Apparatus and method for manufacturing optical fibers

Abstract

The device for producing an optical fiber 10 includes a drawing furnace 11 that heats a glass base material 101 to draw an optical fiber 102 and an annealing furnace 12 that is capable of heat treating the optical fiber 102 drawn by the drawing furnace 11 by providing an ambient temperature Tz that decreases at a constant gradient toward downstream and controlling the gradient of the ambient temperature Tz. The fictive temperature Tf of the optical fiber 102 is lowered without extending the length of the annealing furnace 12 while maintaining the drawing speed of the optical fiber 102 to reduce transmission loss due to Rayleigh scattering.

Description

[Technical Field]

[0001] This disclosure relates to an apparatus and method for manufacturing optical fibers, and more particularly to an apparatus and method for manufacturing optical fibers with reduced loss achieved by controlling the virtual temperature of the optical fiber. [Background technology]

[0002] Conventionally, with the increasing data demand for optical communications, there has been a need to reduce transmission loss in optical fibers in order to increase the speed and capacity of optical communications. Rayleigh scattering, which is the main cause of transmission loss in optical fibers that serve as the transmission medium for optical communications, is known to be able to be reduced by lowering the virtual temperature at which the optical fiber solidifies from a glassy state. From this perspective, a technique has been studied in which a slow-cooling furnace is placed immediately after the drawing furnace that heat-draws the glass matrix, and the optical fiber is heat-treated at a predetermined temperature to reduce the virtual temperature (see Non-Patent Literature 1). The structural relaxation time of optical fibers, which is related to the virtual temperature of optical fibers, has also been studied (see Non-Patent Literature 2). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Kazumasa Ozono, et al., "Study of Optical Fibers with Reduced Rayleigh Scattering Loss," IEICE Technical Report, IEICE, May 2005, Vol. 105, No. 38, pp. 11-16. [Non-Patent Document 2] K. Saito et al., “Limit of the Rayleigh scattering loss in silica fiber,” Applied Physics Letters, American Institute of Physics, December 22, 2003, Vol. 83, No. 25, pp. 5175-5177. [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] In the aforementioned technique of placing an annealing furnace immediately after the drawing furnace, the optical fiber drawn by heating the glass base material to, for example, 2000°C in the drawing furnace is heat-treated at a predetermined temperature in the annealing furnace. However, at high temperatures of, for example, around 1500°C, the viscosity of the optical fiber is low, so the time required for structural relaxation is short, and the drawing speed can be maintained. However, the virtual temperature does not reach below the ambient temperature, so the transmission loss cannot be sufficiently reduced. Conversely, if the temperature for heat treatment in the annealing furnace is set to a low temperature of around 1300°C, the achievable virtual temperature is lower, but the viscosity of the optical fiber is high, so the time required for structural relaxation is longer, which necessitates either reducing the drawing speed or extending the length of the annealing furnace, resulting in decreased productivity.

[0005] This disclosure is proposed in view of the above circumstances and aims to provide an apparatus and method for manufacturing optical fibers that can reduce the virtual temperature of optical fibers without extending the length of the slow-cooling furnace, while ensuring the optical fiber drawing speed, by using a technique in which optical fibers drawn in a drawing furnace are heat-treated in a slow-cooling furnace in order to reduce the virtual temperature of the optical fibers. [Means for solving the problem]

[0006] To solve the above-mentioned problems, the optical fiber manufacturing apparatus according to this application includes a drawing furnace for heating a glass base material to draw optical fibers, and a slow-cooling furnace that can heat-treat the optical fibers drawn in the drawing furnace by providing an ambient temperature that decreases at a constant gradient downstream, and can control the gradient of the ambient temperature.

