Optical fiber production method and optical fiber production device

Abstract

The optical fiber production method includes: a step for forming a glass fiber containing a plurality of cores and cladding surrounding the plurality of cores and containing silica glass as a main component by melting a glass base material using a drawing furnace and spinning the glass base material to a desired diameter; a step for warming the glass fiber using a warming furnace after the forming step; and a step for applying a resin coating material to the periphery of the glass fiber to surround the cladding after the warming step.

Description

Optical fiber manufacturing method and optical fiber manufacturing apparatus

[0001] This disclosure relates to an optical fiber manufacturing method and an optical fiber manufacturing apparatus. This application claims priority to Japanese Patent Application No. 2024-088513, filed May 31, 2024, and incorporates by reference all of the contents of said Japanese application.

[0002] A multi-core optical fiber (MCF) has multiple glass cores, a glass cladding surrounding the multiple cores, and a resin layer covering the cladding. Among MCFs, uncoupled multi-core fiber (UMCF), which reduces mode coupling of guided modes between the multiple cores, can transmit optical signals using the same transmission equipment as that used to transmit optical signals to conventional single-mode optical fiber (SMF), thereby achieving higher spatial density at lower cost than SMF. UMCF is also called weakly coupled multi-core fiber (WC-MCF). In MCF, a glass composition with a larger refractive index difference between the cores and the cladding than SMF is used to reduce crosstalk caused by mode coupling between the cores. The cores are positioned away from the central axis of the cladding.

[0003] The MCF's structure described above generates large residual stress in the core due to the difference in glass composition between the core and the cladding. As a result, the MCF may experience large polarization mode dispersion (PMD). PMD is the group delay difference between the two inherent polarization modes propagating through each core of the MCF, and is caused by the anisotropy of the residual stress in the core and the non-rotational symmetry of the core's shape. In direct detection optical communications, large PMD leads to a decrease in transmission capacity due to waveform distortion. In coherent detection optical communications, large PMD increases the computational cost of digital signal processing.

[0004] In order to reduce PMD, the method described in Patent Document 3 involves inserting multiple core canes without a central hole into holes in a soot blank that will become the cladding when manufacturing an MCF preform, and then sintering the core canes and the soot blank together, thereby reducing the non-axisymmetric nature of the core cane near the center.

[0005] To reduce PMD, Patent Document 4 imparts three-fold or greater rotational symmetry to the multiple multi-core units that make up an MCF. The multi-core unit includes multiple cores arranged closely together so that coupling occurs actively. One multi-core unit forms a coupled multi-core.

[0006] International Publication No. 2011 / 102191 International Publication No. 2022 / 210786 U.S. Patent Application Publication No. 2015 / 0307387 U.S. Patent Application Publication No. 2011 / 0206330 JP 2000-335934 A

[0007] An optical fiber manufacturing method according to one embodiment of the present disclosure includes the steps of: forming a glass fiber, which includes a plurality of cores and a cladding surrounding the plurality of cores, and is composed primarily of silica glass, by melting a glass preform using a drawing furnace and spinning it to a desired diameter; after the forming step, keeping the glass fiber warm using a heat-retaining furnace; and after the heat-retaining step, applying a resin coating material surrounding the cladding to the periphery of the glass fiber.

[0008] FIG. 1 is a diagram schematically illustrating a multi-core optical fiber (MCF) as an example of an optical fiber manufactured by an optical fiber manufacturing method according to an embodiment of the present disclosure. FIG. 2 is a diagram illustrating an example of a refractive index profile of an MCF. FIG. 3 is a diagram illustrating an example of a refractive index profile of an MCF. FIG. 4 is a diagram schematically illustrating a configuration of a manufacturing apparatus according to an embodiment. FIG. 5 is a flowchart illustrating a manufacturing method according to an embodiment. FIG. 6 is a diagram illustrating the relationship between the time integral of the temperature of a glass fiber and PMD when the temperature of the glass fiber is 800°C or higher and lower than 1600°C. FIG. 7 is a graph illustrating the relationship between the time integral of the temperature of a glass fiber and PMD when the temperature of the glass fiber is 800°C or higher and lower than 1600°C. FIG. 8 is a diagram illustrating the time integral of the temperature of a glass fiber at an upstream portion of a heat-retaining furnace, the time integral of the temperature of a glass fiber at a downstream portion of the heat-retaining furnace, PMD, and transmission loss. FIG. 9 is a graph illustrating the relationship between the time integral of the temperature of a glass fiber at an upstream portion of the heat-retaining furnace and both PMD and transmission loss. FIG. 10 is a graph showing the relationship between the time integral value of the temperature of the glass fiber at the downstream part of the heat-retaining furnace and both the PMD and the transmission loss.

