Entitled manufacturing method of the optical fiber

Technical field

[0001]

　The present invention relates to a method of manufacturing an optical fiber.

Background technique

[0002]

　To achieve long-distance and speed of the optical transmission rate of optical transmission distance in optical fiber communication systems, optical signal to noise ratio is not free if elevated. Therefore, reduction in transmission loss of the optical fiber has been demanded. In the current manufacturing process of the optical fiber is highly sophisticated, transmission loss due to impurities contained in the optical fiber is believed to be reduced to substantially limit. The major cause of transmission loss remains, the scattering loss due to fluctuations in the structure and composition of glass constituting the optical fiber. This is but the optical fiber is made of glass because those inevitable.

[0003]

　As a method of reducing the fluctuation of the structure of the glass, it is known to slowly cool on cooling the molten glass. As a method of thus molten glass slowly cooled, it has been tried to slow cooling the optical fiber immediately after being drawn from the drawing furnace. Specifically, or by heating the optical fiber drawn from the drawing furnace at annealing furnace, the optical fiber immediately after drawing by Guests enclosed in insulation, reducing the cooling rate of the optical fiber has been studied.

[0004]

　The following Patent Document 1, the temperature of the optical fiber from a position at which the outer diameter is smaller than 500% of the final outer diameter of an optical fiber having a core and a cladding mainly composed of silica glass of up to a position to be 1400 ° C. in more than 70% of the area, to set the temperature of the heating furnace so that the ± 100 ° C. or less (annealing furnace) disclosed the target temperature determined by the recursion formula. By thus temperature history of the optical fibers is controlled, the transmission loss is to be reduced fictive temperature of the glass constituting the optical fiber is lowered.

[0005]

Patent Document 1: JP 2014-62021 JP

Summary of the Invention

[0006]

　However, in the technique disclosed in Patent Document 1, it is required to repeat the complex calculations in order to adjust the temperature of the optical fiber to the ideal temperature changes sought recurrence formula. In the technique disclosed in Patent Document 1, the temperature of the optical fiber is allowed to deviate even ± 50 ℃ ~ 100 ℃ the target temperature determined by the recursion formula. The deviation of the temperature of the optical fiber in such a wide range is acceptable, it is hard to say that the temperature history is sufficiently optimized. For example, the temperature of the optical fiber to be slow cooling varies from ± 100 ° C., the fictive temperature of the glass constituting the optical fiber is also changed in a similar range, can be reached at the target temperature determined by the recursion formula fictive temperature When not only obtained 100 ° C. higher fictive temperature of the optical fiber than the transmission loss due to light scattering of the resulting optical fiber would increase even about 0.007dB / km. In such an optical fiber the conventional manufacturing method in which the temperature history is not well optimized for, or excessive capital investment is made to be longer than necessary annealing furnace, the productivity lowers the drawing speed than necessary It is or impaired.

[0007]

　The present inventors have, by appropriately setting the temperature of the annealing furnace, and therefore the temperature difference between the temperature of the fictive temperature and the optical fiber of glass constituting the optical fiber is properly controlled, structural relaxation of the glass constituting the optical fiber There is promoted, transmission loss due to light scattering of the optical fiber is found that is easy to be reduced.

[0008]

　Accordingly, the present invention is able to reduce the transmission loss of the optical fiber and to provide a method for producing easily fiber.

[0009]

　To solve the above problems, an optical fiber manufacturing method of the present invention, the annealing and drawing step of drawing the optical fiber preform drawing furnace, the optical fiber drawn in the wire drawing step annealing comprising a step of, in the annealing step, the optical fiber is passed through a plurality of the annealing furnace, the constant tau (T when the structural relaxation of the glass constituting the core included in the optical fiber n ), the slow cooling temperature T of the optical fiber at the time of the incoming line from the upstream side to the n-th said annealing furnace in step n , the virtual temperature T of the glass constituting the core at the time of the incoming fn , the time of the incoming line the fictive temperature T of the glass constituting the core after time Δt has elapsed from f when a, characterized in that the following equation (1) holds in any period of the slow cooling step.

[0010]

　The present inventors have found that by the optical fiber is gradually cooled while the temperature difference between the fictive temperature of the glass constituting the core contained in the temperature and the optical fiber of the optical fiber is controlled to the predetermined range, the core structural relaxation of the glass constituting the found that is facilitated. By structural relaxation of the glass is accelerated constituting the core, since the scattering loss due to fluctuations in the structure of the glass optical core constitutes the core as it is transmitted is reduced, the transmission loss of the optical fiber It is reduced. Further, a plurality of the annealing furnace is used in the annealing step as described above, by setting the temperature of the annealing furnace are properly controlled, the fictive temperature of the glass constituting the core contained in the temperature and the optical fiber of the optical fiber and temperature difference is easily controlled to the predetermined range. As a result, the structural relaxation of the glass is accelerated constituting the core, easily transmission loss of the optical fiber is reduced.

[0011]

　The manufacturing method for the optical fiber of the present invention, it is preferable that the following formula (2) is established in any period of the slow cooling step.

[0012]

　The temperature T of the optical fiber in this way slow cooling step n virtual temperature T of the glass constituting the core included in the said optical fiber f temperature difference between (T f -T n ) is controlled to a more appropriate range it allows easily the structural relaxation of the glass constituting the core included in the optical fiber is further promoted, the transmission loss of the optical fiber is easily further reduced.

[0013]

　Further, the method of manufacturing an optical fiber of the present invention, the n-th set temperature of the annealing furnace T sn when the, it is preferable that the relationship of the following equation (3) holds.

[0014]

　A plurality of the annealing furnace is used in the annealing step as described above, by being controlled to a predetermined range with respect to the virtual temperature of the glass setting temperature of each annealing furnace constituting the core at the inlet of each lehr, the optical fiber the temperature difference between the fictive temperature of the glass constituting the core contained in the temperature and the optical fiber is easily controlled to a predetermined range. As a result, the structural relaxation of the glass is accelerated constituting the core, easily transmission loss of the optical fiber is reduced.

