JP2007197273A - Optical fiber strand and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a production method of efficiently producing a low loss optical fiber strand. <P>SOLUTION: In the method for producing an optical fiber strand, an optical fiber preform is heated and melted by a heating furnace, a bare optical fiber is drawn , the obtained bare optical fiber is annealed, the bare optical fiber is cooled to a temperature capable of resin coating, a coating liquid is applied to the circumference of the bare optical fiber, so as to be cured, and the obtained optical fiber strand is wound up, upon the annealing of the bare optical fiber, an annealing furnace is used, He gas or He-containing gas is used as atmospheric gas in the annealing furnace, and also, the temperature in the annealing furnace is set to 1,200 to 1,500°C, thus the optical fiber having a residual stress distribution where the residual stress of cladding in the vicinity of the core is a compressive stress, and the residual stress of the core is any of the compressive stress of the cladding or below, the residual stress of zero or tensile stress can be obtained. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a low-loss optical fiber and a manufacturing method thereof, and in particular, is applied to a manufacturing method that performs slow cooling to draw a low-loss optical fiber. In the manufacturing method of the present invention, even if slow cooling spinning is performed, the temperature distribution in the radial direction of the optical fiber becomes uniform, so there is little change in residual stress in the optical fiber glass, and there is little structural irregular loss component. Can be obtained. Furthermore, the structure relaxation of the optical fiber is advanced by slow cooling, the fictive temperature is lowered, and as a result, the Rayleigh scattering loss component is also reduced, so that an optical fiber strand having a small transmission loss as a whole is obtained, and a long distance transmission fiber. As a result, a suitable optical fiber can be obtained.

  In recent years, studies on reducing the loss of an optical fiber have been actively conducted. As a method for reducing the loss, mainly in the cooling process after heating and melting the optical fiber preform in the drawing process of the optical fiber strand, the bare optical fiber drawn from the preform is gradually cooled to reduce the optical fiber loss. Among the components, a method of reducing the total loss by reducing the Rayleigh scattering loss is generally performed.

On the other hand, in the drawing process, the drawing speed tends to increase from the viewpoint of improving productivity. When spinning at such a high speed, in order to coat the bare optical fiber in a spinning tower (building) with a limited height, the cooling tube is brought to a temperature that allows the bare optical fiber to be coated. Need to be quickly cooled. However, if the optical fiber temperature is high and the rapid cooling starts, the optical fiber transmission loss deteriorates.
Therefore, in order to reduce transmission loss and ensure productivity, optimization of the optical fiber bare wire slow cooling method is mainly performed in the drawing process of the optical fiber preform. The technique described in 4 is proposed.

In the method disclosed in Patent Document 1, a slow cooling furnace is installed in the spinning cooling process, and the slow cooling temperature is in the range of 500 ° C to 1500 ° C. Further, the annealing furnace rush fiber temperature is set to 500 ° C. to 1500 ° C., and the annealing time is set to 0.1 to 10 seconds.
In the method disclosed in Patent Document 2, the section in which the temperature difference is 50 ° C. or higher among the portions where the optical fiber temperature is 1300 to 1700 ° C. is gradually cooled at 1000 ° C./second or lower.
In the method disclosed in Patent Document 3, the lowest cooling temperature of the portion where the outer diameter of the melt-deformed portion is 1 mm or less and the portion where the deformation is substantially finished is set to 4000 ° C./second or less.
In the method disclosed in Patent Document 4, in order to enhance the slow cooling effect, a gap is provided between the heating furnace and the slow cooling furnace, the heating furnace atmosphere containing He and the slow cooling furnace atmosphere are distinguished, and as the slow cooling furnace gas, N ₂ , Ar, Either one of air is allowed to flow, and the optical fiber inlet temperature is set to 1400 to 1800 ° C.
JP-A-4-59631 JP 2000-335933 A JP 2003-335545 A JP 2000-335935 A

