JP2023114345A - Method of producing optical fiber glass body - Google Patents

Abstract

To provide a method of producing an optical fiber glass body that allows the production of an optical fiber glass body having a shallow relative refractive index difference with respect to pure silica glass in at least a part of a layer of a clad glass body serving as a clad, and having a small variation in the relative refractive index difference in the layer, the method inhibiting the decline in production efficiency.SOLUTION: The method of producing an optical fiber glass body includes a deposition step P11 of depositing soot that will become a predetermined glass body on an outer surface of a starting substrate, and a diffusion step P12 of heating a porous glass body 20 in a fluoride atmosphere to diffuse fluorine. A bulk density of the soot decreases from the inner to the outer circumference, and is between 0.63 g/cm3 and 0.87 g/cm3 at the inner circumference and between 0.54 g/cm3 and 0.79 g/cm3 at the outer circumference, the difference between the maximum and minimum bulk density is between 0.05 g/cm3 and 0.09 g/cm3; and in the diffusion step P12, the fluorine concentration in the atmosphere is between 1.8% and 7.4%.SELECTED DRAWING: Figure 5

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing an optical fiber glass body.

An optical fiber is known in which at least part of the clad is doped with fluorine. Patent Literature 1 listed below describes such an optical fiber and a method for manufacturing the preform for the optical fiber. In this document, a depressed layer is formed in the innermost region of the clad. When producing a glass body to be a depressed layer in an optical fiber preform for manufacturing this optical fiber, soot to be a depressed layer is deposited on the outer peripheral surface of the core glass body to be the core, and the deposited soot is is heated in a fluorine atmosphere to diffuse the fluorine into the soot. Then, by sintering the soot in which the fluorine has diffused, the soot turns into transparent glass, forming a glass body that serves as a depressed layer. In this document, when depositing the soot to be the depressed layer, it is preferable that the bulk density of the soot is 0.1 g/cm ³ to 0.4 g/cm ³ .

Japanese Patent No. 5342614

In the case of manufacturing a clad glass body that will be the clad of an optical fiber, when fluorine is diffused into the soot that will be the clad glass body, the degree of diffusion changes depending on the bulk density of the soot and the fluorine concentration in the atmosphere. In the case of the bulk density of the soot described in Patent Document 1, the relative refractive index difference of the depressed layer with respect to pure silica becomes too large on the negative side due to excessive diffusion of fluorine, that is, the relative refractive index difference becomes deep. I have too many concerns. On the other hand, there are cases where it is desired to make the relative refractive index difference with respect to pure silica glass shallow in at least some layers of the clad. Therefore, it is conceivable to lower the concentration of fluorine in the atmosphere when diffusing fluorine when fabricating the layer. However, in this case, the diffusion of fluorine to the innermost peripheral portion of the layer becomes difficult, and the refractive index does not sufficiently decrease in this portion, resulting in large fluctuations in the refractive index between the inner peripheral side and the outer peripheral side of the relevant region. I have too many concerns. Therefore, in order to sufficiently diffuse fluorine to the innermost portion of the layer while reducing the fluorine concentration in the atmosphere, it is considered to heat the soot that will be the layer deposited in the fluorine atmosphere for a long time. be done. However, in this case, there is a concern that the manufacturing efficiency of the glass body for optical fiber may be lowered.

Therefore, the present invention provides an optical fiber glass body having a shallow relative refractive index difference with respect to pure silica glass in at least a part of the layer of the cladding glass body to be the cladding of the optical fiber, and a small variation in the relative refractive index difference in the layer. can be manufactured while suppressing a decrease in manufacturing efficiency.