[0007] The method for manufacturing optical fibers according to this application includes the steps of heating a glass base material in a drawing furnace to draw optical fibers, and heat-treating the drawn optical fibers by providing an ambient temperature that decreases at a constant gradient downstream in an annealing furnace, thereby enabling control of the ambient temperature gradient. [Effects of the Invention]

[0008] According to this disclosure, by controlling the ambient temperature gradient in the slow-cooling furnace, it is possible to reduce the virtual temperature without extending the length of the slow-cooling furnace while ensuring the optical fiber drawing speed, thereby reducing transmission loss due to Rayleigh scattering. [Brief explanation of the drawing]

[0009] [Figure 1] This is a diagram showing the schematic configuration of an optical fiber manufacturing apparatus. [Figure 2] This figure shows the optical fiber manufacturing apparatus for Example 1. [Figure 3] This is a perspective view showing the elements of an optical fiber corresponding to a small time interval Δt. [Figure 4] This graph shows the distribution of virtual temperature at a line drawing speed of 20 m / min in Example 2. [Figure 5] This graph shows the distribution of virtual temperature at a line drawing speed of 40 m / min in Example 3. [Figure 6] This graph approximates the regions in the graphs of Examples 2 and 3 of Example 4 with triangles. [Figure 7] This graph shows the relationship between the optical fiber drawing speed and the minimum virtual temperature in Example 5. [Figure 8] This graph shows the distribution of virtual temperature at a line drawing speed of 60 m / min for Comparative Example 1. [Figure 9] This graph shows the time change of the virtual temperature at a fixed point on the optical fiber in Comparative Example 2. [Modes for carrying out the invention]

[0010] The embodiments of the optical fiber manufacturing apparatus and method will be described in detail below with reference to the drawings. Figure 1 is a diagram showing the schematic configuration of the optical fiber manufacturing apparatus of this embodiment.

[0011] In an optical fiber manufacturing apparatus 10, a slow cooling furnace 12 is provided directly below a drawing furnace 11. The drawing furnace 11 heats, for example, a silica-based glass preform 101, and draws an optical fiber 102 downward from the lower portion of the glass preform 101 at a predetermined speed. The drawing furnace 11 is filled with an inert gas such as helium (He) or argon (Ar), and is maintained at an ambient temperature of, for example, 2000°C by a heater (not shown).

[0012] The optical fiber 102 drawn from the glass preform 101 in the drawing furnace 11 passes through an opening 11a at the lower part of the drawing furnace 11 and proceeds into the slow cooling furnace 12 which is provided directly below in contact with the drawing furnace 11. The slow cooling furnace 12 has a longitudinal direction along the downward direction in which the optical fiber 102 travels, an upper opening 12a as the start end, and a lower opening 12b as the terminal end, and is controlled such that the ambient temperature decreases at a predetermined gradient in the downstream direction in which the optical fiber 102 travels along the longitudinal direction from the start end to the terminal end. The slow cooling furnace 12 is also filled with an inert gas such as helium (He) or argon (Ar), and a heater (not shown) provides an ambient temperature having a predetermined gradient. The ambient temperature in the slow cooling furnace 12 can be set, for example, in a range from 1600°C to 800°C, and the gradient of the ambient temperature may be, for example, a maximum temperature of 1600°C at the start end of the slow cooling furnace 12 and a minimum temperature of 800°C at the lower end of the terminal end.

[0013] In the present embodiment, in the slow cooling furnace 12 provided immediately after the drawing furnace 11, the optical fiber 102 is subjected to heat treatment by being provided with a graded ambient temperature that decreases as it travels downstream, and the gradient of the ambient temperature can be controlled. This makes it possible to appropriately control properties such as the viscosity and structural relaxation time of the optical fiber 102 in the slow cooling furnace 12, and to reduce the fictive temperature of the optical fiber 102 while maintaining the length of the drawing furnace 11 and ensuring the drawing speed of the optical fiber 102. Therefore, in the present embodiment, it is possible to manufacture the optical fiber 102 with reduced transmission loss due to Rayleigh scattering while ensuring productivity, without reducing the drawing speed of the optical fiber 102 or increasing the length of the slow cooling furnace 12.

Examples

[0014] Figure 2 is a diagram showing the optical fiber manufacturing apparatus 10 of Example 1. The optical fiber manufacturing apparatus 10 of Example 1 has the same structure as the optical fiber manufacturing apparatus 10 of the present embodiment shown in Figure 1, but the ambient temperature Tz in the optical fiber manufacturing apparatus 10 is specifically set.

