A boron-germanium co-doped polarization maintaining photosensitive optical fiber and a preparation method thereof

Abstract

The application discloses a boron-germanium co-doped polarization maintaining photosensitive optical fiber and a preparation method thereof, and belongs to the technical field of optical fibers. The boron-germanium co-doped polarization maintaining photosensitive optical fiber is prepared by co-doping boron oxide and germanium oxide in the fiber core, preferably setting the co-doping combination ratio of the two, and setting a fluorine-doped layer between the fiber core and the cladding. On the basis of ensuring the forming quality of the optical fiber, the boron-germanium co-doped polarization maintaining photosensitive optical fiber can meet the use requirements of photosensitivity and the use requirements of the birefringence of the polarization maintaining optical fiber. The boron-germanium co-doped polarization maintaining photosensitive optical fiber has a simple structure design and a convenient preparation method, and can quickly complete the design and preparation of the polarization maintaining photosensitive optical fiber. The optical fiber can meet the requirements of photosensitivity and the requirements of birefringence performance, simplifies the preparation process of the photosensitive optical fiber, effectively expands the application range of the polarization maintaining optical fiber, reduces the manufacturing cost of the optical fiber grating, and has good practical value and application prospect.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber technology, specifically relating to a boron-germanium co-doped polarization-maintaining optical fiber and its preparation method. Background Technology

[0002] In the field of fiber optic sensing and communication, fiber Bragg gratings have been widely used due to their advantages such as small size, low insertion loss, and simple structure.

[0003] Fiber Bragg gratings are fabricated based on the photosensitivity of fiber optic materials. Specifically, the interaction between incident photons and germanium ions in the fiber core causes a permanent change in refractive index, forming a spatial phase grating within the fiber core. Generally, the ultraviolet photosensitivity in germanium-doped fibers is limited. To improve the photosensitivity, the usual method is to increase the germanium doping concentration, thereby increasing germanium-related defects. However, photosensitive fibers obtained through this method often have excessively high refractive indices, leading to incompatibility with conventional standard communication fiber parameters and excessive joint loss during use. These drawbacks significantly limit the applicability of photosensitive fibers and restrict their widespread application.

[0004] As a special type of single-mode fiber, polarization-maintaining fiber (PSB) enhances the fiber's inherent birefringence by introducing sufficient stress on both sides of the fiber core, overcoming the influence of external factors on the fiber's polarization state during transmission, thereby achieving its polarization-maintaining performance. Due to its strong polarization-maintaining capability and good compatibility with ordinary single-mode fibers, PSB is widely used in aerospace, aviation, marine, industrial manufacturing, and communications fields. Common PSB fiber types used in PSB gratings include panda-type, bow-tie-type, and elliptical-clad type PSB. Panda-type PSB is currently the main type of PSB used in China due to its excellent polarization-maintaining characteristics, structural geometric symmetry, and longitudinal uniformity.

[0005] With the increasing demand for fiber optic functionality, more and more multifunctional optical fibers are attracting widespread attention and research from researchers, including polarization-maintaining fibers with photosensitive properties. In the fabrication of conventional polarization-maintaining fiber gratings, a step-index fiber profile is typically used, with a pure silicon cladding and germanium doping in the core to increase its refractive index and achieve light guiding. During grating fabrication, hydrogen loading technology is used to further enhance the fiber's sensitivity. The entire fabrication process is complex and requires extremely high precision control at each stage, resulting in high manufacturing costs for polarization-maintaining photosensitive fibers, making it difficult to meet the demands of large-scale, practical applications. Furthermore, due to limitations in the fabrication process, the content of doping elements in traditional polarization-maintaining photosensitive fibers cannot be precisely controlled, leading to significant performance fluctuations after fabrication and unreliable product performance, thus affecting product application and promotion. Summary of the Invention

[0006] In view of one or more of the above-mentioned defects or improvement needs of the prior art, the present invention provides a boron-germanium co-doped polarization-maintaining optical fiber and its preparation method, which can realize the rapid and accurate preparation of polarization-maintaining optical fiber, while meeting the requirements for optical fiber photosensitivity and birefringence performance, and reducing the manufacturing cost and application cost of fiber Bragg grating.

[0007] To achieve the above objectives, one aspect of the present invention provides a boron-germanium co-doped polarization-maintaining optical fiber, comprising a core located in the middle and a cladding disposed on the outer periphery of the core, wherein two circular stress zones are symmetrically arranged in the cladding with respect to the center of the core. The fiber core is doped with boron oxide and germanium oxide, with germanium oxide concentration of 10%~15% and boron oxide concentration of 5%~10%, resulting in a polarization-maintaining optical fiber NA value of 0.1~0.2 and a relative refractive index difference Δ1 value of 0.8%~1%; and The outer periphery of the fiber core is further covered with a fluorine-doped layer of a certain thickness; the diameter of the fluorine-doped layer is... d 2 and the diameter of the fiber core d The ratio of 1 to 1 is not greater than 5.0, and the molar percentage of fluorine doping concentration in the fluorine-doped layer is limited to below 1.5, so that the relative refractive index difference Δ2 of the fluorine-doped layer is -0.12% to -0.2%.

[0008] As a further improvement of the present invention, the boron doping concentration in the stress region is 20%~25%.

[0009] As a further improvement of the present invention, the distance from the edge of the stress zone to the center of the fiber core r With core radius a The ratio is not less than 2.

[0010] As a further improvement of the present invention, the diameter of the fluorine-doped layer d 2 and the diameter of the fiber core d The ratio of 1 is between 2.5 and 5.0.

