Optical fiber, optical fiber preform, and method for manufacturing optical fiber

Abstract

To provide an optical fiber having a lower transmission loss.SOLUTION: An optical fiber includes an inner portion including a core portion and an inner cladding portion surrounding the core portion and having a refractive index lower than a maximum refractive index of the core portion, and an outer layer surrounding the inner portion, wherein a pressurization region capable of applying a pressure to the inner portion by applying a pressure from the outside is provided between the inner portion and the outer layer, and an average viscosity of the inner portion is lower than an average viscosity of the outer layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical fiber, an optical fiber preform, and a method for manufacturing an optical fiber. [Background technology]

[0002] One method for reducing the transmission loss of optical fibers is to use fluorine-doped silica glass for the cladding and pure silica glass or silica glass containing a small amount of dopant for the core, and this method is currently being actively investigated.

[0003] On the other hand, the main cause of transmission loss in optical fibers is Rayleigh scattering loss. A major cause of Rayleigh scattering loss is minute density fluctuations in the glass that constitutes the core. One method for controlling such density fluctuations is to control the glass viscosity using a core dopant material. Other methods include controlling the fictive temperature by, for example, adjusting the drawing conditions or introducing an annealing furnace, and many reports have been published on this method, achieving a transmission loss of, for example, about 0.14 dB / km. Still another method is to suppress the occurrence of large voids due to density fluctuations in the glass by applying pressure during the glass transition when drawing an optical fiber from an optical fiber preform (Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] M. Ono, S. Aoyama, M. Fujinami, and S. Ito, “Significant suppression of Rayleigh scattering loss in silica glass formed by the compression of its melted phase,” Optics Express, vol. 26, pp. 7942-7948, 2018. Summary of the Invention [Problem to be solved by the invention]

[0005] As mentioned above, it is important to reduce the transmission loss in optical fibers.

[0006] The present invention has been made in view of the above, and an object of the present invention is to provide an optical fiber having a lower transmission loss, and an optical fiber preform and an optical fiber manufacturing method for achieving the same. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is an optical fiber comprising: an inner portion having a core portion; an inner cladding portion surrounding the core portion and having a refractive index lower than the maximum refractive index of the core portion; and an outer layer surrounding the inner portion, wherein a pressurized region is provided between the inner portion and the outer layer, whereby pressure can be applied to the inner portion by applying pressure from the outside, and the average viscosity of the inner portion is lower than the average viscosity of the outer layer.

[0008] The pressure region may be hollow and further include a fixing portion that fixes the inner portion and the outer layer together.

[0009] The core may be made of silica glass containing at least one of potassium and sodium.

[0010] The inner cladding may contain more fluorine than the outer layer.

[0011] The transmission loss of light with a wavelength of 1550 nm in the core may be 0.15 dB / km or less.

[0012] The ratio of the thickness of the outer layer to the radius of the inner portion may be 1 or greater.

[0013] The ratio of the thickness of the outer layer to the radius of the inner portion may be 1.5 or greater.

[0014] The ratio of the thickness of the outer layer to the radius of the inner portion may be 2 or greater.

[0015] One aspect of the present invention is an optical fiber preform comprising: an inner portion having a core portion; an inner cladding portion surrounding the core portion and having a refractive index lower than the maximum refractive index of the core portion; and an outer layer surrounding the inner portion, wherein a pressure region is provided between the inner portion and the outer layer where pressure can be applied to the inner portion by applying pressure from the outside, and the average viscosity of the inner portion is lower than the average viscosity of the outer layer.

