阅读说明：本技术 一种保偏光纤及其制备方法 (Polarization maintaining optical fiber and preparation method thereof ) 是由 徐律 缪振华 冯术娟 赵霞 侯树虎 卞新海 韩婷婷 于 2019-09-25 设计创作，主要内容包括：一种保偏光纤及其制备方法,包括位于中心的纤芯,所述纤芯的截面为椭圆形；所述纤芯的外侧为包围所述纤芯的环状内包层,所述内包层的外侧为截面为矩形的应力区；所述应力区的外侧为与应力区的形状相匹配的截面为环形的外包层；所述外包层的外侧为包层；所述纤芯、内包层、应力区、外包层以及包层均同心设置；并且应力区所包围的部分的截面面积与光纤截面的面积之比小于10％。该保偏光纤及其制备方法,将纤芯设计成椭圆型,使光纤同时具备几何型双折射以及应力型双折射,两者现象叠加,可以在保证同等的双折射情况下,继续减少应力区面积,优化其温度稳定性。且椭圆度也不需要太大既能保持同等的双折射,不影响光纤光学参数。(A polarization maintaining optical fiber and a preparation method thereof comprise a fiber core positioned in the center, wherein the section of the fiber core is elliptical; the outer side of the fiber core is an annular inner cladding surrounding the fiber core, and the outer side of the inner cladding is a stress region with a rectangular cross section; the outer side of the stress region is an outer cladding layer which is matched with the stress region in shape and has an annular cross section; the outer side of the outer cladding layer is a cladding layer; the fiber core, the inner cladding, the stress region, the outer cladding and the cladding are all concentrically arranged; and the ratio of the cross-sectional area of the portion surrounded by the stress region to the area of the cross-section of the optical fiber is less than 10%. According to the polarization maintaining optical fiber and the preparation method thereof, the fiber core is designed into an elliptical shape, so that the optical fiber has geometric birefringence and stress birefringence simultaneously, the two phenomena are superposed, the area of a stress area can be continuously reduced under the condition of ensuring the same birefringence, and the temperature stability of the optical fiber is optimized. And the ovality is not required to be too large, the equal birefringence can be kept, and the optical parameters of the optical fiber are not influenced.)

1. A polarization maintaining optical fiber, comprising: comprises that

The fiber core (1) is positioned in the center, and the cross section of the fiber core (1) is oval;

the outer side of the fiber core (1) is provided with an annular inner cladding (2) surrounding the fiber core, and the outer shape of the inner cladding is an ellipse matched with the cross-sectional shape of the fiber core (1);

the outer side of the inner cladding (2) is a stress region (3) with a rectangular cross section;

the outer side of the stress region (3) is provided with an outer cladding layer (4) which is matched with the stress region in shape and has an annular cross section;

the outer side of the outer cladding (4) is provided with a cladding (5);

the cross section of the cladding (5), namely the cross section of the whole optical fiber, is circular;

the fiber core (1), the inner cladding (2), the stress region (3), the outer cladding (4) and the cladding (5) are all concentrically arranged;

and the ratio of the cross-sectional area of the portion surrounded by the stress region (3) to the area of the cross-section of the optical fiber is less than 10%.

2. The polarization maintaining optical fiber of claim 1, wherein: the cross section of the inner cladding (2) is in a ring shape with uniform thickness; the cross section of the outer cladding (4) is in a ring shape with uniform thickness.

3. The polarization maintaining optical fiber of claim 1, wherein:

the long axis (D1) of the fiber core (1) is 5um-8um, and the short axis (D2) is 3um-6 um;

the thickness of the inner cladding (2) is 0.5um-1.2 um;

the stress region (3) has a length (L1) of 20-25 μm and a width (L2) of 7-10 μm;

the thickness of the outer cladding layer (4) is 0.5-1.2 um;

the diameter of the whole optical fiber is 50um-70 um.

4. The polarization maintaining optical fiber of claim 1, wherein: the fiber core is made of SiO ₂、GeO ₂And F, a homogeneous doping scheme is adopted, wherein SiO ₂80 to 95 percent of GeO ₂5 to 20 percent of the total mol percent of the catalyst, and 0.5 to 2 percent of the total mol percent of the F;

the inner cladding comprises SiO ₂、GeO ₂And F, a homogeneous doping scheme is adopted, wherein SiO ₂95 to 98 percent of GeO ₂The mol percent of the F is 0.1 to 2 percent, and the mol percent of the F is 0.1 to 5 percent;

the stress region has SiO as its component ₂、GeO ₂And B ₂O ₃By homogeneous doping, wherein SiO ₂In mole percent of SiO ₂Accounting for 64 to 80 percent of the mol percentage, GeO ₂0.1-2% of the total mole percentage, B ₂O ₃The molar percentage of the compound is 20 to 35 percent;

the outer cladding layer has SiO composition ₂P and F, by homogeneous doping, wherein SiO ₂96 to 98 percent of the total mol percentage, 0.1 to 3 percent of P and 0.1 to 2 percent of F.