[0015]

　The manufacturing method for the optical fiber of the present invention, it is preferable that the following formula (4) holds.

[0016]

　By thus setting the temperature of a plurality of the annealing furnace is controlled to a suitable range from each easily promoting effect of the glass structure relaxation constituting the cores included in the optical fiber can be more increased, the transmission loss of the optical fiber but likely to be further reduced.

[0017]

　The manufacturing method for the optical fiber of the present invention, than the annealing furnace provided on the upstream side towards the downstream side in the annealing furnace provided the temperature difference between the fictive temperature of the glass constituting the core in the set temperature and the inlet it is preferable that the small.

[0018]

　The present inventors have found that the better the temperature of the glass has reduced the temperature difference between the temperature of the fictive temperature and the glass of the glass becomes lower is likely to be promoted structural relaxation of the glass. Thus, by than annealing furnace provided upstream to set the temperature of the lehr so ​​that the temperature difference between the fictive temperature of the glass towards the annealing furnace provided in the downstream side to configure the core in the set temperature and the inlet is reduced, efficient structural relaxation of the glass constituting the core may be promoted. As a result, it is easy transmission loss of the optical fiber can be further reduced.

[0019]

　Further, it is preferable that the optical fiber in at least one time when the temperature of the optical fiber is in the range of 1500 ° C. 1300 ° C. or higher to stay in one of the plurality of the annealing furnace.

[0020]

　By optical fiber is gradually cooled when the temperature of the optical fiber is in this range, tends fictive temperature of the glass constituting the core included in the optical fiber is reduced in a short time, the transmission loss of the optical fiber is reduced likely to be.

[0021]

　As described above, according to the present invention, a method of manufacturing easy optical fiber to reduce the transmission loss of the optical fiber is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]

FIG. 1 is a flow chart illustrating the steps of the method of manufacturing an optical fiber of the present invention.
2 is a diagram schematically showing a structure of an apparatus for use in the production method of the optical fiber of the present invention.
3 is a graph showing the relationship between the virtual temperature and annealing time temperature and the glass of the glass.
[4] the temperature difference between the fictive temperature and the glass temperature of the glass (T f 0 and -T), the rate of decrease per virtual unit temperature time of the glass ((T f -T f 0 and) / Δt), the relationship is a graph showing schematically. 5 is a graph showing temporal changes of the temperature difference between the fictive temperature and the temperature of the glass of the glass. The [6] 5 is a graph showing temporal changes of the temperature difference between the temperature of the fictive temperature and the glass of the glass at different initial conditions. The [7] FIGS. 5 and 6 is a graph showing temporal changes in temperature of the glass at different initial conditions. [8] optimized temperature difference shown by the solid line in FIG. 6 (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) is a graph showing the upper and lower limits. [9] the set temperature of each annealing furnace, the fictive temperature of the optimized glass at the inlet of each annealing furnace, and is a graph showing the virtual temperature at the inlet of each annealing furnace of glass undergoing a virtual temperature history.

DESCRIPTION OF THE INVENTION

[0023]

　It will be described in detail with reference to the accompanying drawings preferred embodiments of the method of manufacturing an optical fiber according to the present invention.

[0024]

　Figure 1 is a flow chart illustrating the steps of the method of manufacturing an optical fiber of the present invention according to one embodiment. As shown in FIG. 1, an optical fiber manufacturing method of the present embodiment includes a drawing step P1, the pre-cooling step P2, the annealing step P3, the quenching step P4, the. The following describes these steps. Incidentally, FIG. 2 is a diagram schematically showing the configuration of an apparatus used for the method of manufacturing an optical fiber according to the present embodiment.

[0025]

　
　drawing process P1 is a step of drawing the end of the preform 1P optical fiber in a drawing furnace 110. First, a preform 1P for constructed optical fiber of glass with the same refractive index distribution as the core and cladding constituting the desired optical fiber 1. Optical fiber 1 has a cladding which surrounds with no gap the one or more cores and the outer peripheral surface of the core. The core and cladding are made from each silica glass, the refractive index of the core is higher than the refractive index of the cladding. For example, if made of silica glass dopants such as germanium to the core to increase the refractive index is added, the cladding is composed of pure silica glass. For example, when the core is made of pure silica glass, the cladding is composed of silica glass dopant such as fluorine to lower the refractive index is added.

[0026]

　Then, the preform 1P optical fiber in the longitudinal direction to suspend to be perpendicular. Then, the preform 1P optical fiber disposed drawing furnace 110, by heating the heating section 111 to heat the lower end of the preform 1P optical fiber. The lower end of this time the optical fiber preform 1P is heated and becomes a molten state, for example, in 2000 ° C.. Then, draw from the drawing furnace 110 glass melted from the lower end of the preform 1P heating optical fiber at a predetermined drawing speed.

[0027]

　
　precooling step P2 is a step of cooling to a predetermined temperature suitable for optical fiber drawn from the drawing furnace 110 in drawing process P1 is sent to the lehr 121 to be described later. For a given temperature of the optical fiber suitable for being fed to annealing furnace 121 will be described in detail later.

[0028]

　In the method for manufacturing an optical fiber according to the present embodiment, row by precooling step P2 is the optical fiber drawn by the drawing step P1 in the hollow portion of the cylindrical body 120 provided immediately below the drawing furnace 110 is passed divide. By providing the tubular body 120 immediately below the drawing furnace 110, the atmosphere in the hollow portion of the cylindrical body 120 is substantially the same as the atmosphere in the drawing furnace 110. Therefore, it is prevented that the atmosphere and the temperature around the optical fiber immediately after being drawn rapidly changes.