[0009] Even MCFs with reduced non-axial symmetry near the core center can still have large PMD because the stresses generated by the arrangement of each core do not have rotational symmetry. The present disclosure aims to provide an optical fiber manufacturing method and an optical fiber manufacturing apparatus that can reduce PMD.

[0010] According to the present disclosure, it is possible to provide an optical fiber manufacturing method and an optical fiber manufacturing apparatus that can reduce PMD.

[0011] First, the contents of an embodiment of the present disclosure will be described. [1] An optical fiber manufacturing method according to one embodiment of the present disclosure includes the steps of: forming a glass fiber containing a plurality of cores and a cladding surrounding the plurality of cores, the glass fiber being composed primarily of silica glass, by melting a glass preform in a drawing furnace and spinning it to a desired diameter; after the forming step, keeping the glass fiber warm in a heat-retaining furnace; and, after the heat-retaining step, applying a resin coating material surrounding the cladding to the periphery of the glass fiber. According to this manufacturing method, keeping the glass fiber warm in the heat-retaining furnace can reduce PMD.

[0012] [2] In the optical fiber manufacturing method according to [1] above, the temperature may be adjusted in the step of keeping the glass fiber in a heat-retention furnace so that the integrated value of the temperature (°C) of the glass fiber and the heat-retention time (seconds) when the temperature of the glass fiber during its stay in the heat-retention furnace is 800°C or higher but lower than 1600°C is 800°C·s or higher. In this case, the PMD can be further reduced. In this specification, the time during which a certain point in the spun glass fiber stays in the heat-retention furnace is defined as the time during which the glass fiber is kept in the heat-retention furnace.

[0013] [3] In the optical fiber manufacturing method described in [2] above, when the temperature of the glass fiber in the heat-retaining furnace is 800° C. or higher but lower than 1600° C., the time integral of the temperature (° C.) of the glass fiber and the heat-retaining time (seconds) may be set to 1000° C. s or higher. In this case, the PMD can be further reduced.

[0014] [4] In the optical fiber manufacturing method according to any one of [1] to [3] above, the temperature setting in the upstream part of the heat-retaining furnace may be higher than that in the downstream part of the heat-retaining furnace or may be the same as that in the downstream part of the heat-retaining furnace, so that the temperature of the glass fiber in the heat-retaining furnace is higher in the upstream part and gradually decreases downstream. In this case, the PMD can be further reduced.

[0015] [5] In the optical fiber manufacturing method of [1] above, the temperature setting in the upstream part of the heat-retaining furnace may be maintained higher than the temperature setting in the downstream part of the heat-retaining furnace, and the time integral of the temperature of the glass fiber and the heat-retaining time (seconds) when the temperature is 1200° C. or higher and lower than 1600° C. may be 200° C.-s or higher, and the time integral of the temperature of the glass fiber and the heat-retaining time (seconds) when the temperature is 800° C. or higher and lower than 1200° C. may be 700° C.-s or higher. In this case, low PMD and low transmission loss can be achieved at the same time.

[0016] [6] In the optical fiber manufacturing method according to [5] above, the plurality of cores may further contain an alkali metal.

[0017] [7] An optical fiber manufacturing apparatus according to an embodiment of the present disclosure includes a drawing furnace that forms a glass fiber composed primarily of silica glass, including multiple cores and a cladding surrounding the multiple cores, by melting a glass preform and spinning it to a desired diameter; a heat-retention furnace located downstream of the drawing furnace that keeps the glass fiber warm; and an application unit located downstream of the heat-retention furnace that applies a resin coating material surrounding the cladding to the periphery of the glass fiber. With this manufacturing apparatus, PMD can be reduced by using the heat-retention furnace to keep the glass fiber warm. [Details of the embodiment of the present disclosure]

[0018] Specific examples of the present disclosure will be described below with reference to the drawings. 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 of the claims. In the following description, the same elements in the description of the drawings will be given the same reference numerals, and duplicate explanations will be omitted.