In order to solve the above object, the present invention has a relative refractive index difference of −0.20% or more and −0.05% or less with respect to pure silica glass, and a difference between the maximum value and the minimum value of the relative refractive index difference of 0.03. % or more and 0.04% or less and containing a predetermined glass body to be a predetermined layer disposed outside the core of the optical fiber, wherein the cross-sectional shape is circular. A deposition step of depositing soot to be the predetermined glass body on the outer peripheral surface of the base material; and a sintering step of sintering the soot in which the fluorine has been diffused into transparent glass, wherein the bulk density of the soot decreases from the inner peripheral side to the outer peripheral side, and the It is 0.63 g/cm ³ or more and 0.87 g/cm ³ or less on the inner peripheral side, and 0.54 g/cm ³ or more and 0.79 g/cm ³ or less on the outer peripheral side. value is 0.05 g/cm ³ or more and 0.09 g/cm ³ or less, and in the diffusion step, the fluorine concentration in the atmosphere is 1.8% or more and 7.4% or less. It is something to do.

According to such a method for manufacturing an optical fiber glass body, since the bulk density of the soot that becomes a predetermined glass body decreases toward the outer peripheral side, cracking or the like of the predetermined glass body is suppressed during manufacturing. be able to. Further, as a result of studies by the present inventors, it has been found that the relative refractive index difference of the predetermined glass body is It has been found that even in the shallow case as described above, it is possible to appropriately add fluorine to the soot that becomes a predetermined glass body while suppressing an increase in the diffusion process time. It is considered that this is because fluorine, which enters from the outer peripheral side of the deposited soot, appropriately penetrates to the inner peripheral side by satisfying the above conditions. Therefore, according to the method for manufacturing an optical fiber glass body of the present invention, the specific refractive index difference with respect to the pure silica glass of the predetermined glass body, which is the predetermined layer arranged outside the core, is shallow, and the specific refractive index difference is shallow. It is possible to suppress a decrease in the manufacturing efficiency of an optical fiber glass body having a small variation in index difference. The optical fiber glass body is a glass body used for manufacturing an optical fiber, and is an optical fiber preform or an optical fiber intermediate preform before becoming an optical fiber preform.

Further, in the sintering step, the fluorine concentration in the atmosphere in which the porous glass body is placed may be 0.41% or less.

In this case, it is possible to prevent the refractive index from decreasing unnecessarily on the outer peripheral side of the predetermined glass body. In addition, if the fluorine concentration is 0.18% or more, it is possible to suppress an unnecessary increase in the refractive index on the outer peripheral side of the predetermined glass body in the sintering process.

Moreover, the starting base material may be a core glass body that becomes the core of the optical fiber.

In this case, it is possible to produce an optical fiber glass body capable of producing an optical fiber having a shallow refractive index depressed layer relative to pure silica.

Further, the starting base material includes a core glass body serving as the core of the optical fiber, and a trench layer glass body serving as a trench layer having a lower refractive index than the cladding of the optical fiber and surrounding the core glass body, The soot may be deposited on the outer peripheral surface of the trench layer glass body.

In this case, in the trench optical fiber, a glass body for an optical fiber that has a shallow relative refractive index difference with respect to the pure silica glass of the layer surrounding the trench layer and a small variation in the relative refractive index difference is provided. can be manufactured.

In addition, the starting base material includes a core glass body that serves as the core of the optical fiber, and an inner clad glass body that surrounds the core and has a higher refractive index than the predetermined glass body, and the soot is the inner glass body. It may be deposited on the outer peripheral surface of the clad glass body.

In this case, since the predetermined glass body has a lower refractive index than the inner clad glass body, the trench layer has a shallow relative refractive index difference with respect to pure silica glass, and the trench optical fiber has a small variation in the relative refractive index difference. A manufacturable optical fiber glass body can be produced.

As described above, according to the present invention, at least a portion of the layer of the clad glass body, which is the clad of the optical fiber, has a shallow relative refractive index difference with respect to pure silica glass, and the variation of the relative refractive index difference in the layer is small. Provided is a method for manufacturing an optical fiber glass body that can manufacture an optical fiber glass body while suppressing a decrease in manufacturing efficiency.

