Optical fiber

Abstract

An optical fiber (100) according to an embodiment has a structure capable of reducing an increase in transmission loss. The optical fiber (100) includes a glass portion extending in the direction of a central axis, the glass portion is made of silica-based glass, and includes a core (10) and a cladding (20). The core (10) includes the central axis and contains chlorine at a mass percentage of 1% or more. The cladding (20) surrounds the core (10) and has a refractive index lower than the maximum refractive index of the core (10). Further, the optical fiber has substantially uniform residual stress in the entire region of a cross section of the glass portion orthogonal to the central axis.

Description

Technical Field

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

[0002] This application claims the priority of Japanese Patent Application No. 2020-028324 filed on February 21, 2020, the content of which is incorporated herein by reference in its entirety. Background Art

[0003] For example, an optical fiber made of silica-based glass with germanium (Ge) added to the core is known. In such an optical fiber having a core with added Ge, the Rayleigh scattering loss caused by the concentration fluctuation (variation from the average value) of Ge is large, and adding Ge to the core becomes an obstacle to reducing transmission loss. As a countermeasure, for example, an optical fiber in which Ge is not added to the core but fluorine (F) is added to the cladding has been fabricated for low loss.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: International Publication WO2016 / 007806A1

[0007] Non-Patent Documents

[0008] Non-Patent Document 1: Libert et al, IWCS_Proceedings (1998) 375

[0009] Non-Patent Document 2: M.P. Varnham et al, Electron Lett 20, 1034 (1984) Summary of the Invention

[0010] The optical fiber of the present invention includes a glass part made of silica-based glass. The glass part includes a core including a central axis and a cladding surrounding the core. Chlorine (Cl) is added to the core at a mass percentage of 1% or more. The cladding has a refractive index lower than the maximum refractive index of the core. In addition, in the entire region of the cross-section of the glass part orthogonal to the central axis, the residual stress is substantially uniform. Brief Description of the Drawings

[0011] Figure 1 It is a diagram showing the cross-sectional structure of an optical fiber according to an embodiment of the present disclosure.

[0012] Figure 2 It is a graph showing the specifications of each of Samples 1 to 5 of an optical fiber according to an embodiment of the present disclosure.

[0013] Figure 3It is a graph showing the relationship between the mass percentage (%) of chlorine (Cl) in each of Samples 1 to 5 and the transmission loss (dB / km).

[0014] Figure 4 It is a chart showing the specifications of each of Samples 6 to 9 of the optical fiber according to an embodiment of the present disclosure.

[0015] Figure 5 It is a graph showing the relationship between the mass percentage (%) of fluorine (F) in each of Samples 6 to 9 and the transmission loss (dB / km).

[0016] Figure 6 It is a chart showing the specifications of each of Samples 10 to 13 of the optical fiber according to an embodiment of the present disclosure and the specification of Sample 5 (reference).

[0017] Figure 7 It is a graph showing the relationship between the difference (MPa) between the maximum and minimum values of the residual stress in each of Sample 5 (reference) and Samples 10 to 13 and the transmission loss (dB / km).

[0018] Figure 8 It is a chart showing the specifications of each of Samples 14 to 16 of the optical fiber according to an embodiment of the present disclosure and the specification of Sample 5 (reference).

[0019] Figure 9 It is a graph showing the relationship between Sample 5 (reference) and the effective cross-sectional area A eff (μm 2 ) of each of Samples 14 to 16 and the transmission loss (dB / km).

[0020] Figure 10 It is a chart showing the specifications of each of Samples 17 to 23 of the optical fiber according to an embodiment of the present disclosure and the specification of Sample 5 (reference).

[0021] Figure 11 It is a graph showing the relationship between Sample 5 (reference) and the α value of each of Samples 17 to 23 and the transmission loss (dB / km).

[0022] Figure 12 It is a chart showing the specifications of each of Samples 24 to 26 of the optical fiber according to an embodiment of the present disclosure and the specification of Sample 5 (reference).

[0023] Figure 13 It is a graph showing the relationship between Sample 5 (reference) and the difference (MPa) in residual stress between specific parts of each of Samples 24 to 26 and the transmission loss (dB / km).

