JP2007238354A - Method for manufacturing optical fiber and wire drawing furnace - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wire drawing furnace manufacturing an optical fiber having a small Rayleigh scattering loss by simple temperature control performed with a small energy consumption for a wire drawing furnace having an arbitrary length without necessitating a concomitant slow cooling furnace. <P>SOLUTION: The wire drawing furnace 20 has a wire drawing furnace body 10 in which an optical fiber preform 1 is arranged, and a heater 13 which is arranged in the wire drawing furnace body 10 and heats the optical fiber preform 1. In the wire drawing furnace 20, the optical fiber is manufactured by cooling an optical fiber body 2, obtained by heating and drawing the optical fiber preform 1, to room temperature. The wire drawing furnace body 10 has a slow cooling zone 11, where the temperature is continuously lowered from 1,600 °C in accordance with the distance in the longitudinal direction, at a lower part of the heater 13, and can variably regulate the setting temperature Tmin at the lowermost part 10a of the slow cooling zone 11 within a range not higher than 1,600°C. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical fiber manufacturing method and a drawing furnace, and more particularly to an optical fiber manufacturing method and a drawing furnace that reduce optical loss of various optical fibers used for optical communication.

  In recent years, the deployment of optical fiber networks in access systems (FTTH: Fiber To The Home) and the development of optical LAN technologies represented by Ethernet (registered trademark) have greatly expanded the network of user systems built by users themselves. Is showing. In these regions, 1.3 μm band zero-dispersion single mode optical fiber (SMF), multimode optical fiber (MMF), and 1.55 μm band dispersion shifted single mode optical fiber (DSF) for long-distance transmission. Various optical fibers made from quartz glass are used.

  Further, an optical wavelength division multiplexing (WDM) technique for transmitting a plurality of optical signals having different wavelengths through a single optical fiber has been advanced, and a wavelength region used for optical signal communication has been expanded. In the future, in order to expand the distance and wavelength region used for optical communication, it is essential to reduce optical loss in the optical fiber, which is a signal transmission medium.

An optical fiber made of silica-based glass is composed mainly of SiO ₂ and usually contains a small amount of GeO ₂ or F to form a structure for guiding light. Among these, optical fibers having a structure in which pure silica glass is used for the core are known as optical fibers having the lowest loss (hereinafter, these are collectively referred to as silica-based optical fibers).

The most dominant loss factor of such a silica-based optical fiber is Rayleigh scattering (hereinafter referred to as Rayleigh scattering), the intensity of which is proportional to the fourth power of the wavelength, and increases in the shorter wavelength region. To do. This Rayleigh scattering is caused by density fluctuations and density fluctuations. It is known that the contribution of the density fluctuation component Rd is large in the silica-based optical fiber, and it has been experimentally clarified that the density fluctuation component Rd increases in proportion to the virtual temperature T _f (non-conversion). (See Patent Document 1). Further, this Non-Patent Document 1 also describes that the fictive temperature T _f can be reduced from 900 ° C. to near 1000 ° C. if it is processed under optimum heat treatment conditions in the quartz glass in the bulk state. ing.

The glass is considered to have a frozen high-temperature liquid structure, and the fictive temperature T _f is a temperature corresponding to the frozen structure. The time τ necessary for relaxation of this structure (hereinafter referred to as relaxation time) τ becomes longer as the viscosity of the glass increases. Accordingly, as described in Non-Patent Document 2, a silica-based optical fiber is usually drawn at a high speed from a high temperature of about 2000 ° C., so the fictive temperature T _f is 1600 ° C., which is the softening point temperature of silica-based glass. It stays high in the vicinity.

In view of the above, in Non-Patent Documents 1 and 2, a slow cooling furnace for gradually cooling the optical fiber is separately provided below the drawing furnace to promote relaxation of the glass structure, and the fictive temperature T _f and A method for reducing light loss due to Rayleigh scattering has been proposed.