However, in the prior art as described above, as a result of intensive studies by the present inventors, it has been found that the following problems occur.
If the annealing temperature is increased and the annealing time is lengthened for the purpose of further reducing transmission loss, the Rayleigh scattering loss component is reduced, but the atmosphere in the annealing furnace is reduced in N ₂ , Ar, air with a poor convective heat transfer coefficient. Etc., heat accumulates in the optical fiber, and the difference in temperature distribution in the radial direction of the optical fiber becomes large and non-uniform, thereby changing the residual stress distribution in the optical fiber, and as a result As a result, there occurs a phenomenon in which structural loss, which is another loss component, increases.

  FIG. 2 shows the residual stress distribution of an optical fiber strand annealed slowly in an Ar atmosphere, and FIG. 3 shows the residual stress distribution of an optical fiber strand manufactured without ordinary slow cooling. As shown in FIG. 2, when the glass fiber is gradually cooled in an Ar atmosphere, the residual stress of the optical fiber is shifted to the tensile stress side as a whole, the core portion tensile stress is large, and the innermost cladding of the core outer peripheral portion is also pulled. This is stress, and the change in stress from the center to the outer periphery of the outer cladding is very large. When compared with the residual stress distribution of the optical fiber manufactured without the usual slow cooling shown in FIG. 3, there is a clear change. There is also a correlation between the residual stress and the structural fraud loss, and it is assumed that the structural fraud loss is increased due to the residual stress.

  From the above, for the purpose of reducing Rayleigh scattering, in the cooling process after heating and melting of the optical fiber in the drawing process with the method shown in the prior art, depending on the conditions, the radial direction of the optical fiber As a result, the temperature distribution of the optical fiber becomes non-uniform, the residual stress increases, and the structural fraud loss increases. Therefore, it is impossible to stably obtain an optical fiber with reduced loss as intended.

  This invention is made | formed in view of the said situation, and aims at provision of the manufacturing method which can manufacture a low-loss optical fiber strand efficiently.

  In order to achieve the above object, the present invention provides an optical fiber preform that is heated and melted in a heating furnace, then the bare optical fiber is drawn out, and then the obtained bare optical fiber is gradually cooled, and then the bare optical fiber In a method for manufacturing an optical fiber, the optical fiber is wound around the bare optical fiber, and then the coating liquid is applied and cured around the bare optical fiber. By using a slow cooling furnace when slowly cooling, using He gas or He-containing gas as the atmosphere gas in the slow cooling furnace, and setting the inside of the slow cooling furnace to a temperature of 1200 ° C. to 1500 ° C., the vicinity of the core The residual stress of the clad is the compressive stress, and the residual stress of the core is one of the compressive stress, residual stress zero, or tensile stress smaller than the clad. It provides a method for manufacturing an optical fiber, characterized in that to obtain an optical fiber having.

  In the method for manufacturing an optical fiber according to the present invention, it is preferable that the temperature of the slow cooling furnace is provided with a temperature gradient in the entire region or a partial temperature range within a range of 1200 ° C to 1500 ° C.

  The present invention also provides a core, at least one clad surrounding the core, and at least one synthetic resin provided on the outer periphery of the core, manufactured by the above-described method for manufacturing an optical fiber according to the present invention. An optical fiber comprising a coating layer comprising: a clad residual stress in the vicinity of the core is a compressive stress, and the core residual stress is any of a compressive stress, a residual stress of zero, or a tensile stress smaller than the clad. An optical fiber having a residual stress distribution is provided.

  In the optical fiber of the present invention, the transmission loss is preferably 0.180 dB / km or less.

  In the optical fiber of the present invention, the Rayleigh scattering loss is preferably 0.165 dB / km or less.

