JP2020090418A - Method for manufacturing optical fiber preform and method for manufacturing optical fiber using the same - Google Patents

Abstract

To provide a method and the like for manufacturing an optical fiber preform capable of improving a yield even when an addition concentration of rare-earth elements becomes high.SOLUTION: A method for manufacturing an optical fiber preform having a core part including a rare-earth element and a clad part surrounding the core part includes a hollow preform formation step for forming a hollow preform by forming a core layer containing a rare-earth element on an inner face of a quartz tube forming the clad part. The hollow preform formation step is a step for forming the core layer on the inner face of the quartz tube by performing at least once a step including a soot formation step for forming soot containing glass particles by supplying rare-earth element containing gas and raw material gas of glass particles in the quartz tube, and an exhaust step for exhausting inside of the quartz tube. In the exhaust step, gas containing inert gas is supplied in the quartz tube.SELECTED DRAWING: None

Description

The present invention relates to an optical fiber preform manufacturing method and an optical fiber manufacturing method using the same, and more particularly to a manufacturing method of an optical fiber preform containing a rare earth element in a core portion and an optical fiber manufacturing method using the same. ..

Optical fibers containing rare earth elements such as ytterbium (Yb) and erbium (Er) are widely used in the field of fiber lasers as amplifiers for optical signals. A vapor phase method is known as a method of manufacturing an optical fiber preform for forming such an optical fiber.

As a method for producing a rare earth element-doped optical fiber preform by a vapor phase method, a production method described in, for example, Patent Document 1 below is conventionally known. In this document, SiCl ₄ and Yb(DPM) ₃ (β-diketone metal complex) are supplied while heating a quartz tube, and a soot composed of SiO ₂ to which Yb is added is deposited on the inner surface of the quartz tube. , A method of producing a hollow optical fiber preform by collapsing the hollow preform after sintering the soot to obtain a hollow preform.

JP, 2014-143287, A

However, in the method for manufacturing an optical fiber preform described in Patent Document 1, if the rare earth element addition concentration in the optical fiber preform is increased, defective members increase, and there is room for improvement in terms of yield improvement. Was.

The present invention has been made in view of the above circumstances, and an optical fiber preform manufacturing method and an optical fiber manufacturing method using the same, which can improve the yield even when the concentration of rare earth elements added is high. The purpose is to provide.

The present inventor has conducted extensive research to find out the cause of the above problems. As a result, in the process of forming soot in the quartz tube, heat may be generated due to a rapid oxidation reaction in the quartz tube, and this heat may be a factor that leaves bubbles in the optical fiber preform. The inventor thought. Therefore, the present inventor examined the correlation between heat generation and bubbles in the optical fiber preform. As a result, when a rare earth element-containing gas containing a rare earth element is supplied at the time of soot formation, decomposition of the rare earth element-containing gas usually starts and progresses to become hydrocarbons, and eventually carbon oxidation occurs. At this time, if carbon is oxidized, heat is not generated. However, when the supply amount of the rare earth element-containing gas is increased to increase the added concentration of the rare earth element, the carbon is not fully oxidized, the hydrocarbon remains, and the present inventor considers that the hydrocarbon may cause heat generation. It was Then, when heat is generated in this way, a sudden pressure change occurs in the quartz tube, causing separation of the glass particles forming the soot and separation between the soot and the inner surface of the quartz tube, and the resulting light is obtained. The present inventor considered that the bubbles might remain in the fiber preform as bubbles. Further, when heat is generated, a local pressure fluctuation in the quartz tube occurs due to the heat generation, a backflow of the glass particles formed in the downstream of the quartz tube occurs, and in the soot, there is a place where the glass particles are not sufficiently adhered to each other. The present inventor has considered that the number of glass particles is likely to increase and peeling easily occurs, and bubbles may remain in the obtained optical fiber preform easily. Then, as a result of further earnest studies, the present inventor has found that the above problems can be solved by the following invention.

That is, the present invention is a method of manufacturing an optical fiber preform for producing an optical fiber preform including a core part containing a rare earth element and a clad part surrounding the core part, and quartz for forming the clad part. On the inner surface of the tube, including a hollow base material forming step of forming a hollow base material by forming a core layer containing the rare earth element, the hollow base material forming step, in the quartz tube, a raw material gas of glass particles, And performing at least once a step including a soot forming step of supplying a rare earth element-containing gas containing the rare earth element to form a soot containing the glass particles and an exhaust step of exhausting the inside of the quartz tube. Is a step of forming the core layer on the inner surface of the quartz tube by the method, and is a method for manufacturing an optical fiber preform in which, in the evacuation step, an exhaust gas containing an inert gas is supplied into the quartz tube.

According to the above manufacturing method, in the hollow base material forming step, the core layer containing a rare earth element is formed on the inner surface of the quartz tube to form the hollow base material, and the core portion containing a rare earth element and the core portion are surrounded. And an optical fiber preform having a clad portion. The hollow base material is formed by performing at least one step including a soot forming step and an exhaust step. At this time, when the rare earth element-containing gas is supplied into the quartz tube in the soot forming step, decomposition of the rare earth element-containing gas usually starts and proceeds to become hydrocarbons, and eventually carbon oxidation occurs. At this time, if carbon is oxidized, heat is not generated. However, if the supply amount of the rare earth element-containing gas is increased to increase the added concentration of the rare earth element, the carbon is not completely oxidized, the hydrocarbon remains, and the hydrocarbon is heated in the soot forming process, so that heat is naturally generated. I have something to do. On the other hand, according to the method for producing an optical fiber preform of the present invention, even if the supply amount of the rare earth element-containing gas is large in the soot forming step, the inside of the quartz tube is exhausted containing an inert gas in the exhausting step. Since it is exhausted by the working gas, the hydrocarbons remaining in the quartz tube are exhausted, and the amount of hydrocarbons that can cause heat generation decreases. Therefore, heat generation is suppressed in the quartz tube. When heat generation is suppressed in the quartz tube in this way, separation of glass particles from each other and separation between the quartz tube and soot are less likely to occur, and bubbles are less likely to remain in the obtained optical fiber preform. Further, when the heat generation is suppressed, the local pressure fluctuation in the quartz tube that may occur due to the heat generation is suppressed, and the phenomenon that the glass particles formed downstream of the quartz tube flow backward due to the pressure fluctuation is less likely to occur. Therefore, in the soot, the places where the glass particles are not sufficiently adhered to each other are reduced, the glass particles are less likely to be separated from each other, and bubbles are less likely to remain in the obtained optical fiber preform. As described above, the method for producing an optical fiber preform of the present invention can reduce the defective preform and improve the yield even if the rare earth element addition concentration is high.