[0019] [Explanation of Optical Fiber] Fig. 1 is a diagram schematically illustrating a multi-core optical fiber (MCF) 1 as an example of an optical fiber manufactured by an optical fiber manufacturing method according to an embodiment of the present disclosure. The MCF 1 has a structure extending along a central axis 10. The MCF 1 includes a glass fiber 11 and a coating layer 13. The glass fiber 11 is mainly composed of silica glass, and contains, for example, 95% or more silica glass. The glass fiber 11 includes two cores 111 and 112 and a cladding 12 surrounding the two cores 111 and 112. The coating layer 13 surrounds the outer periphery of the cladding 12. The coating layer 13 mainly contains a resin such as an acrylate.

[0020] 2 and 3 are diagrams showing examples of the refractive index profile of the MCF 1. In FIGS. 2 and 3, the horizontal axis indicates the position along the diameter of the MCF 1, and the vertical axis indicates the refractive index. As shown in these figures, the refractive index of the cores 111 and 112 is greater than that of the cladding 12. The refractive index of the core 112 may be the same as or different from that of the core 111. As shown in FIGS. 2(b), 3(a), and 3(b), the cladding 12 may have a multilayer structure of two or more layers with different refractive indices. In this case, the cladding 12 includes an inner first cladding 121 and an outer second cladding 122. As shown in FIGS. 3(a) and 3(b), the MCF 1 may have a pit 123 between the cores 111 and 112, the pit 123 having a refractive index lower than that of the cores 111 and 112 and that of the cladding 12. 3( b), the MCF 1 may have a structure (core shift structure) in which the center position between the cores 111 and 112 is shifted from the center position of the clad 12 in order to distinguish the cores 111 and 112 from each other. The MCF 1 may also have a structure (marker structure) in which a marker is provided in the clad 12 outside the cores 111 and 112. The marker is a glass portion provided separately from the cores 111 and 112 and having a refractive index different from that of the clad 12.

[0021] The cores 111 and 112 have a higher refractive index than the cladding 12, thereby guiding light. The relative refractive index difference of the cores 111 and 112 with respect to the cladding 12 is, for example, 0.3% to 0.6%. The diameter of the cores 111 and 112 is, for example, 7 μm to 14 μm. In this case, a single linear polarization mode (LP mode) is guided in the cores 111 and 112. The LP mode includes two polarization modes. The polarization mode is expressed as a linear combination of two basis sets, and there is an eigenmode with the maximum group delay and an eigenmode with the minimum group delay. Polarization mode dispersion (PMD) is the difference between the group delays of these eigenmodes.

[0022] The cores 111 and 112 may further contain an alkali metal. In this case, the alkali metal is, for example, one or more elements of Na, K, and Rb. The concentration of the alkali metal contained in the cores 111 and 112 is 1 wtppm or more and 3000 wtppm or less, or 10 wtppm or more and 300 wtppm or less. This sufficiently reduces the viscosity of the cores 111 and 112, thereby reducing transmission loss.

[0023] The cores 111 and 112 may contain chlorine (Cl), fluorine (F), or both Cl and F in addition to alkali metals. The F concentration is, for example, 1000 wtppm or more and 5000 wtppm or less. The Cl concentration is, for example, 100 wtppm or more and 3000 wtppm or less. When the cores 111 and 112 contain Cl, F, or both Cl and F within such concentration ranges, viscosity is reduced, thereby reducing transmission loss due to density fluctuations. The F concentration of the first cladding 121 included in the cladding 12 is, for example, 8000 wtppm or more and 16000 wtppm or less. The F concentration of the second cladding 122 located outside the first cladding 121 is, for example, 0.5 to 0.9 times the F concentration of the first cladding 121. The ratio (L1 / L2) of the diameter L1 of the first cladding 121 to the diameter L2 of the second cladding 122 is, for example, not less than 0.4 and not more than 0.7.

