Optical fiber, tapered optical fiber and preparation method thereof

Abstract

The invention belongs to the technical field of optical fibers, and particularly discloses an optical fiber, a tapering optical fiber and a preparation method of the tapering optical fiber. The core layer comprises a first region, a second region and a third region; the second area and the third area are sequentially coated outside the first area; the refractive index of the first region is distributed in a linear declining manner outwards along the center of the region; the refractive index of the second region is uniformly distributed; the refractive index of the third area is distributed in a rising manner from inside to outside; the inner cladding layer is doped with Cl and F, the outer cladding layer is doped with Cl, and the content of Cl in the inner cladding layer is lower than that of Cl in the outer cladding layer. Through the application, the refractive indexes of the core layer and the inner cladding layer are designed, the bending resistance of the optical fiber can be improved, the refractive index design can promote the uniform change of the refractive index when the optical fiber is tapered, and through the design of raw materials and components of the inner cladding layer and the outer cladding layer, the viscosity difference between the inner cladding layer and the outer cladding layer is reduced; and one-time tapering can be successful when the optical fiber is tapered.

Description

Technical Field

[0001] The present application belongs to the field of optical fiber technology, and more specifically, relates to an optical fiber, a tapered optical fiber, and a preparation method thereof. Background Art

[0002] With the in-depth development of optical fiber communication technology and applications, single-mode optical fibers for various devices have been widely used. Single-mode optical fibers for such devices are mainly used for fiber coupling applications, such as couplers, wavelength division multiplexers, splitters, combiners, etc. Usually, since optical devices require delicate manual operations and various heat treatment processes, high requirements have been put forward for the optical properties, geometric properties and bending performance of such optical fibers themselves. At present, the mass production of mature optical devices is facing the demand for production with a higher degree of automation. Therefore, in the process of taper drawing, higher requirements are put forward for the adaptability of optical fibers to the same taper drawing process, so as to promote more automated taper drawing control.

[0003] In the existing taper optical fiber, the viscosity matching of the core layer and the inner cladding material is often used in the doping design to achieve synchronous deformation of the core and the cladding, hoping to achieve the purpose of stabilizing the waveguide structure; in fact, only by matching the viscosity of the material, it is often ignored that after the optical fiber is thinned, the size of the core layer is reduced and it cannot bind the light, becoming an evanescent core, and the core layer light leaks into the inner cladding. In this process, the doped ions of the core layer, such as Ge or F, will also diffuse to the cladding synchronously, making the inner cladding and the outer cladding transmit light as a new waveguide structure; the difference in expansion coefficient between the pure silicon outer cladding and the inner cladding material, as well as the different speeds of material molecular movement or diffusion, cause abnormal changes in the waist refractive index of the two optical fibers being synchronously tapered during the taper process, thereby causing abnormal fluctuations in the output splitting ratio at both ends of the coupler, which cannot be stabilized. Therefore, the existing taper optical fiber must be pre-tightened by human intervention, or multiple firepower corrections are made to achieve the required splitting ratio during the period, and it is impossible to successfully taper at one time. The degree of automation of the taper process is too low. In addition, the existing waveguide structure design generally adopts a core / cladding pure step structure, which easily overlooks the boundary between the core and the inner cladding during the actual tapering process. The difference in doping concentration is large. During the tapering process, the Ge / F diffuses faster near the cladding, and the core refractive index changes from the expected step shape to a triangle, resulting in greater tapering losses. As a result, the performance of the tapered optical fiber obtained after tapering does not meet expectations. Summary of the invention

[0004] In view of the defects of the prior art, the purpose of the present application is to provide an optical fiber, a tapered optical fiber and a preparation method thereof, aiming to solve the problem that the existing optical fiber cannot be successfully tapered in one go and the performance of the obtained tapered optical fiber does not meet the expected performance.

[0005] To achieve the above objectives, in a first aspect, the present application provides an optical fiber, comprising a core layer, the core layer comprising: a first zone, a second zone and a third zone; the second zone and the third zone are sequentially coated outside the first zone; the refractive index of the first zone is linearly decreased from the center of the zone to the outside; the refractive index of the second zone is uniformly distributed; the refractive index of the third zone is increased from the inside to the outside.

