US20180290914A1 - Optical fiber production method - Google Patents

Optical fiber production method Download PDF

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Publication number
US20180290914A1
US20180290914A1 US15/575,690 US201615575690A US2018290914A1 US 20180290914 A1 US20180290914 A1 US 20180290914A1 US 201615575690 A US201615575690 A US 201615575690A US 2018290914 A1 US2018290914 A1 US 2018290914A1
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Prior art keywords
optical fiber
temperature
glass
core
glass forming
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Takayuki Kitamura
Akihito IMASE
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Fujikura Ltd
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Fujikura Ltd
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    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/02—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor
    • C03B37/025—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor from reheated softened tubes, rods, fibres or filaments, e.g. drawing fibres from preforms
    • C03B37/027—Fibres composed of different sorts of glass, e.g. glass optical fibres
    • C03B37/02718—Thermal treatment of the fibre during the drawing process, e.g. cooling
    • C03B37/02727—Annealing or re-heating
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/02—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor
    • C03B37/025—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor from reheated softened tubes, rods, fibres or filaments, e.g. drawing fibres from preforms
    • C03B37/0253—Controlling or regulating
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/02—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor
    • C03B37/025—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor from reheated softened tubes, rods, fibres or filaments, e.g. drawing fibres from preforms
    • C03B37/027—Fibres composed of different sorts of glass, e.g. glass optical fibres
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02395—Glass optical fibre with a protective coating, e.g. two layer polymer coating deposited directly on a silica cladding surface during fibre manufacture
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B2205/00—Fibre drawing or extruding details
    • C03B2205/55—Cooling or annealing the drawn fibre prior to coating using a series of coolers or heaters

Definitions

  • the present invention relates to an optical fiber production method.
  • optical fiber communication systems in order to increase the reach and the rate of optical transmission, the optical signal-to-noise ratio has to be increased. Thus, a decrease in transmission losses in optical fibers is demanded.
  • transmission losses caused by impurities contained in optical fibers are closed to the lower limits.
  • a remaining main cause of transmission losses is scattering losses in association with fluctuations in the structure or composition of glass forming optical fibers. This is inevitable, because optical fibers are formed of glass.
  • a method is known to cool molten glass slowly.
  • a method of slowly cooling molten glass an attempt is made to slowly cool an optical fiber drawn from a drawing furnace immediately. Specifically, it is investigated to decrease the cooling rate of the optical fiber that an optical fiber drawn from a drawing furnace is heated in an annealing furnace, or an optical fiber drawn from a drawing furnace is surrounded by a heat insulator immediately.
  • Patent Literature 1 discloses a method of setting the temperature of a heating furnace (an annealing furnace) is ⁇ 100° C. or less of the target temperature found by a recurrence formula in 70% or more of a region from a position at which the outer diameter of a silica based optical fiber having a core and a cladding becomes smaller than 500% of the final outer diameter to a position at which the temperature of the optical fiber is 1,400° C. Since the temperature history of the optical fiber is controlled in this manner, the fictive temperature of glass forming the optical fiber is decreased to reduce transmission losses.
  • Patent Literature 1 is required to repeat complex calculations in order to adjust the temperature of the optical fiber to an ideal temperature change found by the recurrence formula.
  • the technique disclosed in Patent Literature 1 permits the temperature of the optical fiber to have a temperature shift of as large as ⁇ 50° C. to 100° C. with respect to the target temperature found by the recurrence formula.
  • the temperature shift of the optical fiber is permitted in such a large deviation, it is difficult to say that the temperature history is sufficiently optimized. For example, supposing that the temperature of the optical fiber slowly cooled is changed in a range of ⁇ 100° C.
  • the present inventors found that the transmission losses in the optical fiber are reduced easily by appropriately setting the temperature of the annealing furnace and appropriately controlling the temperature difference between the fictive temperature of glass forming the optical fiber and the temperature of the optical fiber, because of promoting the relaxation of the structure of glass forming the optical fiber.
  • One or more embodiments of the present invention provide an optical fiber production method that easily reduces transmission losses in the optical fiber.
  • An optical fiber production method includes: a drawing process of drawing an optical fiber from an optical fiber preform in a drawing furnace; and a slow cooling process of slowly cooling the optical fiber drawn in the drawing process.
  • the optical fiber is passed through a plurality of annealing furnaces.
  • Equation (1) is held in a given period in the slow cooling process, where a time constant of relaxation of a structure of glass forming a core included in the optical fiber is defined as ⁇ (T), a temperature of the optical fiber at a point in time in the slow cooling process is defined as T, a fictive temperature of glass forming the core at the point in time is defined as T f 0 , and a fictive temperature of glass forming the core after a lapse of time ⁇ t from the point in time is defined as T f .
