Technical field
[0001]
　The present invention relates to a method for manufacturing an optical fiber.
Background technology
[0002]
　In an optical fiber communication system, the optical signal-to-noise ratio must be increased in order to increase the optical transmission distance and the optical transmission speed. Therefore, it is required to reduce the transmission loss of the optical fiber. Now that the manufacturing method of optical fibers is highly sophisticated, it is considered that the transmission loss due to impurities contained in optical fibers is reduced to almost the limit. The main cause of the remaining transmission loss is scattering loss due to fluctuations in the structure and composition of the glass constituting the optical fiber. This is unavoidable because the optical fiber is made of glass.
[0003]
　As a method for reducing fluctuations in the structure of glass, it is known that the molten glass is cooled gently when it is cooled. As a method for gently cooling the molten glass in this way, attempts have been made to slowly cool the optical fiber immediately after being drawn from the drawing furnace. Specifically, it has been studied to reduce the cooling rate of the optical fiber by heating the optical fiber drawn from the drawing furnace in a slow cooling furnace or surrounding the optical fiber immediately after drawing with a heat insulating material.
[0004]
　The following Patent Document 1 describes from a position where the outer diameter of an optical fiber having a core and a clad containing silica glass as a main component is smaller than 500% of the final outer diameter to a position where the temperature of the optical fiber becomes 1400 ° C. It is disclosed that the temperature of the heating furnace (slow cooling furnace) is set so as to be ± 100 ° C. or less with respect to the target temperature obtained by the gradual equation in the region of 70% or more. By controlling the temperature history of the optical fiber in this way, the virtual temperature of the glass constituting the optical fiber is lowered, and the transmission loss is reduced.
[0005]
Patent Document 1: Japanese Unexamined Patent Publication No. 2014-62021
Outline of the invention
[0006]
　However, in the technique disclosed in Patent Document 1, it is required to repeat a complicated calculation in order to adjust the temperature of the optical fiber to the ideal temperature change obtained by the recurrence formula. Further, in the technique disclosed in Patent Document 1, it is allowed that the temperature of the optical fiber deviates by ± 50 ° C. to 100 ° C. with respect to the target temperature obtained by the recurrence formula. If the temperature deviation of the optical fiber is allowed in such a wide range, it cannot be said that the temperature history is sufficiently optimized. For example, if the temperature of the slowly cooled optical fiber changes in the range of ± 100 ° C. and the virtual temperature of the glass constituting the optical fiber also changes in the same range, the transmission loss due to light scattering of the obtained optical fiber is It will fluctuate by about ± 0.007 dB / km. In the conventional manufacturing method in which the temperature history of the optical fiber is not sufficiently optimized, excessive capital investment for making the slow cooling furnace longer than necessary is made, or the drawing speed is lowered more than necessary for productivity. Is damaged.
[0007]
　The present inventors appropriately set the temperature of the slow cooling furnace, and appropriately control the temperature difference between the virtual temperature of the glass constituting the optical fiber and the temperature of the optical fiber to appropriately control the structure of the glass constituting the optical fiber. It has been found that mitigation is promoted and the transmission loss of the optical fiber is easily reduced.
[0008]
　Therefore, the present invention aims to provide a method for manufacturing an optical fiber, which makes it easy to reduce the transmission loss of the optical fiber.
[0009]
　In order to solve the above problems, the method for manufacturing an optical fiber of the present invention includes a drawing step of drawing a base material for an optical fiber in a drawing furnace and a slow cooling of slowly cooling the optical fiber drawn in the drawing step. In the slow cooling step, the optical fiber is passed through a plurality of slow cooling furnaces, the time constant of structural relaxation of the glass constituting the core contained in the optical fiber is τ (T), and the slow cooling step is provided. The temperature of the optical fiber at a certain point in time is T, the virtual temperature of the glass constituting the core at a certain point in time is T f 0 , and the virtual temperature of the glass constituting the core after a lapse of time Δt from the certain point in time. When T f , the following equation (1) holds in any period of the slow cooling step.

[0010]
　The present inventors slowly cool the optical fiber while the temperature difference between the temperature of the optical fiber and the virtual temperature of the glass constituting the core contained in the optical fiber is controlled within the above-mentioned predetermined range, thereby causing the core. It was found that the structural relaxation of the glass constituting the above was promoted. By promoting the relaxation of the structure of the glass constituting the core, the scattering loss due to the fluctuation of the structure of the glass constituting the core when light is transmitted to the core is reduced, so that the transmission loss of the optical fiber is reduced. It will be reduced. Further, as described above, a plurality of slow cooling furnaces are used in the slow cooling process, and the set temperature of each slow cooling furnace is appropriately controlled to control the temperature of the optical fiber and the virtual temperature of the glass constituting the core contained in the optical fiber. The temperature difference between the two is easily controlled within the above-mentioned predetermined range. As a result, the structural relaxation of the glass constituting the core is promoted, and the transmission loss of the optical fiber is easily reduced.
[0011]
　Further, in the method for producing an optical fiber of the present invention, it is preferable that the following formula (2) holds in an arbitrary period of the slow cooling step.

　In this way, in the slow cooling step, the temperature difference (T f − T) between the temperature T of the optical fiber and the virtual temperature T f of the glass constituting the core contained in the optical fiber is controlled within a more suitable range. , The structural relaxation of the glass constituting the core included in the optical fiber is more likely to be promoted, and the transmission loss of the optical fiber is more likely to be reduced.
[0012]
　Further, in the method for manufacturing an optical fiber of the present invention, the set temperature of the slow cooling furnace nth from the upstream side is set to T sn , and the glass constituting the core contained in the optical fiber at the outlet of the slow cooling furnace nth from the upstream side. the fictive temperature T en when a, it is preferable that the relationship of the following equation (3) holds.