[0015] In the drawing furnace 11, let L be the length from the neck-down end point 102a, where the glass preform 101 is drawn to form the optical fiber 102 with a predetermined diameter, toward the downstream side where the optical fiber 102 is drawn, to the lower opening 11a of the drawing furnace 11 through which the optical fiber 102 is drawn out. The ambient temperature Tz in the drawing furnace 11 decreases at a constant gradient from Tzstart at the neck-down end point 102a until the optical fiber 102 reaches Tzend at the lower opening 11a in the downstream direction. The gradient of the ambient temperature Tz in the drawing furnace 11 can be set, for example, such that the ambient temperature Tzstart at the neck-down end point 102a is 2000°C, and the ambient temperature Tzend at the lower opening 11a is 600°C.

[0016] In the slow cooling furnace 12 provided at the lower portion in contact with the drawing furnace 11, let La be the length from the upper opening 12a as the start end to the lower opening 12b as the terminal end. The ambient temperature Taz in the slow cooling furnace 12 decreases at a constant gradient from Tazstart at the upper opening 12a until reaching Tazend at the downstream lower opening 12b. The gradient of the ambient temperature Taz in the slow cooling furnace 12 can be controlled, for example, such that the ambient temperature Tazstart at the upper opening 12a and the ambient temperature Tazend at the lower opening 12b are each between 800°C and 1600°C.

[0017] Figure 3 is a perspective view showing the elements of the optical fiber 102 corresponding to a small time interval Δt. If the speed at which the optical fiber 102 is drawn is v, then the length of the optical fiber elements corresponding to the movement over a small time interval Δt is vΔt. Also, let the diameter of the optical fiber 102 be d. We derive the virtual temperature Tf(t) of the optical fiber 102 by assuming such elements of the optical fiber 102.

[0018] When an object is sufficiently small or has a sufficiently large heat transfer coefficient, there is almost no temperature difference within the object due to heating and cooling, and the temperature change occurs while maintaining a nearly uniform temperature. In this case, using the concentrated heat capacity model, the time change of temperature T in the elements of the optical fiber 102 can be approximately given by equation (1). In equation (1), the specific heat c, density ρ, and heat transfer coefficient h of the optical fiber 102, the volume V and surface area S of the elements of the optical fiber 102, and the ambient temperature Tz of the drawing furnace 11 are used.

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[0019] In equation (1), if the initial condition is T(0) = Ts, the solution is given by equation (2). Note that in equations (1) and (2), the ambient temperature Tz of the drawing furnace 11 is used when the elements of the optical fiber 102 are inside the drawing furnace 11, and when the elements of the optical fiber 102 are inside the annealing furnace 12, it is replaced with the ambient temperature Taz of the annealing furnace 12.

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[0020] Furthermore, the volume V and surface area S of the elements of the optical fiber 102 are given by equations (3) and (4), respectively, where v is the drawing speed of the optical fiber 102 and d is its diameter.

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[0021] Substituting equations (3) and (4) into equation (2) yields equation (5).

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[0022] When the ambient temperature Tz in the drawing furnace 11 decreases at a constant gradient to room temperature along the direction in which the optical fiber 102 is drawn from the neck-down end point 102a to the lower opening 11a, the ambient temperature Tz in the drawing furnace 11 is given by equations (6) and (7). Here, m is the rate of change over time of the ambient temperature Tz at a fixed point on the optical fiber 102 inside the drawing furnace 11.

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[0023] The ambient temperature Taz in the annealing furnace 12 decreases at a constant gradient in the downstream direction as the optical fiber 102 is routed, from the upper opening 12a which is the starting point to the lower opening 12b which is the ending point. Therefore, the ambient temperature Taz in the annealing furnace 12 is given by equations (8) and (9). Here, n is the rate of change over time of the ambient temperature Taz at a fixed point on the optical fiber 102 inside the annealing furnace 12.

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[0024] The time-dependent temperature T(t) of the optical fiber 102 can be derived using equations (5) to (9).

[0025] Next, based on the derived temperature T(t) of optical fiber 102, we derive the virtual temperature Tf(t) of optical fiber 102. The virtual temperature Tf(t) can be expressed by the recurrence relation in equation (10).

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[0026] It is important to note that Equation 10 is a recurrence relation that changes over a small time interval Δt. The interval Δt can be arbitrarily determined. In other words, if the structural relaxation time τ for a specific optical fiber 102 at a specific temperature T can be determined, then a virtual temperature Tf can be derived based on the time rate of change n of the ambient temperature Taz at a fixed point on the optical fiber 102 in the slow-cooling furnace 12 and the small time interval Δt.