  • the present inventors found that the optical fiber is slowly cooled with the temperature difference between the temperature of the optical fiber and the fictive temperature of glass forming the core included in the optical fiber being controlled in the predetermined range and hence the relaxation of the structure of glass forming the core is promoted.
  • the relaxation of the structure of glass forming the core scattering losses caused by fluctuations in the structure of glass forming the core in the transmission of light through the core are reduced, and hence transmission losses in the optical fiber are reduced.
  • the plurality of annealing furnaces is used in the slow cooling process, the preset temperatures of the annealing furnaces are appropriately controlled, and hence the temperature difference between the temperature of the optical fiber and the fictive temperature of glass forming the core included in the optical fiber is easily controlled in the predetermined range. As a result, the relaxation of the structure of glass forming the core is promoted, and transmission losses in the optical fiber are easily reduced.
  • Equation (2) below is held in a given period in the slow cooling process.
  • the temperature difference (T f ⁇ T) between the temperature T of the optical fiber and the fictive temperature T f of glass forming the core included in the optical fiber is controlled in a more suitable range, and hence the relaxation of the structure of glass forming the core included in the optical fiber is more easily promoted, and transmission losses in the optical fiber are more easily reduced.
  • Equation (3) a relationship of Equation (3) below is held, where a preset temperature of an nth annealing furnace of the plurality of annealing furnaces from an upstream side is defined as T sn and a fictive temperature of glass forming a core included in the optical fiber at an outlet port of the nth annealing furnace of the plurality of annealing furnaces from the upstream side is T en .
  • the plurality of annealing furnaces is used in the slow cooling process, the preset temperatures of the annealing furnaces are controlled in a predetermined range with respect to the fictive temperature of glass forming the core at the outlet ports of the annealing furnaces, and hence the temperature difference between the temperature of the optical fiber and the fictive temperature of glass forming the core included in the optical fiber is easily controlled in a predetermined range. As a result, the relaxation of the structure of glass forming the core is promoted, and transmission losses in the optical fiber are easily reduced.
  • Equation (4) is held.
  • the preset temperatures of the plurality of the annealing furnaces are individually controlled in a more suitable range, and hence the effect of promoting the relaxation of the structure of glass forming the core included in the optical fiber is easily increased, and transmission losses in the optical fiber are more easily reduced.
  • a temperature difference between a preset temperature and a fictive temperature of glass forming the core at an outlet port is smaller in the annealing furnace provided on a downstream side than in the annealing furnace provided on an upstream side.
  • the present inventors found that when the temperature of glass becomes low, a small temperature difference between the fictive temperature of glass and the temperature of glass easily promotes the relaxation of the structure of glass.
  • the temperature of the annealing furnace is set so that the temperature difference between the preset temperature and the fictive temperature of glass forming the core at the outlet port is smaller in the annealing furnace provided on the downstream side than in the annealing furnace provided on the upstream side.
  • the relaxation of the structure of glass forming the core can be efficiently promoted. As a result, transmission losses in the optical fiber are more easily reduced.
  • the optical fiber is in any one of the plurality of annealing furnaces during at least certain period for which a temperature of the optical fiber is in a range of 1,300° C. or more and 1,500° C. or less.
  • the optical fiber is slowly cooled when the temperature of the optical fiber is in this range, and hence the fictive temperature of glass forming the core included in the optical fiber is easily decreased for a shorter time, and transmission losses in the optical fiber are easily reduced.
  • an optical fiber production method that easily reduces transmission losses in the optical fiber is provided.
  • FIG. 1 is a flowchart of the processes of an optical fiber production method according to one or more embodiments of the present invention.
  • FIG. 2 is a schematic diagram of the configuration of devices for use in an optical fiber production method according to one or more embodiments of the present invention.
  • FIG. 3 is a graph of the relationship of the temperature of glass and the fictive temperature of the glass with slow cooling time according to one or more embodiments of the present invention.
  • FIG. 4 is a graph showing the relationship of the temperature difference (T f 0 ⁇ T) between the fictive temperature of glass and the temperature of glass with the decrease rate ((T f ⁇ T f 0 )/ ⁇ t) of the fictive temperature of glass per unit time according to one or more embodiments of the present invention.
  • FIG. 5 is a graph of a temporal change in the temperature difference between the fictive temperature of glass and the temperature of glass according to one or more embodiments of the present invention.
  • FIG. 6 is a graph of the upper limit and the lower limit of the variation over time in the optimized temperature difference (T f ⁇ T) depicted by a solid line in FIG. 5 and the temperature difference (T f ⁇ T) in which a transmission loss caused by scattering is not increased by 0.001 dB/km or more according to one or more embodiments of the present invention.
  • FIG. 7 is a graph of the preset temperatures of annealing furnaces, the optimized fictive temperature of glass at the outlet ports of the annealing furnaces, and the fictive temperature of glass through the fictive temperature history at the outlet ports of the annealing furnaces according to one or more embodiments of the present invention.