　As described above, a plurality of slow cooling furnaces are used in the slow cooling step, and the set temperature of each slow cooling furnace is controlled within a predetermined range with respect to the virtual temperature of the glass constituting the core at the outlet of each slow cooling furnace. The temperature difference between the temperature and the virtual temperature of the glass constituting the core contained in the optical fiber can be easily controlled within a predetermined range. As a result, the structural relaxation of the glass constituting the core is promoted, and the transmission loss of the optical fiber is easily reduced.
[0013]
　Further, in the method for producing an optical fiber of the present invention, it is preferable that the following formula (4) holds.

　By controlling the set temperatures of the plurality of slow cooling furnaces to more appropriate ranges in this way, the effect of promoting structural relaxation of the glass constituting the core contained in the optical fiber is more likely to be increased, and the transmission loss of the optical fiber is easily increased. Is more likely to be reduced.
[0014]
　Further, in the method for manufacturing an optical fiber of the present invention, the temperature difference between the set temperature and the virtual temperature of the glass constituting the core at the outlet of the slow cooling furnace provided on the downstream side is larger than that of the slow cooling furnace provided on the upstream side. Is preferably small.
　The present inventors have found that when the temperature of the glass is lowered, the structural relaxation of the glass is more likely to be promoted by reducing the temperature difference between the virtual temperature of the glass and the temperature of the glass. Therefore, by setting the temperature of the slow cooling furnace so that the temperature difference between the set temperature and the virtual temperature of the glass constituting the core at the outlet is smaller in the slow cooling furnace provided on the downstream side than in the slow cooling furnace provided on the upstream side. It is possible to efficiently promote the structural relaxation of the glass constituting the core. As a result, the transmission loss of the optical fiber is more likely to be reduced.
[0015]
　Further, it is preferable that the optical fiber stays in one of the plurality of slow cooling furnaces at least for a period of time when the temperature of the optical fiber is in the range of 1300 ° C. or higher and 1500 ° C. or lower. By slowly cooling the optical fiber when the temperature of the optical fiber is in this range, the virtual temperature of the glass constituting the core contained in the optical fiber is likely to be lowered in a short time, and the transmission loss of the optical fiber is reduced. It becomes easy to be done.
[0016]
　As described above, according to the present invention, there is provided a method for manufacturing an optical fiber in which it is easy to reduce the transmission loss of the optical fiber.
A brief description of the drawing
[0017]
FIG. 1 is a flowchart showing a process of the optical fiber manufacturing method of the present invention.
FIG. 2 is a diagram schematically showing a configuration of an apparatus used in the method for manufacturing an optical fiber of the present invention.
FIG. 3 is a graph showing the relationship between the temperature of glass and the virtual temperature of the glass and the slow cooling time.
FIG. 4 shows the temperature difference between the virtual temperature of glass and the temperature of glass (T f 0 −T), the rate of decrease of the virtual temperature of glass per unit time ((T f −T f 0 ) / Δt), and It is a graph which shows the relationship of.
[Fig. 5] Fig. 5 is a graph showing the time change of the temperature difference between the virtual temperature of glass and the temperature of glass.
[6] optimized temperature difference shown by the solid line in FIG. 5 (T f and -T), transmission loss due to scattering is 0.001 dB / miles or losses do not increase the temperature difference (T f of aging of -T) It is a graph which shows the upper limit and the lower limit.
FIG. 7 is a graph showing the set temperature of each slow cooling furnace, the virtual temperature of the optimized glass at the outlet of each slow cooling furnace, and the virtual temperature at the outlet of each slow cooling furnace of glass through a virtual temperature history.
Mode for carrying out the invention
[0018]
　Hereinafter, preferred embodiments of the optical fiber manufacturing method according to the present invention will be described in detail with reference to the drawings.
[0019]
　FIG. 1 is a flowchart showing a process of a method for manufacturing an optical fiber of the present invention according to one embodiment. As shown in FIG. 1, the method for manufacturing an optical fiber of the present embodiment includes a drawing step P1, a precooling step P2, a slow cooling step P3, and a quenching step P4. Hereinafter, each of these steps will be described. Note that FIG. 2 is a diagram schematically showing the configuration of an apparatus used in the method for manufacturing an optical fiber of the present embodiment.
[0020]
　 The
　drawing step P1 is a step of drawing one end of the base material 1P for an optical fiber in the drawing furnace 110. First, an optical fiber base material 1P made of glass having the same refractive index distribution as the core and clad constituting the desired optical fiber 1 is prepared. The optical fiber 1 has one or more cores and a clad that surrounds the outer peripheral surface of the core without gaps. Further, the core and the clad are each made of silica glass, and the refractive index of the core is higher than the refractive index of the clad. For example, if the core is made of silica glass to which a dopant such as germanium, which increases the refractive index, is added, the clad is made of pure silica glass. Further, for example, when the core is made of pure silica glass, the clad is made of silica glass to which a dopant such as fluorine that lowers the refractive index is added.
[0021]
　Next, the base material 1P for an optical fiber is suspended so that the longitudinal direction is vertical. Then, the optical fiber base material 1P is arranged in the drawing furnace 110, and the heating unit 111 is heated to heat the lower end portion of the optical fiber base material 1P. At this time, the lower end of the base material 1P for an optical fiber is heated to, for example, 2000 ° C. to be in a molten state. Then, the molten glass is pulled out from the drawing furnace 110 at a predetermined drawing speed from the lower end portion of the heated base material 1P for the optical fiber.
[0022]
　<
　Pre- cooling step P2> The pre-cooling step P2 is a step of cooling the optical fiber drawn from the drawing furnace 110 in the drawing step P1 to a predetermined temperature suitable for being sent to the slow cooling furnace 121 described later. The predetermined temperature of the optical fiber suitable for being sent to the slow cooling furnace 121 will be described in detail later.
[0023]
　In the method for manufacturing an optical fiber of the present embodiment, the precooling step P2 is performed by passing the optical fiber drawn in the drawing step P1 through the hollow portion of the tubular body 120 provided directly under the drawing furnace 110. It is said. By providing the tubular body 120 directly below the drawing furnace 110, the atmosphere in the hollow portion of the tubular body 120 becomes substantially the same as the atmosphere in the drawing furnace 110. Therefore, it is possible to suppress a sudden change in the atmosphere and temperature around the optical fiber immediately after the line is drawn.
[0024]
　The temperature of the optical fiber sent to the slow cooling furnace 121 is mainly determined by the drawing speed and the atmosphere in the drawing furnace 110. By providing the precooling step P2, the cooling rate of the optical fiber is further finely adjusted, and the temperature of the optical fiber entering the slow cooling furnace 121 can be easily adjusted within an appropriate range. Based on the temperature of the optical fiber drawn from the drawing furnace 110 and the temperature of the optical fiber suitable for being sent to the slow cooling furnace 121, the distance between the slow cooling furnace 121 and the drawing furnace 110 and the length of the tubular body 120 are appropriately adjusted. You can choose. The tubular body 120 is composed of, for example, a metal tube or the like. The cooling rate of the optical fiber may be adjusted by air-cooling the metal tube or arranging a heat insulating material around the metal tube.
[0025]
　 The
　slow cooling step P3 is a step of slowly cooling the optical fiber drawn out in the drawing step P1. In the method for manufacturing an optical fiber of the present embodiment, the temperature of the optical fiber is adjusted through the precooling step P2, and the optical fiber is slowly cooled in the slow cooling step P3. In the slow cooling step P3, the optical fiber is passed through a plurality of slow cooling furnaces 121a, 121b, 121c, 121d. In the description of the method for manufacturing an optical fiber of the present embodiment, when all of these slow cooling furnaces are included or when it is not necessary to distinguish each slow cooling furnace, it may be simply referred to as "slow cooling furnace 121". Although FIG. 2 shows four slow cooling furnaces 121a, 121b, 121c, and 121d, the number of slow cooling furnaces in the present invention is not particularly limited as long as there are a plurality of slow cooling furnaces. Having a plurality of slow cooling furnaces means that there are a plurality of heat generating parts that can be set to different temperatures. For example, even if the temperature is housed in one housing, it can be said that there are a plurality of slow cooling furnaces if a plurality of heat generating portions capable of setting different temperatures are provided.
[0026]
　The inside of the slow cooling furnace 121 has a predetermined temperature different from the temperature of the optical fiber entering the slow cooling furnace 121, and the cooling rate of the optical fiber is lowered by the temperature around the optical fiber entering the slow cooling furnace 121. By reducing the cooling rate of the optical fiber in the slow cooling furnace 121, the structure of the glass constituting the core contained in the optical fiber is relaxed and the optical fiber 1 with reduced scattering loss can be obtained, as described below.
[0027]
　In the conventional method for manufacturing an optical fiber having a slow cooling step, the temperature of the optical fiber at the time of entering the slow cooling furnace is not sufficiently optimized. Specifically, the optical fiber may be entered into the slow cooling furnace in a state where the temperature of the optical fiber is too high or too low. If the temperature of the optical fiber entering the slow cooling furnace is too high, the structure of the glass constituting the optical fiber is relaxed at a very high speed, so that the effect of slowly cooling the optical fiber can hardly be expected. On the other hand, if the temperature of the optical fiber entering the slow cooling furnace is too low, the speed at which the structure of the glass constituting the optical fiber is relaxed becomes slow, so that it may be necessary to reheat the optical fiber in the slow cooling furnace. As described above, in the conventional slow cooling process, it cannot be said that the structure of the glass constituting the optical fiber is efficiently relaxed. Therefore, there is a risk that excessive capital investment may be made to lengthen the slow cooling furnace more than necessary, or the drawing speed may be slower than necessary to impair productivity.
[0028]
　According to the method for manufacturing an optical fiber of the present embodiment, in the slow cooling step P3, the temperature of the slow cooling furnace 121 is appropriately set as described below, and the virtual temperature and light of the glass constituting the core contained in the optical fiber are set appropriately. By appropriately controlling the temperature difference from the temperature of the fiber, the structural relaxation of the glass constituting the core is promoted. As a result, it is possible to obtain an optical fiber 1 that does not require excessive capital investment, has good productivity, and has reduced transmission loss. Further, according to the method for manufacturing an optical fiber of the present embodiment, a complicated calculation as in the technique disclosed in the above-mentioned Cited Document 1 is not required at the time of manufacturing.
[0029]
　In silica glass, which is classified as so-called strong glass, the time constant τ (T) of structural relaxation, which is considered to be due to the viscous flow of glass, follows Arrhenius' equation. Therefore, the time constant τ (T) is expressed by the following equation (5) as a function of the temperature T of the glass using the constant A determined by the composition of the glass and the activation energy Eact . In addition, k B is Boltzmann constant.