[0016] One aspect of the present invention is a method for manufacturing an optical fiber, which comprises applying pressure to the inner portion by applying pressure to the pressurized region of the optical fiber preform from the outside, thereby drawing the optical fiber. [Effects of the Invention]

[0017] The present invention has the effect of realizing an optical fiber with lower transmission loss. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic cross-sectional view of an optical fiber according to a first embodiment, taken along a plane perpendicular to the longitudinal direction thereof. [Figure 2] FIG. 2 is a schematic cross-sectional view of an optical fiber preform used in manufacturing the optical fiber shown in FIG. 1, taken along a plane perpendicular to the longitudinal direction. [Figure 3] FIG. 3 is an explanatory diagram of a process for drawing an optical fiber from an optical fiber preform. [Figure 4] FIG. 4 is a diagram showing an example of the relationship between the ratio of the thickness of the outer layer to the radius of the inner portion and the noncircularity of the optical fiber. [Figure 5] FIG. 5 is a diagram showing an example of the relationship between the K concentration in the core portion, the reduction in the relative refractive index difference due to F in the inner cladding portion, and the improvement in transmission loss. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments described below. Furthermore, in each drawing, the same or corresponding components are appropriately designated by the same reference numerals, and duplicate explanations are appropriately omitted. Furthermore, terms not specifically defined in this specification shall follow the definitions and measurement methods in ITU-T G.650.1 and G.650.2 of the International Telecommunications Union (ITU).

[0020] (Embodiment 1) 1 is a schematic cross-sectional view of an optical fiber in a plane perpendicular to the longitudinal direction of the optical fiber according to embodiment 1. The optical fiber 1 includes an inner portion 1a and an outer layer 1c.

[0021] The inner portion 1a has a core portion 1aa and an inner cladding portion 1ab. The core portion 1aa is made of, for example, silica glass containing at least one of potassium (K) and sodium (Na). However, the material constituting the core portion 1aa is not limited to this, and it may be, for example, pure silica glass. Pure silica glass is extremely high-purity silica glass that contains substantially no dopants that change the refractive index and has a refractive index of approximately 1.444 at a wavelength of 1550 nm. However, the pure silica glass that constitutes the elements of the optical fiber 1 may contain a certain amount of chlorine and other substances used in the manufacturing process.

[0022] The inner cladding 1ab is disposed to surround the core. The inner cladding 1ab has a refractive index lower than the maximum refractive index of the core 1aa. The inner cladding 1ab is made of silica glass doped with a dopant, such as fluorine, that lowers the refractive index.

[0023] The outer layer 1c is a tubular portion surrounding the inner portion 1a. The outer layer 1c is made of, for example, pure silica glass. Alternatively, the outer layer 1c may be made of silica glass doped with a dopant that lowers the refractive index, such as fluorine. In this case, for example, the inner cladding portion 1ab contains more fluorine than the outer layer 1c.

[0024] Between the inner portion 1a and the outer layer 1c, there is provided a pressurized region 1b that can apply pressure to the inner portion 1a by applying pressure from the outside. In this embodiment, the pressurized region 1b is hollow. The optical fiber 1 also has a fixing portion 1d. The fixing portion 1d is welded to the outer peripheral surface of the inner portion 1a and the inner peripheral surface of the outer layer 1c, fixing the inner portion 1a and the outer layer 1c together. In this embodiment, the fixing portion 1d is a rod-shaped portion with a circular cross section. The fixing portion 1d is made of, for example, pure silica glass, but is not particularly limited as long as it is made of silica glass. Although there is one fixing portion in this embodiment, multiple fixing portions may be provided. In this case, it is preferable that the multiple fixing portions are arranged at equal angles around the central axis of the optical fiber. An example of the number of multiple fixing portions is, for example, three or more.

[0025] Here, in the optical fiber 1, the average viscosity of the inner portion 1a is lower than the average viscosity of the outer layer 1c. The average viscosity of the inner portion 1a is expressed as the viscosity of the core portion 1aa expressed as A (Pa·s) and the cross-sectional area of ​​the core portion 1aa perpendicular to the longitudinal direction of the optical fiber 1 expressed as X (μm 2 ), the viscosity of the inner cladding 1ab is B (Pa·s), and the cross-sectional area of ​​the inner cladding 1ab perpendicular to the longitudinal direction of the optical fiber 1 is Y (μm 2 ), it can be defined as the value of (AX+BY) / (X+Y)(Pa·s).

