Manufacturing method for optical fiber and manufacturing apparatus for optical fiber

A manufacturing method for an optical fiber, includes: drawing, while heating in a heating furnace, a lower end of an optical fiber preform that is to be an optical fiber having a core consisting of silica glass containing a rare earth element compound. The heating furnace has a temperature profile in which a temperature of the heating furnace increases to a maximum temperature Tmax and then decreases from an upstream side of the heating furnace toward a downstream side of the heating furnace. The temperature profile has a changing point at which the temperature decreases more steeply on the downstream side from a position where the maximum temperature Tmax is reached. At the maximum temperature, a temperature of the silica glass is higher than or equal to a glass transition temperature and the silica glass is in a single phase.

TECHNICAL FIELD

The present invention relates to a manufacturing method for an optical fiber and a manufacturing apparatus for an optical fiber.

BACKGROUND

A fiber laser device is used in various fields such as a laser processing field and a medical field because it is excellent in light condensing property, has a high power density, and can obtain a light beam having a small beam spot. In such a fiber laser device, a rare earth-added optical fiber is used having a core to which a rare earth element is added.

By the way, the optical fiber is obtained by heating an optical fiber preform in a heating furnace and drawing the optical fiber preform. A method for manufacturing an optical fiber in this way is described in Patent Literature 1 below. In the manufacturing method for an optical fiber of Patent Literature 1 below, to prevent the surface of the drawn optical fiber from being scratched, a blow of a gas is applied perpendicularly to the side surface of the drawn optical fiber, whereby the optical fiber is rapidly cooled, and compressive stress is applied to the surface of the optical fiber to strengthen the optical fiber.

However, when it is required to reduce optical transmission loss over a long distance as in an optical fiber used for optical communication, the optical transmission loss may be reduced by slowly cooling the drawn optical fiber to reduce a Rayleigh scattering coefficient. For example, in Patent Literature 2 below, a manufacturing method for an optical fiber is described capable of rapidly cooling only the surface of the optical fiber and slowly cooling the inside of the optical fiber.

PATENT LITERATURE

SUMMARY

The core of the optical fiber described in Patent Literature 1 and Patent Literature 2 does not contain a rare earth element. The core consisting of silica glass that does not contain rare earth elements is unlikely to undergo crystallization during the manufacturing process of the optical fiber. On the other hand, in the case of a rare earth-added optical fiber, the core consisting of silica glass to which a rare earth element is added is likely to crystallize in the manufacturing process. When such crystallization occurs in the core of the rare earth-added optical fiber, there is a concern that loss of light propagating through the core increases. Furthermore, the silica glass to which the rare earth element is added may cause phase separation in which separation occurs into a plurality of liquid phases having different composition ratios at a predetermined temperature. The occurrence of such phase separation also causes a concern that the loss of the light propagating through the core increases.

Since the manufacturing method for an optical fiber of Patent Literature 1 and Patent Literature 2 is premised on manufacture of an optical fiber containing no rare earth elements in the core, the crystallization and phase separation in the core as described above have not been studied. Thus, when an optical fiber having a core containing a rare earth element compound is manufactured by the manufacturing method for an optical fiber described in Patent Literature 1 and Patent Literature 2, there is a concern that the loss of the light propagating through the core cannot be sufficiently suppressed.

The present invention provides a manufacturing method for an optical fiber and a manufacturing apparatus for an optical fiber capable of suppressing the loss of the light propagating through a core consisting of silica glass containing a rare earth element compound.

A manufacturing method for an optical fiber of the present invention includes a drawing process of drawing while heating, in a heating furnace, a lower end portion of an optical fiber preform that is to be an optical fiber having a core consisting of silica glass containing a rare earth element compound, in which a temperature profile in the heating furnace is a profile in which temperature is increased from an upstream side toward a downstream side to a maximum temperature and then decreased and that has a changing point at which a temperature decrease becomes steep on the downstream side from a place where the maximum temperature is reached, and the maximum temperature is a temperature at which temperature of the silica glass is higher than or equal to a glass transition temperature and the silica glass is in a single phase.

In the silica glass containing the rare earth element compound, the crystallization and phase separation of the rare earth element compound occur at a temperature lower than a temperature at which the silica glass containing the rare earth element compound is in a single phase at higher than or equal to the glass transition temperature. Thus, the silica glass containing the rare earth element compound is once heated to the temperature that is higher than or equal to the glass transition temperature and at which the silica glass is in a single phase, and then cooled to a predetermined temperature at an increased cooling rate, whereby the crystallization and phase separation of the rare earth element compound can be suppressed. The manufacturing method for an optical fiber of the present invention can therefore suppress the loss of the light propagating through the core containing the rare earth element compound.

Furthermore, the temperature profile in the heating furnace may be set to cause the cooling rate to be maximized at a temperature at which separation occurs into a plurality of liquid phases respectively having different composition ratios between the rare earth element compound and a pure silica glass in an equilibrium state.

The cooling rate is maximized in a temperature range in which the phase separation occurs in the silica glass containing the rare earth element compound, whereby the phase separation of the silica glass containing the rare earth element compound can be further suppressed.

Furthermore, the temperature profile on the upstream side from the changing point in the temperature profile in the heating furnace may be approximated to a part of a normal distribution represented by a following equation (1), and the temperature profile on the downstream side from the changing point is approximated to a part of a normal distribution represented by a following equation (2), and a temperature of the heating furnace is set to cause a ratio σt/σbof a dispersion σtof the normal distribution represented by the equation (1) to a dispersion σbof the normal distribution represented by the equation (2) to be greater than or equal to 2.

Here, in the above equations (1) and (2), T is a temperature at an arbitrary point in the heating furnace, A and B are constants, x is a distance to the arbitrary point from a reference point when the reference point is a position where the maximum temperature is reached and a direction from the reference point to the downstream side is the positive direction, xcis a distance from the reference point to a position where the changing point is reached, and x0is a distance from the reference point to a position where T calculated by the equation (2) is a calculated maximum temperature.

The larger the ratio σt/σb, the steeper the temperature decrease after the changing point of the temperature profile in the heating furnace. Furthermore, the present inventor has found that the ratio σt/σbis set to be greater than or equal to 2, whereby the cooling rate of a glass wire drawn from the optical fiber preform can be made sufficiently large on the downstream side from the place where the maximum temperature is reached in the heating furnace.

Furthermore, the temperature of the heating furnace may be set to cause the ratio σt/σbto be greater than or equal to 3.