In the method for manufacturing an optical fiber according to the present invention, the convective heat transfer coefficient between the bare optical fiber, the gas atmosphere in the slow cooling furnace, and the slow cooling furnace heater is improved by containing He gas in the optical fiber slow cooling furnace. Since the temperature can be changed to a desired slow cooling furnace temperature (1200 ° C to 1500 ° C) with good responsiveness, it can be gradually cooled in a temperature range (1200 ° C to 1500 ° C) that is efficient in reducing Rayleigh scattering. , Rayleigh scattering can be reduced. Furthermore, the temperature distribution in the radial direction of the optical fiber tends to be uniform.
Further, in the present invention, by providing a gradient in the slow cooling furnace temperature, it is possible to prevent the virtual temperature from staying high and to lower the Rayleigh scattering more efficiently.
Further, in the present invention, by containing He in the optical fiber annealing furnace, the convective heat transfer coefficient between the optical fiber bare wire, the gas atmosphere in the annealing furnace, and the annealing furnace heater is improved. Since the temperature distribution tends to be uniform and the change in the residual stress distribution is small, it is possible to obtain a low-loss optical fiber that does not increase the structural loss.
Further, in the present invention, since it is manufactured by slowly cooling efficiently, a low-loss optical fiber with low Rayleigh scattering can be obtained.
In the present invention, since the residual stress distribution in the radial direction of the optical fiber is optimized, it is possible to obtain a low-loss optical fiber with low structural loss.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a configuration diagram showing an example of an optical fiber manufacturing apparatus for carrying out the optical fiber manufacturing method of the present invention. In FIG. 1, 10 is an optical fiber preform, 11 is a spinning furnace, 12 is an optical fiber bare wire, 13 is a slow cooling furnace, 14 is a cooling cylinder, 15 is a coater, 16 is a bridging cylinder, 17 is an optical fiber, 18 Is a turn pulley, 19 is a take-up portion, 20 is a dancer roll, and 21 is a take-up bobbin.

  In the method of manufacturing an optical fiber according to the present invention, an optical fiber preform 10 is set in a spinning furnace 11, one end side thereof is heated and melted with a heater, and then an optical fiber bare wire 12 is drawn out. The bare fiber 12 is gradually cooled while passing through the slow cooling furnace 13, and then the optical fiber bare wire 12 is cooled to a temperature capable of resin coating while passing through the cooling cylinder 14, and then the cooled optical fiber The bare wire 12 is introduced into the coater 15 to apply the coating liquid, then this is introduced into the cross-linking cylinder 16 to cure the coating liquid, and a coating layer made of a synthetic resin is formed on the outer periphery of the optical fiber bare wire 12. Thus, the optical fiber 17 is manufactured. The obtained optical fiber 17 is moved through a turn pulley 18, a take-up portion 19, and a dancer roll 20, and is taken up on a take-up bobbin 21.

  In the manufacturing method of the present invention, the optical fiber preform 10 is made of quartz glass, and various optical fiber preforms for manufacturing optical fibers having a core and a clad surrounding the core can be used. The various optical fibers are not particularly limited, and examples thereof include a single mode optical fiber, a multimode optical fiber, a rare earth doped optical fiber, a rare earth doped double clad fiber, and a holey fiber.

  In the manufacturing method of the present invention, the material of the coating layer provided on the outer periphery of the bare optical fiber 12 can be appropriately selected and used from conventionally known various synthetic resin materials, such as an ultraviolet curable resin, A thermosetting resin or the like is preferably used. Further, the cross-linking cylinder 16 for effecting the coating liquid applied to the outer periphery of the bare optical fiber 12 is changed according to the resin used for the coating layer, and when the ultraviolet curable resin is used, an ultraviolet lamp is incorporated. A cross-linking cylinder 16 that irradiates the coating resin with ultraviolet rays is used. When the thermosetting resin is used, a cross-linking cylinder 16 that incorporates a heater and heat cures the coating resin is used. The coating layer provided on the outer periphery of the bare optical fiber 12 is not limited to one layer, and two or more coating layers made of different resins may be provided.

