Optical fiber core wire and method for producing optical fiber core wire

Abstract

To provide an optical fiber core wire with high water resistance even under low loads where delamination may occur.SOLUTION: There is provided an optical fiber core wire including: an optical fiber bare wire; a primary layer which coats the optical fiber bare wire and is formed of a first ultraviolet curable resin; and a secondary layer which coats the primary layer and is formed of a second ultraviolet curable resin. When a load is applied to the optical fiber core wire perpendicular to its longitudinal direction, delamination occurs at a load of 330 g or less with a ratio of 50%. When the optical fiber core wire is immersed in 60°C warm water for 200 days, delamination does not occur.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical fiber and a method for manufacturing the optical fiber. [Background technology]

[0002] In an optical fiber core having a primary layer covering the bare optical fiber and a secondary layer covering the primary layer, a technique for improving the adhesion between the bare optical fiber and the primary layer is known (Patent Document 1). By improving the adhesion between the bare optical fiber and the primary layer, it is possible to suppress delamination between the bare optical fiber and the primary layer and reduce the transmission loss of the optical fiber core.

[0003] It is generally known that delamination of the optical fiber occurs even when the optical fiber is placed under high-temperature and high-humidity conditions. The technology described in Patent Document 2 improves the water resistance of the optical fiber and also improves the adhesion between the bare optical fiber and the primary layer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-4481 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-171072 Summary of the Invention [Problem to be solved by the invention]

[0005] The required adhesion between the bare optical fiber and the primary layer may vary depending on the application of the optical fiber. For example, when the optical fiber is used as a load sensor, it is preferable that the adhesion between the bare optical fiber and the primary layer is low. However, in the optical fiber cores of Patent Documents 1 and 2, if the adhesion between the bare optical fiber and the primary layer is reduced, the water resistance of the optical fiber core may also be reduced.

[0006] An object of the present invention is to provide an optical fiber core that has high water resistance even when the load at which delamination may occur is low. [Means for solving the problem]

[0007] According to one aspect of the present invention, there is provided an optical fiber core wire comprising a bare optical fiber, a primary layer formed of a first ultraviolet-curable resin covering the bare optical fiber, and a secondary layer formed of a second ultraviolet-curable resin covering the primary layer, wherein when a load is applied to the optical fiber core wire from a direction perpendicular to the longitudinal direction of the optical fiber core wire, the load at which delamination occurs in 50% of cases is 330 g or less, and when the optical fiber core wire is immersed in warm water at 60°C for 200 days, no delamination occurs.

[0008] According to another aspect of the present invention, there is provided a method for manufacturing an optical fiber core wire, comprising the steps of drawing a bare optical fiber from an optical fiber preform, applying a first ultraviolet-curable resin around the bare optical fiber to form a primary layer, and applying a second ultraviolet-curable resin around the primary layer to form a secondary layer, wherein when a load is applied to the optical fiber core wire perpendicular to the longitudinal direction of the optical fiber core wire, the load at which delamination occurs in 50% of cases is 330 g or less, and when the optical fiber core wire is immersed in warm water at 60°C for 200 days, no delamination occurs. [Effects of the Invention]

[0009] According to the present invention, an optical fiber core having high water resistance can be provided even when the load at which delamination may occur is low. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view of an optical fiber core according to an embodiment of the present invention. [Figure 2] 1 is a schematic diagram showing an optical fiber coated wire manufacturing apparatus according to an embodiment of the present invention; [Figure 3] 3 is a flowchart of a method for manufacturing an optical fiber according to the present embodiment. [Figure 4] 1 is a schematic diagram of a measuring device for measuring delamination of an optical fiber core according to an embodiment of the present invention. [Figure 5] 1 is a schematic diagram of a load sensor including an optical fiber core according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Elements having common functions throughout the drawings will be designated by the same reference numerals, and duplicate descriptions may be omitted or simplified.

