Optical Fiber and Method for Manufacturing the Same

Abstract

This optical fiber has a central axis. The optical fiber is provided with: a core (11) that is formed from silica glass, while extending along the central axis; a cladding (12) that is formed from silica glass so as to surround the core, while extending along the central axis; and coating layers (13, 14) that are formed from a resin so as to surround the cladding, while extending along the central axis. The outer diameter (f) of the cladding changes along the central axis. The residual stress (s) in the direction along the central axis, said residual stress being averaged over the core and the cladding in a cross-section that is perpendicular to the central axis, changes along the central axis. The deviation of the outer diameter (f) from the average and the deviation of the residual stress (s) from the average have mutually opposite signs.

Description

Technical Field

[0001] The present disclosure relates to an optical fiber and a method for manufacturing the optical fiber. This application claims priority based on Japanese Application No. 2020-101719 filed on June 11, 2020, and incorporates all the descriptions described in the above Japanese application by reference.

Background Art

[0002] Non-Patent Documents 1 to 3 disclose optical fibers used for long-distance transmission such as undersea cable transmission.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

[0004] An optical fiber according to an embodiment of the present disclosure has a central axis. The optical fiber is made of silica glass and includes a core made of silica glass that extends along the central axis, a cladding made of silica glass that surrounds the core and extends along the central axis, and a coating layer made of resin that surrounds the cladding and extends along the central axis. The outer diameter of the cladding varies along the central axis. The residual stress in the direction along the central axis, averaged over the core and the cladding in a single cross-section perpendicular to the central axis, varies along the central axis. The deviation from the average value of the outer diameter and the deviation from the average value of the residual stress have opposite signs to each other.

[0005] A method for manufacturing an optical fiber according to an embodiment of the present disclosure includes heating a tip portion of an optical fiber preform made of glass, pulling out a glass fiber from the softened tip portion by heating, and forming a coating layer made of resin on the glass fiber to obtain an optical fiber. Pulling out includes changing the tension applied to the glass fiber periodically so that the diameter of the glass fiber and the residual stress in the axial direction of the glass fiber change in antiphase with each other along the axial direction. [Brief Description of the Drawings]

[0006]

Figure 1A

Figure 1B

Figure 1C

Figure 1D

Figure 2A

Figure 2B

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0007] [Problems to be Solved by the Present Disclosure] In an optical fiber used for long-distance transmission, in addition to the spontaneous emission optical noise of an optical amplifier and the non-linear noise due to the non-linear optical effect in the optical fiber, noise due to guided acoustic wave Brillouin scatter (GAW BS: Guided Acoustic Wave Brillouin Scatter) causes a decrease in transmission performance.

[0008] Non-cited Document 1 discloses GAWBS as follows. That is, in an optical fiber formed of glass, guided modes of acoustic waves are generated inside due to reflection on the outer peripheral surface of the glass. GAWBS is a phenomenon in which thermally excited guided modes randomly scatter the light propagating through the core of the optical fiber. The frequency spectrum of the scattered light due to GAWBS has a plurality of discrete peaks centered on the frequency of the original light. The center frequency of each peak corresponds to the guided mode of the acoustic wave. The frequency shift from the frequency of the original light is from 20 MHz to 800 MHz. The line width of the peak is from 165 kHz to 1000 kHz.

[0009] Non-Patent Document 2 discloses that when signal light is transmitted over a long distance by an optical fiber, the signal light scattered by GAWBS accumulates as noise, so GAWBS has a non-negligible impact on the signal-to-noise ratio.

[0010] Therefore, an object of the present disclosure is to provide an optical fiber and a method for manufacturing an optical fiber that can improve the transmission performance in long-distance transmission by suppressing GAWBS.

[0011] [Effects of the Present Disclosure] According to the present disclosure, GAWBS can be suppressed and the transmission performance in long-distance transmission can be improved.

[0012] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described. An optical fiber according to an embodiment has a central axis. The optical fiber is made of silica glass and includes a core made of silica glass and extending along the central axis, a cladding made of silica glass surrounding the core and extending along the central axis, and a coating layer made of resin surrounding the cladding and extending along the central axis. The outer diameter of the cladding varies along the central axis. The residual stress in the direction along the central axis, averaged over the core and the cladding in a single cross-section perpendicular to the central axis, varies along the central axis. The deviation from the average value of the outer diameter and the deviation from the average value of the residual stress have opposite signs to each other.

[0013] In the optical fiber according to the above embodiment, the line width of the peak of the frequency spectrum of the scattered light due to GAWBS can be effectively expanded. Thereby, GAWBS can be suppressed. As a result, the transmission performance in long-distance transmission can be improved.

[0014] The outer diameter and the residual stress may change so as to be in antiphase with each other along the central axis. In this case, the deviation from the average value of the outer diameter of the cladding and the deviation from the average value of the residual stress can have opposite signs to each other.