(Here, T is the absolute temperature of the glass.)
[0030]
　From the above equation (5), it can be seen that the higher the temperature of the glass, the faster the structure of the glass relaxes and the faster the equilibrium state at that temperature is reached. That is, the higher the temperature of the glass, the faster the virtual temperature of the glass approaches the temperature of the glass.
[0031]
　FIG. 3 shows the relationship between the temperature of the glass when the glass is slowly cooled and the virtual temperature of the glass and the time. In the graph shown in FIG. 3, the horizontal axis represents time and the vertical axis represents temperature. In FIG. 3, the solid line shows the temperature transition of the glass under a certain slow cooling condition, and the broken line shows the transition of the virtual temperature of the glass at that time. The dotted line shows the temperature transition of the glass when the cooling rate is slower than the slow cooling condition shown by the solid line, and the alternate long and short dash line shows the transition of the virtual temperature of the glass at that time.
[0032]
　As shown by the solid line and the broken line in FIG. 3, when the temperature of the glass decreases with the passage of time in a high temperature region, the virtual temperature of the glass also decreases. When the temperature of the glass is sufficiently high as described above, the rate of structural relaxation of the glass is very high. However, as the temperature of the glass decreases, the rate of structural relaxation of the glass decreases. Eventually, the decrease in the virtual temperature of the glass cannot follow the decrease in the temperature of the glass. Then, the temperature difference between the temperature of the glass and the virtual temperature of the glass becomes large. Here, if the cooling rate of the glass is slowed down, the glass is held in a relatively high temperature state for a long time as compared with the case where the cooling rate is high. Therefore, as shown by the dotted line and the alternate long and short dash line in FIG. 3, the temperature difference between the temperature of the glass and the virtual temperature of the glass becomes small, and the virtual temperature of the glass becomes lower than the example described above. That is, if the cooling rate of the glass is slowed down, the structural relaxation of the glass is likely to be promoted.
[0033]
　As mentioned above, when the temperature of the glass is high, the structure of the glass relaxes quickly. However, since the virtual temperature of the glass is never lower than the temperature of the glass, when the temperature of the glass is high, the virtual temperature of the glass also remains high. That is, if the temperature of the glass is too high, the effect of slow cooling is small. From this point of view, the temperature of the optical fiber to stay in the slow cooling furnace 121 is preferably 1600 ° C. or lower, and more preferably 1500 ° C. or lower. On the other hand, when the temperature of the glass is low, the virtual temperature is lowered to a lower temperature, but the rate of decrease of the virtual temperature is slowed down. That is, if the temperature of the glass is too low, it takes time for slow cooling to sufficiently lower the virtual temperature. From this point of view, the temperature of the optical fiber to stay in the slow cooling furnace 121 is preferably 1300 ° C. or higher, and more preferably 1400 ° C. or higher. Therefore, it is preferable that the optical fiber stays in the slow cooling furnace 121 at least for a period of time when the temperature of the optical fiber is in the range of 1300 ° C. or higher and 1500 ° C. or lower. As described above, in the slow cooling step P3, when the temperature of the optical fiber is within a predetermined range, the optical fiber is slowly cooled, so that the virtual temperature of the glass constituting the core contained in the optical fiber is lowered in a short time. This makes it easier to reduce the transmission loss of the optical fiber.
[0034]
　Next, from the relationship between the glass temperature and the virtual temperature of the glass, how can the optical fiber be slowly cooled to efficiently promote the structural relaxation of the glass constituting the core and reduce the transmission loss of the optical fiber? explain.
[0035]
　The time constant of structural relaxation of the glass constituting the core contained in the optical fiber is τ (T), the temperature of the optical fiber at a certain point in the slow cooling step P3 is T, and the virtual glass constituting the core at that point in time is virtual. When the temperature is T f 0 , the virtual temperature T f of the glass constituting the core after the lapse of time Δt from the certain time point is expressed by the above equation (5) to the following equation (6). It is assumed that Δt is a minute time and T during that time is constant.

