JP5948094B2 - Optical fiber and optical fiber manufacturing method - Google Patents

Description

The present invention relates to an optical fiber typified by a silica glass-based optical fiber and a method for manufacturing the optical fiber, and in particular, a technique for reducing polarization mode dispersion (hereinafter referred to as “PMD”) of an optical fiber. especially for producing how lateral pressure or bending the optical fiber and the optical fiber is small increase of PMD even subjected to any disturbance such as.

  As is well known, PMD is a phenomenon in which a propagation time difference (delay difference) occurs between two orthogonal polarization mode components in an optical fiber. In addition, when PMD increases, waveform deterioration occurs in signal light transmitted through the fiber in digital transmission, and separation of adjacent pulses becomes difficult, causing problems such as limiting transmission capacity. Therefore, it is desired to suppress PMD as small as possible.

  PMD is caused by the optical anisotropy of the optical fiber. The generation factor is an internal factor in which the optical anisotropy is caused by the structure or material inside the optical fiber and the outside of the optical fiber. And external factors that cause optical anisotropy due to the stress from

  Of the internal factors, the factor having the greatest influence is the cross-sectional shape of the optical fiber. On the other hand, in the production of an optical fiber, any fiber preform manufacturing method and any method of producing a bare optical fiber by spinning (drawing) any fiber preform can be used. It is practically difficult to realize a perfect circular cross-sectional shape including the core portion of the strand and the surrounding cladding portion. Therefore, the actual product has a cross-sectional shape slightly distorted into an elliptical shape or other shapes. When the anisotropy of such a cross-sectional shape increases, the refractive index distribution in the cross-section is not completely concentric, and birefringence occurs and PMD increases.

  On the other hand, among the external factors, as a factor having a great influence, stress applied anisotropically such as bending or lateral pressure applied to the optical fiber from the outside can be cited. Birefringence also occurs due to the isotropic stress and PMD increases.

  By the way, in order to reduce PMD of an optical fiber, it is effective to add a twist to the optical fiber, and methods as disclosed in Patent Documents 1 to 5 have been proposed.

Among these patent documents, in Patent Document 1 and Patent Document 2, when spinning an optical fiber bare wire, a method of adding a twist before the optical fiber preform is solidified and thereby permanently fixing the twist. It is shown. In the above method, twisting as plastic deformation is applied to the bare optical fiber (plastic twist), so that the twist is maintained as it is even when the external force to the optical fiber is released, that is, twisted state as permanent deformation. It is a method to maintain.
Hereinafter, the plastic twist remaining as such permanent deformation may be referred to as “span”.

  On the other hand, Patent Documents 3 to 5 show methods for twisting an optical fiber after the optical fiber is spun and solidified. The twist in this case is generated by elastic deformation. In other words, the twist in this case is an elastic twist in which the twist returns when the external force is released and the optical fiber is in a free state (external force released state). In this case, with the elastic twist held, it is finally used for a product in a final use form such as a cable, that is, as an optical fiber used in the product in a final use form such as a cable. It is assumed that the optical fiber strand in a state where the elastic twist is held is used. Hereinafter, such an elastic twist may be referred to as “twist”.

  As described above, the causes of PMD are broadly classified into internal factors and external factors, and PMD due to internal factors is a span (plastic twist) as shown in Patent Document 1 and Patent Document 2. Is effective to give to the optical fiber. However, it is known that such a method of giving a span to an optical fiber is not effective for suppressing an increase in PMD due to an external factor (see, for example, Patent Document 3).

  On the other hand, as shown in Patent Documents 3 to 5, a method of giving a twist (elastic twist) is effective for suppressing an increase in PMD due to an external factor such as a lateral pressure or bending. However, when the external force is released, the twist returns elastically torsion. Here, for example, a coloring step, a step of arranging a plurality of optical fiber strands in a tape shape, and a step of forming an optical fiber cable in order to use the twisted optical fiber strand as a product of an end use form such as an optical cable. In an actual mass production process such as the above, and between the processes, an external force such as a frictional force applied to the optical fiber may be released or an external force such as a frictional force may be significantly reduced. In that case, the twist is released or the twist becomes extremely small, and the effect of suppressing the increase in PMD due to an external factor is lost. Therefore, in the final product such as a cable, there is a problem that it is difficult to reliably and stably suppress an increase in PMD due to an external factor.

  As described above, conventionally, the increase in PMD due to external factors such as an anisotropic external force such as a lateral pressure or bending applied to an optical fiber is reliably and stably suppressed in a product in a final use form. It was difficult.

JP-A-8-295528 US Pat. No. 6,324,872 International Publication No. 2009/107667 JP 2010-122666 A US Pat. No. 7,317,855

The present invention has been made in view of the circumstances as described above, and an increase in PMD caused by an external factor such as an anisotropic external force such as a lateral pressure or bending can be achieved even in a product of a final use form such as a cable. reliable and stable optical fiber which may be inhibited, and it is an object to provide a manufacturing how.

  The present inventors have conducted various experiments and studies to solve the above-described problems. As a result, when the optical fiber bare wire drawn and solidified from the heated and melted optical fiber preform is coated with a liquid (uncured) curable resin and the coating resin is cured, the bare optical fiber is coated. It has been found that the elastic twist is fixed (held) by the cured coating resin by applying an elastic twist to the bare optical fiber between the solidification of the wire and the curing of the coating resin. Furthermore, even in the case of optical fiber strands that are used inside products such as optical cables, the use of the above-described optical fiber strands makes it possible to retain elastic twist (PMD) due to external factors. It was found that the increase in the amount can be suppressed.

  Here, the cured coating resin also has elasticity, and its Young's modulus is generally smaller than that of glass. Therefore, even if an elastic twist is applied to the bare optical fiber between the time when the bare optical fiber is solidified and the coating resin is cured as described above, the twist is directly fixed by the covering resin, that is, Thus, it is difficult to completely prevent the action (return of twist) from returning to the state before the elastic twist is twisted by the restoring force. Further, if the external force is released after the twist is applied and the free state is entered, it is inevitable that the twist of the bare portion of the optical fiber returns to some extent. However, when the twist of the bare optical fiber portion returns (restores to the state before being twisted), the return direction (the bare optical fiber portion of the optical fiber bare wire portion) returns to the coating resin layer as the twist of the bare optical fiber portion returns. Torsion in the direction of restoring to the state before being twisted. As a result, the elastic repulsion force of the coating resin against the twist in the return direction applied to the coating resin layer and the return force of the twist of the bare optical fiber portion (the restoring force to return to the state before the elastic twist is twisted) ) Is balanced, the twist return of the bare optical fiber portion stops. Therefore, the elastic twist of the bare optical fiber portion when the external force is released after applying the twist is not eliminated 100%, but the twist of the bare optical fiber portion is always maintained at a certain rate by the elastic repulsive force of the coating resin. Is done. The maintained twist is held by the coating resin even in the external force released state, and functions as an elastic twist (twist). As will be described later, it is usually confirmed that at least about 20 to 30% of the applied elastic twist remains and is retained by the coating resin. The elastic twist (twist) held and fixed by the coating resin in this way is further used in a final use form through a process of arranging a plurality of optical fiber strands in a tape shape, a process of forming an optical fiber cable, and the like. Even when the external force is released when the product is used, the elastic torsion (twist) is securely held, and is stably effective in suppressing the increase in PMD due to external factors.

  Next, each aspect in the present invention will be described.

  The optical fiber strand according to the first aspect of the present invention includes an optical fiber bare wire portion to which an elastic twist is applied, and the optical fiber bare wire portion, which is formed of a curable resin, And a coating layer that generates an elastic repulsion force against a restoring force generated in the bare optical fiber portion so as to hold the elastic twist applied to the optical fiber.

  By using the above optical fiber, the elastic twist (twist) of the bare fiber portion is held by the elastic repulsive force of the coating layer against the force in the direction in which the twist returns (restores). Even in the state of an optical cable or the like as a usage form, the elastic twist of the bare fiber portion is reliably and stably maintained. Therefore, an increase in PMD due to an external factor can be reliably and stably suppressed.