[0012] [First embodiment] 1 is a cross-sectional view of an optical fiber core according to the present embodiment. The optical fiber core 1 includes a bare optical fiber 2, a primary layer 3 that covers the outer periphery of the bare optical fiber 2, and a secondary layer 4 that covers the outer periphery of the primary layer 3.

[0013] The bare optical fiber 2 is made of, for example, silica-based glass and transmits light. The primary layer 3 is a soft layer and functions to buffer external forces applied to the bare optical fiber 2. The elastic modulus of the primary layer 3 is preferably 0.10 MPa or more and 0.30 MPa or less. The secondary layer 4 is a hard layer and functions to protect the bare optical fiber 2 and the primary layer 3 from external forces. The elastic modulus of the secondary layer 4 is preferably 500 MPa or more and 2000 MPa or less.

[0014] The diameter of the optical fiber 1 is 185 μm or more and 260 μm or less. The diameter of the bare optical fiber 2 is 80 μm or more and 150 μm or less, preferably 124 μm or more and 126 μm or less. The thickness of the primary layer 3 is 5 μm or more and 60 μm or less. The thickness of the secondary layer 4 is 5 μm or more and 60 μm or less. The sum of the thicknesses of the primary layer 3 and the secondary layer 4 is 25 μm or more and 90 μm or less. Here, the diameter of the optical fiber 1 is determined by the sum of the diameter of the bare optical fiber 2, the length twice the thickness of the primary layer 3, and the length twice the thickness of the secondary layer 4. Therefore, the diameter of the bare optical fiber 2, the thickness of the primary layer 3, and the thickness of the secondary layer 4 are each selected so that the diameter of the optical fiber 1 is 185 μm or more and 260 μm or less.

[0015] The modulus of elasticity of the primary layer 3 of the optical fiber is ISM (In Situ Modulus), and the modulus of elasticity of the primary layer 3 can be measured by the following method.

[0016] First, a commercially available stripper is used to strip a few millimeters of the coating layer from the middle of a sample optical fiber. One end of the optical fiber with the coating layer is then fixed to a glass slide with adhesive, and a load F is applied to the other end of the optical fiber with the coating layer. In this state, the displacement δ of the primary layer 3 at the boundary between the stripped portion and the remaining portion is measured using a microscope. The rate (slope) of change in load F with respect to displacement δ is calculated by varying load F to 10, 20, 30, 50, and 70 gf (i.e., 98, 196, 294, 490, and 686 mN, respectively). The primary modulus is calculated using the calculated slope and the following equation (1). The calculated primary modulus is the so-called ISM, and hereinafter, the primary modulus will be referred to as P-ISM. Note that when drawing the optical fiber, the drawing speed and UV irradiance are controlled to adjust the P-ISM. P-ISM=(3F / δ)*(1 / 2πl)*ln(DP / DG) ···(1)

[0017] The unit of P-ISM is [MPa]. On the right side of equation (1), F / δ is the rate (slope) of change in load (F) [gf] with respect to displacement (δ) [μm], l is the sample length (e.g., 10 mm), and DP / DG is the ratio of the outer diameter (DP) [μm] of the primary layer 3 to the outer diameter (DG) [μm] of the cladding of the optical fiber. Therefore, when calculating P-ISM using equation (1) from the F, δ, and l used, a predetermined unit conversion is required. The outer diameter of the primary layer 3 and the outer diameter of the cladding can be measured by observing the cross section of the optical fiber cut with a fiber cutter under a microscope.

[0018] Furthermore, the elastic modulus of the secondary layer 4 of the optical fiber is ISM (In Situ Modulus), and the elastic modulus of the secondary layer 4 can be measured by the following method.