[0027] According to Non-Patent Literature 2, the structural relaxation time τ[s] of the optical fiber 102 can be expressed by equation (11) in terms of the temperature T[K] of the optical fiber 102. Here, k in equation (11) B is the Boltzmann constant, k B = 8.617333262 × 10 -5 The value is [eV / K]. Note that in equation (2) of Non-Patent Document 2, "τ=" is likely to be "1 / τ=", so in equation (11), "τ=" is replaced with "1 / τ=".

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[0028] The virtual temperature Tf of the optical fiber 102 can be derived from equations (10) and (11). A specific example of the derivation of the virtual temperature Tf will be explained in Comparative Example 2 below. [Examples]

[0029] Figure 4 is a graph showing the distribution of the virtual temperature Tf when the drawing speed v in Example 2 is 20 m / min. In Example 2, the relationship between the gradient of the ambient temperature Taz, which is set by the ambient temperature Tazstart at the upper opening 12a of the drawing furnace 11 and the temperature Tazend at the lower opening 12b of the drawing furnace 11, and the virtual temperature Tf of the optical fiber 102 was determined using the virtual temperature Tf of the optical fiber 102 derived in Example 1. Since the optical fiber 102 is rapidly cooled to room temperature when it comes out of the annealing furnace 12, the value of the virtual temperature Tf derived in Example 1 when the optical fiber 102 reaches the end of the drawing furnace 11 can be used.

[0030] In Example 2, the first drawing speed of the optical fiber 102 was v = 20 m / min, the length L from the neck-down end point 102a to the lower opening 11a in the drawing furnace 11 was L = 0.3 m, and the length La of the annealing furnace 12 was 1 m. It is preferable that the drawing furnace 11 be 1 m or less in length to avoid increasing the size of the drawing equipment. Furthermore, the ambient temperature Tzstart = 1600°C at the neck-down end point 102a of the drawing furnace 11 and the ambient temperature Tzend = 500°C at the lower opening 11a were set. For the optical fiber 102, the diameter d = 125 μm and the heat transfer coefficient h = 210 W / m 2 ℃, density ρ=2203kg / m 3 The specific heat c was assumed to be 1000 J / kg / ℃.

[0031] Under these conditions, as shown in Figure 4, the distribution of virtual temperature Tf was obtained with the vertical axis representing the ambient temperature Tazstart at the upper opening 12a, which is the starting point of the annealing furnace 12, and the horizontal axis representing the ambient temperature Tazend at the lower opening 12b, which is the ending point. Since it is undesirable for the optical fiber 102 to soften if the ambient temperature Taz of the annealing furnace 12 exceeds 1600°C, the upper limit of the virtual temperature Tf was set to 1600°C for both the vertical and horizontal axes. Furthermore, the lower limit of the virtual temperature Tf was set to 800°C for both the vertical and horizontal axes.

[0032] In Figure 4, in the region U enclosed by the dashed line, the virtual temperature Tf of the optical fiber 102 is 1300°C or less. Therefore, by setting the gradient of the ambient temperature Taz so that the ambient temperature Tazatart at the beginning and the ambient temperature Tazand at the end of the annealing furnace 12 correspond to the coordinates within region U, a virtual temperature Tf of 1300°C or less for the optical fiber 102 can be achieved.

[0033] In Example 2, it was revealed that by providing an ambient temperature Taz with an appropriate gradient in the annealing furnace 12, a virtual temperature Tf of 1300°C can be achieved using an annealing furnace 12 with a length La of 1m at a line drawing speed v of 20m / min. Therefore, in Example 2, it is possible to ensure productivity while keeping the line drawing equipment from becoming large. [Examples]

[0034] Figure 5 is a graph showing the distribution of the virtual temperature Tf when the wire drawing speed v in Example 3 is 40 m / min. In Example 3 as well, the relationship between the gradient of the ambient temperature Taz, which is set by the ambient temperature Tazstart at the starting end and the temperature Tazend at the ending end in the wire drawing furnace 11, and the virtual temperature Tf of the optical fiber 102 was determined using the virtual temperature Tf(t) of the optical fiber 102 derived in Example 1.