[0006] It should be noted that the refractive index profile design of the core layer provided in the present application is different from that of the ordinary step type. Considering that the deformation of the core layer gradually decreases from the outside to the inside during the tapering process, the doping elements of the core layer diffuse into the inner cladding during the deformation process, resulting in a corresponding decrease in the refractive index. Among them, the refractive index change in the third zone is the largest, changing from the previous rising distribution to a curved descending distribution from the inside to the outside; the refractive index change in the second zone is relatively small, and also changes into a curved descending distribution; the refractive index change in the first zone is the smallest, and changes from a linear descending distribution from the center to the outside to a curved descending distribution from the center to the outside. The distributions from the first to the third zones can be combined to form a Gaussian distribution. Therefore, when the optical fiber provided in the present application is tapered, in the early stage of tapering, the refractive index profile of the core layer can gradually transform into a Gaussian distribution. The Gaussian distribution can ensure that the energy of the light transmitted by the optical fiber is concentrated in the core layer in the early stage of tapering, thereby ensuring the transmission performance of the optical fiber during the tapering process; and through the above-mentioned design of the refractive index of the core layer, it can be ensured that the refractive index distribution of the core layer gradually becomes a Gaussian distribution during the tapering process, thereby promoting the controllable regularity of the entire deformation process, making the mode field of the optical fiber in the early stage of tapering change uniformly, and effectively promoting the successful one-time tapering of the optical fiber.

[0007] In a possible implementation, the optical fiber further includes: an inner cladding and an outer cladding; the inner cladding and the outer cladding are sequentially coated outside the core layer;

[0008] The refractive index of the inner cladding layer is lower than the lowest refractive index of the core layer.

[0009] Optionally, the refractive index of the outer cladding is between the refractive index of the inner cladding and the lowest refractive index of the core layer. Further optionally, the outer cladding may be a pure silica outer cladding, which may refer to pure silica quartz glass slightly doped with Cl. For optical fibers, the refractive index of the core layer is higher than that of the inner cladding, and the refractive index of the inner cladding is lower than that of the outer cladding in order to improve the bending resistance of the optical fiber, as described in the following paragraph.

[0010] It can be understood that the refractive index of the inner cladding is designed to be smaller than that of the core layer and the outer cladding in the present application, forming a refractive index lower limit structure, which can constrain the light transmitted in the optical fiber to be transmitted in the core layer, and because the refractive index of the first zone is triangularly distributed, it is more possible to constrain most of the transmitted light to the first zone, that is, constrain the transmission to be concentrated inside the optical fiber. When the above optical fiber is bent, since the deformation during bending focuses on the periphery of the optical fiber, the bending deformation inside the optical fiber is small. Therefore, through the above design, the optical fiber can be made insensitive to bending when transmitting light, thereby ensuring the transmission performance of the optical fiber.

[0011] In a possible implementation manner, the inner cladding is doped with Cl and F, the outer cladding is doped with Cl, and the content of Cl in the inner cladding is lower than the content of Cl in the outer cladding.

[0012] It can be understood that since Cl can reduce the viscosity of pure silica, F can also reduce the viscosity of pure silica. The present application dopes a small amount of F on the basis of a small amount of Cl doping in the inner cladding, which can reduce the viscosity difference between the inner cladding and the outer cladding (the outer cladding is only doped with Cl) under the premise of ensuring that the refractive indices of the inner cladding and the outer cladding are inconsistent; in the later stage of tapering, since the viscosity difference between the outer cladding and the inner cladding is small, the physical dimensions of the inner and outer claddings change almost consistently, so the refractive index change trends of the inner cladding and the outer cladding in the later stage of tapering are consistent, that is, the refractive index of the fiber waist region (that is, the inner cladding and the outer cladding) changes uniformly, so that the mode field of the optical fiber in the later stage of tapering changes uniformly, effectively promoting the successful one-time tapering of the optical fiber.

[0013] In a possible implementation, the content of SiO2 in the inner cladding is 99.82mol% to 99.87mol%, the content of F is 0.074mol% to 0.22mol%, and the content of Cl is 0.05mol% to 0.1mol%; the content of SiO2 in the outer cladding is 95.7mol% to 97.8mol%, and the content of Cl is 0.3mol% to 0.4mol%.

[0014] It can be understood that when the inner cladding is doped with 0.074mol% to 0.22mol% F and 0.05mol% to 0.1mol% Cl and the outer cladding is doped with 0.3mol% to 0.4mol% Cl, the refractive index of the inner cladding can be lower than that of the outer cladding, and the viscosities of the inner and outer claddings are almost consistent.

[0015] In a possible implementation, the first zone accounts for 30% to 35% of the core layer, the second zone accounts for 35% to 40% of the core layer, the third zone accounts for 25% to 30% of the core layer, and the third zone accounts for the smallest proportion of the core layer.

[0016] It should be noted that the refractive index of the second zone is uniformly distributed, which can avoid a sudden change in the refractive index between the first zone and the third zone, causing stress imbalance to affect the performance of the optical fiber; and the refractive index of the second zone is uniformly distributed, and the proportions of the first zone, the second zone and the third zone can be effectively controlled by controlling the diameter of the second zone. Since the refractive indexes of the first zone and the third zone are higher than those of the second zone, light is more inclined to be transmitted in the first zone and the third zone. By designing the proportion of the third zone to be minimum, more light can be concentrated in the first zone of the core layer for transmission, thereby ensuring the transmission performance of the optical fiber; in addition, the refractive index of the second zone is uniformly distributed, which can be used as a benchmark for the design of the refractive index of the core layer, and even as a benchmark for the change in the refractive index during the tapering of the core layer. The above benchmark can be used to more effectively design the change trend and parameters of the refractive index of the first zone and the third zone, which helps the core layer of the tapered optical fiber obtained after tapering to form a Gaussian distributed refractive index profile, thereby ensuring the transmission performance of the tapered optical fiber.