It is a figure which shows the appearance of the cross section perpendicular|vertical to the longitudinal direction of the optical fiber manufactured by the manufacturing method of the optical fiber which concerns on embodiment of this invention. 2 is a diagram showing the refractive index profile of the optical fiber of FIG. 1; FIG. FIG. 2 is a view showing a cross section perpendicular to the longitudinal direction of an optical fiber preform for manufacturing the optical fiber shown in FIG. 1; 4 is a flow chart showing steps of an optical fiber manufacturing method including the method of manufacturing the optical fiber preform of FIG. 3; It is a figure which shows the mode of a deposition process. It is a figure which shows the mode of a diffusion process and a sintering process. 3 is a diagram similar to FIG. 2 showing the refractive index distribution of the optical fiber of the second embodiment; FIG. 3 is a diagram similar to FIG. 2 showing the refractive index distribution of the optical fiber of the third embodiment; FIG. FIG. 4 is a diagram showing a refractive index distribution of an inner clad glass body in Examples. FIG. 4 is a diagram showing a refractive index distribution of an inner clad glass body in a comparative example;

Hereinafter, preferred embodiments of the method for manufacturing an optical fiber glass body according to the present invention will be described in detail with reference to the drawings. The embodiments illustrated below are intended to facilitate understanding of the present invention, and are not intended to limit and interpret the present invention. The present invention can be modified and improved without departing from its spirit. Note that in the drawings referred to below, the dimensions of each member may be changed to facilitate understanding.

In the following embodiments, an example in which an optical fiber glass body used for manufacturing an optical fiber is an optical fiber preform will be described.

(First embodiment)
FIG. 1 is a diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber manufactured from an optical fiber preform according to this embodiment. As shown in FIG. 1, in this embodiment, the optical fiber 1 mainly includes a core 10, a clad 11 surrounding the outer peripheral surface of the core 10, and a coating layer 12 covering the outer peripheral surface of the clad 11. . The clad 11 has an inner clad 11a that is a layer that contacts and surrounds the core 10, and an outer clad 11b that is a layer that contacts and surrounds the inner clad 11a. The outer clad 11b includes a first region b1 that is a layer that contacts the inner clad 11a and surrounds the inner clad 11a, and a second region b2 that is a layer that surrounds the first region b1 and extends to the outermost periphery of the clad 11. .

FIG. 2 is a diagram showing the refractive index profile of the optical fiber 1 of FIG. In the following figures showing refractive indices, the same reference numerals as those corresponding to the refractive indices are attached. The refractive index of the core 10 is higher than the refractive index of the clad 11, and in this embodiment, the core 10 is made of silica glass to which chlorine or the like is added to increase the refractive index. Therefore, the refractive index of the core 10 is higher than the refractive index n0 of pure silica glass containing no impurities.

The inner cladding 11a is made of fluorine-doped silica glass and has a refractive index lower than the refractive index n0 of pure silica glass. The refractive index of the inner clad 11a generally decreases from the inner peripheral side, which is the core 10 side, toward the outer peripheral side, and the relative refractive index difference with respect to pure silica glass is generally −0.20% or more and −0.08% or less. be. Thus, the inner cladding 11a of this embodiment has a relatively shallow relative refractive index difference with respect to pure silica. In this embodiment, the thickness of the inner cladding 11a is slightly larger than the radius of the core 10. As shown in FIG. However, the thickness of inner clad 11 a may be equal to or smaller than the radius of core 10 . In the following description, simply referring to a relative refractive index difference means a relative refractive index difference with respect to pure silica glass. The thickness of the inner clad 11a is, for example, 20 mm or more.