  • FIG. 1 is a flowchart of the processes of an optical fiber production method according to one or more embodiments of the present invention.
  • the optical fiber production method according to one or more embodiments includes a drawing process P 1 , a precooling process P 2 , a slow cooling process P 3 , and a rapid cooling process P 4 .
  • FIG. 2 is a schematic diagram of the configuration of devices for use in the optical fiber production method according to one or more embodiments.
  • the drawing process P 1 is a process in which one end of an optical fiber preform 1 P is drawn in a drawing furnace 110 .
  • the optical fiber preform 1 P is prepared.
  • the optical fiber preform 1 P is formed of glass having refractive index profiles the same as the refractive index profiles of a core and a cladding forming an optical fiber 1 .
  • the optical fiber 1 includes one or a plurality of cores and a cladding surrounding the outer circumferential surface of the core with no gap.
  • the core and the cladding are formed of silica glass.
  • the refractive index of the core is higher than the refractive index of the cladding.
  • the cladding is formed of pure silica glass.
  • the cladding is formed of silica glass doped with a dopant, such as fluorine, which decreases the refractive index.
  • the optical fiber preform 1 P is suspended so that the longitudinal direction is perpendicular.
  • the optical fiber preform 1 P is disposed in the drawing furnace 110 , a heating unit 111 is caused to generate heat, and then the lower end portion of the optical fiber preform 1 P is heated.
  • the lower end portion of the optical fiber preform 1 P is heated at a temperature of 2,000° C., for example, to be molten. From the heated lower end portion of the optical fiber preform 1 P, molten glass is drawn out of the drawing furnace 110 at a predetermined drawing rate.
  • the precooling process P 2 is a process in which the optical fiber drawn out of the drawing furnace 110 in the drawing process P 1 is cooled to a predetermined temperature suitable for delivering the optical fiber into an annealing furnace 121 , described later.
  • a predetermined temperature of the optical fiber suitable for delivering the optical fiber into the annealing furnace 121 will be described later in detail.
  • the precooling process P 2 is performed by passing the optical fiber drawn in the drawing process P 1 through the hollow portion of a tubular body 120 provided directly below the drawing furnace 110 .
  • the tubular body 120 is provided directly below the drawing furnace 110 , causing the atmosphere in the inside of the hollow portion of the tubular body 120 to be almost the same as the atmosphere in the inside of the drawing furnace 110 .
  • a sudden change in the atmosphere and the temperature around the optical fiber immediately after drawn is reduced.
  • the temperature of the optical fiber to be delivered into the annealing furnace 121 is mainly determined by the drawing rate and the atmosphere in the drawing furnace 110 .
  • the precooling process P 2 is provided, which further finely adjusts the cooling rate of the optical fiber for easy adjustment of the incoming temperature of the optical fiber to be delivered into the annealing furnace 121 to a suitable range. Based on the temperatures of the optical fiber drawn out of the drawing furnace 110 and suitable for delivering the optical fiber into the annealing furnace 121 , the distance from the annealing furnace 121 to the drawing furnace 110 and the length of the tubular body 120 can be appropriately selected.
  • the tubular body 120 is formed of a metal tube, for example.
  • the cooling rate of the optical fiber may be adjusted by air-cooling the metal tube or by providing a heat insulator around the metal tube.
  • the slow cooling process P 3 is a process in which the optical fiber, which is drawn out in the drawing process P 1 , is slowly cooled.
  • the temperature of the optical fiber is adjusted through the precooling process P 2 , and then the optical fiber is slowly cooled in the slow cooling process P 3 .
  • the optical fiber is passed through a plurality of annealing furnaces 121 a , 121 b , 121 c , and 121 d .
  • the annealing furnace 121 in the case in which all the annealing furnaces are collectively referred or in the case in which it is unnecessary to distinguish between the annealing furnaces, they are sometimes simply referred to as “the annealing furnace 121 ”. Note that, in FIG. 2 , four annealing furnaces 121 a , 121 b , 121 c , and 121 d are shown. However, in one or more embodiments of the present invention, the number of the annealing furnaces is not limited specifically as long as the number is more than one.
  • the presence of more than one annealing furnace means that there is a plurality of heat generating units whose temperatures can be set differently. For example, it can be said that a plurality of annealing furnaces is present when a plurality of heat generating units whose temperatures can be set differently is provided even though the heat generating units are housed in one enclosure.
  • the temperature in the inside of the annealing furnace 121 is a predetermined temperature different from the temperature of the optical fiber to be delivered into the annealing furnace 121 .
  • the cooling rate of the optical fiber is decreased by the temperature around the optical fiber delivered into the annealing furnace 121 .