The present inventor has found that the ratio σt/σbis set to be greater than or equal to 3, whereby the cooling rate of the glass wire drawn from the optical fiber preform can be made larger on the downstream side of the place where the maximum temperature is reached in the heating furnace.

Moreover, the temperature of the heating furnace may be set to cause the ratio σt/σbto be less than or equal to 8.

As described above, the ratio σt/σbis set to be greater than or equal to 2, whereby the cooling rate of the glass wire drawn from the optical fiber preform can be made sufficiently large on the downstream side from the place where the maximum temperature is reached in the heating furnace. However, the present inventor has found that when the ratio σt/σbbecomes larger than 8, the maximum value of the cooling rate does not change much. Furthermore, as the ratio σt/σbhas a lower value, setting of the temperature in the heating furnace tends to be easier. Thus, the temperature of the heating furnace is set so that the ratio σt/σbis less than or equal to 8, whereby the cooling rate of the glass wire can be made larger while the setting of the temperature of the heating furnace is facilitated.

Furthermore, the dispersion σtmay be set to be greater than or equal to 100 mm and less than or equal to 300 mm.

As described above, the larger the ratio σt/σb, the steeper the temperature decrease after the changing point of the temperature profile in the heating furnace. Thus, as the dispersion σtis smaller, the ratio σt/σbis smaller, and the temperature decrease is gentle after the changing point of the temperature profile in the heating furnace. That is, as the dispersion σtis smaller, it is easier for the cooling rate of the glass wire drawn from the optical fiber preform to asymptotically approach a constant value. The present inventor has found that the cooling rate of the glass wire can be made sufficiently high when the dispersion σtis greater than or equal to 100 mm. On the other hand, the larger the dispersion σt, the slower the cooling rate of the glass. To obtain a high cooling rate, the dispersion σtmay be less than or equal to 300 mm.

Furthermore, a ratio Tc/Tmaxof a temperature Tcof the changing point to a maximum temperature Tmaxmay be set to be greater than or equal to 0.5.

When the ratio Tc/Tmaxis greater than or equal to 0.5, it becomes easier to rapidly cool the glass wire drawn from the optical fiber preform after heating the glass wire to the maximum temperature. Thus, the crystallization and phase separation are more likely to be suppressed of the rare earth element compound in the silica glass constituting the core.

Furthermore, the ratio Tc/Tmaxmay be set to be greater than or equal to 0.7.

When the ratio Tc/Tmaxis greater than or equal to 0.7, it becomes easier to rapidly cool the glass wire drawn from the optical fiber preform after heating the glass wire to the maximum temperature. Thus, the crystallization and phase separation are further likely to be suppressed of the rare earth element compound in the silica glass constituting the core.

Furthermore, a rare earth element contained in the rare earth element compound may be ytterbium (Yb), and a concentration of the rare earth element in the core may be greater than or equal to 2.0 wt % and less than or equal to 3.1 wt %.

Since Yb is a rare earth element, the greater the concentration of Yb added to the core, the more likely it is that the crystallization and phase separation occur in the core. For example, when the concentration of Yb in the core is greater than or equal to 2.0 wt % and less than or equal to 3.1 wt %, the crystallization and phase separation are likely to occur in the core. However, as described above, in the manufacturing method for an optical fiber, the temperature in the heating furnace is steeply decreased on the downstream side from a position where the temperature in the heating furnace is maximized, so that even when Yb of greater than or equal to 2.0 wt % and less than or equal to 3.1% is added, the crystallization and phase separation of Yb can be suppressed, and the loss of the light propagating through the core can be suppressed.

When the concentration of Yb added to the core is greater than or equal to 2.0 wt % and less than or equal to 3.1 wt %, the core may further contain aluminum (Al) of greater than or equal to 3.0% wt and less than or equal to 5.3 wt %, and phosphorus (P) of greater than or equal to 1.7 wt % and less than or equal to 5.6 wt %.

When Al and P are co-added with Yb, the crystallization and phase separation can be suppressed in the core to which Yb is added. In addition to this, in the manufacturing method for an optical fiber, as described above, the temperature in the heating furnace can be steeply decreased on the downstream side from the position where the temperature in the heating furnace is maximized. The crystallization and phase separation in the core can therefore be further suppressed.

Furthermore, a manufacturing apparatus for an optical fiber of the present invention includes a heating furnace that heats, with a heating element, an optical fiber preform that is to be optical fiber having a core consisting of silica glass containing a rare earth element compound, in which a temperature profile in the heating furnace is a profile in which temperature is increased from an upstream side toward a downstream side to a maximum temperature and then decreased and that has a changing point at which a temperature decrease becomes steep on the downstream side from a place where the maximum temperature is reached, and the maximum temperature is a temperature at which temperature of the silica glass is higher than or equal to a glass transition temperature and the silica glass is in a single phase.

As described above, the silica glass is once heated to the temperature at which temperature of the silica glass containing the rare earth element compound is higher than or equal to the glass transition temperature and the silica glass is in a single phase, and then cooled to the predetermined temperature at the increased cooling rate, whereby the crystallization and phase separation of the rare earth element compound can be suppressed. The manufacturing apparatus for an optical fiber of the present invention can therefore suppress the loss of the light propagating through the core containing the rare earth element compound.

Furthermore, a cooling member that cools the glass wire drawn from the optical fiber preform may be provided below the heating element.

The cooling member that cools the glass wire drawn from the optical fiber preform is provided below the heating element, whereby it becomes easier to decrease the temperature in the heating furnace, in the lower side of the heating furnace. It is therefore possible to easily form the temperature profile having the changing point at which the temperature decrease becomes steep on the downstream side of the place where the maximum temperature is reached, in the heating furnace.

Furthermore, the cooling member may surround the glass wire, and a blow of a cooling gas may be applied from the bottom toward the top between the inner peripheral surface of the cooling member and the surface of the glass wire.

The blow of the cooling gas is applied as described above, whereby the temperature decrease can be made steeper on the downstream side of the place where the maximum temperature is reached, in the heating furnace. Furthermore, the blow of the cooling gas is applied from the bottom toward the top, whereby the cooling gas flows along the glass wire. In this case, shaking of the glass wire can be suppressed as compared with a case where the cooling gas is applied perpendicularly to the side surface of the glass wire as in the methods described in Patent Literature 1 and Patent Literature 2. Thus, the optical fiber can be manufactured with high accuracy as compared with the methods described in Patent Literature 1 and Patent Literature 2.

Furthermore, a heat radiating material that transfers heat inside the heating furnace to the outside of the heating furnace may be provided below the heating element.