[0029]

　Temperature of the optical fiber to be sent to the annealing furnace 121 is determined primarily by the atmosphere of the drawing speed and the drawing furnace 110. By providing pre-cooling step P2, the cooling rate of the optical fiber is finely adjusted, easily adjust the entering temperature of the optical fiber to the annealing furnace 121 to the appropriate range. Based on the temperature of the optical fiber suitable for being sent from the drawing furnace 110 to a temperature and annealing furnace 121 of an optical fiber drawn, suitably a length of distance and the tubular member 120 between the annealing furnace 121 and the drawing furnace 110 it can be selected. Cylindrical body 120 is constituted by, for example, a metal tube or the like. Or cooling the metal pipe, and or arranged insulation material around the metal pipe may be adjusted to the cooling rate of the optical fiber.

[0030]

　
　annealing step P3 is a step of annealing the optical fiber drawn at drawing step P1. In the manufacturing method of the optical fiber of this embodiment, the optical fiber is temperature adjusted through a pre-cooling step P2, is gradually cooled in the slow cooling step P3. In slow cooling step P3, the optical fiber is a plurality of the annealing furnace 121a, 121b, 121c, passed through the 121d. In the description of the method of manufacturing an optical fiber of this embodiment, when there is no need to distinguish between the case and the annealing furnace to cover all these annealing furnace simply referred to as "annealing furnace 121". In FIG. 2, four lehr 121a, 121b, 121c, while indicating 121d, the number of the annealing furnace in the present invention is not particularly limited as long as it is plural. Xu because cooling furnace there is a plurality, it is meant that there are a plurality of heat generating portions which may be set to different temperatures. For example, even are contained in one casing, if provided with plurality of heat generating portions which can be set to different temperatures, it can be said that the annealing furnace there are multiple.

[0031]

　The annealing furnace 121 are different from the predetermined temperature and the temperature of the optical fiber to the incoming lines, the temperature around the optical fibers incoming to the annealing furnace 121, the cooling rate of the optical fiber is lowered. By cooling rate of the optical fiber is lowered at annealing furnace 121, as described below, the structure of the glass constituting the core included in the optical fiber is reduced, the optical fiber 1 is obtained scattering loss is reduced.

[0032]

　In the method of manufacturing an optical fiber having a conventional annealing step, the temperature of the optical fiber at the input line to the annealing furnace is not sufficiently optimized. Specifically, the temperature of the optical fiber is input lines to the annealing furnace in a state of too high or too low. When the temperature of the optical fiber to the incoming line to the annealing furnace is too high, since the rate at which the structure of the glass constituting the optical fiber to relax very fast, can hardly be expected to obtain the effect of slow cooling the optical fiber. Meanwhile, since the speed of relaxation structure of the glass constituting the optical fiber when the temperature is too low for optical fiber incoming to the annealing furnace becomes slow, it may be necessary, such as reheating the optical fiber at the annealing furnace occurs. Thus, the conventional annealing step, it is hard to say that the structural relaxation of glass constituting the optical fiber is efficiently performed. Therefore, there is a fear that or annealing furnace or conducted excessive capital investment for longer than necessary, the productivity is impaired by slow drawing speed than necessary.

[0033]

　According to the manufacturing method for the optical fiber of this embodiment, in a slow cooling step P3 as described below, by appropriately setting the temperature of the annealing furnace 121, the glass constituting the core included in the optical fiber fictive temperature and light and therefore the temperature difference between the temperature of the fiber is properly controlled, structural relaxation of the glass constituting the core is facilitated. As a result, without requiring excessive capital investment, and, with good productivity, it is possible to obtain an optical fiber 1 which transmission loss is reduced. Further, according to the manufacturing method for the optical fiber of this embodiment does not require the time of manufacture of complex calculations, such as the technique disclosed in the cited document 1 described above.

[0034]

　The silica glass is classified into a so-called strong glass, the time constant of by the considered structure relaxation viscous flow of the glass tau (T) follows the equation Arrhenius. Therefore, the time constant tau (T) is a constant A and the activation energy E depends on the composition of the glass act with, is expressed by the following equation (5) as a function of the temperature T of the glass. Incidentally, k B is the Boltzmann constant.

(Here, T is the absolute temperature of the glass.)

[0035]

　From the formula (5), the temperature of the glass relaxes the structure of higher fast glass, it can be seen that reach faster equilibrium at that temperature. That is, the fictive temperature of the glass as the temperature of the glass is high is increased that approaches the temperature of the glass.

[0036]

　The relationship between the virtual temperature and time of the temperature and the glass of the glass at the time of annealing the glass shown in Fig. In the graph shown in FIG. 3, the horizontal axis represents time, the vertical axis represents the temperature. 3, the solid line indicates the temperature transition of the glass in the gradual cooling conditions with, a broken line indicates a transition of the glass of the virtual temperature at that time. The dotted line shows the temperature transition of the glass in the case of gradual cooling rate than the slow cooling conditions shown by the solid line, chain line indicates the transition of the glass of the virtual temperature at that time.

[0037]

　As shown by the solid line and the broken line in FIG. 3, decreases as well fictive temperature of the glass when the temperature of the glass at high temperatures is reduced over time. The temperature is sufficiently high state of glass in this way, a very high rate of structural relaxation of the glass. However, the speed of the glass structure relaxation as the temperature of the glass is lowered slowly. Before long decline of the fictive temperature of the glass will not be able to follow the lowering of the temperature of the glass. Then, the temperature difference between the virtual temperature of the glass of the glass increases. Here, when the gentle cooling rate of the glass, the glass will be held for a long time in a relatively high temperature conditions than when the cooling rate is high. Therefore, as shown by a dotted line and a dashed line in FIG. 3, the temperature difference between the virtual temperature of the glass of the glass decreases, the fictive temperature of the glass is lower than the example described above. In other words, when the moderate cooling rate of the glass, structural relaxation of the glass tends to be promoted.