[0036]
　According to the equation (6), the virtual temperature T of the glass constituting the core f is not only dependent on the constant tau (T) when the structural relaxation, the fictive temperature T of the glass constituting the core f of the optical fiber temperature difference between the temperature T (T f -T) is the virtual temperature T of the glass constituting the core at a certain point in time f 0 temperature difference between the temperature T of the optical fiber (T f 0 to be dependent on -T) The time constant τ (T) of the structural relaxation is the temperature difference (T f − T) between the virtual temperature T f of the glass and the temperature T of the glass when the temperature of the glass where the virtual temperature is T f 0 is T. Is defined as the time until 1 / e, and the larger the temperature difference (T f 0 −T) is, the larger the change in the virtual temperature T f per unit time becomes.
[0037]
The temperature difference (T f 0- T) when the temperature of the optical fiber including the core made of glass having a 　virtual temperature of T f 0 is T, and the change of the virtual temperature T f per unit time ((T). The relationship between f− T f 0 ) / Δt) is schematically shown in FIG. As shown in FIG. 4, under the condition that the virtual temperature T f 0 of the glass constituting the core and the temperature T of the optical fiber match (T f 0 = T), the structural relaxation of the glass constituting the core occurs. However, the change in virtual temperature per unit time is 0 ((T f −T f 0 ) / Δt = 0). Considering the condition that the temperature T of the optical fiber is lowered and the temperature difference (T f 0 − T) between the virtual temperature T f 0 of the glass constituting the core and the temperature T of the optical fiber becomes large, the core is configured. Although the time constant τ (T) of the structural relaxation of the glass increases, the rate of change of the virtual temperature T f per unit time ((T) f− T f 0 ) / Δt) becomes negatively large. However, considering the condition that the temperature difference (T f 0 − T) between the virtual temperature T f 0 of the glass constituting the core and the temperature T of the optical fiber is further increased by further lowering the temperature T of the optical fiber , this time. Gradually, the time constant τ (T) of the structural relaxation of the glass constituting the core increases , and the absolute value of the change in the virtual temperature T f ((T f −T f 0 ) / Δt) per unit time decreases. That is, like the peak shown in the graph of FIG. 4, when the temperature difference (T f 0 − T) between the virtual temperature T f 0 of the glass constituting the core and the temperature T of the optical fiber is a value, it is virtual. It can be seen that the change in temperature per unit time ((T f −T f 0 ) / Δt) takes a minimum value.
[0038]
　Here, when the above equation (6) is solved, the relationship of the following equation (7) is established between the glass temperature T and the virtual temperature T f when the rate of decrease of the virtual temperature T f of the glass is maximized. Understand.

[0039]
　Further, when the above equation (7) is solved for T as in the following equation (8), the glass temperature T when the virtual temperature T f of the glass can be lowered most efficiently can be obtained. Hereinafter, the temperature of the glass when the virtual temperature T f of the glass can be lowered most efficiently is sometimes referred to as the "optimized glass temperature", and the most efficiently lowered virtual temperature is "optimized". It is sometimes called "virtual temperature".