The method for manufacturing the optical fiber preform may further include a collapsing step of collapsing the hollow preform.

In the method for manufacturing an optical fiber preform, it is preferable that the temperature inside the quartz tube in the exhaust step is lower than the temperature inside the quartz tube in the soot forming step.

In this case, since the temperature in the quartz tube in the exhaust step is lower than the temperature in the quartz tube in the soot forming step, even if hydrocarbon remains in the quartz tube in the exhaust step, it is caused by the hydrocarbon. Fever is less likely to occur.

In the method for manufacturing an optical fiber preform, in the soot forming step, the quartz tube is supplied with the rare earth element-containing gas by using a gas supply tube, and in the exhaust step, the inside of the gas supply tube is also described. It is preferable to exhaust with an exhaust gas.

In this case, in the exhaust step, not only the inside of the quartz tube but also the inside of the gas supply tube is exhausted by the exhaust gas, so that hydrocarbons due to the rare earth element-containing gas in the gas supply tube are also exhausted sufficiently. Therefore, heat generation due to hydrocarbons is less likely to occur in the quartz tube.

In the above-described method for manufacturing an optical fiber preform, it is preferable that in the exhausting step, the volume content of the inert gas in the exhaust gas is reduced stepwise as the exhausting step progresses.

In this case, when an inert gas, such as helium or argon, which is more expensive than oxygen is used, the manufacturing cost of the optical fiber preform can be reduced.

In the method for manufacturing an optical fiber preform, the step in the hollow preform forming step further includes a sintering step of sintering the glass particles of the soot to make the soot transparent glass after the exhaust step. May be included.

In the method for producing an optical fiber preform, the core portion further contains a dopant different from the rare earth element, and a dopant-containing gas containing the dopant is introduced into the quartz tube between the exhaust step and the sintering step. It is preferable to further include a diffusion step of supplying and diffusing into the soot.

In this case, unlike the case where the diffusion step is performed after the sintering step, the addition efficiency of the dopant in the diffusion step decreases due to the progress of the sintering of the soot, and the concentration of the dopant in the core portion of the obtained optical fiber preform is reduced. It can be prevented from becoming insufficient.

It is preferable that the method for producing an optical fiber preform further includes a treatment step of supplying at least one of oxygen gas and chlorine atom-containing gas into the quartz tube between the exhaust step and the sintering step.

In this case, by supplying at least one of the oxygen gas and the chlorine atom-containing gas into the quartz tube, the decomposition of hydrocarbons caused by the rare earth element-containing gas is promoted and the hydrocarbons become carbon, so that heat generation is further suppressed. To be done. Further, by supplying at least one of the oxygen gas and the chlorine atom-containing gas into the quartz tube, even if a transition metal impurity such as Fe ²⁺ is mixed when the rare earth element-containing gas is fed into the quartz tube, the transition It is possible to shift the absorption wavelength band of transition metal impurities from the operating wavelength band of the optical fiber by changing the valence of the metal impurity by an oxidation reaction, and to contribute to the reduction of loss of the optical fiber. The material can be obtained.

Further, the present invention is a base material preparing step of preparing an optical fiber base material manufactured by the above-described optical fiber base material manufacturing method, and drawing the optical fiber base material prepared in the base material preparing step. And a drawing step for obtaining an optical fiber.

According to this optical fiber manufacturing method, the optical fiber base material yield can be improved by the above-described optical fiber base material manufacturing method even when the rare earth element addition concentration is high. Therefore, the optical fiber obtained by drawing the optical fiber preform can be efficiently manufactured at low cost.

The present invention provides a method for manufacturing an optical fiber preform and a method for manufacturing an optical fiber using the same, which can improve the yield even when the rare earth element addition concentration is high.

It is an end view which shows an example of the optical fiber preform manufactured by the manufacturing method of the optical fiber preform of this invention. It is an end view which shows an example of the hollow preform manufactured by the manufacturing method of the optical fiber preform of the present invention. It is a figure which shows an example of the manufacturing apparatus which enforces the manufacturing method of the optical fiber preform of this invention. It is a figure which shows the soot formation process of the manufacturing method of the optical fiber preform of this invention. It is a figure which shows the exhaust process of the manufacturing method of the optical fiber preform of this invention. It is a figure which shows the sintering process of the manufacturing method of the optical fiber preform of this invention.

<Method of manufacturing optical fiber preform>
Hereinafter, embodiments of the method for producing an optical fiber preform of the present invention will be described in detail.

First, an embodiment of the optical fiber preform of the present invention will be described with reference to FIG. FIG. 1 is an end view showing an example of an optical fiber preform manufactured by the method for manufacturing an optical fiber preform of the present invention.

As shown in FIG. 1, the optical fiber preform 10 is a solid preform and includes a core part 1 containing a rare earth element and a clad part 2 surrounding the core part 1.