[0038]

　As described above, when the temperature of the glass is high, the structure of the glass relaxes faster. However, the fictive temperature of the glass is so not become lower than the temperature of the glass, when the temperature of the glass is high, remains higher fictive temperature of the glass. That is, the effect due to the slow cooling when the temperature of the glass is too high is small. From this viewpoint, it is preferable that the temperature of the optical fiber to be stayed annealing furnace 121 is 1600 ° C. or less, more preferably 1500 ° C. or less. On the other hand, when the temperature of the glass is low is reduced to a lower temperature fictive temperature, the rate of decrease in fictive temperature slows. That is, it takes time to slow cooling to reduce sufficiently the fictive temperature and the temperature of the glass is too low. From this viewpoint, it is preferable that the temperature of the optical fiber to be stayed annealing furnace 121 is 1300 ° C. or higher, more preferably 1400 ° C. or higher. Therefore, it is preferable to stay optical fibers to the annealing furnace 121 at least one time when the temperature of the optical fiber is in the range of 1300 ° C. or higher 1500 ° C. or less. In this way, by the optical fiber is gradually cooled when the temperature of the optical fiber is in a predetermined range in the slow cooling step P3, the fictive temperature of the glass constituting the core included in the optical fiber is reduced in a short time easily, easily transmission loss of the optical fiber is reduced.

[0039]

　Then, one of the relationship between the virtual temperature of the glass of the glass, of the structural relaxation of the glass constituting how the core by slowly cooling the optical fiber efficiently promoted, thereby reducing the transmission loss of the optical fiber explain.

[0040]

　Virtual glass constituting the core of the time constant of the structural relaxation of the glass constituting the core included in the optical fiber tau (T), the temperature of the optical fiber at some point in the annealing step P3 T, when the certain the temperature T f 0 when the fictive temperature T of the glass constituting the core after time Δt has elapsed from the time when the certain f is expressed by the equation (5) the following formula (6). Incidentally, Delta] t is a minute time, is assumed to be between the T is constant.

[0041]

　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) When structural relaxation constant tau (T), the fictive temperature T f 0 when the temperature of the glass is is T, the virtual temperature T of the glass f temperature difference between the temperature T of the glass (T f -T) There is defined as the time until the 1 / e, the temperature difference (T f 0 fictive temperature T per unit as -T) is somewhat greater time f change of increase.

[0042]

　Fictive temperature T f 0 temperature difference when the temperature of the optical fiber comprising a core composed of a glass which is in T (T f 0 and -T), fictive temperature T f change per unit time ((T f -T f 0 ) / Delta] t) and the relationship is shown schematically in Figure 4 of. As shown in FIG. 4, the virtual temperature T of the glass constituting the core f 0 conditions and the temperature T of the optical fiber is coincident (T f 0 in = T), structural relaxation of the glass constituting the core occur not, change per unit time of the fictive temperature is 0 ((T f -T f 0 ) / Delta] t = 0). Lowering the temperature T of the optical fiber from which the virtual temperature T of the glass constituting the core f 0 the temperature difference (T between the temperature T of the optical fiber f 0 Given -T) f the rate of change of ((T f -T f 0 ) / Delta] t) increases in the negative. However, further virtual temperature T of the glass constituting the core by lowering the temperature T of the optical fiber f 0 temperature difference between the temperature T of the optical fiber (T f 0 Given -T) f changes in ((T f -T f 0 is the absolute value of the) / Delta] t) becomes smaller. That is, as the downward peak that appears in the graph in FIG. 4, the virtual temperature T of the glass constituting the core f 0 the temperature difference (T between the temperature T of the optical fiber f 0 when -T) ((T on f -T f 0 ) / Delta] t) it can be seen that the minima.

[0043]

　Now solving the above equation (6), the virtual temperature T of the glass f temperature T and the virtual temperature T of the glass when the rate of decrease is maximized f that holds true relationship of the following equation (7) between the Understand.

[0044]

　Furthermore the above formula (7) and solving for T as Equation (8), the virtual temperature T of the glass f may determine a temperature T of the glass when it is possible to most efficiently reduce. Hereinafter, the virtual temperature T of the glass f the temperature of the glass at which it is possible to most efficiently reduce the sometimes referred to as "the temperature of the optimized glass", "optimizing the most efficient virtual temperature was reduced It has been sometimes referred to as virtual temperature ".

[0045]

　As so far described, the virtual temperature T of the glass at a given point in time f 0 the temperature difference (T between the temperature T of the glass f 0 fictive temperature T of the glass when the -T) is a predetermined value f unit time change per becomes the largest. That is, the virtual temperature T f 0 fictive temperature T after a predetermined time Δt has elapsed glass f when considering the virtual temperature T f so that the temperature T of the glass which can be a minimum value exists.

[0046]

　G core e O 2 for general single mode optical fiber doped with a virtual temperature T of the glass constituting the core obtained from the above equation (6) f and the value of when the the lowest value, the light at that time temperature difference between the temperature T of the fiber (T f of measuring the change in the -T). Here, it is assumed a case in which the slow cooling step P3 immediately after heating and melting an optical fiber preform in a drawing step P1. The temperature T of the optical fiber in the slowly cooling the initial (when annealing time is 0 seconds) 0 When is assumed to be 1800 ° C., the structural relaxation time of the glass constituting the core at this temperature and very less than 0.001 seconds short. Therefore, the virtual temperature T of the annealing initial glass constituting the core f 0 may be considered is equally 1800 ° C.. That, T f 0 -T 0 = 0 ° C. and the initial value is assumed. Further, the formula (5) and (7) constant A and the activation energy E in act is non-patent document 1 (K. Saito, et al ., Journal of the American Ceramic Society, Vol.89, pp.65- 69 (2006)), and non-patent Document 2 (K. Saito, et al ., Applied Physics Letters, Vol.83, pp.5175-5177 (2003) employs the values listed in) 0 Δt the results calculated as .0005 seconds shown in FIG. In the graph of FIG. 5, the vertical axis, the virtual temperature T of the glass constituting the core f temperature difference between the temperature T of the optical fiber at that time (T f -T), the horizontal axis is annealing time of the optical fiber is there. 5, the solid line constants A and activation energy E described in Non-Patent Document 1 act shows the results using a broken line constants A and activation energy E described in Non-Patent Document 2 act with It shows the results. In these conditions, the fictive temperature of the glass constituting the core when the annealing time of 0.5 seconds, 1390 ° C. respectively, obtained as 1322 ° C..