[0040]
　As described above, the unit time of the virtual temperature T f of the glass when the temperature difference (T f 0 − T) between the virtual temperature T f 0 of the glass and the temperature T of the glass at a certain time point is a predetermined value. The change in hit is the largest. That is, when considering the virtual temperature T f after a certain period of time Δt of the glass having the virtual temperature T f 0 , there exists a glass temperature T that can make the virtual temperature T f the minimum value.
[0041]
　For a general-purpose single-mode optical fiber in which the core is doped with Ge O 2 , the value when the virtual temperature T f of the glass constituting the core obtained from the above equation (6) becomes the minimum value and the light at that time. The time course of the temperature difference (T f− T) from the temperature T of the fiber is shown in FIG. In the graph of FIG. 5, the vertical axis represents the temperature difference (T f− T) between the value when the virtual temperature T f of the glass constituting the core becomes the minimum value and the temperature T of the optical fiber at that time, and the horizontal axis. Is the slow cooling time of the optical fiber. The graph shown by the solid line shows the constant A and activation described in Non-Patent Document 1 (K. Saito, et al., Journal of the American Ceramic Society, Vol.89, pp.65-69 (2006)). energy E act is the result of using, the graph shown by a broken line non-patent document 2 (K. Saito, et al ., Applied Physics Letters, Vol.83, pp.5175-5177 (2003)) have been described in This is the result of setting Δt to 0.0005 seconds using the constant A and the activation energy Eact .
[0042]
　Here, it is assumed that the slow cooling step P3 is performed immediately after the base material 1P for optical fiber is heated and melted in the drawing step P1. Assuming that the temperature T 0 of the optical fiber at the initial stage of slow cooling (when the slow cooling time is 0 seconds) is 1800 ° C., the structural relaxation time of the glass constituting the core at this temperature is very less than 0.001 seconds. short. Therefore, it can be considered that the virtual temperature T f 0 at the initial stage of slow cooling of the glass constituting the core is also 1800 ° C. That is, the initial value is assumed to be T f 0 −T 0 = 0 ° C.
[0043]
Looking at the change over time in 　the temperature difference ( Tf− T) between the virtual temperature of the glass constituting the core and the temperature of the optical fiber, which is obtained from the above assumption, the temperature is approximately 0.01 seconds in the time domain. It can be seen that the difference (T f− T) may be gradually increased, and the temperature difference (T f− T) may be gradually decreased in the time region after about 0.01 seconds . Further, it is preferable that the temperature difference (T f− T) is approximately less than 60 ° C. in all time regions, and the temperature difference (T f− T) is generally higher than approximately 40 ° C. and less than approximately 60 ° C. in most time regions. It can be seen that by controlling the temperature T of the optical fiber so as to keep the temperature T, the virtual temperature T f of the glass constituting the core is efficiently lowered. The time at which the temperature difference (T f −T) shown in FIG. 5 becomes maximum is the constant A in the above formula (5), the activation energy Eact , and the initial stage of slow cooling (when the slow cooling time is 0 seconds). It takes about 0.01 seconds, although it varies slightly depending on the temperature T 0 of the optical fiber and the virtual temperature T f 0 of the glass constituting the core .
[0044]
　It can be seen from the above assumption that the precooling step P2 is provided following the drawing step P1 so that a temperature difference (T f− T) between the virtual temperature of the glass constituting the core and the temperature of the optical fiber occurs to some extent. If the slow cooling step P3 is performed, the structure of the glass constituting the core can be efficiently relaxed by effectively utilizing the length of the slow cooling furnace 121. For example, in FIG. 5, until the temperature difference ( Tf− T) between the virtual temperature of the glass constituting the core and the temperature of the optical fiber becomes higher than about 40 ° C. and less than 60 ° C. at the time when about 0.01 seconds have elapsed. It is preferable to perform the precooling step P2 and then start the slow cooling step P3 because the length of the slow cooling furnace 121 can be effectively used.
[0045]
　In addition, the following can be seen from the results shown in FIG. That is, the constant A and the activation energy E is determined based on the composition act even if some differences in the value of the temperature difference between the temperature of the fictive temperature and the glass of the glass in the annealing step P3 (T f - It can be seen that when T) is in the range higher than 40 ° C. and lower than 60 ° C., the virtual temperature of the glass can be efficiently lowered. Therefore, in the case of a general optical fiber having a low concentration of so-called dopant and the main component being silica glass, the temperature difference ( Tf− T) between the virtual temperature of the glass constituting the optical fiber and the temperature of the optical fiber is 40. By slowly cooling the optical fiber under the condition of higher than ° C. and lower than 60 ° C., the virtual temperature of the glass constituting the optical fiber can be efficiently lowered. For example, G e O 2 core and a dopant, such as is made of doped silica glass, in any of the cladding consisting essentially of pure silica glass, are also effectively lowers the fictive temperature.
[0046]
　Further, the temperature difference (T f - T) between the temperature T of the optical fiber and the virtual temperature T f of the glass constituting the core contained in the optical fiber in an arbitrary period from the start to the end of the slow cooling step P3. ) Is controlled within the above-mentioned predetermined range, so that the structural relaxation of the glass constituting the core included in the optical fiber is easily promoted, and the transmission loss of the optical fiber is easily reduced. That is, the time constant of structural relaxation of the glass constituting the core is τ (T), the temperature of the optical fiber at a certain point in the slow cooling step P3 is T, and the virtual temperature of the glass constituting the core at that point is T. When f 0 , where T f is the virtual temperature of the glass constituting the core after the lapse of time Δt from the certain point in time, it is preferable that the following equation (2) holds. 　In this way, in the slow cooling step P3, the temperature difference (T f− T) between the temperature T of the optical fiber and the virtual temperature T f of the glass constituting the core contained in the optical fiber is controlled within a predetermined range. , The structural relaxation of the glass constituting the core contained in the optical fiber is more likely to be promoted. Therefore, the transmission loss of the optical fiber is likely to be reduced.

