optical fiber

Abstract

The present disclosure relates to an optical fiber having a structure with low transmission loss and capable of being produced with high productivity. An optical fiber according to an embodiment includes a core and a cladding. The core is formed of silica glass to which bromine is added, and the cladding has a refractive index lower than the maximum refractive index of the core. In addition, the core has a compressive stress.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an optical fiber.

[0002] This application claims priority to Japanese Patent Application No. 2019-225471 filed on December 13, 2019, the contents of which are incorporated herein by reference in its entirety and for all purposes. BACKGROUND

[0003] The demand for transmission capacity is increasing, and in order to meet this demand, the laying of optical fiber cables is constantly being carried out. At this time, it is possible to achieve an improvement in signal-to-noise ratio or a reduction in optical amplifiers by laying optical fibers with low transmission loss, and in particular, in long distance transmission, there is an effect of improving the cost effectiveness of the system. Therefore, the use of a pure silica core fiber (PSCF) having low transmission loss instead of a standard single mode fiber (SSMF) having a core with added GeO2 in long distance transmission is increasing.

[0004] The PSCF has a core formed of silica glass that does not contain GeO2, and a cladding formed of silica glass whose refractive index is lowered by the addition of fluorine (F). The addition of F to the silica glass is achieved by heating and sintering a soot body of silica glass in an atmosphere of a fluorine-containing gas such as SiF4 or CF4, but this process of adding F is generally less productive and more costly than the process of manufacturing pure silica glass. The higher the F concentration, the more pronounced this tendency. In addition, in the PSCF and the SSMF, the outer diameter of the cladding is 125 μm, while the outer diameter of the core is only about 10 μm. Therefore, the productivity of the cladding, which occupies more than 99% of the volume, can greatly affect the productivity of the optical fiber as a whole. As a result, the PSCF is more expensive than the SSMF, and the current production volume in the entire industry is only about 1 / 100 of that of the SSMF.

[0005] On the other hand, the SSMF has a cladding formed of pure silica glass or silica glass containing a very small amount of F, and a core formed of silica glass whose refractive index is raised by the addition of GeO2. Since the productivity of the cladding is high, the productivity is high compared to the PSCF, but the transmission loss increases due to the addition of GeO2 in the core. When comparing the transmission loss in a wavelength of 1550 nm, the PSCF is 0.15 dB / km or more and 0.17 dB / km or less, while the SSMF is as high as 0.18 dB / km or more and 0.20 dB / km or less.

[0006] In view of this, as one of the fiber structures that take into account both low transmission loss and high productivity, Patent Document 1 below proposes an optical fiber in which the refractive index is increased by adding a high concentration of chlorine (Cl) instead of GeO2 in the core, and a method for manufacturing the same. However, in order to add Cl at a concentration sufficient to guide light, it is necessary to sinter a soot body of silica glass in an atmosphere containing SiCl4 gas several times the atmospheric pressure. Moreover, in a post-process after sintering, the possibility of bubbles being generated in the glass due to vaporization of SiCl4 becomes high.

[0007] As another of the fiber structures that take into account both low transmission loss and high productivity, Patent Document 2 below proposes an optical fiber in which the refractive index is increased by adding bromine (Br) instead of GeO2 in the core, and a method for manufacturing the same. By sintering a soot body of silica glass in an atmosphere containing SiBr4 at approximately the same pressure as the atmospheric pressure, it is possible to add Br at a concentration sufficient to guide light, and SiBr4 has the characteristic of having a large molecular weight and being less likely to vaporize compared to SiCl4.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: U.S. Patent Publication No. 2019 / 0119143

[0011] Patent Document 2: U.S. Patent Publication No. 2017-0176673 SUMMARY

[0012] The optical fiber according to an embodiment of the present disclosure has a core that extends along a central axis, and a cladding that surrounds the core. The core is formed of silica glass to which bromine is added. The cladding is formed of silica glass having a lower refractive index than the maximum refractive index of the core. In addition, the residual stress of the core is a compressive stress. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 is a diagram showing an outline structure of a manufacturing apparatus for manufacturing the optical fiber according to each of the embodiments of the present disclosure.

[0014] Figure 2 is a diagram showing the cross-sectional structure of each of the optical fibers (Types A to C) according to the first to third embodiments of the present disclosure.

[0015] Figure 3 is a diagram showing the refractive index profile, the viscosity distribution, and the stress distribution of the optical fiber (Type A) according to the first embodiment of the present disclosure, shown along a common straight line orthogonal to the central axis of the optical fiber.

[0016] Figure 4 is Figure 3An enlarged view of the region R1 in the refractive index profile and the viscosity distribution.

[0017] Figure 5 is a refractive index profile, a viscosity distribution, and a stress distribution shown along a common straight line orthogonal to a central axis of an optical fiber (Type B) related to a second embodiment of the present disclosure.

[0018] Figure 6 is a refractive index profile, a viscosity distribution, and a stress distribution shown along a common straight line orthogonal to a central axis of an optical fiber (Type C) related to a third embodiment of the present disclosure.

[0019] Figure 7 is a diagram showing a cross-sectional structure of an optical fiber related to a fourth embodiment (Type D) of the present disclosure.

[0020] Figure 8 is a refractive index profile, a viscosity distribution, and a stress distribution shown along a common straight line orthogonal to a central axis of an optical fiber related to a fourth embodiment (Type D) of the present disclosure. DETAILED DESCRIPTION

[0021] [PROBLEMS TO BE SOLVED BY THE INVENTION]

[0022] The inventors have conducted research on the above-described related art, and as a result, have found the following technical problem. That is, the transmission loss of a fiber in which Br is added to the core tends to be high, and a transmission loss higher than that of GeO2 is also reported in the above-described Patent Literature 2. Thus, it is a technical problem in the related art to reduce the transmission loss by a fiber in which Br is added to the core.

[0023] The present disclosure has been made to solve the above-described technical problem, and aims to provide an optical fiber having a structure that has low transmission loss and can be produced with high productivity.

[0024] [EFFECTS OF THE INVENTION]

[0025] According to the present disclosure, by providing an optical fiber having a core in which Br is added and having compressive stress, reduction of transmission loss and high productivity can be achieved.