  Further, in the optical fiber strand according to the first aspect of the present invention, as the elastic twist given to the bare optical fiber portion, the first twist for each predetermined length in the longitudinal direction of the optical fiber strand, It is preferable that a second twist generated in a direction opposite to a direction in which the first twist is generated is alternately given to the bare optical fiber portion.

  By using the above-described optical fiber, the first twist and the second twist generated in the direction opposite to the direction in which the first twist occurs for each predetermined length in the longitudinal direction of the bare optical fiber as the elastic twist. When twist is alternately applied to the bare optical fiber portion, the increase in PMD due to external factors can be further increased compared to the case where elastic twist is continuously applied only in one direction. It can be reliably and stably suppressed.

  In the optical fiber strand according to the first aspect of the present invention, the coating layer is formed of a primary coating layer formed of a resin having a relatively low Young's modulus and a resin formed of a resin having a relatively high Young's modulus. It is preferable that it is comprised by the next coating layer.

  By using the above-mentioned optical fiber, a resin having a low Young's modulus is used as a primary coating layer that is in contact with the outer peripheral surface of the bare optical fiber. A high elastic repulsive force can be obtained by using a resin having a high Young's modulus as the secondary coating layer. Accordingly, it is advantageous to hold the elastic twist (twist) of the bare part of the optical fiber by the coating layer, and an increase in PMD due to an external factor can be more reliably and stably suppressed.

  Further, in the optical fiber strand according to the first aspect of the present invention, in the bare optical fiber portion in a state where the elastic twist applied to the bare optical fiber portion is held by the elastic repulsive force generated in the coating layer. The reversal period T of the residual elastic twist is preferably 5 to 30 m, and the maximum amplitude MA of the cumulative twist angle in the reverse twist profile is preferably 100 × T (°) to 1200 × T (°).

  By using the above optical fiber, the twist inversion period T is in the above range, and the maximum amplitude of the accumulated twist angle is in the above range, so that the remaining elastic twist is a sufficient amount. The increase in PMD due to external factors can be reliably and stably suppressed.

  Furthermore, the second aspect of the present invention is a method for manufacturing the optical fiber strand according to the first aspect described above.

  That is, in the method for manufacturing an optical fiber in the second aspect, an optical fiber preform is heated and melted, an optical fiber bare wire having a predetermined diameter is drawn from the melted optical fiber preform, and the drawn optical fiber bare wire is taken out. By solidifying and propagating an elastic twist to the bare optical fiber toward the upstream side in the drawing direction of the bare optical fiber, the twisted optical fiber is imparted to the solidified optical fiber by solidification, and the solidified A coating layer before solidification is formed by coating the outer periphery of the bare optical fiber with a curable resin in a liquid state, and the coating layer is formed on the outer periphery of the bare optical fiber to which an elastic twist is applied Forming a twisted optical fiber so that the elastic twist of the bare optical fiber is maintained, and drawing the twisted optical fiber. That.

  By using the above-described optical fiber manufacturing method, after the elastic twist applied to the solidified bare optical fiber releases the optical fiber held by the cured coating layer, that is, external force In addition, an optical fiber strand in which elastic twist remains in the bare portion of the optical fiber can be manufactured.

  In the method of manufacturing an optical fiber according to the second aspect of the present invention, a twist is imparted to the bare optical fiber by using a twist imparting device, and the optical fiber is disposed upstream of the twist imparting device. It is preferable that the optical fiber bare wire is twisted in a state where there is no member that prevents the propagation of the twist of the bare fiber wire.

  When the above-described optical fiber manufacturing method is used, the twist is smoothly transmitted from the twist imparting device to the upstream side of the twist imparting device, so that the elastic twist can be imparted to the bare optical fiber reliably and stably. it can.

  In the method for manufacturing an optical fiber according to the second aspect of the present invention, when twisting is applied to the optical fiber, the direction of twist applied to the bare optical fiber is periodically reversed. preferable.

  By using the above-described optical fiber manufacturing method, it is possible to obtain an optical fiber in which the direction of twist applied is periodically reversed, and more effectively suppress an increase in PMD due to external factors. be able to.

  Furthermore, in the manufacturing method according to the second aspect of the present invention, when the curable resin is coated on the bare optical fiber, the viscosity at the time of coating the curable resin in a liquid state is 0.1 to 3 Pa · sec. It is preferable.

  When the above-described optical fiber manufacturing method is used, the viscosity of the resin in the liquid state at the time of coating is 0.1 Pa · sec or more, thereby suppressing variation in the outer diameter of the optical fiber strand and making it uniform. An optical fiber strand having a coating outer diameter can be obtained. Furthermore, when the viscosity of the resin in the liquid state at the time of coating is 3 Pa · sec or less, the resin in the liquid state is prevented from becoming a resistance to torsion propagation, and in particular, the direction of the twist is periodically reversed. In, the propagation of twist and the direction of twist can be reliably reversed.

  Furthermore, in the method for manufacturing an optical fiber according to the second aspect of the present invention, the inversion period T of the twist applied to the optical fiber in the longitudinal direction of the optical fiber is 5 to 30 m, and the inversion twist profile is used. The maximum amplitude MA of the cumulative torsion angle is preferably 500 × T (°) to 4000 × T (°).

  When the above optical fiber manufacturing method is used, the inversion period T of the twist is in the above range, and the maximum amplitude of the accumulated twist angle is in the above range, so that the external force on the optical fiber is released. It is possible to sufficiently secure the elastic torsion remaining when the heat is applied. Furthermore, it is possible to prevent the coating layer from peeling or cracking due to excessive stress.

  Furthermore, in the optical fiber manufacturing method according to the second aspect of the present invention, the elastic twist imparted to the optical fiber in the longitudinal direction of the optical fiber is held by the elastic repulsive force generated in the coating layer. The twist inversion period T remaining in the bare optical fiber portion of the optical fiber is 5 to 30 m, and the maximum amplitude MA of the cumulative torsion angle in the inversion twist profile is 100 × T (°) to 1200 × T (°). It is preferable that

  When the above-described optical fiber manufacturing method is used, the reversal period T of the twist is in the above range, and the maximum amplitude of the cumulative twist angle is in the above range, so that the remaining elastic twist is a sufficient amount. Therefore, the increase in PMD due to external factors can be reliably and stably suppressed.

Furthermore, the third aspect of the present invention is an apparatus for manufacturing the optical fiber strand according to the first aspect.
That is, an optical fiber manufacturing apparatus according to a third aspect of the present invention includes a spinning heating furnace that heats and melts an optical fiber preform, and an optical fiber bare wire drawn linearly downward from the spinning heating furnace. A cooling device that forcibly cools and solidifies, a coating device that forms a coating layer by coating the cooled and solidified optical fiber bare wire with a curable resin for protective coating, and a coating device that is coated with the coating device. A coating curing apparatus that cures the uncured coating layer, and an optical fiber bare wire after solidification by propagating an elastic twist to the bare optical fiber toward the upstream side in the drawing direction of the bare optical fiber A torsion imparting device for imparting a torsion.

  By using the above-described optical fiber manufacturing apparatus, the elastic twist imparted to the solidified optical fiber bare wire is retained by the hardened coating layer, that is, even after the external force is released. An optical fiber strand in which the elastic twist remains in the bare optical fiber portion can be manufactured.