[0017] Optionally, the proportion of the first zone in the core layer needs to be greater than the proportion of the third zone in the core layer, so as to ensure that more light is concentrated in the first zone for transmission, thereby improving the transmission performance of the optical fiber.

[0018] In a possible implementation, the relative refractive index difference Δ1 of the first zone at the maximum refractive index relative to the outer cladding is 0.65% to 0.75%; the relative refractive index difference Δ2 of the second zone relative to the outer cladding is 0.6% to 0.68%; the relative refractive index difference Δ3 of the edge of the third zone relative to the outer cladding is 0.7% to 0.82%; preferably, Δ1=(Δ2+Δ3) / 2, 0.05%≤Δ3-Δ1≤0.07%;

[0019] and / or

[0020] The relative refractive index difference Δ4 of the inner cladding with respect to the outer cladding is -0.02% to -0.06%, so as to be used as a lower limit inner cladding, which helps to improve the bending resistance of the optical fiber.

[0021] It should be noted that the designs of the above-mentioned relative refractive index differences are determined based on the wavelength band of the optical fiber application, so that the optical fiber can meet the requirements of the application band; further, Δ1=(Δ2+Δ3) / 2, 0.05%≤Δ3-Δ1≤0.07%, is to keep the maximum refractive index of the first zone between the maximum refractive index of the third zone and the refractive index of the second zone, and to control the difference between the maximum refractive index of the third zone and the maximum refractive index of the first zone, all of which are to promote the refractive index distribution of the core layer to gradually become a Gaussian distribution during the tapering process.

[0022] In a possible implementation, the content of SiO2 in the core layer is 92.16 mol% to 92.78 mol%, the content of Ge is 7 mol% to 7.5 mol%, the content of F is 0.12 mol% to 0.18 mol%, and the content of Cl is 0.08 mol% to 0.12 mol%.

[0023] It is understandable that those skilled in the art can refer to the specific refractive index design of each region of the core layer and adjust the content of raw materials in each region of the core layer to meet the corresponding refractive index. The present application does not specifically limit the specific refractive index and content.

[0024] In a possible implementation, the radius of the core layer is 2.75 microns to 3.25 microns; and / or the radius of the inner cladding layer is 4.2 microns to 16.3 microns, and 1.5≤the radius ratio of the inner cladding layer to the core layer≤5.0.

[0025] In a second aspect, the present application provides a tapered optical fiber, wherein the tapered optical fiber is obtained by tapering the optical fiber provided by the first aspect or any possible implementation of the first aspect.

[0026] In a third aspect, the present application provides a method for preparing an optical fiber provided by the first aspect or any possible implementation of the first aspect, comprising the following steps:

[0027] A core layer is deposited in a quartz tube; the raw materials of the core layer include silicon tetrachloride, germanium tetrachloride, oxygen and hexafluoroethane; in the first zone, the volume flow ratio of the four raw materials is controlled in the range of (15-27.5): (15.6-28.5): (48-137.5): (0.2-0.4); in the second zone, the volume flow ratio of the four raw materials is controlled in the range of (15-27.5): (15.4-28.3): (48-137.5): (0.2-0.4); in the third zone, the volume flow ratio of the four raw materials is controlled in the range of (15-27.5): (15.8-28.7): (48-137.5): (0.2-0.4);

[0028] When the third zone is deposited from the outside to the inside, the volume flow rate of germanium tetrachloride is controlled to be gradually reduced;

[0029] When depositing the second zone, the volume flow rate of germanium tetrachloride is controlled to remain constant;

[0030] When the first zone is deposited from the outside to the inside, the volume flow rate of germanium tetrachloride is controlled to increase gradually.

[0031] It can be understood that the above refractive index is regulated by germanium, and the refractive index is proportional to germanium doping.

[0032] In a possible implementation, when the core layer is deposited in the quartz tube, the volume flow rate of germanium tetrachloride is controlled to change continuously, so that the refractive index of the core layer changes continuously.

[0033] In a possible implementation, the method further includes the following steps:

[0034] An inner cladding layer is deposited on the periphery of the core layer; the raw materials of the inner cladding layer include: silicon tetrachloride, oxygen and hexafluoroethane; the volume flow ratio of the above three raw materials is controlled in the range of (27.5-37.5): (220-450): (0.4-0.6);

[0035] and / or depositing an outer cladding layer outside the inner cladding layer; the raw materials of the outer cladding layer include: silicon tetrachloride and oxygen; the volume flow ratio of the above two raw materials is controlled in the range of (50-105): (125-294).