  • the cooling rate of the optical fiber is decreased in the annealing furnace 121 .
  • the temperature of the optical fiber is not sufficiently optimized when the optical fiber is delivered into the annealing furnace.
  • the optical fiber is sometimes delivered into the annealing furnace with the temperature of the optical fiber being too high or too low.
  • the rate to relax the structure of glass forming the optical fiber is too fast, hardly expecting the effect of slowly cooling the optical fiber.
  • the temperature of the optical fiber to be delivered into the annealing furnace is too low, the rate to relax the structure of glass forming the optical fiber is decreased, sometimes causing a necessity to heat up again the optical fiber in the annealing furnace, for example.
  • the annealing furnace is elongated more than necessary, which might demand an excessive capital investment, or the drawing rate is decreased more than necessary, which might degrade productivity.
  • the temperature of the annealing furnace 121 is appropriately set, the temperature difference between the fictive temperature of glass forming the core included in the optical fiber and the temperature of the optical fiber is appropriately controlled, and hence the relaxation of the structure of glass forming the core is promoted.
  • the optical fiber 1 having decreased transmission losses can be obtained with no requirement of excessive capital investment and with excellent productivity.
  • complex calculation is unnecessary unlike the technique disclosed in Patent Literature 1 described above.
  • Equation (5) the time constant ⁇ (T) of the structural relaxation, which is thought to correspond to the viscosity flow of glass, follows the Arrhenius equation.
  • Equation (5) the time constant ⁇ (T) is expressed as Equation (5) as a function of the temperature T of glass using a constant A and an activation energy E act determined by the composition of glass.
  • k B is Boltzmann constant.
  • T absolute temperature of glass
  • Equation (5) above shows that the structure of glass is relaxed faster as the temperature of glass is higher and reached faster in the equilibrium state at the given temperature. That is, the fictive temperature of glass more quickly comes close to the temperature of glass faster as the temperature of glass is higher.
  • FIG. 3 shows the relationship of the temperature of glass and the fictive temperature of the glass with time in slowly cooling glass.
  • the horizontal axis expresses time
  • the vertical axis expresses temperature.
  • a solid line expresses the transition of the temperature of glass under certain slow cooling conditions
  • a broken line expresses the transition of the fictive temperature of glass at that time.
  • a dotted line expresses the transition of the temperature of glass in the case in which the cooling rate is decreased more slowly than under the slow cooling conditions expressed by the solid line
  • an alternate long and short dash line expresses the transition of the fictive temperature of glass at that time.
  • the fictive temperature of glass is also similarly decreased.
  • the rate of the relaxation of the structure of glass forming the optical fiber is very fast.
  • the rate of the relaxation of the structure of glass is decreased, and the fictive temperature of glass fails to follow a decrease in the temperature of glass after a while. The temperature difference between the temperature of glass and the fictive temperature of glass is then increased.
  • the optical fiber when the cooling rate of glass is slowed, the optical fiber is held in a relatively higher temperature state for a longer time, compared with the case in which the cooling rate is fast.
  • the temperature difference between the temperature of glass and the fictive temperature of glass becomes smaller, and the fictive temperature of glass is lower than the example described above. That is, when the cooling rate of glass is slowed, the relaxation of the structure of glass is easily promoted.
  • the fictive temperature of glass does not reach to the below of the temperature of glass.
  • the fictive temperature of the glass also remains high. That is, when the temperature of glass is too high, the effects obtained by slow cooling are poor.
  • the temperature of the optical fiber in the annealing furnace 121 is 1,600° C. or less, or 1,500° C. or less.
  • the fictive temperature can be decreased to a lower temperature, but the decrease rate of the fictive temperature is slowed.
  • the temperature of the optical fiber in the annealing furnace 121 is 1,300° C. or more, or 1,400° C. or more. Therefore, the optical fiber stays in the annealing furnace 121 at least one period during which the temperature of the optical fiber is in a range of temperatures of 1,300° C. to 1,500° C., both inclusive.
  • the optical fiber is slowly cooled when the temperature of the optical fiber is in a predetermined range.
  • the fictive temperature of glass forming the core included in the optical fiber is easily decreased for a shorter time, and transmission losses in the optical fiber are easily reduced.
  • the time constant of the relaxation of the structure of glass forming the core included in the optical fiber is defined as ⁇ (T)
  • T the temperature of the optical fiber at a certain point in time in the slow cooling process P 3
  • T f 0 the fictive temperature of glass forming the core at that certain point in time
  • T f 0 the fictive temperature of glass forming the core after a lapse of time ⁇ t from the certain point in time
  • T f ⁇ T ( T f 0 ⁇ T )exp( ⁇ t / ⁇ ( T )) (6)
  • Equation (6) above shows that the temperature difference (T f ⁇ T) between the fictive temperature T f of glass forming the core and the temperature T of the optical fiber depends on the temperature difference (T f 0 ⁇ T) between the fictive temperature T f 0 of glass forming the core and the temperature T of the optical fiber at a certain point in time as well as the fictive temperature T f of glass forming the core depends on the time constant ⁇ (T) of the relaxation of the structure.