K.Saito et al., "Limit of the Rayligh scattering loss in silica faiber", Appl.Phys.Lett., Vol.83, no.25, pp.5175-5177, 2003 Sakaguchi Shigeki, "Reduction of Rayleigh scattering by heat treatment of silica core optical fiber", IEICE Transactions C.I. , Vol.J83-C, No.1, pp.30-36, January 2000 UCPeak et al., "Calculation of Cooling Rate and Induced Stresses in Drawing of Optical Fibers", Journal of The American Ceramic Society-Paek and Kurkjian., Vol.58, No.7-8, pp.330-335, Jul -Aug.1975 K. Tsujikawa et al., "Intrinsic loss of optical fibers", Opt. Fiber. Technol., Vol. 11, pp. 319-331, 2005

  However, in the techniques described in Non-Patent Documents 1 and 2 described above, there is a problem that the drawing furnace and the slow cooling furnace are used in combination, and the equipment cost increases.

  From the economical point of view, the drawing speed is fast and the slow cooling furnace should be small. However, in this method, the length of the slow cooling furnace must be increased according to the speed in order to cope with the high speed drawing. It was.

  In addition to these points, in Non-Patent Document 1, the temperature of the slow cooling furnace is set to a constant slow cooling temperature Ta, the optical fiber is cooled, and the slow cooling furnace is installed at a position where the temperature T becomes T = Ta. Is described. However, as will be described later, the temperature T of the optical fiber is a function of the cooling time. In the above method, the installation position of the slow cooling furnace is changed according to the drawing speed, or the slow cooling furnace itself is enlarged, Complicated control such as changing the spatial region in the furnace that performs slow cooling with respect to the drawing speed is required. Further, since it is necessary to set the annealing temperature Ta to about 1000 ° C. or more in principle, a heater for holding a specific wide region in the annealing furnace at the annealing temperature Ta is separately required. Energy consumption such as electric power will increase.

  In addition, in the technique described in Non-Patent Document 2, it is assumed that a slow cooling furnace is installed immediately below the drawing furnace, and although the problem of the installation position of the slow cooling furnace is solved, in order to reduce Rayleigh scattering at an arbitrary drawing speed, There is no quantitative guideline on how to adjust the length of the drawing furnace or the temperature distribution in the furnace for various drawing speeds.

  Thus, the present inventors have conducted intensive research, found quantitative guidelines for the length of the drawing furnace and the temperature distribution in the furnace for an arbitrary drawing speed, and have reached the present invention.

  That is, the present invention has been proposed in view of the above-described problems, and suppresses an increase in manufacturing cost and reduces an optical loss due to Rayleigh scattering, and a drawing furnace, specifically, An optical fiber manufacturing method capable of manufacturing an optical fiber in which light loss due to Rayleigh scattering is reduced by a simple temperature control that can be performed with a small energy consumption for a drawing furnace of an arbitrary length without requiring an additional slow cooling furnace, and The purpose is to provide a drawing furnace.

An optical fiber manufacturing method according to a first invention for solving the above-described problem is an optical fiber obtained by drawing a silica-based optical fiber preform at a high temperature of 1600 ° C. or higher by a heating means disposed in a drawing furnace. An optical fiber manufacturing method for manufacturing an optical fiber by cooling a main body to room temperature, wherein the time when the temperature of the optical fiber main body reaches around 1600 ° C. in the cooling process of cooling the optical fiber main body to room temperature is t When T = 0, the temperature T1 of the optical fiber body that reaches after the predetermined slow cooling time t1 is controlled to a temperature of 900 ° C. or more, and the predetermined slow cooling time t1 is relaxed in the structure of the optical fiber body at T1. It is characterized in that it is in the range of 10 ⁻⁵ times to 10 times the time τ required for.
In practice, t1 may be determined experimentally by setting the position where the outer diameter (usually 125 μm) of the optical fiber is constant as the origin Z = 0, as will be described later. This is because the position (Z = 0) corresponds to the temperature at which the viscous flow of the silica-based glass does not occur (around 1600 ° C.).