  The manufacturing method of the present invention uses a slow cooling furnace 13 when gradually cooling the bare optical fiber 12, uses He gas or He-containing gas as the atmospheric gas in the slow cooling furnace 13, and the inside of the slow cooling furnace 13 is 1200 ° C. to 1500 ° C. By performing slow cooling at the temperature of the core, the residual stress of the cladding in the vicinity of the core is a compressive stress, and the residual stress of the core is one of the compressive stress, zero residual stress, or tensile stress smaller than the cladding. An optical fiber 17 having a residual stress distribution is obtained.

  Since the He gas has a thermal conductivity that is 10 times greater than that of the argon (Ar) gas, the responsiveness of heat transfer between the optical fiber and the gas and between the gas and the annealing furnace heater is improved compared to the argon gas. That is, the heat generated from the heater of the slow cooling furnace 13 is transferred to the atmospheric gas, and the warmed gas is transferred to the fiber. On the contrary, the heat of the fiber is quickly transferred to the atmospheric gas, and is transferred from the gas to the inner wall of the annealing furnace. As a result, the difference in temperature distribution in the radial direction of the optical fiber (for example, the temperature between the center and the outer periphery of the optical fiber) is reduced. As a result, the difference in viscosity of the glass is reduced, the drawing tension is uniformly applied, and the residual stress is compared. The structural loss will not increase if the product is not slowly cooled.

  However, since the responsiveness to heat is improved when He gas is introduced, the temperature of the optical fiber changes rapidly to the same temperature as the annealing furnace temperature. That is, in order to obtain the slow cooling effect performed for reducing the loss, it is necessary to match the slow cooling temperature to a temperature range in which the structural relaxation of the optical fiber occurs in a sufficiently short time. This temperature is desirably in the range of 1200 ° C to 1500 ° C. As a result, structural relaxation of the optical fiber occurs, the fictive temperature decreases, and as a result, the Rayleigh scattering loss decreases.

  Furthermore, it is desirable to provide a temperature gradient within the temperature range for the annealing furnace temperature. This is because the fictive temperature decreases in a short time at high temperatures, but this time becomes longer at low temperatures, but by slow cooling at a temperature corresponding to the fictive temperature of the fiber, it can be reduced in the shortest time. In addition to being able to lower the virtual temperature, the virtual temperature can be prevented from staying high at high temperatures. When the temperature of the slow cooling furnace is constant, if the set temperature is high, the rate of reduction of Rayleigh scattering is fast, but only the structural relaxation occurs according to the set temperature, the fictive temperature stays high, and the Rayleigh scattering is sufficient. It does not fall. On the other hand, at a low temperature, it takes time to relax the structure as described above, and it takes time to lower the virtual temperature. Therefore, by providing a temperature gradient to the slow cooling furnace temperature, it is possible to prevent the virtual temperature from decreasing at a high rate while maintaining the virtual temperature decreasing rate, and therefore, it is possible to efficiently reduce Rayleigh scattering.

On the other hand, in the temperature range where the optical fiber temperature is 1500 ° C. or more, the viscosity of the bare optical fiber 12 is low, and the fiber is stretched violently in the slow cooling process.
Furthermore, in the temperature range where the optical fiber temperature is 1200 ° C. or lower, the time required for lowering the Rayleigh scattering becomes longer, the drawing speed needs to be extremely slowed, and the productivity deteriorates, which is not practical. Rayleigh scattering depends on a fictive temperature that depends on the optical fiber annealing process. The fictive temperature depends on the structure relaxation time, and the structure relaxation is fast at high temperatures and slow at low temperatures.