[0026] For example, assume that the outer layer 1c is made of pure silica glass and the inner cladding portion 1ab is made of silica glass doped with fluorine so that the relative refractive index difference with respect to pure silica glass is −0.4%. In this case, the viscosity of the outer layer 1c at 1200°C is 3×10 11 The viscosity of the inner cladding 1ab at 1200°C is 4×10 10The viscosity is Pa·s. Here, 1200°C is the temperature reached when the optical fiber 1 is heat-softened during the manufacturing process (drawing process). In this case, if the core 1aa is made of pure silica glass, the condition that the average viscosity of the inner portion 1a is lower than that of the outer layer 1c is met. Furthermore, if the core 1aa is made of silica glass containing at least one of potassium and sodium, the viscosity of the core 1aa is even lower than when made of pure silica glass, and therefore the condition that the average viscosity of the inner portion 1a is lower than that of the outer layer 1c is met. If the average viscosity of the outer layer 1c at 1200°C is 1, the average viscosity of the inner portion 1a at 1200°C is, for example, 0.9 or less.

[0027] In the optical fiber 1 configured as described above, density fluctuations in the glass in the region where light propagates (mainly the core 1aa) can be suppressed by applying pressure to the inner portion 1a using the pressurized region 1b during manufacturing, thereby reducing transmission loss. Furthermore, in the optical fiber 1, the average viscosity of the inner portion 1a is lower than that of the outer layer 1c, so that the effect of suppressing density fluctuations due to pressure can be more effectively exerted on the inner portion 1a. As a result, transmission loss is further reduced. In this case, the transmission loss of light with a wavelength of 1550 nm in the core 1aa of the optical fiber 1 is preferably 0.15 dB / km or less.

[0028] The optical fiber 1 can be manufactured, for example, as follows: First, an optical fiber preform is prepared for manufacturing the optical fiber 1. Figure 2 is a schematic cross-sectional view of the optical fiber preform used to manufacture the optical fiber 1, taken along a plane perpendicular to the longitudinal direction.

[0029] The optical fiber preform 10 shown in FIG. 2 has a cross-sectional structure similar to that of the optical fiber 1. That is, the optical fiber preform 10 includes an inner portion 10a having a core portion 10aa and an inner cladding portion 10ab, an outer layer 10c, and a fixing portion 10d. A hollow pressurized region 10b is provided between the inner portion 10a and the outer layer 10c, allowing pressure to be applied to the inner portion 10a by applying pressure from the outside. The constituent materials of the optical fiber preform 10 are the same as the constituent materials of the corresponding constituent materials of the optical fiber 1. For example, if the core portion 1aa is made of silica glass containing potassium, the core portion 10aa is also made of silica glass containing potassium. Therefore, the average viscosity of the inner portion 10a is lower than that of the outer layer 10c.

[0030] The optical fiber preform 10 can be manufactured by preparing the inner portion 10a, the outer layer 10c, and the fixing portion 10d, and then heating and bonding them with an oxyhydrogen flame. Furthermore, the manufacturing process is facilitated by aligning the inner portion 10a, the outer layer 10c, and the fixing portion 10d using an alignment jig.

[0031] Next, an optical fiber is drawn from this optical fiber preform 10 to manufacture the optical fiber 1. FIG.

[0032] In the process of drawing an optical fiber, the optical fiber preform 10 is placed in a drawing furnace of a drawing apparatus 100, and one end of the optical fiber preform 10 is heated and softened by a heater 101 in the drawing furnace, and the optical fiber 1 is drawn vertically downward. A pressure device 102 is connected to the upper end of the optical fiber preform 10 via a pipe 103 and a connecting jig 104. During drawing, an inert gas such as N2 or Ar is fed from the pressure device 102 into the pressure region 10b of the optical fiber preform 10. This allows the optical fiber 1 to be drawn while applying pressure to the inner region 10a by applying pressure (e.g., 10,000 Pa or more) to the pressure region 10b from the outside. Here, since the average viscosity of the inner region 10a is lower than that of the outer layer 10c of the optical fiber preform 10, the inner region 10a is pressurized more effectively than the outer layer 10c. This effectively suppresses density fluctuations in the inner region 10a.