[0026] [EXPLANATION OF EMBODIMENTS OF THE PRESENT APPLICATION]

[0027] Hereinafter, the content of the embodiments of the present disclosure will be explained, respectively and independently.

[0028] First, the words common to the embodiments of the present disclosure will be explained, and the content of the embodiments of the present disclosure will be explained, respectively and independently.

[0029] (Definition of Words)

[0030] In the present specification, the difference Δ in the relative refractive index of a certain medium (refractive index n) with respect to pure silica glass (refractive index n0) is provided by

[0031] Δ = (n / n0) - 1

[0032]

[0033] In addition, unless otherwise specified, an "optical fiber" is assumed to be a structure having one central axis, being substantially rotationally symmetric around the central axis, and being translationally symmetric along the central axis. As to constituent elements of the optical fiber such as a core, a cladding, a coating, and the like, unless otherwise specified, a structure substantially rotationally symmetric around the central axis and translationally symmetric along the central axis is assumed. In a case where these assumptions can be applied, a physical property value of a constituent element of the optical fiber is defined in an arbitrary cross section orthogonal to the central axis. When a statistical value such as an average value, a maximum value, a percentage value, or the like of a physical property value is defined, the physical property value in the above cross section can be replaced with a statistical value of a set of measured values obtained by measuring at a uniform frequency in space at a prescribed spatial resolution. Furthermore, unless otherwise specified, the above spatial resolution is assumed to be approximately the action wavelength of the optical fiber, that is, a circle having a radius of 1 μm.

[0034] In an outer region of the core that surrounds an inner region in the vicinity of the central axis, the refractive index profile of the core has a shape in which a difference Δ0 in the relative refractive index at a position separated from the central axis by an amount of a distance r0 in the radial direction, a difference Δ1 in the relative refractive index at a position separated from the central axis by an amount of a distance r1 longer than the distance r0, and a difference Δr in the relative refractive index at a position separated from the central axis by an amount of a distance r between the distance r0 and the distance r1 satisfy the following expression (1):

[0035] Δr = Δ0 + (Δ1 - Δ0) x ((r - r0) / (r1 - r0))α... (1)

[0036] an approximate relationship. The shape is adjusted by changing the value of the index α (as an example, α = 2.0). The inner region in the vicinity of the central axis including the central axis is difficult to control to have a correct refractive index profile in a manufacturing process of the optical fiber, and thus the outer region of the core that surrounds the inner region is controlled to have a correct refractive index profile.

[0037] ​(1) As one aspect of an optical fiber according to an embodiment of the present disclosure, there is provided a fiber having a core portion extending along a central axis, and a cladding portion surrounding the core portion. The core portion is formed of silica glass to which bromine is added. The cladding portion is formed of silica glass having a lower refractive index than the maximum refractive index of the core portion. In addition, the residual stress of the core portion is a compressive stress. With this configuration, low transmission loss and high productivity can be achieved.

[0038] (2) As one aspect of the present disclosure, the cladding portion can have a multi-layer structure. As one example, the cladding portion is composed of a first cladding portion surrounding the core portion in contact with the outer peripheral surface of the core portion, and a second cladding portion surrounding the first cladding portion in contact with the outer peripheral surface of the first cladding portion. Note that the first cladding portion is formed of silica glass to which fluorine is added. The second cladding portion is formed of pure silica glass or silica glass to which fluorine is added at a lower concentration than the fluorine concentration of the first cladding portion. In addition, the second cladding portion has a tensile stress. This configuration can achieve low transmission loss, and can achieve both the low transmission loss and high productivity. In particular, the second cladding portion is preferably pure silica glass in which the concentration of halogen elements is suppressed to less than 0.1 wt%. Thus, a large adhesion difference between the second cladding portion and the core portion is achieved, a tensile stress is formed in the second cladding portion, and a compressive stress is formed in the core portion.

[0039] (3) As one aspect of the present disclosure, the multi-layer structure of the cladding portion is composed of a first cladding portion surrounding the core portion in contact with the outer peripheral surface of the core portion, a second cladding portion surrounding the first cladding portion in contact with the outer peripheral surface of the first cladding portion, and a third cladding portion surrounding the second cladding portion in contact with the outer peripheral surface of the second cladding portion. Note that the first cladding portion is formed of silica glass to which fluorine is added. The second cladding portion is formed of pure silica glass or silica glass to which fluorine is added at a lower concentration than the fluorine concentration of the first cladding portion. According to this configuration, the residual stress of the second cladding portion is a tensile stress. The third cladding portion is formed of pure silica glass or silica glass to which fluorine is added at a lower concentration than the fluorine concentration of the first cladding portion. According to this configuration, the residual stress of the third cladding portion is a compressive stress. With this configuration, low transmission loss can be achieved, and both the low transmission loss and high productivity can be achieved.

[0040] (4) As one aspect of the present disclosure, it is preferred that the core further contains chlorine, and the optical fiber has a viscosity adjustment region. The viscosity adjustment region is a region defined on a cross section of the optical fiber orthogonal to the central axis, and is constituted by a portion of the core and a portion of the cladding that are adjacent to a boundary portion of the core and the cladding (the first cladding in the case where the cladding has a multilayer structure). Specifically, the viscosity adjustment region has a shape that surrounds the central axis in a state of being separated from the central axis, and the shape (planar shape defined on the cross section) of the viscosity adjustment region has an inner peripheral portion and an outer peripheral portion that are arranged in a manner of sandwiching the boundary portion of the core and the cladding in a state where the separation distance (corresponding to the width of the viscosity adjustment region defined in the radial direction) is 2 pm or more. In the viscosity adjustment region having such a shape, the viscosity distribution (distribution defined in the radial direction) of the optical fiber has a viscosity distribution that continuously changes in the radial direction. Note that the radial direction coincides with the direction from the central axis to the outer periphery of the optical fiber on the cross section of the optical fiber.

[0041] In the above, each aspect listed in the column of [Explanation of Embodiments of the Present Disclosure] can be applied to each of the remaining aspects, or all combinations of these remaining aspects.

[0042] [Detailed Contents of Embodiments of the Present Disclosure]

[0043] Hereinafter, the specific structure of the optical fiber according to the embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to these examples, and is intended to include all modifications within the meaning and scope equivalent to those shown by the claims. In addition, the same reference numerals are given to the same elements in the explanation of the drawings, and the repeated explanation is omitted.