[0015] When monochromatic light is propagated through the core, the effective line width of the peak of the frequency spectrum of the scattered light that is scattered forward by the acoustic wave thermally excited in the optical fiber and propagates through the core may be larger than 1.5 MHz. In this case, according to the relationship between the SNR (signal to noise ratio) decrease and the line width disclosed in Non-Patent Document 3, the SNR decrease due to GAWBS can be effectively suppressed.

[0016] When the deviation of the outer diameter is δf and the deviation of the residual stress is δσ,

Equation

[0017] The method for manufacturing an optical fiber according to an embodiment includes heating a tip portion of an optical fiber preform made of glass, pulling out a glass fiber from the softened tip portion by heating, and forming a coating layer made of resin on the glass fiber to obtain an optical fiber. Pulling out includes changing the tension applied to the glass fiber periodically so that the diameter of the glass fiber and the residual stress in the axial direction of the glass fiber change in antiphase with each other along the axial direction.

[0018] In the method for manufacturing an optical fiber according to the above embodiment, an optical fiber can be obtained in which the outer diameter of the cladding and the residual stress change in antiphase with each other along the axial direction. Therefore, the line width of the peak of the frequency spectrum of the scattered light due to GAWBS can be effectively expanded. Thus, GAWBS can be suppressed. As a result, the transmission performance in long-distance transmission can be improved.

[0019] The method for manufacturing the optical fiber further includes guiding an optical fiber continuous with the glass fiber to a winding machine, and guiding may include changing the tension applied in pulling out by changing the length of the traveling path of the optical fiber periodically. In this case, by changing the length of the traveling path of the optical fiber, the tension applied to the glass fiber can be changed as a result.

[0020] Guiding may include changing the length of the traveling path periodically by moving a roller that changes the traveling direction of the optical fiber periodically. In this case, since the optical fiber is protected by the coating layer, it is difficult to be damaged by traveling on the outer peripheral surface of the roller.

[0021] The method for manufacturing the optical fiber may further include measuring at least one of the diameter and the tension of the glass fiber. In this case, based on the measurement result, the tension applied to the glass fiber can be adjusted.

[0022] The method for manufacturing the optical fiber may further include gripping the optical fiber preform and inserting the optical fiber preform into a heating furnace at a constant speed. The heating may be performed by heating the tip portion with a heating furnace. In this case, glass fibers can be stably drawn from the optical fiber preform.

[0023] [Details of Embodiments of the Present Disclosure] Specific examples of the optical fiber and the method for manufacturing the optical fiber of the present disclosure will be described below with reference to the drawings. It should be noted that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant descriptions are omitted.

[0024] In this specification, when the refractive index of a certain medium is n and the refractive index of pure silica glass is n0, the relative refractive index difference Δ of the medium is [Equation] is defined as. Unless otherwise specified, it is assumed that the optical fiber has one central axis, is substantially rotationally symmetric about the central axis, and has a structure that is translationally symmetric along the central axis. Regarding the components of the optical fiber such as the core, cladding, and coating, unless otherwise specified, it is assumed that they are substantially rotationally symmetric about the central axis and have a structure that is translationally symmetric along the central axis. When this assumption is applicable, the physical property values of the components of the optical fiber can be defined by the values in any cross section perpendicular to the central axis. When defining statistical values such as the average value, maximum value, and percentile value of the physical property values, the physical property values in the above cross section are replaced by the statistical values for the set of measurement values obtained by measuring the physical property values in the above cross section at a spatially uniform frequency with a predetermined spatial resolution. Unless otherwise specified, the above spatial resolution assumes a circle with a radius of 1 μm, which is an approximate value of the operating wavelength of the optical fiber.

[0025] Let the radial coordinate of the optical fiber be r, and the relative refractive index differences in the regions with the inner radius r0 and the outer radius r1 are [Equation] When represented by the following formula, the specific refractive index of the region has a shape of the α01 power. Here, Δ0 is the specific refractive index difference at the radius r = r0, that is, at one end of the region, and Δ1 is the specific refractive index difference at the radius r = r1, that is, at the other end of the region.

[0026] (Optical fiber) As shown in FIG. 1A, the optical fiber 1 includes a central axis 10, a core 11, a cladding 12, a first coating layer 13, and a second coating layer 14. The core 11 is made of glass and extends along the central axis 10. The cladding 12 is made of glass, surrounds the core 11, and extends along the central axis 10. The first coating layer 13 is made of resin, surrounds the cladding 12, and extends along the central axis 10. The first coating layer 13 is made of, for example, an acrylate-based ultraviolet curable resin. The second coating layer 14 is made of resin, surrounds the first coating layer 13, and extends along the central axis 10. The second coating layer 14 is made of, for example, an acrylate-based ultraviolet curable resin having a higher elastic modulus than the first coating layer 13.

[0027] The specific refractive index difference of the core 11 is higher than that of the cladding 12, and the difference is 0.2% or more and 2.0% or less. The core 11 does not contain GeO2 as an additive and contains one or more additives selected from Cl, F, P, Br, Na, K, and Rb. The cladding 12 contains one or more of F and Cl as additives. Thereby, the optical fiber 1 can achieve low transmission loss and is suitable for long-distance optical communication. The transmission loss is preferably 0.17 dB / km or less, more preferably 0.16 dB / km or less, and even more preferably 0.15 dB / km or less. On the other hand, when the transmission loss is 0.10 dB / km or more, the productivity can be improved.

[0028] The diameter of the core 11 is 7 μm or more and 14 μm or less. The average outer diameter of the cladding 12 is 123 μm or more and 127 μm or less, more preferably 124 μm or more and 126 μm or less. The average diameter of an optical fiber that has already been widely used is 125 μm. Therefore, since the average outer diameter of the cladding 12 of the optical fiber 1 is 125 μm, the cost required for connection with widely used optical fibers can be reduced. The outer diameter of the second coating layer 14 is 170 μm or more and 270 μm or less. Thereby, the optical fiber 1 can achieve both sufficient mechanical strength and cable accommodation at a high density.

[0029] The optical fiber 1 is manufactured by heating and drawing the tip portion 201b (see FIG. 4) of the optical fiber preform 201. The optical fiber preform 201 is made of silica glass and has a shape similar to that of the optical fiber 1 in a cross section perpendicular to the axial direction. When the optical fiber 1 is drawn, a tension is applied to the optical fiber 1. Due to this tension and the thermal contraction accompanying the cooling of the optical fiber 1 during the drawing process, stress remains in the glass of the drawn optical fiber 1 (that is, the core 11 and the cladding 12).

[0030] As shown in FIGS. 1B and 1C, the outer diameter f(z) of the cladding 12 changes as a function of the axial position z of the optical fiber 1. That is, the outer diameter f(z) of the cladding 12 changes along the central axis 10. Hereinafter, the outer diameter of the cladding 12 is also referred to as the cladding outer diameter.

[0031] The average value of the cladding outer diameter f(z) <f>Taking the length of the optical fiber 1 as L,

Number

Number

[0032] As shown in FIGS. 1B and 1D, in the optical fiber 1, the residual stress s(z) in the silica glass also changes as a function of z. That is, the residual stress s(z) changes along the central axis 10. In the present disclosure, the residual stress s(z) in the silica glass is defined as the value of the component in the direction along the central axis 10 averaged over the core 11 and the cladding 12 in a single cross section perpendicular to the central axis 10. That is, the residual stress s(z) in the silica glass is

Number

[0033] The average value of the residual stress s(z) in the silica glass <s>is

Number

Number

[0034] As shown in FIGS. 1C and 1D, the average value of the cladding outer diameter f(z) <f>The deviation δf from <s>The deviation δs from [a certain reference] has opposite signs. The cladding outer diameter f(z) and the residual stress s(z) change along the central axis 10 such that they are in opposite phases with each other (the phase difference is 180 degrees when each of the cladding outer diameter f(z) and the residual stress s(z) is approximated by a trigonometric function).

[0035] The fluctuation period of the deviation δf is defined as the reciprocal of the centroid of the power spectrum obtained by Fourier-transforming the deviation δf with respect to the position z and squaring the amplitude. The fluctuation period of the deviation δs is defined as the reciprocal of the centroid of the power spectrum obtained by Fourier-transforming the deviation δs with respect to the position z and squaring the amplitude. The fluctuation period of the deviation δf and the fluctuation period of the deviation δs are equal to each other. Each fluctuation period is preferably 0.01 m or more and 100 m or less, more preferably 0.02 m or more and 50 m or less. When each fluctuation period is long, the variation in transmission performance for each section of the transmission line increases. When each fluctuation period is short, an increase in transmission loss due to mode coupling to higher-order modes occurs. Therefore, it is preferable to be within the above range.

[0036] In addition to the above range of the fluctuation period, as in the optical fiber 1A according to the modified example shown in FIGS. 2A and 2B, it is more preferable to include at least two layers, an inner cladding 120 that surrounds the core 11 and an outer cladding 121 that surrounds the inner cladding 120, and the inner cladding 120 has a lower refractive index than the outer cladding 121. Thereby, the refractive index difference between the fundamental guided mode and the higher-order mode can be enlarged. Therefore, mode coupling to higher-order modes caused by short fluctuation period components can be suppressed. As a result, for example, even when a microbend is applied to the optical fiber, an increase in transmission loss due to mode coupling can be suppressed.

[0037] Assuming that the Young's modulus averaged over the core 11 and the cladding 12 in a single cross-section perpendicular to the central axis 10 is E and the residual strain is ε, since ε = s / E, the average value <ε> of the residual strain and the deviation δε from the average are approximately

Equation

Number

[0038] As disclosed in Non-Patent Document 4, the longitudinal wave sound velocity Vd in an optical fiber, with the density being ρ and the Poisson's ratio being κ,

Number

Number

[0039] Therefore, the sound velocity deviation δVd caused by the residual stress deviation δs is

Number

[0040] As disclosed in Non-Patent Document 1, the frequency Ωm of the m-th peak in the spectrum of scattered light by GAWBS is given as follows. That is, with the longitudinal wave velocity of the corresponding acoustic wave mode being Vd, the transverse wave velocity being Vs, and the sound velocity ratio being α = Vs / Vd,

Number

Number

[0041] Therefore, when the cladding outer diameter f and the residual stress s are changed along the central axis 10, the deviation δΩm generated in the m-th peak frequency is

Number

[0042] By changing the frequency Ωm of the m-th peak longitudinally by about 1 / 2 or more of the full width at half maximum ΔΩm of its linewidth, the effective linewidth can be enlarged when the fiber length including the longitudinal change is regarded as a uniform fiber. In other words, by changing the peak frequency Ωm along the central axis 10 and making the change amount about 1 / 2 or more of the full width at half maximum ΔΩm, the linewidth of the optical fiber 1 can be effectively enlarged.

[0043] Non-Patent Document 3 discloses that the smaller the linewidth of the peak in the spectrum of the scattered light by GAWBS, the lower the signal-to-noise ratio by GAWBS. That is, by effectively enlarging the linewidth, the decrease in the signal-to-noise ratio by GAWBS can be suppressed. As shown in Equation (A), by changing the deviation from the average value of the residual stress and the deviation from the average value of the cladding outer diameter longitudinally so that they have opposite signs to each other, the effective linewidth of GAWBS can be enlarged more effectively.

[0044] On the other hand, when the change in the cladding outer diameter of the optical fiber 1 is excessive, the connection loss in the connection using a ferrule and a V-groove as the centering means increases. In addition, it becomes difficult to detect abnormal parts such as bubbles in the glass based on the measured value of the cladding outer diameter. Therefore, three times the standard deviation (3σ) of the deviation of the cladding outer diameter may be 1.0 μm or less, more preferably 0.5 μm or less. When the residual stress is excessive, a decrease in the breaking strength and an increase in the connection loss due to a decrease in the flatness of the end face when the optical fiber 1 is cleaved occur. For this reason, three times the standard deviation (3σ) of the deviation of the residual stress may be 150 MPa or less, more preferably 100 MPa.

[0045] FIG. 3 is a graph showing a range in which the effective line width is expanded to 1 / 2 or more of the full width at half maximum, and the adverse effects due to excessive deviations in the cladding outer diameter and residual stress are suppressed. The horizontal axis of FIG. 3 indicates the residual stress deviation [MPa], and the vertical axis of FIG. 3 indicates the cladding outer diameter deviation [μm]. In FIG. 3, based on Non-Patent Document 1, the characteristics of GAWBS when there is no change in the cladding outer diameter and residual stress in the longitudinal direction of the optical fiber are shown based on Non-Patent Document 1, when the peak frequency is 500 MHz and the full width at half maximum of the line width is 1 MHz.

[0046] Specifically, a range that satisfies the following condition (1) is preferable, and a range that satisfies condition (2) is more preferable. Condition (1)

Number

Number

[0047] In order to expand the line width of the peak of the frequency spectrum of the scattered light by GAWBS, in addition to the change in the opposite phase of the cladding outer diameter and residual stress as described above, it is more preferable to eliminate the degeneracy of the acoustic mode by imparting non-circularity to the outer diameter of the cladding 12 of the optical fiber 1 to eliminate rotational symmetry. Specifically, the cladding non-circularity may be 0.1% or more, more preferably 0.2% or more. On the other hand, an excessive cladding non-circularity increases the connection loss, so the cladding non-circularity may be 1.5% or less, more preferably 1% or less. Here, the outer diameter of the cladding 12 having non-circularity means that the outer peripheral portion of the cladding 12 is not a perfect circle. The cladding non-circularity is a value obtained by dividing the difference in the lengths of the major axis and the minor axis by the length of the major axis when the outer peripheral portion of the cladding 12 is approximated by an ellipse.

[0048] When the core 11 is eccentric from the center of gravity of the cladding 12, the overlap between the mode amplitude of the acoustic wave and the mode amplitude of the optical electric field is reduced. Thereby, the line width of the peak of the frequency spectrum of the scattered light by the GAWBS can be expanded. Specifically, the core eccentricity may be 0.1 μm or more, more preferably 0.2 μm or more. On the other hand, excessive core eccentricity increases the connection loss. Therefore, the core eccentricity may be 1.0 μm or less, more preferably 0.8 μm or less.

[0049] In the optical fiber 1, when monochromatic light is propagated through the core 11, the line width of the peak at 500 MHz or more of the frequency spectrum of the scattered light that is scattered forward by the acoustical wave thermally excited in the optical fiber 1 and propagates through the core 11 is larger than 1.5 MHz.

[0050] (Method for manufacturing an optical fiber) Hereinafter, a method for manufacturing the optical fiber 1 according to the embodiment will be described. FIG. 4 is a configuration diagram of a manufacturing apparatus for an optical fiber according to the embodiment. FIG. 5 is a diagram for explaining the operation of the roller. The manufacturing apparatus 2 shown in FIG. 4 is an apparatus for manufacturing the optical fiber 1 from an optical fiber preform 201 through a glass fiber 204. The manufacturing apparatus 2 includes a gripping portion 202, a heating furnace 203, a heat-insulating furnace 205, a measuring device 206, a cooler 207, a die 208, an ultraviolet irradiator 209, a roller 211, a capstan 212, and a take-up machine 213.

[0051] The gripping portion 202 grips the optical fiber preform 201 and feeds it into the heating furnace 203 at a constant speed. The optical fiber preform 201 has a base end portion 201a gripped by the gripping portion 202 and a tip end portion 201b inserted into the heating furnace 203. The gripping portion 202 functions as a supply portion that supplies the optical fiber preform 201 to the heating furnace 203.

[0052] The heating furnace 203 has an opening 203a into which the optical fiber preform 201 is inserted, and an opening 203b that faces the opening 203a and from which the glass fiber 204 is drawn out. The heating furnace 203 heats and softens the tip 201b of the optical fiber preform 201 supplied into the interior of the heating furnace 203. The glass fiber 204 is drawn out from the tip 201b softened by heating. The glass fiber 204 is drawn out to the outside of the heating furnace 203 through the opening 203b.

[0053] The heat-insulating furnace 205 keeps the glass fiber 204 warm and relaxes the structure of the glass. The measuring device 206 measures at least one of the diameter and the tension of the glass fiber 204 in a state where the structure of the glass is relaxed. Examples of the measuring device 206 include a measuring device that measures the diameter by irradiating the glass fiber 204 with a laser, and a measuring device that measures the tension by irradiating the glass fiber 204 with ultrasonic waves.

[0054] The cooler 207 is arranged downstream of the measuring device 206 and cools the glass fiber 204. The die 208 applies a resin to the outer peripheral surface of the incoming glass fiber 204 to form a coating resin. The resin contains an acrylate-based ultraviolet curable resin. The ultraviolet irradiator 209 irradiates the coating resin formed on the glass fiber 204 with ultraviolet rays to cure the coating resin. Thereby, the glass fiber is coated with the resin. As a result, the optical fiber 210 is manufactured.

[0055] In FIG. 4, a set of the die 208 and the ultraviolet irradiator 209 is shown, but the manufacturing apparatus 2 may include two sets of the die 208 and the ultraviolet irradiator 209 arranged along the axial direction of the glass fiber 204. In that case, the die 208 and the ultraviolet irradiator 209 arranged in the front stage function as a first coating layer forming portion that forms the first coating layer 13. The die 208 and the ultraviolet irradiator 209 arranged in the rear stage function as a second coating layer forming portion that forms the second coating layer 14. Thereby, the first coating layer 13 and the second coating layer 14 are formed, and the optical fiber 1 is obtained.

[0056] The roller 211 changes the traveling direction of the optical fiber 1. The roller 211 moves so as to change the angle or position of the roller 211. As a result, the length of the traveling path (pass line) of the optical fiber 1 and the glass fiber 204 from the tip 201b to the take-up machine 213 changes periodically.

[0057] As shown in FIG. 5, the roller 211 reciprocates, for example, along the axial direction of the roller 211. In this case, the traveling position of the optical fiber 1 on the outer peripheral surface of the roller 211 reciprocates between one end side and the other end side in the axial direction of the roller 211. As a result, the pulling direction and the pulling angle of the glass fiber 204 from the tip 201b of the optical fiber preform 201 also change periodically. The pulling angle is the angle formed by the pulling direction of the glass fiber 204 and the axial direction of the optical fiber preform 201.

[0058] The length of the traveling path of the optical fiber 1 and the glass fiber 204 becomes shorter when the optical fiber 1 travels through the center in the axial direction of the roller 211, and becomes longer when traveling through one end side and the other end side. In the process of the traveling path becoming longer, the tension applied to the glass fiber 204 increases. Therefore, the diameter of the glass fiber 204 is reduced and the residual stress of the glass fiber 204 increases. As a result, the clad outer diameter is reduced and the residual stress of the optical fiber 1 in the direction along the central axis 10 increases.

[0059] On the contrary, in the process of the traveling path becoming shorter, the tension applied to the glass fiber 204 decreases. The diameter of the glass fiber 204 increases and the residual stress of the glass fiber 204 decreases. As a result, the clad outer diameter increases and the residual stress of the optical fiber 1 in the direction along the central axis 10 decreases. As a result, an optical fiber 1 having longitudinal variations in the clad outer diameter and the residual stress is obtained. That is, an optical fiber 1 is obtained in which the clad outer diameter and the residual stress change in antiphase with each other along the central axis 10.

[0060] The roller 211 guides the optical fiber 1 to the capstan 212 while periodically changing the length of the running paths of the glass fiber 204 and the optical fiber 1 in this way. Thereby, the roller 211 imparts a longitudinally changing inverse phase between the cladding outer diameter and the residual stress of the optical fiber 1. The roller 211 may move, for example, along the central axis 10 of the optical fiber 1, and periodically change only the length of the running path of the optical fiber 1 while maintaining the length of the running path of the glass fiber 204. In this case, the drawing direction and the drawing angle of the glass fiber 204 are also maintained. Even in this case, since the length of the running path of the optical fiber 1 changes, the tension applied to the glass fiber 204 changes as a result. Therefore, the tension applied to the glass fiber 204 can be periodically changed.

[0061] The period of changing the length of the running path is 0.01 m or more and 100 m or less, more preferably 0.02 m or more and 50 m or less, in terms of the length of the optical fiber 1. Thereby, the suppression effect of GAWBS can be enhanced. For that purpose, for example, while the optical fiber 1 is being drawn at 50 m / s, the position or angle of the roller 211 is changed at 0.5 Hz or more, more preferably 1 Hz or more.

[0062] The capstan 212 pulls the optical fiber 1 at a predetermined speed and tension. The winder 213 winds up the optical fiber 1 pulled by the capstan 212.

[0063] FIG. 6 is a flowchart showing a method for manufacturing an optical fiber according to an embodiment. The method for manufacturing the optical fiber 1 includes a step S1 of inserting the optical fiber preform 201 into the heating furnace 203, a step S2 of heating the tip portion 201b of the optical fiber preform 201, a step S3 of drawing out the glass fiber 204 from the tip portion 201b, a step S4 of keeping the glass fiber 204 warm, a step S5 of measuring at least one of the diameter and the tension of the glass fiber 204, a step S6 of cooling the glass fiber 204, a step S7 of forming a coating resin on the glass fiber 204 to obtain the optical fiber 1, a step S8 of guiding the optical fiber 1, and a step S9 of winding up the optical fiber 1.

[0064] In step S1, the optical fiber preform 201 is inserted into the heating furnace 203 at a constant speed by the gripping part 202. The optical fiber preform 201 is fed into the heating furnace 203 with the base end portion 201a being gripped and the tip end portion 201b passing through the opening 203a of the heating furnace 203. In step S2, the tip end portion 201b is heated and softened by the heating furnace 203.

[0065] In step S3, the glass fiber 204 is drawn out from the softened tip end portion 201b through the opening 203b. In step S3, by periodically changing the tension applied to the glass fiber 204, the diameter of the glass fiber 204 and the residual stress in the axial direction of the glass fiber 204 are changed so as to be out of phase with each other along the axial direction. The insertion speed of the optical fiber preform 201 in step S1 can be set according to the drawing speed of the glass fiber 204 in step S3.

[0066] In step S4, the drawn glass fiber 204 is heat-insulated by the heat-insulating furnace 205. Thereby, the structure of the glass is relaxed. In step S5, at least one of the diameter and the tension of the glass fiber 204 is measured by the measuring device 206. In step S6, the glass fiber 204 is cooled.

[0067] In step S7, first, resin is applied to the outer peripheral surface of the glass fiber 204 by the die 208 to form a coating resin. Subsequently, the coating resin is cured by ultraviolet rays irradiated from the ultraviolet irradiator 209. By repeating step S7, the first coating layer 13 and the second coating layer 14 are formed, and as a result, the optical fiber 1 is obtained.

[0068] In step S8, the optical fiber 1 continuous with the glass fiber 204 is pulled by the capstan 212 at a predetermined speed and tension, travels on the outer peripheral surface of the roller 211, and then is guided to the winder 213. The traveling direction of the optical fiber 1 is changed by the roller 211. In step S8, the tension applied to the glass fiber 204 in step S3 is periodically changed by periodically changing at least the length of the traveling path of the optical fiber 1. In step S8, the length of the traveling path of the optical fiber 1 is periodically changed by periodically moving the roller 211. In step S8, the total length of the traveling paths of the glass fiber 204 and the optical fiber 1 is periodically changed.

[0069] The changes in the clad outer diameter and the residual stress are caused by the movement of the roller 211 and the change in the pulling speed by the capstan 212. Therefore, based on the clad outer diameter or the tension measured by the measuring device 206, the movement of the roller 211 and the rotation of the capstan 212 can be controlled so that the fluctuation range falls within the target range.

[0070] The period for changing the lengths of the traveling paths of the glass fiber 204 and the optical fiber 1 is 0.01 m or more and 100 m or less, more preferably 0.02 m or more and 50 m or less, in terms of the length of the optical fiber 1. Thereby, the suppression effect of GAWBS is improved. In order to change the lengths of the traveling paths of the glass fiber 204 and the optical fiber 1 with such a period, for example, while the optical fiber 1 is being drawn at 50 m / s, the position or angle of the roller 211 may be changed at 0.5 Hz or more and 5 kHz or less, more preferably 1 Hz or more and 2.5 kHz or less.

[0071] In step S9, the optical fiber 1 is wound by the winder 213.

[0072] As described above, in the optical fiber 1, the clad outer diameter and the residual stress change along the central axis 10 of the optical fiber 1, and the average value of the clad outer diameter f(z) <f>The deviation δf from <s>The deviation δs from [the reference] has opposite signs to each other. Therefore, the line width of the peak of the frequency spectrum of the scattered light due to GAWBS can be effectively broadened. As a result, GAWBS can be suppressed. Consequently, the transmission performance in long-distance transmission can be improved.

[0073] The cladding outer diameter f(z) and the residual stress s(z) change along the central axis 10 so as to have opposite phases to each other. Therefore, the average value of the cladding outer diameter f(z) <f>The deviation δf from <s>The deviation δs from can have opposite signs to each other.

[0074] When monochromatic light is propagated through the core 11, the line width of the peak of the frequency spectrum of the scattered light scattered forward by the acoustical wave thermally excited in the optical fiber 1 and propagating through the core 11 is greater than 1.5 MHz. Therefore, GAWBS can be reliably suppressed.

[0075] In the optical fiber 1, the above condition (1) is satisfied. Therefore, the adverse effects due to excessive deviation of the cladding outer diameter and deviation of the residual stress can be suppressed.

[0076] In the manufacturing method of the optical fiber 1, in step S3, by periodically changing the tension applied to the glass fiber 204, the diameter of the glass fiber 204 and the residual stress in the axial direction of the glass fiber 204 are changed so as to have opposite phases along the axial direction. As a result, an optical fiber 1 is obtained in which the cladding outer diameter and the residual stress change so as to have opposite phases along the axial direction. Therefore, the line width of the peak of the frequency spectrum of the scattered light due to GAWBS can be effectively increased. Thus, GAWBS can be suppressed. As a result, the transmission performance in long-distance transmission can be improved.

[0077] In step S8, by periodically changing the length of the running path of the optical fiber 1, the tension applied in step S3 is periodically changed. Therefore, by changing the length of the running path of the optical fiber 1, the tension indirectly applied to the glass fiber can be changed.

[0078] In step S8, the length of the running path of the optical fiber 1 is periodically changed by periodically moving the roller 211 that changes the running direction of the optical fiber 1. If a roller for changing the running direction of the glass fiber 204 is provided and the tension applied to the glass fiber 204 is changed by the movement of this roller, the glass fiber 204 may be damaged due to contact with the roller. Since the optical fiber 1 is protected by the first coating layer 13 and the second coating layer 14, it is difficult to be damaged by the roller 211.

[0079] Since the manufacturing method of the optical fiber 1 includes step S5, at least one of the diameter and the tension of the glass fiber 204 Measurement Based on the determined result, the tension applied to the glass fiber 204 can be adjusted.

[0080] Since the manufacturing method of the optical fiber 1 includes step S1, the glass fiber 204 can be stably drawn from the optical fiber preform 201.

Explanation of Reference Numerals

[0081] 1, 1A… Optical fiber 2… Manufacturing apparatus 10… Central axis 11… Core 12… Cladding 13… First coating layer 14… Second coating layer 201… Optical fiber preform 201a… Base end portion 201b… Tip end portion 202… Gripping portion 203… Heating furnace 203a… Opening 203b… Opening 204… Glass fiber 205… Heat insulation furnace 206… Measuring instrument 207… Cooler 208… Die 209… Ultraviolet irradiator 211… Roller 212… Capstan 213… Take-up machine< / s> < / f> < / s> < / f> < / s> < / f> < / s> < / s> < / f> < / f>

Claims

1. An optical fiber having a central axis, comprising a silica glass, a core extending along the central axis, comprising a silica glass, a cladding surrounding the core and extending along the central axis, comprising a resin, a coating layer surrounding the cladding and extending along the central axis, and comprising: the outer diameter of the cladding varies along the central axis, the residual stress in the direction along the central axis, averaged over the core and the cladding in a single cross-section perpendicular to the central axis, varies along the central axis, the deviation from the average value of the outer diameter and the deviation from the average value of the residual stress have opposite signs to each other, the diameter of the core is 7 μm or more and 14 μm or less, the fluctuation period of the deviation of the outer diameter and the fluctuation period of the deviation of the residual stress are each 0.01 m or more and 100 m or less, an optical fiber.

2. An optical fiber having a central axis, comprising a silica glass, a core extending along the central axis, comprising a silica glass, a cladding surrounding the core and extending along the central axis, comprising a resin, a coating layer surrounding the cladding and extending along the central axis, and comprising: the outer diameter of the cladding varies along the central axis, the residual stress in the direction along the central axis, averaged over the core and the cladding in a single cross-section perpendicular to the central axis, varies along the central axis, the deviation from the average value of the outer diameter and the deviation from the average value of the residual stress have opposite signs to each other, the diameter of the core is 7 μm or more and 14 μm or less, the cladding includes an inner cladding surrounding the core and an outer cladding surrounding the inner cladding, the refractive index of the inner cladding is lower than the refractive index of the outer cladding, an optical fiber.

3. An optical fiber having a central axis, comprising a silica glass, a core extending along the central axis, comprising a silica glass, a cladding surrounding the core and extending along the central axis, comprising a resin, a coating layer surrounding the cladding and extending along the central axis, and comprising: the outer diameter of the cladding varies along the central axis, the residual stress in the direction along the central axis, averaged over the core and the cladding in a single cross-section perpendicular to the central axis, varies along the central axis, The deviation from the average value of the outer diameter and the deviation from the average value of the residual stress have opposite signs to each other. The diameter of the core is 7 μm or more and 14 μm or less. The non-circularity of the cladding is 0.1% or more and 1.5% or less. Optical fiber.

4. The outer diameter and the residual stress change along the central axis so as to have opposite phases to each other. The optical fiber according to any one of Claims 1 to 3.

5. When monochromatic light is propagated through the core, the line width of the peak of the frequency spectrum of the scattered light that is scattered forward by the acoustic waves thermally excited in the optical fiber and propagates through the core is greater than 1.5 MHz. The optical fiber according to any one of Claims 1 to 4.

6. When the deviation of the outer diameter is δf and the deviation of the residual stress is δσ, 【Number 1】 is substantially established over the entire length. The optical fiber according to any one of Claims 1 to 5.

7. Three times the standard deviation of the deviation of the outer diameter is 1.0 μm or less. The optical fiber according to any one of Claims 1 to 6.

8. Three times the standard deviation of the deviation of the residual stress is 150 MPa or less. The optical fiber according to any one of Claims 1 to 7.

9. The transmission loss is 0.17 dB / km or less. The optical fiber according to any one of Claims 1 to 8.

10. Heating the tip of an optical fiber preform made of glass, drawing out glass fibers from the tip softened by heating, forming a coating layer made of resin on the glass fibers to obtain an optical fiber, including the drawing out includes changing the tension applied to the glass fibers periodically, so as to change the diameter of the glass fibers and the residual stress in the axial direction of the glass fibers so as to have opposite phases to each other along the axial direction, the diameter of the core of the optical fiber is 7 μm or more and 14 μm or less, the fluctuation period of the diameter of the glass fibers and the fluctuation period of the residual stress in the axial direction of the glass fibers are each 0.01 m or more and 100 m or less. Method for manufacturing an optical fiber.

11. Heating the tip of an optical fiber preform made of glass, drawing out glass fibers from the tip softened by heating, forming a coating layer made of resin on the glass fibers to obtain an optical fiber, including The pulling includes changing the tension applied to the glass fiber periodically so that the diameter of the glass fiber and the residual stress in the axial direction of the glass fiber change in antiphase with each other along the axial direction. The diameter of the core of the optical fiber is 7 μm or more and 14 μm or less. The cladding of the optical fiber includes an inner cladding surrounding the core and an outer cladding surrounding the inner cladding. The refractive index of the inner cladding is lower than that of the outer cladding. Method for manufacturing an optical fiber.

12. Heating the tip of an optical fiber preform made of glass. Pulling out a glass fiber from the tip softened by heating. Forming a coating layer made of resin on the glass fiber to obtain an optical fiber. The pulling includes changing the tension applied to the glass fiber periodically so that the diameter of the glass fiber and the residual stress in the axial direction of the glass fiber change in antiphase with each other along the axial direction. The diameter of the core of the optical fiber is 7 μm or more and 14 μm or less. The non-circularity of the cladding of the optical fiber is 0.1% or more and 1.5% or less. Method for manufacturing an optical fiber.

13. Further including guiding the optical fiber continuous with the glass fiber to a winding machine. The guiding includes changing the tension applied in the pulling periodically by changing the length of the traveling path of the optical fiber periodically. The method for manufacturing an optical fiber according to any one of Claims 10 to 12.

14. The guiding includes changing the length of the traveling path periodically by moving a roller for changing the traveling direction of the optical fiber periodically. The method for manufacturing an optical fiber according to Claim 13.

15. Further including measuring at least one of the diameter and the tension of the glass fiber. The method for manufacturing an optical fiber according to any one of Claims 10 to 14.

16. Further including gripping the optical fiber preform and inserting the optical fiber preform into a heating furnace at a constant speed. The heating is heating the tip by the heating furnace. The method for manufacturing an optical fiber according to any one of Claims 10 to 15.

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