Such a heat radiating material is provided, whereby it becomes easier to radiate heat to the outside below the heating element in the heating furnace. It is therefore possible to easily form the temperature profile having the changing point at which the temperature decrease becomes steep on the downstream side of the place where the maximum temperature is reached, in the heating furnace.

As described above, according to the present invention, the manufacturing method for an optical fiber and the manufacturing apparatus for an optical fiber are provided capable of suppressing the loss of the light propagating through the core containing the rare earth element compound.

DETAILED DESCRIPTION

Hereinafter, embodiments of a manufacturing method for an optical fiber and a manufacturing apparatus for an optical fiber according to the present invention will be described in detail with reference to the drawings. The embodiments exemplified below are for facilitating understanding of the present invention, and are not for limiting interpretation of the present invention. The present invention can be modified and improved without departing from the spirit of the present invention.

FIG.1is a diagram illustrating a cross section perpendicular to the longitudinal direction of an optical fiber according to one or more embodiments of the present invention. An optical fiber1according to one or more embodiments is an amplification optical fiber. As illustrated inFIG.1, the optical fiber1according to one or more embodiments includes a core10, an inner clad11that is a clad surrounding the outer peripheral surface of the core10without a gap, an outer clad12that coats the outer peripheral surface of the inner clad11, and a protective layer13that coats the outer peripheral surface of the outer clad12, as main components. As described above, the optical fiber1has a so-called double clad structure. A refractive index of the inner clad11is lower than a refractive index of the core10, and a refractive index of the outer clad12is lower than the refractive index of the inner clad11. Furthermore, the core10is disposed at the center of the inner clad11.

Examples of a material constituting the core10include silica glass to which a rare earth element such as ytterbium (Yb) is added. Examples of such a rare earth element include thulium (Tm), cerium (Ce), neodymium (Nd), europium (Eu), erbium (Er), and the like, in addition to Yb described above. Note that, the rare earth element forms a compound and is contained in the material constituting the core10. For example, Yb forms an oxidized compound such as Yb2O3and exists in the core10. Furthermore, an element such as germanium (Ge) that increases the refractive index, or an element such as aluminum (Al) or phosphorus (P) that can suppress crystallization and photodarkening may be further added to the material constituting the core10. Moreover, to adjust the refractive index, an element such as fluorine (F) or boron (B) that decreases the refractive index may be added to the material constituting the core10.

Examples of a material constituting the inner clad11include pure silica glass to which no dopant is added. Note that, an element such as fluorine (F) that decreases the refractive index may be added to the material constituting the inner clad11.

The outer clad12includes, for example, a resin, and examples of the resin include an ultraviolet curable resin and a thermosetting resin.

Examples of a material constituting the protective layer13include an ultraviolet curable resin and a thermosetting resin. When the outer clad12includes a resin, the material constituting the protective layer13is a resin different from the resin constituting the outer clad12.

Next, a description will be given of the manufacturing apparatus for an optical fiber according to one or more embodiments of the present invention.

FIG.2is a diagram schematically illustrating the manufacturing apparatus for an optical fiber according to one or more embodiments. A manufacturing apparatus100for the optical fiber1illustrated inFIG.2includes a preform feeding device21, a heating furnace20, a coating device50, a turn pulley60, a drawing device61, and a winding device62, as main components. The optical fiber1is manufactured by the manufacturing apparatus100for an optical fiber.

The preform feeding device21is a device attached to the upper end portion of an optical fiber preform1P that is to be the optical fiber1, and including a motor that feeds the optical fiber preform1P from the lower end side into the heating furnace20at a predetermined speed.

The heating furnace20according to one or more embodiments includes a housing23, a core tube22, a heating element30, a heat insulating material25, and a cooling member40, as main components.

A refrigerant flow path24through which a refrigerant flows is formed in the outer wall of the housing23. The housing23is cooled by the flow of the refrigerant through the refrigerant flow path24, and damage to the housing23due to heat is suppressed. Furthermore, the housing23includes a through hole penetrating in the vertical direction in the central portion, and the core tube22is inserted into the through hole. In one or more embodiments, the core tube22protrudes from each of the upper end and the lower end of the housing23. However, the core tube22does not have to protrude from at least one of the upper end or the lower end of the housing23. Moreover, a hollow portion20H communicating with the through hole is formed in the housing23, and the heating element30is provided so that the core tube22can be heated from the outer peripheral surface side of the core tube22in the hollow portion20H.

The heating element30according to one or more embodiments generates heat due to electric resistance when energized. The heating element30may be a part of the core tube22. To effectively use the heat generated by the heating element30, the heating element30and the core tube22are surrounded by the heat insulating material25in the hollow portion20H. The number of the heat insulating materials25is not particularly limited, and the heat insulating material25may be divided into a plurality of parts.

The maximum temperature in the heating furnace20may be set to a high temperature of about 2000° C. although it depends on the size of the optical fiber preform1P, a target outer diameter of a bare optical fiber1E drawn from the optical fiber preform1P, tension applied to the bare optical fiber1E, and the like. Materials constituting the core tube22, the heating element30, and the heat insulating material25may therefore be carbon, for example. When carbon is used for the core tube22, the heating element30, and the heat insulating material25, the inside of the heating furnace20may have an inert atmosphere. Thus, the inner peripheral surface side of the core tube22and the hollow portion20H of the housing23may be filled with an inert gas such as argon (Ar) or helium (He).

The cooling member40is a member that cools a glass wire drawn from the optical fiber preform1P, and is provided below the heating element30on the inner peripheral surface side of the core tube22in the heating furnace20. Furthermore, the cooling member40according to one or more embodiments surrounds the glass wire drawn from the optical fiber preform1P. Such a cooling member40may be configured such that cooling water flows inside, for example. The temperature of the cooling water may be a temperature at which boiling does not occur while the cooling water flows inside the cooling member40, and a temperature at which excessive dew condensation does not occur on the cooling member40. The cooling water may be supplied to the cooling member40at a constant temperature within a range of, for example, higher than or equal to 10° C. and less than or equal to 70° C. The temperature of the cooling water supplied to the cooling member40increases while the cooling water flows inside the cooling member40, but the temperature of the cooling water when supplied to the cooling member40is kept constant, whereby a change over time can be suppressed in the temperature of the cooling water at each part in the cooling member40.