The first region b1 of the outer clad 11b is made of silica glass doped with fluorine at a concentration lower than that added to the inner clad 11a, and has a refractive index lower than the refractive index n0 of pure silica glass. The refractive index of the first region b1 is highest on the inner peripheral side and gradually decreases from the inner peripheral side toward the outer peripheral side. The lowest refractive index of the first region b1 is higher than the lowest refractive index of the inner clad 11a. Therefore, the first region b1 has a shallower relative refractive index difference than the inner clad 11a. Also, the thickness of the first region b1 is greater than the thickness of the inner clad 11a. Therefore, the gradient of the decreasing refractive index from the inner peripheral side to the outer peripheral side of the first region b1 is gentler than the decreasing gradient of the refractive index from the inner peripheral side to the outer peripheral side of the inner clad 11a.

The second region b2 is made of silica glass doped with fluorine at a concentration lower than that added to the first region b1. The refractive index of the second region b2 is higher than the lowest refractive index of the first region b1 and lower than the refractive index n0 of pure silica glass. Therefore, the second region b2 of this embodiment has a shallower relative refractive index difference than the first region b1.

The covering layer 12 is made of resin. Examples of the resin forming the coating layer 12 include thermosetting resins and ultraviolet curable resins. The coating layer 12 may have a single-layer structure consisting of one resin layer surrounding the clad 11, or may have a multi-layer structure consisting of a plurality of resin layers.

Next, the optical fiber preform used to manufacture the optical fiber 1 according to this embodiment will be described.

FIG. 3 is a view showing a cross section perpendicular to the longitudinal direction of the optical fiber preform for manufacturing the optical fiber 1 shown in FIG. As shown in FIG. 3, the optical fiber preform 1P is composed of a rod-shaped core glass body 10P that serves as the core 10, and a clad glass body 11P that surrounds the outer peripheral surface of the core glass body 10P and serves as the clad 11. . The clad glass body 11P includes an inner clad glass body 11Pa serving as the inner clad 11a and an outer clad glass body 11Pb serving as the outer clad 11b. , and a second region glass body Pb2 that becomes the second region b2.

The refractive index distribution of the optical fiber preform 1</b>P is substantially the same as the refractive index distribution of the optical fiber 1 . The inner clad glass body 11Pa is in contact with and surrounds the core glass body 10P. The relative refractive index difference of the inner clad glass body 11Pa is -0.20% or more and -0.05% or less. The position where the relative refractive index difference of the inner clad glass body 11Pa is maximized is generally located on the inner peripheral side, and the maximum value is −0.16% or more and −0.05% or less, and is the minimum relative refractive index difference. This position is generally located on the outer peripheral side, and the minimum value is -0.20% or more and -0.08% or less. Moreover, the difference between the maximum value and the minimum value of the relative refractive index difference is 0.03% or more and 0.04% or less.

In this embodiment, the relative refractive index difference of the outer clad glass body 11Pb is -0.20% or more and -0.05% or less. Moreover, the difference between the maximum value and the minimum value of the relative refractive index difference of the outer clad glass body 11Pb is 0.03% or more and 0.04% or less.

The first region glass body Pb1 of the outer clad glass body 11Pb is in contact with and surrounds the inner clad glass body 11Pa. The relative refractive index difference of the first region glass body Pb1 is, for example, -0.20% or more and -0.05% or less. In this embodiment, the relative refractive index difference on the inner peripheral side of the first region glass body Pb1 is, for example, −0.16% or more and −0.05% or less, and the relative refractive index difference on the outer peripheral side is, for example, −0. .20% or more and -0.08% or less. The difference between the maximum value and the minimum value of this relative refractive index difference is, for example, 0.03% or more and 0.04% or less. In this embodiment, it is smaller than the difference between the maximum value and the minimum value of the relative refractive index difference in the inner clad glass body 11Pa. Therefore, the average refractive index of the first region glass body Pb1 is greater than the average refractive index of the inner clad glass body 11Pa.