Next, a method for manufacturing the optical fiber preform 10 will be described with reference to FIGS. FIG. 2 is an end view showing an example of a hollow preform manufactured by the method for manufacturing an optical fiber preform of the present invention. 3 is a diagram showing an example of a manufacturing apparatus for carrying out the method for manufacturing an optical fiber preform of the present invention, FIG. 4 is a view showing a soot forming step of the method for manufacturing an optical fiber preform of the present invention, and FIG. FIG. 6 is a diagram showing an exhaust step of the method for producing an optical fiber preform of the present invention, and FIG. 6 is a diagram showing a sintering step of the method for producing an optical fiber preform of the present invention.

First, in order to manufacture the optical fiber preform 10, as shown in FIG. 2, the hollow preform 20 is formed by forming the core layer 21 containing a rare earth element on the inner surface of the quartz tube 22 forming the cladding part 2. Is formed (hollow base material forming step). At this time, the hollow base material 20 is formed so that the hollow portion 23 is formed inside the core layer 21.

At this time, the hollow base material 20 is specifically formed as follows.

First, as shown in FIG. 3, a quartz tube 22 is prepared. The quartz tube 22 is formed by welding dummy glass tubes 24A and 25A made of quartz to both ends of a main body 22A made of quartz.

The quartz tube 22 prepared as described above is installed in the manufacturing apparatus 100. Specifically, the dummy glass tube 25A of the quartz tube 22 is held by a bearing 34, a seal box 27 is hermetically connected to the dummy glass tube 25A, and a gas exhaust pipe 29 is inserted and fixed in the seal box 27. It On the other hand, the dummy glass tube 24A is connected to the grip (chuck) 26 of the lathe, and the gas supply pipe 28 penetrates the grip 26 of the lathe and is inserted into the quartz tube 22. In this way, the quartz tube 22 is rotatably installed around the central axis along the longitudinal direction thereof by the bearing 34 and the grip portion 26 of the lathe. The gas supply pipe 28 is connected to a raw material gas source 30 for glass particles, a rare earth element-containing gas source 31, and an inert gas source 32, respectively. The inert gas source 32 is connected to the rare earth element addition gas source 32 via a pipe. A heating source 33 is arranged so as to face the quartz tube 22, and the heating source 33 is movable along the longitudinal direction of the quartz tube 22. As the heating source 33, for example, an oxyhydrogen burner is used.

Next, as shown in FIG. 4, the raw material gas for the glass particles is supplied into the quartz tube 22 from the raw material gas source 30 for the glass particles through the gas supply pipe 28, and the rare earth element-containing gas source is supplied. A rare earth element-containing gas is supplied from 31 through the gas supply pipe 28 to form a porous soot 21A containing glass particles in the quartz pipe 22 (soot forming step).

Next, as shown in FIG. 5, an exhaust gas containing an inert gas is supplied from the inert gas source 32 into the quartz pipe 22 through the gas supply pipe 28. In this way, the inside of the quartz pipe 22 and the inside of the gas supply pipe 28 are exhausted (exhaust step).

Next, as shown in FIG. 6, the glass particles of the soot 21A are sintered to make the soot 21A transparent glass (sintering step).

Then, the hollow matrix 20 is obtained by forming the core layer 21 on the inner surface of the quartz tube 22 by performing the step including the soot forming step, the exhaust step, and the sintering step at least once.

Finally, the hollow base material 20 is collapsed, and the portions of the quartz tube 22 corresponding to the dummy glass tubes 24A and 25A are melt-cut to obtain the optical fiber preform 10 (collapse step).

According to the above manufacturing method, in the hollow base material forming step, the core layer 21 containing a rare earth element is formed on the inner surface of the quartz tube 22 to form the hollow base material 20, and the hollow base material 20 is collapsed in the collapse step. Thus, a solid optical fiber preform 10 including the core portion 1 containing the rare earth element and the clad portion 2 surrounding the core portion 1 is obtained. The hollow base material 20 is formed by performing at least one step including a soot forming step, an exhaust step, and a sintering step. At this time, when the rare earth element-containing gas is supplied into the quartz tube 22 in the soot forming step, decomposition of the rare earth element-containing gas usually starts and progresses to become hydrocarbons, and eventually carbon oxidation occurs. At this time, if carbon is oxidized, heat is not generated. However, if the supply amount of the rare earth element-containing gas is increased in order to increase the added concentration of the rare earth element, the carbon may not be completely oxidized, the hydrocarbon remains, and the hydrocarbon may spontaneously generate heat in the soot forming step. On the other hand, according to the method for manufacturing the optical fiber preform 10, even if the supply amount of the rare earth element-containing gas is large in the soot forming step, the inside of the quartz tube 22 for exhaust gas containing an inert gas is exhausted in the exhaust step. Since the gas is exhausted, the hydrocarbons remaining in the quartz tube 22 are exhausted, and the amount of hydrocarbons that can cause heat generation is reduced. Therefore, heat generation is suppressed in the quartz tube 22. When heat generation is suppressed in the quartz tube 22 as described above, peeling between glass particles and peeling between the quartz tube 22 and the soot 21A are less likely to occur, and voids due to the above peeling are less likely to be formed in the sintering step. It becomes difficult for the voids to remain as bubbles in the obtained optical fiber preform 10. Further, when the heat generation is suppressed, the local pressure fluctuation in the quartz tube that may occur due to the heat generation is suppressed, and the backflow due to the pressure fluctuation of the glass particles formed downstream of the quartz tube 22 is less likely to occur. Therefore, in the soot, the number of places where the glass particles are not sufficiently adhered to each other is reduced, and the glass particles are less likely to be peeled from each other, and in the sintering step, voids due to the peeling are less likely to be formed, and the voids are It becomes difficult for bubbles to remain in the obtained optical fiber preform 10. From the above, the manufacturing method of the optical fiber preform 10 can reduce the defective preform and improve the yield even if the rare earth element addition concentration is high.