[0024] The diameter L2 of the second cladding 122 is, for example, 124 μm or more and 126 μm or less. This allows the use of a standard fusion splicer and a standard optical connection component (e.g., an optical connector). The distance between the cores 111 and 112 is preferably large to reduce crosstalk. The distance between the cores 111 and 112 is preferably small to facilitate connection. The distance between the cores 111 and 112 is, for example, 15 μm or more and 60 μm or less, and may be 25 μm or more and 50 μm or less.

[0025] The compositions of the cores 111 and 112 and the cladding 12 are calculated based on the atomic ratio measured using a known method such as an EPMA (Electron Probe Micro Analyzer) and the known weight per atom. The refractive indices of the cores 111 and 112 and the cladding 12 are measured using a known method such as the RNF (Refracted Near Field) method. In the MCF 1, the boundaries between regions with different refractive indices (for example, the boundaries between both the cores 111 and 112 and the cladding 12) are defined by curves that are the set of points where the gradient of the refractive index is greatest. The refractive index of a certain region is defined as the average value of the refractive index within that region.

[0026] PMD is measured using a known method such as Jones Matrix Eigenanalysis (JME). When measuring PMD, measurement light is input to a first end of the core 111 or 112 to be measured in the MCF 1, and the intensity of the measurement light output from a second end of the core 111 or 112 is measured. A known input / output device, such as a fan-out device, may be used to selectively input and output measurement light to and from the core 111 or 112 to be measured. The PMD of such an input / output device is negligibly small or is appropriately compensated for in the measurement results.

[0027] Typically, the PMD of a long MCF 1 is measured while it is wound on a reel. When the MCF 1 is wound on a reel, it is subjected to lateral pressure. This causes mode coupling between the two polarization modes. As a result, the group delay difference between the polarization modes may be averaged, resulting in a lower PMD. In this case, the measured PMD value is lower than the PMD of the original MCF 1. This poses a practical problem. To avoid this problem, it is desirable to measure PMD under conditions similar to the actual operating conditions in which the MCF 1 is housed in a cable. Therefore, the actual PMD values ​​described below are measured not in a reel-wound state, but in a bundle of MCFs wound into a coil with a predetermined diameter without applying tension to the MCF 1. The actual PMD values ​​described below are average values ​​within the wavelength range of 1510 nm to 1640 nm.

[0028] [Explanation of Optical Fiber Manufacturing Apparatus and Manufacturing Method] FIG. 4 is a diagram schematically illustrating the configuration of a manufacturing apparatus 20 according to one embodiment. This manufacturing apparatus 20 manufactures an MCF 1. As shown in FIG. 4 , the manufacturing apparatus 20 of this embodiment includes a drawing furnace 4, a heat-retaining furnace 5, and a coating unit 6. The drawing furnace 4 melts a glass preform 2 and spins it to a desired diameter to form a glass fiber 11. To this end, the drawing furnace 4 includes a container 41 in which the glass preform 2 is placed and a heat source 42 surrounding the container 41. The glass preform 2 includes a portion made of the same material as the cores 111 and 112 and a portion made of the same material as the cladding 12. The heat-retaining furnace 5 is located downstream of the drawing furnace 4 and keeps the glass fiber 11 warm. The coating unit 6 is located downstream of the heat-retaining furnace 5 and applies a coating material to form the coating layer 13 to the periphery of the glass fiber 11.

[0029] The temperature of the heat-retaining furnace 5 is set so that the temperature-retaining time (seconds) when the temperature of the glass fiber 11 staying in the heat-retaining furnace 5 is at least 800°C and less than 1600°C and the time integral of the temperature (°C) of the glass fiber 11 is at least 800°C·s or at least 1000°C·s. The set temperature at the upstream portion 51 of the heat-retaining furnace 5 may be higher than or equal to the set temperature at the downstream portion 52 of the heat-retaining furnace 5.

[0030] Alternatively, by setting the set temperature of the upstream section 51 of the heat-retaining furnace 5 higher than that of the downstream section 52, the time integral of the heat-retaining time (seconds) and the temperature (°C) of the glass fiber 11 while it is in the heat-retaining furnace 5 when the temperature is 1200°C or higher and lower than 1600°C can be 200°C·s or higher, and the time integral of the heat-retaining time (seconds) and the temperature (°C) of the glass fiber 11 when the temperature is 800°C or higher and lower than 1200°C can be 700°C·s or higher.