[0011] Another aspect of the present invention provides a method for preparing a boron-germanium co-doped polarization-maintaining optical fiber, which includes the following preparation process: (1) Conduct single-doped flow rate opening tests for boron and germanium respectively to determine the linear relationship between the flow rate opening of reactant raw materials and refractive index and doping amount; (2) Based on the results of the single-doped flow rate opening test in (1) and the design specifications of the optical fiber, determine the doping concentration of germanium oxide and boron oxide in the core layer, and complete the design of the core rod size specifications. (3) Based on the results of the single-doped flow rate opening test in (1) and the doping concentration determined in (2), determine the flow rate opening of the reactant raw materials of boron oxide and germanium oxide, and sequentially complete the deposition of pure silicon, fluorine-doped inner cladding, and boron-germanium co-doped core layer to obtain a core rod, and then melt and shrink the core rod. (4) Deposition preparation of boron-doped stress rods; (5) Select a suitable combination of quartz sleeve and mandrel, and melt, shrink, stretch and cut to obtain a solid rod of the corresponding size; (6) Drill holes at symmetrical positions on both sides of the solid rod core area according to the design dimensions, and insert the stress rods made into the two holes to form a preformed rod to be drawn. (7) Use the obtained preform to draw polarization-maintaining optical fiber.

[0012] As a further improvement of the present invention, in the preparation process (3), the deposition preparation process parameters include a low deposition rate of 0.5 g / min ~ 1 g / min, a temperature of less than 1200℃, and a gas pressure range of 10 ~ 20 mbar.

[0013] As a further improvement of the present invention, in the preparation process (3), the process control parameters for mandrel melting and shrinking include low speed of 8mm / min~10mm / min, low power of 5kw~10kw, and maintaining a slight positive pressure of 10~30mbar for oxygen in the tube.

[0014] As a further improvement of the present invention, in the preparation process (7), the fiber drawing process control includes a drawing speed of 450 m / min to 550 m / min, a drawing temperature of 1900℃ to 2100℃, and a drawing tension of 100g to 200g.

[0015] As a further improvement of the present invention, in the preparation process (1), the single-doped flow rate opening test includes: Using the process parameters of mandrel deposition in preparation process (3), single doping deposition of boron oxide and germanium oxide was carried out respectively. GeCl4 and BCl3 were deposited at different flow rates. The flow rates of the two single doped materials were adjusted from low to high values ​​to obtain single doped mandrels at different flow rates. By detecting the mandrels, the deposition amount of single doping of the materials at the corresponding flow rates and the relationship between the flow rate and the refractive index of the mandrel were obtained.

[0016] As a further improvement of the present invention, in the preparation process (5), the selection process of the quartz sleeve is completed by the following formula:

[0017] In the formula, CSA 套管 To select the cross-sectional area of ​​the sleeve; d2 a and D These refer to the core layer diameter of the core rod, the fiber core diameter, and the core rod diameter, respectively.

[0018] The aforementioned improved technical features can be combined with each other as long as they do not conflict with each other.

[0019] In summary, the beneficial effects of the above-described technical solutions conceived by this invention compared with the prior art include: (1) The boron-germanium co-doped polarization-maintaining optical fiber of the present invention, by co-doping boron oxide and germanium oxide in the fiber core and preferably setting the co-doping concentration of the two, can make the prepared optical fiber meet both the photosensitive requirements and the birefringence requirements of polarization-maintaining optical fiber, effectively adapting to the use requirements of polarization-maintaining optical fiber in specific application scenarios. Without hydrogen loading, the preparation of polarization-maintaining optical fiber can be completed quickly, reducing the preparation cost of polarization-maintaining optical fiber and reducing the preparation cost and application cost of polarization-maintaining optical fiber.

[0020] Meanwhile, by setting a fluorine-doped inner cladding between the cladding and the core, direct coupling between the cladding mode and the core can be suppressed in terms of fiber performance. In terms of manufacturing, fluorine doping allows for viscosity matching between the cladding and the core, reducing the viscosity difference between them and avoiding problems such as core cracking and significant fiber attenuation caused by large residual stress differences between the boron-doped core and the cladding. Compared to conventional fluorine-doped layer designs, the viscosity of boron-doped silica glass is significantly lower than that of conventional germanium-doped cores due to the boron oxide doping in the core. Therefore, a higher fluorine concentration doping design ensures better viscosity matching between the cladding and the boron-germanium co-doped core, achieving higher reliability and yield in fiber drawing. (2) The boron-germanium co-doped polarization-maintaining optical fiber of the present invention, by symmetrically setting circular stress regions in the cladding on both sides of the fluorine-doped layer, and by optimizing the size of the stress regions and the boron doping concentration in the stress regions, can effectively meet the birefringence performance of the optical fiber, and fully avoid excessive stress caused by excessive boron, thus avoiding the breakage of the core rod and even the optical fiber, and meeting the requirements of the final beat length and crosstalk of the optical fiber. At the same time, by optimizing the setting position of the stress regions, the ratio of the distance from the edge of the stress region to the center of the fiber core to the radius of the fiber core is not less than 2, and the ratio of the diameter of the fluorine-doped layer to the diameter of the fiber core is limited to 2.5~5.0, which can better guarantee the birefringence performance of the optical fiber, reduce the attenuation of the optical fiber performance, and also fully consider the difficulty of the forming process of the preform and the optical fiber.

[0021] (3) The method for preparing boron-germanium co-doped polarization-maintaining optical fiber of the present invention can accurately determine the linear relationship between the raw material flow rate and refractive index and doping amount by designing a single doping flow rate opening test, thereby accurately determining the doping concentration of the doped material and the flow rate opening during deposition preparation under the corresponding optical fiber specifications, ensuring the accuracy of boron-germanium doping control in the preform and optical fiber, improving the product performance of the optical fiber, ensuring the yield of polarization-maintaining optical fiber preparation, and reducing the preparation cost and application cost of polarization-maintaining optical fiber.