[0024] Figure 14 It is a diagram showing the specifications of each of samples 27 to 31 of the optical fiber related to one embodiment of the present disclosure and the specification of sample 5 as a reference example.

[0025] Figure 15 It is a diagram showing the specifications of each of samples 32 to 38 of the optical fiber related to one embodiment of the present disclosure.

[0026] Figure 16 It is a diagram showing the specifications of each of samples 39 to 41 of the optical fiber related to one embodiment of the present disclosure. Detailed Embodiments

[0027] [Technical Problems to be Solved by the Present Disclosure]

[0028] The inventors studied the conventional method for manufacturing an optical fiber preform and found the following technical problems. That is, Patent Document 1 describes the following: By setting the viscosity of the core region (the region that should become the core of the optical fiber after drawing) of the preform to be equal to or lower than the viscosity of the cladding region (the region that should become the cladding of the optical fiber after drawing), and making the coefficient of thermal expansion (CTE) between the core region and the cladding region consistent, it is possible to reduce transmission loss. However, as described in the above Patent Document 1, just by making the values of viscosity and coefficient of thermal expansion between the core region and the cladding region in the preform close to each other, the reduction of transmission loss is not sufficient. This is because the stress remaining in the glass region is affected not only by characteristics of the preform such as viscosity and coefficient of thermal expansion, but also by the tension, wire speed, and slow cooling method during drawing.

[0029] The present disclosure was completed to solve the above technical problems, and an object thereof is to provide an optical fiber having a structure capable of reducing an increase in transmission loss.

[0030] [Effects of the Present Disclosure]

[0031] According to the optical fiber of the present disclosure, transmission loss can be reduced.

[0032] [Description of Embodiments of the Invention of the Present Application]

[0033] First, the content of the embodiments of the present disclosure will be separately described individually.

[0034] (1) One mode of the optical fiber involved in the embodiment of the present disclosure includes a glass portion composed of a silica-based glass. The glass portion includes a core including a central axis and a cladding surrounding the core. Chlorine (Cl) is added to the core in an amount of 1% by mass or more. The cladding has a refractive index lower than the maximum refractive index of the core. In addition, the residual stress is basically consistent throughout the entire area of ​​the cross section of the glass portion orthogonal to the central axis. Specifically, "a state in which the residual stress is basically consistent" refers to a state in which the difference between the maximum and minimum values ​​of the residual stress is less than 230 MPa or less than 200 MPa. In addition, as one mode of the present disclosure, it is more preferred that the difference between the maximum and minimum values ​​of the residual stress is less than 100 MPa. In the present application, "residual stress" is the axial stress σz described in Non-Patent Document 2. This is the residual stress acting on the axis corresponding to the central axis. Figure 1 The component of stress parallel to the AX direction on the cross section perpendicular to the AX direction is tensile stress when it has a positive value, and is compressive stress when it has a negative value.

[0035] As described above, in an optical fiber to which Cl is added at a mass percentage of 1% or more in the core, when the difference (absolute value) between the maximum and minimum values ​​of the residual stress in the glass portion is 230 MPa or less or 200 MPa or less, an optical fiber with lower loss can be obtained compared to the prior art. When the difference (absolute value) between the maximum and minimum values ​​of the residual stress is 100 MPa or less, the effect of reducing the loss is more significant. It is believed that this is because a high Cl concentration reduces the viscosity of the glass, and therefore the Rayleigh scattering loss is easily reduced by utilizing the slow cooling effect when cooling the optical fiber (optical fiber after drawing) that has been temporarily heated in the drawing furnace.

[0036] (2) As one embodiment of the present disclosure, the core may further contain fluorine (F). That is, by adding an appropriate amount of F, the glass viscosity is further reduced, thereby achieving an effect of reducing Rayleigh scattering loss.

