17 μm.

3. A double air hole solid core polarization maintaining optical fiber according to claim 1, characterized in that: The diameter D3 of the circular fiber core (3) satisfies 2.5 μm≤D3≤3.5 μm.

4. A double air hole solid core polarization maintaining optical fiber according to claim 1, characterized in that: The outer diameter D1 of the quartz cladding (1) is 40 μm.

5. A double air hole solid core polarization maintaining optical fiber according to claim 1, characterized in that: The distance L between the circular fiber core (3) and the circular air hole (2) is ≤0.5 μm.

6. A double air hole solid core polarization maintaining optical fiber according to claim 1, characterized in that: The working wavelength of the optical fiber is 820nm, and the birefringence coefficient can reach 10 -4 Magnitude.

7. A method for preparing an optical fiber as claimed in claim 1, characterized in that The following steps are involved: First, the parameters of the optical fiber are proportionally enlarged to design appropriate parameters of the optical fiber preform; second, the GeO2-doped quartz glass rod, the thin-walled hollow tube, and the quartz capillary rod are arranged in an outer sleeve according to the designed structure by using a stacking method, and then the optical fiber preform is formed by high-temperature treatment; third, the optical fiber preform is placed in an optical fiber drawing tower and heated to 1400-2000°C for high-temperature melt drawing to obtain the optical fiber as claimed in claim 1.

8. The method for preparing an optical fiber according to claim 7, characterized in that: The outer diameter of the outer sleeve is 15 mm.

9. The method for preparing an optical fiber according to claim 7, characterized in that: The thin-walled hollow tube has a duty ratio of 34 / 38 and a diameter of 1-1.4 mm.

10. The method for preparing an optical fiber according to claim 7, characterized in that: The diameter of the quartz capillary rod is 0.2-4.5 mm.

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