[0036] In general, the above technical solutions conceived by the present application have at least the following beneficial effects compared with the prior art:

[0037] The present application provides an optical fiber, a tapered optical fiber and a preparation method thereof. Considering that the deformation of the core layer gradually decreases from the outside to the inside during the tapering process, the refractive index of the optical fiber core layer is designed. In the early stage of tapering, the refractive index profile of the core layer can gradually deform into a Gaussian distribution, thereby ensuring the transmission performance of the optical fiber after tapering. The entire deformation process is controllable, so that the mode field of the optical fiber in the early stage of tapering changes uniformly, effectively promoting the one-time successful tapering of the optical fiber.

[0038] The present application provides an optical fiber, a tapered optical fiber, and a preparation method thereof, wherein a small amount of F is doped in the inner cladding, and the viscosity difference between the inner cladding and the outer cladding is reduced while ensuring that the refractive index of the inner cladding and the outer cladding are inconsistent, so that the mode field of the optical fiber changes uniformly in the later stage of tapering, and the one-time successful tapering of the optical fiber is effectively promoted.

[0039] The present application provides an optical fiber, a tapered optical fiber, and a preparation method thereof. By designing the refractive index of the inner cladding of the optical fiber to be smaller than that of the core layer, the light transmitted in the optical fiber can be constrained to be transmitted in the core layer, and the refractive index of the middle region of the core layer is triangularly distributed, which further constrains most of the transmitted light in the middle region of the core layer, making the optical fiber insensitive to bending when transmitting light, having good macrobending performance, and effectively ensuring the transmission performance of the optical fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 is a schematic diagram of the structure of an optical fiber provided in an embodiment of the present application;

[0041] Figure 2 is a schematic diagram of the structure of the core layer provided in an embodiment of the present application;

[0042] Figure 3is a schematic cross-sectional diagram of the relative refractive index difference of the optical fiber provided in an embodiment of the present application;

[0043] In all the drawings, the same reference numerals are used to denote the same elements or structures, wherein: 10 is a core layer, 20 is an inner cladding layer, 30 is an outer cladding layer; 101 is a first zone, 102 is a second zone, and 103 is a third zone. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0045] First, the technical terms involved in the embodiments of the present application are introduced.

[0046] (1) Refractive index profile

[0047] The refractive index profile mentioned in the embodiments of the present application refers to the relationship between the refractive index and radius of the optical fiber at a wavelength of 633 nm.

[0048] (2) Relative refractive index difference

[0049] Assume that the refractive index of material layer A is n i , the refractive index of material layer B is n0; then the relative refractive index difference between material layer A and material layer B is Δ i is:(n i -n0) / n0*100%.

[0050] It should be noted that the relative refractive index difference mentioned in the embodiments of the present application specifically refers to the relative refractive index difference of each layer (such as the core layer, the inner cladding layer or each area within the core layer) relative to the outer cladding layer, and will not be specifically explained below.

[0051] Next, the embodiments of the present application are described in conjunction with the drawings in the embodiments of the present application.

[0052] Figure 1 Schematic diagram of the structure of the optical fiber provided in the embodiment of the present application; Figure 1 As shown, it comprises: a core layer 10, an inner cladding layer 20 and an outer cladding layer 30. The inner cladding layer 20 and the outer cladding layer 30 are coated on the outside of the core layer 10 in sequence.

[0053] like Figure 2 As shown, the core layer 10 includes: a first zone 101, a second zone 102 and a third zone 103; the second zone 102 and the third zone 103 are sequentially coated on the outside of the first zone 101; the refractive index of the first zone 101 decreases linearly from the center of the region to the outside; the refractive index of the second zone 102 is uniformly distributed; the refractive index of the third zone 103 increases from the inside to the outside.

[0054] Further optionally, the refractive index of the inner cladding 20 is less than the lowest refractive index of the core layer 10 ; and the refractive index of the outer cladding 30 is between the refractive index of the inner cladding 20 and the lowest refractive index of the core layer 10 .

[0055] Further optionally, the refractive index difference profile of the core layer is different from the ordinary step-type, such as Figure 3 As shown, it can be divided into three areas, the refractive index of the central area, namely the first area 101, is triangularly distributed, the refractive index of the two sides of the center, namely the second area 102, is flatly distributed, and there is an ascending distribution outside, namely the third area 103.