According to the optical fiber of the present invention, the elastic twisting of the bare optical fiber portion causes the elastic repulsion of the coating layer to resist the force in the direction in which the elastic twist returns (restores) even when the external force is released. Held by force. Therefore, the elastic twist of the bare fiber portion can be reliably and stably held even in the state of the optical cable that is the final use form. As a result, the increase in PMD due to external factors such as bending or lateral pressure can be reliably and stably suppressed.
Further, according to the method and apparatus for manufacturing an optical fiber of the present invention, an optical fiber that can reliably and stably suppress an increase in PMD due to external factors such as bending or lateral pressure as described above can be reliably obtained. And it can be manufactured easily.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows the whole structure of an example of the apparatus for manufacturing the optical fiber strand of this invention. It is the top view which looked at the twist imparting apparatus which shows an example of the twist imparting apparatus used for the apparatus for manufacturing the optical fiber strand of this invention from upper direction. It is a front view of the twist provision apparatus which shows an example of the twist provision apparatus used for the apparatus for manufacturing the optical fiber strand of this invention. It is a front view which shows the other example of the twist provision apparatus used for the apparatus for manufacturing the optical fiber strand of this invention. FIG. 2 is a perspective view schematically showing an example of the state of an optical fiber in the stage immediately after the coating layer is cured in the manufacturing process of the optical fiber of the present invention, and is a diagram for explaining the internal structure of the optical fiber It is. It is typical sectional drawing of the optical fiber strand which shows typically the condition at the time of releasing external force about the optical fiber strand of this invention. 4A and 4B are diagrams for explaining the internal structure of the optical fiber, in which FIG. 4A is a perspective view showing a state immediately after the coating layer of the optical fiber shown in FIG. 4 is cured, and FIG. It is a perspective view which shows the state after the external force provided to the optical fiber strand shown is released. It is a graph which shows an example of the profile of inversion twist in the manufacturing method of the optical fiber strand of this invention. It is a schematic diagram which shows the whole structure of the other example of the apparatus for manufacturing the optical fiber strand of this invention. It is a schematic diagram which shows the optical fiber strand manufacturing apparatus used in the comparative example 2.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an example of an apparatus for manufacturing the optical fiber of the present invention.
In FIG. 1, an optical fiber manufacturing apparatus 10 includes a spinning heating furnace 14 for heating and melting an optical fiber preform 12 formed of, for example, quartz glass, and a linear shape downward from the spinning heating furnace 14. A cooling device 18 for forcibly cooling and solidifying the bare optical fiber 16 drawn out to the optical fiber, and the curable and solidified optical fiber bare wire 16 for a protective coating such as an ultraviolet curable resin or a thermosetting resin. Coating device 20 for coating with resin, coating curing device 22 for curing uncured (liquid state) curable resin coated with coating device 20 by ultraviolet irradiation or heating, and curable resin for protective coating The twist imparting device 26 for imparting twist to the optical fiber 24 in a state where the optical fiber is cured, and the guide fiber 28 or the optical fiber strand 24 to which the twist is imparted are not shown. Comprising a take-up device (not shown) taken off via Nsarora, finally a take-up device (not shown) winding the optical fiber.

  Here, the twist imparting device 26 may be configured to continuously impart a twist in a certain direction to the optical fiber 24, but normally, as will be described later, the twist direction (clockwise) Direction or counterclockwise) is preferably reversed. The specific configuration of the twist imparting device 26 is not particularly limited. For example, the twist imparting device as shown in FIGS. 2A and 2B (the same device as the twist imparting device shown in FIG. 11 of Patent Document 4) or FIG. What is necessary is just to apply the twist provision apparatus as shown (apparatus similar to the twist provision apparatus shown by FIG. 2 of patent document 1, or FIG. 10 of patent document 4).

  The twist imparting device 26 shown in FIGS. 2A and 2B includes two pairs of twist imparting rollers 26Aa, 26Ab, 26Ba, and 26Bb that rotate while sandwiching the optical fiber 24 from both sides. The second set is located on the downstream side of the first set of torsion imparting rollers 26Aa and 26Ab (the side on which the guide pulley 28 in FIG. 1 is provided) close to the first set of torsion imparting rollers 26Aa and 26Ab. Torsion imparting rollers 26Ba and 26Bb are provided. The second set of twist imparting rollers 26Ba and 26Bb is centered on a cross section perpendicular to the longitudinal direction (drawing direction) of the optical fiber 24, and the first set of twist imparting rollers 26Aa and 26Ab. It is arranged with a 90 ° offset. When each of the twist imparting rollers 26Aa, 26Ab, 26Ba, and 26Bb rotates while sandwiching the optical fiber strand 24 from both sides thereof, the rotation axis of each of the twist imparting rollers 26Aa, 26Ab, 26Ba, and 26Bb is used as the optical fiber strand 24. By twisting the optical fiber 24 by a predetermined small angle with respect to a direction orthogonal to the length direction of the optical fiber 24, twist can be imparted. And the direction of the twist given to the optical fiber strand 24 can be changed by changing the inclination direction of each twist provision roller 26Aa, 26Ab, 26Ba, 26Bb with respect to the optical fiber strand 24 to the opposite direction.

  Further, the twist imparting device 26 shown in FIG. 3 includes a twist imparting roller 26 </ b> C that is wound around the outer periphery of the optical fiber 24 and rotates about a rotation axis that is inclined with respect to the drawing direction, and a downstream side thereof. And a fixed position roller 26D that rotates about a rotation axis that is orthogonal to the drawing direction. Further, when the optical fiber 24 rolls on the outer periphery of the twist applying roller 26C along the rotation axis direction, the optical fiber 24 is twisted and the inclination direction of the twist applying roller 26C is reversed. By swinging in this manner, the twisting direction can be reversed.

  The twist imparting device 26 is preferably installed at a position where the cooled and solidified optical fiber 16 is covered with a curable resin for protective coating, and the twist can be imparted after the curable resin is cured. . However, it is desirable not to provide a mechanism or member that is in contact with the optical fiber 24 or the bare optical fiber 16 and prevents the transmission of twist on the upstream side of the twist applying device 26. In the optical fiber manufacturing apparatus shown in FIG. 1, the twist imparting device 26 is disposed between the coating curing device 22 and the guide roller 28 in order to satisfy the above conditions. In this case, there is no member that physically contacts the surface of the optical fiber 24 or the bare optical fiber 16 on the upstream side of the twist imparting device 26 except for the curable coating resin. Accordingly, the twist imparted by the twist imparting device 26 can be continuously and smoothly propagated to the upstream side thereof, and the elastic twist (twist) intended in the present invention can be imparted. However, if the member allows the rolling of the optical fiber, such as a flat groove pulley having a certain groove width, there is little risk of hindering the propagation of torsion even if the member is in contact with it. It is allowed to be provided upstream of the twist imparting device 26.

  Further, the curable resin coated by the coating apparatus 20 may be a single layer, but generally, a two-layer structure of a primary coating layer (primary material) and a secondary coating layer (secondary material) is often adopted, Also in the present invention, it is desirable to form a resin coating layer having a two-layer structure. That is, the primary coating layer is formed of an ultraviolet curable resin such as epoxy acrylate resin or urethane acrylate resin, or a thermosetting resin such as silicon resin, and has a Young's modulus after curing of about 5 MPa or less (generally, Preferably has a Young's modulus at room temperature of 0.3 to 1,5 MPa. On the other hand, the secondary coating layer is formed of an ultraviolet curable resin such as an epoxy acrylate resin or a urethane acrylate resin, or a thermosetting resin such as a modified silicon resin, and has a high Young's modulus after curing of about 100 MPa or more. It is desirable to use a material having a modulus (generally a Young's modulus at room temperature of 300 to 1500 MPa). Thus, by using a material having a low Young's modulus as the primary coating layer, it is possible to exhibit a good cushioning effect on the bare optical fiber and to improve the adhesion of the coating layer to the bare optical fiber. On the other hand, by using a material having a high Young's modulus as the secondary coating layer, it can sufficiently withstand external damage, friction, or lateral pressure. In particular, in the case of the optical fiber of the present invention, it is possible to increase the adhesion to the bare optical fiber portion and at the same time increase the apparent Young's modulus of the entire coating layer. This is advantageous in maintaining the twist. Therefore, also from this viewpoint, it is desirable to form a coating layer having a two-layer structure with different Young's modulus after curing as described above.

As a coating method and a curing method for forming a coating layer having such a two-layer structure, as shown in FIG. 1, the coating device 20 and the coating curing device 22 are provided only in one place, The two-layer coating may be performed by the base coating apparatus 20, and the obtained two-layer coating layer may be collectively cured by the three coating curing apparatuses 22. Alternatively, as shown in FIG. 8, which will be described later, the coating device 20 and the coating curing device 22 are provided in two places, respectively, and the resin of the primary coating layer is coated and cured, and then the resin of the secondary coating layer May be coated and cured.
In addition, the viscosity of the resin in the liquid state when the curable resin is coated on the bare optical fiber is also a factor that affects the application state of the elastic twist (twist) and the like will be described later.

  Next, a description will be given of a method for manufacturing an optical fiber with an elastic twist (twist) according to the present invention using the above-described optical fiber manufacturing apparatus.