The optical fiber 17 manufactured by the above manufacturing method has the following characteristics.
The obtained optical fiber 17 is composed of a core, at least one cladding layer surrounding the core, and one or more coating layers provided on the outer periphery thereof, and the residual stress in the optical axis direction in the outermost cladding layer near the core. The radial distribution of is a compressive stress in almost the entire radial direction (excluding the layer boundary), and the residual stress of the core is smaller than that of the innermost clad or reversed to the tensile stress side. is doing. With such a residual stress distribution, the stress distribution is the same in the vicinity of the core as in the case of the conventional drawing, so that the optical fiber 17 does not increase in structural fraud loss.

  As described above, since the optical fiber manufactured by this manufacturing method is gradually cooled, the Rayleigh scattering loss is low, and further, the temperature distribution in the radial direction of the optical fiber at the time of slow cooling is uniform. Since the distribution becomes relatively uniform and the structural irregularity loss does not increase, as a result, the optical fiber 17 having a loss of 0.180 dB / km or less, particularly a Rayleigh scattering loss of 0.165 dB / km or less can be obtained. .

[Example 1]
The optical fiber preform was heated and melted, and then, the drawn optical fiber bare wire was gradually cooled by a slow cooling furnace, and the atmosphere in the slow cooling furnace was He gas, and the slow cooling furnace temperature was fixed at 1500 ° C. The Rayleigh scattering loss and structural loss of the obtained optical fiber at a wavelength of 1.55 μm were 0.160 dB / km and 0.01 dB / km, respectively, and the total loss was 0.179 dB / km. It was. Also, the residual stress distribution in the radial direction of the obtained optical fiber is shown in the graph of Example 1 in FIG. From the graph of Example 1 in FIG. 4, the residual stress of the cladding near the core is a compressive stress, and the residual stress of the core is shifted to the low compressive stress side compared to the residual stress in the vicinity of the core (approximately zero stress). The difference was 10 MPa or less.

[Example 2]
The optical fiber preform was heated and melted, and then, the drawn optical fiber bare wire was gradually cooled by a slow cooling furnace, and the atmosphere in the slow cooling furnace was He gas, and the slow cooling furnace temperature was kept constant at 1200 ° C. The obtained optical fiber had a Rayleigh scattering loss at a wavelength of 1.55 μm and a structural irregularity loss of 0.161 dB / km and 0.01 dB / km, respectively, and the total loss was 0.180 dB / km. It was. Also, the residual stress distribution in the radial direction of the obtained optical fiber is shown in the graph of Example 2 in FIG. From the graph of Example 2 in FIG. 4, the residual stress of the cladding near the core is a compressive stress, and the residual stress of the core is a tensile stress, and the difference is 20 MPa or less.

[Example 3]
The optical fiber preform is heated and melted, and then the drawn optical fiber bare wire is gradually cooled by a slow cooling furnace, the atmosphere in the slow cooling furnace is a mixed gas of He gas 50% and Ar gas 50%, and the slow cooling furnace temperature is 1300 ° C. Draw as constant. The Rayleigh scattering loss and structural loss of the obtained optical fiber at a wavelength of 1.55 μm were 0.160 dB / km and 0.01 dB / km, respectively, and the total loss was 0.179 dB / km. It was. Also, the residual stress distribution in the radial direction of the obtained optical fiber is shown in the graph of Example 3 in FIG. From the graph of Example 3 in FIG. 4, the residual stress of the cladding near the core is a compressive stress, and the residual stress of the core is a tensile stress, and the difference is 20 MPa or less.

[Example 4]
The optical fiber preform is heated and melted, and then the drawn optical fiber bare wire is gradually cooled by a slow cooling furnace, the atmosphere in the slow cooling furnace is He gas, and the gradient of the slow cooling furnace temperature is in the range of 1200 ° C to 1500 ° C. I drew it. The obtained optical fiber had a Rayleigh scattering loss at a wavelength of 1.55 μm and a structural irregularity loss of 0.156 dB / km and 0.01 dB / km, respectively, and the total loss was 0.175 dB / km. It was. Also, the residual stress distribution in the radial direction of the obtained optical fiber is shown in the graph of Example 4 in FIG. From the graph of Example 4 in FIG. 5, the residual stress of the cladding near the core is a compressive stress, the residual stress of the core is almost 0, and the difference is 10 MPa or less.