5. A method for preparing a polarization maintaining optical fiber is characterized by comprising the following steps:

1) base pipe pretreatment;

2) depositing an outer cladding layer on the inner side of the base tube;

3) depositing a stress area on the inner side of the outer cladding;

4) directionally etching the stress region so that the stress region forms two separate halves;

5) depositing an inner cladding layer on the inner side of the stress region;

6) depositing a fiber core on the inner side of the inner cladding;

7) positively collapsing a base pipe;

8) reversely collapsing the base tube to manufacture a solid polarization maintaining rod;

9) polishing the polarization maintaining rod to prepare a core rod;

10) and drawing the core rod into the optical fiber.

6. The method of claim 5, wherein: in the step 1), the base tube is subjected to acid washing and preheating, and impurities and bubbles on the inner wall of the base tube are removed.

7. The method of claim 5, wherein: the directional etching in the step 4) is to arrange oxyhydrogen metal lamp caps at two ends of the base tube to generate heat sources, move the lamp caps along the axial direction of the base tube, and introduce SF into the base tube ₆So that the inner wall of the tube is subject to corrosion reaction and the stress area is gradually etched.

8. The method of claim 5, wherein: the number of times of forward collapsing in the step 7) is 4-6 times, and the number of times of reverse collapsing in the step 8) is 1-2 times.

Technical Field

The invention relates to a polarization maintaining optical fiber and a preparation method thereof.

Background

Polarization maintaining optical fibers, i.e., polarization maintaining optical fibers, are used for transmitting linearly polarized light and are widely used in various fields of national economy, such as aerospace, aviation, navigation, industrial manufacturing technology, communication and the like. In an interference type optical fiber sensor based on optical coherent detection, the polarization maintaining optical fiber is used to ensure that the linear polarization direction is unchanged, and the coherent signal-to-noise ratio is improved, so that high-precision measurement of physical quantity is realized; the polarization maintaining fiber is used as a special fiber, is mainly applied to sensors such as fiber current transformers, fiber optic gyroscopes and fiber optic hydrophones and fiber optic communication systems such as DWDM and EDFA, and is a special fiber type with wide application value.

The mechanism of birefringence of a polarization maintaining fiber mainly refers to thermal stress from the inside of a material and mechanical stress from the outside of the material, and birefringence is generated by a photoelastic effect, which is a change in refractive index of the material caused by stress. The design principle is that stress is applied to the optical fiber core area, and the polarization-maintaining optical fiber product comprises a stress double-refraction polarization-maintaining optical fiber and a geometric double-refraction polarization-maintaining optical fiber.

The example of the geometric birefringence polarization maintaining optical fiber is an elliptical core polarization maintaining optical fiber, the core is made into an ellipse, the circular symmetry of the optical fiber is damaged, the birefringence of the optical fiber is improved, the phase velocity difference of two orthogonal polarization modes is increased, and the polarization maintaining effect is achieved. Because of no stress region structure, the temperature stability of the optical fiber is better, however, the birefringence of the geometric polarization-maintaining optical fiber is generally not high, and the geometric polarization-maintaining optical fiber is not suitable for being used in an optical fiber gyroscope during interference.

The stress double refraction polarization maintaining fiber can be divided into various structures such as panda type, bow tie type, I shape and the like according to different structures of stress applying areas. The optical fiber is characterized in that a stress area with a high expansion coefficient is introduced into the cladding of the optical fiber to extrude the fiber core to generate a double refraction effect. The birefringence of the fiber can be greatly increased by introducing a boron material of high expansion coefficient as the most stressed region. However, the stress of boron on the fiber core also changes with the change of temperature, and further influences the temperature stability of the fiber-optic gyroscope.

During the preparation process of the panda type polarization maintaining fiber, mechanical punching needs to be carried out on two sides of a fiber core, and then a stress rod is inserted. In order to avoid damaging the fiber core in the punching process, the punching position cannot be too close to the fiber core; to provide sufficient birefringence, the stress region area must be increased appropriately. The temperature stability of the polarization maintaining fiber is affected by the overlarge proportion (about 20%) of the stress area in the sectional area of the fiber, so the panda type polarization maintaining fiber can only be applied to a medium-low precision fiber-optic gyroscope at present.