  • the time constant ⁇ (T) of the relaxation of the structure is defined as time until the temperature difference (T f ⁇ T) between the fictive temperature T f of glass and the temperature T of glass reaches 1/e when the temperature of glass whose fictive temperature is T f 0 is T.
  • a change in the fictive temperature T f per unit time is greater as the temperature difference (T f 0 ⁇ T) is greater to some extent.
  • FIG. 4 schematically shows the relationship between the temperature difference (T f 0 ⁇ T) where the temperature of the optical fiber including the core formed of glass whose fictive temperature is T f 0 is T and a change ((T f -) T f 0 / ⁇ t) in the fictive temperature T f per unit time.
  • the conditions are thought in which the temperature T of the optical fiber is decreased from this point and the temperature difference (T f 0 ⁇ T) between the fictive temperature T f 0 of glass forming the core and the temperature T of the optical fiber is increased.
  • the time constant ⁇ (T) of the relaxation of the structure of glass forming the core is increased, the change rate of the fictive temperature T f per unit time ((T f ⁇ T f 0 )/ ⁇ t) is negatively increased.
  • the conditions are thought in which the temperature T of the optical fiber is further decreased and the temperature difference (T f 0 ⁇ T) between the fictive temperature T f 0 of glass forming the core and the temperature T of the optical fiber is further increased.
  • FIG. 4 shows that as a peak expressed in the graph, a change in the fictive temperature per unit time ((T f ⁇ T f 0 )/ ⁇ t) takes a minimum value when the temperature difference (T f 0 ⁇ T) between the fictive temperature T f 0 of glass forming the core and the temperature T of the optical fiber is a certain value.
  • Equation (7) the relationship of Equation (7) below is held between the temperature T of glass and the fictive temperature T f when the decrease rate of the fictive temperature T f of glass is the maximum.
  • the temperature T of glass can be found, at which the fictive temperature T f of glass can be most efficiently decreased.
  • the temperature of glass, at which the fictive temperature T f of glass can be most efficiently decreased is sometimes referred to as “the optimized temperature of glass”, and the fictive temperature that has been most efficiently decreased is sometimes referred to as “the optimized fictive temperature”.
  • FIG. 5 shows, on a standard single-mode optical fiber having a core doped with G e O 2 , a variation over time of the temperature difference (T f ⁇ T) between the value where the fictive temperature T f of glass forming the core, which is found from Equation (6) above, takes the lowest value and the temperature T of the optical fiber at that value.
  • the vertical axis expresses the temperature difference (T f ⁇ T) between the value where the fictive temperature T f of glass forming the core takes the lowest value and the temperature T of the optical fiber at that value
  • the horizontal axis expresses the slow cooling time of the optical fiber.
  • the slow cooling process P 3 is performed immediately after the optical fiber preform 1 P is heated and molten in the drawing process P 1 .
  • a temperature T 0 of the optical fiber is 1,800° C. at the beginning of slow cooling, at which slow cooling time is zero second
  • time required for relaxing the structure of glass forming the core at this temperature is as very short as less than 0.001 second.
  • the temperature difference (T f ⁇ T) has to be gradually increased, whereas in the time domain from about 0.01 second and later, the temperature difference (T f ⁇ T) has to be gradually decreased. It is shown that in all the time domains, the temperature difference (T f ⁇ T) has to be less than about 60° C., the temperature T of the optical fiber is controlled so that in almost all the time domains, the temperature difference (T f ⁇ T) is kept higher than about 40° C.
  • Time at which the temperature difference (T f ⁇ T) shown in FIG. 5 is the maximum, is about 0.01 second, although the time is varied more or less depending on the constant A and the activation energy E act in Equation (5) above, and the temperature T 0 of the optical fiber and the fictive temperature T f 0 of glass forming the core at the beginning of slow cooling, at which slow cooling time is zero second.
  • the precooling process P 2 is provided subsequent to the drawing process P 1 so that the temperature difference (T f ⁇ T) between the fictive temperature of glass forming the core and the temperature of the optical fiber is provided to some extent, and then the slow cooling process P 3 is performed, which allows the efficient relaxation of the structure of glass forming the core with the advantageous use of the length of the annealing furnace 121 .
  • the precooling process P 2 is performed until a point in time at which the temperature difference (T f ⁇ T) between the fictive temperature of glass forming the core and the temperature of the optical fiber is reached at above 40° C. and less than 60° C. at a point in time after a lapse of about 0.01 second FIG. 5 , and then the slow cooling process P 3 is started so that the length of the annealing furnace 121 can be advantageously used.