  An optical fiber manufacturing method according to a second invention for solving the above-described problem is the optical fiber manufacturing method described in the first invention, wherein the drawing is performed below the heating means in the drawing furnace. A constant-length slow cooling section in which the temperature in the furnace continuously decreases from 1600 ° C. with respect to the distance in the longitudinal direction, and according to the drawing speed and the time t1 during which the optical fiber body stays in the slow cooling section, The setting temperature Tmin at the lowermost part of the slow cooling section is changed.

An optical fiber manufacturing method according to a third aspect of the present invention for solving the above-mentioned problem is an optical fiber formed by heating a silica-based optical fiber preform at a high temperature of 1600 ° C. or higher by heating means disposed in a drawing furnace. An optical fiber manufacturing method for manufacturing an optical fiber by cooling a main body to room temperature, wherein the time when the temperature of the optical fiber main body reaches around 1600 ° C. in the cooling process of cooling the optical fiber main body to room temperature is t When T = 0, the temperature T1 of the optical fiber body that reaches after the predetermined slow cooling time t1 is controlled to a temperature of 900 ° C. or more, and the predetermined slow cooling time t1 is relaxed in the structure of the optical fiber body at T1. and 2 times or less in the range 10 ^-2 times or more time τ required for, below the heating means in the drawing furnace, 1 temperature of the drawing furnace with respect to longitudinal distance A constant-length slow cooling section that continuously decreases from 00 ° C. is provided, and the set temperature Tmin at the bottom of the slow cooling section changes according to the drawing speed and the time t1 during which the optical fiber body stays in the slow cooling section. It is characterized by making it.

  A drawing furnace according to a fourth aspect of the present invention that solves the above-described problem includes a drawing furnace body in which an optical fiber preform is disposed, and a heating unit that is disposed in the drawing furnace body and that heats the optical fiber preform. A drawing furnace for manufacturing an optical fiber by cooling an optical fiber body formed by heating and drawing the optical fiber preform to room temperature, wherein the drawing furnace body has a temperature at a distance in a longitudinal direction at a lower portion thereof. On the other hand, it has a certain length of slow cooling section that continuously decreases from 1600 ° C., and the set temperature Tmin at the bottom of the slow cooling section can be variably adjusted within a range of 1600 ° C. or less.

  According to the optical fiber manufacturing method of the present invention, an optimum temperature gradient in the furnace is given for an arbitrary slow cooling time found from the temperature change of the relaxation time τ of the silica-based optical fiber as a function of temperature. Based on the guideline, it is done by controlling the temperature at the bottom of the furnace, so there is no need to use an additional annealing furnace, and it is easy to reduce the Rayleigh scattering loss of the optical fiber with a small energy consumption. Can be done. In addition, strict control of temperature conditions is not necessary as compared with the case of using a constant temperature slow cooling furnace.

  Furthermore, when drawing at a certain range of speed, it becomes possible to determine the optimum furnace length based on the above guidelines, so design and prepare a drawing furnace of a size suitable for the drawing speed in advance. This makes it possible to manufacture an optical fiber with low loss efficiently and economically.

According to the drawing furnace according to the present invention, it is not necessary to use an accompanying slow cooling furnace, so that an increase in equipment cost can be suppressed, and the virtual temperature T _f of the optical fiber can be reduced to reduce its Rayleigh scattering. By doing so, a low-loss optical fiber can be manufactured at low cost.

  Hereinafter, an optical fiber manufacturing method and a drawing furnace according to the best mode of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic diagram for explaining a drawing furnace according to the principle and the best mode of the present invention. FIG. 1A shows a state in which an optical fiber is drawn in the drawing furnace, and FIG. Shows the temperature distribution at that time. FIG. 2 is a diagram showing an example of the relationship between the time during which the optical fiber body stays in the slow cooling section of the drawing furnace according to the best mode of the present invention and the temperature at that time, and FIG. It is a figure which shows an example of the temperature dependence of relaxation time (tau).

  The principle of the optical fiber manufacturing method according to the best mode of the present invention will be described using a theoretical model. The following models are also used in Non-Patent Documents 2 and 3, and a good correspondence with actual drawing conditions can be expected.