The I-shaped polarization maintaining optical fiber has the stress area closer to the fiber core, so that high birefringence can be realized only by the smaller stress area, and the I-shaped polarization maintaining optical fiber can be applied to a high-precision optical fiber gyroscope.

Disclosure of Invention

The invention aims to provide an elliptic core linear polarization maintaining optical fiber which ensures the birefringence performance of the optical fiber and improves the temperature stability of the optical fiber.

The technical scheme adopted by the invention for solving the technical problems is as follows: a polarization maintaining optical fiber comprises

The fiber core is positioned in the center, and the cross section of the fiber core is oval;

the outer side of the fiber core is provided with an annular inner cladding surrounding the fiber core, and the shape of the inner cladding is an ellipse matched with the cross section shape of the fiber core;

the outer side of the inner cladding is a stress region with a rectangular cross section;

the outer side of the stress region is an outer cladding layer which is matched with the stress region in shape and has an annular cross section;

the outer side of the outer cladding layer is a cladding layer;

the cross section of the cladding, namely the cross section of the whole optical fiber, is circular;

the fiber core, the inner cladding, the stress region, the outer cladding and the cladding are all concentrically arranged;

and the ratio of the cross-sectional area of the portion surrounded by the stress region to the area of the cross-section of the optical fiber is less than 10%.

Preferably, the cross section of the inner cladding is annular with uniform thickness; the cross section of the outer cladding layer is in a ring shape with uniform thickness.

Preferably, the long axis of the fiber core is 5um-8um, and the short axis is 3um-6 um;

the thickness of the inner cladding is 0.5um-1.2 um;

the length of the stress area is 20-25 μm, and the width is 7-10 μm;

the thickness of the outer cladding layer is 0.5-1.2 um;

the diameter of the whole optical fiber is 50um-70 um.

Preferably, the fiber core comprises SiO2, GeO2 and F, and adopts a homogeneous doping design, wherein the molar percentage of SiO2 is 80-95, the molar percentage of GeO2 is 5-20, and the molar percentage of F is 0.5-2;

the inner cladding comprises SiO2, GeO2 and F, and adopts a homogeneous doping design, wherein the molar percentage of SiO2 is 95-98, the molar percentage of GeO2 is 0.1-2, and the molar percentage of F is 0.1-5;

the stress region comprises SiO2, GeO2 and B2O3, and adopts a homogeneous doping design, wherein SiO2 accounts for 64-80 mol%, GeO2 accounts for 0.1-2 mol%, and B2O3 accounts for 20-35 mol%;

the outer cladding layer comprises the components of SiO2, P and F, and adopts a homogeneous doping design, wherein the molar percentage of SiO2 is 96-98, the molar percentage of P is 0.1-3, and the molar percentage of F is 0.1-2.

The preparation method of the polarization maintaining optical fiber is characterized by comprising the following steps:

1) base pipe pretreatment;

2) depositing an outer cladding layer on the inner side of the base tube;

3) depositing a stress area on the inner side of the outer cladding;

4) directionally etching the stress region so that the stress region forms two separate halves;

5) depositing an inner cladding layer on the inner side of the stress region;

6) depositing a fiber core on the inner side of the inner cladding;

7) positively collapsing a base pipe;

8) reversely collapsing the base tube to manufacture a solid polarization maintaining rod;

9) polishing the polarization maintaining rod to prepare a core rod;

10) and drawing the core rod into the optical fiber.

Preferably, in the step 1), the substrate tube is subjected to acid washing and preheating to remove impurities and bubbles on the inner wall of the substrate tube.

Preferably, the directional etching in the step 4) is to arrange oxyhydrogen metal lamp caps at two ends of the base tube to generate heat sources, move the lamp caps back and forth along the length direction of the base tube, and introduce SF into the base tube ₆So that the inner wall of the tube is subject to corrosion reaction and the stress area is gradually etched.

Preferably, the number of forward collapsing passes in the step 7) is 4 to 6, and the number of reverse collapsing passes in the step 8) is 1 to 2.