[0035] In Example 3, the same conditions as in Example 2 were used, except that the wire drawing speed v was 40 m / min. Specifically, the length L = 0.3 m from the neck-down end point 102a to the lower opening 11a in the wire drawing furnace 11, the length La = 1 m of the annealing furnace 12, the ambient temperature Tzstart = 1600°C at the neck-down end point 102a of the wire drawing furnace 11, the ambient temperature Tzend = 500°C at the lower opening 11a, and for the optical fiber 102, the diameter d = 125 μm and the heat transfer coefficient h = 210 W / m 2 ℃, density ρ=2203kg / m 3 The specific heat c was assumed to be 1000 J / kg / ℃.

[0036] Under these conditions, as shown in Figure 5, the distribution of the virtual temperature Tf was obtained with the vertical axis representing the ambient temperature Tazstart at the upper opening 12a, which is the starting point of the annealing furnace 12, and the horizontal axis representing the ambient temperature Tazend at the lower opening 12b, which is the ending point. In Figure 5, as in Figure 4, the upper limit of the virtual temperature Tf was set to 1600°C and the lower limit to 800°C for both the vertical and horizontal axes.

[0037] In Figure 5, in the region V enclosed by the dashed line, the virtual temperature Tf of the optical fiber 102 is 1300°C or less. Therefore, by setting the gradient of the ambient temperature Taz so that the ambient temperature Tazatart at the beginning and the ambient temperature Tazand at the end of the annealing furnace 12 correspond to the coordinates within region V, a virtual temperature Tf of 1300°C or less for the optical fiber 102 can be achieved. It can be observed that region U of Example 2, also shown in Figure 5, is included within region V.

[0038] In Example 3, it was revealed that by providing an ambient temperature Taz with an appropriate gradient in the annealing furnace 12, a virtual temperature Tf of 1300°C can be achieved using an annealing furnace 12 with a length La of 1m at a line drawing speed v of 40m / min. Therefore, in Example 3, the line drawing equipment can be kept from becoming large, and productivity can be further improved compared to Example 2. [Examples]

[0039] Figure 6 shows a graph in which the regions in the graphs of Examples 2 and 3 of Example 4 are approximated by triangles. In Example 4, the region V in the graph of Figure 5 of Example 3 where the virtual temperature Tf of the optical fiber 102 is 1300°C or less, and the region U in Example 2 where the virtual temperature Tf is 1300°C or less, are approximated by regions enclosed by triangles. The approximated region V' of the triangle corresponding to region V is determined by vertices A'(1060,1600), B'(1220,1600), and C'(1280,1300). The approximated region U' of the triangle corresponding to region U is determined by vertices A''(800,1600), B''(1260,1600), and C''(1310,930).

[0040] Furthermore, we assume that each point in the approximate regions U' and V' of the triangle changes linearly with respect to the drawing speed v of the optical fiber 102. Since the drawing speed v is 20 m / min in the approximate region U' corresponding to Example 2 and 40 m / min in the approximate region corresponding to Example 3, the approximate region W of the triangle at a drawing speed v is determined by the following vertices A, B, and C. A(13V+540,1600) B(-2v+1300,1600) C(-1.5V+1340, 18.5V+560)

[0041] Therefore, by setting the gradient of the ambient temperature Taz so that the ambient temperature Tazatart at the beginning and the ambient temperature Tazand at the end of the slow-cooling furnace 12 correspond to coordinates within the approximate region W, a virtual temperature Tf of the optical fiber 102 of 1300°C or less can be achieved. The approximate region W is determined according to the line drawing speed v. [Examples]

[0042] In Example 4, by approximating regions U and V with triangular approximation regions U' and V', the triangular approximation region W where the virtual temperature Tf is 1300°C or less could be expressed as a function of the drawing speed v of the optical fiber 102. Therefore, the relationship between the virtual temperature Tf and the drawing speed v in the optical fiber manufacturing apparatus 10 becomes clear, and the operation of the optical fiber manufacturing apparatus 10 becomes easier. [Examples]