  As shown in FIG. 1, a drawing furnace 20 according to the best mode of the present invention is arranged in a drawing furnace main body 10 and the drawing furnace main body 10, and a silica-based glass base material (optical fiber base material) 1 is 1600. And a heating furnace 13 which is a heating means for heating at a high temperature of not less than 0 ° C., and a drawing furnace body 10 draws the heated optical fiber preform 1 and ends the neck down 1a. In the lower part of the heater 13, there is provided a slow cooling section 11 having a fixed length in which the temperature is continuously cooled from 1600 ° C. with respect to the distance in the longitudinal direction. The optical fiber body 2 is rapidly cooled to room temperature outside the furnace 12 to become an optical fiber.

That is, in the optical fiber manufacturing method using the drawing furnace 10, the position where the neck down 1 a of the optical fiber preform 1 ends in the drawing furnace 10 and the outer diameter d becomes 125 μm is set to Z = 0. The temperature T of the optical fiber body 2 is set to 1600 ° C., the softening point of pure quartz glass. The furnace temperature Tz is also assumed to be T ₀ = 1600 ° C. at the position of Z = 0, and a slow cooling zone (slow cooling section) 11 in which Tz linearly decreases until the temperature Tmin at the lowest point (Z = L) is assumed. To do. Assuming that the drawing speed is v, the optical fiber main body 2 stays within the total length L of the slow cooling section 11 for a time tl (= L / v), and then rapidly cooled to room temperature outside the furnace 12 to become an optical fiber.

The temperature distribution Tz in the slow cooling section 11 in the wire drawing furnace 10 is expressed by equations (1) and (2). Further, when the optical fiber main body 2 passes through the drawing furnace 10, this temperature is smaller in the longitudinal direction than in the convection heat transfer on the surface of the optical fiber main body 2, and the heat capacity is small. Since the temperature is uniform, the relationship between the temperature T of the optical fiber body 2 and the time t can be expressed by equation (3). h is a heat transfer coefficient, and c and ρ are specific heat and density of pure quartz glass. However, in the following formula, it calculated as c = 1.0 kJ / kgK and ρ = 2200 kg / m ³ .

Here, as shown in FIG. 1, even in a general case where the heater 13 is installed only above the drawing furnace 10, the temperature distribution linearly changing in the longitudinal direction as in the formula (1) is obtained. It is well established as a primary approximation. In addition, if a separate heater is installed at the bottom or middle of the drawing furnace and appropriate control is performed, it is possible to achieve a temperature distribution that changes almost completely linearly over the entire slow cooling zone. is there. FIG. 2 shows the calculation results of T (t) when the following equations (1) to (3) are solved under the condition of Tmin = 1000 ° C. and tl (s) is set to 0.5, 1, and 2. However, 210 W / m ² K described in Non-Patent Document 2 was used as the heat transfer coefficient h. The heat transfer coefficient h slightly changes depending on the conditions such as the atmospheric gas in the drawing furnace, but is a constant that can be experimentally determined from the time change of the temperature T of the optical fiber.

Tz = T ₀ −rt (1)
r = v (T ₀ −Tmin) / L = (T ₀ −Tmin) / tl (2)
dT / dt = −4h / cρd (T−Tz) (3)

FIG. 2 shows the temperature change of the optical fiber main body only in the drawing furnace. Thereafter, the optical fiber main body is rapidly cooled outside the furnace at room temperature to become an optical fiber. Since the temperature change of the optical fiber main body occurs very rapidly outside the drawing furnace, the glass structure is hardly relaxed, that is, the fictive temperature _Tf is hardly reduced. This is supported by the fact that the light loss due to Rayleigh scattering becomes a substantially constant value in a commercially available silica-based optical fiber drawn at a high speed of about 10 m / s. Therefore, in the following, it is limited only to the change of the virtual temperature T _f in the drawing furnace.