[0047]

　Obtained by the above assumptions, the temperature difference between the temperature of the fictive temperature and the optical fiber of the glass constituting the core (T f time domain looking at aging of the -T), may be gradually increased -T), the temperature difference (T in approximately 0.01 seconds after the time area from the slow cooling starting f it can be seen that it is sufficient to -T) The temperature difference at all in the time domain (T f -T) a generally well when made to be less than 60 ° C., the temperature difference (T in most of the time domain f -T) a generally 40 ° C. higher than generally less than 60 ° C. by controlling the temperature T of the optical fiber so as to maintain the virtual temperature T of the glass constituting the core f it can be seen that decreases efficiently. The temperature difference shown in FIG. 5 (T f -T) time is maximized, the above equation (5) constant A and the activation energy E in act in and annealing the initial (when annealing time is 0 seconds) the temperature T of the optical fiber 0 fictive temperature T of the glass constituting the core and f 0 somewhat back and forth by, but becomes approximately 0.01 seconds.

[0048]

　Here the temperature difference seen (T f -T) from slow cooling starts until the reaches approximately 60 ° C. until after 0.01 seconds, change per virtual unit temperature time shown in the graph of FIG. 4 ((T f -T f 0 in) / Delta] t) temperature conditions indicated on the left side than the minimum value of. Thus, T as above f 0 -T 0 when starting annealing at = 0 ° C., the structural relaxation due to slow cooling is considered not performed efficiently. Therefore, the pre-cooling step P2 before the annealing step P3 provided, the temperature difference (T in pre-cooling step P2 f is assumed to carry out slow cooling step P3 after performing pre-cooling to -T)

[0049]

Therefore,　T f 0 -T 0 further verified by assuming an initial value of a = 60 ° C.. That is, the temperature T of the optical fiber in the slowly cooling the initial (when annealing time is 0 seconds) 0 is 1500 ° C., the fictive temperature T of the glass constituting the core of this case f 0 is assumed initial value of 1560 ° C.. Then, similar to the results shown in FIG. 5, virtual temperature T of the glass constituting the core f temperature difference between the value when the the lowest value, and the temperature T of the optical fiber at that time (T f of -T) In the graph of FIG. 6, the vertical axis, the virtual temperature T of the glass constituting the core f temperature difference between the temperature T of the value as the optical fiber at the time of when the the lowest value (T f -T), the horizontal axis is a slow cooling time of the optical fiber. The solid line is a constant A and the activation energy E described in Non-Patent Document 1 act shows the results using a broken line constants A and activation energy E described in Non-Patent Document 2 act shows the result of using there. As shown in FIG. 6, the temperature difference at all in the time domain (T f -T) has continued to decrease monotonically, the temperature T of the optical fiber virtual temperature T of the glass constituting the core f suitable for reduction of it can be seen that are kept in the range. In this condition, the fictive temperature of the glass constituting the core when the annealing time of 0.5 seconds, respectively 1387 ° C., determined to be 1321 ° C., a virtual glass constituting the core than in the condition shown in FIG. 5 temperature can be further reduced.

[0050]

　Then, T f 0 -T 0 as = 120 ° C., the fictive temperature T of the glass constituting the core f further verify the temperature difference between the temperature T of the optical fiber assuming the large initial value. That is, the temperature T of the optical fiber in the slowly cooling the initial (when annealing time is 0 seconds) 0 is 1500 ° C., the fictive temperature T of the glass constituting the core of this case f 0 is assumed initial value of 1620 ° C.. The fictive temperature T of the glass constituting the core f indicate when is the minimum value, the time course of the temperature T of the optical fiber in FIG. Constants A and activation energy E act used a value described in Non-Patent Document 1. The initial temperature difference (T f if -T) is large, the change per virtual unit temperature time shown in the graph of FIG. 4 ((T f -T f 0 temperature represented by the right side of the minimum value of) / Delta] t) in the conditions. That is, the virtual temperature T of the glass constituting the core f if is higher than the temperature T of the optical fiber, the virtual temperature T by raising the temperature T of the optical fiber f temperature difference close to (T f -T) is less who is faster structural relaxation. Therefore, as can be seen from Figure 7, the temperature T of the optical fiber is increased once from start slowly cooled generally 0.01 seconds. Then, the temperature difference (T f after -T) becomes appropriate, the temperature of the optical fiber T is likewise monotonically decreases with FIG. Thus gradual cooling initial temperature difference (T f if -T) Accordingly, to the waste heating, it can not be performed efficiently annealing within the time which the optical fiber to stay lehr. In this condition, the fictive temperature of the glass constituting the core when the annealing time of 0.5 seconds determined to be 1389 ° C., the fictive temperature of the glass constituting the core is reduced than the case of the condition shown in FIG. 5 that although it falls short when the conditions shown in FIG.

[0051]

　It can be seen from the above assumption, the temperature difference between the temperature of the fictive temperature and the optical fiber of the glass constituting the core (T f such that -T) becomes appropriate, after having a pre-cooling step P2 subsequent to the drawing step P1 by performing the annealing step P3 to is that it is possible to effectively utilize the time that the optical fibers stay in the annealing furnace to efficiently structural relaxation of the glass constituting the core. That is, the temperature difference (T between the temperature of the fictive temperature and the optical fiber of the glass constituting the core f performs pre-cooling step P2 until -T)

[0052]

　Further, the following can be known from the results shown in FIGS. That is, the constant A and the activation energy E is determined based on the composition of the glass act somewhat on the value of the difference even if the temperature of the fictive temperature and the glass of the glass at the time the slow cooling step P3 is started the temperature difference (T f if -T) Therefore, if the concentration of the so-called dopant is composed mainly low general optical fiber is silica glass, the temperature difference between the temperature of the fictive temperature and the optical fiber of glass constituting the optical fiber (T f -T) is generally by slow cooling of the optical fiber under the condition of 60 ° C. it is started, the virtual temperature of the glass constituting the optical fiber is reduced efficiently. 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 also or cores and consisting of substantially pure silica glass, fluorine, etc. in any of the dopant of the cladding made of doped silica glass, it is also effectively lowers the fictive temperature.