[0033] During drawing, it is desirable to monitor the outer diameter and non-circularity of the optical fiber and optimize the pressure conditions so that these are at desired values.

[0034] (Favorable characteristics of optical fiber) Next, preferred characteristics of the optical fiber according to the embodiment will be described. The inventors first investigated the effect of the ratio of the thickness of the outer layer to the radius of the inner portion of the optical fiber on the noncircularity of the optical fiber. For the investigation, an optical fiber preform was manufactured that had the same basic structure as the optical fiber preform 10 but differed in that four fixing portions 10d were provided equiangularly around the central axis. The radius of the cross section perpendicular to the longitudinal direction of the manufactured optical fiber preform was 8 mm to 30 mm. In this case, the core was made of silica glass doped with 100 ppm potassium, the inner cladding was made of silica glass doped with fluorine so that the relative refractive index difference with respect to pure silica glass was −0.4%, and the fixing portion and the outer layer were made of pure silica glass. The thickness of the outer layer was 3 mm to 16 mm, and the thickness of the pressurized region was half the thickness of the outer layer.

[0035] The optical fiber preform manufactured as described above was drawn while applying pressure as described with reference to Figure 3 to manufacture an optical fiber, and the noncircularity of the cross section perpendicular to the longitudinal direction of the manufactured optical fiber was measured. On the other hand, for comparison, an optical fiber was manufactured by drawing without applying pressure, and the noncircularity of the cross section perpendicular to the longitudinal direction of the manufactured optical fiber was measured. It was confirmed that the potassium added to the core diffused to a certain extent into the inner cladding due to heating during drawing, etc. of the optical fiber.

[0036] Figure 4 shows an example of the relationship between the ratio of the thickness of the outer layer to the radius of the inner portion and the noncircularity of an optical fiber. Note that the data shown in Figure 4 is selected from optical fibers in which the transmission loss at 1550 nm of the optical fiber was reduced by 0.01 dB / km or more compared to when no pressure was applied. As shown in Figure 4, a small ratio increases the noncircularity, but a ratio of 1 or more is preferable because it can reduce the noncircularity to 2% or less, a ratio of 1.5 or more is more preferable because it can reduce the noncircularity to 1% or less, and a ratio of 2 or more is even more preferable because it can reduce the noncircularity to 1% or less and the change in noncircularity relative to changes in the ratio is stable.

[0037] Furthermore, according to the study by the present inventors, it has been found that the number of fixing portions 1d is preferably three or more in order to stabilize the non-circularity.

[0038] Next, the ratio of the thickness of the outer layer to the radius of the inner part was fixed at 1.5, and the potassium concentration in the core and the fluorine concentration in the inner cladding were varied to investigate the change in transmission loss at a wavelength of 1550 nm in the optical fiber using simulation calculations and experiments. The optical fiber used had the same basic structure as optical fiber 1, but differed in that four fixing parts 1d were provided equiangularly around the central axis. The thickness of the pressurized region was set to half that of the outer layer. Optimal drawing conditions were used during the experiments.

[0039] 5 shows an example of the relationship between the reduction in relative refractive index difference due to the potassium (K) concentration in the core and the fluorine (F) concentration in the inner cladding, and the improvement in transmission loss. Here, the improvement in transmission loss is the reduction in transmission loss when pressurized from the transmission loss when no pressurization is applied at a wavelength of 1550 nm, and is expressed in dB / km.

[0040] As shown in Figure 5, it was confirmed that the improvement was 0.01 dB / km or more when the decrease in the relative refractive index difference (hereinafter simply referred to as the decrease) was -0.6% or less when the K concentration was 0 ppm, when the decrease was -0.3% or less when the K concentration was about 10 ppm, when the decrease was -0.15% or less when the K concentration was about 20 ppm, when the decrease was -0.08% or less when the K concentration was about 30 ppm, when the decrease was -0.02% or less when the K concentration was about 40 ppm, and when the decrease was 0% (i.e., no F was added) when the K concentration was about 45 ppm.