Further, in the above-mentioned manufacturing method, the rare earth element-containing gas is supplied into the quartz tube 22 by using the gas supply pipe 28 in the soot forming step, and the inside of the gas supply tube 28 is also exhausted by the exhaust gas in the exhaust step. It Therefore, in the exhaust step, not only the interior of the quartz tube 22 but also the interior of the gas supply pipe 28 is exhausted by the exhaust gas, so that the hydrocarbons due to the rare earth element-containing gas in the gas supply pipe 28 are also exhausted sufficiently. To be done. Therefore, heat generation due to hydrocarbons is less likely to occur in the quartz tube 22.

Next, the hollow base material forming step and the collapse step will be described in detail.

<Hollow base material forming step>
The hollow base material forming step is a step of obtaining the hollow base material 20 by forming the core layer 21 inside the quartz tube 22 by performing at least one step including the soot forming step, the exhausting step and the sintering step. is there. The number of the above steps may be once or plural times, and may be appropriately determined according to the desired thickness of the core layer 21. Hereinafter, the soot forming step, the exhaust step, and the sintering step will be described in detail.

(Soot formation process)
The soot forming step is a step of supplying the raw material gas of the glass particles and the rare earth element-containing gas containing the rare earth element into the quartz tube 22 to form the porous soot 21A containing the glass particles. At this time, the raw material gas for the glass particles and the rare earth element-containing gas are each supplied by a carrier gas. Further, at this time, in order to form the soot 21A, the heating source 33 is moved in the longitudinal direction of the quartz tube 22 while rotating the quartz tube 22 around the central axis line along the longitudinal direction thereof. To heat. At this time, the heating source 33 may be moved once or plural times.

For example, SiCl ₄ is used as the raw material gas for the glass particles.

As the rare earth element-containing gas, for example, a gas obtained by vaporizing a β-diketone metal complex such as Re(DPM) ₃ is used. Here, examples of Re include Yb, Nd, and Er. DPM is a _C 11 _H 19 _{O 3.}

The supply amount of the raw material gas of the glass particles and the rare earth element-containing gas can be adjusted by the temperature of the raw material gas source 30 of the glass particles or the rare earth element-containing gas source 31 and the flow rate of the carrier gas. The supply amount increases as the temperature of the raw material gas source 30 of the glass particles or the rare earth element-containing gas source 31 increases, and the supply amount increases as the flow rate of the carrier gas increases. The temperature of the source gas source 30 for the glass particles may be 20° C. to 40° C. when the source gas for the glass particles is SiCl ₄ , for example. Further, the temperature of the rare earth element-containing gas source 31 may be about 200° C. to 260° C. when Re(DPM) ₃ is Yb(DPM) ₃ , for example.

The temperature of the quartz tube 22 may be lower than the sintering temperature of the glass particles, for example, 1450 to 1650°C. In this case, as compared with the case where the temperature of the quartz tube 22 exceeds 1650° C., the soot 21A is less likely to be sintered, and the soot 21A and the quartz tube 22 are less likely to be sintered than when the temperature of the quartz tube 22 is less than 1450° C. Of the soot 21A and the quartz tube 22 are less likely to be peeled off, voids are less likely to be formed in the sintering process, and voids are less likely to remain as bubbles in the obtained optical fiber preform 10. ..

Examples of the carrier gas include oxygen (O ₂ ), helium (He) and argon (Ar). Further, the carrier gas such as Re(DPM) ₃ which is used together with the rare earth element-containing gas having a temperature higher than room temperature is preferably preheated upstream of the rare earth element-containing gas source 31.

(Exhaust process)
The exhaust process is a process of exhausting the inside of the quartz pipe 22 and the gas supply pipe 28, and is performed after the soot forming process. In the exhaust step, instead of supplying the raw material gas for the glass particles and the rare earth element-containing gas into the quartz tube 22, an exhaust gas containing an inert gas is supplied. Here, examples of the inert gas include He, Ar, and nitrogen (N ₂ ).

The volume content (inert gas concentration) of the inert gas in the exhaust gas is not particularly limited, but is preferably 50 vol% or more, more preferably 70 vol% or more, and 90 It is particularly preferable that the content is at least volume %. Further, the exhaust process may be divided into a plurality of times. In this case, it is preferable that the concentration of the inert gas be gradually reduced as the exhaust process proceeds. For example, the inert gas concentration is preferably reduced stepwise such as 90% by volume in the first exhaust step, 70% by volume in the second exhaust step, and 50% by volume in the third exhaust step. In this case, the manufacturing cost of the optical fiber preform 10 can be reduced when an inert gas that is more expensive than oxygen, such as helium or argon, is used.

The temperature in the quartz tube 22 in the exhaust step may be higher or lower than the temperature in the quartz tube 22 in the soot forming step, but is preferably low.

In this case, since the temperature in the quartz tube 22 in the exhaust step is lower than the temperature in the quartz tube 22 in the soot forming step, even if hydrocarbons remain in the quartz tube 22 in the exhaust step, Heat generation due to the hydrocarbon is less likely to occur.

At this time, the temperature (T2) of the quartz tube 22 in the exhaust step is not particularly limited, but it is preferably 700° C. or higher. However, T2 is preferably 1300° C. or lower.

At this time, the heating of the quartz tube 22 can be performed by moving the heating source 33 in the longitudinal direction of the quartz tube 22 while rotating the quartz tube 22 around the central axis line along the longitudinal direction thereof. At this time, the heating source 33 may be moved once or plural times.

The temperature of the quartz tube 22 is set to a temperature at which the temperature of the quartz tube 22 becomes 300 to 800° C. when the quartz tube 22 has an outer diameter of 32 mm and a thickness of 2.5 mm, for example. In this case, as compared with the case where the temperature of the quartz tube 22 is lower than 300° C., the rare earth element existing gas is less likely to be solidified in the evacuation process and is less likely to be attached to the inner surface of the quartz tube 22, and more sufficient evacuation is performed. Further, the quartz tube 22 is prevented from being rapidly cooled, and cracks due to strain of the quartz tube 22 are suppressed. On the other hand, as compared with the case where the temperature of the quartz tube 22 exceeds 800° C., the temperature inside the quartz tube 22 does not rise and heat generation is further suppressed.