[0031] 5 is a flowchart showing a manufacturing method according to one embodiment. This manufacturing method is a method for manufacturing an MCF 1, and is performed using, for example, the manufacturing apparatus 20 shown in FIG. 4. As shown in FIG. 5, the manufacturing method according to this embodiment includes a process ST1, a process ST2, and a process ST3.

[0032] In step ST1, a glass fiber 11 is formed by melting a glass preform 2 and spinning it to a desired diameter. Step ST2 is a step that follows step ST1. In step ST2, the glass fiber 11 is kept warm using a heat-retaining furnace 5. Step ST3 is a step that follows step ST2. In step ST3, a resin coating material for a coating layer 13 that surrounds the cladding 12 is applied to the periphery of the glass fiber 11.

[0033] In step ST2, the temperature of the glass fiber 11 is set to 800° C. or higher and lower than 1600° C., and the time integral value of the temperature (° C.) of the glass fiber 11 during the heat-retention time (seconds) is set to 800° C. s or higher or 1000° C. s or higher. In step ST2, the set temperature in the upstream section 51 of the heat-retention furnace 5 may be higher than or equal to the set temperature in the downstream section 52 of the heat-retention furnace 5.

[0034] Alternatively, in step ST2, the time integral of the heat-retention time (seconds) and the temperature (°C) of the glass fiber 11 when the temperature of the glass fiber 11 is 1200°C or higher and lower than 1600°C may be set to 200°C·s or higher, and the time integral of the heat-retention time (seconds) and the temperature (°C) of the glass fiber 11 when the temperature of the glass fiber 11 is 800°C or higher and lower than 1200°C may be set to 700°C·s or higher.

[0035] [Problem to be Solved and Effects] When the glass fiber 11 melted in the drawing furnace 4 is drawn out of the drawing furnace 4 and cooled, residual strain occurs in the radial direction at the boundary between the cores 111 and 112 and the cladding 12 due to the difference in the linear expansion coefficient between the cores 111 and 112 and the cladding 12, which is caused by the difference in the glass composition between the cores 111 and 112 and the cladding 12 surrounding them. In the case of a single-core fiber, the core is disposed on the central axis of the glass fiber, so the composition of the cladding surrounding the core is approximately uniform in the circumferential direction of the core. Therefore, residual strain does not pose a problem because it occurs uniformly in the circumferential direction around the core, including the interface between the core and the cladding. In contrast, in the MCF 1, the cores 111 and 112 are disposed away from the central axis 10, so the composition of the cladding 12 is not uniform in the circumferential direction of the core 111 or the core 112. That is, the difference in linear expansion between both the cores 111 and 112 and the cladding 12 is not uniform in the circumferential direction of the core 111 and the circumferential direction of the core 112. The residual strain at the interface between both the cores 111 and 112 and the cladding 12 is not uniform in the circumferential direction of the core 111 and the circumferential direction of the core 112. As a result, birefringence occurs, which causes deterioration of PMD.

[0036] Therefore, in this embodiment, after the glass fiber 11 is formed by melting the glass preform 2 in the drawing furnace 4 and spinning it to a desired diameter, the glass fiber 11 is kept warm in the heat-retention furnace 5 until the coating material for the coating layer 13 is applied. This prevents the glass from being rapidly cooled and reduces the residual strain that is non-uniform in the radial direction at the interface between the core and the cladding. Therefore, birefringence and PMD can be reduced.

[0037] Experiments by the inventors have revealed that by maintaining the temperature of the glass fiber 11 at 800° C. or higher, the residual stress generated around the cores 111 and 112, including the interfaces between the cores 111 and 112 and the cladding 12, can be further reduced, thereby effectively reducing PMD. In addition to the maintaining temperature, it has become clear that ensuring a sufficient maintaining time is important for reducing PMD.

[0038] 6 and 7 are diagrams and graphs showing the relationship between the time integral value (unit: °C·s) of the temperature (°C) of the glass fiber 11 during the heat-retention time (seconds) and the PMD (unit: ps / √km) when the temperature of the glass fiber 11 is 800°C or higher and lower than 1600°C. As is clear from these figures, in order to reduce the PMD to 0.4 ps / √km or lower, the time integral value should be, for example, 800°C·s or higher. In order to reduce the PMD to 0.2 ps / √km or lower, the time integral value should be, for example, 1000°C·s or higher. In order to reduce the PMD to 0.1 ps / √km or lower, the time integral value should be, for example, 1200°C·s or higher.