  • the results shown in FIG. 5 reveal the following. It is revealed that even though slight differences are present in the values of the constant A and the activation energy E act determined based on the composition of glass, when the temperature difference (T f ⁇ T) between the fictive temperature of glass and the temperature of glass is in a range of above 40° C. and less than 60° C. in the slow cooling process P 3 , the fictive temperature of glass is efficiently decreased.
  • the optical fiber is slowly cooled under the conditions in which the temperature difference (T f ⁇ T) between the fictive temperature of glass forming the optical fiber and the temperature of the optical fiber is in a range of above 40° C.
  • the fictive temperature of glass forming the optical fiber is efficiently decreased.
  • the fictive temperature is efficiently decreased.
  • the temperature difference (T f ⁇ T) between the temperature T of the optical fiber and the fictive temperature T f of glass forming the core included in the optical fiber is controlled in the predetermined range, and hence the relaxation of the structure of glass forming the core included in the optical fiber is easily promoted and transmission losses in the optical fiber are easily reduced.
  • T the time constant of the relaxation of the structure of glass forming the core
  • T f the temperature of the optical fiber at a certain point in time in the slow cooling process P 3
  • T f 0 the fictive temperature of glass forming the core at that certain point in time
  • T f the fictive temperature of glass forming the core after a lapse of time ⁇ t from the certain point in time
  • the temperature difference (T f ⁇ T) between the temperature T of the optical fiber and the fictive temperature T f of glass forming the core included in the optical fiber is controlled in a predetermined range, and hence the relaxation of the structure of glass forming the core included in the optical fiber is more easily promoted. Therefore, transmission losses in the optical fiber are easily reduced.
  • the plurality of the annealing furnaces 121 is used, the preset temperatures of the annealing furnaces 121 are appropriately controlled, and hence the temperature difference (T f ⁇ T) between the temperature of the optical fiber and the fictive temperature of glass forming the core included in the optical fiber is easily controlled in a predetermined range. As a result, the relaxation of the structure of glass forming the core is promoted, and transmission losses in the optical fiber are reduced.
  • the conditions for the temperature difference (T f ⁇ T) between the temperature T of the optical fiber and the fictive temperature T f of glass forming the core included in the optical fiber in order to most efficiently decrease the fictive temperature T f of glass forming the core are as described above.
  • transmission losses in the optical fiber can also be sufficiently reduced under the conditions described below.
  • the fictive temperature T f of glass forming the core included in the optical fiber can be tied to transmission losses in the optical fiber by a relational expression below.
  • a Rayleigh scattering coefficient R r is proportional to the fictive temperature T f of glass forming the core, and a transmission loss ⁇ T caused by Rayleigh scattering is expressed by Equation (9) below where the wavelength of light to be transmitted is A ( ⁇ m).
  • Non-Patent Literature 2 K. Saito, et al., Applied Physics Letters, Vol. 83, pp. 5175-5177 (2003)
  • B 4.0 ⁇ 10 ⁇ 4 dB/km/ ⁇ m 4 /K.
  • T f of glass forming the core When the fictive temperature T f of glass forming the core is increased by 14° C., the Rayleigh transmission loss ⁇ T caused by Rayleigh scattering is increased by about 0.001 dB/km.
  • the optical fiber only has to be delivered into the annealing furnace 121 under the temperature conditions in which the temperature difference (T f ⁇ T) between the fictive temperature T f of glass forming the core and the temperature of the optical fiber is higher than 20° C. and lower than 100° C. as described below.
  • FIG. 6 shows the upper limit expressed by a broken line and the lower limit expressed by an alternate long and short dash line of a variation over time of the temperature difference (T f ⁇ T) at which the transmission loss caused by scattering is not increased by 0.001 dB/km or more.
  • the values described in Non-Patent Literature 1 K. Saito, et al., Journal of the American Ceramic Society, Vol. 89, pp. 65-69 (2006) are used.
  • the result shown in FIG. 6 reveals the following.
  • the temperature of the annealing furnace 121 only has to be set so as to control the temperature history of the optical fiber in which the temperature difference (T f ⁇ T) is in a range of above about 20° C. and less than about 100° C. in the time domain from about 0.01 second and later during the slow cooling process P 3 , an increase in the fictive temperature of glass forming the core is restricted up to a rise of about 14° C. with respect to the fictive temperature T f of glass forming the core, at which the fictive temperature T f is most efficiently decreased. As a result, an increase can be restricted to an increase of 0.001 dB/km or less with respect to the values under the optimized conditions under which transmission losses are most decreased.