[0044] (Optical fiber manufacturing apparatus)

[0045] Figure 1 is a diagram showing an outline structure of a manufacturing apparatus for manufacturing the optical fiber according to each of the embodiments of the present disclosure. Figure 1The shown optical fiber manufacturing apparatus 1 is provided with a drawing furnace 23 that heats one end of an optical fiber preform 10, a heating furnace 24 that performs temperature control, a cooling device 25 that cools a bare fiber drawn out in a He atmosphere, a mold 26 for applying a resin for coating to the outer peripheral surface of the cooled bare fiber, an ultraviolet light source 27 that outputs ultraviolet light for curing the resin, a roller 28, a capstan 29, and a winder 30. Note that the resin coating device 21 is constituted by the mold 26 and the ultraviolet light source 27, and a primary coating is provided on the outer peripheral surface of the bare fiber by the resin coating device 21. A resin coating device 22 (including a mold and an ultraviolet light source) having the same structure as the resin coating device 21 on the upstream side is arranged downstream of the resin coating device 21, and a secondary coating is provided on the outer peripheral surface of the primary coating provided by the resin coating device 21 on the upstream side.

[0046] Specifically, one end of the prepared optical fiber preform 10 is heated by the drawing furnace 23, and a bare fiber is drawn from the heated end. The bare fiber drawn from the drawing furnace 23 gradually decreases in temperature in the heating furnace 24 that performs temperature control. During this period, structural relaxation of glass occurs in the bare fiber, and by this structural relaxation, an increase in transmission loss caused by the related art is eliminated. The bare fiber drawn from the heating furnace 24 is cooled in the He atmosphere of the cooling device 25, and then passes through the mold 26. When passing through the mold 26, a coating resin is applied to the outer peripheral surface of the bare fiber (primary coating). The applied coating resin is cured by being irradiated with ultraviolet light from the ultraviolet light source 27. Furthermore, a secondary coating is provided on the primary coating provided by the resin coating device 21, and thus an optical fiber 100 is obtained.

[0047] The roller 28 has a rotation surface that is inclined with respect to the running direction of the optical fiber 100 obtained by passing through the resin coating device 21 and the resin coating device 22. Thus, a twist is applied to the optical fiber 100, and polarization mode dispersion can be reduced. The capstan 29 is located downstream of the roller 28, and applies a prescribed tension to the optical fiber 100. Thus, a compressive stress and a tensile stress remain in the core and the cladding of the drawn optical fiber 100. The optical fiber 100 that has passed through the capstan 29 is wound by the winder 30 that rotates in the direction indicated by an arrow S in the figure.

[0048] Note that, in the example of Figure 1 the resin coating device 21 that provides the primary coating and the resin coating device 22 that provides the secondary coating are arranged in this order along the fiber running direction, but the secondary coating can also be provided after the optical fiber provided with the primary coating is wound by the winder 30. In this case, the resin coating device 22 is not needed. That is, the secondary coating is provided in the optical fiber that is replaced from the winder 30 and wound in another winder.

[0049] Furthermore, the cross-sectional structure of the optical fiber 100 obtained by the optical fiber manufacturing apparatus 1 with the above-described structure is similar to that of the optical fiber parent material 10. Therefore, by placing optical fiber parent materials 10 with different cross-sectional structures into the optical fiber manufacturing apparatus 1, optical fibers with various cross-sectional structures, such as the optical fiber 100a according to the first embodiment, the optical fiber 100b according to the second embodiment, and the optical fiber 100c according to the third embodiment, can be obtained.

[0050] (First Implementation)

[0051] Figure 2 The type A optical fiber shown in the upper section is the optical fiber 100a according to the first embodiment of this disclosure. Furthermore, the refractive index curve 150a, viscosity distribution 151a, and stress distribution 152a shown along a common straight line orthogonal to the central axis AX of this optical fiber 100a are... Figure 3 As shown in the image. It should be noted that... Figure 4 yes Figure 3 Enlarged view of region R1 in the refractive index curve 150a and viscosity distribution 151a shown.

[0052] like Figure 2 As shown in the upper paragraph, the optical fiber 100a of type A according to the first embodiment includes: a core 110 extending along the central axis AX of the optical fiber 100a; a cladding 120 surrounding the core 110; a primary cladding 210 surrounding the cladding; and a secondary cladding 220 surrounding the primary cladding 210.

[0053] The core 110 is formed of silica glass (SiO2) containing bromine (Br). In this core 110, the Br concentration is 0.8 wt% or more and 2.6 wt% or less, preferably 1.6 wt% or more and 2.6 wt% or less. Furthermore, the maximum relative refractive index difference of the core 110 is 0.1% or more and 0.3% or less, preferably 0.2% or more and 0.3% or less. The cladding 120 is formed of pure silica glass or silica glass containing trace amounts of fluorine (F) of 3000 ppm or less. More preferably, it is formed of silica glass in which the total concentration of chlorine, fluorine, and other halogen elements is suppressed to 0.1 wt% or less. Figure 3 The simplified refractive index profile 150a of the optical fiber 100a according to the first embodiment is shown. Here, the curve shape in the outer region of the core 110 is provided by the above formula (1), but in the refractive index profile 150a, the curve shape of the core 110 shows a general shape.

[0054] In the first embodiment, the diameter of the core 110 is 6 μm or more and 10 μm or less. With this configuration, the optical fiber 100a according to the first embodiment has one or more waveguide modes (a group of two polarization modes is defined as one waveguide mode) in the lowest loss band of silica glass, that is, the 1550 nm band. In addition, the effective area of the fundamental mode in the wavelength of 1550 nm is preferably 60 μm 2 and 120 μm 2 Hereinafter. The outer diameter of the cladding 120 is preferably 125 ± 1 μm. The outer diameter of the entire cladding including the primary cladding 210 and the secondary cladding 220 (substantially the outer diameter of the secondary cladding 220) is 245 ± 5 μm, and more preferably 200 ± 5 μm.