The second region glass body Pb2 is in contact with the first region glass body Pb1 to surround the first region glass body Pb1. The refractive index of the second region glass body Pb2 gradually increases from the inner peripheral side. Further, the second region glass body Pb2 is in contact with the first region glass body Pb1 to surround the first region glass body Pb1. The relative refractive index difference of the second region glass body Pb2 is, for example, -0.20% or more and -0.05% or less. Moreover, the difference between the maximum value and the minimum value of the relative refractive index difference is, for example, 0.03% or more and 0.04% or less.

Assuming that the inner clad 11a of the optical fiber 1 is a predetermined layer, the inner clad glass body 11Pa can be understood as a predetermined glass body that is the predetermined layer arranged outside the core 10 . Further, if the first region b1 and the second region b2 of the optical fiber 1 are formed of predetermined layers different from the above-described predetermined layers, the first region glass body Pb1 and the second region glass body Pb2 are formed from the core 10 respectively. can also be understood as a given glass body with given layers arranged on the outside. Further, when the outer clad 11b, which is a combination of the first region b1 and the second region b2, is a predetermined layer different from the above-described predetermined layer, the first region glass body Pb1 and the second region glass body Pb2 are combined. The outer cladding glass body 11Pb can be understood as a prescribed glass body serving as a prescribed layer arranged outside the core 10 .

Next, a method for manufacturing the optical fiber preform 1P and a method for manufacturing the optical fiber 1 will be described.

FIG. 4 is a flow chart showing the steps of the method for manufacturing the optical fiber 1 including the method for manufacturing the optical fiber preform 1P according to this embodiment. As shown in FIG. 4, the method of manufacturing the optical fiber preform 1P of the present embodiment includes an inner clad glass body forming step P1 and an outer clad glass body forming step P2. and a drawing step P3 for drawing the manufactured optical fiber preform 1P. In this embodiment, the first region glass body Pb1 and the second region glass body Pb2 are formed by the outer clad glass body forming step P2.

(Inner clad glass body forming step P1)
This process is a process for forming the inner clad glass body 11Pa on the outer peripheral surface of the core glass body 10P, and includes a deposition process P11, a diffusion process P12, and a sintering process P13.

<Deposition step P11>
This step is a step of forming a porous glass body by depositing soot, which is glass particles, on the outer peripheral surface of a core glass body 10P having a circular cross section by an external deposition method. Therefore, in this step, the core glass body 10P is the starting substrate. Note that the circular shape means that it is circular in appearance, and for example, has a non-circularity of 2.0% or less. FIG. 5 is a diagram showing the state of this step. In this step, the core glass body 10P is prepared, and the soot 11Sa that becomes the inner clad glass body 11Pa is deposited on the surface of the core glass body 10P. The soot 11Sa is jetted from the burner 50 together with the flame. The material of soot 11Sa is preferably SiCl ₄ , OMCTS (octamethylcyclotetrasiloxane), or HMDSO (hexamethylsiloxane). At this time, the deposition temperature is lowered each time the burner 50 is traversed. The deposition temperature for depositing the innermost soot 11Sa is preferably 1060° C. or higher and 1230° C. or lower. By setting the temperature within such a range, the innermost bulk density of the soot 11Sa that forms the inner clad glass body 11Pa can be set to 0.63 g/cm ³ or more and 0.87 g/cm ³ or less. The deposition temperature for depositing the outermost soot 11Sa is preferably 1000° C. or more and 1175° C. or less. By setting such a temperature range, the outermost bulk density of the soot 11Sa can be set to 0.54 g/cm ³ or more and 0.79 g/cm ³ or less. Since the deposition temperature decreases from the inner peripheral side to the outer peripheral side, the bulk density of the deposited soot decreases from the inner peripheral side to the outer peripheral side. Also, the difference between the deposition temperature on the inner peripheral side and the deposition temperature on the outer peripheral side is preferably 40 degrees or more and 55 degrees or less. Since the temperature is within such a range, the difference between the maximum value and the minimum value of the bulk density of the soot forming the inner clad glass body 11Pa can be 0.05 g/cm ³ or more and 0.09 g/cm ³ or less. . The deposition temperature is the surface temperature of the deposited soot 11Sa.