[0037] (3) The mass percentage of chlorine added to the core may be 1.5% or more. In this case, the Rayleigh scattering loss can be further reduced. In addition, as one embodiment of the present disclosure, the mass percentage of chlorine added to the core is preferably 5% or less, and more preferably 3% or less. It should be noted that if the mass percentage of chlorine exceeds 5% (or more than 3% in some cases), bubbles may be generated when chlorine is added, making the manufacture of the optical fiber base material difficult.

[0038] (4) As one aspect of this embodiment, it is preferred that the optical fiber has a 70 μm 2 Above and 150μm 2 The following effective cross-sectional area A eff In this case, a sufficient reduction in transmission loss can be expected.

[0039] (5) As one aspect of the present disclosure, the refractive index profile of the core preferably follows the α-power profile, and the α value defining its shape is 150 or less. This is because if the α value exceeds 150, the increase in transmission loss becomes significant. Further, as one aspect of the present disclosure, the α value is preferably 3 or more and 99 or less. When the α value ranges from 60 or more to 80 or less, the effect of reducing transmission loss is significant, but an α value of 3 or more and 99 or less is practical.

[0040] (6) As one aspect of the present disclosure, in the cross-section of the glass part, it is preferable that the average value of the residual stress in the region where the distance from the center of the cross-section along the radial direction is 50 μm or more and 62.5 μm or less is lower than the average value of the residual stress in the region where the distance from the center of the cross-section along the radial direction is 45 μm or more and 55 μm or less. By satisfying the above relationship between such different regions, a sufficient effect of reducing Rayleigh scattering loss can be obtained.

[0041] (7) As one aspect of the present disclosure, the optical fiber preferably has a microbending loss of 1 dB / km or less at a wavelength of 1550 nm. By adjusting the Young's modulus of the coating provided on the outer peripheral surface of the glass part, the control of microbending loss becomes easy.

[0042] (8) As one aspect of the present disclosure, it is preferable that in the optical fiber after being exposed to a hydrogen atmosphere at a partial pressure of 1.5 kPa and a temperature of 25°C for 720 hours, the increase in transmission loss is 0.005 dB / km or less at a wavelength of 1550 nm. By controlling the fictive temperature to be low (for example, 2000°C or less), the increase in transmission loss after hydrogen atmosphere treatment becomes small (the crystal defects in the glass structure become fewer).

[0043] As described above, each of the aspects listed in the [Description of the Embodiments of the Present Disclosure] column can be applied to each of all the remaining aspects or a combination of all these remaining aspects.

[0044] [Details of the Embodiments of the Present Disclosure]

[0045] Hereinafter, with reference to the drawings, the specific structure of the optical fiber according to the embodiments of the present disclosure will be described in detail. It should be noted that the present invention is not limited to these examples, but is indicated by the claims and is intended to include all modifications within the meaning and scope equivalent to the claims. Further, in the description of the drawings, the same reference numerals are assigned to the same elements, and duplicate descriptions are omitted.

[0046] Figure 1 is a cross-sectional view showing an example of the structure of the optical fiber according to the present disclosure. In Figure 1In this case, the optical fiber 100 includes: a glass optical fiber (glass portion) 100a made of silica-based glass; a main coating layer 210 provided on the outer peripheral surface of the glass optical fiber 100a; and a sub-coating layer 220 provided on the outer peripheral surface of the main coating layer 210. The glass optical fiber 100a includes a core 10 including a central axis (optical axis) AX and a cladding 20 provided on the outer peripheral surface of the core. Chlorine (Cl) is added to the core 10 in a mass percentage of 1% or more. In addition, an appropriate refractive index reducer such as F may be added to the cladding 20, and the refractive index of the cladding 20 is set to be lower than the maximum refractive index of the core 10.

[0047] The main coating layer 210 has a thickness of 18 μm or more and 33 μm or less (the width of the main coating layer 210 defined along the radial direction orthogonal to the central axis AX). In addition, the main coating layer 210 has a Young's modulus of 0.05 MPa or more and 0.6 MPa or less. On the other hand, the sub-coating layer 220 has a thickness of 20 μm or more and 30 μm or less. In addition, the sub-coating layer 220 has a Young's modulus of 1200 MPa or more and 1500 MPa or less. It should be noted that the ratio of the thickness of the main coating layer 210 to the thickness of the sub-coating layer 220 ("main thickness" / "sub-thickness") is 0.3 or more and 1.8 or less, preferably 0.9 or more and 1.8 or less. In this case, the microbending loss (dB / km) of the optical fiber 100 in the state where the main coating layer 210 and the sub-coating layer 220 are provided can be controlled within an appropriate range.