[0043] FIG. 7 is a graph showing the relationship between the drawing speed v of the optical fiber 102 of Example 5 and the minimum value of the fictive temperature Tf. In Example 7, the fictive temperature Tf of the optical fiber 102 derived in Example 1 was used to derive the minimum value Tfmin of the fictive temperature Tf with respect to the drawing speed v of the optical fiber 102. Also in Example 5, the same conditions as in Example 2 were used except that the drawing speed v was taken as a variable. That is, the length L from the neck-down end point 102a in the drawing furnace 11 to the lower opening 11a is 0.3 m, the length La of the slow cooling furnace 12 is 1 m, the ambient temperature Tzstart at the neck-down end point 102a of the drawing furnace 11 is 1600°C, the ambient temperature Tzend at the lower opening 11a is 500°C, and for the optical fiber 102, the diameter d is 125 μm, and the heat transfer coefficient h is 210 W / m 2 °C, and the density ρ is 2203 kg / m 3 , and the specific heat c is 1000 J / kg / °C.

[0044] Under such conditions, as shown in FIG. 7, the dependence of the minimum value Tfmin of the fictive temperature Tf of the optical fiber 102 on the drawing speed v was obtained. In FIG. 7, it is observed that the minimum value Tfmin of the fictive temperature Tf increases monotonically from a drawing speed v of 10 m / min, and reaches 1300°C at a drawing speed v of 57 m / min.

[0045] From this, it is clear that if the drawing speed v of the optical fiber 102 is 57 m / min or less, the fictive temperature Tf of the optical fiber 102 becomes 1300°C or less. In other words, when the drawing speed v of the optical fiber 102 exceeds 57 m / min, the fictive temperature Tf of the optical fiber 102 exceeds 1300°C.

[0046] (Comparative Example 1) FIG. 8 is a graph showing the distribution of the fictive temperature Tf when the drawing speed v of Comparative Example 1 is 60 m / min. Also in Comparative Example 1, using the fictive temperature Tf(t) of the optical fiber 102 derived in Example 1, the relationship between the gradient of the ambient temperature Taz set by the ambient temperature Tazstart at the start end and the temperature Tazend at the end end in the drawing furnace 11 and the fictive temperature Tf of the optical fiber 102 was determined.

[0047] In Comparative Example 1, the same conditions as in Example 2 were used, except that the wire drawing speed v was 60 m / min. Specifically, the length L = 0.3 m from the neck-down end point 102a to the lower opening 11a in the wire drawing furnace 11, the length La = 1 m of the annealing furnace 12, the ambient temperature Tzstart = 1600°C at the neck-down end point 102a of the wire drawing furnace 11, the ambient temperature Tzend = 500°C at the lower opening 11a, and for the optical fiber 102, the diameter d = 125 μm and the heat transfer coefficient h = 210 W / m 2 ℃, density ρ=2203kg / m 3 The specific heat c was assumed to be 1000 J / kg / ℃.

[0048] Under these conditions, as shown in Figure 8, the distribution of the virtual temperature Tf was obtained with the vertical axis representing the ambient temperature Tazstart at the upper opening 12a, which is the starting point of the annealing furnace 12, and the horizontal axis representing the ambient temperature Tazend at the lower opening 12b, which is the ending point. In Figure 8, as in Figure 4, the upper limit of the virtual temperature Tf was set to 1600°C and the lower limit to 800°C for both the vertical and horizontal axes.

[0049] In Figure 8, there was no region where the virtual temperature Tf of optical fiber 102 was below 1300°C. This is consistent with the result shown in Example 3, where the minimum value Tfmin of the virtual temperature Tf of optical fiber 102 exceeds 1300°C when the drawing speed v of optical fiber 102 exceeds 57 m / min, considering that the drawing speed v of optical fiber 102 in Comparative Example 1 was 60 m / min.

[0050] (Comparative Example 2) Figure 9 is a graph showing the time evolution of the temperature and virtual temperature Tf of the optical fiber 102 at a fixed point on the optical fiber 102 in Comparative Example 2. In Comparative Example 2, the temperature T(t) at a fixed point on the optical fiber 102 was derived from equation (5), and the virtual temperature Tf was derived from equations (10) and (11). In Comparative Example 2, similar to Example 2, the first drawing speed of the optical fiber 102 was v = 20 m / min, the length L = 0.3 m from the neck-down end point 102a to the lower opening 11a in the drawing furnace 11 was set to 1 m, and the length La = 1 m of the annealing furnace 12 was set to 1600°C at the neck-down end point 102a of the drawing furnace 11, 500°C at the lower opening 11a, and the annealing furnace 12 was set to a constant Tzstart = 1600°C and Tzend = 500°C.