[0055] Next, the intermediate value of the viscosity of the core 110 becomes lower than the maximum value of the viscosity of the cladding 120 due to the difference in the concentration of the above-described additive. More preferably, the intermediate value of the viscosity of the core 110 is lower than the 75% percentile value of the viscosity in the cladding 120. Further preferably, it is lower than the intermediate value of the viscosity of the cladding 120. Due to this difference in the viscosity of each portion, the tension at the time of manufacturing the optical fiber 100a, particularly, at the time of drawing the parent material, is supported by the cladding 120, and as a result, the residual tensile stress is left in the cladding 120 of the optical fiber 100a after drawing, and on the other hand, the residual compressive stress is left in the core 110. In Figure 3 In addition to the refractive index profile 150a, the simplified viscosity distribution 151a and the simplified stress distribution 152a of the optical fiber 100a according to the first embodiment are shown. Note that, Figure 3 The positions of the respective axes of the refractive index profile 150a, the viscosity distribution 151a, and the stress distribution 152a on the cross section orthogonal to the central axis AX of the optical fiber 100a (positions on the straight line passing through the central axis AX) are shown to be uniform.

[0056] The compressive stress itself depends not only on the viscosity difference between different parts, but also on the drawing conditions such as the tension during drawing of the base material. Specifically, to suppress the increase in transmission loss, the absolute value of the compressive stress of the core 110 (the absolute value of the average value of the residual stress within the core 110) is preferably 15 MPa or more, more preferably 30 MPa or more. Further preferably, the absolute value of the compressive stress of the core 110 is the absolute value of 75% of the residual stress within the core 110, preferably 30 MPa or more. When tensile stress remains in the glass, an increase in transmission loss due to glass defects is likely to occur. However, when the residual stress of the core 110 is a sufficiently large absolute value of the average value or the absolute value of 75% of the residual stress, as described above, the increase in transmission loss due to localized tensile stress can be effectively suppressed. It should be noted that residual stress is represented by a positive sign for tensile stress and by a negative sign for compressive stress, and the percentage value is defined as a proportion when the signed values ​​are arranged from smallest to largest.

[0057] Furthermore, in the optical fiber 100a according to this first embodiment, it is preferable that although the viscosity differs between the core 110 and the cladding 120, the spatial variation is continuous and gradual. When the viscosity difference between the core 110 and the cladding 120 is abrupt, unexpected changes in temperature or tension during fiber drawing can cause large variations in structure or residual stress at the boundary between the core 110 and the cladding 120. This can lead to increased transmission loss. Therefore, at the boundary between the core 110 and the cladding 120, since the spatial variation in viscosity is gradual, the increase in transmission loss can be suppressed. More preferably... Figure 4 As shown, in the viscosity adjustment region AD, which includes the boundary between the core 110 and the cladding 120 (the point P0 where the absolute value of the refractive index gradient is the largest), and has a width of 2 μm or more, more preferably 3 μm or more, the viscosity preferably changes continuously. When defined on the cross-section of the optical fiber 100a orthogonal to the central axis AX, the viscosity adjustment region AD is an annular region having an inner periphery and an outer periphery, which are arranged such that they sandwich the boundary between the core 110 and the cladding 120, with a distance of 2 μm or more, preferably separated by 3 μm or more. Therefore, the distance between the inner and outer peripheries corresponds to the width of the viscosity adjustment region AD defined along the radial direction.

[0058] It should be noted that, in order to... Figure 4 This control method, as shown, makes the viscosity change near the boundary between the core 110 and the cladding 120 more gradual. It is preferable to add Cl and Br together to the core 110. It should be noted that it is desirable that at least one or more of the additives (besides Br, F, Cl, etc., as needed) added to the core 110 and cladding 120 of the optical fiber 100a according to this first embodiment are obtained through... Figure 1The soot deposition process is added in the manufacturing process of the optical fiber preform 10 shown.

[0059] In addition to the above-described gradual change in the shape of the compressive stress and the viscosity distribution, the average value of the Cl concentration in the core 110 is preferably 100 ppm or more. By containing Cl, an increase in transmission loss due to glass defects is further suppressed. More preferably, the average value of the Cl concentration in the core 110 is 200 ppm or more. Also, the 75% percentile value of the Cl concentration in the core 110 is preferably 200 ppm or more. In this case, an increase in transmission loss due to glass defects is further suppressed.

[0060] (Second Embodiment)

[0061] Figure 2 The optical fiber 100b of Type B shown in the middle of the drawing is an optical fiber 100b related to a second embodiment of the present disclosure. In addition, the refractive index profile 150b, the viscosity distribution 151b, and the stress distribution 152b shown along a common straight line orthogonal to the central axis AX of the optical fiber 100b are shown in the middle of the drawing of Figure 5 It should be noted that, Figure 5 The distribution shape of the region R2 in the viscosity distribution 151b shown is substantially similar to the distribution shape shown in the middle of the drawing of Figure 4

[0062] As shown in the middle of the drawing of Figure 2 The optical fiber 100b of Type B related to the second embodiment has a core 110 extending along the central axis AX of the optical fiber 100b, a first cladding 120a surrounding the core 110, a second cladding 120b surrounding the first cladding 120a, a primary coating 210 surrounding the second cladding 120b, and a secondary coating 220 surrounding the primary coating 210. In addition, the cladding 120 is constituted by the first cladding 120a and the second cladding 120b.

[0063] The core 110 is formed of silica glass (SiO2) containing bromine (Br) similarly to the optical fiber 100a related to the first embodiment. In the core 110, the Br concentration is 0.8 wt% or more and 2.6 wt% or less, and is preferably 1.6 wt% or more and 2.6 wt% or less. In addition, the maximum relative refractive index difference of the core 110 is 0.1% or more and 0.3% or less, and is preferably 0.2% or more and 0.3% or less. The first cladding 120a is formed of silica glass containing 1000 ppm or more and 3000 ppm or less of a trace amount of fluorine (F). The second cladding 120b is formed of pure silica glass or silica glass containing F at a lower concentration than the F concentration of the first cladding 120a. In Figure 5 ​In the second embodiment, a simplified refractive index profile 150b of the optical fiber 100b is shown. Here, the profile shape in the outer region of the core 110 is provided by the above-described formula (1), but in the refractive index profile 150b, the profile shape of the core 110 shows an outline shape.