[0022] (4) The method for preparing boron-germanium co-doped polarization-maintaining optical fiber of the present invention optimizes the deposition process, melting and shrinking process and drawing process of the core rod, effectively matching the product design characteristics of polarization-maintaining optical fiber, reducing the core rod defects that may be caused by inaccurate process control during the core rod preparation process, ensuring the roundness and final quality of the core rod, and fully improving the quality of the optical fiber after drawing.

[0023] (5) The boron-germanium co-doped polarization-maintaining photosensitive fiber and its preparation method of the present invention utilize the co-doping of boron and germanium elements in the fiber core, combined with the corresponding setting of the fluorine-doped layer on the outer periphery of the fiber core and the two symmetrical stress regions on the outer periphery of the fluorine-doped layer, to realize the corresponding setting of the panda-type polarization-maintaining fiber. Combined with the optimal design of the doping ratio control parameters such as the doping amount of boron oxide and germanium oxide in the fiber core, the molar percentage of fluorine doping concentration in the fluorine-doped layer, and the doping amount of boron in the stress region, a boron-germanium co-doped polarization-maintaining photosensitive fiber that meets both the photosensitive requirements and the birefringence performance requirements can be obtained, thereby improving the applicability of polarization-maintaining fiber, reducing the manufacturing cost of fiber grating, and having good practical value and application prospects. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the cross-sectional structure of the boron-germanium co-doped polarization-maintaining optical fiber in an embodiment of the present invention. Figure 2 This is a process flow diagram of the preparation of boron-germanium co-doped polarization-maintaining optical fiber in the embodiments of the present invention; Figure 3 This is a design diagram of the refractive index profile of the boron-germanium co-doped polarization-maintaining optical fiber in this embodiment of the invention. Figure 4 This is a graph showing the relationship between the flow rate of reactant GeCl4 and the refractive index in an embodiment of the present invention. Figure 5 This is a graph showing the relationship between the flow rate of reactant BCl3 and the refractive index in an embodiment of the present invention. Figures 6-8 These are test diagrams of the fiber optic gratings in specific embodiments 1-3 of the present invention; In all the accompanying drawings, the same reference numerals denote the same technical features, specifically: 1. Fiber core; 2. Fluorine-doped layer; 3. Cladding layer; 4. Stress zone; 5. Inner coating layer; 6. Outer coating layer. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and 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.

[0026] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0027] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0028] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0029] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0030] Please see Figure 1 In a preferred embodiment of the present invention, the boron-germanium co-doped polarization-maintaining optical fiber includes a core 1 located at the center and a cladding 3 covering the outer periphery of the core 1. A fluorine-doped layer 2 of a certain thickness is formed in the cladding 3 surrounding the core 1. Two circular stress regions 4 are symmetrically arranged around the center of the core 1 in the cladding 3 outside the fluorine-doped layer 2. The line connecting the centers of the two stress regions 4 forms the slow axis of signal transmission on the cross-section of the optical fiber, and the direction perpendicular to the line connecting the two stress regions forms the fast axis of signal transmission on the cross-section of the optical fiber.

[0031] In actual installation, it is preferable to sequentially cover the outer side of the cladding layer 3 with an inner coating layer 5 and an outer coating layer 6.

[0032] In the preferred embodiment of the polarization-maintaining optical fiber, the core 1 is germanium-doped quartz glass. To improve the photosensitivity of the optical fiber, more germanium doping is performed in the core 1, and the refractive index of the core 1 increases with the increase of germanium doping. At the same time, a certain amount of boron doping is performed in the core 1 to increase the photosensitivity of the optical fiber while controlling the refractive index.

[0033] It is understandable that, compared to pure quartz, the refractive index of the resulting material layers changes with variations in doping elements and doping concentration, thus leading to significant differences in the relative refractive index difference Δ between the functional layers. In a preferred embodiment, the relative refractive index difference Δ between the layers is preferably calculated using the following simplified formula:

[0034] In the formula, n 0 is the refractive index of the pure quartz layer (i.e., cladding 3 in the preferred embodiment), which is usually a fixed value; n i This represents the actual refractive index of the corresponding functional layer.

[0035] For example, when calculating the relative refractive index difference Δ1 between core layer 1 and pure quartz, the formula above... n i for n1, which is the refractive index of core layer 1; when calculating the relative refractive index difference Δ2 between fluorine-doped layer 2 and pure quartz, the above formula... n i for n 2, which is the refractive index of the fluorine-doped layer 2.

[0036] Meanwhile, to suppress direct coupling between the cladding mold and the core 1, a matching recessed cladding design is incorporated in the preferred embodiment. This involves a fluorine-doped structure of a certain thickness, namely the fluorine-doped layer 2, located on the outer periphery of the core 1. This design not only suppresses direct coupling between the cladding mold and the core 1 but also reduces the difference through viscosity matching, effectively preventing core cracking caused by a large difference in residual stress between the boron-doped core 1 and the cladding 3.

[0037] In practical applications, Freon is more prone to saturation as a fluorine source, and excessive fluorine content hinders silica deposition, impacting deposition efficiency. Furthermore, a viscosity mismatch between the core and cladding can lead to residual stress and bond breakage, significantly affecting fiber attenuation and causing a substantial increase in attenuation. A viscosity-matched design ensures minimal stress difference while optimizing fiber attenuation. Compared to conventional fluorine-doped layer designs, boron-doped silica glass, with its boron oxide core, exhibits significantly lower viscosity than conventional germanium-doped cores.

[0038] Therefore, in the preferred embodiment, by designing a higher fluorine concentration doping in the outer cladding of the fiber core 1, the viscosity of the cladding 3 and the boron-germanium co-doped fiber core 1 can be better matched.