[0019] First, an optical fiber is immersed in liquid nitrogen and the coating layer is stripped off using a stripper, creating a hollow cylindrical sample with only the coating layer. The end of the sample is fixed to an aluminum plate using adhesive. The aluminum plate is chucked using a Tensilon universal tensile tester in an atmosphere of 23°C and 50% relative humidity. Next, the sample is stretched at a gauge spacing of 25 mm and a tensile speed of 1 mm / min. The force at 2.5% elongation is measured to calculate the S-ISM (2.5% secant modulus) of the secondary layer 4. The coating layer sample includes the primary layer 3 and secondary layer 4. Because the S-ISM is sufficiently larger than the P-ISM, the influence of the P-ISM on the measured elastic modulus can be ignored and the measured elastic modulus can be considered the S-ISM.

[0020] The primary layer 3 and the secondary layer 4 are formed by curing an ultraviolet curable resin by irradiating it with ultraviolet light. The ultraviolet curable resin will be described in detail below.

[0021] The ultraviolet curable resin is a resin that polymerizes and hardens when irradiated with ultraviolet light. The ultraviolet curable resin is not particularly limited as long as it can be polymerized when irradiated with ultraviolet light. The ultraviolet curable resin can be polymerized by, for example, photoradical polymerization.

[0022] The ultraviolet-curable resin is, for example, an ultraviolet-curable resin having a polymerizable unsaturated group such as an ethylenically unsaturated group that polymerizes and hardens when exposed to ultraviolet light, such as urethane (meth)acrylates such as polyether-based urethane (meth)acrylates and polyester-based urethane (meth)acrylates, epoxy (meth)acrylates, and polyester (meth)acrylates, and preferably has at least two polymerizable unsaturated groups.

[0023] The polymerizable unsaturated group in the ultraviolet-curable resin may be, for example, a group having an unsaturated double bond such as a vinyl group, an allyl group, an acryloyl group, or a methacryloyl group, or a group having an unsaturated triple bond such as a propargyl group, etc. Among these, an acryloyl group or a methacryloyl group is preferred in terms of polymerizability.

[0024] The ultraviolet-curable resin may be a monomer, oligomer, or polymer that initiates polymerization and hardens upon irradiation with ultraviolet light, but is preferably an oligomer. An oligomer is a polymer with a degree of polymerization of 2 to 100. In this specification, "(meth)acrylate" refers to either or both of acrylate and methacrylate. The ultraviolet-curable resin contains any photopolymerization initiator (photoinitiator) that is sensitive in the ultraviolet region.

[0025] Polyether-based urethane (meth)acrylates are compounds having a polyether segment, a (meth)acrylate, and a urethane bond, such as the reaction product of a polyol having a polyether skeleton with an organic polyisocyanate compound and a hydroxyalkyl (meth)acrylate. Polyester-based urethane (meth)acrylates are compounds having a polyester segment, a (meth)acrylate, and a urethane bond, such as the reaction product of a polyol having a polyester skeleton with an organic polyisocyanate compound and a hydroxyalkyl (meth)acrylate.

[0026] The UV-curable resin of the primary layer 3 (first UV-curable resin) and the UV-curable resin of the secondary layer 4 (second UV-curable resin) may contain, in addition to an oligomer and a photoinitiator, a diluent monomer, a photosensitizer, a UV absorber, an antioxidant, a chain transfer agent, a silane coupling agent, a lubricant such as silicone, and various additives. The diluent monomer may be a monofunctional (meth)acrylate or a polyfunctional (meth)acrylate. Here, the diluent monomer refers to a monomer for diluting the UV-curable resin.

[0027] Next, a manufacturing apparatus used in the manufacturing method of the optical fiber according to this embodiment will be described. Fig. 2 is a schematic diagram showing a manufacturing apparatus 10 for the optical fiber 1 according to this embodiment. The manufacturing apparatus 10 includes a heating device 20, a primary layer coating device 30, a secondary layer coating device 40, a guide roller 50, and a winding device 60. The manufacturing apparatus 10 shown in Fig. 2 manufactures the optical fiber 1 from the optical fiber preform BM.