  When an optical fiber is manufactured by the optical fiber manufacturing apparatus as described above, an optical fiber preform 12 such as a quartz glass preform, which is a raw material for an optical fiber bare wire, is heated to 2000 ° C. in a spinning heating furnace 14. It melts by being heated to the above high temperature, and is drawn out downward from the lower part of the heating furnace 14 for spinning as an optical fiber bare wire 16 in a high temperature state, and the optical fiber bare wire 16 is cooled by a cooling device 18. To solidify. The optical fiber bare wire 16 cooled to a required temperature by the cooling device 18 and solidified has, for example, two types of curable resins such as an ultraviolet curable resin or a thermosetting resin in the coating device 20 for two-layer coating. It coat | covers as a primary coating layer and a secondary coating layer in a liquid state. Further, the obtained coating resin is cured by an appropriate curing method in accordance with the type of resin such as heat curing or ultraviolet curing in the coating curing device 22 to obtain an optical fiber 24 having two coating layers. It is done. Further, after the predetermined twists TW1 and TW2 are applied to the obtained optical fiber 24 by, for example, a twist applying device 26 as shown in FIG. 2A, FIG. 2B or FIG. It is taken up at a predetermined speed by a take-up device that is not, and is further taken up by a take-up device that is not shown.

  In the apparatus shown in FIG. 1, the twists TW1 and TW2 applied to the optical fiber 24 by the twist imparting device 26 are the front and rear (upstream side, upstream) of the twist imparting device 26 as indicated by arrows Y1 and Y2 in FIG. Here, attention is particularly paid to the twist TW1 that propagates to the optical fiber preform side (upstream side). In this case, the twist TW1 is propagated through the coating curing device 22 through the coating device 20 and further toward the cooling device 18 thereabove. Accordingly, after the bare optical fiber 16 is solidified by the cooling device 18, an uncured (liquid state) curable resin is coated on the outer periphery of the bare wire by the coating device 20, and the coating resin is further coated with the coating curing device. Twist is applied during the period until it is hardened by 22 (near the area of S1 in FIG. 1). Here, the twist applied after the bare optical fiber is solidified is a twist that returns when the external force is released, that is, an elastic twist (twist). The twist applied to the optical fiber 24 by the twist applying device 26 after the coating resin is cured is naturally applied to the coating layer integrated with the bare optical fiber portion. On the other hand, since the coating resin is in a state where it can flow during the period from the time when it is coated in the liquid state in the coating apparatus 20 until the resin is cured (in the vicinity of the region S2 in FIG. 1), it does not exhibit elastic behavior. . Accordingly, in the meantime, in S2, the elastic twist is not substantially applied to the coating layer. When the resin coated on the outer circumference of the bare optical fiber is cured in a liquid state, the elastic twist (twist) of the bare optical fiber applied so far is fixed (retained) by the resin of the coating layer. )

  Here, an example of the optical fiber 24 at the stage where the coating layer is cured by the coating curing device 22 in the manufacturing process of the optical fiber 24 manufactured by the apparatus of FIG. Is shown schematically. In FIG. 4, reference numeral 32 </ b> A is a primary covering layer, 32 </ b> B is a secondary covering layer, and a thick solid line and a broken line drawn on the outer periphery of the bare optical fiber 16 in FIG. It represents. This figure shows a state in which a clockwise twist as viewed from the downstream side in the manufacturing process of the optical fiber is added, and the portion of the bare optical fiber 16 has a clockwise twist as viewed from below. Yes. As described above, the coating layers 32A and 32B do not exhibit an elastic behavior after being coated on the outer periphery of the bare optical fiber 16 in a liquid state until cured. Therefore, at the stage shown in FIG. 4, the coating layers 32A and 32B are not substantially twisted. However, what is shown in FIG. 4 is an optical fiber at a stage where an external force such as friction is not released, as will be described below. In addition, even after the coating layer is cured by the coating curing device 22 and reaches the twist applying device 26, the optical fiber strand is twisted, and also on the downstream side of the twist applying device 26. Twist is applied, but the twist added after these coating layers are cured is an elastic twist applied globally to both the bare optical fiber portion and the coating layer. Therefore, if an external force such as friction with respect to the optical fiber is released, the twist applied after the coating layer is cured is released and is not a reliable factor for obtaining an effect of suppressing an increase in PMD due to an external factor. Therefore, since it does not contribute to the solution of the problem of the present invention, the description thereof is omitted here.

  By the way, the cured coating resin is softer and less rigid than the bare optical fiber portion, so that the optical fiber strand is formed after the bare optical fiber is solidified until the coating resin is cured as described above. Even if elastic torsion is applied, the applied elastic torsion is completely fixed by the coating resin as it is, that is, the return (restoration) due to the elastic force of the torsion when the external force is released is completely prevented by the coating resin. It is difficult. In other words, if an external force such as a frictional force is released after the twisted optical fiber strand, the resin coating layer is formed by the elastic return force of the bare optical fiber portion inside the optical fiber strand. It is unavoidable that the elastic twisting of the bare fiber portion returns to some extent by twisting in the return direction of the portion. However, since the cured coating resin also has elasticity, the twist in the return direction applied to the coating resin layer when the twist of the bare optical fiber portion returns also functions as an elastic twist, and repulsion against the elastic twist of the coating resin layer. In the state where the force and the return force of the twist of the bare optical fiber portion (restoring force to return to the state before the elastic twist is twisted) are balanced, the return of twist of the bare optical fiber portion stops. Therefore, the elastic twist imparted to the bare optical fiber portion when the external force is released does not disappear 100%, but the elasticity imparted to the bare optical fiber portion at a certain rate by the elastic repulsion force of the coating resin. The twist remains. The remaining twisted portion is held and fixed by the coating resin, and functions as an elastic twist (twist) even in the product in the final use form.

  FIG. 5 schematically shows the relationship between the balance of force and twist when an external force such as friction with respect to the optical fiber is released as described above. FIG. 6B schematically shows a twisted state of the optical fiber in a free state (a state where no external force is applied to the optical fiber) after the external force such as friction on the optical fiber is released. Show. For comparison, FIG. 6A shows a twisted state immediately after the coating layer is cured (substantially the same as FIG. 4). In FIGS. 6A and 6B, a thick solid line and a thick broken line indicate a twisted state. However, in FIG. 5 and FIGS. 6A and 6B, the case where the coating layer is one layer (reference numeral 32) is shown for simplification of description.

  In FIG. 5, for example, a counterclockwise elastic twist TP <b> 1 is given to the portion of the bare optical fiber 16 until immediately before the external force on the optical fiber is released. On the other hand, when the external force is released to enter the free state, the elastic return force F1 acts in the clockwise direction, and the counterclockwise elastic twist TP1 decreases. This means that the bare optical fiber 16 is twisted clockwise when the external force is released. Along with this, the coating layer 32 in close contact with the bare optical fiber 16 is also twisted clockwise (twist TP2). At this time, since the covering layer 32 also has elasticity, an elastic repulsive force F2 in the opposite direction (counterclockwise) to the clockwise twist TP2 is generated. The elastic twist TP1 of the bare optical fiber 16 is held in a state where the counterclockwise elastic repulsive force F2 of the coating layer 32 and the clockwise elastic repulsive force F1 of the bare optical fiber 16 are balanced. Is done. Therefore, in an optical fiber in a free state after an external force such as friction with respect to the optical fiber is released, as shown in FIG. 6B, the portion of the bare optical fiber 16 and the coating layer 32 are formed. In the portion, twists TP1 and TP2 in opposite directions exist, and the twist TP1 remaining in the portion of the bare optical fiber 16 is twisted immediately after the coating layer is cured (the thick solid line in FIG. 6A, the broken line) Smaller than).