[0064] In the second embodiment, the diameter of the core 110 is 6 μm or more and 12 μm or less. With this configuration, the optical fiber 100b of the second embodiment has one or more waveguide modes (a group of two polarization modes is defined as one waveguide mode) in the lowest loss band of silica glass, that is, the 1550 nm band. In addition, the effective area of the fundamental mode in the wavelength 1550 nm is preferably 60 μm 2 and 160 μm 2 Hereinafter. The outer diameter of the cladding 120 including the first cladding 120a and the second cladding 120b (substantially the outer diameter of the second cladding 120b) is 125 ± 1 μm, and the outer diameter of the entire coating including the primary coating 210 and the secondary coating 220 (in practice, the outer diameter of the secondary coating 220) is 245 ± 5 μm, and more preferably 200 ± 5 μm.

[0065] Next, due to the difference in the concentration of the above-described additive, the intermediate value of the viscosity of each of the core 110 and the first cladding 120a becomes lower than the maximum value of the viscosity of the second cladding 120b. More preferably, the intermediate value of the viscosity of each of the core 110 and the first cladding 120a is lower than the 75% percentile value of the viscosity of the second cladding 120b. Further preferably, the intermediate value of the viscosity of each of the core 110 and the first cladding 120a is lower than the intermediate value of the viscosity in the second cladding 120b. Due to this difference in the viscosity of each portion, the tension applied at the time of manufacturing the optical fiber 100b, particularly at the time of drawing the preform, is supported by the second cladding 120b, and as a result, the second cladding 120b of the optical fiber 100b after drawing has a residual tensile stress, and on the other hand, the core 110 and the first cladding 120a have a residual compressive stress. In Figure 5 In the second embodiment, in addition to the refractive index profile 150b, a simplified viscosity distribution 151b and a simplified stress distribution 152b of the optical fiber 100b are shown. Note that, Figure 5 The positions of the respective horizontal axes of the refractive index profile 150b, the viscosity distribution 151b, and the stress distribution 152b on the cross section orthogonal to the central axis AX of the optical fiber 100b (positions on a straight line passing through the central axis AX) are shown to be uniform.

[0066] The compressive stress itself depends not only on the difference in viscosity of the respective portions, but also on the tension and the like during drawing of the parent material. Among them, in order to suppress an increase in transmission loss, the absolute value of the compressive stress of the core portion 110 (the absolute value of the average of the stresses remaining in the core portion 110) is preferably 15 MPa or more, and more preferably 30 MPa or more. Further preferably, the absolute value of the compressive stress of the core portion 110 is the absolute value of 75% of the percentage value of the stresses remaining in each of the core portion 110 and the first clad layer 120a, and is preferably 30 MPa or more. When a tensile stress remains in the glass, an increase in transmission loss due to a defect in the glass easily occurs. However, as described above, in the case where the residual stresses of the core portion 110 and the first clad layer 120a are compressive stresses whose absolute values are sufficiently large, either the average or 75% of the percentage value, an increase in transmission loss due to a local tensile stress is effectively suppressed.

[0067] In the optical fiber 100b according to the second embodiment as well, as with the optical fiber 100a according to the first embodiment described above, it is preferable that, although the viscosity is different between the core portion 110 and the first clad layer 120a, the spatial change thereof is continuous and gentle. When the difference in viscosity between the core portion 110 and the first clad layer 120a is sharp, a large variation in structure or residual stress occurs at the boundary portion between the core portion 110 and the first clad layer 120a due to unexpected variations in temperature or tension during drawing. This can become a cause of an increase in transmission loss. Thus, at the boundary portion between the core portion 110 and the first clad layer 120a, since the spatial change in viscosity is gentle, an increase in transmission loss is suppressed. Note that the distribution shape of the region R2 of the viscosity distribution 151b is substantially similar to the shape shown in FIG. 6. That is, in the optical fiber 100b according to the second embodiment as well, in the viscosity adjustment region AD (circular ring region) including the boundary portion between the core portion 110 and the first clad layer 120a (the point P0 at which the absolute value of the refractive index gradient is the largest) having a width of 2 μm or more, and more preferably 3 μm or more, it is preferable that the viscosity continuously changes. Figure 4

[0068] In order to control so that the viscosity change near the boundary portion between the core portion 110 and the first clad layer 120a becomes gentle as with the first embodiment described above, it is preferable that Cl is added to the core portion 110 together with Br. Note that it is desirable that at least one or more of the additives (in addition to Br, F, Cl, and the like as needed) added to the core portion 110 and the first clad layer 120a of the optical fiber 100b according to the second embodiment are added during the manufacturing process of the optical fiber parent material 10, such as the soot deposition process, shown in FIG. 8. Figure 1

[0069] ​​In addition to the aforementioned gradual shape changes in compressive stress and viscosity distribution, the average Cl concentration in the core 110 is preferably 100 ppm or more. By containing Cl, the increase in transport loss caused by glass defects can be further suppressed. More preferably, the average Cl concentration in the core 110 is 200 ppm or more. Moreover, the 75% percentage value of the Cl concentration in the core 110 is preferably 200 ppm or more. In this case, the increase in transport loss caused by glass defects can be further suppressed.

[0070] As described above, in the second embodiment, the second cladding 120b supporting the drawing tension is separated from the core 110 compared to the optical fiber 100a in the first embodiment. This structure allows for greater freedom in selecting the composition of the core 110 and the first cladding 120a of the optical fiber 100b in the second embodiment. In particular, by adding fluorine (F), the relative refractive index difference of the first cladding 120a is reduced, thereby creating a refractive index difference between the core 110 and the first cladding 120a. This allows for keeping the necessary concentration of Br or Cl added to the core 110 low. This suppresses the yield reduction caused by foaming within the core 110 due to high concentrations of Br or Cl.

[0071] (Third Implementation)

[0072] Figure 2 The lower section shows type C optical fiber, which is optical fiber 100c according to the third embodiment of this disclosure. Furthermore, the refractive index curve 150c, viscosity distribution 151c, and stress distribution 152c shown along a common straight line orthogonal to the central axis AX of this optical fiber 100c are... Figure 6 As shown in the image. It should be noted that... Figure 6 The distribution shape of region R3 in the viscosity distribution 151c shown is substantially similar to Figure 3 The distribution shape shown.