[0039] In other words, it is advisable to set the temperature retention time so that the value A1 calculated by the following formula (1) and the value A2 calculated by the formula (2) are both 1 or more, or both 10 or more. t is the time (seconds) that the glass fiber 11 stays in the temperature retention furnace 5. T(t) is the temperature (°C) of the glass fiber 11 at that time. t1 is the time (seconds) for T(t) to decrease to 1000°C. τ1(T) is the relaxation time of the cores 111 and 112 at that temperature. τ2(T) is the relaxation time of the cladding 12 at that temperature.

[0040] The relaxation times τ1(T) and τ2(T) are calculated by the following formulas (3) and (4), respectively. Here, η1(T) is the viscosity of the cores 111 and 112 at temperature T (°C). η2(T) is the viscosity of the cladding 12 at temperature T (°C). E1(T) is the Young's modulus of the cores 111 and 112 at temperature T (°C). E2(T) is the Young's modulus of the cladding 12 at temperature T (°C).

[0041] Conventionally, in the manufacture of single-core fibers, a heat-retention furnace has been used for the purpose of promoting structural relaxation and reducing transmission loss (see, for example, Patent Document 5). In a drawing furnace, a glass preform is melted at a high temperature of approximately 2000°C, causing atoms to vibrate violently due to thermal energy, resulting in a more disordered atomic arrangement compared to low-temperature glass. When high-temperature glass is rapidly cooled, the atomic arrangement is cooled and fixed before reaching an equilibrium state corresponding to the temperature. Therefore, the atomic arrangement of the solidified glass is in a disordered state. When high-temperature glass is slowly cooled, the atoms are cooled while being randomly arranged corresponding to the temperature. As a result, the atomic disorder in the glass reaches a state corresponding to the lowest temperature at which structural relaxation proceeds.

[0042] For example, when an alkali metal such as K, Na, Ca, or Rb is added to the core, the structural relaxation rate is high at temperatures of 1200°C or higher. Therefore, if the glass fiber is heated to a temperature of 1200°C or higher, the structural relaxation of the glass is inhibited, and the transmission loss does not decrease. Therefore, the temperature is preferably 800°C or higher but lower than 1200°C. In other words, when an alkali metal is added to the core, if the glass fiber is heated in a heating furnace to a high temperature of 1200°C or higher, the PMD decreases and the transmission loss increases.

[0043] If the glass fiber is kept at a temperature of 800°C or higher and 1200°C or lower, both low transmission loss and low PMD can be achieved. At a low temperature, a long temperature-keeping time is required to sufficiently reduce the PMD, and adjustments such as increasing the length of the temperature-keeping furnace or slowing down the drawing speed are necessary. This increases the equipment costs and manufacturing costs or reduces productivity.

[0044] To avoid an increase in transmission loss and simultaneously achieve both low equipment costs, low manufacturing costs, and high productivity, the following method can be considered: In the MCF 1 having the cores 111 and 112 doped with an alkali metal, the glass fiber 11 is kept at a high temperature of 1200° C. or higher in the upstream section 51 of the heat-retaining furnace 5 in order to reduce PMD. In the downstream section 52 of the heat-retaining furnace 5, the glass fiber 11 is kept at a low temperature of 800° C. or higher but lower than 1200° C. in order to reduce transmission loss. That is, the set temperature of the heat-retaining furnace 5 may be gradually lowered from the upstream section to the downstream section.