On the other hand, using the relaxation time τ (T), it is known that the relationship between the virtual temperature T _f of the optical fiber body and the actual temperature T of the optical fiber body is approximately described by equation (4). . Here, T _f0 is the initial value of the fictive temperature T _f , 1600 ° C. The relaxation time τ (T) can be determined from an infrared absorption spectrum or the like. Here, the result shown in Non-Patent Document 1 is cited and shown in FIG. In this figure, the horizontal axis represents the temperature T of the optical fiber body, and the vertical axis represents the relaxation time τ. As shown in this figure, the relaxation time τ (T) increases as the temperature T of the optical fiber body decreases.

Also, from equation (4), if the optical fiber is held for a time t at a constant temperature T _k as in Non-Patent Document 1, the relaxation time τ (T _k ) is about the same as t as the holding temperature T _k. It is necessary to choose the temperature that becomes.

T _f = T + (T _f0 −T) exp (−t / τ (T)) (4)

On the other hand, in the present invention, holding / slow cooling at a constant temperature is not performed, and a temperature change as shown in FIG. At this time, the longer the tl is, the more gradually the optical fiber can be cooled, while the temperature T of the optical fiber body approaches Tmin. However, since tl (= L / v) is determined by the slow cooling section length L and the drawing speed v, it is effective for an arbitrary drawing speed v value when the slow cooling section length L is constant. In order to reduce the fictive temperature _Tf , it is necessary to optimize the temperature T (tl) of the optical fiber body after tl seconds. For that purpose, as in the case described above, it is a guideline to set tl and the relaxation time τ (T (tl)) to the same extent. As described in the first embodiment, the conditions are greatly relaxed as compared with the case of holding and slow cooling at a constant temperature, and as a result, strict temperature control becomes unnecessary in the present invention.

  In the present invention, the above condition is realized by setting the temperature Tmin at the lowest point of the drawing furnace to an appropriate value Tp. In this case, Tp is given from the parameters h, c, ρ, d and τ (T) in equation (3) and the value of tl. Therefore, when the slow cooling section length L is constant, by changing Tmin according to the drawing speed v, it becomes possible to manufacture an optical fiber with a low loss economically with minimum equipment. In addition, when a drawing condition with a constant drawing speed v is assumed in advance, a drawing furnace having an optimum slow cooling section length L is designed and prepared according to the time the optical fiber body stays in the slow cooling section. Thus, Tmin can be set to an appropriate value.

  Therefore, according to the method of manufacturing an optical fiber according to the best mode of the present invention, the temperature in the furnace is increased with respect to an arbitrary slow cooling time found from the temperature change of the relaxation time τ of the silica-based optical fiber as a function of temperature. Since it is performed by controlling the temperature of the lowest part in the furnace based on the guideline that gives the optimum temperature gradient, there is no need to use an additional annealing furnace, and the Rayleigh scattering loss of the optical fiber can be easily reduced with low energy consumption. Reduction can be done at any drawing speed. In addition, strict control of temperature conditions is not necessary as compared with the case of using a constant temperature slow cooling furnace.

  Furthermore, when drawing at a certain range of speed, it becomes possible to determine the optimum furnace length based on the above guidelines, so design and prepare a drawing furnace of a size suitable for the drawing speed in advance. This makes it possible to manufacture an optical fiber with low loss efficiently and economically.

Moreover, according to the drawing furnace mentioned above, since it is not necessary to use an accompanying slow cooling furnace, the increase in equipment cost can be suppressed, the virtual temperature _Tf of an optical fiber can be reduced, and the Rayleigh scattering can be reduced. Thus, a low-loss optical fiber can be manufactured at a low cost.

  Below, the manufacturing method of the optical fiber which concerns on the 1st Example of this invention is demonstrated using drawing. The present embodiment relates to a specific setting value and setting method of the temperature T (tl) of the optical fiber body after the slow cooling time tl.

FIG. 4 shows the relationship between the parameter m in the relational expression (5) described later, the temperature T (tl) of the optical fiber body after the slow cooling time t1, and the set value Tp of the temperature Tmin at the lowest point of the furnace at this time. FIG. 5 is a diagram showing the relationship between the parameter m and the virtual temperature T _f , and FIG. 6 is a diagram showing the relationship between the set value Tp of the temperature Tmin at the lowest point of the drawing furnace and the virtual temperature T _f . It is.