[0039] More specifically, in the preferred embodiment, the fluorine doping concentration should be as close as possible to the thermal equilibrium molar percentage limit of 1.5, further limiting the refractive index dip depth of the fluorine-doped layer 2 to approximately 0.2%. Furthermore, researchers have found that the smaller the fluorine-doped matching channel, the greater the impact of the liner purity on fiber attenuation. Considering the mechanical properties of the fiber and the requirement that the screening tension meet 100 kpsi, in the preferred embodiment, the design ratio of the fluorine-doped matching channel is preferably no greater than 5.0, i.e., the diameter of the fluorine-doped layer 2... d 2 and the diameter of fiber core 1 d The ratio of 1 to 1 is not greater than 5.0, and is more preferably 2.5 to 5.0.

[0040] More specifically, in the preferred embodiment, the germanium oxide concentration in the core 1 of the polarization-maintaining optical fiber is 10%~15%, the boron oxide concentration is 5%~10%, and the NA (numerical aperture) of the fiber is designed to be 0.1~0.2. Simultaneously, the relative refractive index difference Δ1 of the core 1 is 0.8%~1%, and the relative refractive index difference Δ2 of the fluorine-doped layer 2 is -0.12%~-0.2%. Furthermore, in the preferred embodiment, the boron doping concentration in the stress region 4 is 20%~25%.

[0041] In another aspect of the present invention, a preferred embodiment also provides a method for fabricating a boron-germanium co-doped polarization-maintaining optical fiber, the fabrication process of which is as follows: Figure 2 As shown, a further preferred method includes the following process: I. Preparation process of preforms The actual preparation process of the preform mainly includes the design process of the mandrel, the selection process of the sleeve, the design process of the stress bar, the preparation process of the mandrel, the preparation process of the stress bar, the melting and stretching process of the mandrel, and the drilling and assembly process of the preform.

[0042] (1) Design process of mandrel The polarization-maintaining optical fiber in the preferred embodiment is a special single-mode fiber structure that can be matched with conventional communication optical fibers.

[0043] In a preferred embodiment, the fiber core 1 has a diameter of 8 μm, the cladding 3 has a diameter of 125 μm, the numerical aperture NA is 0.1~0.2, and the cutoff wavelength is 1~1.4 μm. By doping the fiber core with boron and germanium, defects are formed in the glass core under ultraviolet light irradiation, thereby increasing the refractive index.

[0044] The design of step-index single-mode fiber mainly involves controlling the core layer geometry and relative refractive index difference. The fiber specifications typically include mode field diameter (MFD), relative refractive index difference (Δ), and numerical aperture (NA). The relationship between these parameters can be expressed by the following equation:

[0045]

[0046]

[0047] In the formula, n 1, n 0 represents the refractive index of the fiber core and the pure quartz layer, respectively; NA represents the numerical aperture. a Δ is the core diameter; Δ is the relative refractive index difference. V c This is the normalized cutoff frequency, which takes a value of approximately 2.405. λ c The cutoff wavelength; λ This is the operating wavelength. In actual setup, the performance parameters required for optical fibers are usually known, that is, the geometric parameters of the fiber core, the relative refractive index difference, and the fiber specifications are usually known. With the refractive index of the base layer (silica layer) known, the relative refractive index Δ1 of the fiber core can be estimated by reversing the above formula.

[0048] (2) Single-doped flow rate opening test In a preferred embodiment, the deposition and preparation of the preform is preferably carried out using a plasma chemical vapor deposition (PCVD) platform. The preparation principle is as follows: using SiCl4 as the main material and GeCl4 and BCl3 as dopants, the preform is transported into the interior of a quartz liner under high-purity oxygen. Under high temperature, a chemical reaction occurs to form oxide particles, which are deposited on the inner wall of the liner. After melting and shrinking, the core rod of the photosensitive fiber is prepared.

[0049] More specifically, the deposition process mainly proceeds through the following reaction: SiCl4(g)+O2(g) =SiO2(s)+2Cl2(g) GeCl4(g)+O2(g) = GeCl4(s)+2Cl2(g) 4BCl3(g)+3O2(g) =2B2O3(s)+6Cl2(g) For conventional carrier gas deposition methods, the carrier gas flow rates of SiCl4 and BCl3 are generally determined based on their respective flow rates. V i Assuming the carrier gas vapor conforms to the ideal gas law PV=(m / M)RT, the mass flow rate per unit time can be obtained. m i Then, based on the aforementioned deposition reaction, the doping ratio of each component product is obtained.

[0050] However, traditional component doping methods rely heavily on numerous assumptions, and discrepancies exist between actual processes and theoretical calculations, making them unsuitable for fabricating optical fibers with high precision requirements for doping levels. Therefore, in a preferred embodiment, a single-doping flow rate test is designed to accurately reflect the relationship between doping level and flow rate.

[0051] Specifically, in a preferred embodiment, both boron oxide and germanium oxide are doped into the fiber core 1. Since the two substances have different mechanisms of action on the refractive index of the fiber core, to investigate the effects of germanium doping and boron doping on the refractive index respectively, flow rate tests of the two reactants were conducted in the preferred embodiment. Under otherwise unchanged operating conditions, the linear relationship between the flow rate of a single dopant element and the change in refractive index was investigated by controlling the flow rate of that single dopant element.

[0052] During the experiment, for the germanium-doped system, the opening degree of the mass flow controller (MFC) of GeCl4 was changed from 0% to 60% in 10% increments; while for the boron-doped system, the opening degree of the mass flow controller (MFC) of BCl3 was changed from 0% to 60% in 5% increments.

[0053] Furthermore, the results of the flow rate opening test for a single doped element are as follows: Figure 4 , Figure 5 As shown in the figure, based on the experimental results, the linear relationship between the flow rate opening of a single doped element and the refractive index and the element doping concentration can be accurately obtained. It is easy to see from the figure that as the flow rate opening of germanium increases, both the refractive index and the doping concentration increase; in contrast, as the flow rate opening of boron increases, the refractive index decreases while the doping concentration increases.