[0045] Here, the inventors conducted an experiment in which the furnace 5 was divided into an upstream section 51 (high-temperature section) and a downstream section 52 (low-temperature section). While maintaining constant drawing conditions and a total insulation length, the ratio of the length of the upstream section 51 to the length of the downstream section 52 was varied to change the ratio of the length in which the fiber temperature was 1200°C or higher and 1600°C or lower and the length in which the fiber temperature was 800°C or higher and lower than 1200°C. Figure 8 is a table showing the experimental results, including the time-integrated value of the temperature of the glass fiber 11 in the upstream section 51, the time-integrated value of the temperature of the glass fiber 11 in the downstream section 52, PMD, and transmission loss at a wavelength of 1550 nm. Figure 9(a) is a graph showing the relationship between the time-integrated value of the temperature of the glass fiber 11 and PMD in the upstream section 51. Figure 9(b) is a graph showing the relationship between the time-integrated value of the temperature of the glass fiber 11 and transmission loss in the upstream section 51. Figure 10(a) is a graph showing the relationship between the time-integrated value of the temperature of the glass fiber 11 and PMD in the downstream section 52. FIG. 10B is a graph showing the relationship between the time integral value of the temperature of the glass fiber 11 and the transmission loss in the downstream portion 52 .

[0046] As is clear from these figures, in order to reduce PMD, it is preferable that the time integral value of the temperature of the glass fiber 11 at an insulation temperature of 1200°C or higher but lower than 1600°C is large. For example, in order to achieve a PMD of 0.2 ps / √km or less, it is preferable that the time integral value of the temperature of the glass fiber 11 in the upstream portion 51 is 200°C·s or higher. In order to achieve a PMD of 0.1 ps / √km or less, it is preferable that the time integral value of the temperature of the glass fiber 11 in the upstream portion 51 is 400°C·s or higher. In order to reduce transmission loss, it is preferable that the time integral value of the temperature of the glass fiber 11 in the downstream portion 52 (in other words, at an insulation temperature of 800°C or higher but lower than 1200°C) is large. For example, in order to achieve a transmission loss of 0.156 dB / km or less, it is preferable that the time integral value of the temperature of the glass fiber 11 in the downstream portion 52 is 700°C·s or higher. In order to make the transmission loss 0.154 dB / km or less, it is preferable that the time integral value of the temperature of the glass fiber 11 in the downstream portion 52 is 800° C.·s or more.

[0047] That is, in order to make the PMD equal to or less than 0.2 ps / √km and the transmission loss equal to or less than 0.156 dB / km, it is preferable to adjust the temperature of the upstream section 51 so that the time integral value of the temperature of the glass fiber 11 at an insulation temperature of 1200°C or more and less than 1600°C is 200°C·s or more, and to adjust the temperature of the downstream section 52 to a temperature lower than that of the upstream section 51 so that the time integral value of the temperature of the glass fiber 11 at an insulation temperature of 800°C or more and less than 1200°C is 700°C·s or more.

[0048] As described above, the manufacturing method according to this embodiment includes step ST1 of forming the glass fiber 11 by melting the glass preform 2 and spinning it to a desired diameter, step ST2 of keeping the glass fiber 11 warm using a heat-retaining furnace 5 after step ST1, and step ST3 of applying a resin coating material that surrounds the clad 12 to the periphery of the glass fiber 11 after step ST2. The manufacturing apparatus 20 according to this embodiment includes: a drawing furnace 4 that forms the glass fiber 11 by melting the glass preform 2 and spinning it to a desired diameter, a heat-retaining furnace 5 that is provided downstream of the drawing furnace 4 and that keeps the glass fiber 11 warm, and an application unit 6 that is provided downstream of the heat-retaining furnace 5 and that applies the resin coating material that surrounds the clad 12 to the periphery of the glass fiber 11. According to this manufacturing method and manufacturing apparatus 20, PMD can be reduced by keeping the glass fiber 11 warm using the heat-retaining furnace 5.

[0049] As in the present embodiment, in step ST2, the set temperature of the heat-retaining furnace 5 may be adjusted so that the time integral of the temperature (°C) of the glass fiber 11 and the heat-retaining time (seconds) when the temperature of the glass fiber 11 staying in the heat-retaining furnace 5 is equal to or higher than 800°C and lower than 1600°C is 800°C·s or more. In this case, the PMD can be further reduced.

[0050] As in this embodiment, when the temperature of the glass fiber 11 during its stay in the heat-retaining furnace 5 is equal to or higher than 800° C. and lower than 1600° C., the time integral of the temperature (° C.) of the glass fiber 11 and the heat-retaining time (seconds) may be set to 1000° C. s or more. In this case, the PMD can be further reduced.