[0073] like Figure 2 As shown in the lower paragraph, the optical fiber 100c of type C according to the third embodiment includes: a core 110 extending along the central axis AX of the optical fiber 100c; a first cladding 120a surrounding the core 110; a second cladding 120b surrounding the first cladding 120a; a third cladding 120c surrounding the second cladding 120b; a primary cladding 210 surrounding the third cladding 120c; and a secondary cladding 220 surrounding the primary cladding 210. The cladding 120 is constituted by the first cladding 120a, the second cladding 120b, and the third cladding 120c.

[0074] The core 110 is formed of silica glass (Si02) containing bromine (Br) like the optical fiber 100a according to the first embodiment and the optical fiber 100b according to the second embodiment. In the core 110, the Br concentration is 0.8 wt% or more and 2.6 wt% or less, and preferably 1.6 wt% or more and 2.6 wt% or less. In addition, the maximum relative refractive index difference of the core 110 is 0.1% or more and 0.3% or less, and preferably 0.2% or more and 0.3% or less. The first cladding layer 120a is formed of silica glass containing fluorine (F) in a small amount of 1000 ppm or more and 3000 ppm or less. The second cladding layer 120b is formed of pure silica glass or silica glass containing F at a lower concentration than the F concentration of the first cladding layer 120a. The third cladding layer 120c contains F or OH groups and has a lower viscosity than the viscosity of the second cladding layer 120b. Figure 6 A simplified refractive index profile 150c of the optical fiber 100c according to the third embodiment is shown in FIG. 15. Here, the curve shape in the outer region of the core 110 is provided by the above-described formula (1), but in the refractive index profile 150c, the curve shape of the core 110 shows an outline shape.

[0075] In the third embodiment, the diameter of the core 110 is 6 μm or more and 12 μm or less. With this configuration, the optical fiber 100c according to the third embodiment has one or more waveguide modes (a group of two polarization modes is defined as one waveguide mode) in the lowest loss band of silica glass, that is, the 1550 nm band. The effective area of the fundamental mode in the wavelength 1550 nm is preferably 60 μm 2 or more and 160 μm 2 or more. The outer diameter of the cladding 120 including the first cladding layer 120a, the second cladding layer 120b, and the third cladding layer 120c (substantially the outer diameter of the third cladding layer 120c) is 125 ± 1 μm. The outer diameter of the entire coating including the primary coating 210 and the secondary coating 220 (substantially the outer diameter of the secondary coating 220) is 245 ± 5 μm, and more preferably 200 ± 5 μm.

[0076] Next, due to the difference in the concentration of the above-mentioned additive, the intermediate value of the viscosity of each of the core 110, the first cladding 120a, and the third cladding 120c becomes lower than the maximum value of the viscosity of the second cladding 120b. More preferably, the intermediate value of the viscosity of each of the core 110, the first cladding 120a, and the third cladding 120c is lower than the 75% percentile value of the viscosity of the second cladding 120b. Further preferably, the intermediate value of the viscosity of each of the core 110, the first cladding 120a, and the third cladding 120c is lower than the intermediate value of the viscosity of the second cladding 120b. Due to this difference in the viscosity of each portion, the tension applied at the time of manufacturing the optical fiber 100c, particularly at the time of drawing the preform, is supported by the second cladding 120b, and as a result, the second cladding 120b of the drawn optical fiber 100c is left with a tensile stress, whereas each of the core 110, the first cladding 120a, and the third cladding 120c is left with a compressive stress. In Figure 6 In addition to the refractive index profile 150c, a simplified viscosity distribution 151c and a simplified stress distribution 152c of the optical fiber 100c according to the third embodiment are shown. Note that, Figure 6 The positions of the respective horizontal axes of the refractive index profile 150c, the viscosity distribution 151c, and the stress distribution 152c on the cross section orthogonal to the central axis AX of the optical fiber 100c (positions on a straight line passing through the central axis AX) are shown to be uniform.

[0077] The compressive stress itself depends not only on the difference in the viscosity of each portion but also on the tension at the time of drawing the preform and the like. Among them, in order to suppress an increase in the transmission loss, the absolute value of the compressive stress in each of the core 110, the first cladding 120a, and the third cladding 120c (the absolute value of the average value of the residual stress in each portion) is preferably 15 MPa or more, and more preferably 30 MPa or more. Further preferably, the absolute value of the compressive stress in each of the core 110, the first cladding 120a, and the third cladding 120c is the absolute value of the 75% percentile value of the stress left in each portion and is preferably 30 MPa or more. When a tensile stress is left in glass, an increase in the transmission loss due to a glass defect is likely to occur. However, as described above, since the average value or the 75% percentile value of the residual stress left in each of the core 110, the first cladding 120a, and the third cladding 120c is a compressive stress that is sufficiently large, an increase in the transmission loss due to a local tensile stress is suppressed.

[0078] Note that, also in the optical fiber 100c according to the third embodiment, as with the optical fiber 100a according to the first embodiment and the optical fiber 100b according to the second embodiment, it is preferable that, although the viscosity is different between the core 110 and the first cladding 120a, the spatial change thereof is continuous and gentle. When the difference in viscosity between the core 110 and the first cladding 120a is sharp, a large variation in structure or residual stress occurs at the boundary portion between the core 110 and the first cladding 120a due to unexpected variations in temperature or tension in drawing. This can become a cause of an increase in transmission loss. Thus, at the boundary portion between the core 110 and the first cladding 120a, the increase in transmission loss is suppressed due to the gentle spatial change in viscosity. Note that, the distribution shape of the region R3 of the viscosity distribution 151c is substantially similar to the shape shown in FIG. 6. That is, also in the optical fiber 100c according to the third embodiment, in the viscosity adjustment region AD (annular region) including the boundary portion between the core 110 and the first cladding 120a (the point P0 at which the refractive index gradient is the largest) having a width of 2 μm or more, and more preferably 3 μm or more, it is preferable that the viscosity continuously change. Figure 4

[0079] In order to control so that the viscosity change near the boundary portion between the core 110 and the first cladding 120a becomes gentle as with the first embodiment and the second embodiment, it is preferable that Cl be added to the core 110 together with Br. Note that, it is desirable that at least one or more of the additives (in addition to Br, F, Cl, or the like as needed) added to the core 110 and the first cladding 120a of the optical fiber 100c according to the third embodiment be added in the manufacturing process of the optical fiber preform 10, such as the soot deposition process, shown in FIG. 7. Figure 1

[0080] In addition to the compressive stress and the gentle change in the shape of the viscosity distribution described above, the average value of the Cl concentration in the core 110 is preferably 100 ppm or more. By containing Cl, an increase in transmission loss due to glass defects is further suppressed. More preferably, the average value of the Cl concentration in the core 110 is 200 ppm or more. Also, the 75% percentile value of the Cl concentration in the core 110 is preferably 200 ppm or more. In this case, an increase in transmission loss due to glass defects is further suppressed.