  In order to determine T (t1) and Tp when tl and relaxation time τ (T (tl)) are set to similar values, equation (5) is assumed, and m is in the range of 0.0001 to 5 Changed.

  tl = m × τ (T (tl)) (5)

In the above equation (5), as an example, when tl = 1 (s) and the parameter m is changed, T (tl) and the setting value Tp of the lowest temperature Tmin required at this time are shown in FIG. _Tf obtained from the equation (4) is shown in FIG. Furthermore, the relationship between Tp and _Tf is shown in FIG. In addition, about the value of relaxation time (tau), it estimated from FIG. 3 (nonpatent literature 1).

As shown in FIG. 4, by controlling the value of the temperature Tp at the lowest point, it becomes possible to obtain a desired T (tl). Further, as shown in FIG. 5, it has been found that the fictive temperature change is very gradual when the parameter m is in the range of 10 ⁻⁵ or more and 10 or less. That is, from FIG. 5 and equation (5), t1 is in the range of 10 ^{−5 to} 10 times the relaxation time τ (T) of the optical fiber, preferably in the range of 10 ^{−2 to 2} times. Thus, it was found that an appropriate fictive temperature T _f can be obtained. Further, as shown in FIG. 6, it has been found that the change of the fictive temperature T _f with respect to Tp is very gradual, and even if Tp changes by 500 ° C., T _f changes only from the minimum value to about 20 ° C. Therefore, it was found that the conditions for controlling Tp are very easy.

As can be understood from FIG. 2 and FIG. 3, in the present invention, slow cooling starts from a high temperature range where the value of the relaxation time τ is extremely small, and finally the temperature of the optical fiber body reaches T (tl). This is because the influence of the change in the value of Tp on T _f becomes small. As shown in Non-Patent Document 1, when the optical fiber body is held at a constant temperature, this is a significant advantage of the present invention compared to the fact that the optimum annealing furnace temperature is limited to a narrow range of about 50 ° C. Is a point. Regarding the control of Tp, depending on the size of the drawing furnace, a heater for heating is installed at the lowermost part of the drawing furnace or in the middle part of the slow cooling section, or forced cooling from the outside of the drawing furnace. Can be appropriately performed.

Therefore, according to the optical fiber manufacturing method of the first embodiment of the present invention, when the time when the temperature of the optical fiber body reaches around 1600 ° C. is t = 0, the predetermined slow cooling time t1 By setting the structure of the optical fiber at T1 within the range of 10 ⁻⁵ times to 10 times the time τ required for relaxation, preferably within the range of 10 ⁻² times to 2 times, an accompanying slow cooling furnace Therefore, it is possible to manufacture an optical fiber in which light loss due to Rayleigh scattering is reduced by a simple temperature control that can be performed with a small energy consumption with respect to a drawing furnace having an arbitrary length.

  Below, the manufacturing method of the optical fiber which concerns on the 2nd Example of this invention is demonstrated concretely using drawing. This embodiment relates to an effect of reducing optical loss when an optical fiber is drawn under optimum temperature conditions for different values of tl.

  FIG. 7 is a diagram illustrating the relationship between the slow cooling time and the Rayleigh scattering coefficient of a single mode optical fiber and a multimode optical fiber.

  In the optical fiber manufacturing method according to the second embodiment of the present invention, 0.5 is used as the optimum value of the parameter m from the result of FIG. Here, Table 1 shows tl, T (tl), and Tp when m = 0.5 in the equation (5). However, by limiting the limiting condition of Tp to room temperature 25 ° C. or more, m = 2 was set only for the case of tl = 0.1.

In the non-patent document 4, the present inventors have found that the Rayleigh scattering coefficient R of an optical fiber using quartz glass doped with GeO ₂ as a core can be satisfactorily approximated by the following equation (6). Here, Δ is a relative refractive index difference (%) with respect to pure quartz glass.