[0056] Among them, the regional center of the first zone 101 can be the regional center point or any point on a line segment at the regional center including the regional center point, that is, the maximum refractive index point of the first zone 101 can be the center point of the first zone or a point near the center point. Therefore, the above-mentioned triangular distribution can be an equilateral triangular distribution, or other non-equilateral triangular distributions. The above-mentioned rising distribution can be a linear rising distribution, a parabolic rising distribution or other distribution showing an upward trend, etc., and technical personnel in this field can make a selection according to their needs, and they only need to ensure that the core refractive index of the tapered optical fiber can ultimately present an approximate Gaussian distribution.

[0057] Combination Figure 3 As shown, let R0 be the radius of the core layer, R1 be the radius of the first zone, and R2 be the outer radius of the second zone. Then, the proportion of the triangular distribution (first zone) to the entire core layer can be R1 / R0=30% to 35%, and the relative refractive index difference at the center of the zone can be Δ1=0.65% to 0.75%; the proportion of the flat area (second zone) adjacent to the triangular zone to the entire core layer can be: (R2-R1) / R0=35% to 40%, and the relative refractive index difference of the core in this area can be Δ2=0.6% to 0.68%; the proportion of the rising area (third zone) to the entire core layer can be (R0-R2) / R0=25% to 30%, and the relative refractive index difference at the highest point of the edge can be Δ3=0.7% to 0.82%, and 0.05%≤Δ3-Δ1≤0.07%, Δ1=(Δ2+Δ3) / 2 is satisfied.

[0058] It should be noted that the design of the above relative refractive index difference is determined based on the wavelength band of the optical fiber application. Taking the above parameters as an example, the optical fiber corresponding to the above parameters can be applied to the 1310nm band for transmission.

[0059] It can be understood that the refractive index of the second zone is uniformly distributed, which can avoid the sudden change of refractive index between the first zone and the third zone, control the proportion of the first zone and the third zone, and help the tapered optical fiber obtained after tapering to form a Gaussian distribution refractive index profile, thereby ensuring the transmission performance of the tapered optical fiber; since the deformation of the core layer gradually decreases from the outside to the inside during the taper process, designing the refractive index of the third zone to increase can promote the uniform change of the refractive index of the optical fiber waist region during the taper process.

[0060] Furthermore, in one example, the core layer 10 is a Ge and F co-doped silica quartz glass layer; wherein the Ge element may account for 7 mol% to 7.5 mol%, the F element may account for 0.12 mol% to 0.18 mol%, the SiO2 element may account for 92.16 mol% to 92.78 mol%, and the Cl element may account for 0.08 mol% to 0.12 mol%.

[0061] The proportion of the above materials is to adapt to the relative refractive index difference between the first zone to the third zone of the core layer mentioned above, wherein the higher the refractive index, the greater the corresponding proportion of the Ge element.

[0062] For example, the core layer radius R0 may be 2.75 micrometers to 3.25 micrometers.

[0063] It should be noted that the core radius and the relative refractive index difference of each region of the core determine the cutoff wavelength of the optical fiber, which are related to the applicable wavelength band of the optical fiber. Please refer to the above records and will not be elaborated here.

[0064] Further, in one example, the inner cladding 20 is a F-doped silica quartz glass layer; wherein the F element may account for 0.074 mol% to 0.22 mol%, the SiO2 may account for 99.82 mol% to 99.87 mol%, and the Cl element may account for 0.05 mol% to 0.1 mol%.

[0065] For example, the inner cladding radius may be 4.2 micrometers to 16.3 micrometers, and satisfy 1.5≤inner cladding radius / core radius≤5.0.

[0066] Optionally, the relative refractive index difference of the inner cladding is Δ4=-0.02% to -0.06%.

[0067] Among them, the relative refractive index difference of the inner cladding and the design of the inner cladding radius are both for confining the light transmitted in the optical fiber to the core layer for transmission, making the optical fiber transmission light insensitive to bending and reducing the bending loss of the optical fiber; in addition, when the ratio of the inner cladding radius to the core layer radius is small, the core layer will not be able to confine the light for transmission well, and when the ratio of the inner cladding radius to the core layer radius is large, the waveguide structure of the tapered optical fiber obtained after tapering will not reach the expected structure, so the above-mentioned limitation is made on the radius ratio of the inner cladding and the core layer.

[0068] Furthermore, in one example, the outer cladding layer 30 is a silicon dioxide quartz glass layer, wherein SiO2 may account for 95.7 mol% to 97.8 mol%, and Cl element may account for 0.3 mol% to 0.4 mol%.

[0069] It is understandable that those skilled in the art can select the material ratio of the inner cladding and the outer cladding according to actual conditions, so that the viscosities of the prepared inner cladding and the outer cladding are consistent or nearly consistent.

[0070] Illustratively, the outer cladding diameter may be 79 microns to 81 microns.