Compared with the prior art, the polarization maintaining optical fiber and the preparation method thereof have the advantages that the fiber core is designed into an elliptical shape, so that the optical fiber has geometric birefringence and stress birefringence simultaneously, the two phenomena are superposed, the area of a stress area can be continuously reduced under the condition of ensuring the same birefringence, and the temperature stability of the polarization maintaining optical fiber is optimized. The ovality of the elliptical core can be flexibly adjusted by regulating and controlling the pressure in the pipe and the collapse speed in the collapse process. Under the condition of reducing the occupied cross section ratio of the stress area, and due to the superposition of the elliptical core effect, the birefringence effect cannot be weakened, and the temperature stability of the optical fiber is greatly improved. And the ovality is not required to be too large, the equal birefringence can be kept, and the optical parameters of the optical fiber are not influenced.

Drawings

FIG. 1 is a schematic cross-sectional view of a polarization maintaining optical fiber according to an embodiment of the present invention;

FIG. 2 is a flow chart of the preparation of a polarization maintaining optical fiber according to an embodiment of the present invention.

Fig. 3 is a schematic diagram of a deposition process of a polarization maintaining optical fiber according to an embodiment of the present invention, fig. 3a is a schematic diagram after an outer cladding layer is deposited, fig. 3b is a schematic diagram after a stress region is deposited, fig. 3c is a schematic diagram after a stress region is etched, fig. 3d is a schematic diagram after an inner cladding layer is deposited, and fig. 3e is a schematic diagram after a fiber core is deposited.

FIG. 4 is a refractive index profile of a cross-section of a polarization maintaining fiber according to an embodiment of the present invention.

Detailed Description

The invention is described in further detail below with reference to the embodiments of the drawing, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention.

The terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless specifically defined otherwise.

In the present invention, unless otherwise expressly stated or limited, the terms "mounted," "connected," "secured," and the like are to be construed broadly and can, for example, be fixedly connected, detachably connected, or integrally formed; can be mechanically or electrically connected; either directly or indirectly through intervening media, either internally or in any other relationship. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.

As shown in fig. 1, a cross-sectional view of the polarization maintaining fiber includes a fiber core 1 located at the center, the cross-section of the fiber core 1 is elliptical, the major axis D1 of the fiber core 1 is 5um to 8um, the minor axis D2 is 3um to 6um, the outer side of the fiber core 1 is provided with an inner cladding 2, the inner cladding 2 surrounds the fiber core 1, the outer shape of the inner cladding 2 is elliptical matching with the cross-sectional shape of the fiber core 1, and therefore the inner cladding 2 is in a ring structure with the same width, and the cross-section of the overall shape of the fiber core composed of the inner cladding 2 and the fiber core 1 is matched with the fiber core 1 and is elliptical. The thickness D3 of the inner cladding 3 is 2um-5 um. The outer side of the inner cladding 2, i.e. the outer side of the core, is a stress region 3, the cross-sectional shape of the stress region 3 is a straight strip, i.e. a rectangle, the length L1 is 20 μm to 25 μm, the width L2 is 7 μm to 10 μm, and the area of the portion surrounded by the stress region 3, i.e. the area ratio of the total cross-sectional area of the core and the stress region in the stress region to the cross-sectional area of the optical fiber is less than or equal to 10%. The outer cladding layer 4 is located outside the stress region 3 and is shaped to match the stress region 3, the outer cladding layer 4 being annular and of uniform thickness and being about 0.5um to 1.2um thick, the outer cladding layer 4 being a cladding layer 5 outside the cladding layer 4, the cladding layer diameter D4 being 50um to 70 um. The centers of the fiber core, the inner cladding, the stress region, the outer cladding and the cladding are all superposed, namely concentrically arranged.

In the polarization maintaining optical fiber, the fiber core 1 comprises SiO2, GeO2 and F, and adopts a homogeneous doping design, wherein the SiO2 accounts for 80-95 mol%, the GeO2 accounts for 5-20 mol%, and the F accounts for 0.5-2 mol%; the refractive index difference delta 1 between the fiber core 1 and the pure quartz glass is 0.0120-0.0165; the inner cladding 2 comprises the components of SiO2, GeO2 and F, and adopts a homogeneous doping design, wherein the molar percentage of SiO2 is 95-98%, the molar percentage of GeO2 is 0.1-2%, and the molar percentage of F is 0.1-5%; the refractive index difference delta 2 between the inner cladding 2 and the pure quartz glass is-0.0005 to-0.0025; the stress region 3 comprises SiO2, GeO2 and B2O3, and adopts a homogeneous doping design, wherein SiO2 accounts for 64-80 mol%, GeO2 accounts for 0.1-2 mol%, and B2O3 accounts for 20-35 mol%; the refractive index difference delta 3 between the stress area 3 and the pure quartz glass is-0.0150 to-0.0195. The outer cladding layer 4 comprises the components of SiO2, P and F, and adopts a homogeneous doping design, wherein the mole percentage of SiO2 is 96-98%, the mole percentage of P is 0.1-3%, and the mole percentage of F is 0.1-2%; the outer cladding 4 has an index of refraction almost equal to that of pure quartz glass.