Here, the Young's modulus of the cured coating resin is generally considerably lower than that of the optical fiber glass, but is not zero. Therefore, when the external force is released, an elastic repulsive force due to the twist of the resin coating layer accompanying the return of the twist of the bare portion of the optical fiber is always generated. Therefore, in the state where the repulsive force is balanced as described above, The elastic torsion of the line part remains.
In a two-layer coating layer used in a general optical fiber, a material having a Young's modulus of about 0.3 to 1.5 MPa at room temperature is used as the resin (primary material) for the primary coating layer. As the resin (secondary material) for the next coating layer, a material having a Young's modulus at room temperature of about 300 to 1500 MPa is used. The diameter of the bare optical fiber portion is about 125 μm. Of the outer diameter of the coating layer, the outer diameter of the primary coating layer (primary layer) is about 170 to 210 μm, and the outer diameter of the secondary coating layer (secondary layer). Is about 230 to 260 μm. Also, for such an optical fiber, an elastic twist is applied to the optical fiber as described above, and then the external force applied to the optical fiber is released, and the elastic twist of the remaining bare optical fiber is removed. As a result of the examination, it has been confirmed that an elastic twist of about 20 to 30% of the twist imparted to the optical fiber remains.

  In addition, the twisting device may be used to continuously twist the optical fiber in one direction, but as described above, the twist direction is periodically reversed clockwise and counterclockwise, That is, for each predetermined length in the longitudinal direction of the optical fiber, the first twist and the second twist that occurs in the direction opposite to the direction in which the first twist occurs are alternately given. It is more effective for suppressing the increase of.

  When the twist direction is periodically reversed in this way, the viscosity of the coating resin in the liquid state when coating with the coating apparatus is within a range of 0.1 to 3 Pa · sec including each coating layer of the two-layer coating. It is desirable to be. When the viscosity of the resin in the liquid state at the time of coating is less than 0.1 Pa · sec, the viscosity is too low, and it is difficult to coat uniformly and obtain a coating layer having a uniform film thickness. Therefore, the fluctuation amount of the coating outer diameter of the optical fiber strand exceeds ± 2 μm, and there is a possibility that it becomes a defective product as the optical fiber strand. On the other hand, when the viscosity of the resin in a liquid state at the time of coating exceeds 3 Pa · sec, the viscosity of the coating resin is resistant to the propagation of twist in the bare optical fiber from the twist imparting device to the upstream side of the twist imparting device. Acts as As a result, a phenomenon in which twist is accumulated between the twist imparting device and the coating device becomes remarkable, and accordingly, the propagation of twist between the coating curing device and the coating device tends to be slow. In that case, the twist in the opposite direction (e.g. counterclockwise) has been propagated before the twist in one direction (e.g. clockwise) is reliably held by the coating layer between the coating curing device and the coating device, The clockwise twist is returned. As a result, the twist remaining after the coating layer is cured may be reduced or the twist may be almost completely lost. Therefore, when the twist direction is periodically reversed, it is desirable to adjust the viscosity of the resin in the liquid state at the time of coating within the appropriate range as described above.

  Further, when the twist direction is periodically reversed as described above, the twist angle (or cumulative twist angle) with respect to the distance in the longitudinal direction of the optical fiber can be drawn as an inverted twist profile. The waveform of the inverted twist profile may generally be a sine wave shape, but may be a triangular wave shape or a trapezoidal wave shape, and is not particularly limited. An example of the inverted twist profile when a sine wave is employed is shown in FIG. In FIG. 7, the solid line shows the transition of the twist angle (twist angle per unit length) with respect to the longitudinal distance of the optical fiber, and the broken line shows the transition of the cumulative twist angle with respect to the longitudinal distance of the optical fiber. Show.

  Here, in the inverted twist profile, the twist inversion period (a twist in a certain direction, for example, clockwise, is started and the twist in the clockwise direction is applied, and then the twist direction is reversed in the counterclockwise direction. The length (T) on the optical fiber until the twist is applied and the counterclockwise twist is completed is preferably in the range of 5 to 30 m. When the inversion period T of the twist is less than 5 m, the clockwise twist and the counterclockwise twist are easily canceled during propagation. On the other hand, when the twist inversion period T exceeds 30 m, there is a possibility that the effect of suppressing the increase in PMD due to external factors cannot be obtained unless more twists are added.

  Further, in the inverted twist profile, the maximum amplitude MA (see FIG. 7) of the cumulative twist angle is preferably in the range of 500 × T to 4000 × T (°). When the maximum amplitude MA of the cumulative twist angle is less than 500 × T (°), the elastic twist of the bare optical fiber remaining after releasing the external force on the optical fiber is reduced, and PMD due to external factors is reduced. The effect of suppressing the increase is reduced. On the other hand, when the maximum amplitude MA of the cumulative torsion angle exceeds 4000 × T (°), the stress applied to the coating layer from the bare portion of the optical fiber when the external force on the optical fiber is released is so great that the light There is a possibility that peeling occurs between the bare fiber portion and the coating layer, or cracking occurs in the coating layer.

  Further, in the inverted twist profile, the reversal period T of the residual elastic twist in the bare optical fiber portion in a state where it is held by the elastic twist applied to the bare optical fiber portion and the elastic repulsive force generated in the coating layer, It is desirable that the maximum amplitude MA of the cumulative torsional angle is in the range of 5 to 30 m and the range of 100 × T (°) to 1200 × T (°). When the maximum amplitude of the cumulative torsion angle is within the above range, the remaining elastic torsion is a sufficient amount, so that an increase in PMD due to an external factor can be reliably and stably suppressed.

  FIG. 8 shows another embodiment of an apparatus for manufacturing the optical fiber of the present invention. The optical fiber manufacturing apparatus shown in FIG. 8 has a configuration in which a coating apparatus and a coating curing apparatus are respectively installed at two locations in order to manufacture an optical fiber having a two-layer coating layer. That is, a primary coating device 20A is installed immediately below a cooling device 18 that cools and solidifies the bare optical fiber 16 drawn from the spinning heating furnace 14, and a primary coating curing device 22A is installed downstream thereof. First, the primary coating layer is coated and cured. Further, a secondary coating device 20B and a secondary coating curing device 22B are installed in the order of 20B and 22B downstream of the primary coating curing device, and the secondary coating layer is coated and cured on the primary coating layer. And it is comprised so that a twist may be provided by the twist provision apparatus 26 downstream of the secondary coating curing apparatus 22B. When manufacturing an optical fiber having a coating layer having a two-layer structure, even when the coating layer is coated and cured separately at two locations as described above, the twisting, holding and remaining are shown in FIG. This is the same as the case described based on this, and the desirable conditions are the same as described above.

  Below, the measuring method of the elastic twist (twist) angle currently hold | maintained by the coating layer in this invention is shown.

First, as a sample, about 1 m of an optical fiber produced by the above-described method is collected. Subsequently, one end of the collected sample is fixed, and the sample is suspended in the vertical direction. Subsequently, the other end of the sample opposite to the fixed side is fixed to the clip, and the clip is fixed in a state where the twist of the suspended sample is released. Subsequently, 1 m of the coating layer of the sample is removed. Subsequently, the clip is opened, and the rotation angle of the clip in the released state from the fixed state is measured.
The rotation angle measured by the above method is the elastic twist angle (° / m) per unit length.
Furthermore, when measuring the amount of elastic twist of another optical fiber, it can be measured by repeating the above method.

  Examples of the present invention will be described below together with comparative examples. The following examples are examples for clarifying the effects of the present invention, and the technical scope of the present invention is not limited by the conditions described in the examples.

[Example 1]
A two-layer coating structure having the characteristics of a general single mode fiber using the optical fiber manufacturing apparatus as shown in FIG. 1 and using the apparatus as shown in FIG. 3 as a twist imparting apparatus in the manufacturing apparatus. An optical fiber was prepared by applying an elastic twist (twist) to the silica glass optical fiber in accordance with the above-described method of the present invention. The spinning speed (drawing speed) from the optical fiber preform was 2000 mm / min. Moreover, the coating apparatus applied the two-layer simultaneous coating system (wet on wet system) which coats two types of coating resin in one place. As a resin (primary material) for the primary coating layer, a UV curable urethane acrylate resin (Young's modulus at curing 0.5 MPa) is used, and as a resin (secondary material) for the secondary coating layer, a UV curable urethane acrylate. System resin (Young's modulus at the time of curing 1000 MPa) was used. Further, the viscosity of the liquid resin at the time of coating with the above materials was adjusted to 1 Pa · sec. After being applied by a coating apparatus, it was cured by a UV lamp as a coating curing apparatus. Twist was imparted immediately after the step of curing the coating resin with a coating curing device. In addition, on the upstream side of the twist imparting device, the wires other than the coating resin were drawn so as not to physically contact the optical fiber.