[0047]
　At this time, according to the method for manufacturing an optical fiber of the present embodiment, a plurality of slow cooling furnaces 121 are used in the slow cooling step P3, and the set temperature of each slow cooling furnace 121 is appropriately controlled to obtain the temperature and light of the optical fiber. The temperature difference (T f− T) from the virtual temperature of the glass constituting the core included in the fiber can be easily controlled within the above-mentioned predetermined range. As a result, the structural relaxation of the glass constituting the core is promoted, and the transmission loss of the optical fiber is easily reduced.
[0048]
　Note that the virtual temperature T of the glass constituting the core f for most efficiently lower the fictive temperature T of the glass constituting the core contained in the temperature T and the optical fiber of the optical fiber f temperature difference between (T f The conditions of −T) are as described above, but the transmission loss of the optical fiber can be sufficiently reduced even under the conditions described below.
[0049]
　The virtual temperature T f of the glass constituting the core included in the optical fiber and the transmission loss of the optical fiber are linked by the following relational expression. The Rayleigh scattering coefficient R r is proportional to the virtual temperature T f of the glass constituting the core, and the transmission loss α T due to Rayleigh scattering is expressed by the following equation (9) in which the wavelength of the light to be transmitted is λ [μm].

[0050]
　Here, according to Non-Patent Document 2 (K. Saito, et al., Applied Physics Letters, Vol.83, pp.5175-5177 (2003)), B = 4.0 × 10 -4 dB / km / μm. It is 4 / K. Considering the transmission loss at the wavelength λ = 1.55 μm, when the virtual temperature T f of the glass constituting the core rises by 14 ° C., the transmission loss α T due to Rayleigh scattering increases by about 0.001 dB / km. That is, if the error from the virtual temperature T f of the glass constituting the core when it is most efficiently lowered can be suppressed to less than 14 ° C. , the increase in transmission loss α T due to Rayleigh scattering is less than 0.001 dB / km. Can be suppressed to.
[0051]
　As described above, most efficiently fictive temperature T of the glass constituting the core when being lowered f when considering the error acceptable from, as described below, the fictive temperature T of the glass constituting the core f The optical fiber may be entered into the slow cooling furnace 121 under the temperature condition that the temperature difference ( Tf− T) between the temperature of the optical fiber and the temperature of the optical fiber is higher than 20 ° C. and less than 100 ° C.
[0052]
　The increase in the scattering loss expected from the virtual temperature T f of the glass constituting the core when the temperature difference (T f − T) shown by the solid line in FIG. 5 has passed the slow cooling time of 0.5 seconds is less than 0.001 dB / km. The temperature difference when suppressed to is can be predicted from the above recurrence formula (6). The virtual temperature T f 0 of the glass constituting the core of the optical fiber at the initial stage of slow cooling (when the slow cooling time is 0 seconds) is 1800 ° C., and the temperature difference (T f −T) is almost constant during the slow cooling step P3. When the recurrence formula (6) is solved assuming that, the graph shown in FIG. 6 is obtained. In FIG. 6, the temperature difference (T f− T) shown by the solid line in FIG. 5 is shown again by the solid line. Further, in FIG. 6, the upper limit of the change with time of the temperature difference ( Tf− T) in which the transmission loss due to scattering does not increase by 0.001 dB / km or more is shown by a broken line, and the lower limit is shown by a dashed line. Here, the constant A and the activation energy E act NPL 1 (K. Saito, et al ., Journal of the American Ceramic Society, Vol.89, pp.65-69 (2006)) are shown in Use the value. The following can be seen from the results shown in FIG. During the slow cooling step P3, the temperature difference (T f) is approximately 0.01 seconds or later. If the temperature of the slow cooling furnace 121 is set to control the temperature history of the optical fiber so that −T) is higher than 20 ° C and less than 100 ° C, the glass constituting the core can be lowered most efficiently. With respect to the virtual temperature T f , the virtual temperature of the glass constituting the core is suppressed within a range that does not rise by about 14 ° C. or more. As a result, the transmission loss can be suppressed to an increase of 0.001 dB / km or less with respect to the value under the optimized conditions where the transmission loss is most reduced.
[0053]
　Therefore, the virtual temperature T of the glass constituting the core contained in the temperature T and the optical fiber of the optical fiber even in an arbitrary period from the start to the end of the annealing step P3 f the temperature difference between (T f - By maintaining T) in a range higher than 20 ° C. and lower than 100 ° C., structural relaxation of the glass constituting the core contained in the optical fiber is easily promoted, and transmission loss of the optical fiber is easily reduced. That is, it is preferable that the following equation (1) holds.

[0054]
　Next, a specific example for facilitating the satisfaction of the above equations (2) and (1) will be described. In the method for manufacturing an optical fiber of the present embodiment, four slow cooling furnaces 121a, 121b, 121c, 121d are used in the slow cooling step P3. By using the plurality of slow cooling furnaces 121 in this way, it becomes easy to control the temperature difference between the temperature of the optical fiber and the virtual temperature of the glass constituting the core within a predetermined range. That is, in the slow cooling step P3, the optical fiber is passed through a plurality of slow cooling furnaces 121, the set temperature of the nth slow cooling furnace 121 from the upstream side is set to T sn , and the slow cooling time until the temperature reaches the outlet of the nth slow cooling furnace 121 from the upstream side. the fictive temperature of the glass constituting the core T in the en when a, so that satisfied the relationship of the following formula (3).