FIG. 4 is a schematic diagram showing the difference between refractive indexes.

The refractive index n0 of the pure quartz glass material is 1.457, the refractive index n1 of the core, the refractive index n2 of the inner cladding and the refractive index n3 of the stress region;

Δ ₁＝n ₁-n ₀；Δ ₂＝n ₂-n ₀；Δ ₃＝n ₃-n ₀。

the preparation method of the elliptical core I-shaped polarization maintaining optical fiber comprises the following steps:

(1) pretreatment of the substrate tube 10: the acid cleaning of the base tube and the preheating of the base tube effectively eliminate the impurities and the bubbles on the inner wall of the base tube. This substrate tube is the cladding 5 in the above structure.

(2) Depositing an outer cladding layer 4, the outer cladding layer 4 being of SiO composition ₂P and F, by homogeneous doping, wherein SiO ₂The mol percentage of P is 92-98%, the mol percentage of P is 0.1-2%, and the mol percentage of F is 0.1-2%. The substrate tube 10 after deposition is provided with an outer cladding 4 as shown in figure 3 a.

(3) Depositing a stress region 3, wherein the stress region is SiO ₂、GeO ₂And B ₂O ₃By homogeneous doping, wherein SiO ₂In mole percent of SiO ₂Occupied mole percentageThe percentage of the component is 64 percent to 80 percent, and GeO ₂0.1-2% of the total mole percentage, B ₂O ₃The mol percentage of the compound is 20 to 35 percent. The cross section ratio of the area of the stress region can be regulated and controlled by changing the number of deposited layers of the stress region. The structure after the stress region is deposited is shown in fig. 3 b.

(4) Directional etching: after the stress area deposition process is finished, directional etching is carried out, and the size of an etching light spot is adjusted to 1/4 of the diameter of the base tube 10 by adjusting the flow rate and the moving speed of oxyhydrogen metal lamp caps symmetrically arranged at two ends of the base tube. Generating a heat source by etching the lamp head, moving the lamp head longitudinally along the base tube, and introducing SF into the base tube ₆The inner wall of the tube is subjected to a corrosion reaction, and the stress area is gradually etched until the stress area is completely separated into two halves. Because the etching lamp holders on the two sides cannot be completely consistent, in order to ensure the symmetry of etching, the etching is divided into a plurality of times, and the lamp holders are turned over by 180 degrees every time of etching, and the other side of the base tube is etched. Since the stress region contains boron, flow will occur during etching, the stress region 3 after etching, as shown in fig. 3 c.

(5) Depositing an inner cladding layer: the material composition of the inner cladding is SiO ₂、GeO ₂F, adopting a homogeneous doping design, wherein SiO ₂95 to 98 percent of GeO ₂The mol percentage of the F is 0.1 to 2 percent, and the mol percentage of the F is 0.1 to 5 percent. As shown in fig. 3 d.

(6) Deposition of core, the composition of core 1 being SiO ₂、GeO ₂And F, a homogeneous doping scheme is adopted, wherein SiO ₂80 to 95 percent of GeO ₂5 to 20 percent of the total mol percent of the catalyst, and 0.5 to 2 percent of the total mol percent of the F. As shown in fig. 3 e.

(7) The base pipe is positively collapsed for 4-6 times, and the inner cladding is easily extruded and deformed due to small viscosity of a stress area and can be driven, so that the shape of the fiber core is changed. The smaller the pressure in the tube, the faster the lamp head speed, and the larger the ellipticity of the fiber core. Controlling the pressure in the tube to be 0.40-0.60 torr according to different ovalities; the speed of the lamp head is 15mm/min-20mm/min, and the forward collapse is carried out for multiple times so as to ensure that the overall shape of the base tube is circular.

(8) Reversely collapsing, controlling the pressure in the tube to be 0.10-0.30 torr and the lamp holder speed to be 6-10 mm/min, and manufacturing the solid polarization maintaining rod with the stress area in a straight shape.

(9) Polishing the polarization maintaining rod to obtain a core rod; as shown in fig. 1.

(10) And drawing the fiber drawing tower on the core rod to manufacture the elliptical core I-shaped polarization maintaining fiber.

The properties of the fiber are shown in table 1: