Optical fiber and method for manufacturing optical fiber

Abstract

An optical fiber has a central axis. The optical fiber has a core (11) composed of quartz glass and extending along the central axis, a cladding (12) composed of quartz glass and extending along the central axis while surrounding the core, and a coating layer (13, 14) composed of resin and extending along the central axis while surrounding the cladding. An outer diameter (f) of the cladding varies along the central axis. A residual stress (s) in the direction of the central axis, averaged over the entire core and cladding, varies along the central axis in one cross section perpendicular to the central axis. Deviation of the outer diameter (f) from an average value and deviation of the residual stress (s) from an average value are opposite in sign to each other.

Description

TECHNICAL FIELD

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

[0002] Optical fibers for long distance transmission such as submarine cable transmission are disclosed in Non-Patent Literatures 1 to 3.

[0003] Prior Art Documents

[0004] Non-Patent Literature

[0005] Non-Patent Literature 1: R. M. Shelby et al., “Guided acoustic-wave Brillouin scattering”, Physical Review B, vol. 31, no. 8, p 5244 (1985)

[0006] Non-Patent Literature 2: M. A. Bolshtyansky et al., “Impact of Spontaneous Guided Acoustic-Wave Brillouin Scattering on Long-haul Transmission”, OFC 2018, M4B.3 (2018)

[0007] Non-Patent Literature 3: M. Paskov et al., “Observation and Compensation of Guided Acoustic-Wave Brillouin Scattering in Modulated Channels”, OFC 2019, Tu3J.3 (2019)

[0008] Non-Patent Literature 4: T. Horiguchi et al. “Tensile strain dependence of Brillouin frequency shift in silica optical fibers”, IEEE Photonics Technology Letters vol. 1, no. 5, p. 107 (1989)

[0009] Non-Patent Literature 5: Andrew D. Yablon, "Advanced Fiber Characterization Technologies for Fiber Lasers and Amplifiers", Advanced Solid State Lasers (ASSL), Th2A.45 (2014) SUMMARY

[0010] One embodiment of the present disclosure relates to an optical fiber having a central axis. The optical fiber includes a core composed of silica glass and extending along the central axis, a cladding composed of silica glass and extending along the central axis while surrounding the core, and a coating layer composed of resin and extending along the central axis while surrounding the cladding. An outer diameter of the cladding varies along the central axis. A residual stress in a direction along the central axis, which is averaged over the entire core and the cladding in one cross section perpendicular to the central axis, varies along the central axis. Deviation of the outer diameter from an average value and deviation of the residual stress from an average value are opposite in sign to each other.

[0011] One embodiment of the present disclosure relates to a method of manufacturing an optical fiber. The method includes heating a front end portion of an optical fiber preform composed of glass, drawing a glass fiber from the front end portion softened by the heating, and forming a coating layer composed of resin around the glass fiber to become the optical fiber. The drawing is performed by periodically changing a tension applied to the glass fiber so that a diameter of the glass fiber and a residual stress in an axial direction of the glass fiber vary in a manner that they become out of phase with each other in the axial direction. BRIEF DESCRIPTION OF DRAWINGS

[0012] [ Figure 1A ] Figure 1A is a perspective view showing a structure of an optical fiber according to an embodiment.

[0013] [ Figure 1B ] Figure 1B is a graph showing an outer diameter of a cladding and a residual stress in an optical fiber according to an embodiment.

[0014] [ Figure 1C ] Figure 1C is a graph showing a relationship between a position in an axial direction of an optical fiber according to an embodiment and an outer diameter of a cladding.

[0015] [ Figure 1D ] Figure 1D is a graph showing a relationship between a position in an axial direction of an optical fiber according to an embodiment and a residual stress.

[0016] [ Figure 2A ] Figure 2A is a perspective view showing a structure of an optical fiber according to a modified example.

[0017] [ Figure 2B ] Figure 2B is a graph showing the outer diameter of the cladding and the residual stress in the optical fiber to which the modification relates.

[0018] [ Figure 3 ] Figure 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 in which the adverse effects due to the excessive deviation of the outer diameter of the cladding and the residual stress can be suppressed.

[0019] [ Figure 4 ] Figure 4 is a configuration diagram of a manufacturing apparatus of the optical fiber to which the embodiment relates.

[0020] [ Figure 5 ] Figure 5 is a graph for explaining the operation of the roller.

[0021] [ Figure 6 ] Figure 6 is a flowchart showing the manufacturing method of the optical fiber to which the embodiment relates. DETAILED DESCRIPTION

[0022] [Problem to be solved by the disclosure]

[0023] In an optical fiber for long distance transmission, in addition to the spontaneous emission noise of an optical amplifier and the nonlinear noise due to the nonlinear optical effect in the optical fiber, the noise due to Brillouin scattering of guided acoustic wave (GAWBS) causes a decrease in transmission performance.

[0024] In Non-Patent Literature 1, the following is disclosed about GAWBS. That is, in an optical fiber formed of glass, a guided wave mode of an acoustic wave is generated inside due to the reflection at the outer periphery of the glass. GAWBS is a phenomenon in which a thermally excited guided wave mode randomly scatters light propagating in the core of the optical fiber. The spectrum of the scattered light of 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 wave mode of the acoustic wave. The frequency shift from the frequency of the original light is 20 MHz to 800 MHz. The line width of the peak is 165 kHz to 1000 kHz.

[0025] In Non-Patent Literature 2, it is disclosed that, in the case where signal light is transmitted over a long distance through an optical fiber, the signal light scattered by GAWBS accumulates as noise, and thus GAWBS has a non-negligible influence on the signal-to-noise ratio.

[0026] Therefore, an object of the present disclosure is to provide an optical fiber and a manufacturing method of an optical fiber in which the transmission performance in long distance transmission can be improved by suppressing GAWBS.

[0027] [Effects of the present disclosure]

[0028] According to the present disclosure, it is possible to improve the transmission performance in long distance transmission by suppressing GAWBS.

[0029] [Explanation of embodiments of the present disclosure]

[0030] First, an embodiment of the present disclosure is cited and explained. One embodiment relates to an optical fiber having a central axis. The optical fiber has: a core portion composed of silica glass and extending along the central axis; a cladding layer composed of silica glass and extending along the central axis while surrounding the core portion; and a coating layer composed of resin and extending along the central axis while surrounding the cladding layer. An outer diameter of the cladding layer varies along the central axis. In one cross section perpendicular to the central axis, a residual stress in a direction along the central axis after averaging the entire core portion and the cladding layer varies along the central axis. Deviation of the outer diameter from an average value and deviation of the residual stress from an average value have opposite signs to each other.

[0031] In the optical fiber according to the above embodiment, it is possible to effectively expand a line width of a peak of a spectrum of scattered light of GAWBS. Thereby, it is possible to suppress GAWBS. As a result, it is possible to improve the transmission performance in long distance transmission.

[0032] The outer diameter and the residual stress can vary in a manner that they become out of phase with each other along the central axis. In this case, it is possible to make the deviation of the outer diameter of the cladding layer from the average value and the deviation of the residual stress from the average value have opposite signs to each other.

[0033] When monochromatic light propagates in the core portion, an effective line width of a peak of a spectrum of scattered light that propagates in the core portion by forward scattering of an acoustic wave excited by heat in the optical fiber can be greater than 1.5 MHz. In this case, according to the relationship between the SNR (signal-to-noise ratio) reduction and the line width disclosed in Non-Patent Literature 3, it is possible to effectively suppress the SNR reduction due to GAWBS.

[0034] When the deviation of the outer diameter is δf and the deviation of the residual stress is δσ, the following formula 1 can substantially hold over the entire length.

[0035] [Formula 1]

[0036] |δf / [μm] - 0.0078 · (δσ / [MPa])| ≥ 0.125, |δf / [μm]| ≤ 1, |(δσ / [MPa])| ≤ 150

[0037] Since the formula holds at more than 99% of points randomly sampled from the entire length of the optical fiber, it is equivalent that the formula holds substantially over the entire length. Furthermore, it is more preferable that the formula holds at more than 99.9% of points randomly sampled from the entire length. In this case, the drawbacks due to the excessive deviation of the outer diameter of the cladding and the residual stress can be suppressed.

[0038] One embodiment relates to a method of manufacturing an optical fiber including: heating a front end portion of an optical fiber preform made of glass; drawing a glass fiber from the front end portion softened by the heating; and forming a coating layer made of resin around the glass fiber to become the optical fiber. The drawing refers to changing the diameter of the glass fiber and the residual stress in the axial direction of the glass fiber in such a manner that they become out of phase with each other along the axial direction by periodically changing the tension applied to the glass fiber.

[0039] In the method of manufacturing an optical fiber according to the above embodiment, the optical fiber in which the outer diameter of the cladding and the residual stress are changed in such a manner that they become out of phase with each other along the axial direction is obtained. Therefore, the line width of the peak of the spectrum of the scattered light caused by the GAWBS can be effectively enlarged. Thus, the GAWBS can be suppressed. As a result, the transmission performance in long distance transmission can be improved.

[0040] The method of manufacturing an optical fiber according to the above embodiment can further include guiding the optical fiber continuous to the glass fiber to a winding machine. The guiding can include changing the length of the travel path of the optical fiber periodically, thereby changing the tension applied in the drawing, by periodically moving a roller that changes the traveling direction of the optical fiber. 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.

[0041] The guiding can include changing the length of the travel path periodically by periodically moving a roller that changes the traveling direction of the optical fiber. 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.

[0042] The method of manufacturing an optical fiber according to the above embodiment can further include measuring at least one of the diameter of the glass fiber and the tension. In this case, based on the measurement result, the tension applied to the glass fiber can be adjusted.

[0043] The method of manufacturing an optical fiber according to the above embodiment can further include holding the optical fiber preform and inserting the optical fiber preform into a heating furnace at a certain speed. The heating can heat the front end portion by the heating furnace. In this case, the glass fiber can be stably drawn from the optical fiber preform.

[0044] [Detailed Description of Embodiments of the Present Disclosure]

[0045] Hereinafter, specific examples of the optical fiber and the manufacturing method of the optical fiber according to the present disclosure will be described with reference to the accompanying drawings. Note that the present disclosure is not limited to these examples, but is represented by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims. In the description of the drawings, the same symbols are attached to the same elements, and overlapping description is omitted.

[0046] In the present specification, when the refractive index of a certain medium is set as n, and the refractive index of pure quartz glass is set as n0, the relative refractive index difference Δ of the medium is set as the following formula 2.

[0047] [mathematical formula 2]

[0048] Δ = (n / n0) - 1

[0049] Without being particularly described, it is assumed that the optical fiber is a structure having one central axis, and the circumference of the central axis is approximately rotationally symmetric, and is translationally symmetric along the central axis. Without being particularly described, it is assumed that the constituent elements of the optical fiber such as the core, the cladding, and the coating are structures in which the circumference of the central axis is approximately rotationally symmetric, and is translationally symmetric along the central axis. In a case where this assumption can be applied, the physical property value of the constituent element of the optical fiber can be defined as a value in an arbitrary cross section perpendicular to the central axis. In defining a statistical value such as an average value, a maximum value, and a percentile value of the physical property value, a statistical value of a set of measured values obtained by measuring with a predetermined spatial resolution and at a spatially uniform frequency is used instead of the physical property value in the above cross section. Without being particularly described, the above spatial resolution assumes a circle having a radius of 1 μm as an approximate value of the operating wavelength of the optical fiber.

[0050] When the radial coordinate of the optical fiber is set as r, and the relative refractive index difference in the region of the inner radius r0 and the outer radius r1 is represented by the following formula 3,

[0051] [mathematical formula 3]

[0052] Δ = Δ0 + (Δ1 - Δ0) * ((r - r0)(r1 - r0))^α01

[0053] The relative refractive index of the region has a shape of the α01th power. Here, Δ0 is the relative refractive index difference at the radius r = r0, that is, one end of the region, and Δ1 is the relative refractive index difference at the radius r = r1, that is, the other end of the region.

[0054] (Optical fiber)

[0055] As Figure 1AAs shown, the optical fiber 1 has a center axis 10, a core 11, a cladding 12, a first coating layer 13, and a second coating layer 14. The core 11 is composed of glass and extends along the center axis 10. The cladding 12 is composed of glass and extends along the center axis 10 while surrounding the core 11. The first coating layer 13 is composed of resin and extends along the center axis 10 while surrounding the cladding 12. The first coating layer 13 is composed of, for example, an ultraviolet-curable resin of acrylate type. The second coating layer 14 is composed of resin and extends along the center axis 10 while surrounding the first coating layer. The second coating layer 14 is composed of, for example, an ultraviolet-curable resin of acrylate type having a higher modulus of elasticity than the first coating layer 13.

[0056] The relative 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. In the core 11, as an additive, none of GeO2 is contained, and one or more additives among Cl, F, P, Br, Na, K, and Rb are contained. In the cladding 12, as an additive, one or more among F and Cl is contained. Thus, the optical fiber 1 can achieve low transmission loss, and thus 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 further preferably 0.15 dB / km or less. On the other hand, by making the transmission loss 0.10 dB / km or more, productivity can be improved.

[0057] 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, and more preferably 124 μm or more and 126 μm or less. The average diameter of the optical fiber that has been widely used is 125 μm. Thus, by making the average outer diameter of the cladding 12 of the optical fiber 1 125 μm, the cost required for connection with the widely used optical fiber can be reduced. The outer diameter of the second coating layer 14 is 170 μm or more and 270 μm or less. Thus, the optical fiber 1 can have both sufficient mechanical strength and high-density cable accommodation.

[0058] The optical fiber 1 is manufactured by heating the front end portion 201b (see Figure 4 ) of the optical fiber preform 201 and drawing. The optical fiber preform 201 is composed of quartz glass and has a similar shape to 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 the tension and thermal shrinkage that occurs with cooling of the optical fiber 1 during the drawing, a stress remains in the glass (i.e., the core 11 and the cladding 12) of the drawn optical fiber 1.

[0059] As shown in Figure 1B and Figure 1CAs shown, the outer diameter f(z) of the cladding 12 varies as a function of the position z in the axial direction of the optical fiber 1. That is, the outer diameter f(z) of the cladding 12 varies along the central axis 10. Hereinafter, the outer diameter of the cladding 12 is also referred to as the cladding outer diameter.

[0060] The length of the optical fiber 1 is set to L, and the average value of the cladding outer diameter f(z) is set to f0. <f>is defined as the following formula 4.

[0061] [mathematical formula 4]

[0062]

[0063] Outer diameter f(z) of the clad layer with respect to the average value <f>The deviation δf and the standard deviation σf of the frequency f are defined by the following equation 5.

[0064] [Equation 5]

[0065] δf = f - f <f>,

[0066] like Figure 1B and Figure 1D As shown, in optical fiber 1, the residual stress s(z) within the quartz glass also varies as a function of z. That is, the residual stress s(z) varies along the central axis 10. In this disclosure, the residual stress s(z) within the quartz glass is defined as the average value of the component along the central axis 10 of the entire core 11 and cladding 12 in a cross-section perpendicular to the central axis 10. That is, the residual stress s(z) within the quartz glass is defined by the following equation 6.

[0067] [Mathematical Expression 6]

[0068]

[0069] Regarding the sign of stress, tensile stress is designated as positive and compressive stress as negative. As a method for measuring residual stress, for example, the interferometric measurement method described in Non-Patent Document 5 can be used. That is, by irradiating a measurement light with controlled polarization from the side of the optical fiber, interference is caused between the measurement light passing through the fiber and a reference light, thereby measuring the spatial distribution of the phase change during transmission through the fiber, and based on this distribution, the distribution of refractive index and birefringence within the fiber cross-section is obtained. Based on this distribution, the residual stress inside the optical fiber can be measured. Non-Patent Document 5 is incorporated herein by reference.

[0070] The average value of residual stress s(z) in quartz glass <s>defined as the following formula 7.

[0071] [mathematical formula 7]

[0072]

[0073] Residual stress s(z) in quartz glass with respect to average value <s>The deviation δs and the standard deviation σs of the signal S are defined by the following equation 8.

[0074] [Equation 8]

[0075] δs = s- <s>,

[0076] As Figure 1C and Figure 1D shown, the cladding outer diameter f(z) is relative to the average value <f>deviation δf of the residual stress s(z) from the average value <s>The deviations δs are opposite in sign to each other. The cladding outer diameter f(z) and the residual stress s(z) vary in such a manner that they are out of phase with each other along the central axis 10 (the phase difference when each of the cladding outer diameter f(z) and the residual stress s(z) is approximated by a trigonometric function is 180 degrees).

[0077] The fluctuation period of the deviation δf is defined as the inverse of the center of gravity of a power spectrum obtained by performing Fourier transform of the deviation δf with respect to the position z and squaring the amplitude. The fluctuation period of the deviation δs is defined as the inverse of the center of gravity of a power spectrum obtained by performing Fourier transform of 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 can be equal to each other. Each of the fluctuation periods is preferably 0.01 m or more and 100 m or less, and more preferably 0.02 m or more and 50 m or less. In a case where each of the fluctuation periods is longer, the deviation of the transmission performance of each section of the transmission path increases. In a case where each of the fluctuation periods is shorter, an increase in transmission loss occurs due to mode coupling to a high-order mode. Therefore, it is preferable to be within the above range.

[0078] In addition to the above range of the fluctuation period, it is more preferable that Figure 2A and Figure 2B A modification as shown in FIG. 1A, in which the cladding 12 includes at least two layers of an inner cladding 120 surrounding the core 11 and an outer cladding 121 surrounding the inner cladding 120, the inner cladding 120 having a lower refractive index than the outer cladding 121. Thereby, the refractive index difference between the fundamental guided mode and the high-order mode can be enlarged. Therefore, the mode coupling to the high-order mode due to the short fluctuation period component can be suppressed. As a result, for example, even in a case where a microbend is applied to the optical fiber, an increase in transmission loss due to the mode coupling can be suppressed.

[0079] In one cross section perpendicular to the central axis 10, when the Young's modulus after averaging the entire core 11 and the cladding 12 is set as E and the residual strain is set as ε, ε = s / E, and thus the average <ε> of the residual strain and the deviation δε from the average are approximately expressed by the following formula 9 and the following formula 10.

[0080] [Formula 9]

[0081] <ε> = <s> / E,

[0082] [Math. 10]

[0083] δε= δs / E,

[0084] As disclosed in Non-Patent Literature 4, the sound velocity Vd of a longitudinal wave in an optical fiber is expressed by the following formula 11, with the density set to p and the Poisson's ratio set to κ.

[0085] [Math. 11]

[0086]

[0087] Therefore, when the differential d / dε due to strain is represented by the superscript "'", the approximation is known to be the following formula 12.

[0088] [Math. 12]

[0089]

[0090] Therefore, the deviation δVd of the sound velocity due to the deviation δs of the residual stress is expressed by the following formula 13.

[0091] [Math. 13]

[0092]

[0093] As disclosed in Non-Patent Literature 1, the frequency Ωm of the mth peak in the spectrum of scattered light due to GAWBS is given as follows. That is, with the velocity of a longitudinal wave of the mode of a corresponding acoustic wave set to Vd, the velocity of a transverse wave set to Vs, the sound velocity ratio set to a = Vs / Vd, and the mth zero of the following formula 14 set to y = ym,

[0094] [Math. 14]

[0095] (1 - a 2 )J0(y) - a 2 J2(y) = 0

[0096] The frequency Ωm of the mth peak is expressed by the following formula 15.

[0097] [Math. 15]

[0098] Ω m = 2V d y m / f

[0099] Therefore, when the cladding outer diameter f and the residual stress s vary along the center axis 10, the deviation δΩm of the frequency of the mth peak is expressed by the following formula 16.

[0100] [Math. 16]

[0101]

[0102] By longitudinally varying the frequency Ωm of the m-th peak to more than approximately half of its full width at half the half-value ΔΩm, the effective linewidth can be increased when the fiber length, including the longitudinal variation, is considered a uniform fiber. In other words, by varying the peak frequency Ωm along the central axis 10, while setting the amount of variation to more than approximately half of the full width at half the half-value ΔΩm, the linewidth of optical fiber 1 can be effectively increased.

[0103] Non-Patent Document 3 discloses that the smaller the linewidth of the peaks in the spectrum of scattered light caused by GAWBS, the lower the signal-to-noise ratio caused by GAWBS. That is, by effectively increasing the linewidth, the decrease in signal-to-noise ratio caused by GAWBS can be suppressed. As shown in Equation (A), the effective linewidth of GAWBS can be increased more effectively by making the deviation of residual stress relative to the average value and the deviation of cladding outer diameter relative to the average value vary longitudinally in a manner that are opposite in sign to each other.

[0104] On the other hand, when the outer diameter of the cladding of optical fiber 1 varies too much, the connection loss increases in connections using ferrules and V-grooves as self-aligning methods. Furthermore, it becomes difficult to detect abnormalities such as bubbles in the glass based on the measured value of the cladding outer diameter. Therefore, three times (3σ) of the standard deviation of the cladding outer diameter deviation can be 1.0 μm or less, more preferably 0.5 μm or less. When the residual stress is too high, the connection loss increases due to the decrease in fracture strength and the decrease in the flatness of the end face when optical fiber 1 is cut. Therefore, three times (3σ) of the standard deviation of the residual stress deviation can be 150 MPa or less, more preferably 100 MPa.

[0105] Figure 3 It is a graph showing the range in which the effective linewidth is expanded to more than 1 / 2 of the full width at half the value, and the drawbacks caused by excessive deviations in the outer diameter of the cladding and the residual stress are suppressed. Figure 3 The horizontal axis represents the residual stress deviation [MPa]. Figure 3 The vertical axis represents the deviation of the cladding outer diameter [μm]. Figure 3 In this paper, the characteristics of GAWBS based on the optical fiber in the longitudinal direction are shown when the peak frequency is set to 500MHz and the half-width of the linewidth is set to 1MHz.

[0106] Specifically, it is preferable to satisfy the range of the following condition (1), and more preferably to satisfy the range of condition (2).

[0107] Condition (1)

[0108] [Mathematical Expression 17]

[0109] |δf / [μm]-0.0078·(δσ / [MPa])|≥0.125, |δf / [μm]|≤1, |(δσ / [MPa])|≤150

[0110] Condition (2)

[0111] [Mathematical Expression 18]

[0112] |δf / [μm]-0.0078·(δσ / [MPa])|≥0.125, |δf / [μm]|≤0.5, |(δσ / [MPa])|≤100

[0113] To broaden the linewidth of the peaks in the spectrum of scattered light caused by GAWBS, it is more preferable that, in addition to the anti-phase changes in cladding outer diameter and residual stress as described above, the degeneracy of the acoustic mode can be eliminated by imparting non-circularity to the outer diameter of the cladding 12 of the fiber 1 and losing rotational symmetry. Specifically, the cladding non-circularity can be 0.1% or more, more preferably 0.2% or more. On the other hand, since an excessively large cladding non-circularity will increase connection loss, the cladding non-circularity can be 1.5% or less, more preferably 1% or less. Here, the non-circularity of the outer diameter of the cladding 12 means that the outer periphery of the cladding 12 is not a perfect circle. The cladding non-circularity is a value obtained by dividing the difference between the lengths of the major and minor axes when the outer periphery of the cladding 12 is approximately elliptical by the length of the major axis.

[0114] When the core 11 is offset from the centroid of the cladding 12, the overlap between the mode amplitudes of the acoustic wave and the mode amplitudes of the photoelectric field is reduced. This increases the linewidth of the peak in the spectrum of scattered light caused by GAWBS. Specifically, the core offset can be 0.1 μm or more, more preferably 0.2 μm or more. On the other hand, excessive core offset increases connection loss. Therefore, the core offset can be 1.0 μm or less, more preferably 0.8 μm or less.

[0115] In optical fiber 1, when monochromatic light propagates in the core 11, the linewidth of the peak above 500MHz in the spectrum of the scattered light propagating in the core 11 after being scattered forward by the thermally excited acoustic wave in optical fiber 1 can be greater than 1.5MHz.

[0116] (Methods for manufacturing optical fibers)

[0117] The manufacturing method of the optical fiber 1 according to the embodiments will be described below. Figure 4 This is a configuration diagram of the optical fiber manufacturing apparatus according to the implementation method. Figure 5 This is a diagram used to illustrate the operation of the roller. Figure 4 The manufacturing apparatus 2 shown is an apparatus for manufacturing an optical fiber 1 from an optical fiber preform 201 via a glass fiber 204. The manufacturing apparatus 2 is provided with a holding section 202, a heating furnace 203, a holding furnace 205, a measurer 206, a cooler 207, a mold 208, an ultraviolet irradiation machine 209, a roller 211, a winch 212, and a winding machine 213.

[0118] The holding section 202 holds the optical fiber preform 201 and feeds it into the heating furnace 203 at a certain speed. The optical fiber preform 201 has a base end section 201a held by the holding section 202 and a front end section 201b inserted into the inside of the heating furnace 203. The holding section 202 is provided with a function as a supply section that supplies the optical fiber preform 201 into the heating furnace 203.

[0119] The heating furnace 203 has an opening 203a into which the optical fiber preform 201 is inserted and an opening 203b from which the glass fiber 204 is drawn out opposite the opening 203a. The heating furnace 203 heats and softens the front end section 201b of the optical fiber preform 201 supplied into the inside of the heating furnace 203. The glass fiber 204 is drawn out from the front end section 201b softened by the heating. The glass fiber 204 is drawn out to the outside of the heating furnace 203 through the opening 203b.

[0120] The holding furnace 205 holds the glass fiber 204 and relaxes the structure of the glass. The measurer 206 measures at least one of the diameter and the tension of the glass fiber 204 in a state in which the structure of the glass is relaxed. As the measurer 206, for example, a measurer that irradiates laser light to the glass fiber 204 to measure the diameter and a measurer that irradiates ultrasonic waves to the glass fiber 204 to measure the tension can be cited.

[0121] The cooler 207 is arranged at a rear stage of the measurer 206 and cools the glass fiber 204. The mold 208 coats a resin on the outer peripheral surface of the glass fiber 204 after the entry to form a coated resin. The resin contains an acrylate-based ultraviolet-curable resin. The ultraviolet irradiation machine 209 irradiates ultraviolet light to the coated resin formed on the glass fiber 204 to cure the coated resin. Thus, the glass fiber is coated with the resin. As a result, the optical fiber 210 is manufactured.

[0122] In Figure 4 The diagram shows one set of molds 208 and ultraviolet irradiation machines 209, but the manufacturing apparatus 2 may also have two sets of molds 208 and ultraviolet irradiation machines 209 arranged along the axial direction of the glass fiber 204. In this case, the molds 208 and ultraviolet irradiation machines 209 arranged at the front section function as a first coating layer forming section for forming the first coating layer 13. The molds 208 and ultraviolet irradiation machines 209 arranged at the rear section function as a second coating layer forming section for forming the second coating layer 14. Thus, the first coating layer 13 and the second coating layer 14 are formed, thereby obtaining the optical fiber 1.

[0123] Roller 211 changes the direction of travel of optical fiber 1. Roller 211 moves by changing its angle or position. As a result, the length of the path-line of optical fiber 1 and glass fiber 204 from the front end 201b to the winding machine 213 changes periodically.

[0124] like Figure 5 As shown, roller 211 moves back and forth, for example, along the axial direction of roller 211. In this case, the traveling position of optical fiber 1 in the outer peripheral surface of roller 211 moves back and forth between one end and the other end of roller 211 along the axial direction. As a result, the pull-out direction and pull-out angle of glass fiber 204 from the front end 201b of optical fiber preform 201 also change periodically. The pull-out angle is the angle formed by the pull-out direction of glass fiber 204 and the axial direction of optical fiber preform 201.

[0125] The length of the travel path of optical fiber 1 and glass fiber 204 decreases as optical fiber 1 travels along the central axis of roller 211, and increases as it travels along one end and the other. During the lengthening of the travel path, the tension imparted to glass fiber 204 increases. Therefore, while the diameter of glass fiber 204 decreases, the residual stress of glass fiber 204 increases. Consequently, while the outer diameter of the cladding decreases, the residual stress of optical fiber 1 along the central axis 10 increases.

[0126] Conversely, as the travel path shortens, the tension imparted to the glass fiber 204 decreases. As the diameter of the glass fiber 204 increases, the residual stress in the glass fiber 204 decreases. Consequently, as the cladding outer diameter increases, the residual stress in the optical fiber 1 along the central axis 10 decreases. As a result, an optical fiber 1 with longitudinal variations in cladding outer diameter and residual stress is obtained. That is, an optical fiber 1 in which the cladding outer diameter and residual stress vary in opposite phases along the central axis 10 is obtained.

[0127] The roller 211 periodically changes the length of the travel path of the glass fiber 204 and the optical fiber 1 in this way while guiding the optical fiber 1 to the capstan 212. Thereby, the roller 211 imparts a longitudinal variation in the anti-phase between the cladding outer diameter of the optical fiber 1 and the residual stress. The roller 211 may, for example, also move along the central axis 10 of the optical fiber 1, periodically changing the length of the travel path of the optical fiber 1 only in a state of keeping the length of the travel path of the glass fiber 204. In this case, the pulling direction and the pulling angle of the glass fiber 204 are also kept. Even in this case, by changing the length of the travel path of the optical fiber 1, the tension imparted to the glass fiber 204 is changed accordingly. Therefore, it is possible to periodically change the tension imparted to the glass fiber 204.

[0128] The period of the variation in the length of the travel path is converted to the length of the optical fiber 1 of 0.01 m or more and 100 m or less, more preferably 0.02 m or more and 50 m or less. Thereby, it is possible to improve the suppression effect of GAWBS. To this end, for example, the position or the angle of the roller 211 is changed at 0.5 Hz or more, more preferably 1 Hz or more during drawing of the optical fiber 1 at 50 m / s.

[0129] The capstan 212 pulls the optical fiber 1 at a predetermined speed and tension. The winder 213 winds the optical fiber 1 pulled by the capstan 212.

[0130] Figure 6 A flowchart of the manufacturing method of the optical fiber involved in the embodiment is shown. The manufacturing method of 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 front end portion 201b of the optical fiber preform 201, a step S3 of pulling out the glass fiber 204 from the front end portion 201b, a step S4 of keeping the glass fiber 204, 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 become the optical fiber 1, a step S8 of guiding the optical fiber 1, and a step S9 of winding the optical fiber 1.

[0131] In the step S1, the optical fiber preform 201 is inserted into the inside of the heating furnace 203 by the holding portion 202 at a certain speed. The optical fiber preform 201 passes through the opening 203a of the heating furnace 203 with the front end portion 201b held at the base end portion 201a and is sent into the inside of the heating furnace 203. In the step S2, the front end portion 201b is heated by the heating furnace 203 and softened.

[0132] In the process S3, the glass fiber 204 is pulled out from the front end portion 201b softened by heating through the opening 203b. In the process S3, by periodically changing the tension imparted 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 in such a manner that they become out of phase with each other along the axial direction. According to the pulling speed of the glass fiber 204 in the process S3, the insertion speed of the optical fiber preform 201 in the process S1 can be set.

[0133] In the process S4, the glass fiber 204 after being pulled out is kept in a heat-insulating furnace 205. Thereby, the structure of the glass is relaxed. In the process S5, at least one of the diameter and the tension of the glass fiber 204 is measured by a measurer 206. In the process S6, the glass fiber 204 is cooled.

[0134] In the process S7, first, a resin is applied to the outer peripheral surface of the glass fiber 204 by a mold 208 to form a coating resin. Next, the coating resin is cured by ultraviolet rays irradiated from an ultraviolet ray irradiator 209. By repeating the process S7, the first coating layer 13 and the second coating layer 14 are formed, and as a result, the optical fiber 1 is obtained.

[0135] In the process S8, the optical fiber 1 continuous to the glass fiber 204 is drawn at a predetermined speed and tension by a capstan 212, and after traveling on the outer peripheral surface of a roller 211, is guided to a winder 213. The traveling direction of the optical fiber 1 is changed by the roller 211. In the process S8, by at least periodically changing the length of the traveling path of the optical fiber 1, the tension imparted to the glass fiber 204 in the process S3 is periodically changed. In the process S8, by periodically moving the roller 211, the length of the traveling path of the optical fiber 1 is periodically changed. In the process S8, the sum of the lengths of the traveling paths of the glass fiber 204 and the optical fiber 1 is periodically changed.

[0136] The changes in the cladding outer diameter and the residual stress occur by the movement (motion) of the roller 211 and the change in the drawing speed of the capstan 212. Therefore, based on the cladding outer diameter or the tension measured by the measurer 206, the movement of the roller 211 and the rotation of the capstan 212 can be controlled so that the variation falls within the target range.

[0137] The period of the variation in the lengths of the traveling paths of the glass fiber 204 and the optical fiber 1 is converted to the length of the optical fiber 1, which is 0.01 m or more and 100 m or less, more preferably 0.02 m or more and 50 m or less. Thereby, the suppression effect of the GAWBS can be improved. In order to vary the lengths of the traveling paths of the glass fiber 204 and the optical fiber 1 at such a period, for example, during drawing of the optical fiber 1 at 50 m / s, the position or the angle of the roller 211 can 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.

[0138] In the process S9, the optical fiber 1 is wound by the winder 213.

[0139] As described above, in the optical fiber 1, the cladding outer diameter and the residual stress vary along the central axis 10 of the optical fiber 1, and the cladding outer diameter f(z) is offset from the average value <f>deviation δf of the residual stress s(z) from the average value <s>The deviations δs of the residual stresses s(z) are opposite in sign to each other. Therefore, the line width of the peak of the spectrum of the scattered light caused by the GAWBS can be effectively enlarged. Thus, the GAWBS can be suppressed. As a result, the transmission performance in long distance transmission can be improved.

[0140] The clad outer diameter f(z) and the residual stress s(z) vary in a manner that they are out of phase with each other along the central axis 10. Therefore, the clad outer diameter f(z) is <f>deviation δf of the residual stress s(z) from the average value <s>The deviations δs can be opposite in sign to each other.

[0141] When single-color light propagates in the core 11, the line width of the peak of the spectrum of the scattered light propagating in the core 11 by the forward scattering of the acoustic wave excited by heat in the optical fiber 1 is greater than 1.5 MHz. Thus, the GAWBS can be reliably suppressed.

[0142] In the optical fiber 1, the above condition (1) is satisfied. Thus, the adverse effects due to the too large deviations of the outer diameter of the cladding and the residual stress can be suppressed.

[0143] In the manufacturing method of the optical fiber 1, in the step S3, the diameter of the glass fiber 204 and the residual stress in the axial direction of the glass fiber 204 are changed in such a manner that they become out of phase with each other along the axial direction by periodically changing the tension applied to the glass fiber 204. Thus, the optical fiber 1 in which the outer diameter of the cladding and the residual stress are changed in such a manner that they become out of phase with each other along the axial direction is obtained. Thus, the line width of the peak of the spectrum of the scattered light caused by the GAWBS can be effectively enlarged. Thus, the GAWBS can be suppressed. As a result, the transmission performance in long distance transmission can be improved.

[0144] In the step S8, the tension applied to the glass fiber in the step S3 can be changed by periodically changing the length of the travel path of the optical fiber 1. Thus, the tension applied to the glass fiber can be indirectly changed by changing the length of the travel path of the optical fiber 1.

[0145] In the step S8, the length of the travel path of the optical fiber 1 is periodically changed by periodically moving the roller 211 that changes the travel direction of the optical fiber 1. In the case where the roller that changes the travel direction of the glass fiber 204 is provided and the tension applied to the glass fiber 204 is changed by the movement of the roller, it is possible that the glass fiber 204 is damaged by the 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.

[0146] Since the manufacturing method of the optical fiber 1 includes the step S5, the tension applied to the glass fiber 204 can be adjusted based on the measurement result of at least one of the diameter of the glass fiber 204 and the tension.

[0147] Since the manufacturing method of the optical fiber 1 includes the step S1, the glass fiber 204 can be stably drawn from the optical fiber preform 201.

[0148] Explanation of symbols

[0149] 1, 1A … optical fiber

[0150] 2 … manufacturing apparatus

[0151] 10 … central axis

[0152] 11 core

[0153] 12 cladding

[0154] 13 first coating layer

[0155] 14 second coating layer

[0156] 201 optical fiber preform

[0157] 201a base end portion

[0158] 201b front end portion

[0159] 202 holding portion

[0160] 203 heating furnace

[0161] 203a opening

[0162] 203b opening

[0163] 204 glass fiber

[0164] 205 heat retaining furnace

[0165] 206 measuring device

[0166] 207 cooler

[0167] 208 mold

[0168] 209 ultraviolet irradiation machine

[0169] 211 roller

[0170] 212 winch

[0171] 213 winding machine< / s> < / f> < / s> < / f> < / s> < / s> < / f> < / s> < / s> < / s> < / f> < / f> < / f>

Claims

1. An optical fiber having a central axis, comprising: A core made of quartz glass and extending along the central axis; A cladding made of quartz glass, extending along the central axis while surrounding the core; and A coating layer made of resin that extends along the central axis while surrounding the cladding layer. The outer diameter of the cladding varies along the central axis. In a cross-section perpendicular to the central axis, the average residual stress along the central axis varies along the central axis throughout the core and the cladding. The deviation of the outer diameter relative to the average value and the deviation of the residual stress relative to the average value are opposite in sign to each other. When the deviation of the outer diameter is set as δf and the deviation of the residual stress is set as δσ, [Mathematical Formula 1] |δf / [μm]-0.0078·(δσ / [MPa])|≥0.125, |δf / [μm]|≤1, |(δσ / [MPa])|≤150 In essence, it holds true throughout the entire length.

2. The optical fiber according to claim 1, wherein, The outer diameter and the residual stress vary in an antiphase manner along the central axis.

3. The optical fiber according to claim 1 or claim 2, wherein, When monochromatic light propagates in the core, the linewidth of the peak of the spectrum of the scattered light propagating in the core is greater than 1.5 MHz, which is scattered forward by the thermally excited sound wave in the optical fiber.

4. The optical fiber according to claim 1 or claim 2, wherein, The variation period of the deviation of the outer diameter and the variation period of the deviation of the residual stress are both between 0.01m and 100m.

5. The optical fiber according to claim 1 or claim 2, wherein, The standard deviation of the outer diameter is less than 1.0 μm if it is three times the standard deviation.

6. The optical fiber according to claim 1 or claim 2, wherein, The standard deviation of the residual stress is less than 150 MPa, which is three times the standard deviation of the residual stress.

7. The optical fiber according to claim 1 or claim 2, wherein, 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 that of the outer cladding.

8. The optical fiber according to claim 1 or claim 2, wherein, The non-circularity of the cladding is between 0.1% and 1.5%.

9. The optical fiber according to claim 1 or claim 2, wherein, The transmission loss is below 0.17 dB / km.

10. A method for manufacturing an optical fiber, comprising the method for manufacturing an optical fiber according to any one of claims 1 to 9, comprising: The front end of the optical fiber preform made of glass is heated; The glass fiber is pulled out from the front end, which has softened due to heating; as well as A resin-based coating is formed on the glass fiber to form an optical fiber. The pulling out includes: periodically varying the tension applied to the glass fiber, such that the diameter of the glass fiber and the residual stress in the axial direction of the glass fiber vary in a manner that is out of phase with each other along the axial direction.

11. The method for manufacturing optical fiber according to claim 10, This further includes guiding the optical fiber, which is continuous with the glass fiber, to a winding machine. The guidance includes: By periodically varying the length of the optical fiber's travel path, the tension applied during the pull-out is thus periodically varied.

12. The method for manufacturing optical fiber according to claim 11, wherein, The guidance includes periodically changing the length of the travel path by periodically moving a roller that converts the travel direction of the optical fiber.

13. The method for manufacturing an optical fiber according to any one of claims 10 to 12, It further includes measuring at least one of the diameter and tension of the glass fiber.

14. The method for manufacturing an optical fiber according to any one of claims 10 to 12, The process further includes holding the optical fiber preform and inserting it into the heating furnace at a certain speed. The heating is performed by heating the front end portion through the heating furnace.

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