Through this step, the porous glass body 20 is obtained in which the soot 11Sa that forms the inner clad glass body 11Pa is deposited on the outer peripheral surface of the core glass body 10P. The thickness of the soot 11Sa deposited in this step is, for example, a thickness that becomes 20 mm or more when vitrified as described later.

<Diffusion step P12>
This step is a step of adding fluorine to the soot 11Sa of the porous glass body formed in the deposition step P11. FIG. 6 is a diagram showing the state of this process. As shown in FIG. 6, in the present embodiment, the soot 11Sa of the porous glass body 20 is heated by using a heating furnace 70 having a furnace core tube 71 having a housing space 75 and a heating element 72 for heating the furnace core tube 71. add fluorine to. First, the porous glass body 20 suspended by the support rod 76 is housed in the housing space 75 of the core tube 71 . Next, the interior of the housing space 75 is heated by the heating element 72 while supplying a diffusion gas containing fluorine into the housing space 75 from an air supply port (not shown). As the diffusion gas, a mixture of inert gas and fluorine gas is used. He, _N2 , Ar can be mentioned as an inert gas. Examples of fluorine gas include CF ₄ , C ₂ F ₆ , SF ₆ and SiF ₄ . At this time, the concentration of fluorine in the diffusion gas atmosphere is 1.8% or more and 7.4% or less. Also, the temperature in the accommodation space 75 is preferably 800° C. or higher and 1250° C. or lower, and more preferably 900° C. or higher and 1100° C. or lower. Under such conditions, a more appropriate amount of fluorine can be added to the soot 11Sa deposited to the bulk density in the deposition step P11.

Further, a dehydration step may be performed simultaneously with this step by mixing a chlorine-based gas with the diffusion gas. Cl ₂ , SiCl ₄ , SOCl ₂ , CCl ₄ and the like can be cited as the chlorine-based gas. Alternatively, a dehydration step may be performed before this step.

Fluorine is thus added to the soot 11Sa of the porous glass body 20 .

<Sintering step P13>
This step is a step of sintering the soot 11Sa to which fluorine has been added in the diffusion step P12 to form transparent glass. In the present embodiment, the heating furnace 70 is used to heat the porous glass body 20 to convert the soot 11Sa into transparent glass. In this step, the porous glass body 20 is heated while supplying the sintering gas into the housing space 75 from an air supply port (not shown). Examples of the sintering gas include the above inert gases. In order to remove unnecessary fluorine in the atmosphere, sintering gas may be flowed into the furnace core tube 71 for a certain period of time, the pressure inside the furnace core tube 71 may be reduced for a certain period of time, or the soot deposited at 1000° C.-1200° C. may be removed. Heating 11Sa can be mentioned. Moreover, the temperature for heating the porous glass body 20 is not particularly limited as long as it is a temperature at which the porous glass body 20 is sintered and turned into transparent glass. Fluorine gas may be mixed with the sintering gas. In this case, the concentration of fluorine in the sintering gas is preferably 0.41% or less, and more preferably 0.18% or more from the viewpoint of adding an appropriate amount of fluorine to the inner clad glass body 11 Pa. preferable. In this step, it is sufficient that the porous glass body 20 can be sintered, and the configuration of the heating furnace 70 and the sintering method are not particularly limited.

Note that the concentration of fluorine in the gas can be replaced with the partial pressure of fluorine in the atmosphere to obtain the same result.

Thus, a glass body is obtained in which the inner clad glass body 11Pa is formed on the outer peripheral surface of the core glass body 10P.

(Outer clad glass body forming step P2)
In this step, an outer clad glass body 11Pb composed of a first region glass body _Pb1 and a second region glass body _Pb2 is formed on the outer peripheral surface of the inner clad glass body 11Pa formed on the outer peripheral surface of the core glass body 10P. It is a forming process, and includes a deposition process P21, a diffusion process P22, and a sintering process P23.