[0054] Based on the design requirements of the optical fiber, the Δ1 value can be obtained. The addition of GeCl4 forms GeO2, which increases the refractive index; the addition of BCl3 generates B2O3, which both decreases the refractive index and improves photosensitive performance. Therefore, the refractive index of the B / Ge co-doped core layer... n 1 can be represented as: n 1= n Ge - n B By using GeCl4 and BCl3 as dopants to react with high-purity oxygen to generate germanium oxide and boron oxide, the refractive index difference of the core layer can be adjusted, ultimately obtaining the required doping level.

[0055] However, excessive total doping of either compound will inevitably lead to an excessively high decay level, failing to meet practical application requirements. Therefore, a suitable balance needs to be found in the experiment, and the final total doping should not exceed 25%. In a preferred embodiment, the germanium oxide concentration is 10%~15%, and the boron oxide concentration is 5%-10%.

[0056] In the corresponding Figure 4 , Figure 5 In a preferred embodiment, mass flow controllers (MFCs) for GeCl4 and BCl3 are designed with step sizes of 10% and 5%, respectively, and an opening degree ranging from 0% to 60%. Analysis of the linearly doped preform confirms the deposition efficiency of GeCl4 and BCl3 and establishes the relative relationship between doping concentration and refractive index. According to the design principles, the total doping of the core layer refractive index in GeCl4 and BCl3 should not exceed 25%, with a Δ1 value of 0.8% to 1%.

[0057] (3) Selection process of quartz sleeve To obtain suitable preforms for the fiber core and cladding materials, a quartz sleeve with a suitable cross-sectional area (CSA) needs to be selected. Based on the CSA size, the diameter of the core and core layer in the core design can be calculated using the following formula:

[0058] In the formula, CSA 套管 The cross-sectional area of ​​the sleeve is typically a known parameter; for example, in a preferred embodiment, its CSA = 4600 mm².2 ; d 2 a and D These are the core layer diameter of the core rod and the fiber core diameter, respectively. a (where is the radius of the fiber core) and the diameter of the core rod.

[0059] Based on the calculations of the above formula, it can be deduced that the diameter of the core rod in the preferred embodiment is 20~22mm, and the diameter of the core layer of the core rod is 2~4mm.

[0060] (4) Stress bar design process The panda-type polarization-maintaining fiber exerts stress on the fiber core 1 through two symmetrical circular stress regions 4 on both sides of the core 1, affecting the fiber's birefringence performance. Geometrically, the preferred birefringence is expressed as:

[0061] In the formula, L 0 represents a material parameter, which is related to the material composition; R The radius of the stress zone; l This is the distance from the center of the stress zone to the center of the fiber core. c denoted as the cladding radius of the optical fiber.

[0062] As can be seen from the formula, the magnitude of birefringence is related to the geometric parameters and structure of the end face; different... r / a This represents the different distances between stress zone 4 and fiber core 1; among them, r The distance from the edge of the stress zone to the center of the fiber core. a Let be the core radius. According to the formula, the closer stress region 4 is to the core 1, the better the birefringence performance. r / a The smaller the better. However, considering the overall process design, when stress region 4 is close to fiber core 1, the attenuation performance of the optical fiber and the presence of air lines caused by boron diffusion make the process more difficult. Therefore, in the preferred embodiment, it is generally designed... r / a ≥2.

[0063] Furthermore, the optimal solution of the birefringence model satisfies Equation 2. R + r =0.76 c .

[0064] Taking a specific preferred embodiment as an example, the optical fiber in this embodiment is a 125μm diameter optical fiber, when the core radius... a When the diameter is 4μm, the appropriate stress zone diameter is 2. RAt a depth of 39.5 μm, the distance from the inner edge of stress zone 4 to the center of core 1 is 8 μm, which satisfies the requirement that the distance between the edge of the stress zone and the core is approximately twice the core radius. Simultaneously, according to the birefringence model, the radius of the stress zone R ≤ 19.75 μm. Therefore, based on the aforementioned core rod proportions, the maximum design size of the stress rod can be calculated to be 15.8 mm.

[0065] Furthermore, because the fiber core of this invention features boron doping, its viscosity and coefficient of thermal expansion are significantly different from other doped materials. The core 1 experiences more pronounced stress in the stress zones on both sides compared to conventional cores. Therefore, a lower boron concentration can be used to achieve the desired birefringence performance while avoiding the high stress and breakage risks associated with excessive boron. In a preferred embodiment, the boron doping concentration is controlled within the range of 20-25 mol%. Within this range, the final beat length and crosstalk requirements of the fiber can be met.

[0066] (5) Mandrel preparation process In a preferred embodiment, the preparation process of the mandrel mainly includes two parts: the deposition process of the mandrel and the melting and shrinking process of the mandrel.

[0067] a. Deposition of the core rod First, a pure silicon cladding layer (corresponding to cladding layer 3) is deposited inside the quartz liner, then a fluorine-doped inner cladding layer (corresponding to fluorine-doped layer 2) is deposited, and finally a boron-germanium co-doped core layer (corresponding to fiber core 1) is deposited.

[0068] During core deposition, the viscosity of boron-doped quartz glass is significantly lower than that of other doped glasses due to the boron doping in the core layer. Furthermore, experiments revealed that defects such as bubbles, inclusions, or inhomogeneities caused mandrel defects during deposition. Therefore, to ensure mandrel roundness and deposition consistency, the following deposition process parameters were used: low deposition rate (0.5 g / min ~ 1 g / min), temperature less than 1200℃, and internal gas pressure range of 10 ~ 20 mbar.