[0028] The optical fiber preform BM is made of, for example, silica-based glass and is manufactured by a well-known method such as the VAD method, the OVD method, or the MCVD method. The heating device 20 has a heater 21. The heater 21 is any heat source such as a tape heater, a ribbon heater, a rubber heater, an oven heater, a ceramic heater, or a halogen heater. The end of the optical fiber preform BM is heated and melted by the heater 21 arranged around the optical fiber preform BM, and is then drawn to extract a bare optical fiber 2.

[0029] A primary layer coating device 30 is provided below the heating device 20. The primary layer coating device 30 has a resin applicator 31 and an ultraviolet ray irradiator 32. The resin applicator 31 holds an ultraviolet ray curable resin for the primary layer 3. The resin applicator 31 applies the ultraviolet ray curable resin for the primary layer 3 to the bare optical fiber 2 drawn out from the optical fiber preform BM.

[0030] An ultraviolet irradiation device 32 is provided below the resin application device 31. The ultraviolet irradiation device 32 is equipped with any ultraviolet light source such as a metal halide lamp, a mercury lamp, or a UV-LED. The resin application device 31 applies an ultraviolet-curable resin for the primary layer 3 to the bare optical fiber 2, and the bare optical fiber 2 then enters the ultraviolet irradiation device 32, where ultraviolet light is irradiated onto the ultraviolet-curable resin for the primary layer 3. As a result, the ultraviolet-curable resin for the primary layer 3 is cured, and the primary layer 3 is formed.

[0031] A secondary layer coating device 40 is provided below the primary layer coating device 30. The secondary layer coating device 40 has a resin coating device 41 and an ultraviolet ray irradiation device 42. The resin coating device 41 holds the ultraviolet ray curable resin of the secondary layer 4. The ultraviolet ray curable resin of the secondary layer 4 is applied to the primary layer 3 by the resin coating device 41.

[0032] An ultraviolet irradiation device 42 is provided below the resin application device 41. The ultraviolet irradiation device 42 may be configured similarly to the ultraviolet irradiation device 32. The bare optical fiber 2, which has the primary layer 3 coated with the ultraviolet-curable resin of the secondary layer 4, enters the ultraviolet irradiation device 42, and ultraviolet light is irradiated onto the ultraviolet-curable resin of the secondary layer 4. As a result, the ultraviolet-curable resin of the secondary layer 4 is cured, and the secondary layer 4 is formed. The bare optical fiber 2 is coated with the primary layer 3 and the secondary layer 4, thereby forming the optical fiber core 1.

[0033] The resin applicator 31 may be configured to separately hold the UV-curable resin for the primary layer 3 and the UV-curable resin for the secondary layer 4. In this case, the resin applicator 31 applies the UV-curable resin for the primary layer 3 to the bare optical fiber 2, and then applies the UV-curable resin for the secondary layer 4 around the UV-curable resin for the primary layer 3. Furthermore, in this case, the UV irradiation device 32 irradiates the UV-curable resin for the primary layer 3 and the UV-curable resin for the secondary layer 4 that have been applied to the bare optical fiber 2 with UV light. This forms the primary layer 3 and the secondary layer 4. In this case, the manufacturing apparatus 10 does not necessarily need to include the secondary layer coating device 40.

[0034] A guide roller 50 and a winding device 60 are provided below the secondary layer coating device 40. The manufactured optical fiber 1 is guided by the guide roller 50 and wound around the winding device 60.

[0035] 3 is a flowchart of the method for manufacturing the optical fiber 1 according to this embodiment. First, the optical fiber preform BM is placed in the manufacturing apparatus 10 (step S101).

[0036] Next, the heater 21 provided in the heating device 20 heats the optical fiber preform BM, and starts drawing the bare optical fiber 2 (step S102).

[0037] The primary layer coating device 30 applies an ultraviolet curable resin for the primary layer 3 to the periphery of the drawn bare optical fiber 2, and irradiates the ultraviolet curable resin for the primary layer 3 with ultraviolet light to form the primary layer 3 (step S103).