<Deposition step P21>
This step is a step of forming a porous glass body by depositing soot, which will become the outer clad glass body 11Pb, on the outer peripheral surface of the inner clad glass body 11Pa in a manner substantially similar to the deposition process P11. Therefore, in this step, a glass body having a circular cross-section and having an inner clad glass body 11Pa provided on the outer peripheral surface of the core glass body 10 serves as the starting substrate. Also in this step, the deposition temperature is lowered each time the burner 50 is traversed. The deposition temperature for depositing the innermost soot is preferably 1060° C. or more and 1230° C. or less. At this time, the innermost bulk density of the deposited soot can be 0.63 g/cm ³ or more and 0.87/cm ³ or less. Also, the deposition temperature when depositing the soot on the outermost side is preferably 1000° C. or more and 1175° C. or less. By setting the temperature within such a range, the outermost bulk density of the deposited soot can be 0.54 g/cm ³ or more and 0.79/cm ³ or less. The bulk density of the soot deposited also in this step decreases from the inner peripheral side to the outer peripheral side. Also, the difference between the deposition temperature on the inner peripheral side and the deposition temperature on the outer peripheral side is preferably 40° C. or more and 55° C. or less. By setting the temperature within such a range, the difference between the maximum value and the minimum value of the bulk density of the deposited soot can be 0.05 g/cm ³ or more/0.09 cm ³ or less.

Through this step, a porous glass body is formed in which soot, which will be the outer clad glass body 11Pb, is deposited on the outer peripheral surface of the inner clad glass body 11Pa. It should be noted that the bulk density of the soot to be the first region glass body Pb1 deposited in this step is higher than the bulk density of the soot 11Sa deposited in the deposition step P11 within the above range. This is preferable from the viewpoint of making the relative refractive index difference shallower than the relative refractive index difference of the inner clad glass body 11Pa.

<Diffusion step P22>
This step is a step of adding fluorine to the soot of the porous glass body formed in the deposition step P21 in substantially the same manner as in the diffusion step P12. In this embodiment, a heating furnace 70 is used. In this step, the porous glass body obtained in the deposition step P21 is accommodated in the accommodation space 75 of the furnace core tube 71, and fluorine is diffused into the soot in the same manner as in the diffusion step P12. The concentration of fluorine in the atmosphere of the diffusion gas in this step is 1.8% or more and 7.4% or less as in the diffusion step P12. Moreover, it is preferable that the temperature in the accommodation space 75 in this step and the time of this step be the same as those in the diffusion step P12. A dehydration step may be performed simultaneously with or before this step. Fluorine is thus added to the soot of the porous glass body formed in the deposition step P21. The fluorine concentration in the atmosphere of the diffusion gas in this step must be lower than the fluorine concentration in the atmosphere of the diffusion gas in the diffusion step P12 within the above range. This is preferable from the viewpoint of making the difference shallower than the relative refractive index difference of the inner clad glass body 11Pa.

<Sintering process P23>
This step is a step of sintering the soot to which fluorine has been added in the diffusion step P22 to form transparent glass. In this embodiment, the sintering step P13 is performed in the same manner. When fluorine gas is mixed in the sintering gas in this step, the concentration of fluorine is preferably 0.41% or less, and more preferably 0.18% or more in the outer clad glass body 11Pb. This is preferable from the viewpoint of adding an appropriate amount of fluorine. In this way, the soot is vitrified to become the outer clad glass body 11Pb, and the optical fiber preform 1P shown in FIG. 3 is obtained in which the core glass body 10P, inner clad glass body 11Pa, and outer clad glass body 11Pb are integrated. Note that even when fluorine gas is mixed with the sintering gas in this step, the fluorine concentration is lower than the fluorine concentration in the atmosphere of the sintering gas used in the sintering step P13. This is preferable from the viewpoint of making the relative refractive index difference of the clad glass body 11Pb shallower than the relative refractive index difference of the inner clad glass body 11Pa. Further, in this step, fluorine gas may not be mixed.