[0048] Hereinafter, using Figures 2 to 16 , the evaluation results of each of Samples 1 to 41 of the optical fiber 100 of the present disclosure will be described. As a prerequisite, first, Figure 2 , Figure 4 , Figure 6 , Figure 8 , Figure 10 , Figure 12 , Figures 14 to 16 The specifications shown in each will be described. It should be noted that F is added to the cladding of the optical fiber involved in each sample.

[0049] (1) "Core Δ (%)":

[0050] Item (1) is the maximum relative refractive index difference (%) of the core in each sample, based on the refractive index n0 of pure silica glass. For example, the relative refractive index difference Δ of a portion having a refractive index n can be given by the formula (n / n0) - 1.

[0051] (2) "α value of the core":

[0052] The item (2) is equivalent to the exponent in the definition formula of the "α-power profile" described in the above-mentioned Patent Document 1, and the profile shape of the core is represented by this α value. That is, the refractive index profile of the core in each sample (the change in the relative refractive index difference along the radial direction orthogonal to the central axis AX) is the following definition formula of the "α-power profile":

[0053] [Mathematical formula 1]

[0054]

[0055] Among them,

[0056] r0: The position where Δ(r) is the maximum

[0057] r1: Applicable to the boundary between the core and the cladding, and the exponent α value is determined by the least squares method.

[0058] (3) "Dopant in the core":

[0059] The item (3) is an element added to the core in each sample in addition to Cl.

[0060] (4) "Mass percentage (%) of Cl in the core":

[0061] The item (4) is the mass percentage (%) of chlorine (Cl) added to the core in each sample. It should be noted that when measuring the mass percentage of elements, for the polished cross-section of the optical fiber, EPMA (Electron Probe MicroAnalyzer) is used to measure along the radial direction from the central axis of the optical fiber. The measurement conditions are, for example, setting the acceleration voltage to 20 kV, setting the probe beam diameter to 1 μm or less, setting the measurement interval to 100 nm or less, and calculating the mass percentage using the measured values and the calibration curve obtained in advance.

[0062] (5) "Mass percentage (%) of F in the core":

[0063] The item (5) is the mass percentage (%) of fluorine (F) added to the core in each sample, and the measurement of the mass percentage (%) is the same as that in the case of the above item (4).

[0064] (6) "Core outer diameter (μm)":

[0065] The item (6) is the outer diameter (μm) of the core in each sample.

[0066] (7) "Glass part outer diameter (μm)":

[0067] The item (7) is the glass part in each sample, that is, equivalent to Figure 1The outer diameter (μm) of the portion of the glass optical fiber 100a (the portion composed of the core 10 and the cladding 20) shown.

[0068] (8) "Dispersion @ 1550 nm (ps / nm / km)":

[0069] Item (8) is the chromatic dispersion (unit: ps / nm / km) of each sample at a wavelength of 1550 nm.

[0070] (9) "MFD (μm)":

[0071] Item (9) is the mode field diameter (unit: μm) of each sample at a wavelength of 1550 nm.

[0072] (10) "A eff (μm 2 )":

[0073] Item (10) is the effective area (unit: μm 2 ) of each sample at a wavelength of 1550 nm.

[0074] In addition,

[0075] (11) "22 m optical cable cut-off wavelength λ cc (μm)":

[0076] Item (11) is the optical cable cut-off wavelength (μm) defined by ITU-T G650.1.

[0077] (12) "MFD / λ cc ":

[0078] Item (12) is the ratio of "MFD" to "22 m optical cable cut-off wavelength λ cc ".

[0079] (13) "Bending loss @ 1550 nm (dB / turn) (bending diameter 30 mm)":

[0080] Item (13) is the increase in loss per turn (dB / turn) measured when light with a wavelength of 1550 nm is input to each sample wound around a mandrel with a diameter of 30 mm.