[0038] Next, the secondary layer coating device 40 applies an ultraviolet curable resin for the secondary layer 4 to the periphery of the primary layer 3, and irradiates the ultraviolet curable resin for the secondary layer 4 with ultraviolet light to form the secondary layer 4 (step S104). This results in the optical fiber 1. The manufactured optical fiber 1 is wound up by the winding device 60.

[0039] It is not necessary to irradiate the primary layer 3 with ultraviolet light in the step (step S103) of forming the primary layer 3. In this case, the primary layer 3 may be cured in the step (step S104) of forming the secondary layer 4.

[0040] 4 is a schematic diagram of a measuring device for measuring delamination of the optical fiber 1 according to this embodiment. The measuring device 70 includes a stage 71, a rod 72, a load unit 73, and a drive unit 74.

[0041] The stage 71 is in the form of a long plate and is set horizontally relative to the ground surface. The stage 71 is made of a material such as metal or glass. The optical fiber 1 is placed on the upper surface of the stage 71 along the longitudinal direction of the stage 71. The optical fiber 1 is cut into short pieces and fixed on the stage 71.

[0042] The rod 72 has a cylindrical shape and is provided on the optical fiber 1. The rod 72 is perpendicular to the longitudinal direction of the optical fiber 1 and is provided parallel to the upper surface of the stage 71.

[0043] The load unit 73 is spaced apart from the stage 71 and is provided on the rod 72. The weight of the load unit 73 may be changed. The load unit 73, together with the rod 72, can apply a load in the vertical direction to the surface of the optical fiber core 1. The drive unit 74 is provided to hold the load unit 73. The drive unit 74 is configured using any drive method, such as hydraulic or electric.

[0044] When the driving unit 74 releases the load unit 73, the rod 72 and the load unit 73 move toward the stage 71 due to their own weight. The stage 71 and the rod 72 sandwich the optical fiber 1, thereby starting to apply a load to the optical fiber 1. After a predetermined time has elapsed, the driving unit 74 moves the rod 72 and the load unit 73 in a direction away from the optical fiber 1, and the application of the load to the optical fiber 1 ends.

[0045] The start and end of applying a load to the optical fiber 1 may be manually controlled by the user, or may be automatically performed by providing a control unit. The measuring device 70 is not limited to the device configuration shown in Fig. 4, as long as it applies a predetermined load to one point on the optical fiber 1 in the longitudinal direction for a predetermined time.

[0046] D50 is an index that indicates the likelihood of delamination. D50 represents the load at which delamination occurs 50% of the time when a load is applied to the optical fiber 1 in a direction perpendicular to the longitudinal direction of the optical fiber 1. The lower the D50, the smaller the load required to cause delamination, i.e., the more likely delamination is to occur. A method for measuring D50 using the measuring device 70 will be described in detail below.

[0047] First, the optical fiber core 1 to be measured is fixed on the stage 71. Then, a predetermined load is applied at a certain point on the optical fiber core 1 for 4.5 seconds, and this load is then applied to 30 points at intervals of approximately 6 mm, and this process is repeated. After that, the presence or absence of delamination at each point is observed using an optical microscope. The load is then increased, and each time, the load is applied and observed at 30 points different from the previous observation. This allows a plot of the magnitude of the load and the number of delaminations to be created. Note that the number of points to which the load is applied is not limited to 30.

[0048] The lowest load at which delamination is observed at 50% or more of the total number of points where the load is applied is defined as D50. In other words, the lowest load at which delamination is observed at 50% (15) or more of the 30 points where the load is applied is defined as D50. This measurement method is described in Charles Aloisio et al., "Optical Fiber Coating Delamination Using Model Coating Materials," Proceedings of the 51st IWCS, 2002, pp. 738-747.

[0049] 5 is a schematic diagram of a load sensor including an optical fiber core according to this embodiment. The load sensor 80 includes an optical fiber core 1, a light source 81, an optical circulator 82, and a light receiver 83.