[0041] In the optical fiber according to the embodiment, the outer diameter of the outer layer is, for example, 125 μm, but from the viewpoint of mechanical reliability, it is preferably 250 μm or less.

[0042] In the optical fiber according to the embodiment, the effective core area (Aeff) is set to 170 μm from the viewpoint of suppressing an increase in transmission loss due to the influence of microbending loss. 2 It is preferable that:

[0043] Furthermore, the optical fiber according to the embodiment is preferably a single-mode optical fiber from the viewpoint of suppressing changes in transmission loss due to the influence of higher-order modes, but is not limited to this, and even if it is a multi-mode optical fiber, the effect of reducing transmission loss by suppressing density fluctuations can be obtained.

[0044] Moreover, the optical fiber according to the embodiment preferably satisfies the optical fiber characteristics specified in ITU-T G.652 or related recommendations.

[0045] In the optical fiber and optical fiber preform according to the embodiment, the pressurized region is hollow, but the pressurized region may be hollow as long as it can be pressurized from the outside. For example, the pressurized region may be porous glass through which a pressurizing gas can flow, or may be a solid medium whose volume expands when heated. In this case, the fixing portion may not be necessary.

[0046] Furthermore, the present invention is not limited to the above-described embodiments. Configurations in which the above-described components are appropriately combined are also included in the present invention. For example, the refractive index profile of the inner portion is a step type in embodiment 1, but it may be replaced with any refractive index profile applied to optical fibers, such as a trench type or a W-type. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible. [Explanation of symbols]

[0047] 1: Optical fiber 1a, 10a: inner part 1aa, 10aa: Core part 1ab, 10ab: inner cladding 1b, 10b: Pressurized area 1c, 10c: outer layer 1d, 10d: Fixed part 10: Optical fiber preform 100:Drawing device 101: Heater 102: Pressure device 103: Pipe 104: Connection jig

Claims

1. an inner portion having a core portion and an inner cladding portion surrounding the core portion and having a refractive index lower than the maximum refractive index of the core portion; an outer layer surrounding the inner portion; Equipped with a pressure region is provided between the inner portion and the outer layer, where pressure can be applied to the inner portion by applying pressure from the outside; The average viscosity of the inner portion is lower than the average viscosity of the outer layer. Optical fiber.

2. The pressure region is hollow, The inner layer and the outer layer are further provided with a fixing portion. The optical fiber of claim 1 .

3. The core is made of silica glass containing at least one of potassium and sodium. The optical fiber of claim 1 .

4. The inner cladding contains more fluorine than the outer cladding. The optical fiber of claim 1 .

5. The transmission loss of light at a wavelength of 1550 nm in the core is 0.15 dB / km or less. The optical fiber of claim 1 .

6. The ratio of the thickness of the outer layer to the radius of the inner portion is 1 or greater. The optical fiber of claim 1 .

7. The ratio of the thickness of the outer layer to the radius of the inner portion is 1.5 or greater. The optical fiber of claim 1 .

8. The ratio of the thickness of the outer layer to the radius of the inner portion is 2 or greater. The optical fiber of claim 1 .

9. an inner portion having a core portion and an inner cladding portion surrounding the core portion and having a refractive index lower than the maximum refractive index of the core portion; an outer layer surrounding the inner portion; Equipped with a pressure region is provided between the inner portion and the outer layer, where pressure can be applied to the inner portion by applying pressure from the outside; The average viscosity of the inner portion is lower than the average viscosity of the outer layer. Optical fiber base material.

10. 9. The method for producing an optical fiber according to claim 1, The optical fiber preform according to claim 9 is drawn while applying pressure to the inner portion by applying pressure to the pressurized region from the outside. Optical fiber manufacturing method.

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