In the exhaust step, the amount of gas exhausted is the total volume of the pipe connecting the raw material gas source 30 of glass particles and the gas supply pipe 28, the gas supply pipe 28 and the quartz pipe 22. It is preferable to adjust the moving speed and the number of times of movement of the heating source 33 so that the volume becomes twice or more. In this case, the amount of gas exhausted is less than twice the total volume of the pipe connecting the glass particle source gas source 30 and the gas supply pipe 28, the gas supply pipe 28, and the quartz pipe 22. As compared with the case of adjusting to 1, the concentration of the raw material gas remaining in the pipe can be diluted to a lower concentration, and the exothermic point can be lowered.

(Sintering process)
The sintering step is a step of sintering the glass particles of the soot 21A to make the soot 21A transparent glass. The sintering process is performed while heating the quartz tube 22. At this time, the temperature inside the quartz tube 22 may be changed depending on the thickness of the soot 21A as long as it is a temperature at which the soot 21A becomes vitrified (for example, 2000° C.). Further, it is preferable that the sintering process is performed while flowing O ₂ and He in the quartz tube 22.

(Treatment process)
The step including the soot forming step, the exhaust step, and the sintering step further includes a processing step of supplying at least one of oxygen gas and chlorine atom-containing gas into the quartz tube 22 between the exhaust step and the sintering step. But it's okay.

In this case, by supplying at least one of the oxygen gas and the chlorine atom-containing gas into the quartz tube 22, the decomposition of hydrocarbons due to the rare earth element-containing gas is promoted and heat generation is further suppressed. Further, by supplying at least one of the oxygen gas and the chlorine atom-containing gas into the quartz tube 22, a transition metal such as Fe ²⁺ mixed when the rare earth element-containing gas or the AlCl ₃ -containing gas is fed into the quartz tube 22. It becomes possible to shift the absorption wavelength band of transition metal impurities from the operating wavelength band of the optical fiber by changing the valence of impurities from Fe ²⁺ to Fe ³⁺ by an oxidation reaction, and to reduce the loss of the optical fiber. The optical fiber preform 10 that can contribute can be obtained.

Examples of the chlorine atom-containing gas include Cl ₂ , SOCl ₂ , BCl _{3 and the} like.

An inert gas such as He may be supplied in the processing step.

In the processing step, it is preferable to heat the quartz tube 22. In this case, as compared with the case where the quartz tube 22 is not heated, the decomposition of hydrocarbons due to the rare earth element-containing gas is more sufficiently promoted, and heat generation is further suppressed.

The temperature of the quartz tube 22 in the treatment step is not particularly limited, but is preferably 1100 to 1700°C, more preferably 1200 to 1600°C.

At this time, the heating of the quartz tube 22 can be performed by moving the heating source 33 in the longitudinal direction of the quartz tube 22 while rotating the quartz tube 22 around the central axis line along the longitudinal direction thereof. At this time, the heating source 33 may be moved once or plural times.

The flow rate of oxygen gas is not particularly limited, but is preferably 10 to 5000 sccm. The flow rate of the chlorine atom-containing gas is not particularly limited, but is preferably more than 0 sccm and 1000 sccm or less.

(Diffusion process)
In the optical fiber preform 10, when the core portion 1 further contains a dopant different from the rare earth element, the steps including the soot forming step, the exhaust step, and the sintering step are performed by using the quartz tube 22 containing the dopant-containing gas containing the dopant. A diffusing step of diffusing the soot into the soot 21A may be further included.

At this time, the diffusion step may be performed in the soot forming step, after the exhaust step and before the sintering step, or in the sintering step, but after the exhaust step and the baking step. It is preferably performed before the binding step. In this case, unlike the case where the diffusion step is performed after the sintering step, the sintering efficiency of the soot 21A progresses, so that the doping efficiency of the dopant in the diffusion step decreases and the dopant in the core portion 1 of the obtained optical fiber preform 10 is reduced. Can be prevented from becoming insufficient.

Note that a part of the dopant diffusion step may be performed after the exhaust step and before the sintering step, and the remaining dopant diffusion step may be performed in the soot forming step or the sintering step. Examples of the dopant include Al, P, B, Yb and F. These may be used alone or in combination of two or more. When Al is used as the dopant, AlCl ₃ is used as the dopant-containing gas, and when P is used as the dopant, POCl ₃ is used as the dopant-containing gas and B is used as the dopant. When used, BBr ₃ or BCl ₃ is used as the dopant-containing gas, and when Yb is used as the dopant, Yb(DPM) ₃ is used as the dopant-containing gas. When F is used as the dopant, SiF ₄ , C ₂ F ₆ , CF ₄ or the like is used as the dopant-containing gas.

In the diffusion step, it is preferable to heat the quartz tube 22. In this case, the reaction between the dopant-containing gas and oxygen sufficiently proceeds as compared with the case where the quartz tube 22 is not heated.

The temperature of the quartz tube 22 in the diffusion step depends on the type and amount of the dopant to be added, but may be lower than the sintering temperature of the soot 21A. The temperature of the quartz tube 22 in the diffusion step is preferably lower than the temperature of the quartz tube 22 in the soot forming step. When the dopant is added to the soot 21A, the sintering temperature of the soot 21A decreases. Therefore, as compared with the case where the temperature of the quartz tube 22 in the diffusion step is set to be equal to or higher than the temperature of the quartz tube 22 in the soot forming step, the soot 21A is less likely to shrink and vitrify, and the dopant-containing gas diffuses. It is possible to suppress the difficulty of permeating into the soot 21A in the latter half of the process.