  Here, the profile in the longitudinal direction of the twisted optical fiber strand applied to the optical fiber strand by the twist imparting device is a sine wave that periodically inverts the twist direction, and the cycle T is 20 m, and the cumulative twist angle. The swing angle and swing speed of the torsion imparting device were set so that the maximum amplitude MA of 10000 was 10000 °. The optical fiber after passing through the twist imparting device is taken up by the take-up machine through the guide pulley, and further taken up by the take-up machine through the dancer pulley, and an elastic twist (twist) is given to the bare optical fiber portion. I got an optical fiber. The obtained optical fiber had a bare wire diameter of 125 μm, a coating outer diameter primary diameter (primary coating layer outer diameter) of 200 μm, and a secondary diameter (secondary coating layer outer diameter) of 250 μm. .

About the optical fiber strand of Example 1 manufactured and wound as described above, a rewinding device is used while securing a distance (free length) of 30 m that does not contact a member such as a pulley to which an external force such as friction resistance is applied. The sample was prepared by winding up and releasing the twist applied to the optical fiber. About the prepared sample, a sample having a length of 1000 m is intentionally wound around an optical fiber by forcibly winding it on a 400 mmφ iron bobbin with a winding tension of 200 gf in one layer so that the fibers do not overlap each other. Lateral pressure was applied. That is, the conditions are such that PMD is likely to occur due to external factors. Then, PMD measurement was performed after leaving it to stand for 1 hour or more for stabilization of fiber temperature. For PMD measurement, an HP 8509B measuring instrument manufactured by Hewlett-Packard was used, and the PMD measurement was performed by the JME method (Jones Matrix Eigenanalysis method). The measurement wavelength was 1510 to 1600 nm, and scanning was performed in 2 nm steps. As a result, a very small value of 0.05 ps / √km was obtained as the PMD value (PMD1) when the side pressure was applied.
On the other hand, when the same wire as that used in the sample was placed in a free coil state (side pressure removed state) and PMD was measured again under the same conditions as described above, the PMD value (PMD2) in the free coil state was It was 0.02 ps / √km. Here, the difference (0.03 ps / √km) between PMD1 and PMD2 can be regarded as a PMD increase due to the addition of lateral pressure, that is, a PMD increase due to an external factor.
Moreover, when the coating layer was actually removed and the residual twist was measured, the reversal period T of the residual elastic twist was 20 m, and the maximum amplitude MA of the cumulative twist angle was 3000 °.
Thus, the PMD increase at the time of applying side pressure (external factor) is remarkably small, even after the twist added during the manufacturing process of the wire becomes free (external force released) due to the aforementioned rewinding. It is understood that the increase in PMD at the time of lateral pressure application (external factor) could be suppressed by the residual elastic torsion (twist) retained by the coating layer and remaining as elastic torsion (twist).

[Comparative Example 1]
An optical fiber was manufactured in the same manner as in Example 1 except that the elastic twist (twist) was not applied. Further, as in Example 1, PMD values were measured in each of a state where a side pressure was intentionally applied and a free coil state where the side pressure was removed. As a result, the PMD value (PMD1) in the state where the lateral pressure was applied was an extremely high value of 0.62 ps / √km. The PMD value (PMD2) in the side pressure removal state (free coil state) was 0.02 ps / √km, as in Example 1. In this case, the difference between PMD1 and PMD2, that is, the PMD increase due to the external factor of the side pressure application, was a large value of 0.60 ps / √km.

  Comparing Example 1 with elastic twist as described above and Comparative Example 1 without elastic twist, PMD1 when applying a side pressure is much stronger in Example 1 than in Comparative Example 1. The difference between PMD1 and PMD2 (PMD increase due to the addition of side pressure) is much smaller in Example 1 than in Comparative Example 1. From this, in Comparative Example 1, the increase in PMD due to external factors was increased because the elastic twist was not applied, whereas in Example 1, the elastic twist was applied as described above, and the external force ( It is clear that the increase in PMD due to external factors can be suppressed to a very small amount by maintaining the elastic twist even after the release of the side pressure).

[Comparative Example 2]
As shown in FIG. 9, the twist imparting device 26 is installed downstream of the take-up capstan 36 to impart twist to the optical fiber 24 after passing through the take-up capstan 36. Other configurations are the same as those of the first embodiment. The manufactured optical fiber was wound back in the same manner as in Example 1 to release the twist applied to the optical fiber, and a sample was produced. About the produced sample, similarly to the case of Example 1, the PMD value in each state of the state where the side pressure was intentionally applied and the free coil state where the side pressure was removed was measured. As a result, the PMD value (PMD1) with a lateral pressure applied was a remarkably high value of 0.58 ps / √km. The PMD value (PMD2) in the side pressure removal state (free coil state) was 0.016 ps / √km. In this case, the difference between PMD1 and PMD2, that is, the PMD increase due to the external factor of the side pressure application was about 0.56 ps / √km. As described above, in the case of Comparative Example 2, the PMD1 value was close to that of Comparative Example 1, and the difference between PMD1 and PMD2, that is, the increase in PMD due to external factors was also a large value close to that of Comparative Example 1. This means that the increase in PMD due to external factors was not sufficiently suppressed in the optical fiber according to Comparative Example 2, even when twist was added during the manufacturing process. This is because a twist is added on the downstream side of the take-up capstan 36 so that the take-up capstan 36 becomes a resistance to the propagation of the twist. For this reason, the added twist is not sufficiently propagated upstream of the take-up capstan 36, so that the optical fiber bare wire is formed in the vicinity of the space between the coating device 20 located upstream of the take-up capstan 36 and the coating curing device 22. The twist is not given. As a result, it is understood that the elastic torsion remaining on the optical fiber has almost become zero.
Moreover, when the residual twist was measured after actually removing the coating, it was not possible to measure the twist with a regular period.

[Example 2]
A two-layer coating having the characteristics of a general single mode fiber using an optical fiber manufacturing apparatus as shown in FIG. 8 and an apparatus as shown in FIG. 3 as a twist imparting apparatus 26 in the manufacturing apparatus. An optical fiber strand was manufactured by applying an elastic twist (twist) to the quartz glass-based optical fiber strand having the structure according to the above-described method of the present invention. The spinning speed (drawing speed) from the optical fiber preform was 1500 mm / min. Further, as the coating-curing method, as shown in FIG. 8, a method (wet on dry method) in which different coating resins are coated at two locations, respectively, was applied. As a resin (primary material) for the primary coating layer, a UV curable urethane acrylate resin (Young's modulus at the time of curing: 1.0 MPa) is used. As a resin (secondary material) for the secondary coating layer, a UV curable urethane acrylate is used. System resin (Young's modulus at the time of curing 500 MPa) was used. The viscosity of the liquid resin during the coating of the above materials is adjusted such that the primary material has a viscosity of 3 Pa · sec, the secondary material has a viscosity of 0.1 Pa · sec, and the primary material in the liquid state is coated by the primary coating apparatus 20A. It was cured by a UV lamp which is a primary coating curing apparatus 22A. Subsequently, the secondary material was coated with the secondary coating device 20B, and cured with a UV lamp as the secondary coating curing device 22B. The twist was applied immediately after the step of curing the secondary material by the secondary coating curing device 22B. In addition, on the upstream side of the twist imparting device 26, the wires other than the coating resin were drawn so as not to physically contact the optical fiber.

  Here, the profile in the longitudinal direction of the twisted optical fiber strand applied to the optical fiber strand 24 by the twist imparting device 26 is a triangular wave that periodically inverts the twist direction, the period T is 5 m, and the cumulative twist angle is The swing angle and swing speed of the torsion imparting device were set so that the maximum amplitude MA was 2500 °. The optical fiber 24 after passing through the twist imparting device 26 is taken up by a take-up machine (not shown) through a guide pulley 28, further taken up by a winder through a dancer pulley, and an elastic twist ( An optical fiber with a twist was obtained. The obtained optical fiber had a bare wire diameter of 125 μm, a primary outer diameter (primary coating layer outer diameter) of 190 μm, and a secondary diameter (secondary coating layer outer diameter) of 240 μm.