[0081] ​​As shown above, in the optical fiber 100c according to the third embodiment, the outermost third cladding 120c in the multilayer structure of the cladding 120 retains compressive stress. Therefore, even if mechanical damage is applied to the outer surface of the cladding 120, the rate of damage progression is suppressed. As a result, the optical fiber 100c achieves a high fatigue coefficient, thereby improving long-term reliability. Preferably, the dynamic fatigue coefficient is 20 or higher.

[0082] (Fourth Implementation)

[0083] Figure 7 The optical fiber of type D shown is the optical fiber 100d according to the fourth embodiment of this disclosure. Furthermore, the refractive index curve 150d, viscosity distribution 151d, and stress distribution 152d shown along a common straight line orthogonal to the central axis AX of this optical fiber 100d are... Figure 8 As shown in the image. It should be noted that... Figure 8 The distribution shape of region R4 in the viscosity distribution 151d shown is substantially similar to Figure 3 The distribution shape shown.

[0084] like Figure 7 As shown, the optical fiber 100d of type D according to the fourth embodiment includes: a core 110d extending along the central axis AX of the optical fiber 100d; a first cladding 120a surrounding the core 110d; a second cladding 120b surrounding the first cladding 120a; a primary cladding 210 surrounding the second cladding 120b; and a secondary cladding 220 surrounding the primary cladding 210. The cladding 120 is constituted by the first cladding 120a and the second cladding 120b.

[0085] The core 110d is formed of a first core 111d extending along the central axis AX and a second core 112d surrounding the first core 111d and extending along the central axis AX. The first core 111d is formed of silica glass to which an alkali element is added. The alkali element is one or more of sodium (Na), potassium (K), rubidium (Rb), or cesium (Cs). The atomic concentration of the alkali element in the first core 111d is 1 ppm or more and 100 ppm or less with respect to the silicon (Si) atoms of the silica glass, whereby the viscosity of the first core 111d can be effectively reduced while suppressing an increase in transmission loss caused by the addition. In the first core 111d, in addition to the alkali element, chlorine (Cl) and fluorine (F) can also be added in common, whereby the viscosity can be further effectively reduced. The second core 112d is likewise formed of silica glass (SiO2) containing bromine (Br) as in the optical fiber 100a related to the first embodiment or the optical fiber 100b related to the second embodiment. In the second core 112d, the Br concentration is 0.8 wt% or more and 2.6 wt% or less, and is preferably 1.6 wt% or more and 2.6 wt% or less. Further, the maximum relative refractive index difference of the core 110d is 0.1% or more and 0.3% or less, and is preferably 0.2% or more and 0.3% or less. The first cladding 120a is formed of silica glass containing 1000 ppm or more and 3000 ppm or less of a trace amount of fluorine (F). The second cladding 120b is formed of pure silica glass or silica glass containing F at a lower concentration than the F concentration of the first cladding 120a. In Figure 8 In the fourth embodiment, a simplified refractive index profile 150d of the optical fiber 100d is shown. Here, the curve shape in the outer region of the core 110d is provided by the above-described formula (1), but in the refractive index profile 150d, the curve shape of the core 110d shows an outline shape.

[0086] In the fourth embodiment, the diameter of the core 110d is 6 μm or more and 12 μm or less. With this configuration, the optical fiber 100d related to the fourth embodiment has one or more waveguide modes (defining a group of two polarization modes as one waveguide mode) in the lowest loss band of silica glass, that is, the 1550 nm band. The effective area of the fundamental mode at a wavelength of 1550 nm is preferably 60 μm 2 or more and 160 μm 2 or more. The outer diameter of the cladding 120 including the first cladding 120a and the second cladding 120b is 125 ± 1 μm. The outer diameter of the entire coating including the primary coating 210 and the secondary coating 220 (substantially the outer diameter of the secondary coating 220) is 245 ± 5 μm, and is preferably 200 ± 5 μm.

[0087] Next, due to the difference in the concentration of the above-mentioned additive, the intermediate value of the viscosity of each of the first core portion 111d, the second core portion 112d, and the first cladding portion 120a becomes lower than the maximum value of the viscosity of the second cladding portion 120b. More preferably, the intermediate value of the viscosity of each of the first core portion 111d, the second core portion 112d, and the first cladding portion 120a is lower than the 75% percentile value of the viscosity of the second cladding portion 120b. Further preferably, the intermediate value of the viscosity of each of the first core portion 111d, the second core portion 112d, and the first cladding portion 120a is lower than the intermediate value of the viscosity of the second cladding portion 120b. Due to the difference in the viscosity of each portion, the tension applied at the time of manufacturing the optical fiber 100d, particularly at the time of drawing the preform, is supported by the second cladding portion 120b, and as a result, the residual tensile stress in the second cladding portion 120b of the drawn optical fiber 100d, and on the other hand, the residual compressive stress in each of the first core portion 111d, the second core portion 112d, and the first cladding portion 120a are left. In Figure 8 In addition to the refractive index profile 150d, a simplified viscosity distribution 151d and a simplified stress distribution 152d of the optical fiber 100d according to the fourth embodiment are shown. Note that, in the viscosity distribution 151d and the stress distribution 152d, the horizontal axis indicates the position on the cross section orthogonal to the central axis AX of the optical fiber 100d (the position on the straight line passing through the central axis AX). Figure 8 The positions of the horizontal axes of the refractive index profile 150d, the viscosity distribution 151d, and the stress distribution 152d on the cross section orthogonal to the central axis AX of the optical fiber 100d (the positions on the straight line passing through the central axis AX) are shown to be uniform.