  • the temperature difference (T f ⁇ T) between the temperature T of the optical fiber and the fictive temperature T f of glass forming the core included in the optical fiber is maintained in a range of above 20° C. and less than 100° C. also in a given period from the start to the end of the slow cooling process P 3 , and hence the relaxation of the structure of glass forming the core included in the optical fiber is easily promoted, and transmission losses in the optical fiber are easily reduced. That is, Equation (1) below is held.
  • Equation (2) or (1) above a specific example for easily satisfying the conditions of Equation (2) or (1) above will be described.
  • four annealing furnaces 121 a , 121 b , 121 c , and 121 d are used in the slow cooling process P 3 .
  • the plurality of the annealing furnaces 121 is used in this manner, and hence the temperature difference between the temperature of the optical fiber and the fictive temperature of glass forming the core is easily controlled in a predetermined range.
  • the preset temperature of the nth annealing furnace 121 from the upstream side is defined as T sn
  • T f of glass forming the core in slow cooling time until which the optical fiber is reached at the outlet port of the nth annealing furnaces 121 from the upstream side is defined as T sn
  • the optical fiber is slowly cooled with the temperature difference between the temperature of the optical fiber and the fictive temperature of glass forming the core included in the optical fiber being controlled in a predetermined range, and hence the relaxation of the structure of glass forming the core is promoted.
  • the relaxation of the structure of glass forming the core scattering losses caused by fluctuations in the structure of glass forming the core in the transmission of light through the core are reduced, and hence transmission losses in the optical fiber are reduced.
  • the plurality of the annealing furnaces 121 is used, and the preset temperatures of the annealing furnaces 121 is controlled in a predetermined range with respect to the fictive temperature of glass forming the core in slow cooling time until which the optical fiber is reached at the outlet port of each of the annealing furnaces 121 , and hence the temperature difference between the temperature of the optical fiber and the fictive temperature of glass forming the core included in the optical fiber is easily controlled in a predetermined range. As a result, the relaxation of the structure of glass forming the core is promoted, and transmission losses in the optical fiber are reduced. Referring to FIG. 7 , this will be described more in detail below.
  • FIG. 7 shows a change in the optimized fictive temperature of glass forming the core calculated from Equation (5) where the temperature and fictive temperature of the optical fiber are 1,800° C. as the initial values (a solid line), the preset temperatures of the annealing furnaces 121 a , 121 b , 121 c , and 121 d (an alternate long and short dash line), and the expected fictive temperature of glass forming the core in slow cooling time until which the optical fiber is reached at the outlet ports of the annealing furnaces 121 a , 121 b , 121 c , and 121 d .
  • the temperature and fictive temperature of the optical fiber are 1,800° C. as the initial values (a solid line)
  • the preset temperatures of the annealing furnaces 121 a , 121 b , 121 c , and 121 d an alternate long and short dash line
  • the expected fictive temperature of glass forming the core in slow cooling time until which the optical
  • the lengths of the annealing furnaces 121 are each 0.5 m and the drawing rate is 20 m/second.
  • the optimized fictive temperature T f of glass forming the core when the optical fiber is delivered out of each of the annealing furnaces 121 i.e. when the slow cooling time is 0.025 second, 0.050 second, 0.075 second, and 0.100 second
  • the optimized fictive temperature T f is calculated as temperatures of 1,556° C., 1,515° C., 1,492° C., and 1,476° C., respectively.
  • the preset temperatures of the annealing furnaces 121 a , 121 b , 121 c , and 121 d are then set as expressed by the alternate long and short dash line in FIG. 7 . That is, the temperatures of the annealing furnaces 121 are set to a temperature lower by 50° C. than the optimized fictive temperature T f of glass forming the core at the slow cooling time at which the optical fiber is reached at the outlet port of each of the annealing furnaces 121 .
  • the actual fictive temperature of glass is more gently decreased and close to the preset temperature of the annealing furnace, the actual fictive temperature is slightly higher than optimized fictive temperatures expressed by the solid triangles and slightly lower than the fictive temperatures expressed by the solid circles. However, this is errors in a tolerable range.
  • the temperature difference between the fictive temperature of glass through a fictive temperature history after slow cooling for 0.100 second and the optimized fictive temperature is 12° C., which the different in the scattering loss is only less than 0.001 dB/km.
  • Equation (4) Based on the viewpoint described above, from the viewpoint of controlling the temperature difference between the temperature of the optical fiber and the fictive temperature of glass forming the core in a more appropriate range, i.e. from the viewpoint of easily satisfying Equation (2) above, Equation (4) below is held.
  • the preset temperature of the annealing furnace 121 is controlled in a more appropriate range, and hence the effect of promoting the relaxation of the structure of glass forming the core included in the optical fiber is easily increased, and transmission losses in the optical fiber are easily reduced.
  • the temperature difference between the preset temperature and the fictive temperature of glass forming the core at the outlet port is smaller in the annealing furnace 121 provided on the downstream side than in the annealing furnace 121 provided on the upstream side.