[0071] In a specific embodiment, the embodiment of the present application provides a method for preparing an optical fiber, comprising the following steps: using a plasma chemical vapor deposition (PCVD) process to sequentially deposit an inner cladding and a core layer in a quartz tube, wherein the deposition raw materials are silicon tetrachloride, germanium tetrachloride, high-purity oxygen, and hexafluoroethane; melting the deposited quartz tube at a high temperature of 1850°C to 2100°C on an electric melting lathe to form a solid rod, wherein the temperature range is adapted to the component structure of the outermost quartz tube, the core layer, and the inner cladding of F-doped quartz, so that the quartz tube collapses uniformly in a molten state and is finally sintered into an optical rod, i.e., an optical fiber preform; and drawing the optical fiber preform into an optical fiber at a high temperature of 1950°C to 2100°C on a drawing tower.

[0072] It can be understood that the above-mentioned quartz tube finally exists as a part of the outer cladding of the optical fiber, wherein the Cl content in the quartz tube can be 0.3 mol / % to 0.4 mol / %, and the hydroxyl content is less than 0.1 PPM.

[0073] In the above scheme, the volume flow ratio of silicon tetrachloride and oxygen introduced into the outer layer is in the range of (50-105): (125-294); the volume flow ratio of silicon tetrachloride, oxygen and hexafluoroethane introduced into the inner layer is in the range of:

[0074] (27.5~37.5):(220~450):(0.4~0.6); By controlling the component ratio during the above inner cladding deposition process, the inner cladding of the optical fiber obtains a lower Cl element content. Even if F is doped, its viscosity will not be greatly reduced, thereby reducing the viscosity difference between the inner cladding F-doped quartz and the outer cladding pure silica quartz (here, quartz is assumed to be Cl-doped silicon dioxide).

[0075] Silicon tetrachloride and germanium tetrachloride are introduced into the starting part of the core layer (i.e., the refractive index rising area, the third area), and the volume flow ratio range of oxygen and hexafluoroethane is: (15-27.5): (15.8-28.7): (48-137.5): (0.2-0.4), and the germanium tetrachloride is gradually reduced when depositing toward the flat part (the second area); Silicon tetrachloride and germanium tetrachloride are introduced into the core layer refractive index flat area, and the volume flow ratio range of oxygen and hexafluoroethane is the same as the end point of the refractive index decreasing area, and the flow ratio range is: (15~27.5):(15.4~28.3):(48~137.5):(0.2~0.4); the core layer refractive index triangular area (i.e., it is linearly decreasing from the center of the area to the outside, also known as the triangular distribution, i.e., the first area), the corresponding flow ratio range: (15~27.5):(15.6~28.5):(48~137.5):(0.2~0.4), through the above core layer deposition process, the refractive index design of three different areas of the core layer can be realized.

[0076] Based on the above scheme, deposition is carried out in layers: first, silicon tetrachloride and oxygen enter the tube to react and deposit the outer cladding; silicon tetrachloride, hexafluoroethane, and oxygen enter the tube to react and deposit the inner cladding; silicon tetrachloride, germanium tetrachloride, hexafluoroethane, and oxygen enter the tube to react and deposit the core layer.

[0077] The deposited quartz tube is melted and shrunk multiple times at 1850℃~2100℃ on a melting lathe and finally sintered into a preform rod; the preform rod is drawn into an optical fiber at a high temperature of 1950℃~2100℃ in a drawing tower.

[0078] It can be understood that, through the preparation method provided in the embodiment of the present application, the inner cladding is doped with F, which effectively lowers the refractive index, and the formed channel waveguide can enhance the ability of light field confinement and reduce macrobending loss; and when the inner cladding is doped with a small amount of F, the viscosity difference between the inner and outer claddings is reduced, which effectively avoids the large difference in expansion coefficient between the outer pure silicon layer and the inner cladding during the tapering process, and at the same time avoids the situation in the previous technology where the inner cladding has a large variety of doping elements such as Ge, F, P and a high concentration, resulting in different speeds of material molecular movement or diffusion.

[0079] In addition, the core layer provided by the present application has the largest difference in Ge doping concentration at the boundary between the core layer and the inner cladding, so the Ge diffusion rate is the fastest; the present application deliberately increases the Ge doping from the inside to the outside at the core layer near the cladding, that is, the third area of ​​the core layer mentioned above, to form a refractive index distribution that changes linearly from the inside to the outside, thereby avoiding inconsistent Ge diffusion rates (the deformation of the core layer gradually decreases from the inside to the outside during the tapering process), so that the waist refractive index of the optical fiber changes uniformly during the heating and natural cooling of the tapering, thereby ensuring that the output splitting ratio at both ends of the coupler changes normally and stably with the change of the waist length, meeting the different splitting ratio requirements of the client.