R (dB / km / μm ⁴ ) = 4.1 × 10 ^-4 (T _f + 273.1) (1 + 0.62Δ) (6)

Rayleigh of single mode optical fiber SMF (Δ = 0.35%) and multimode optical fiber MMF (Δ = 1.0%) with a GeO ₂ doped quartz core using the equation (6) under the temperature conditions in Table 1. The scattering coefficient R is calculated, and the result is shown in FIG. Note that commercially available optical fibers are generally drawn at a high speed of about 10 m / s, and the temperature control at the bottom of the drawing furnace is not performed, so when not slowly cooled (tl = 0, T _f = 1600 ° C.). It seems that it corresponds to the value. Actually, the calculated value 0.93 of the Rayleigh scattering coefficient R when tl = 0 of the SMF almost coincides with the average value of the Rayleigh scattering coefficient R of the commercially available SMF. The calculated value of the Rayleigh scattering coefficient R when MMF t1 = 0 was 1.24. Also, as shown in FIG. 7, the Rayleigh scattering coefficient R of SMF and MMF decreases linearly with respect to log (tl). In each optical fiber, the amount of decrease from the value when not slowly cooled (t1 = 0) indicated by the dotted line in the figure is about 8 to 18%.

  As described above, in the present invention, by controlling Tp, it is possible to obtain a loss reduction effect at an arbitrary drawing speed for various types of silica-based optical fibers.

For example, assuming that the slow cooling section length L is 50 cm, v = 5 m / s when converted from the value of the Rayleigh scattering coefficient R to the Rayleigh scattering loss (= R / λ ⁴ , λ: wavelength) on the short wavelength side. When tl = 0.1 s, the loss reduction effect of about 0.03 dB / km at a wavelength of 1.3 μm can be obtained with SMF, compared with the value when tl = 0, and v = 0.5 m In the case of / s, tl = 1s. Similarly, with MMF, a loss reduction effect of about 0.4 dB / km can be obtained at a wavelength of 0.8 μm.

  On the contrary, when the range of the drawing speed v is determined, tl can be determined from the target Rayleigh scattering coefficient R and the loss value, so that it is possible to calculate the optimal slow cooling section length L in advance. It can be used for the design preparation and manufacture of a drawing furnace.

The optical fiber manufacturing method according to the third embodiment of the present invention will be specifically described below with reference to the drawings. The present embodiment relates to a change in the fictive temperature _Tf with respect to the values of the slow cooling zone length L and the slow cooling time t1, and the effect of reducing the Rayleigh scattering coefficient R.

FIG. 8 is a diagram showing the relationship between the temperature T of the optical fiber body and the fictive temperature _Tf with respect to the slow cooling time t in the furnace.

The SMF of a GeO ₂ doped quartz core having a relative refractive index difference Δ = 0.33% in a drawing furnace that actually satisfies the conditions of the slow cooling section length L = 30 cm, the drawing speed v = 1 m / s, and t1 = 0.3 s. As a result of drawing and calculating the Rayleigh scattering coefficient R from the loss wavelength characteristic, it was 0.8 dB / km / μm ⁴ . On the other hand, when the temperature distribution in the drawing furnace was measured, in the high temperature region, the temperature decreased at a rate of about 100 ° C./cm in the longitudinal direction.

Therefore, as shown in FIG. 8, the temperature T of the optical fiber and the fictive temperature T _f with respect to the slow cooling time t (position Z) of the optical fiber main body were calculated assuming a drawing furnace temperature distribution Tz. As shown in this figure, the value of T _f was about 1400 ° C. At this time, the value of the Rayleigh scattering coefficient R obtained from the equation (6) was 0.83, and the actually obtained value almost coincided with 0.8.

  INDUSTRIAL APPLICABILITY The present invention can be used for an optical fiber manufacturing method and a drawing furnace, and in particular, can be used for an optical fiber manufacturing method and a drawing furnace for reducing optical loss of various optical fibers used for optical communication.