[0053]

　According to the manufacturing method for the optical fiber of this embodiment, a plurality of the annealing furnace 121 are use to from the start to the end annealing step P3, the optical fiber in each annealing furnace 121 sequentially while lowering the temperature T gradually the incoming line. From the results shown in FIGS. 5 and 6, the temperature difference between the fictive temperature of the glass constituting the core contained in the temperature and the optical fibers of the optical fiber when causing efficient structural relaxation by slow cooling (T f -T) is monotonically decreases over the course of the annealing time. That is, the time constant of the structural relaxation of the glass tau (T), the temperature of the optical fiber at some point in the annealing step P3 T, the fictive temperature of the glass constituting the core at the time the said certain T constituting the core f 0 , the fictive temperature of the glass constituting the core after time Δt has elapsed from the time when the certain T f when a, it is preferable that the following formula (2 ') holds.

[0054]

　Accordingly, the temperature T of the optical fiber at the time of the incoming line from the upstream side to the n-th annealing furnace 121 at a slow cooling step P3 n , the virtual temperature T of the glass constituting the core at the time the incoming line to the annealing furnace fn , its the fictive temperature of the glass constituting the core after time Δt has elapsed from the time that the incoming lines to the annealing furnace T f when a, it is preferable that the following formula (2) holds.

[0055]

　The case of using the plurality of the annealing furnace 121 at slow cooling step P3, by setting the temperature of the annealing furnace 121 is properly controlled, the temperature T and the optical fiber of the optical fiber when the incoming line to each of the annealing furnace 121 fictive temperature T of the glass constituting the cores included f temperature difference between (T f -T) is controlled to the predetermined range, structural relaxation of the glass constituting the core included in the optical fiber is further promoted liable Become. Therefore, it is easy transmission loss of the optical fiber is reduced.

[0056]

　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 conditions -T) is is as described above, it is possible to sufficiently reduce the transmission loss of the optical fiber in conditions described below.

[0057]

　Fictive temperature T of the glass constituting the core included in the optical fiber f with transmission loss of the optical fiber is tied by the following relation. Rayleigh scattering coefficient R r virtual temperature T of the glass constituting the core f proportional to the transmission loss α by Rayleigh scattering T is represented by the following formula (9) to lambda [[mu] m] of the wavelength of light to be transmitted.
Arufa T = R R / Ramuda 4 = BT F / Ramuda 4 · · · (9)

[0058]

　Here, according to Non-Patent Document 2, B = 4.0 × 10 -4 dB / miles / [mu] m 4 is / K. Given the transmission loss at a wavelength lambda = 1.55 .mu.m, the fictive temperature T of the glass constituting the core f When 14 ° C. increases, the transmission loss α by Rayleigh scattering T is substantially increased 0.001 dB / miles. That is, the virtual temperature T is optimized f if it is possible to suppress errors from below 14 ° C., the transmission loss α by Rayleigh scattering T it is possible to suppress an increase in the less than 0.001 dB / miles.

[0059]

　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 and the temperature difference (T between the temperature of the optical fiber f at temperature conditions that -T)

[0060]

　Temperature difference indicated by the solid line in FIG. 6 (T f virtual temperature T of the glass constituting the core when slowly cooled 0.5 seconds -T) f increases relative to scattering losses expected from the less than 0.001 dB / miles temperature difference when the suppressing (T f -T) can be predicted from the recursion formula (6). Similar to the assumptions shown in FIG. 6, the virtual temperature T of the glass constituting the core of the optical fiber in the slowly cooling the initial (when annealing time is 0 seconds) f 0 to 1560 ° C., the temperature difference (T f the -T) The constants A and activation energy E act Solving recurrence formula (6) using the values as shown in Non-Patent Document 1, the graph of FIG. 8 is obtained. 8, the temperature difference indicated by the solid line in FIG. 6 (T f virtual temperature T of the glass constituting the core when slowly cooled 0.5 seconds -T) f increment to the transmission losses expected from There the temperature difference in the case of equal to or less than 0.001 dB / miles (T f indicates the upper limit of the time course of -T) by a broken line shows the lower limit by the chain line. Furthermore, the temperature difference indicated by the solid line in FIG. 6 in FIG. 8 (T f is again shown -T)

[0061]

　The following are clear from the results shown in FIG. In slow cooling step P3, the temperature difference (T f if -T) f with respect to a virtual temperature of the glass constituting the core is suppressed to a range that does not rise above about 14 ° C.. As a result, the transmission loss can be suppressed to increase the 0.001 dB / miles or less with respect to the value when optimizing conditions is decreased most.

[0062]

　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 T) is maintained generally in the range of approximately less than 100 ° C. higher than 20 ° C., tends glass structure relaxation constituting the cores included in the optical fiber is promoted, easy transmission loss of the optical fiber is reduced Become. That is, it is preferable that the following formula (1 ') is satisfied.

[0063]

　Therefore, temperature T of the optical fiber at the time of the incoming line from the upstream side to the n-th annealing furnace at annealing step P3 n , the virtual temperature T of the glass constituting the core at the time the incoming line to the annealing furnace fn , the annealing furnace the fictive temperature of the glass constituting the core of the time Δt elapses after the time of incoming T to f when a, it is preferable that the following formula (1) holds.

[0064]

　That is, the virtual temperature T of the glass constituting the core f temperature difference (T between the temperature of the optical fiber f with -T)

[0065]

　Next, a specific example of order to facilitate satisfying the condition of formula (2) or the formula (1). In the manufacturing method of the optical fiber of this embodiment, four lehr 121a in slow cooling step P3, 121b, 121c, and 121d is used. By using a plurality of the annealing furnace 121 Thus, it is easy to control the temperature difference between the fictive temperature of the glass constituting the temperature and the core of the optical fiber in a predetermined range. That is, in the annealing step P3 through an optical fiber to a plurality of the annealing furnace 121, the set temperature of the n-th annealing furnace 121 from the upstream side T sn when the, so that satisfied the relationship of the following formula (3).