[0069] b. Core rod melting and shrinkage The viscosity properties of quartz glass are significantly affected by doping, and the viscosity of boron-germanium co-doped quartz glass is significantly lower than that of germanium-doped quartz glass. Therefore, the rod-shrinking process of boron-germanium co-doped rods and highly germanium-doped rods will differ accordingly, especially in controlling ellipticity. By controlling the pressure inside the tube during the actual melting and shrinking process, and combining this with power control of the heating system, the process difficulties under the abnormal viscosity characteristics of boron-germanium co-doped glass were overcome, and qualified boron-germanium co-doped rods were successfully prepared.

[0070] Specifically, in the preferred embodiment, to ensure the roundness of the core layer and the quality of the final mandrel, a low-speed (8mm / min~10mm / min) and low-power (5kW~10kW) method is preferably used during the melting and shrinking process, while maintaining a slight positive pressure (10~30mbar) of oxygen inside the tube. This allows surface tension to play a dominant role during the melting and shrinking process, reducing the influence of pressure inside the tube. Through the optimization of the mandrel preparation method, the influence of boron doping on the core roundness is avoided, resulting in a core roundness of ≤3%, ultimately yielding a mandrel with a core diameter of 2~4mm and a rod diameter of 20~22mm.

[0071] (7) Preparation process of stress rod Boron-doped stress rods were prepared using PCVD, with a boron doping concentration of approximately 20–25 mol.

[0072] (8) The stretching process of the mandrel In a preferred embodiment, a quartz sleeve with an outer diameter × wall thickness × length of 80mm × 28mm × 1060mm is preferably used. The prepared mandrel and sleeve are combined and subjected to melt shrinkage and stretching. The resulting solid rod has a core diameter of 3.2±0.2 mm and a rod diameter of 50±0.5 mm. After slitting, a solid rod with a length of approximately 400mm is obtained.

[0073] (9) Drilling and assembling precast rods To introduce birefringence, two circular stress zones need to be added symmetrically on both sides of the core region.

[0074] In a preferred embodiment, a preformed rod to be drawn is formed by drilling holes on both sides of the solid rod core area and inserting the prepared stress rod into the two holes.

[0075] II. Optical Fiber Drawing Process Based on the photosensitivity mechanism of optical fibers, increasing the stress in the fiber core can improve the photosensitivity of silica optical fibers due to the stress-photoelastic effect and the core compression effect caused by different forces on different parts. Therefore, adjusting appropriate drawing process conditions such as drawing temperature and drawing tension is crucial in the fabrication of polarization-maintaining optical fibers.

[0076] During the fiber preform drawing process, the drawing tension on the deformed region of the fiber preform is primarily applied to the fiber core. Under this strong drawing tension, the core glass structure generates numerous defects, further enhancing its photosensitive properties. However, excessively high drawing temperatures and tensions will affect optical performance parameters such as fiber attenuation, cutoff wavelength, and mode field diameter.

[0077] Therefore, in the preferred embodiment, the drawing process designed for polarization-maintaining photosensitive fiber is as follows: drawing speed (450 m / min~550 m / min), drawing temperature (1900℃~2100℃), and drawing tension (100g~200g).

[0078] In summary, it is easy to see that the preferred method for preparing the polarization-maintaining optical fiber in the preferred embodiment includes the following process: (1) Conduct single-doped flow rate opening tests for boron and germanium respectively. Design mass flow controllers (MFC) for GeCl4 and BCl3 with step sizes of 10% and 5% respectively, and opening from 0 to 60% to determine the linear relationship between the flow rate opening of reactant raw materials and refractive index and doping amount. (2) Based on the design specifications / performance parameters of the optical fiber, and complete the design of the core rod size specifications, determine the co-doping concentration of germanium oxide and boron oxide in the core layer based on the results of the single-doped flow rate opening test in (1); (3) Based on the results of the single-doped flow rate opening test in (1) and the co-doping concentration determined in (2), determine the co-doped flow rate opening of boron oxide and germanium oxide, complete the deposition preparation of the core rod, and melt-shrink the deposited core rod. The deposition sequence is as follows: first, pure silicon is deposited; then, a fluorine-doped inner cladding layer is deposited; and finally, a boron-germanium co-doped core layer is deposited. The deposition process parameters include a low deposition rate (0.5 g / min ~ 1 g / min), a temperature less than 1200℃, and an internal gas pressure range of 10 ~ 20 mbar.

[0079] For the core rod that has completed deposition, in a preferred embodiment, the finished end face can be further analyzed by electron probe X-ray microscopy (EPMA) to characterize the doping concentration of the fiber core.

[0080] Accordingly, for the single-doped flow rate opening test in process (1), the specific process preferably includes: selecting the aforementioned deposition process parameters, and performing single-doped deposition of boron oxide and germanium oxide respectively, and performing deposition of GeCl4 and BCl3 at different flow rates. The flow rates of the two single-doped materials are adjusted from low to high values ​​respectively, preferably 0~60% in the preferred embodiment, and then obtaining single-doped core rods deposited at different flow rates. By detecting the core rods, the deposition amount of single-doped materials at the corresponding flow rates and the relationship between the flow rate opening and the refractive index of the core rod can be obtained.

[0081] The process control of the melting and shrinking process includes low speed (8mm / min~10mm / min), low power (5kw~10kw), and maintaining a slight positive pressure of oxygen (10~30mbar) inside the tube.

[0082] (4) Deposit and prepare boron-doped stress rods, with a boron doping concentration of 20-25 mol%; (5) Select a suitable combination of quartz sleeve and mandrel, and melt, shrink, stretch and cut to obtain a solid rod of the corresponding size; (6) Drill holes at symmetrical positions on both sides of the solid rod core area according to the design dimensions, and insert the stress rods made into the two holes to form a preformed rod to be drawn. (7) The obtained preform is used to prepare polarization-maintaining optical fiber by drawing. The drawing process is controlled as follows: drawing speed (450 m / min~550 m / min), drawing temperature (1900℃~2100℃) and drawing tension (100g~200g).