[0050] The load sensor 80 is used, for example, as an impact sensor that detects abnormalities during installation of the optical fiber 1, a weight sensor that detects the intrusion of people or vehicles into restricted areas, and a sensor that detects natural disasters such as landslides and earthquakes. The load sensor 80 can measure the transmission loss of the optical fiber 1 by, for example, an optical time-domain reflectometer (OTDR), and detect the application of a load to the optical fiber 1.

[0051] The light source 81 is connected to the optical fiber 1. The light source 81 may be a laser diode. The light source 81 outputs inspection light to the optical fiber 1. The inspection light may be continuous light or pulsed light output at predetermined time intervals.

[0052] The optical circulator 82 may include a polarizing beam splitter. The optical circulator 82 has an input port, an input / output port, and an output port. The input port is connected to the light source 81, the input / output port is connected to the optical fiber core 1, and the output port is connected to the optical receiver 83. The optical receiver 83 may be a photodiode. The optical receiver 83 converts the input light into an electrical signal.

[0053] Inspection light emitted from a light source 81 enters the optical fiber 1 via an optical circulator 82. The inspection light generates backscattered light at various positions in the optical fiber 1, and the backscattered light travels in the opposite direction to the direction of transmission of the inspection light. The backscattered light enters a photoreceiver 83 via the optical circulator 82. The photoreceiver 83 measures the intensity of the backscattered light and acquires the time waveform of the backscattered light.

[0054] When a load is applied to the optical fiber 1, delamination occurs, which may result in a transmission loss of the backscattered light. Therefore, the load sensor 80 can detect whether a load that causes delamination has been applied to the optical fiber 1 by detecting the transmission loss of the backscattered light. In addition, the position where the load has been applied can be identified by measuring the transmission distance of the backscattered light from the time waveform of the backscattered light.

[0055] When the optical fiber 1 is used as a low-load sensor, it is preferable that the load at which delamination occurs be low. However, if the load at which delamination can occur is low, the adhesion between the bare optical fiber 2 and the primary layer 3 is weakened, and delamination may occur due to factors other than external forces. For example, if the optical fiber 1 is placed under high-temperature and high-humidity conditions, delamination may occur due to the formation of water bubbles between the bare optical fiber 2 and the primary layer 3. This may cause delamination even when no load is applied to the optical fiber 1, resulting in erroneous detection by the load sensor 80.

[0056] In this embodiment, even when the load at which delamination may occur is low, an optical fiber 1 having high water resistance can be realized. The load at which delamination may occur and the water resistance can be changed by the elastic modulus of the primary layer 3, the UV-curable resin of the primary layer 3, the viscoelasticity, and the additives added to the primary layer 3. By appropriately changing these, the load at which delamination may occur and the water resistance can be changed.

[0057] The length of the delamination may vary depending on the cause of the delamination. When delamination is caused by an external force, it generally occurs only in the area where the external force is applied. On the other hand, when the adhesion between the bare optical fiber 2 and the primary layer 3 is weak, partial delamination caused by an external force or natural factors such as temperature or humidity may grow to a large length. In view of these factors, it is more preferable that the delamination occurs due to an external force and does not easily grow. As a judgment indicator, it is desirable that the length of the delamination caused by the above-mentioned delamination measurement method is 50 μm or more and 1000 μm or less.

[0058] The results of experiments on the optical fiber core wire according to the embodiment of the present invention will be described below.

[0059] [Table 1]

[0060] Table 1 shows the pull-out ratio of the optical fiber core, the elastic modulus (MPa) of the primary layer, D50 (g), the load sensor test evaluation (Evaluation 1), and the water resistance evaluation (Evaluation 2).