[0081] (14) "Bending loss @ 1550 nm (dB / turn) (bending diameter 60 mm)":

[0082] Item (14) is the increase in loss (dB / reel) per reel (one turn) measured when light with a wavelength of 1550 nm is input to each sample wound around a mandrel with a diameter of 60 mm.

[0083] (15) "Transmission loss @1550 nm (dB / km)":

[0084] Item (15) is the transmission loss (dB / km) of each sample at a wavelength of 1550 nm.

[0085] (16) "Difference between maximum residual stress and minimum residual stress (MPa)":

[0086] Item (16) is a value (MPa) representing the variation state of the residual stress in the entire cross-sectional area of the glass part (corresponding to the Figure 1 shown glass optical fiber 100a) of each sample.

[0087] (17) "Microbend loss (dB / km)":

[0088] Item (17) is the microbend loss (dB / km) of each sample. It should be noted that the microbend loss is evaluated with reference to Non-Patent Document 1. Specifically, it is obtained from the increase in loss when light with a wavelength of 1550 nm is input to each 500 m sample wound around a drum under a tension of 80 gf, and the drum has a main body diameter of 405 mm and a wire with a cross-sectional diameter of 50 μm woven into a mesh with a pitch of 100 μm on the main body surface.

[0089] (18) "Main thickness (μm)":

[0090] Item (18) is the thickness (μm) of the main coating layer (corresponding to the Figure 1 shown main coating layer 210) of each sample, that is, the cross-sectional width of the main coating layer along the radial direction.

[0091] (19) "Sub-thickness (μm)":

[0092] Item (19) is the thickness (μm) of the sub-coating layer (corresponding to the Figure 1 shown sub-coating layer 220) of each sample, that is, the cross-sectional width of the sub-coating layer along the radial direction.

[0093] In addition,

[0094] (20) "Main Young's modulus (MPa)":

[0095] Item (20) is the Young's modulus (MPa) of the main coating layer in each sample.

[0096] (21) "Sub Young's modulus (MPa)":

[0097] Item (21) is the Young's modulus (MPa) of the secondary coating in each sample.

[0098] (22) "Ratio of coating thicknesses":

[0099] Item (22) is the ratio of the thickness of the primary coating to the thickness of the secondary coating in each sample ("primary thickness (μm)" / "secondary thickness (μm)").

[0100] (23) "Increase in hydrogen loss (dB / km)":

[0101] Item (23) is the increase in transmission loss (dB / km) at a wavelength of 1550 nm measured for the optical fiber after each sample has been exposed to a hydrogen atmosphere at a partial pressure of 1.5 kPa and a temperature of 25 °C for 720 hours.

[0102] (24) "Average residual stress of 55 - 62.5 (55 - 62.5 average)":

[0103] Item (24) is the average residual stress (MPa) in the circular ring region sandwiched between the inner peripheral part with a radius of 55 μm and the outer peripheral part with a radius of 62.5 μm in the cross-section of each sample (the cross-section orthogonal to the axis corresponding to Figure 1 the central axis AX shown).

[0104] (25) "Average residual stress of 45 - 55 (45 - 55 average)":

[0105] Item (25) is the average residual stress (MPa) in the circular ring region sandwiched between the inner peripheral part with a radius of 45 μm and the outer peripheral part with a radius of 55 μm in the cross-section of each sample.

[0106] (26) "Average core pseudo-temperature (°C)":

[0107] Item (26) is the average of the pseudo-temperatures (°C) of the cores in each sample.

[0108] Next, Figure 2 is a graph showing the specifications of each of Samples 1 to 5 prepared as optical fibers according to an embodiment of the present disclosure. Additionally, Figure 3 is a graph showing the relationship between the mass percentage (%) of chlorine (Cl) and the transmission loss (dB / km) for each of Samples 1 to 5.