The temperature of the quartz tube 22 in the diffusion step is preferably 1100 to 1600°C, and more preferably 1200 to 1500°C. In this case, the reaction between the dopant-containing gas and oxygen proceeds more sufficiently than when the temperature of the quartz tube 22 is lower than 1100°C. On the other hand, as compared with the case where the temperature of the quartz tube 22 exceeds 1600° C., the soot 21A is less likely to be sintered during the diffusion process, and is less likely to be a transparent glass. Therefore, when the diffusion process is separately performed for each of a plurality of types of dopants, a decrease in the diffusion efficiency of the dopant is suppressed in the diffusion process performed for each type of dopant, and a desired dopant concentration is easily obtained.

At this time, the heating of the quartz tube 22 can be performed by moving the heating source 33 in the longitudinal direction of the quartz tube 22 while rotating the quartz tube 22 around the central axis line along the longitudinal direction thereof. At this time, the heating source 33 may be moved once or plural times.

Note that the diffusion step may be performed before the treatment step or after the treatment step, but when the dopant to be added is easily volatilized in the treatment step, the diffusion step is performed after the treatment step. Preferably. In this case, since the treatment step is not performed after the diffusion step, the element added in the diffusion step is not desorbed during the treatment step. Examples of such a dopant include P and B.

Moreover, when Yb(DPM) ₃ is used as the dopant-containing gas, the diffusion step is preferably performed before the treatment step. In this case, a small amount of residual organic matter derived from the Yb raw material, which remains after the diffusion step and the exhaust step, is processed by the processing step of supplying oxygen gas, so that the core layer 21 that is a glass deposition layer with few impurities is obtained.

In addition, the diffusion step is preferably performed before the treatment step even when AlCl ₃ is used as the dopant-containing gas. In this case, if Fe is released from a pipe or the like when AlCl ₃ is supplied in the diffusion step, the valence of Fe is changed to make it harmless by a treatment step of supplying oxygen gas, or a treatment step using a chlorine-containing gas is performed. By using FeCl ₃ as FeCl ₃ and volatilizing and removing it, the core layer 21 which is a glass with a small absorption loss derived from the transition metal can be obtained.

(Quartz tube inner surface cleaning process)
The hollow base material forming step may or may not include the quartz tube inner surface cleaning treatment step of cleaning the inner surface of the quartz tube 22 before the above step, but it is preferable to further include the quartz tube inner surface cleaning treatment step. In this case, the inner surface of the quartz tube 22 is cleaned to remove impurities on the inner surface of the quartz tube 22. Therefore, in the optical fiber obtained from the optical fiber preform 10, the loss caused by the impurities can be reduced.

The cleaning process can be performed by etching, for example. For the etching, specifically, while heating the quartz tube 22, a mixed gas containing a fluorine-based gas as an etching gas and oxygen may be supplied into the quartz tube 22. At this time, as the fluorine-based gas, for example, SF ₆ , CF ₄ , C ₂ F ₆ can be used. The temperature of the quartz tube 22 may be set to about 2000° C., for example. When SF ₆ is used as the fluorine-based gas, the flow rate of SF ₆ may be 10 to 1000 cc/min, and the flow rate of O ₂ may be 1000 to 3000 cc/min. The mixed gas may further contain He.

<Collaps process>
The collapsing step is a step of collapsing the hollow base material 20 to obtain a solid base material. In the collapsing step, the quartz tube 22 is heated by the heating source 33 to reduce its diameter to crush the hollow portion 23 of the hollow base material 20.

At this time, the heating of the quartz tube 22 can be performed by moving the heating source 33 in the longitudinal direction of the quartz tube 22 while rotating the quartz tube 22 around the central axis line along the longitudinal direction thereof. At this time, the heating source 33 may be moved once or plural times.

The pressure of the hollow portion 23 of the hollow base material 20 may be higher or lower than the atmospheric pressure outside the hollow base material 20, but the pressure of the hollow portion 23 is increased to a pressure higher than atmospheric pressure (for example, about 40 Pa). From this state, the pressure of the hollow portion 23 may be adjusted to be lowered according to the degree of collapse of the hollow portion 23. Moreover, the pressure of the hollow portion 23 may be equal to or higher than the atmospheric pressure, or may be lower than the atmospheric pressure.

<Jacket process>
The solid base material obtained in the collapsing step is made into a large-diameter base material by covering it with a quartz tube and collapsing it into a large diameter by a jacket method. In this case, the solid base material may be jacketed and integrated with a quartz tube on a lathe. If necessary, a stretching step of stretching the solid base material may be performed before the jacket step. As a result, it is possible to obtain a cylindrical large-diameter base material in which the ratio of the core clad portion, which is the ratio (D2/D1) of the diameter (D1) of the core portion and the diameter (D2) of the clad portion, becomes a desired value. . The ratio of the core portion to the clad portion may be a value different from the ratio (d1/d2) of the diameter (d1) of the core and the diameter (d2) of the clad in the optical fiber after drawing in consideration of the grinding and polishing steps described below. I do not care.

Moreover, you may perform a grinding process and a polishing process with respect to the large diameter base material after a jacket process.
The cross-sectional shape of the grinding base material obtained by grinding is different depending on the coupling method of the excitation light and the core in the optical fiber, but an asymmetrical shape such as D-shape and a polygon such as hexagon, heptagon, octagon, etc. Be done.

When the grinding process and the polishing process are performed, the preform finally obtained by the grinding process and the polishing process becomes the optical fiber preform 10. When the grinding process and the polishing process are not performed, the large-diameter base material becomes the optical fiber base material 10.

<Optical fiber manufacturing method>
Next, a method for manufacturing the optical fiber of the present invention will be described. The optical fiber manufacturing method of the present invention includes a base material preparing step for preparing the optical fiber base material 10 manufactured by the above-described optical fiber base material manufacturing method, and the optical fiber base material 10 prepared in the base material preparing step. And a drawing step for obtaining an optical fiber.