[0061] The measurement method for the "pullout ratio" in Table 1 is described below. First, an approximately 15 cm sample of optical fiber is prepared, and the primary and secondary layers are removed from one end, leaving a 1 cm coating length, to expose the bare optical fiber. Next, the outer periphery of the coating is fixed to an auxiliary jig with adhesive or other means, and the bare optical fiber is pulled longitudinally at a specified speed. The tension at this time is measured, and the maximum tension until the glass portion is pulled out of the coating is defined as the pullout force. Here, the pullout force when the glass portion is pulled at a speed of 50 mm / min is defined as the high-speed pullout force, and the pullout force when the glass portion is pulled at a speed of 0.5 mm / min is defined as the low-speed pullout force. The pullout ratio represents the speed dependency of the pullout force on the pulling speed. A higher pullout ratio indicates a smaller speed dependency of the pullout force and a stronger adhesive strength between the primary layer and the bare optical fiber.

[0062] The evaluation method for "Evaluation 1" in Table 1 will be explained below. An optical fiber sample was prepared by cutting a portion of the optical fiber. The longitudinal length of the optical fiber sample is preferably 2 m or more. The optical fiber sample was placed on a glass plate, and a stainless steel plate was used to apply a load of 80 N to the 5 cm length of the optical fiber sample for 5 seconds. This operation was performed at 20 different locations along the longitudinal direction of the optical fiber sample. The increase in transmission loss of the optical fiber sample was measured before and after this series of processes. If the increase in transmission loss meets the standard (0.1 dB or more), Evaluation 1 is judged as good (OK); if the transmission loss does not meet the standard, Evaluation 1 is judged as poor (NG).

[0063] The evaluation method for "Evaluation 2" in Table 1 is explained below. The optical fiber core was immersed in 60°C warm water for 200 days. After that, approximately 10 cm of the optical fiber core was cut out longitudinally. The cut optical fiber core was observed at three locations (3 cm, 6 cm, and 9 cm) approximately 3 cm apart from one end. Furthermore, for each of the three locations, the optical fiber core was rotated 90 degrees circumferentially around the cross section and observed. The presence or absence of delamination was measured at these six locations. If delamination did not occur at any of the six locations, Evaluation 2 was judged to be good (OK); if delamination occurred at one or more locations, Evaluation 2 was judged to be poor (NG).

[0064] In Example 1, the pullout ratio was 0.40. The elastic modulus of the primary layer was 0.30 MPa. D50 was 330 g. The increase in transmission loss in the load sensor test was 0.1 dB or more, and Evaluation 1 was good (OK). Furthermore, after the optical fiber core was immersed in hot water at 60°C for 200 days, no delamination occurred, and Evaluation 2 was good (OK).

[0065] In Example 2, the pullout ratio was 0.45. The elastic modulus of the primary layer was 0.18 MPa. D50 was 250 g. The increase in transmission loss in the load sensor test was 0.1 dB or more, and Evaluation 1 was good (OK). Furthermore, after the optical fiber was immersed in 60°C hot water for 200 days, no delamination occurred, and Evaluation 2 was good (OK).

[0066] In Example 3, the pullout ratio was 0.42. The elastic modulus of the primary layer was 0.18 MPa. D50 was 200 g. The increase in transmission loss in the load sensor test was 0.1 dB or more, and Evaluation 1 was good (OK). Furthermore, after the optical fiber was immersed in hot water at 60°C for 200 days, no delamination occurred, and Evaluation 2 was good (OK).

[0067] In Comparative Example 1, the pullout ratio was 0.31. The elastic modulus of the primary layer was 0.45 MPa. D50 was 770 g. The increase in transmission loss in the load sensor test was less than 0.1 dB, and Evaluation 1 was poor (NG). Furthermore, after the optical fiber was immersed in 60°C hot water for 200 days, no delamination occurred, and Evaluation 2 was good (OK).

[0068] In Comparative Example 2, the pullout ratio was 0.28. The elastic modulus of the primary layer was 0.45 MPa. D50 was 650 g. The increase in transmission loss in the load sensor test was less than 0.1 dB, and Evaluation 1 was poor (NG). Furthermore, after the optical fiber was immersed in 60°C hot water for 200 days, no delamination occurred, and Evaluation 2 was good (OK).