[Example 5]
The optical fiber preform is heated and melted, and then, the drawn optical fiber bare wire is gradually cooled by a slow cooling furnace, the atmosphere in the slow cooling furnace is He gas, and the gradient of the slow cooling furnace temperature is in the range of 1300 ° C to 1400 ° C. I drew it. The obtained optical fiber had a Rayleigh scattering loss and a structural irregularity loss at a wavelength of 1.55 μm of 0.158 dB / km and 0.01 dB / km, respectively, and the total loss was 0.177 dB / km. It was. Further, the residual stress distribution in the radial direction of the obtained optical fiber is shown in the graph of Example 5 in FIG. From the graph of Example 5 in FIG. 5, the residual stress of the cladding near the core is a compressive stress, and the residual stress of the core is almost 0, and the difference is 15 MPa or less.

[Comparative Example 1]
The optical fiber preform was heated and melted, and then the drawn optical fiber bare wire was drawn without being gradually cooled by a slow cooling furnace. The obtained optical fiber has a Rayleigh scattering loss at a wavelength of 1.55 μm and a structural irregularity loss of 0.170 dB / km and 0.01 dB / km, respectively, and the total loss was 0.188 dB / km. It was. Moreover, the residual stress distribution of the radial direction of the obtained optical fiber strand is shown in FIG. From the graph of FIG. 3, the residual stress of the cladding in the vicinity of the core is a compressive stress, and the residual stress of the core is almost 0 or slightly compressive stress, and the difference is 15 MPa or less. The reason why the tensile stress increases as the residual stress of the outer cladding increases toward the outer side is that the outer cladding is cooled from the outside because it is not gradually cooled.

[Comparative Example 2]
The optical fiber preform was heated and melted, and then, the drawn optical fiber bare wire was gradually cooled by a slow cooling furnace, and the atmosphere in the slow cooling furnace was Ar gas, and the slow cooling furnace temperature was kept constant at 1400 ° C. The obtained optical fiber had a Rayleigh scattering loss and a structural loss of 1.55 μm at a wavelength of 1.55 μm and a structural loss of 0.159 dB / km and 0.02 dB / km, respectively, and the total loss was 0.188 dB / km. It was. FIG. 2 shows the residual stress distribution in the radial direction of the obtained optical fiber. From the graph of FIG. 2, the residual stress of the cladding near the core is tensile stress, and the residual stress of the core is also tensile stress, and the difference is 20 MPa or less.

[Comparative Example 3]
The optical fiber preform was heated and melted, and then the drawn bare optical fiber was gradually cooled in a slow cooling furnace, and the atmosphere in the slow cooling furnace was set to He gas, and the annealing temperature was kept constant at 1600 ° C. Since the temperature was high, the bare optical fiber was drawn inside the slow cooling furnace, so that a stable outer diameter could not be maintained, and drawing was impossible.

[Comparative Example 4]
The optical fiber preform was heated and melted, and then, the drawn optical fiber bare wire was gradually cooled by a slow cooling furnace, and the atmosphere in the slow cooling furnace was He gas and the slow cooling furnace temperature was kept constant at 1100 ° C. The obtained optical fiber had a Rayleigh scattering loss and a structural loss of 1.55 μm at a wavelength of 1.55 μm, respectively, and were 0.169 dB / km and 0.01 dB / km, respectively, and the total loss was 0.188 dB / km. It was. The residual stress distribution in the radial direction of the obtained optical fiber is shown in FIG. From the graph of FIG. 6, the residual stress of the cladding near the core is a compressive stress, and the residual stress of the core is a tensile stress, and the difference is 20 MPa or less.
The above results are summarized in Table 1.

From the results shown in Table 1, when there is no slow cooling furnace at the time of drawing the optical fiber, the Rayleigh scattering loss does not decrease, so a low-loss optical fiber cannot be obtained.
Further, if the atmosphere in the slow cooling furnace is Ar gas, the residual stress waveform becomes abnormal, so that a low-loss optical fiber cannot be obtained. On the other hand, a mixed gas of Ar gas and He gas provides a low-loss optical fiber.
It can also be seen that when the annealing temperature is 1100 ° C. or less, Rayleigh scattering is not sufficiently reduced, and when the annealing temperature is 1600 ° C. or more, spinning becomes difficult, and a low-loss optical fiber cannot be obtained.
Further, in the presence or absence of the temperature gradient of the slow cooling furnace, the loss is 0.180 dB / km or less even if the temperature is constant within the predetermined temperature range, but a lower loss optical fiber can be obtained by providing the temperature gradient. I understand that.

It is a block diagram which shows an example of the optical fiber strand manufacturing apparatus for enforcing the manufacturing method of the optical fiber strand of this invention. It is a graph which shows the residual-stress distribution of the optical fiber strand obtained by the comparative example 2 which annealed slowly using the annealing furnace atmosphere as Ar gas. It is a graph which shows the residual stress distribution of the optical fiber strand obtained by the comparative example 1 performed without slow cooling. It is a graph which shows the residual stress distribution of the optical fiber strand obtained in Examples 1-3 which concern on this invention. It is a graph which shows the residual stress distribution of the optical fiber strand obtained in Example 4, 5 which concerns on this invention. It is a graph which shows the residual-stress distribution of the optical fiber strand obtained by the comparative example 4 which annealed slowly at the annealing furnace temperature at 1100 degreeC.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Optical fiber base material, 11 ... Spinning furnace, 12 ... Bare optical fiber, 13 ... Slow cooling furnace, 14 ... Cooling cylinder, 15 ... Coater, 16 ... Bridging cylinder, 17 ... Optical fiber strand, 18 ... Turn pulley, 19 ... take-up part, 20 ... dancer roll, 21 ... take-up bobbin.

Claims (5)

The optical fiber preform is heated and melted in a heating furnace, then the bare optical fiber is drawn out, and then the obtained bare optical fiber is gradually cooled, and then the bare optical fiber is cooled to a temperature capable of resin coating, In the method of manufacturing an optical fiber, the coating liquid is applied and cured around the bare optical fiber, and the obtained optical fiber is wound up.
A slow cooling furnace is used when the optical fiber bare wire is slowly cooled, He gas or a He-containing gas is used as the atmosphere gas in the slow cooling furnace, and the slow cooling furnace is set to a temperature of 1200 ° C to 1500 ° C to perform the slow cooling. By means of the above, an optical fiber having a residual stress distribution in which the residual stress of the clad in the vicinity of the core is a compressive stress, and the residual stress of the core is any one of the compressive stress, zero residual stress, or tensile stress smaller than the clad A method for manufacturing an optical fiber.   2. The method of manufacturing an optical fiber according to claim 1, wherein the temperature of the annealing furnace is provided with a temperature gradient in a whole range or a partial temperature range within a range of 1200 ° C. to 1500 ° C. 3. A coating comprising the core, at least one clad surrounding the core, and at least one synthetic resin provided on the outer periphery of the core, manufactured by the optical fiber manufacturing method according to claim 1 or 2. An optical fiber comprising a layer,
Light having a residual stress distribution in which the residual stress of the cladding in the vicinity of the core is a compressive stress, and the residual stress of the core is any of a compressive stress, a residual stress of zero, or a tensile stress smaller than the cladding. Fiber strand.   The optical fiber according to claim 3, wherein the transmission loss is 0.180 dB / km or less. The optical fiber strand according to claim 3 or 4, wherein the Rayleigh scattering loss is 0.165 dB / km or less.

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