[0055]
　As described above, the core is configured by slowly cooling the optical fiber while the temperature difference between the temperature of the optical fiber and the virtual temperature of the glass constituting the core contained in the optical fiber is controlled within a predetermined range. The structural relaxation of the glass is promoted. By promoting the relaxation of the structure of the glass constituting the core, the scattering loss due to the fluctuation of the structure of the glass constituting the core when light is transmitted to the core is reduced, so that the transmission loss of the optical fiber is reduced. It will be reduced. As described above, a plurality of slow cooling furnaces 121 are used in the slow cooling step P3, and the set temperature of each slow cooling furnace 121 is predetermined with respect to the virtual temperature of the glass constituting the core in the slow cooling time until reaching the outlet of each slow cooling furnace 121. By being controlled within the range of, the temperature difference between the temperature of the optical fiber and the virtual temperature of the glass constituting the core contained in the optical fiber can be easily controlled within a predetermined range. As a result, the structural relaxation of the glass constituting the core is promoted, and the transmission loss of the optical fiber is reduced. A specific description will be given below with reference to FIG. 7.
[0056]
　FIG. 7 shows the optimized virtual temperature change (solid line) of the glass constituting the core calculated from the equation (5) when the temperature of the glass constituting the core and the virtual temperature are 1800 ° C. as initial values. The set temperature (dashed line) of the slow cooling furnaces 121a, 121b, 121c, 121d and the expected virtual temperature of the glass constituting the core in the slow cooling time until reaching the outlet of the slow cooling furnaces 121a, 121b, 121c, 121d are shown. There is. In the example shown in FIG. 7, it is assumed that the length of each slow cooling furnace 121 is 0.5 m and the drawing speed is 20 m / sec. As shown by a triangle (▲) in FIG. 7, when the optical fiber exits each slow cooling furnace 121, that is, when the slow cooling time is 0.025 seconds, 0.050 seconds, 0.075 seconds, 0.100 seconds. The optimized virtual temperatures T f of the glass constituting the core are calculated to be 1556 ° C, 1515 ° C, 1492 ° C, and 1476 ° C, respectively. The set temperatures of the slow cooling furnaces 121a, 121b, 121c, and 121d are set as shown by the alternate long and short dash line in FIG. That is, the optimized virtual temperature T f of the glass constituting the core during the slow cooling time when the outlet of each slow cooling furnace 121 is reached. The temperature of each slow cooling furnace 121 is set to a temperature 50 ° C. lower. As a result, the temperature of the optical fiber approaches the set temperature of each slow cooling furnace 121 near the outlet of each slow cooling furnace 121, so that the conditions of the above equations (2) and (1) can be easily satisfied near the outlet of each slow cooling furnace 121. Then, the glass that has undergone a virtual temperature history that temporarily deviates from the condition of Eq. (1) with a sudden change that the temperature of the glass that has entered each slow cooling furnace 121 immediately matches the set temperature of each slow cooling furnace 121. , It is expected to have the virtual temperature shown by the circle (●) in FIG. Since the actual glass temperature drops more slowly and approaches the set temperature of the slow cooling furnace, the actual virtual temperature is slightly higher than the ideal virtual temperature indicated by the triangle (▲) and higher than the virtual temperature indicated by the circle (●). It will be slightly lower, but the error will be within an acceptable range. In the example shown in FIG. 7, the difference between the virtual temperature of the glass that has undergone the virtual temperature history after slow cooling for 0.100 seconds and the optimized virtual temperature is 12 ° C., and the scattering loss is 0.001 dB /. There is only a difference of less than km.
[0057]
　From the above-mentioned viewpoint, the following formula (4) is used from the viewpoint of making the temperature difference between the temperature of the optical fiber and the virtual temperature of the glass constituting the core within a more appropriate range, that is, easily satisfying the above-mentioned formula (2). It is preferable that it holds.

　By controlling the set temperature of the slow cooling furnace 121 to a more appropriate range in this way, the effect of promoting structural relaxation of the glass constituting the core contained in the optical fiber is likely to be increased, and the transmission loss of the optical fiber is reduced. It will be easier.
[0058]
　Further, as shown in FIG. 5, when the temperature of the glass is lowered, the structural relaxation of the glass is more likely to be promoted by reducing the temperature difference between the virtual temperature of the glass and the temperature of the glass. Therefore, it is preferable that the slow cooling furnace 121 provided on the downstream side has a smaller difference between the set temperature and the virtual temperature of the glass constituting the core at the outlet than the slow cooling furnace 121 provided on the upstream side. For example, as shown by the solid line in FIG. 5, the optimized glass temperature and the appropriateness of the glass constituting the core at the slow cooling time of 0.025 seconds, 0.050 seconds, 0.075 seconds, and 0.100 seconds. The difference from the virtualized temperature is 59 ° C., 56 ° C., 55 ° C., and 54 ° C., respectively, and it is preferable that the temperature difference becomes smaller toward the downstream side. In this way, the temperature of the slow cooling furnace is set so that the temperature difference between the set temperature and the virtual temperature of the glass constituting the core at the outlet is smaller in the slow cooling furnace provided on the downstream side than in the slow cooling furnace provided on the upstream side. Therefore, it is possible to efficiently promote the structural relaxation of the glass constituting the core. As a result, the transmission loss of the optical fiber is more likely to be reduced.
[0059]
The relationship between 　the virtual temperature T f of the glass constituting the core and the temperature T of the optical fiber when the temperature is lowered most efficiently depends only on the slow cooling time t, the slow cooling time t, and the length L of the slow cooling furnace. And the drawing speed v can be linked by the relation of the following equation (10).