According to this method for producing an optical fiber, the yield of the optical fiber preform 10 can be improved by the above-described method for producing an optical fiber preform even if the rare earth element addition concentration is high. Therefore, an optical fiber obtained by drawing the optical fiber preform 10 can be manufactured efficiently at low cost.

<Drawing process>
In the drawing step, the optical fiber preform 10 is set in a drawing device, heated in a heating furnace and drawn. This produces an optical fiber having a core doped with a rare earth element and a clad not doped with a dopant.

Here, the drawing temperature (furnace temperature) is, for example, 2000 to 2200°C. The drawing speed is, for example, 5 to 300 m/s. In the manufactured optical fiber, for example, the core diameter is 3 to 100 μm, and the clad diameter (fiber diameter) is 80 to 1000 μm.

A coating layer made of a UV curable resin or the like may be provided on the clad simultaneously with the drawing of the optical fiber preform 10. The type of UV curable resin coated on the clad depends on the application of the optical fiber, but for example, when the optical fiber is a single clad fiber, the UV curable resin having a higher refractive index than glass is coated, When the optical fiber is a double clad fiber, a UV curable resin having a lower refractive index than glass is coated as the low refractive index resin layer. When the optical fiber is a double clad fiber, the low refractive index resin layer functions as a second clad for confining the excitation light.

The present invention is not limited to the above embodiment. For example, in the above-described embodiment of the method for manufacturing an optical fiber preform, the collapse process is performed after the hollow base material forming process, but the collapse process may not be necessarily performed. In this case, the hollow preform 20 becomes the optical fiber preform 10.

In the above embodiment, the jacket process may be omitted if necessary. When the jacket process is omitted, the solid preform becomes the optical fiber preform 10 as it is.

Hereinafter, the content of the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples.

(Example 1)
An optical fiber preform was manufactured as follows using the manufacturing apparatus 100 shown in FIG.

First, as the quartz tube 22, by welding two dummy glass tubes 24A, 25A having an outer diameter of 32 mm, an inner diameter of 27 mm, and a length of 300 mm to both ends of a main body 22A having an outer diameter of 32 mm, an inner diameter of 27 mm, and a length of 600 mm, respectively. I prepared. Then, the dummy glass tube 24A of the quartz tube 22 was held by the holding portion 26 of the lathe, and the dummy glass tube 25A was held by the bearing 34.

Next, while moving the oxyhydrogen burner as the heating source 33 along the length direction of the quartz tube 22 to heat the entire length of the quartz tube 22, SF ₆ as an etching gas and O ₂ as a carrier gas are heated. At the same time, the gas was supplied from the gas supply pipe 28 to clean the inner surface of the quartz pipe 22.

Next, while heating the quartz tube 22 with an oxyhydrogen burner as a heating source 33, SiCl ₄ as a source gas is supplied into the quartz tube 22 through a gas supply tube 28 together with O ₂ as a carrier gas, and Yb( DPM) ₃ is supplied together with He as a carrier gas into the quartz pipe 22 through the gas supply pipe 28, and AlCl ₃ is supplied together with He as a carrier gas into the quartz pipe 22 through the gas supply pipe 28, so that the quartz pipe 22 is Heated while rotating. Then, a soot 21A made of SiO ₂ glass to which Yb ₂ O ₃ and Al ₂ O ₃ were added was formed on the inner surface of the quartz tube 22. At this time, the temperature of the quartz tube 22 by the burner was set to 1550°C. Thus, the soot forming step was performed.

Next, He was supplied from the gas supply pipe 28 into the quartz tube 22 as an exhaust gas, and the quartz tube 22 was heated by the oxyhydrogen burner as the heating source 33 while rotating. At this time, the temperature of the quartz tube 22 by the burner was set to 600 to 700°C. At this time, the exhaust time was 20 minutes. Thus, the exhaust process was performed.

Next, while supplying POCl ₃ for adding O ₂ and P and BBr ₃ for adding B into the quartz tube 22 through the gas supply tube 28, the quartz tube 22 is heated by the oxyhydrogen burner as the heating source 33. Was heated while rotating. At this time, the reciprocating movement of the burner was repeated a plurality of times. The temperature of the quartz tube 22 was 1200° C., which is lower than that in the soot forming step. Thus, the diffusion process was performed.

Next, while supplying O ₂ and He from the gas supply pipe 28, the quartz pipe 22 was heated while being rotated by the oxyhydrogen burner as the heating source 33. At this time, the temperature of the quartz tube 22 by the burner was adjusted to 2000°C. As a result, the soot 21A was integrated with the quartz tube 22 and was sintered into a transparent glass. Thus, the core layer 21 was formed on the inner surface of the quartz tube 22. Thus, the sintering process was performed.

The formation of the core layer 21 was repeated as described above until the desired thickness was obtained. Thus, the hollow base material 20 was obtained.

Then, the hollow base material 20 was heated while rotating from the lower end to the upper end. At this time, the pressure of the hollow portion 23 of the hollow base material 20 is set to a pressure higher than the pressure of the outside of the hollow base material 20 by 40 Pa, and the pressure of the hollow portion 23 is adjusted according to the degree of collapse of the hollow portion 23. The hollow portion 23 was extinguished. Then, the portions corresponding to the dummy glass tubes 24A and 25A at both ends were blown. Thus, a solid optical fiber preform having a core diameter of 4 mm was obtained.

(Comparative Example 1)
An optical fiber preform was produced in the same manner as in Example 1 except that the exhaust step was not performed.

With respect to the optical fiber preforms of Example 1 and Comparative Example 1 obtained as described above, the concentrations of Yb, Al and P were measured by elemental analysis. The P concentration was measured while P was added in the diffusion step. Further, an appearance inspection was performed on the obtained optical fiber preform to confirm whether bubbles were mixed in the optical fiber preform. Then, for each concentration of Yb added to the core part, the ratio of the number of defective preforms (numerator) due to bubbles to the number of produced optical fiber preforms (denominator) was calculated in percentage. The results are shown in Table 1.