[0109] By Figure 3From the measurement results of Samples 1 to 5 shown, it can be seen that the higher the chlorine concentration (mass percentage of chlorine), the lower the transmission loss. It can be considered that this is because the higher the chlorine concentration, the lower the glass viscosity. That is, it can be considered that this is because, due to the slow cooling effect on the drawn optical fiber from heating in the drawing furnace to cooling, the Rayleigh scattering loss is easily reduced. It should be noted that when the concentration of Cl added to the core is higher than 3% by mass percentage, there is a tendency that it is difficult to manufacture the optical fiber base material due to the generation of bubbles. Therefore, it is appropriate that the mass percentage of Cl added to the core is 5% or less.

[0110] Figure 4 is a chart showing the specifications of Samples 6 to 9 of the optical fiber according to an embodiment of the present disclosure. In addition, Figure 5 is a graph showing the relationship between the mass percentage (%) of fluorine (F) and the transmission loss (dB / km) for each of Samples 6 to 9. It should be noted that Samples 6 to 9 are all optical fibers in which the same level of concentration of Cl and F are added to the core together.

[0111] From Figure 5 the measurement results of Samples 6 to 9 shown, it can be seen that if the mass percentage of F is 0.5% or less, the transmission loss is approximately the same level, but if the mass percentage of F is more than 0.5%, the transmission loss increases. When the mass percentage of F is 0.5% or less (Samples 6 to 8), the transmission loss hardly changes among the samples. It can be considered that this is because the amount of reduction in Rayleigh scattering loss caused by the decrease in glass viscosity due to the addition of F is the same level as the amount of increase in Rayleigh scattering loss caused by the concentration fluctuation of F (deviation from the designed concentration distribution). On the other hand, when the mass percentage of F is more than 0.5% (Sample 9), the reduction effect of Rayleigh scattering loss caused by the decrease in glass viscosity starts to saturate. Therefore, it can be speculated that the amount of increase in Rayleigh scattering loss caused by the concentration fluctuation of F exceeds the amount of reduction in Rayleigh scattering loss caused by the decrease in glass viscosity. In addition, when comparing Samples 5 to 8, when F is added to the core compared to no addition, sometimes the transmission loss decreases. The mass percentage of this F needs to be 0.1% or more.

[0112] Figure 6 is a chart showing the specifications of Samples 10 to 13 of the optical fiber according to an embodiment of the present disclosure and the specifications of Sample 5 (reference). In addition, Figure 7 is a graph showing the relationship between the difference between the maximum value and the minimum value (MPa) of the residual stress and the transmission loss (dB / km) for each of Sample 5 (reference) and Samples 10 to 13.

[0113] From Figure 7From the measurement results of Sample 5 shown and the measurement results of Samples 10 to 13, it can be seen that the smaller the difference between the maximum residual stress and the minimum residual stress among the stresses remaining in the entire cross-sectional area of each sample, the lower the transmission loss. It is considered that this is because the refractive index difference caused by the deformation inside each sample (optical fiber), especially inside the glass part, causes a change in Rayleigh scattering loss.

[0114] Figure 8 FIG. is a chart showing the specifications of Samples 14 to 16 of the optical fiber according to an embodiment of the present disclosure and the specification of Sample 5 (reference). In addition, Figure 9 FIG. is a chart showing the effective cross-sectional area A eff (μm 2 ) of Sample 5 (reference) and Samples 14 to 16 and the relationship with the transmission loss (dB / km).

[0115] From Figure 9 the measurement results of Sample 5 shown and the measurement results of Samples 14 to 16, it can be seen that the larger the effective cross-sectional area A eff , the lower the transmission loss. It is considered that this is because if the F concentration in the cladding of each sample is reduced in order to increase the effective cross-sectional area A eff , the loss part caused by Rayleigh scattering due to the concentration fluctuation of F allocated to the cladding mode light can be reduced.

[0116] Figure 10 FIG. is a chart showing the specifications of Samples 17 to 23 of the optical fiber according to an embodiment of the present disclosure and the specification of Sample 5 (reference). In addition, Figure 11 FIG. is a chart showing the relationship between the α value of Sample 5 (reference) and Samples 17 to 23 and the transmission loss (dB / km).