[0080] It should be noted that the equipment for optical fiber taper drawing is usually a molten optical fiber taper drawing machine. Its working principle is to use the characteristic of optical fiber material silica to be in a molten state under high temperature conditions, and apply two opposite pulling forces to the two ends of the molten optical fiber to stretch and thin the original optical fiber. The molten state of the optical fiber is also often called "fiber softening". To achieve this molten state, the temperature does not need to reach the melting point of silica (about 1723℃). It is just that in the molten state, as the temperature rises, the viscosity of silica will decrease, and it will show good remodeling and ductility, which is conducive to changing the shape of the optical fiber.

[0081] On the basis of the above technical solution, after the preparation of the optical fiber is completed, when it is necessary to taper the optical fiber, during the taper process, since the viscosity of the inner and outer claddings is basically the same, the inner and outer claddings are guaranteed to shrink synchronously; the core layer accounts for a small proportion and the heat conduction is fast, so the adaptation problem of the viscosity of the core layer and the viscosity of the inner cladding is not considered; usually the viscosity of the core layer is the lowest, but the thermal diffusion effect of the Ge element in the core layer needs to be considered. The above-mentioned multi-region structure of the core layer is to achieve uniform diffusion of the Ge element to the cladding during the core layer thinning process, thereby ensuring that the waist refractive index of the two optical fibers changes uniformly, and the output splitting ratio at both ends of the coupler changes normally and stably with the change of the waist length.

[0082] In order to better illustrate the performance characteristics of the optical fiber provided in the embodiments of the present application, a comparative analysis is further performed through the following specific embodiments. The material parameters in the following embodiments are shown in Table 1 and Table 2:

[0083] It should be noted that since there are many parameters in the multiple embodiments (Example 1 to Example 6) provided in this application, the parameters are divided into Table 1 and Table 2 for display. From the data in Table 1, if the volume flow ratio of oxygen and silicon in the inner cladding exceeds the above range or the proportion of the core refractive index rising region (third region) exceeds the above range, the excess loss (EL) of the taper will increase, and the coupling ratio (CR) will not meet the requirements.

[0084] It should be noted that the outer cladding is doped with 0.3 mol% to 0.4 mol% Cl, which can be easily obtained by those skilled in the art by introducing silicon tetrachloride and oxygen into the outer cladding in a volume flow ratio range of (50 to 105): (125 to 294), so the raw material data of the outer cladding in Table 1 are not shown.

[0085] Furthermore, when the optical fiber is tapered, the mode field of the optical fiber can change evenly during the tapering process, and the splitting ratio of the optical fiber after tapering will be more controllable when multiple optical fibers are tapered. For example, if two optical fibers are tapered, and the mode field of each optical fiber can change evenly during the tapering process, then theoretically the splitting ratio of each tapered optical fiber obtained after tapering will be 1 / 2, that is, about 50%. As can be seen from Table 1, the optical fibers provided in Examples ① to ③ of the present application have a CR after tapering that meets expectations.

[0086] Table 1

[0087]

[0088]

[0089] Table 2

[0090]

[0091] Furthermore, in combination with the embodiments given in the above table, it can be seen that the cutoff wavelength of the optical fiber provided in the embodiments of the present application is ≤1270nm, the mode field diameter of 1310nm wavelength is 6 microns to 6.8 microns, the attenuation at 1310nm is ≤0.5dB / km, and it has good macrobending performance and tapering performance, meeting the use requirements.

[0092] It should be understood that expressions such as "including" and "may include" that may be used in the present application indicate the existence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In the present application, terms such as "including" and / or "having" may be interpreted as indicating specific characteristics, numbers, operations, constituent elements, components, or combinations thereof, but may not be interpreted as excluding the existence or possibility of adding one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof.

[0093] In addition, in the present application, the expression "and / or" includes any and all combinations of the associated listed words. For example, the expression "A and / or B" may include A, may include B, or may include both A and B.

[0094] In the description of the embodiments of the present application, it should be noted that, unless otherwise clearly specified and limited, the term "connection" should be understood in a broad sense. For example, "connection" can be a detachable connection or a non-detachable connection; it can be a direct connection or an indirect connection through an intermediate medium. Among them, "fixed connection" means that the relative position relationship after connection remains unchanged. "Rotational connection" means that the two are connected to each other and can rotate relative to each other after connection. "Sliding connection" means that the two are connected to each other and can slide relative to each other after connection. The directional terms mentioned in the embodiments of the present application, such as "top", "bottom", "inside", "outside", "left", "right", etc., are only reference directions of the accompanying drawings. Therefore, the directional terms used are for better and clearer explanation and understanding of the embodiments of the present application, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0095] In addition, in the embodiments of the present application, the mathematical concepts mentioned are symmetry, equality, parallelism, verticality, etc. These limitations are all for the current state of the art, rather than being absolutely strict definitions in a mathematical sense, and allow a small amount of deviation, approximation to symmetry, approximation to equality, approximation to parallelism, approximation to verticality, etc. are all possible. For example, A and B are parallel, which means that A and B are parallel or approximately parallel, and the angle between A and B can be between 0 and 10 degrees. A and B are perpendicular, which means that A and B are perpendicular or approximately perpendicular, and the angle between A and B can be between 80 and 100 degrees.