From the results shown in Table 1, in Example 1, the ratio of the number of defective base materials due to bubbles was 0% at any Yb concentration. On the other hand, in Comparative Example 1, the ratio of the number of defective base materials due to bubbles increased as the Yb concentration increased, and especially when the Yb concentration became 2.2% by mass or more, the ratio of defective base materials was considerably high. When the Yb concentration was increased to 2.4% by mass or more, the ratio of the defective base material was increased to 50% or more, and half or more of the defective base materials were generated. Carefully observing the process of producing the base material in which the defect due to the bubbles occurred, in the base material having the Yb concentration of 2.5 mass% in Comparative Example 1, the soot 21A was unevenly vitrified or partially formed. There was a behavior such that the soot 21A peeled off was reattached, and it was observed that the reattached portion became a bubble generation point when the glass vitrified in the sintering step.

(Example 2)
An optical fiber preform was obtained in the same manner as in Example 1 except that the diffusion step was not performed.

(Example 3)
An optical fiber preform was obtained in the same manner as in Example 1 except that the treatment step was carried out as follows after the exhaust step and before the diffusion step.

That is, first, while supplying oxygen into the quartz tube through the gas supply tube, the quartz tube was heated by the oxyhydrogen burner as a heating source. The temperature of the quartz tube was 1500° C., which was slightly lower than the temperature of the quartz tube in the soot forming step. The heating of the quartz tube was performed by repeating the reciprocating movement of the burner a plurality of times. Thus, the processing steps were performed.

(Example 4)
An optical fiber preform was produced in the same manner as in Example 1 except that the diffusion process was not performed and the treatment process was performed after the exhaust process and before the sintering process. At this time, the treatment process was performed in the same manner as the treatment process of Example 3.

(Comparative example 2)
An optical fiber preform was produced in the same manner as in Example 3 except that the exhaust step was not performed.

Regarding the optical fiber preforms of Examples 2 to 4 and Comparative Example 2 obtained as described above, the concentrations of Yb, Al and P were measured in the same manner as in Example 1. Further, an appearance inspection was performed on the obtained optical fiber preform to confirm whether bubbles were mixed in the optical fiber preform. Then, for each concentration of Yb added to the core part, the ratio of the number of defective preforms (numerator) due to bubbles to the number of produced optical fiber preforms (denominator) was calculated in percentage. The results are shown in Table 2. In Table 2, for the relatively high Yb concentration in Table 1, the ratio of the number of defective base materials (molecules) due to bubbles is shown as a percentage for each Yb concentration.

From the results shown in Table 2, in Examples 2 to 4, the ratio of the number of defective base materials due to bubbles was 0% at any Yb concentration. On the other hand, in Comparative Example 2, the ratio of the number of defective base materials due to bubbles increased as the Yb concentration increased. Especially, when the Yb concentration became 2.2% by mass or more, the ratio of defective base materials was 50%. It was considerably higher than the above, and more than half of the defective base metal was generated.

From the above, it was confirmed that according to the method for producing an optical fiber preform of the present invention, the yield can be improved even if the concentration of rare earth element added is high.

DESCRIPTION OF SYMBOLS 1... Core part 2... Clad part 10... Optical fiber preform 20... Hollow preform 21... Core layer 21A... Soot 22... Quartz pipe 28... Gas supply pipe

Claims (9)

A method of manufacturing an optical fiber preform for producing an optical fiber preform having a core part containing a rare earth element and a clad part surrounding the core part,
A hollow base material forming step of forming a hollow base material by forming a core layer containing the rare earth element on the inner surface of the quartz tube forming the clad portion;
The hollow base material forming step,
In the quartz tube, a raw material gas of glass particles, and a soot forming step of supplying a rare earth element-containing gas containing the rare earth element to form a soot containing the glass particles,
A step of forming the core layer on the inner surface of the quartz tube by performing at least one step including an exhaust step of exhausting the inside of the quartz tube,
A method of manufacturing an optical fiber preform, wherein an exhaust gas containing an inert gas is supplied into the quartz tube in the exhaust step. The method for manufacturing an optical fiber preform according to claim 1, further comprising a collapse step of collapsing the hollow base material. The method for manufacturing an optical fiber preform according to claim 1, wherein the temperature in the quartz tube in the exhaust step is lower than the temperature in the quartz tube in the soot forming step. In the soot forming step, the rare earth element-containing gas is supplied into the quartz tube by using a gas supply tube,
The method of manufacturing an optical fiber preform according to claim 1, wherein in the exhaust step, the inside of the gas supply pipe is also exhausted by the exhaust gas. The manufacturing of the optical fiber preform according to any one of claims 1 to 4, wherein the volume content of the inert gas in the exhaust gas is reduced stepwise as the exhaust step progresses in the exhaust step. Method. The said step in the said hollow base material formation process further includes the sintering process which after the said exhaust process sinters the said glass particle of the soot, and makes the soot into transparent vitrification. The method for manufacturing an optical fiber preform according to one item. The core portion further includes a dopant different from the rare earth element,
The optical fiber preform according to claim 6, further comprising a diffusion step of supplying a dopant-containing gas containing the dopant into the quartz tube and diffusing the soot into the soot between the exhausting step and the sintering step. Manufacturing method. The manufacturing of the optical fiber preform according to claim 6 or 7, further comprising a treatment step of supplying at least one of an oxygen gas and a chlorine atom-containing gas into the quartz tube between the exhaust step and the sintering step. Method. A preform preparation step of preparing an optical fiber preform manufactured by the method for manufacturing an optical fiber preform according to claim 1.
And a drawing step of drawing the optical fiber preform prepared in the preform preparation step to obtain an optical fiber.

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