[0117] From Figure 11From the measurement results of sample 5 and the measurement results of samples 17 to 23 shown, it can be seen that the transmission loss is the lowest in the range where the α value is 60 or more and 80 or less. When the α value is less than this range and when the α value is greater than this range, the transmission loss tends to increase. It can be considered that the main reason for the increase in the transmission loss when the α value becomes smaller is that since the Cl concentration in the outer periphery of the core decreases, the glass viscosity of this core outer periphery increases. That is, it can be considered that this is because, due to the increase in the glass viscosity of the core outer periphery, the Rayleigh scattering loss increases. On the other hand, regarding the increase in the transmission loss when the α value becomes larger, although the clear reason is not yet known, it can be speculated that the main reason is, for example, that minute bubbles are generated near the interface due to an increase in the Cl concentration at the interface between the core and the cladding. That is, it can be speculated that the phenomenon of an increase in the Rayleigh scattering loss caused by interface inconsistency is the main reason for the increase in the transmission loss. It should be noted that in the drawing of each sample, a holding furnace is used to slowly cool each sample pulled out from the drawing furnace. The temperature of each sample in this holding furnace is different.

[0118] Figure 12 is a graph showing the specifications of each of samples 24 to 26 of the optical fiber according to an embodiment of the present disclosure and the specification of sample 5 (reference). In addition, Figure 13 is a graph showing the relationship between the difference (MPa) in residual stress and the transmission loss (dB / km) between specific parts of sample 5 (reference) and each of samples 24 to 26. It should be noted that the "difference in residual stress between specific parts" is obtained by subtracting the average value of the residual stress in the region where the distance from the center of the cross-section along the radial direction is 55 μm or more and 62.5 μm or less (55 - 62.5 average value) from the average value of the residual stress in the region where the distance from the center of the cross-section along the radial direction is 45 μm or more and 55 μm or less (45 - 55 average value).

[0119] From Figure 13 the measurement results of sample 5 and the measurement results of samples 24 to 26 shown, it can be seen that the larger this difference value is, the lower the transmission loss. Specifically, the difference value defined by "(45 - 55 average value)" - "(55 - 62.5 average value)" is preferably 20 MPa or more. When a stress in the compressive direction is applied to the outermost periphery where the "outer diameter of the glass part" is 110 μm or more and 130 μm or less (radius of 55 μm or more and 62.5 μm or less), tensile stress remains in the cladding, and as a result, compressive stress remains in the core where the optical power is concentrated. Based on this situation, it can be speculated that this is because the Rayleigh scattering loss decreases.

[0120] Next, for each sample after exposure to a hydrogen atmosphere at a partial pressure of 1.5 kPa and a temperature of 25°C for 720 hours, the increase in transmission loss (hydrogen loss increase) at a wavelength of 1550 nm was evaluated. Figure 14 It is a graph showing the specifications of each of Samples 27 to 31 and the specification of Sample 5 (reference) of the optical fiber according to one embodiment of the present disclosure.

[0121] In Figure 14 In the case of Sample 5 and Samples 27 to 31 shown, the smaller the fictive temperature, the smaller the increase in hydrogen loss (increase in transmission loss in hydrogen characteristic evaluation). It is considered that this is because the lower the fictive temperature, the fewer the defects in the glass structure. In addition, comparing Sample 5, Sample 30, and Sample 31, the higher the Cl concentration in the core, the smaller the increase in hydrogen loss. It is considered that this is because if the Cl concentration is small, the number of defects in the glass structure increases. Therefore, it is considered that when the increase in hydrogen loss is 0.008 dB / km or less, it is sufficient that the fictive temperature is 2000°C or less and the mass percentage of Cl is 1% or more.

[0122] Figure 15 It is a graph showing the specifications of each of Samples 32 to 38 of the optical fiber according to one embodiment of the present disclosure. In Figure 15 the microbend loss of Samples 32 to 38 (microbend loss measured in a state where both a main coating layer and a secondary coating layer are provided) is additionally shown.