[0060]
　Therefore, the required length L of the slow cooling furnace can be obtained by setting the target virtual temperature T f of the glass constituting the core contained in the manufactured optical fiber and determining the drawing speed v in consideration of productivity. .. For example, since the slow cooling time t is required to be about 0.1 second in order to set the virtual temperature T f to 1500 ° C., when the drawing speed v is set to 20 m / sec, the length L of the slow cooling furnace must be 2 m. I understand. Further, for example, since the slow cooling time t is required to be about 0.4 seconds in order to set the virtual temperature T f to 1400 ° C., when the drawing speed v is set to 10 m / sec, the length L of the slow cooling furnace needs to be 4 m. It turns out that there is. On the other hand, if the length L of the slow cooling furnace is only 2 m, it can be seen that the drawing speed v needs to be set to 5 m / sec. However, from the viewpoint of productivity and the like, the drawing speed v is preferably selected in the range of about 10 m / sec to 50 m / sec, the slow cooling furnace length L is preferably selected in the range of about 1 m to 10 m, and the slow cooling time t is 1 second or less. It is preferable to do so.
[0061]
　 After the
　slow cooling step P3, the optical fiber is covered with a coating layer in order to improve trauma resistance and the like. This coating layer is usually composed of an ultraviolet curable resin. In order to form such a coating layer, the optical fiber needs to be cooled to a sufficiently low temperature in order to prevent the coating layer from being burnt. The temperature of the optical fiber affects the viscosity of the resin applied and, as a result, the thickness of the coating layer. The appropriate temperature of the optical fiber for forming the coating layer is appropriately determined according to the properties of the resin constituting the coating layer.
[0062]
　In the method for manufacturing an optical fiber of the present embodiment, the section for sufficiently cooling the optical fiber is shortened by providing the slow cooling furnace 121 between the drawing furnace 110 and the coating device 131. In particular, since the optical fiber manufacturing method of the present embodiment also includes the precooling step P2, the section for sufficiently cooling the optical fiber is further shortened. Therefore, the optical fiber manufacturing method of the present embodiment includes a quenching step P4 in which the optical fiber exiting the slow cooling furnace 121 is rapidly cooled by the cooling device 122. In the quenching step P4, the optical fiber is cooled more rapidly than in the slow cooling step P3. By providing such a quenching step P4, the temperature of the optical fiber can be sufficiently lowered in a short section, so that the coating layer can be easily formed. The temperature of the optical fiber when exiting the cooling device 122 is, for example, 40 ° C. to 50 ° C.
[0063]
　The optical fiber cooled to a predetermined temperature through the cooling device 122 as described above passes through the coating device 131 containing the ultraviolet curable resin which is the coating layer covering the optical fiber, and is coated with the ultraviolet curable resin. Will be done. Further, by passing through the ultraviolet irradiation device 132 and being irradiated with ultraviolet rays, the ultraviolet curable resin is cured to form a coating layer, and the optical fiber 1 is formed. The coating layer usually consists of two layers. When forming two coating layers, it is possible to form a two-layer coating layer by coating the optical fiber with the ultraviolet curable resin constituting each layer and then curing the ultraviolet curable resin at once. Further, the second coating layer may be formed after the first coating layer is formed. Then, the direction of the optical fiber 1 is changed by the turn pulley 141, and the optical fiber 1 is wound by the reel 142.
[0064]
　Although a preferred embodiment of the present invention has been described above as an example, the present invention is not limited thereto. That is, the optical fiber manufacturing method of the present invention may include the above-mentioned drawing step and slow cooling step, and the precooling step and the quenching step are not essential components. Further, the method for manufacturing an optical fiber of the present invention is applicable to the manufacture of all kinds of optical fibers. For example, the method for producing an optical fiber of the present invention applies not only to an optical fiber containing silica glass as a main component but also to a method for producing an optical fiber containing other materials such as chalcogenide glass and fluoroglass as a main component. It is applicable if the constant A in the formula (5) and the activation energy Eact are obtained.
[0065]
　According to the present invention, a method for manufacturing an optical fiber capable of manufacturing an optical fiber with reduced transmission loss is provided, and it can be used in the field of optical fiber communication. It can also be used for fiber laser devices and other devices using optical fibers.
Code description
[0066]
1 ... Optical fiber
1P ...
Base material for optical fiber 110 ... Wire drawing furnace
111 ... Heating unit
120 ... Cylindrical body
121 ... Slow
cooling furnace 122 ... Cooling device
131 ... Coating device
132 ・ ・ ・ Ultraviolet irradiation device
141 ・ ・ ・ Turn pulley
142 ・ ・ ・ Reel
P1 ・・・ Drawing process
P2 ・・ ・ Precooling process
P3 ・ ・ ・ Slow cooling process
P4 ・・ ・ Quenching process
The scope of the claims
[Claim 1]
　A drawing step of drawing a base material for an optical fiber in a drawing furnace and
　a slow cooling step of slowly cooling the optical fiber drawn in the drawing step
are provided. In the
　slow cooling step, a plurality of the optical fibers are used. The
　time constant of structural relaxation of the glass constituting the core contained in the optical fiber is τ (T), the temperature of the optical fiber at a certain point in the slow cooling step is T, and the temperature of the optical fiber at a certain point in the slow cooling step is T. When the virtual temperature of the glass constituting the core is T f 0 and the virtual temperature of the glass constituting the core after a lapse of time Δt from a certain point in time is T f, the following is performed in an arbitrary period of the slow cooling step. A
method for manufacturing an optical fiber, characterized in that the formula (1) holds .

[Claim 2]
　The
method for manufacturing an optical fiber according to claim 1, wherein the following formula (2) holds in an arbitrary period of the slow cooling step .

[Claim 3]
　The set temperature T of the n-th of the annealing furnace from the upstream side sn , the fictive temperature of the glass constituting the core included in the optical fiber from the upstream side of the outlet of the n-th of the lehr T en when the following formula ( The
method for manufacturing an optical fiber according to claim 1 or 2, wherein the relationship of 3) is established .

[Claim 4]
　The
method for manufacturing an optical fiber according to claim 3, wherein the following formula (4) holds .

[Claim 5]

Claims 1 to 4 are characterized in that 　the temperature difference between the set temperature and the virtual temperature of the glass constituting the core at the outlet is smaller in the slow cooling furnace provided on the downstream side than in the slow cooling furnace provided on the upstream side. The method for manufacturing an optical fiber according to any one of the above items.
[Claim 6]
　Any one of
claims 1 to 5 , wherein the optical fiber stays in any of the plurality of slow cooling furnaces at least for a period of time when the temperature of the optical fiber is in the range of 1300 ° C. or higher and 1500 ° C. or lower. The method for manufacturing an optical fiber according to.