[0096] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. An optical fiber comprising a core layer, characterized in that: The core layer includes: a first zone, a second zone and a third zone; the second zone and the third zone are sequentially coated outside the first zone; the refractive index of the first zone is linearly decreased from the center of the zone to the outside; the refractive index of the second zone is uniformly distributed; the refractive index of the third zone is increased from the inside to the outside.

2. The optical fiber according to claim 1, characterized in that Also includes: An inner cladding and an outer cladding are sequentially coated on the outer side of the core layer; the inner cladding is doped with Cl and F, the outer cladding is doped with Cl, and the content of Cl in the inner cladding is lower than the content of Cl in the outer cladding.

3. The optical fiber according to claim 2, characterized in that The content of SiO2 in the inner cladding is 99.82mol% to 99.87mol%, the content of F is 0.074mol% to 0.22mol%, and the content of Cl is 0.05mol% to 0.1mol%; the content of SiO2 in the outer cladding is 95.7mol% to 97.8mol%, and the content of Cl is 0.3mol% to 0.4mol%.

4. The optical fiber according to claim 1, characterized in that The proportion of the first zone in the core layer is greater than the proportion of the third zone in the core layer; preferably, the third zone accounts for the smallest proportion of the core layer; further preferably, the first zone accounts for 30% to 35% of the core layer, the second zone accounts for 35% to 40% of the core layer, and the third zone accounts for 25% to 30% of the core layer.

5. The optical fiber according to claim 1, characterized in that The relative refractive index difference Δ1 of the first zone at the maximum refractive index relative to the outer cladding is 0.65% to 0.75%; the relative refractive index difference Δ2 of the second zone relative to the outer cladding is 0.6% to 0.68%; the relative refractive index difference Δ3 of the edge of the third zone relative to the outer cladding is 0.7% to 0.82%; preferably, Δ1=(Δ2+Δ3) / 2, 0.05%≤Δ3-Δ1≤0.07%; And / or a relative refractive index difference Δ4 of the inner cladding with respect to the outer cladding is -0.02% to -0.06%.

6. The optical fiber according to claim 1, characterized in that The content of SiO2 in the core layer is 92.16 mol% to 92.78 mol%, the content of Ge is 7 mol% to 7.5 mol%, the content of F is 0.12 mol% to 0.18 mol%, and the content of Cl is 0.08 mol% to 0.12 mol%.

7. The optical fiber according to claim 1 or 4, characterized in that: The core layer radius is 2.75 microns to 3.25 microns; and / or the inner cladding layer radius is 4.2 microns to 16.3 microns, and the radius ratio of the inner cladding layer to the core layer is 1.5≤≤5.

0.

8. A tapered optical fiber, characterized in that: The tapered optical fiber is obtained by tapering the optical fiber according to any one of claims 1 to 7.

9. A method for preparing the optical fiber according to any one of claims 1 to 7, characterized in that: The following steps are involved: A core layer is deposited in a quartz tube; the raw materials of the core layer include silicon tetrachloride, germanium tetrachloride, oxygen and hexafluoroethane; in the first zone, the volume flow ratio of the four raw materials is controlled in the range of (15-27.5): (15.6-28.5): (48-137.5): (0.2-0.4); in the second zone, the volume flow ratio of the four raw materials is controlled in the range of (15-27.5): (15.4-28.3): (48-137.5): (0.2-0.4); in the third zone, the volume flow ratio of the four raw materials is controlled in the range of (15-27.5): (15.8-28.7): (48-137.5): (0.2-0.4); When the third zone is deposited from the outside to the inside, the volume flow rate of germanium tetrachloride is controlled to be gradually reduced; When depositing the second zone, the volume flow rate of germanium tetrachloride is controlled to remain constant; When the first zone is deposited from the outside to the inside, the volume flow rate of germanium tetrachloride is controlled to increase gradually.

10. The method according to claim 9, characterized in that When the core layer is deposited in the quartz tube, the volume flow rate of germanium tetrachloride is controlled to change continuously; and / or The method further comprises the following steps: An inner cladding layer is deposited on the periphery of the core layer; the raw materials of the inner cladding layer include: silicon tetrachloride, oxygen and hexafluoroethane; the volume flow ratio of the above three raw materials is controlled in the range of (27.5-37.5): (220-450): (0.4-0.6); and / or depositing an outer cladding layer outside the inner cladding layer; the raw materials of the outer cladding layer include: silicon tetrachloride and oxygen; the volume flow ratio of the above two raw materials is controlled in the range of (50-105): (125-294).

Images