[0123] According to Figure 15 the graph of, comparing Samples 32 to 38, the thickness of the main coating layer "main thickness" and the thickness of the secondary coating layer "sub thickness" are of the same order, and the relationship between their Young's modulus and microbend loss can be obtained. When the Young's modulus of the main coating layer "main Young's modulus" is greater than 0.5 MPa and the Young's modulus of the secondary coating layer "sub Young's modulus" is greater than 500 MPa, the microbend loss can be suppressed to 1 dB / km or less.

[0124] Figure 16 It is a graph showing the specifications of each of Samples 39 to 41 of the optical fiber according to one embodiment of the present disclosure. For Figure 16Samples 39 to 41, which have the Young's modulus of the fixed main coating and the Young's modulus of the secondary coating and change the thicknesses of these coatings, are compared. When the outer diameter of the main coating is less than 160 μm (Sample 39: 161 μm, Sample 40: 161 μm, Sample 41: 141 μm), the microbending loss is greater than 1 dB / km. In addition, when the outer diameter of the secondary coating is greater than 200 μm (Sample 39: 201 μm, Sample 40: 181 μm, Sample 41: 201 μm), the microbending loss is greater than 1 dB / km. By comparing Samples 39 to 41, the microbending loss can be reduced when the "ratio of coating thicknesses" is greater than 0.3.

[0125] Symbol Explanation

[0126] 10 cores, 20 claddings, 100 optical fibers, 100a glass optical fibers (glass part), 210 main coating, 220 secondary coating.

Claims

1. An optical fiber, comprising a glass portion extending in the direction of a central axis, wherein the glass portion is made of silica-based glass, includes a core and a cladding, the core includes the central axis and contains chlorine at a mass percentage of 1% or more, the cladding surrounds the core and has a refractive index lower than the maximum refractive index of the core, and has substantially uniform residual stress in the entire region of the cross-section of the glass portion orthogonal to the central axis, the core further contains fluorine, the mass percentage of the fluorine in the core is 0.5% or less, in the cross-section of the glass portion, the first average value of the residual stress in a region where the distance from the center of the cross-section along the radial direction is 50 μm or more and 62.5 μm or less is lower than the second average value of the residual stress in a region where the distance from the center of the cross-section along the radial direction is 45 μm or more and 55 μm or less, the greater the difference value obtained by subtracting the first average value from the second average value, the lower the transmission loss.

2. The optical fiber according to claim 1, wherein, the difference between the maximum value and the minimum value of the residual stress is 200 MPa or less.

3. The optical fiber according to claim 2, wherein, the difference between the maximum value and the minimum value of the residual stress is 100 MPa or less.

4. The optical fiber according to any one of claims 1 to 3, wherein, the mass percentage of the chlorine in the core is 1.5% or more.

5. The optical fiber according to any one of claims 1 to 3, wherein, the mass percentage of the chlorine in the core is 3% or less.

6. The optical fiber according to any one of claims 1 to 3, wherein, Having an effective cross-sectional area of 70 μm 2 or more and 150 μm 2 or less.

7. The optical fiber according to any one of claims 1 to 3, wherein, the α value defining the shape of the refractive index profile of the core is 150 or less, the α corresponds to the exponent in the following definition formula of the "α-th power profile": wherein, r0: the position where Δ(r) is the maximum, r1: applicable to the boundary between the core and the cladding, and the α value is determined by the least squares method.

8. The optical fiber according to claim 7, wherein, the α value is 3 or more and 99 or less.

9. The optical fiber according to any one of claims 1 to 3, wherein, in the cross-section of the glass portion, the value obtained by subtracting the average value of the residual stress in a region where the distance from the center of the cross-section along the radial direction is 55 μm or more and 62.5 μm or less from the average value of the residual stress in a region where the distance from the center of the cross-section along the radial direction is 45 μm or more and 55 μm or less is greater than 20 MPa.

10. The optical fiber according to any one of claims 1 to 3, wherein, in the optical fiber after being exposed in a hydrogen atmosphere at a partial pressure of 1.5 kPa and a temperature of 25 °C for 720 hours, the increase in transmission loss at a wavelength of 1550 nm is 0.005 dB / km or less.

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