About the optical fiber strand of Example 2 manufactured and wound up as mentioned above, it is rewound by a rewinding device while securing a distance (free length) that is not physically in contact with a member such as a pulley. Samples were prepared by releasing the twist applied to the fiber strand.
For the prepared sample, a side pressure was intentionally applied to the optical fiber under the same conditions as in Example 1. That is, PMD measurement was carried out in the same manner as in Example 1 after PMD was allowed to stand for 1 hour or more for stabilization of the fiber temperature under conditions where PMD was likely to occur due to external factors. As a result, a very small value of 0.08 ps / √km was obtained as the PMD value (PMD1) when the side pressure was applied.
On the other hand, when the same wire as that used in the sample was placed in a free coil state (side pressure removed state) and PMD was measured again under the same conditions as described above, the PMD value (PMD2) in the free coil state was It was 0.01 ps / √km. Here, the difference between PMD1 and PMD2 could be suppressed to a very small value of 0.07 ps / √km. When the coating was actually removed and the residual twist was measured, the reversal period T of the residual elastic twist was 5 m, and the maximum amplitude MA of the cumulative twist angle was 750 °. Therefore, when manufacturing an optical fiber having a two-layer coating structure, even if coating and curing are performed at two locations (a configuration in which coating and curing are performed one layer at a time), lateral pressure is applied, that is, PMD due to external factors. It was confirmed that an optical fiber strand with a remarkably small increase can be obtained.

Example 3
In the same manner as in Example 2, an optical fiber having a two-layer coating structure was manufactured while adding an elastic twist (twist). Further, the viscosity of the primary material was adjusted to 3 Pa · sec, and the viscosity of the secondary material was adjusted to 0.1 Pa · sec, as in Example 2, with respect to the viscosity when the coating resin was coated (liquid state). The profile in the longitudinal direction of the optical fiber strand of the twist applied to the optical fiber strand by the twist imparting device is a trapezoidal wave that periodically reverses the twist direction, the period T is 30 m, and the maximum twist angle MA of the cumulative twist angle The swing angle and swing speed of the torsion imparting device were set so that the angle was 120,000 °. An optical fiber in which an optical fiber strand after passing through a twist imparting device is taken up by a take-up machine, further taken up by a winder through a dancer pulley, and an elastic twist (twist) is given to the bare optical fiber portion I got a strand. The obtained optical fiber had a bare wire diameter of 125 μm, a primary outer diameter of 180 μm, and a secondary diameter of 260 μm.

About the optical fiber strand of Example 3 manufactured and wound up as described above, it is rewound by a rewinding device while securing a distance (free length) that is not physically in contact with a member such as a pulley. Samples were prepared by releasing the twist applied to the fiber strand.
For the prepared sample, a side pressure was intentionally applied to the optical fiber under the same conditions as in Example 1. That is, PMD measurement was carried out in the same manner as in Example 1 after PMD was allowed to stand for 1 hour or more for stabilization of the fiber temperature under conditions where PMD was likely to occur due to external factors. As a result, a very small value of 0.06 ps / √km was obtained as the PMD value (PMD1) when the side pressure was applied.
On the other hand, when the same wire as that used in the sample was placed in a free coil state (side pressure removed state) and PMD was measured again under the same conditions as described above, the PMD value (PMD2) in the free coil state was It was 0.03 ps / √km. Here, the difference between PMD1 and PMD2 could be suppressed to an extremely small value of 0.03 ps / √km. When the coating was actually removed and the residual twist was measured, the reversal period T of the residual elastic twist was 30 m, and the maximum amplitude MA of the cumulative twist angle was 24000 °. Therefore, it was confirmed that the optical fiber obtained by Example 3 can also suppress the increase in PMD due to lateral pressure, that is, external factors.

[Comparative Example 3]
In the same manner as in Example 3, an optical fiber having a two-layer coating structure was manufactured while adding an elastic twist (twist). Regarding the viscosity at the time of coating with the coating resin, the viscosity of the primary material was adjusted to 3.5 Pa · sec, and the viscosity of the secondary material was adjusted to 0.5 Pa · sec. The profile in the longitudinal direction of the twisted optical fiber strand applied to the optical fiber strand by the twist imparting device is a triangular wave that periodically reverses the twist direction, the period T is 5 m, and the maximum amplitude MA of the cumulative twist angle is 2500. The rocking angle and rocking speed of the torsion imparting device were set so as to be °. An optical fiber in which an optical fiber strand after passing through a twist imparting device is taken up by a take-up machine, further taken up by a winder through a dancer pulley, and an elastic twist (twist) is given to the bare optical fiber portion I got a strand. The obtained optical fiber had a bare wire diameter of 125 μm, a primary covering layer outer diameter (primary diameter) of 180 μm, and a secondary covering layer outer diameter (secondary diameter) of 260 μm.

About the optical fiber strand of the comparative example 3 manufactured and wound up as mentioned above, it is rewound by a rewinding device while securing a distance (free length) that is not physically in contact with a member such as a pulley. Samples were prepared by releasing the twist applied to the fiber strand.
For the prepared sample, a side pressure was intentionally applied to the optical fiber under the same conditions as in Example 1. That is, PMD measurement was carried out in the same manner as in Example 1 after PMD was allowed to stand for 1 hour or more for stabilization of the fiber temperature under conditions where PMD was likely to occur due to external factors. As a result, the PMD value (PMD1) when the side pressure was applied was 0.25 ps / √km, which is smaller than when no elastic twist (twist) is added, but larger than that in Example 3. In addition, when the same wire as that used in the sample was placed in a free coil state (side pressure removed state) and PMD was measured again under the same conditions as described above, the PMD value (PMD2) in the free coil state was It was 0.025 ps / √km. Here, the difference between PMD1 and PMD2 is about 0.22 ps / √km, which is smaller than those in Comparative Examples 1 and 2, but larger than that in Example 3. When the coating was actually removed and the residual twist was measured, the reversal period T of the residual elastic twist was 5 m, and the maximum amplitude MA of the cumulative twist angle was 100 °. This is because the viscosity at the time of coating the resin (primary material) of the primary coating layer was high, and thus the propagation of the twist and the reversal of the twist direction were hindered, and the twist was offset before the primary material was cured by UV irradiation. It is understood that the twist remaining in the coated optical fiber is reduced.

[Comparative Example 4]
In the same manner as in Example 3, an optical fiber having a two-layer coating structure was manufactured while adding an elastic twist (twist). Regarding the viscosity of the resin in the liquid state at the time of coating, the viscosity of the primary material was adjusted to 2.0 Pa · sec, and the viscosity of the secondary material was adjusted to 0.05 Pa · sec. The profile in the longitudinal direction of the twisted optical fiber imparted to the optical fiber by the twist imparting device is a triangular wave that periodically inverts the twist direction, the period T is 5 m, and the maximum amplitude MA of the cumulative twist angle is 2500 °. Thus, the swing angle and swing speed of the twist imparting device were set. An optical fiber in which an optical fiber strand after passing through a twist imparting device is taken up by a take-up machine, further taken up by a winder through a dancer pulley, and an elastic twist (twist) is given to the bare optical fiber portion I got a strand. The obtained optical fiber had a bare wire diameter of 125 μm, a primary coating layer outer diameter (primary diameter) of the coating layer was 180 μm, and a secondary coating layer outer diameter (secondary diameter) was 260 μm. The fluctuation of the outer diameter (secondary diameter) of the secondary coating layer was remarkably increased to ± 5 μm.
This is because the coating was not stable because the viscosity of the resin (secondary material) of the secondary coating layer in the liquid state was low. Since such an optical fiber is not suitable for practical use, PMD was not evaluated.

[Comparative Example 5]
In the same manner as in Example 1, an optical fiber having a two-layer coating structure was manufactured while adding an elastic twist (twist). However, the twist profile to be applied was different from that in Example 1. That is, the profile in the longitudinal direction of the twisted optical fiber imparted to the optical fiber by the twist imparting device is a sine wave that periodically inverts the twist direction, the period T is 3 m, and the maximum amplitude MA of the cumulative twist angle is The swing angle and swing speed of the torsion imparting device were set so as to be 1500 °. The optical fiber strand after passing through the twist imparting device is taken up by a take-up machine, and further taken up by a winder through a dancer pulley, and an optical twist is applied to the bare portion of the optical fiber. Got a line. The obtained optical fiber had a bare wire diameter of 125 μm, a primary coating layer outer diameter (primary diameter) of 200 μm, and a secondary coating layer outer diameter (secondary diameter) of 250 μm.