Furthermore, a blow of the cooling gas may be applied from the bottom toward the top on the inner peripheral surface side of the cooling member40according to one or more embodiments. The blow of the cooling gas is applied from the bottom toward the top between the inner peripheral surface of the cooling member40and the surface of the glass wire drawn from the optical fiber preform1P. The blow of the cooling gas is applied in this way, whereby the cooling gas easily flows along the surface of the glass wire drawn from the optical fiber preform1P. Shaking of the glass wire can therefore be suppressed as compared with a case where the blow of the cooling gas is applied perpendicularly to the surface of the glass wire. The type of the cooling gas is not particularly limited, but the cooling gas may be He or Ar from viewpoints of thermal conductivity and the like.

The coating device50according to one or more embodiments includes a first coating device51and a second coating device52. The first coating device51is a device that forms the outer clad12that coats the outer peripheral surface of the bare optical fiber1E by causing the bare optical fiber1E including the core10and the inner clad11drawn in the heating furnace20to pass through. The second coating device52is a device that forms the protective layer13that coats the outer peripheral surface of the outer clad12.

The drawing device61is a device that draws the optical fiber1whose direction is changed by the turn pulley60at a predetermined drawing speed, and the winding device62is a device that winds the optical fiber1around a bobbin.

Next, a description will be given of the manufacturing method for an optical fiber according to one or more embodiments of the present invention.

FIG.3is a flowchart illustrating the manufacturing method for an optical fiber according to one or more embodiments of the present invention. According to the manufacturing method for an optical fiber of one or more embodiments, the optical fiber1is manufactured by using the manufacturing apparatus100for an optical fiber. As illustrated inFIG.3, the manufacturing method for the optical fiber1according to one or more embodiments includes a preparation process P1and a drawing process P2, as main processes.

In this process, first, the optical fiber preform1P is prepared including a core glass body that is to be the core10of the optical fiber1and a clad glass body that is to be the inner clad11.FIG.4is a diagram illustrating a cross section perpendicular to the longitudinal direction of the optical fiber preform1P prepared in the preparation process P1. As illustrated inFIG.4, the optical fiber preform1P includes a core glass body10P that is to be the core10of the optical fiber1and a clad glass body11P that is to be the inner clad11. The method for producing the optical fiber preform1P is not particularly limited, and for example, the optical fiber preform1P can be produced by a modified chemical vapor deposition method (MCVD method).

Next, the optical fiber preform1P is set in the heating furnace20. As illustrated inFIG.2, the upper end portion of the optical fiber preform1P is fixed to the preform feeding device21, and the optical fiber preform1P is inserted into the core tube22of the heating furnace20from the lower end portion.

This process is a process of drawing while heating the lower end portion of the optical fiber preform1P in the heating furnace20.

After the optical fiber preform1P is set in the heating furnace20in the preparation process P1as described above, the heating element30of the heating furnace20is caused to generate heat to heat the lower end portion of the optical fiber preform1P. The lower end portion of the optical fiber preform1P is melted by being heated in the heating furnace20, and a tapered neck-down ND is formed and the diameter is reduced. In this way, the lower end portion of the optical fiber preform1P is reduced in diameter, the core glass body10P becomes the core10, the clad glass body11P becomes the inner clad11, and the bare optical fiber1E is obtained including the core10and the inner clad11.

The outer diameter of the bare optical fiber1E, that is, the outer diameter of the inner clad11is adjusted by adjustment of a speed at which the optical fiber preform1P is fed to the downstream side of the heating furnace20by the preform feeding device21and a speed at which the optical fiber1is drawn by the drawing device61. Furthermore, when the bare optical fiber1E is drawn as described above, downward pulling force, that is, drawing tension is applied to the neck-down ND. The drawing tension applied to the neck-down ND is adjusted by adjustment of the temperature in the heating furnace20, and the like.

FIG.5is a diagram illustrating a temperature profile in the heating furnace20. As illustrated by the solid line inFIG.5, the temperature profile in the heating furnace20is a profile in which the temperature is increased from the upstream side toward the downstream side of the heating furnace20to the maximum temperature Tmaxand then decreased and that has a changing point at which a temperature decrease becomes steep on the downstream side from a place where the maximum temperature Tmaxis reached. A temperature of the changing point is set as Tc, and a position of the changing point is set as xc. Note that, the position in the heating furnace20is defined with the place where the maximum temperature Tmaxis reached as a reference point (0), the downstream side from the reference point as the positive direction, and the upstream side as the negative direction.

In the heating furnace20according to one or more embodiments, the cooling member40is disposed below the heating element30of the core tube22, whereby the temperature of the lower part of the heating furnace20is lower than the temperature of the upper part. The temperature in the heating furnace20therefore has a profile having the changing point as described above.

The temperature profile illustrated by the broken line inFIG.5illustrates a temperature profile after the changing point when the cooling member40is not included. When the cooling member40is not included, the temperature profile in the heating furnace20is roughly a normal distribution. That is, when the cooling member40is not included, the temperature profile in the heating furnace20is roughly symmetrical with respect to the reference point at which the maximum temperature Tmaxis reached between the upstream side and the downstream side, but the temperature profile in the heating furnace20according to one or more embodiments is asymmetrical with respect to the reference point between the upstream side and the downstream side.

The maximum temperature Tmaxin the heating furnace20is a temperature at which temperature of the silica glass constituting the core10is higher than or equal to the glass transition temperature and the silica glass is in a single phase. The maximum temperature Tmaxin the heating furnace20will be described with reference to a binary equilibrium diagram of Yb2O3and SiO2.

FIG.6is the binary equilibrium diagram of Yb2O3and SiO2.

Silica glass (SiO2) containing a few percent of Yb2O3is in two liquid phases having composition ratios between Yb2O3and SiO2are different from each other, in a predetermined temperature range of higher than or equal to 1700° C. An upper limit of the temperature at which such phase separation occurs differs depending on an amount of Yb2O3added to the silica glass. The maximum temperature Tmaxin the heating furnace20is the temperature at which the silica glass is in a single phase, and is a temperature higher than the temperature range in which the phase separation occurs. Thus, as illustrated inFIG.6, the maximum temperature Tmaxin the heating furnace20is a temperature at which the silica glass containing Yb2O3is in a liquid phase (Liquid) in an equilibrium state. A few percent of Yb2O3is added to the silica glass constituting the core10, and if the maximum temperature Tmaxin the heating furnace20is higher than or equal to 2200° C., the maximum temperature Tmaxis a temperature at which temperature of the silica glass constituting the core10is higher than or equal to the glass transition temperature and the silica glass is in a single phase. However, as described above, the upper limit of the temperature at which the phase separation occurs differs depending on the amount of Yb2O3added. The maximum temperature Tmaxmay therefore be lower than 2200° C. For example, the maximum temperature Tmaxmay be about 2000° C. to about 1730° C. depending on the composition of the materials constituting the core10.