[0051] Under these conditions, the temperature T and virtual temperature Tf of the optical fiber 102 were derived as functions of time, as shown in Figure 9. The origin of the horizontal axis in Figure 9 is the time when the fixed point of the optical fiber 102 was drawn from the neck-down termination point 102a, and t=3.9s, which is the rightmost point of the data, is the time when the fixed point of the optical fiber 102 reached the end of the annealing furnace 12.

[0052] In Figure 8, it can be observed that during the process of drawing the optical fiber 102 through the drawing furnace 11 and the annealing furnace 12, the temperature T of the optical fiber 102 changes in accordance with the ambient temperature Tz of the drawing furnace 11 and the ambient temperature Taz of the annealing furnace 12. Furthermore, it can be observed that the virtual temperature Tf of the optical fiber 102 decreases in accordance with the temperature T of the optical fiber 102 near the neck-down end point 102a of the drawing furnace 11, and then gradually decreases toward 1290°C through the drawing furnace 11 and the annealing furnace 12. Note that Figure 8 is cut off at 3.9s, but at 3.9s the fixed point of the optical fiber 102 that leaves the drawing furnace 11 is rapidly cooled to room temperature, so the virtual temperature Tf is determined at this point. [Explanation of Symbols]

[0053] 10 Optical fiber manufacturing equipment 11 Line drawing furnace 12 Annealing furnace 101 Glass base material 102 Optical Fiber

Claims

1. A manufacturing apparatus for optical fibers, A drawing furnace that heats a glass base material to draw optical fibers, The system includes a slow-cooling furnace capable of controlling the gradient of the ambient temperature, which is applied to the optical fiber drawn in the aforementioned drawing furnace by providing an ambient temperature that decreases at a constant gradient downstream, thereby performing heat treatment on the optical fiber. The drawing speed of the optical fiber is 57 m / min or less. The virtual temperature of the optical fiber is 1300°C or less. Optical fiber manufacturing equipment.

2. The optical fiber manufacturing apparatus according to claim 1, wherein the ambient temperature provided by the slow-cooling furnace is at a maximum of 1600°C or less at the starting end of the slow-cooling furnace and at a minimum of 800°C or more at the ending end of the slow-cooling furnace.

3. The gradient of the ambient temperature in the slow-cooling furnace is, in a graph where the ambient temperature at the start and end of the slow-cooling furnace is set on the vertical axis and horizontal axis, respectively, with respect to the optical fiber drawing speed v m / min, (13v+540,1600), (-2v + 1300, 1600) and (-1.5v+1340, 18.5v+560) An optical fiber manufacturing apparatus according to claim 1 or 2, controlled by the ambient temperature at the start and end points corresponding to coordinates within a region enclosed by a triangle defined by three points.

4. A method for manufacturing optical fibers, The process involves heating a glass base material in a wire drawing furnace to draw optical fibers, The process includes providing the drawn optical fiber with an ambient temperature that decreases at a constant gradient downstream in a slow-cooling furnace for heat treatment, and controlling the gradient of the ambient temperature. The process of drawing the optical fiber involves drawing the optical fiber at a speed of 57 m / min or less. The virtual temperature of the optical fiber is 1300°C or less. A method for manufacturing optical fibers.

5. The method for manufacturing an optical fiber according to claim 4, wherein the slow cooling furnace has an ambient temperature of 1600°C or less at the starting end of the slow cooling furnace and a minimum ambient temperature of 800°C or more at the ending end of the slow cooling furnace.

6. The gradient of the ambient temperature in the slow-cooling furnace is, in a graph where the ambient temperature at the start and end of the slow-cooling furnace is set on the vertical axis and horizontal axis, respectively, with respect to the optical fiber drawing speed v m / min, (13v+540,1600), (-2v + 1300, 1600) and (-1.5v+1340, 18.5v+560) A method for manufacturing an optical fiber according to claim 4 or 5, further comprising a step controlled by the ambient temperature at the start and end points corresponding to coordinates within a region enclosed by a triangle defined by three points.

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