About the optical fiber strand of the comparative example 5 manufactured and wound up as mentioned above, it rewinds with the rewinding apparatus, ensuring the distance (free length) which does not physically contact members, such as a pulley, and light Samples were prepared by releasing the twist applied to the fiber strand.
For the prepared sample, a side pressure was intentionally applied to the optical fiber under the same conditions as in Example 1. That is, PMD measurement was carried out in the same manner as in Example 1 after PMD was allowed to stand for 1 hour or more for stabilization of the fiber temperature under conditions where PMD was likely to occur due to external factors. As a result, the PMD value (PMD1) when the side pressure was applied was 0.4 ps / √km, which was smaller than those in Comparative Examples 1 and 2, but a relatively large value. In addition, when the same wire as that used in the sample was placed in a free coil state (side pressure removed state) and PMD was measured again under the same conditions as described above, the PMD value (PMD2) in the free coil state was It was 0.02 ps / √km. When the coating was actually removed and the residual twist was measured, the reversal period T of the residual elastic twist was 3 m, and the maximum amplitude MA of the cumulative twist angle was 200 °. Here, the PMD value (PMD1) when the side pressure is applied becomes a relatively large value because the period of twist applied to the optical fiber is relatively short, so that the applied twist is partially released and remains. This is thought to be due to less twisting. However, even in this case, it is clear that the increase in the PMD value (PMD1) when the lateral pressure is applied is smaller than when no twist is added.

[Comparative Example 6]
In the same manner as in Example 1, an optical fiber having a two-layer coating structure was manufactured while adding an elastic twist (twist). However, the twist profile was different from that in Example 1. That is, the profile in the longitudinal direction of the twisted optical fiber imparted to the optical fiber by the twist imparting device is a sine wave that periodically reverses the twist direction, the period T is 15 m, and the maximum amplitude MA of the cumulative twist angle is The swing angle and swing speed of the torsion imparting device were set so as to be 65000 °. An optical fiber in which an optical fiber strand after passing through a twist imparting device is taken up by a take-up machine, further taken up by a winder through a dancer pulley, and an elastic twist (twist) is given to the bare optical fiber portion I got a strand. The obtained optical fiber had a bare wire diameter of 125 μm, a primary coating layer outer diameter (primary diameter) of 200 μm, and a secondary coating layer outer diameter (secondary diameter) of 250 μm.

The optical fiber of Comparative Example 6 manufactured and wound as described above was wound back by a rewinding device while securing a distance (free length) that does not physically contact a member such as a pulley, and the optical fiber was wound. Samples were prepared by releasing the twist applied to the fiber strand.
For the prepared sample, a side pressure was intentionally applied to the optical fiber under the same conditions as in Example 1. That is, PMD measurement was carried out in the same manner as in Example 1 after PMD was allowed to stand for 1 hour or more for stabilization of the fiber temperature under conditions where PMD was likely to occur due to external factors. As a result, the PMD value (PMD1) when the side pressure was applied was as low as 0.04 ps / √km. In addition, when the same wire as that used in the sample was placed in a free coil state (side pressure removed state) and PMD was measured again under the same conditions as described above, the PMD value (PMD2) in the free coil state was It was 0.02 ps / √km. When the coating was actually removed and the residual twist was measured, the reversal period T of the residual elastic twist was 15 m, and the maximum amplitude MA of the cumulative twist angle was 15000 °. Thus, Comparative Example 6 was also effective in reducing PMD when a side pressure was applied. However, when the optical fiber was put in a thermostatic bath and a heat cycle test at -40 ° C. to + 80 ° C. was performed, the coating layer was observed, and it was found that the coating layer was cracked. This is considered to be because the amount of twist applied to the optical fiber was too large, and the stress applied to the coating layer was excessive, leading to the occurrence of cracks.

  DESCRIPTION OF SYMBOLS 10 ... Optical fiber manufacturing apparatus, 12 ... Optical fiber base material, 14 ... Heating furnace for spinning, 16 ... Bare optical fiber, 18 ... Cooling device, 20 ... Coating apparatus, 22 ... Coating hardening apparatus, 24 ... Optical fiber Wire, 26 ... twist imparting device

Claims (7)

An optical fiber,
An optical fiber bare wire part to which an elastic twist is given,
Elasticity against the restoring force generated in the bare optical fiber portion so as to cover the optical fiber bare wire portion and is formed of a curable resin so as to hold the elastic twist applied to the bare optical fiber portion. A coating layer that produces a repulsive force;
Only including,
As the elastic twist applied to the bare optical fiber portion, the first twist and the second twist generated in the direction opposite to the direction in which the first twist occurs at every predetermined length in the longitudinal direction of the optical fiber. Are alternately given to the bare optical fiber portions,
The reversal period T of the residual elastic twist in the bare optical fiber portion held by the elastic twist given to the bare optical fiber portion and the elastic repulsive force generated in the coating layer is 5 to 30 m, And the maximum amplitude MA of the accumulation torsion angle in an inversion twist profile is 100 * T (degree)-1200 * T (degree), The optical fiber strand characterized by the above-mentioned. The optical fiber according to claim 1 ,
The light comprising: a primary coating layer formed of a resin having a relatively low Young's modulus; and a secondary coating layer formed of a resin having a relatively high Young's modulus. Fiber strand. The optical fiber preform is heated and melted,
Pull out the bare optical fiber of the specified diameter from the molten optical fiber preform,
Solidify the drawn optical fiber bare wire,
By giving an elastic twist to the bare optical fiber toward the upstream side in the drawing direction of the bare optical fiber, an elastic twist is imparted to the bare optical fiber after solidification,
Forming a coating layer before curing by coating the outer periphery of the solidified optical fiber bare wire with a curable resin in a liquid state,
An optical fiber element provided with a twist so that the elastic twist of the bare optical fiber is retained by curing the coating layer formed on the outer periphery of the bare optical fiber provided with the elastic twist Forming a line,
A method for manufacturing an optical fiber taking up the optical fiber in which the twist is imparted,
When twisting an optical fiber, the direction of twist applied to the bare optical fiber is periodically reversed, and the inversion period of twist applied to the optical fiber in the longitudinal direction of the optical fiber A method of manufacturing an optical fiber, wherein T is 5 to 30 m, and a maximum amplitude of a cumulative twist angle in an inverted twist profile is 500 × T (°) to 4000 × T (°) . In the manufacturing method of the optical fiber in accordance with claim 3 ,
By using a twist imparting device, imparting twist to the bare optical fiber,
Manufacturing of an optical fiber, wherein the optical fiber bare wire is twisted in a state where there is no member that prevents the twist propagation of the bare optical fiber upstream from the twist applying device. Method. In the manufacturing method of the optical fiber strand of Claim 3 or 4 ,
A method for producing an optical fiber, wherein when the curable resin is coated on the bare optical fiber, the viscosity at the time of coating the curable resin in a liquid state is 0.1 to 3 Pa · sec. In the manufacturing method of the optical fiber strand of Claim 3 or 4 ,
An inversion period T of the twist remaining in the bare optical fiber portion held by the elastic twist applied to the optical fiber in the longitudinal direction of the optical fiber and the elastic repulsion generated in the coating layer is A method of manufacturing an optical fiber, wherein the maximum amplitude MA of the cumulative twist angle in the inverted twist profile is from 100 × T (°) to 1200 × T (°). In the manufacturing method of the optical fiber in accordance with claim 5 ,
An inversion period T of the twist remaining in the bare optical fiber portion held by the elastic twist applied to the optical fiber in the longitudinal direction of the optical fiber and the elastic repulsion generated in the coating layer is A method of manufacturing an optical fiber, wherein the maximum amplitude MA of the cumulative twist angle in the inverted twist profile is from 100 × T (°) to 1200 × T (°).

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