FIG.7is a diagram for explaining the temperature profile in the heating furnace20illustrated inFIG.5in more detail. As illustrated inFIG.7, the temperature profile on the upstream side from the changing point can be approximated as a part of a normal distribution illustrated by the solid line, and the temperature profile on the downstream side from the changing point can be approximated as a part of a normal distribution illustrated by the broken line. That is, the temperature profile on the upstream side from the changing point in the temperature profile in the heating furnace20can be approximated to a part of a normal distribution represented by the following equation (1), and the temperature profile on the downstream side from the changing point can be approximated to a part of a normal distribution represented by the following equation (2).

Here, σtis a dispersion of the normal distribution when the temperature profile on the upstream side from the changing point is approximated to the part of the normal distribution represented by the equation (1), and σbis a dispersion of the normal distribution when the temperature profile on the downstream side from the changing point is approximated to the part of the normal distribution represented by the equation (2). Thus, σt>σb. Furthermore, in the equations (1) and (2), T is a temperature at an arbitrary point in the heating furnace20, A and B are constants, x is a distance from the reference point (0) to an arbitrary point when a position where the maximum temperature Tmaxis reached is the reference point and the downstream side from the reference point is in the positive direction, xcis a distance from the reference point to a position where the changing point is reached, and x0is a distance from the reference point to a position where T represented by the equation (2) is the calculated maximum temperature Tmax.

In this case, a half width of the normal distribution illustrated in the equation (1) is 2√(2 ln 2)σt, and a half width of the normal distribution illustrated in the equation (2) is 2√(2 ln 2)σb.

The ratio σt/σbof the dispersion σtof the equation (1) to the dispersion σbof the equation (2) may be greater than or equal to 2, or may be greater than or equal to 3, and further may be less than or equal to 8, as described in the following calculation examples. Furthermore, the ratio σt/σbmay be less than or equal to 6.

In the calculation examples described below, the temperature profile of the glass constituting the optical fiber1near the neck-down ND and the outer diameter of the neck-down ND are estimated from the outer diameter of the optical fiber preform1P, the outer diameter of the inner clad11, the drawing speed of the optical fiber1, and the temperature profile in the heating furnace20. This estimation calculation is performed by solving relational expressions of equilibrium of forces, mass balance, and heat balance in formation of the neck-down ND. In the following calculation examples, to facilitate the calculation, it is assumed that the optical fiber preform1P is a cylinder made entirely of pure quartz. Note that, in the following description, the glass constituting the optical fiber1near the neck-down ND may be simply referred to as glass.

From the equilibrium of forces in the formation of the neck-down ND, the relational expression of the following equation (3) is derived.

Here, x is a position in the heating furnace20, t is a time, v is a moving speed of the glass at the position x, S is a cross-sectional area of the glass at the position x, F is drawing tension, and β is an elongational viscosity coefficient of the glass. In the case of pure quartz, the elongational viscosity coefficient β is represented by the following equation (4) as a function of a temperature Ta of the glass.

From the mass balance, a relational expression of the following equation (5) is derived.

From the heat balance, a relational expression of the following equation (6) is derived. Note that, g(Ta) in the following equation (6) is obtained by the following equation (7), and p(Ta) is obtained by the following equation (8).

Here, Ta is a temperature of the glass, Tb is a temperature in the heating furnace20, p is specific gravity of the glass, Cpis specific heat of the glass, Emis thermal emissivity of the glass in the radial direction, and Enis thermal emissivity of the glass in the longitudinal direction, and σBis Stefan-Boltzmann constant (5.76×10−8J/sec/m2/K4). In the following calculation examples, ρ=2200 kg/m3, Em=0.2, and En=0.2, and Cpis approximated by the following equations (9) and (10).
Cp=800+0.65(Ta−273)(Ta≤873K)  (9)
Cp=1115+0.1257(Ta−273)(Ta>873K)  (10)

FIG.8illustrates estimation results of the temperature profile in the heating furnace20, the temperature profile of the glass near the neck-down ND, and the outer diameter of the neck-down ND when the outer diameter of the optical fiber preform1P is 30 mm, the outer diameter of the inner clad11is 0.25 mm, the drawing speed of the optical fiber1is 50 m/min, the drawing tension is 50 gf, the constant B of the equations (1) and (2) is 300 K, the dispersion σtis 150 mm, the dispersion σbis 50 mm, and xcis 135 mm. Note that, at this time, the value of the constant A in the equations (1) and (2) is set so that the drawing tension is 50 gf, and the ratio σt/σb=3.0 and the ratio Tc/Tmax=0.70.

Furthermore,FIG.9illustrates a relationship between the temperature of the glass obtained from the estimation results illustrated inFIG.8and a cooling rate of the glass at that time.

Estimation was performed of the temperature profile in the heating furnace20, the temperature profile of the glass near the neck-down ND, and the outer diameter of the neck-down ND by changing the dispersion σtwithin a range from 50 mm to 300 mm and the dispersion σbwithin a range from 20 mm to 300 mm when the outer diameter of the optical fiber preform1P is 30 mm, the outer diameter of the inner clad11is 0.25 mm, the drawing speed is 50 m/min, the drawing tension is 50 gf, the constant B of the equations (1) and (2) is 300 K, and xcis 0 mmFIG.10illustrates the temperature profile in the heating furnace20when the dispersion σtis 150 mm and the dispersion σbis changed within a range from 20 mm to 150 mm,FIG.11illustrates the temperature profile of the glass under the same conditions, andFIG.12illustrates the outer diameter of the neck-down ND under the same conditions.

Furthermore,FIG.13illustrates a relationship between the cooling rate of the glass at each position in the heating furnace20, estimated from the temperature profile of the glass and the outer diameter of the neck-down ND when the dispersion σtis 150 mm, and the temperature of the glass at that time.