It is a schematic diagram for demonstrating the drawing furnace which concerns on the principle and best form of this invention. It is a figure which shows an example of the relationship between the time which an optical fiber main body stays in the slow cooling area of the drawing furnace which concerns on the best form of this invention, and the temperature at that time. It is a figure which shows an example of the temperature dependence of relaxation time (tau) of a silica type optical fiber. Setting of the parameter m in the relational expression between the slow cooling time t1 of the optical fiber body and the relaxation time τ at that time, the temperature T (tl) of the optical fiber body after the slow cooling time t1, and the temperature Tmin at the lowest point at this time It is a figure which shows the relationship of value Tp. It is a figure which shows the relationship between the parameter m of the relational expression of the slow cooling time t1 of the optical fiber main body, and the relaxation time τ at that time, and the fictive temperature _Tf . It is a figure which shows the relationship between the setting value Tp of temperature Tmin of the lowest point of a drawing furnace, and virtual temperature _Tf . It is a figure which shows the relationship between the slow cooling time and the Rayleigh scattering coefficient of single mode optical fiber SMF and multimode optical fiber MMF. It is a figure which shows the relationship between the temperature T of the optical fiber main body with respect to the slow cooling time t of an optical fiber, and the virtual temperature _Tf .

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber base material 2 Optical fiber 10 Drawing furnace main body 11 Slow cooling section 12 Out-of-furnace 13 Heating heater 20 Drawing furnace

Claims (4)

An optical fiber manufacturing method for manufacturing an optical fiber by cooling an optical fiber main body formed by heating a silica-based optical fiber preform at a high temperature of 1600 ° C. or higher by a heating means disposed in a drawing furnace to room temperature. And
In the cooling process of cooling the optical fiber body to room temperature, when the time when the temperature of the optical fiber body reaches around 1600 ° C. is t = 0, the temperature T1 of the optical fiber body reached after a predetermined slow cooling time t1 Is controlled to a temperature of 900 ° C. or higher, and the predetermined slow cooling time t1 is in the range of 10 ⁻⁵ times to 10 times the time τ required for relaxation of the structure of the optical fiber body at T1. An optical fiber manufacturing method. An optical fiber manufacturing method according to claim 1,
Below the heating means in the drawing furnace, there is provided a slow cooling section having a constant length in which the temperature in the drawing furnace continuously decreases from 1600 ° C. with respect to the longitudinal distance, and the drawing speed and the slow cooling section are provided. The method of manufacturing an optical fiber, wherein the set temperature Tmin at the bottom of the slow cooling section is changed according to the time t1 during which the optical fiber main body stays. An optical fiber manufacturing method for manufacturing an optical fiber by cooling an optical fiber main body formed by heating a silica-based optical fiber preform at a high temperature of 1600 ° C. or higher by a heating means disposed in a drawing furnace to room temperature. And
In the cooling process of cooling the optical fiber body to room temperature, when the time when the temperature of the optical fiber body reaches around 1600 ° C. is t = 0, the temperature T1 of the optical fiber body reached after a predetermined slow cooling time t1 Is controlled to a temperature of 900 ° C. or higher, and the predetermined slow cooling time t1 is in a range of 10 ⁻² times to ² times the time τ required for relaxation of the structure of the optical fiber body at T1,
Below the heating means in the drawing furnace, there is provided a slow cooling section having a constant length in which the temperature in the drawing furnace continuously decreases from 1600 ° C. with respect to the longitudinal distance, and the drawing speed and the slow cooling section are provided. A method of manufacturing an optical fiber, wherein the set temperature Tmin at the bottom of the slow cooling section is changed according to the time t1 during which the optical fiber main body stays at the bottom. An optical fiber comprising: a drawing furnace main body on which an optical fiber preform is disposed; and heating means disposed in the drawing furnace main body for heating the optical fiber preform, wherein the optical fiber preform is heated and drawn. A drawing furnace for producing an optical fiber by cooling the main body to room temperature,
The draw furnace body has a constant length slow cooling section in which the temperature continuously decreases from 1600 ° C. with respect to the distance in the longitudinal direction at the lower part thereof,
A drawing furnace characterized in that the lowermost set temperature Tmin of the slow cooling section can be variably adjusted within a range of 1600 ° C. or less.

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