[0083] The following are supplementary explanations of the technical solutions in the preferred embodiments of the present invention through several specific examples.

[0084] Example 1: In this embodiment, the fabrication process of the boron-germanium co-doped polarization-maintaining optical fiber is as follows: Figure 1 As shown, before core rod fabrication, single-doping flow rate opening tests of boron and germanium were conducted to obtain the relationship between the flow rate opening and refractive index of the reactants GeCl4 and BCl3. Then, the core rod doping ratio was designed and the core rod was fabricated. On the other hand, boron-doped stress rods were prepared by doping with boron. After the core rods were stretched through a sleeve, symmetrical holes were drilled on both sides of the fiber core. Two stress rods were inserted into the holes and combined to obtain a complete preform. The preform was then drawn and screened to obtain the finished optical fiber.

[0085] In the above process, the fabrication of the core rod is the most critical, mainly involving process control and the regulation of the core doping ratio. The optical fiber refractive index profile structure design diagram is shown below. Figure 3 As shown, the first step is the deposition process of the core rod, followed by the deposition of the outer cladding, the sunken inner cladding, and the core layer in sequence.

[0086] For the sunken inner cladding, the relative refractive index difference Δ2 is -0.2%, and a wide fluorine-doped matching channel design is employed. d 2 / d=1=3; For the core layer, the germanium oxide concentration is 10%, the boron oxide concentration is 5%, the fiber NA is 0.14, and the relative refractive index difference Δ1 of the core layer is 0.8%; During the core layer deposition process, in order to reduce the core roundness and deposition consistency, a low deposition rate of 1 g / min, a temperature below 1150℃, and an internal gas pressure range of 20 mbar should be maintained. Then, a low-speed (8 mm / min~10 mm / min) and low-power (10 kW) method is used for melting and shrinking, while maintaining a slight positive pressure of oxygen (30 mbar) inside the tube to avoid the influence of boron doping on core roundness, thus preparing a core rod that meets the requirements. In the fiber drawing process, the drawing speed is controlled at (450 m / min~550 m / min), the drawing temperature at (1900℃~2100℃), and the drawing tension at (100 g~200 g) to ensure that the prepared optical fiber meets the conventional geometric and polarization-maintaining optical performance.

[0087] The optical fiber prepared by the above method exhibits photosensitivity. A grating etching experiment was performed directly on the fiber without hydrogen loading. The etching conditions were: laser power 15 MJ, pulse frequency 500 Hz, and 2500 pulses. The transmission spectrum of the resulting fiber grating is as follows: Figure 6 As shown, the transmittance reaches 11.316 dB, and the reflectance is approximately 92.68%.

[0088] Example 2: In this embodiment, the main difference from the preparation process in Example 1 is that the germanium oxide doping concentration of the core layer is 12%, the boron oxide concentration is 5%, the NA of the optical fiber is 0.16, and the Δ1 value of the increase in the refractive index of the core layer is 0.9%.

[0089] Other fabrication processes remain unchanged. The optical fiber prepared by this method exhibits photosensitivity, and the fiber grating obtained through grating can achieve a transmittance of 12.130 dB. Figure 7 As shown, the reflectivity is approximately 93.87%.

[0090] Example 3: In this embodiment, the main difference from the preparation process in Example 1 is that the concentration of germanium oxide doped in the core layer is 15%, the concentration of boron oxide is 10%, the NA of the optical fiber is 0.2, and the Δ1 value of the increase in the refractive index of the core layer is 1%.

[0091] Other fabrication processes remain unchanged. The optical fiber prepared by this method exhibits photosensitivity, and the fiber grating obtained through grating can achieve a transmittance of 14.629 dB. Figure 8 As shown, the reflectivity is approximately 96.5%.

[0092] Example 4: In this embodiment, the main difference from the preparation process in Example 1 is that the concentration of germanium oxide doped in the core layer is 8%, the concentration of boron oxide is 5%, and the NA of the optical fiber is 0.09. The other preparation processes remain unchanged, and the parameters of the prepared optical fiber are shown in Table 1.

[0093] Example 5: In this embodiment, the main difference from the preparation process in Example 1 is that the concentration of germanium oxide doped in the core layer is 20%, the concentration of boron oxide is 15%, the NA of the optical fiber is 0.24, and the core roundness is 3.1%. The other preparation processes remain unchanged, and the parameters of the prepared optical fiber are shown in Table 1.

[0094] Example 6 In this embodiment, the main difference from the preparation process in Example 1 lies in the different process control conditions. In this embodiment, a conventional method is used for rod fabrication, with a deposition rate of 1.5 g / min and an internal gas pressure range of 20 mbar. The rod is then prepared by conventional melting and shrinking processes using a transfer speed of 30 mm / min and a power of 12 kW.

[0095] The parameters of the prepared mandrel are shown in Table 1. Its core roundness index reached 5% as tested by the PK2600 equipment, which does not meet the requirements.

[0096] The test data and grating writing data of the optical fibers prepared in the six embodiments are shown in Tables 1 and 2 below.

[0097] Table 1. Test data of the core rods and optical fibers prepared in the six examples.

[0098] Table 2. Write grating data of the optical fibers prepared in the three embodiments.

[0099] Based on Tables 1 and 2 above, it is easy to see that, according to the process control conditions and core doping ratio adjustment parameters designed in the preferred embodiment, the resulting core rod and preform can meet the requirements for drawing and preparing polarization-maintaining photosensitive fiber, and the polarization-maintaining photosensitive fiber prepared by drawing can meet the design standards of optical fiber, satisfying both the application requirements of photosensitivity and the setting requirements of birefringence performance, thereby satisfying the application of polarization-maintaining photosensitive fiber in multifunctional grating devices for polarization state control.