FIG.14illustrates a relationship between the ratio σt/σband a maximum value of the cooling rate obtained fromFIG.13, andFIG.15illustrates a relationship between a value obtained by normalizing the maximum value of the cooling rate with the maximum value of the cooling rate when σt=σb, and the ratio σt/σb. Note that,FIGS.14and15also illustrate together the result of the same calculation for each case where the dispersion σtis 50 mm, 100 mm, 150 mm, 200 mm, 300 mm, or 400 mm FromFIGS.14and15, it can be seen that the larger the value of the ratio σt/ab, the larger the cooling rate. FromFIG.15, when the ratio σt/σbis less than 2, an increase in the maximum value of the cooling rate is large, and when the ratio σt/σbis greater than or equal to 3, it becomes difficult for the maximum value of the cooling rate to increase. From this fact, it can be seen that the ratio σt/σbmay be greater than or equal to 2, or may be greater than or equal to 3, as described above. Furthermore, as the ratio σt/σbhas a lower value, setting of the temperature in the heating furnace tends to be easier, and fromFIG.14, it can be seen that the maximum value of the cooling rate does not change much when the ratio σt/σbis greater than 8. Thus, the ratio σt/σbmay be less than or equal to 8 as described above from a viewpoint that the cooling rate can be made closer to the maximum value while the ratio σt/σbis kept at a low value. Furthermore, it can be seen that as the dispersion at is smaller, the cooling rate asymptotically approaches a constant value from a value at which the ratio σt/σbis small, but when the dispersion σtis smaller than 100 mm, it is difficult to make the maximum value of the cooling rate large.

On the other hand, it can be seen that the maximum value of the cooling rate is a small value when the dispersion σtis larger than 300 mm Thus, the dispersion σtmay be less than or equal to 300 mm.

Estimation was performed of the temperature profile in the heating furnace20, the temperature profile of the glass near the neck-down ND, and the outer diameter of the neck-down ND by changing xcwithin a range from 0 mm to 225 mm when the outer diameter of the optical fiber preform1P is 30 mm, the outer diameter of the inner clad11is 0.25 mm, the drawing speed is 50 m/min, the drawing tension is 50 gf, the constant B is 300 K, the dispersion σtis 150 mm, and the dispersion m is 50 mm (ratio σt/σb=3.0).FIG.16illustrates the temperature profile in the heating furnace20when xcis changed within the range from 0 mm to 225 mm,FIG.17illustrates the temperature profile of the glass under the same conditions, andFIG.18illustrates the outer diameter of the neck-down ND under the same conditions.

Furthermore,FIG.19illustrates a relationship between the cooling rate of the glass at each position in the heating furnace20, estimated from the temperature profile of the glass and the outer diameter of the neck-down ND when the ratio σt/σb=3.0, and the temperature of the glass at that time.

FIG.20illustrates a relationship between a value obtained by normalizing the maximum value of the cooling rate obtained fromFIG.19with the maximum value of the cooling rate when xc=0, and the ratio Tc/Tmax. FromFIG.20, it can be seen that if the ratio Tc/Tmaxis greater than or equal to 0.5 regardless of the value of the ratio σt/σb, a decrease in the maximum value of the cooling rate can be suppressed to about 10%. Thus, when the ratio Tc/Tmaxis greater than or equal to 0.5, it becomes easier to rapidly cool the glass wire drawn from the optical fiber preform1P after heating the glass wire to the maximum temperature. Furthermore, it can be seen that if the ratio Tc/Tmaxis greater than or equal to 0.7, the decrease in the maximum value of the cooling rate can be suppressed to about 5%. Thus, when the ratio Tc/Tmaxis greater than or equal to 0.7, it becomes much easier to rapidly cool the glass wire drawn from the optical fiber preform1P after heating the glass wire to the maximum temperature. Note that, since the changing point is on the downstream side from the place where the maximum temperature is reached, the temperature Tcof the changing point is lower than the maximum temperature Tmax, and Tc/Tmaxis less than 1.

In the heating furnace20whose temperature is set as described above, the lower end of the optical fiber preform1P is heated and is in a molten state. Then, the glass wire melted is drawn from the optical fiber preform1P. Upon coming out of the heating furnace20, the drawn glass wire in the molten state solidifies immediately, and the core glass body10P becomes the core10, and the clad glass body11P becomes the inner clad11, whereby the bare optical fiber1E is obtained including the core10and the inner clad11.

After the bare optical fiber1E is produced as described above, the bare optical fiber1E is cooled to an appropriate temperature. The bare optical fiber1E may be cooled by a cooling device (not illustrated). The cooled bare optical fiber1E is coated with an ultraviolet curable resin that is to be the outer clad12and then irradiated with ultraviolet rays, in the first coating device51, whereby the outer clad12is formed including the ultraviolet curable resin cured. Next, the bare optical fiber1E coated with the outer clad12is coated with an ultraviolet curable resin that is to be the protective layer13and then irradiated with ultraviolet rays, in the second coating device52, whereby the ultraviolet curable resin is cured and the protective layer13is formed, and the optical fiber1illustrated inFIG.1is obtained.

Then, a direction of the optical fiber1is changed by the turn pulley60, and the optical fiber1is wound by the winding device62.

As described above, the manufacturing method for the optical fiber1according to one or more embodiments includes the drawing process P2that performs drawing while heating, in the heating furnace20, the lower end portion of the optical fiber preform1P that is to be the optical fiber1including the core10consisting of silica glass containing the rare earth element compound. Furthermore, the temperature profile in the heating furnace20is a profile in which the temperature is increased from the upstream side toward the downstream side of the heating furnace20to the maximum temperature Tmaxand then decreased and that has a changing point at which a temperature decrease becomes steep on the downstream side from a place where the maximum temperature Tmaxis reached. Moreover, the maximum temperature Tmaxis a temperature at which temperature of the silica glass is higher than or equal to a glass transition temperature and the silica glass is in a single phase.

In the silica glass containing the rare earth element compound, the crystallization and phase separation of the rare earth element compound occur at a temperature lower than a temperature at which the silica glass containing the rare earth element compound is in a single phase at higher than or equal to the glass transition temperature. Thus, heating is performed once to the temperature at which temperature of the silica glass containing the rare earth element compound is higher than or equal to the glass transition temperature and the silica glass is in a single phase, and then cooling is performed to a predetermined temperature at an increased cooling rate, whereby a time spent in the area indicated by Two liquids illustrated inFIG.6is shortened, so that the crystallization and phase separation of the rare earth element compound can be suppressed. The manufacturing method for the optical fiber1according to one or more embodiments can therefore suppress loss of light propagating through the core10containing the rare earth element compound.

Furthermore, in the manufacturing method for the optical fiber1according to one or more embodiments, the temperature profile in the heating furnace20is set so that the cooling rate is maximized at a temperature at which separation occurs into a plurality of liquid phases respectively having different composition ratios between the rare earth element compound and the pure silica glass in the equilibrium state. The cooling rate is maximized in a temperature range in which the phase separation occurs in the silica glass containing the rare earth element compound, whereby the phase separation of the silica glass containing the rare earth element compound can be further suppressed.