[0051] As in this embodiment, the set temperature in the upstream section 51 of the heat-retaining furnace 5 may be higher than or equal to the set temperature in the downstream section 52 of the heat-retaining furnace 5, so that the temperature of the glass fiber 11 staying in the heat-retaining furnace 5 is higher in the upstream section 51 and gradually decreases downstream. In this case, the PMD can be further reduced.

[0052] As in this embodiment, the set temperature in the upstream section 51 of the heat-retaining furnace 5 may be maintained higher than the set temperature in the downstream section 52 of the heat-retaining furnace 5, and the time integral of the temperature of the glass fiber 11 and the heat-retaining time (seconds) when the temperature is 1200° C. or higher and lower than 1600° C. may be 200° C.·s or higher, and the time integral of the temperature of the glass fiber 11 and the heat-retaining time (seconds) when the temperature is 800° C. or higher and lower than 1200° C. may be 700° C.·s or higher. In this case, low PMD and low transmission loss can be achieved at the same time.

[0053] The optical fiber manufacturing method and optical fiber manufacturing apparatus according to the present disclosure are not limited to the above-described embodiments, and various other modifications are possible. For example, the temperature range of the glass fiber in the heat-retention furnace is not limited to the values ​​in the above-described embodiments. The time-integrated value of the temperature of the glass fiber during the heat-retention time is also not limited to the values ​​in the above-described embodiments.

[0054] DESCRIPTION OF SYMBOLS 1... Multi-core optical fiber (MCF) 2... Glass base material 4... Drawing furnace 5... Heat-retaining furnace 6... Coating section 10... Central axis 11... Glass fiber 12... Cladding 13... Coating layer 20... Manufacturing apparatus 41... Container 42... Heat source 51... Upstream section 52... Downstream section 111, 112... Core 121... First cladding 122... Second cladding 123... Pit

Claims

1. A method for manufacturing an optical fiber, comprising: a step of forming a glass fiber, the glass fiber including a plurality of cores and a cladding surrounding the plurality of cores, and composed primarily of silica glass, by melting a glass preform in a drawing furnace and spinning it to a desired diameter; a step of keeping the glass fiber warm in a heat-retaining furnace after the forming step; and a step of applying a resin coating material surrounding the cladding to the periphery of the glass fiber after the heat-retaining step.

2. The optical fiber manufacturing method according to claim 1, wherein the temperature is adjusted in the heat-retaining step so that the time integral of the temperature (°C) of the glass fiber and the heat-retaining time (seconds) when the temperature of the glass fiber during its stay in the heat-retaining furnace is 800°C or higher but lower than 1600°C is 800°C·s or higher.

3. The optical fiber manufacturing method according to claim 2, wherein the time integral of the temperature (°C) of the glass fiber and the heat-retention time (seconds) when the temperature of the glass fiber during its stay in the heat-retention furnace is 800°C or higher but lower than 1600°C is 1000°C·s or higher.

4. An optical fiber manufacturing method as claimed in any one of claims 1 to 3, wherein the set temperature in the upstream part of the heat-retaining furnace is higher than the set temperature in the downstream part of the heat-retaining furnace or is the same as the set temperature in the downstream part of the heat-retaining furnace, so that the temperature of the glass fiber while it is in the heat-retaining furnace is higher in the upstream part and gradually decreases downstream.

5. An optical fiber manufacturing method as described in claim 1, wherein the set temperature in the upstream part of the heat-retaining furnace is maintained higher than the set temperature in the downstream part of the heat-retaining furnace, and the time integral value of the temperature of the glass fiber and the heat-retaining time (seconds) when the temperature is 1200°C or higher but lower than 1600°C is 200°C·s or higher, and the time integral value of the temperature of the glass fiber and the heat-retaining time (seconds) when the temperature is 800°C or higher but lower than 1200°C is 700°C·s or higher.

6. The method of claim 5, wherein said plurality of cores further comprises an alkali metal.

7. An optical fiber manufacturing apparatus comprising: a drawing furnace that forms a glass fiber composed mainly of silica glass, including a plurality of cores and a cladding surrounding the plurality of cores, by melting a glass preform and spinning it to a desired diameter; a heat-retaining furnace that is provided downstream of the drawing furnace and that keeps the glass fiber warm; and an application unit that is provided downstream of the heat-retaining furnace and that applies a resin coating material that surrounds the cladding to the periphery of the glass fiber.

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