[0066]

　As described above, the temperature difference between the fictive temperature of the glass constituting the core contained in the temperature and the optical fiber of the optical fiber (T f -T) is an optical fiber in a state controlled to a predetermined range is gradually cooled it allows structural relaxation of the glass constituting the core is facilitated. By structural relaxation of the glass is accelerated constituting the core, since the scattering loss due to fluctuations in the structure of the glass optical core constitutes the core as it is transmitted is reduced, the transmission loss of the optical fiber It is reduced. A plurality of the annealing furnace 121 is used in the slow cooling step P3 as described above, predetermined with respect to a virtual temperature of the glass setting temperature of each annealing furnace 121 constitutes the core in the slow cooling time to reach the inlet of the annealing furnace 121 by being controlled to the range of the temperature difference between the fictive temperature of the glass constituting the core contained in the temperature and the optical fiber of the optical fiber is easily controlled to a predetermined range. As a result, structural relaxation of the glass constituting the core is accelerated, the transmission loss of the optical fiber is reduced. Specifically described below with reference to FIG.

[0067]

　9, the temperature T of the optical fiber as the initial value 0 is 1500 ° C., the fictive temperature T of the glass constituting the core of this case f 0 is calculated from equation (5) when assuming an initial value of 1560 ° C., optimized virtual temperature change of the glass constituting the core (solid line), annealing furnace 121a, 121b, 121c, the set temperature of 121d (dashed line), annealing furnace 121a, 121b, 121c, and reaches the inlet or outlet of 121d It shows the expected fictive temperature of the glass constituting the core in the annealing time to. In the example shown in FIG. 9, each length of the annealing furnace 121 is 0.5 m, drawing speed is assumed to 20 m / sec.

[0068]

　As indicated by triangles (▲) in FIG. 9, when the optical fiber to each annealing furnace 121 incoming lines, and when the optical fiber line out most downstream annealing furnace 121d, i.e., the slow cooling time is 0.000 seconds, 0. 025 seconds, 0.050 seconds, 0.075 seconds, the virtual temperature T is properly of the glass constituting the core when the 0.100 seconds f is, 1560 ° C. respectively, 1517 ℃, 1493 ℃, 1477 ℃, 1464 ℃ to be calculated. Then, annealing furnace 121a, 121b, 121c, the set temperature of 121d are set as indicated by one-dot chain lines in FIG. That is, optimized virtual temperature T of the glass constituting the core in the slow cooling time when it reaches the inlet of the annealing furnace 121 f to set the temperature of each annealing furnace 121 from 70 ° C. lower temperature. As a result, the temperature of the optical fiber in the vicinity of the outlet of the respective annealing furnace 121 approaches the set temperature of each annealing furnace 121, the temperature difference (T fictive temperature of the glass in the vicinity of the outlet of the annealing furnace 121 and the temperature of the optical fiber at that time F -T) becomes smaller. The glass passing through the virtual temperature history is temporarily with a rapid change deviates the condition of equation (1) that the temperature of the glass that is the incoming lines to each annealing furnace 121 matches immediately setting temperature of each annealing furnace 121 It is expected to have a virtual temperature indicated by circular (●) in FIG.

[0069]

　Since the actual temperature of the glass approaches the set temperature of the annealing furnace decreases more slowly, actual virtual temperature slightly higher than the ideal virtual temperature indicated by triangles (▲), than the fictive temperature indicated by circular (●) Although is slightly lower, the error acceptable range. In the example shown in FIG. 9, the difference between the virtual temperature and optimized virtual temperature of the glass that has undergone a virtual temperature history after slow cooling 0.100 seconds is 1.1 ° C., the scattering loss 0. the difference of less than 001dB / km only.

[0070]

　In view of the above, to control the temperature difference between the fictive temperature of the glass constituting the temperature and the core of the optical fiber to a more appropriate range, i.e., from the viewpoint of easily satisfying the above expression (2), the following equation ( 4) it is preferred that holds.

[0071]

　By setting the temperature of the thus lehr 121 is controlled to a more appropriate range, easily promoting effect of structural relaxation of the glass constituting the core included in the optical fiber is increased, the transmission loss of the optical fiber is reduced easily.

[0072]

　Further, as shown in FIGS. 5 and 6, easy towards the temperature of the glass has reduced the temperature difference between the temperature of the fictive temperature and the glass of the glass becomes lower is accelerated structural relaxation of the glass. Thus, towards the annealing furnace 121 provided downstream of the lehr 121 provided on the upstream side, and the difference between the virtual temperature of the glass constituting the core in the set temperature and the inlet is small. For example, as shown in FIG. 6 with solid lines, slow cooling time is 0.025 seconds, 0.050 seconds, 0.075 seconds, the glass constituting the optimized temperature and core of the glass at 0.100 seconds proper the difference between the reduction virtual temperature, 55 ° C., respectively, 54 ℃, 53 ℃, was 52 ° C., and the difference in temperature is small enough becomes downstream. Thus, also from the annealing furnace provided upstream to set the temperature of the lehr so ​​that the temperature difference between the fictive temperature of the glass towards the annealing furnace provided in the downstream side to configure the core in the set temperature and the inlet is reduced Accordingly, it is possible to promote the structural relaxation of the glass constituting the efficient core. As a result, it is easy transmission loss of the optical fiber can be further reduced.

[0073]

　Note that the virtual temperature T of the glass constituting the temperature T and the core of the optical fiber f relationship with, if the composition of the optical fiber are the same depends only on slow cooling time t, annealing time t, the annealing furnace length L and the drawing speed v can be linked by the relationship of the following formula (10).