Furthermore, the manufacturing apparatus100for the optical fiber1according to one or more embodiments includes the heating furnace20that heats the optical fiber preform1P that is to be the optical fiber1having the core10including the silica glass containing the rare earth element compound. The temperature profile in the heating furnace20is set as described above. As described above, the silica glass containing the rare earth element compound is once heated to the temperature that is higher than or equal to the glass transition temperature and at which the silica glass is in a single phase, and then cooled to the predetermined temperature at the increased cooling rate, whereby the crystallization and phase separation of the rare earth element compound can be suppressed. The manufacturing apparatus100for the optical fiber1according to one or more embodiments can therefore suppress the loss of the light propagating through the core containing the rare earth element compound.

Furthermore, in the manufacturing apparatus100for the optical fiber1according to one or more embodiments, the cooling member40that cools the glass wire drawn from the optical fiber preform1P is provided below the heating element30that heats the heating furnace20. The cooling member40that cools the glass wire drawn from the optical fiber preform1P is provided below the heating element30that heats the heating furnace20, whereby it becomes easier to decrease the temperature in the heating furnace20, in the lower side of the heating furnace20. It is therefore possible to easily form the temperature profile having the changing point at which the temperature decrease becomes steep on the downstream side of the place where the maximum temperature Tmaxis reached, in the heating furnace20.

Furthermore, in the manufacturing apparatus100for the optical fiber1according to one or more embodiments, the cooling member40may surround the glass wire drawn from the optical fiber preform1P, and the blow of the cooling gas may be applied from the bottom toward the top between the inner peripheral surface of the cooling member40and the surface of the glass wire. The blow of the cooling gas is applied in this way, whereby the temperature decrease can be made steeper on the downstream side of the place where the maximum temperature Tmaxis reached, in the heating furnace20. Furthermore, the blow of the cooling gas is applied from the bottom toward the top, whereby the cooling gas flows along the glass wire. In this case, shaking of the glass wire can be suppressed as compared with a case where the cooling gas is applied perpendicularly to the side surface of the glass wire as in the methods described in Patent Literature 1 and Patent Literature 2. Thus, the optical fiber1can be manufactured with high accuracy as compared with the methods described in Patent Literature 1 and Patent Literature 2.

Although the present invention has been described above by exemplifying embodiments, the present invention is not limited thereto.

For example, a means that makes the temperature decrease steep on the downstream side from the place where the maximum temperature Tmaxis reached in the heating furnace20is not limited to the cooling member40exemplified in the embodiments described above. For example, a heat radiating material that transfers heat inside the heating furnace20to the outside of the heating furnace20may be provided below the heating element30that heats the heating furnace20. A thermal conductivity of the heat radiating material is made higher than a thermal conductivity of the heat insulating material25. Such a heat radiating material is provided, whereby it becomes easier to radiate heat to the outside from the lower side than from the upper side in the heating furnace20. It is therefore possible to easily form the temperature profile having the changing point at which the temperature decrease becomes steep on the downstream side of the place where the maximum temperature Tmaxis reached, in the heating furnace20.

FIGS.21to23are diagrams illustrating a cross section of the heating furnace20according to such a modification.

The heating furnace20illustrated inFIG.21includes a hollow tube26as a heat radiating material below the heat insulating material25in the hollow portion20H of the housing23. The hollow tube26includes, for example, carbon or the like. From a viewpoint of facilitating the transfer of the heat inside the heating furnace20to the outside of the heating furnace20, the hollow tube26may be in contact with the outer peripheral surface of the core tube22and the inner peripheral surface of the housing23.

The heating furnace20illustrated inFIG.22includes a plurality of metal rods27provided to penetrate from the inside to the outside of the heating furnace20at the lower end portion of the heat insulating material25. The metal rod27is a heat radiating material. The metal rod27includes a refractory metal. Examples of the refractory metal constituting the metal rod27include W, Re, Ta, Os, Mo, Nb, Ir, Ru, Hf, and the like. These metals may be used in combination. Furthermore, the number of metal rods27is not particularly limited. However, the metal rods27may be provided at positions that are rotationally symmetric with the axial center of the core tube22as the axis of symmetry. The plurality of metal rods27is provided in this way, whereby it becomes easier to uniformly radiate heat in the circumferential direction of the heating furnace20.

The heating furnace20illustrated inFIG.23includes metal powder28dispersed at the lower end portion of the heat insulating material25. The metal powder28is a heat radiating material, and the metal constituting the metal powder28is the same as that of the metal rod27.

The examples illustrated inFIGS.21to23are only a part of cases where the heat radiating material is provided, and the method of providing the heat radiating material at the lower end portion of the heating furnace20is not limited to these. Furthermore, in the examples illustrated inFIGS.21to23, the cooling member40is not essential.

Furthermore, in the embodiments described above, an example has been described in which the outer periphery of the inner clad11is circular in the cross section perpendicular to the longitudinal direction of the optical fiber1. However, the outer peripheral shape of the inner clad11is not limited to a circular shape, and may be a polygon such as a hexagon, a heptagon, or an octagon, or a non-circular shape such as a shape in which the corners of the polygon are rounded.

Hereinafter, the content of the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

FIG.24is a diagram illustrating a cross section of a heating furnace used in Example 1. InFIG.24, components having the same configurations as those inFIG.2are designated by the same reference numerals. A heating furnace20aused in Example is different from the heating furnace20exemplified in the embodiments in that the heat insulating material25is divided into three and the position of the heating element30is different.

In normal operation, the temperature in the heating furnace20ais about 2000° C., so it is difficult to actually measure the temperature in the heating furnace20a. The temperature in the heating furnace20awas therefore set lower than the temperature during actual operation, and the temperature in the heating furnace20awas actually measured with an Ir/Ir-40% RH thermocouple.

The length of the heating element30in the height direction was 40 mm, and the heat insulating material25having a height of 130 mm was disposed under the heating element30. Furthermore, on the outside of the heating element30, the heat insulating material25was disposed having a height of 300 mm from the lower end of the heating element30. The core tube22extended downward to a position of 160 mm from the lower end of the heat insulating material25. Furthermore, the cooling member40to which cooling water maintained at 26° C. is supplied to the inside was inserted from the lower end of the core tube22. The cooling member40is inserted into the core tube22so that the upper end is disposed at a position of 175 mm from the lower end of the core tube22, and the distance in the height direction between the upper end of the cooling member40and the center of the heating element30was 155 mm. Moreover, He at 6 L/min was caused to flow from the upper side of the core tube22, He at 5 L/min and Ar at 3 L/min were caused to flow between the inner peripheral surface of the core tube22and the outer peripheral surface of the cooling member40from the lower side of the core tube22, and He at 7 L/min was caused to flow from the lower side to the inner peripheral surface side of the tubular cooling member40.