  • the temperature difference between the optimized temperature of glass and the optimized fictive temperature of glass forming the core at slow cooling time of 0.025 second, 0.050 second, 0.075 second, and 0.100 second is temperatures of 59° C., 56° C., 55° C., and 54° C., respectively.
  • the temperature difference is smaller toward the downstream side.
  • the temperature of the annealing furnace is set so that the temperature difference between the preset temperature and the fictive temperature of glass forming the core at the outlet port is smaller in the annealing furnace provided on the downstream side than in the annealing furnace provided on the upstream side.
  • the relaxation of the structure of glass forming the core can be efficiently promoted.
  • transmission losses in the optical fiber are more easily reduced.
  • the relationship between the temperature T of the optical fiber and the fictive temperature T f of glass forming the core, at which the fictive temperature T f is most efficiently decreased depends solely on the slow cooling time t, and the slow cooling time t, the length L of the annealing furnace, and the drawing rate v can be correlated with one another based on the relationship of Equation (10) below.
  • the slow cooling time t needs about 0.1 second to set the fictive temperature T f to 1,500° C.
  • the length L of the annealing furnace needs two meters.
  • the slow cooling time t needs about 0.4 second in order to set the fictive temperature T f to 1,400° C., for example.
  • the drawing rate v is set to 10 m/second
  • the length L of the annealing furnace needs four meters.
  • the drawing rate v is selected in a range of about 10 m/second to 50 m/second
  • the length L of the annealing furnace is selected in a range of about one meter to ten meters
  • the slow cooling time t is one second or less.
  • the optical fiber is covered with a coating layer to enhance the resistance against external flaws, for example.
  • this coating layer is formed of an ultraviolet curable resin.
  • the temperature of the optical fiber affects the viscosity of a resin to be applied, and as a result, this affects the thickness of the coating layer.
  • a suitable temperature of the optical fiber in forming the coating layer is appropriately determined suitable for the properties of a resin forming the coating layer.
  • the optical fiber production method since the annealing furnace 121 is provided between the drawing furnace 110 and a coater 131 , the section for sufficiently cooling the optical fiber is decreased. More specifically, the optical fiber production method according to one or more embodiments also includes the precooling process P 2 , further decreasing the section sufficiently cooling the optical fiber. Thus, the optical fiber production method according to one or more embodiments includes the rapid cooling process P 4 in which the optical fiber delivered out of the annealing furnace 121 is rapidly cooled using a cooling device 122 . In the rapid cooling process P 4 , the optical fiber is rapidly cooled faster than in the slow cooling process P 3 .
  • the temperature of the optical fiber can be sufficiently decreased in a shorter section, easily forming the coating layer.
  • the temperature of the optical fiber when it is delivered out of the cooling device 122 is in a range of temperatures of 40° C. to 50° C., for example.
  • the optical fiber which has been passed through the cooling device 122 and cooled to a predetermined temperature, is passed through a coater 131 containing an ultraviolet curable resin to be the coating layer that covers the optical fiber, and the optical fiber is covered with this ultraviolet curable resin.
  • the optical fiber is further passed through an ultraviolet irradiator 132 , ultraviolet rays are applied to the optical fiber, the coating layer is formed, and then the optical fiber 1 is formed.
  • the coating layer is typically formed of two layers. In the case of forming a two-layer coating layer, after the optical fiber is covered with ultraviolet curable resins forming the respective layers, the ultraviolet curable resins are cured at one time, and then the two-layer coating layer can be formed. Alternatively, after forming a first coating layer, a second coating layer may be formed. The direction of the optical fiber 1 is changed by a turn pulley 141 , and then the optical fiber 1 is wound on a reel 142 .
  • the present invention is described with reference to one or more embodiments taken as examples.
  • the present invention is not limited to the above described embodiments. That is, the optical fiber production method according to one or more embodiments of the present invention only has to include the drawing process and the slow cooling process described above. The precooling process and the rapid cooling process are not essential processes.
  • the optical fiber production method according to one or more embodiments of the present invention is applicable to the manufacture of any types of optical fibers.
  • the optical fiber production method according to one or more embodiments of the present invention is applicable also to production methods for optical fibers having different materials, such as chalcogenide glass and fluorine glass, as a principal component, as well as production methods for optical fibers having silica glass as a principal component, if the constant A and the activation energy E act in Equation (5) above are derived.
  • an optical fiber production method with which an optical fiber with decreased transmission losses can be manufactured, and the method can be used in the field of optical fiber communications.
  • the method can also be used for fiber laser devices and other devices using optical fibers.

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CN108383375A (zh) * 2018-02-12 2018-08-10 江苏富春江光电有限公司 光纤拉丝退火装置及光纤

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