[0074]

　Therefore, the virtual temperature T a target of the glass constituting the core included in the optical fiber manufactured f sets the length L of the required lehr be determined drawing speed v in consideration of productivity is required . For example, the virtual temperature T f that since slow cooling time to the 1500 ° C. t require approximately 0.1 seconds, to set the drawing speed v to 20 m / sec, the length L of the annealing furnace is necessary 2m It is seen. Further, for example, the virtual temperature T f because slow cooling time t required about 0.4 seconds to a 1400 ° C., and when setting the drawing speed v to 10 m / sec, the length L of the annealing furnace is necessary 4m there it can be seen. On the other hand, the length of the annealing furnace L is If there is only 2m, it can be seen that there is a need to drawing speed v and 5 m / sec. However, from the viewpoint of productivity and the like, the drawing speed v is 10 m / sec ~ 50 m / sec about, annealing furnace length L is preferably selected in the range of about 1 m ~ 10 m, annealing time t and less than 1 second it is preferable to.

[0075]

　
　After annealing step P3, the optical fiber is covered with a covering layer in order to increase and external damage resistance. The coating layer is generally composed of a UV-curable resin. Such in order to form a coating layer, so that burning of the coating layer and the like does not occur, it is necessary to the optical fiber is cooled to a sufficiently low temperature. Temperature of the optical fiber affects the viscosity of the resin applied, affects the thickness of the coating layer as a result. Temperature suitable optical fiber in forming the coating layer is appropriately determined according to the properties of the resin constituting the coating layer.

[0076]

　In the manufacturing method of the optical fiber of this embodiment, by lehr 121 between the drawing furnace 110 and the coating unit 131 is provided, the interval for sufficiently cooling the optical fiber becomes short. Especially since the manufacturing method of the optical fiber of this embodiment also comprises pre-cooling step P2, the interval for sufficiently cooling the optical fiber is further reduced. Therefore, in the manufacturing method of the optical fiber of this embodiment, it comprises a rapid cooling step P4 to quench the optical fiber exiting the annealing furnace 121 by the cooling device 122. In rapid cooling step P4, rapid optical fiber is cooled than slow cooling step P3. By providing such a quenching step P4, it is possible to sufficiently lower the temperature of the optical fiber in a short period, it is easy to form a coating layer. Temperature of the optical fiber as it exits the cooling device 122 is, for example, 40 ℃ ~ 50 ℃.

[0077]

　An optical fiber which has been cooled to a predetermined temperature through the cooling unit 122 as described above is passed through the coating device 131 that the ultraviolet curing resin serving as the coating layer covering the optical fiber enters, coated with the UV curable resin It is. Further passing through the ultraviolet irradiation device 132, that the ultraviolet rays are irradiated, the coating layer is formed ultraviolet curable resin is cured, the optical fiber 1. Incidentally, the coating layer is usually composed of two layers. When forming a coating layer of two layers, it is possible to form a coating layer of two layers by curing these UV curable resin at a time after coating the optical fiber with the ultraviolet curing resin constituting each layer. It is also possible to form the second layer of the coating layer after forming the first layer of the coating layer. Then, the optical fiber 1, the direction is converted by the turn pulley 141 and taken up by the reel 142.

[0078]

　Although the preferred embodiments for the present invention has been described as an example, the present invention is not limited thereto. That is, an optical fiber manufacturing method of the present invention has only to comprise a drawing step and annealing step described above, the pre-cooling step and the quenching step is not an essential component. The manufacturing method for the optical fiber of the present invention can be applied to the manufacture of all types of optical fibers. For example, an optical fiber manufacturing method of the present invention is not only an optical fiber mainly composed of silica glass, chalcogenide glasses, and fluorine-based glass, to a method of manufacturing an optical fiber mainly composed of other materials, the constant a in the formula (5), and the activation energy E act is applicable as long sought.

[0079]

　According to the present invention, can be the transmission loss is the production method of the reduced optical fiber can be manufactured optical fiber is provided, utilized in the field of optical fiber communication. It can also be utilized in the manufacture of optical fiber used in the devices using a fiber laser device or other optical fiber.

DESCRIPTION OF SYMBOLS

[0080]

1 ... optical fiber
1P ... preform for optical fiber
110 ... wire drawing furnace
111 ... heating section
120 ... tubular member
121 ... annealing furnace
122 ... cooling device
131 ... coating apparatus
132 ... ultraviolet irradiation device
141 ... turn pulley
142 ... reel
P1 ... drawing process
P2 ... precooling step
P3 ... slow cooling step
P4 ... quenching step

The scope of the claims

[Claim 1]

　A drawing step of drawing an optical fiber preform in a drawing furnace,
　the annealing step of annealing the drawn optical fiber in the wire drawing step
with a
　in the annealing step, the optical fiber more passed through the annealing furnace,
　the constant tau (T when the structural relaxation of the glass constituting the core included in the optical fiber n ), wherein at the time of the incoming line from the upstream side to the n-th the lehr of the said slow cooling step the temperature of the optical fiber T n , the virtual temperature T of the glass constituting the core at the time of the incoming line fn , the fictive temperature of the glass constituting the core after time Δt has elapsed from the time that the incoming line T f when a, holds the following equation (1) in any period of the slow cooling step
method of manufacturing an optical fiber, characterized in that.

[Claim 2]

　The formula in any period of slow cooling step (2) is established
method for manufacturing an optical fiber according to claim 1, characterized in that.

[Claim 3]

　Wherein the n th set temperature of the annealing furnace T sn when the relationship of the following formula (3) holds
an optical fiber manufacturing method according to claim 1 or 2, characterized in that.

[Claim 4]

　Formula (4) holds
an optical fiber manufacturing method according to claim 3, characterized in that.

[Claim 5]

　Towards said annealing furnace provided downstream of the annealing furnace provided on the upstream side, the temperature difference between the fictive temperature of the glass constituting the core in the set temperature and the inlet is smaller
claims 1, wherein the 4 the method of manufacturing an optical fiber according to any one of.

[Claim 6]

　The optical fiber in at least one time when the temperature of the optical fiber is in the range of 1500 ° C. 1300 ° C. or higher to stay in one of the plurality of the annealing furnace
in any one of claims 1 to 5, characterized in that the method of manufacturing an optical fiber according to.