FIG.25illustrates measurement results of the temperature profile in the heating furnace20ain Example 1. InFIG.25, the thick line indicates the measured temperature in the heating furnace20a, and the thin lines indicate normal distributions that approximate the temperature profiles in the heating furnace20a. Note that, inFIG.25, the temperature profile is approximated to the normal distributions that are different between the upstream side and the downstream side from the changing point, and the lines that interpolate the respective normal distributions are indicated by broken lines.

The temperature profile in the heating furnace20awas measured in the same manner as in Example 1 except that the cooling member40was not inserted into the heating furnace20a, and He at 6 L/min was caused to flow from the upper side of the core tube22, and He at 10 L/min and Ar at 3 L/min were caused to flow from the lower side of the core tube22. The results are illustrated inFIG.26.

The temperature profile in the heating furnace of Comparative Example 1 illustrated inFIG.26is almost symmetrical with respect to the place where the maximum temperature is reached between the upstream side and the downstream side, and is approximated to a normal distribution represented by the following equation (11) with a dispersion σ=140 mm, A=1715 K, and B=300 K.

On the other hand, the temperature profile in the heating furnace20aof Example 1 illustrated inFIG.25is approximated to normal distributions represented by the equations (1) and (2) with the dispersion σt=140 mm, the dispersion σb=35 mm, A=1715 K, and B=300 K. Note that, xc=160 mm and Tc=1204 K. Furthermore, the ratio σt/σb=4.0 and the ratio Tc/Tmax=0.70. The temperature profile of Example 1 was similar to that of Comparative Example 1 on the upstream side from the changing point, but the temperature gradient was steep on the downstream side due to influence of the cooling member40.

Using the same heating furnace20aas in Example 1, the optical fiber1was manufactured in which 2.3 wt % Yb, 3.3 wt % Al, and 3.8 wt % P were added to the core10. At the end of the drawing process P2, the feeding of the optical fiber preform1P and the drawing of the optical fiber1were stopped at the same time, and then the temperature in the heating furnace20awas decreased, and a sample of the neck-down ND was collected in a state close to the actual operation. It was assumed that the outer diameter of the optical fiber preform1P was 30 mm, the outer diameter of the inner clad11was 0.28 mm, the drawing speed of the optical fiber1was 50 m/min, and the drawing tension was 50 gf.FIG.27illustrates results of measuring the outer diameter of the neck-down ND, andFIG.28illustrates temperatures of the glass estimated from the outer diameter of the neck-down ND. The horizontal axis ofFIGS.27and28represents a position in the longitudinal direction of the neck-down ND, a position where the temperature in the heating furnace is maximized is set as the reference point (0), and a downstream direction from the reference point is the positive direction.

A sample of the neck-down ND was collected in the same manner as in Example 2 except that the cooling member40was not inserted into the heating furnace20aand the drawing speed of the optical fiber1was set to 80 m/min.FIG.27illustrates results of measuring the outer diameter of the neck-down ND, andFIG.28illustrates temperatures of the glass estimated from the outer diameter of the neck-down ND.

It can be seen that the glass of Example 2 using the cooling member40is cooled more steeply than the glass of Comparative Example 2 on the downstream side from the position where the temperature in the heating furnace is maximized, as illustrated inFIG.28.

The optical fiber was manufactured in the same manner as in Example 2 except that the 2.3 wt % Yb, 3.1 wt % Al, and 4.0 wt % P were used as dopants added to the core, and the cooling member40was not inserted into the heating furnace20a.

The optical fiber1was manufactured in the same manner as in Example 2 except that the length of the heating element30in the heating furnace20a, the distance in the height direction from the upper end of the cooling member40to the center of the heating element30, the content of the dopant added to the core10, and the drawing speed of the bare optical fiber1E were as illustrated in Table 1 below.

The optical fiber1was manufactured in the same manner as in Example 2 except that the cooling member40was not inserted into the heating furnace20a, and the length of the heating element30in the heating furnace20a, the content of the dopant added to the core10, and the drawing speed of the bare optical fiber1E were as illustrated in Table 2 below.

The loss of the light having a wavelength of 1180 nm was measured for each of the optical fibers of Examples 2 and 3-1 to 3-17, and each of the optical fibers of Comparative Examples 3 and 4-1 to 4-11. The results are indicated in Table 1, Table 2, andFIG.29. Note that,FIG.29illustrates a relationship between the concentration of Yb added to the core in Examples 2, Examples 3-1 to 3-17, Comparative Example 3, and Comparative Examples 4-1 to 4-11, and the loss of the light.

Comparing the optical fiber in Example with the optical fiber in Comparative Example having the same Yb concentration in Tables 1 and 2, it has been found that the loss of the light is suppressed in Example 2 and Examples 3-1 to 3-17 in which the temperature in the heating furnace20ais steeply decreased on the downstream side from the position where the temperature in the heating furnace20ais maximized due to insertion of the cooling member40, as compared with Comparative Example 3 and Comparative Examples 4-1 to 4-11 in which the cooling member40is not inserted and the temperature does not steeply decreased. For example, comparing the optical fiber of Comparative Example 4-4 having a Yb concentration of 2.5% with the optical fiber of Example 3-2 having a Yb concentration of 2.5%, the loss of the light in the former is 50 dB, whereas the loss of the light in the latter is only 9 dB, and it can be seen that the loss of the light in Example is smaller than the loss of the light in Comparative Example. It is conceivable that this is because the silica glass constituting the core is heated to a temperature that is higher than or equal to the glass transition temperature of the silica glass and at which the silica glass is in a single phase, and then rapidly cooled, whereby the crystallization and phase separation of the rare earth element compound in the core are suppressed.

Furthermore, as illustrated inFIG.29, when the concentration of Yb is greater than or equal to 2.0 wt % and less than or equal to 3.1 wt %, it has been found that the loss of the light is reduced in Example 2 and Examples 3-1 to 3-17 in which the temperature in the heating furnace20ais steeply decreased on the downstream side from the position where the temperature in the heating furnace20ais maximized due to insertion of the cooling member40, as compared with Comparative Example 3 and Comparative Examples 4-1 to 4-11 in which the cooling member40is not inserted and the temperature does not steeply decreased.

As described above, according to the present invention, the manufacturing method for an optical fiber and the manufacturing apparatus for an optical fiber are provided capable of suppressing the loss of the light propagating through the core containing the rare earth element compound, and are expected to be used in fields of a processing machine, a medical laser device, and the like.

REFERENCE SIGNS LIST