[0088] The absolute value of the compressive stress in each of the first core portion 111d, the second core portion 112d, and the first cladding portion 120a (the absolute value of the average value of the residual stress in each portion) is preferably 15 MPa or more, and more preferably 30 MPa or more, in order to suppress the increase in the transmission loss. Further preferably, the absolute value of the compressive stress in each of the first core portion 111d, the second core portion 112d, and the first cladding portion 120a is the absolute value of the 75% percentile value of the stress left in each portion and is preferably 30 MPa or more. When the tensile stress is left in the glass, the increase in the transmission loss due to the glass defects is likely to occur. However, as described above, since the average value or the 75% percentile value of the residual stress left in each of the first core portion 111d, the second core portion 112d, and the first cladding portion 120a is a sufficiently large compressive stress, the increase in the transmission loss due to the local tensile stress is suppressed.

[0089] Note that, also in the optical fiber 100d according to the fourth embodiment, as with the optical fiber 100a according to the first embodiment and the optical fiber 100b according to the second embodiment, it is preferable that, although the viscosity is different between the second core 112d and the first cladding 120a, the spatial change thereof is continuous and gentle. When the viscosity difference between the second core 112d and the first cladding 120a, which have a large refractive index difference from each other, is sharp, a large variation in structure or residual stress occurs at the boundary portion between the second core 112d and the first cladding 120a due to unexpected variations in temperature or tension in drawing. This can become a cause of an increase in transmission loss. Thus, at the boundary portion between the second core 112d and the first cladding 120a, the increase in transmission loss is suppressed due to the gentle spatial change in viscosity. Note that, the distribution shape of the region R4 of the viscosity distribution 151d is substantially similar to the shape shown in FIG. 15. That is, also in the optical fiber 100d according to the fourth embodiment, in the viscosity adjustment region AD (annular region) including the boundary portion between the second core 112d and the first cladding 120a (the point P0 at which the refractive index gradient is the largest) having a width of 2 μm or more, and more preferably 3 μm or more, it is preferable that the viscosity continuously change. Figure 4

[0090] In order to control so that the viscosity change near the boundary portion between the second core 112d and the first cladding 120a becomes gentle, as with the first embodiment and the second embodiment, it is preferable that Cl be added to the second core 112d together with Br. Note that, it is desirable that at least one or more of the additives (in addition to Br, F, Cl, or the like as needed) added to the second core 112d and the first cladding 120a of the optical fiber 100d according to the fourth embodiment be added in the manufacturing process of the optical fiber preform 10, such as the soot deposition process, shown in FIG. 16. Figure 1

[0091] In addition to the compressive stress and the gentle shape change in the viscosity distribution described above, the average value of the Cl concentration in the core 110 is preferably 100 ppm or more. By containing Cl, an increase in transmission loss due to glass defects is further suppressed. More preferably, the average value of the Cl concentration in the core 110 is 200 ppm or more. Also, the 75th percentile value of the Cl concentration in the core 110 is preferably 200 ppm or more. In this case, an increase in transmission loss due to glass defects is further suppressed.

[0092] ​​As shown above, the optical fiber 100d according to the fourth embodiment can effectively reduce the viscosity of the first core portion by containing an alkali element in the first core portion 111d that forms a part of the core portion 110, as compared with the optical fibers 100a to 100c according to the first to third embodiments. The alkali element can diffuse to the second core portion that surrounds the first core portion, and further diffuse to the first cladding layer that surrounds the second core portion in the drawing process, so that the viscosity reduction effect can be obtained in the second core portion and the first cladding layer. As a result, the compressive stress can be effectively formed in the first core portion, the second core portion, and the first cladding layer regardless of the drawing conditions, so that the drawing speed and the drawing tension can be easily optimized from the viewpoint of productivity, and as a result, the manufacturing cost of the optical fiber can be reduced.

[0093] BRIEF DESCRIPTION OF DRAWINGS

[0094] 1 optical fiber manufacturing apparatus, 10 optical fiber preform, 21, 22 resin coating device, 23 drawing furnace, 24 heating furnace, 25 cooling device, 26 mold, 27 ultraviolet light source, 28 roller, 29 winch, 30 winding machine, 100, 100a, 100b, 100c, 100d optical fiber, 110, 110d core portion, 111d first core portion, 112d second core portion, 120 cladding layer, 120a first cladding layer, 120b second cladding layer, 120c third cladding layer, 210 primary coating, 220 secondary coating, AX central axis, 150a, 150b, 150c, 150d refractive index profile, 151a, 151b, 151c, 151d viscosity distribution, 152a, 152b, 152c, 152d stress distribution, AD viscosity adjustment region, AX central axis, R1, R2, R3, R4 region, S arrow (rotation direction).

Claims

1. An optical fiber, comprising: a core portion extending along a central axis and formed of a silica glass to which bromine is added; and a cladding portion surrounding the core portion and formed of a silica glass having a lower refractive index than a maximum refractive index of the core portion, the core portion having a compressive stress, the cladding portion including: a first cladding portion surrounding the core portion and formed of a silica glass to which fluorine is added; a second cladding portion surrounding the first cladding portion, formed of a pure silica glass or a silica glass to which fluorine is added at a lower concentration than a concentration of fluorine of the first cladding portion, and having a tensile stress; and a third cladding portion surrounding the second cladding portion, formed of a pure silica glass or a silica glass to which fluorine is added at a lower concentration than a concentration of fluorine of the first cladding portion, having a lower viscosity than a viscosity of the second cladding portion, and having a compressive stress.

2. An optical fiber, comprising: a core portion extending along a central axis and formed of a silica glass to which bromine is added; and a cladding portion surrounding the core portion and formed of a silica glass having a lower refractive index than a maximum refractive index of the core portion, the core portion having a compressive stress, the core portion further containing chlorine, a viscosity adjustment region defined on a cross section of the optical fiber orthogonal to the central axis having: a shape surrounding the central axis in a state of being separated from the central axis; an inner peripheral portion and an outer peripheral portion arranged in a manner of sandwiching a boundary portion between the core portion and the cladding portion in a state of being separated by a distance of 2 μm or more; and a viscosity distribution continuously varying along a radial direction from the central axis to an outer periphery of the optical fiber. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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