[0100] The boron-germanium co-doped polarization-maintaining optical fiber of this invention utilizes the co-doping of boron and germanium elements in the fiber core, along with the corresponding setting of a fluorine-doped layer around the core and two symmetrical stress regions around the fluorine-doped layer. This allows for the corresponding setting of a panda-type polarization-maintaining fiber. By optimizing the doping ratio control parameters such as the amount of boron oxide and germanium oxide in the core, the molar percentage of fluorine doping concentration in the fluorine-doped layer, and the amount of boron doping in the stress regions, a boron-germanium co-doped polarization-maintaining optical fiber that meets both photosensitivity and birefringence requirements can be obtained. This expands the applicability of polarization-maintaining fibers, reduces the manufacturing cost of fiber Bragg gratings, and has good practical value and application prospects.

[0101] 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 boron-germanium co-doped polarization-maintaining optical fiber, comprising a core located in the middle and a cladding disposed around the outer periphery of the core, wherein two circular stress regions are symmetrically arranged in the cladding with respect to the center of the core; characterized in that, The fiber core is doped with boron oxide and germanium oxide, with germanium oxide concentration of 10%~15% and boron oxide concentration of 5%~10%, resulting in a polarization-maintaining optical fiber NA value of 0.1~0.2 and a relative refractive index difference Δ1 value of 0.8%~1%; and The outer periphery of the fiber core is further covered with a fluorine-doped layer of a certain thickness; the diameter of the fluorine-doped layer is... d 2 and the diameter of the fiber core d The ratio of 1 to 1 is not greater than 5.0, and the molar percentage of fluorine doping concentration in the fluorine-doped layer is limited to below 1.5, so that the relative refractive index difference Δ2 of the fluorine-doped layer is -0.12% to -0.2%.

2. The boron-germanium co-doped polarization-maintaining optical fiber according to claim 1, characterized in that, The boron doping concentration in the stress zone is 20%~25%.

3. The boron-germanium co-doped polarization-maintaining optical fiber according to claim 1, characterized in that, The distance from the edge of the stress zone to the center of the fiber core r With core radius a The ratio is not less than 2.

4. The boron-germanium co-doped polarization-maintaining optical fiber according to any one of claims 1 to 3, characterized in that, The diameter of the fluorine-doped layer d 2 and the diameter of the fiber core d The ratio of 1 is between 2.5 and 5.

0.

5. A method for preparing a boron-germanium co-doped polarization-maintaining optical fiber, used in the preparation of the boron-germanium co-doped polarization-maintaining optical fiber according to any one of claims 1 to 4, characterized in that, The preparation process includes the following: (1) Conduct single-doped flow rate opening tests for boron and germanium respectively to determine the linear relationship between the flow rate opening of reactant raw materials and refractive index and doping amount; (2) Based on the results of the single-doped flow rate opening test in (1) and the design specifications of the optical fiber, determine the doping concentration of germanium oxide and boron oxide in the core layer, and complete the design of the core rod size specifications. (3) Based on the results of the single-doped flow rate opening test in (1) and the doping concentration determined in (2), determine the flow rate opening of the reactant raw materials of boron oxide and germanium oxide, and sequentially complete the deposition of pure silicon, fluorine-doped inner cladding, and boron-germanium co-doped core layer to obtain a core rod, and then melt and shrink the core rod. (4) Deposition preparation of boron-doped stress rods; (5) Select a suitable combination of quartz sleeve and mandrel, and melt, shrink, stretch and cut to obtain a solid rod of the corresponding size; (6) Drill holes at symmetrical positions on both sides of the solid rod core area according to the design dimensions, and insert the stress rods made into the two holes to form a preformed rod to be drawn. (7) Use the obtained preform to draw polarization-maintaining optical fiber.

6. The method for preparing boron-germanium co-doped polarization-maintaining optical fiber according to claim 5, characterized in that, In the preparation process (3), the deposition preparation process parameters include a low deposition rate of 0.5 g / min ~ 1 g / min, a temperature of less than 1200℃, and a gas pressure range of 10~20 mbar in the tube.

7. The method for preparing boron-germanium co-doped polarization-maintaining optical fiber according to claim 6, characterized in that, In the preparation process (3), the process control parameters for mandrel melting and shrinking include low speed of 8mm / min~10mm / min, low power of 5kw~10kw, and maintaining a slight positive pressure of 10~30mbar for oxygen in the tube.

8. The method for preparing boron-germanium co-doped polarization-maintaining optical fiber according to any one of claims 5 to 7, characterized in that, In the preparation process (7), the fiber drawing process control includes a drawing speed of 450 m / min to 550 m / min, a drawing temperature of 1900℃ to 2100℃, and a drawing tension of 100g to 200g.

9. The method for preparing boron-germanium co-doped polarization-maintaining optical fiber according to claim 6, characterized in that, In the preparation process (1), the single-doped flow rate opening test process includes: Using the process parameters of mandrel deposition in preparation process (3), single doping deposition of boron oxide and germanium oxide was carried out respectively. GeCl4 and BCl3 were deposited at different flow rates. The flow rates of the two single doped materials were adjusted from low to high values ​​to obtain single doped mandrels at different flow rates. By detecting the mandrels, the deposition amount of single doping of the materials at the corresponding flow rates and the relationship between the flow rate and the refractive index of the mandrel were obtained.

10. The method for preparing boron-germanium co-doped polarization-maintaining optical fiber according to any one of claims 5 to 7 and 9, characterized in that, In the preparation process (5), the selection of the quartz sleeve is accomplished by the following formula: In the formula, CSA 套管 To select the cross-sectional area of ​​the sleeve; d 2 a and D These refer to the core layer diameter of the core rod, the fiber core diameter, and the core rod diameter, respectively.

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