[0069] In Comparative Example 3, the pullout ratio was 0.34. The elastic modulus of the primary layer was 0.25 MPa. D50 was 650 g. The increase in transmission loss in the load sensor test was less than 0.1 dB, and Evaluation 1 was poor (NG). Furthermore, after the optical fiber was immersed in 60°C hot water for 200 days, no delamination occurred, and Evaluation 2 was good (OK).

[0070] In Comparative Example 4, the pullout ratio was 0.27. The elastic modulus of the primary layer was 0.50 MPa. D50 was 200 g. The increase in transmission loss in the load sensor test was 0.1 dB or more, and Evaluation 1 was good (OK). Furthermore, after the optical fiber was immersed in 60°C hot water for 200 days, delamination occurred, and Evaluation 2 was poor (NG).

[0071] As described above, according to the present invention, an optical fiber core having high water resistance can be realized even when the load at which delamination may occur is low.

[0072] The pullout ratio is preferably 0.40 or more. When the pullout ratio is 0.40 or more, the adhesive strength between the bare optical fiber and the primary layer is sufficiently large, and the water resistance of the coated optical fiber can be sufficiently ensured.

[0073] The elastic modulus of the primary layer 3 is preferably 0.1 MPa or more and 0.3 MPa or less. If the elastic modulus of the primary layer 3 is more than 0.3 MPa, the primary layer 3 has a large restoring force against a load, making delamination less likely to occur.

[0074] Furthermore, the sum of the thicknesses of the primary layer 3 and the secondary layer 4 is preferably 25 μm or more and 90 μm or less. If the sum of the thicknesses of the primary layer 3 and the secondary layer 4 is less than 25 μm, the optical fiber 1 may break when a load is applied to the optical fiber 1.

[0075] The present invention is not limited to the above-described embodiments and can be modified in various ways. For example, an example in which a part of the configuration of one embodiment is added to another embodiment, or an example in which a part of the configuration of another embodiment is replaced with another embodiment, is also an embodiment of the present invention. Furthermore, with respect to parts not specifically explained or illustrated in the embodiments, well-known or publicly known techniques in the relevant technical field can be applied as appropriate. [Explanation of symbols]

[0076] 1 Optical fiber core 2 Bare optical fiber 3 Primary Layer 4 Secondary Tier

Claims

1. An optical fiber core wire comprising: a bare optical fiber; a primary layer formed of a first ultraviolet-curable resin covering the bare optical fiber; and a secondary layer formed of a second ultraviolet-curable resin covering the primary layer, When a load is applied to the optical fiber in a direction perpendicular to the longitudinal direction of the optical fiber, the load at which delamination occurs at a rate of 50% is 330 g or less; The optical fiber core is characterized in that no delamination occurs when the optical fiber core is immersed in hot water at 60°C for 200 days.

2. 2. The optical fiber according to claim 1, wherein the length of the delamination caused by the load is 50 μm or more and 1000 μm or less.

3. 2. The optical fiber according to claim 1, wherein the pullout ratio is 0.40 or more.

4. 2. The optical fiber according to claim 1, wherein the primary layer has an elastic modulus of 0.30 MPa or less.

5. 2. The optical fiber according to claim 1, wherein the sum of the thickness of the primary layer and the thickness of the secondary layer is 25 [mu]m or more.

6. A step of drawing a bare optical fiber from an optical fiber preform; a step of applying a first ultraviolet curing resin to form a primary layer around the bare optical fiber to form a primary layer; and applying a second ultraviolet curable resin around the primary layer to form a secondary layer, When a load is applied to the optical fiber in a direction perpendicular to the longitudinal direction of the optical fiber, the load at which delamination occurs at a rate of 50% is 330 g or less; A method for manufacturing an optical fiber, characterized in that no delamination occurs when the optical fiber is immersed in hot water at 60°C for 200 days.

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