(Drawing process P3)
This step is a step of drawing the optical fiber preform 1P to obtain the optical fiber 1. As shown in FIG. This step will be described without particular illustration. In this step, the optical fiber preform 1P is heated in a spinning furnace, and glass is drawn from the lower end portion of the optical fiber preform. The drawn glass immediately solidifies, and the core glass body 10P becomes the core 10, the clad glass body 11P becomes the clad 11, and an optical fiber bare wire composed of the core 10 and the clad 11 is obtained. A coating layer 12 is provided on the outer peripheral surface of the bare optical fiber to obtain the optical fiber 1 shown in FIG.

As described above, in this embodiment, the relative refractive index difference with respect to pure silica glass is −0.20% or more and −0.05% or less, and the difference between the maximum value and the minimum value of the relative refractive index difference is 0.03. % or more and 0.04% or less, and a method for manufacturing an optical fiber preform 1P as an optical fiber glass body including a predetermined glass body serving as a predetermined layer arranged outside the core 10 of the optical fiber 1 be. As described above, the inner clad 11a can be regarded as a predetermined layer, the starting substrate as the core glass body 10P, and the inner clad glass body 11Pa as a predetermined glass body. Alternatively, as described above, the outer clad 11b may be a predetermined layer, the starting substrate may be a glass body in which the core glass body 10P is provided with the inner clad 11a, and the outer clad glass body 11Pb may be regarded as a predetermined glass body. can. In the case of these views, this manufacturing method consists of deposition steps P11 and P21 for depositing soot that will become a predetermined glass body on the outer peripheral surface of the starting base material, and a porous glass body in which soot is deposited on the starting base material. Diffusion steps P12 and P22 of heating in a fluorine-containing atmosphere to diffuse fluorine into the soot, and sintering steps P13 and P23 of sintering the fluorine-diffused soot into transparent glass are provided. The bulk density of the deposited soot decreases from the inner peripheral side to the outer peripheral side, being 0.63 g/cm ³ or more and 0.87 g/cm ³ or less on the inner peripheral side and 0.54 g/cm 3 on the outer peripheral side. ³ or more and 0.79 g/cm ³ or less, and the difference between the maximum value and the minimum value of the bulk density is 0.05 g/cm ³ or more and 0.09 g/cm ³ or less. The fluorine concentration is 1.8% or more and 7.4% or less.

According to the method for manufacturing the optical fiber preform 1P, the bulk density of the soot, which is the predetermined glass body, decreases toward the outer periphery, so that cracking or the like of the predetermined glass body is suppressed during manufacturing. be able to. Further, when the bulk density of the soot and the fluorine concentration in the atmosphere in the diffusion steps P12 and P22 satisfy the above conditions, even when the relative refractive index difference of the predetermined glass body is within the above range, the diffusion step P12 , P22 can be prevented from increasing, and fluorine can be appropriately added to the soot. Therefore, according to the method of manufacturing the optical fiber preform 1P of the present embodiment, the relative refractive index difference with respect to pure silica of the predetermined glass body, which is the predetermined layer arranged outside the core 10 of the optical fiber 1, is A decrease in manufacturing efficiency of the optical fiber preform 1P, which is shallow and has a small variation in the relative refractive index difference, can be suppressed.

(Second embodiment)
Next, a second embodiment of the invention will be described in detail. Components that are the same as or equivalent to those of the first embodiment are denoted by the same reference numerals, and overlapping descriptions are omitted unless otherwise specified.

FIG. 7 is a diagram similar to FIG. 2 showing the refractive index profile of the optical fiber of this embodiment. The optical fiber of this embodiment differs from the optical fiber 1 shown in FIG. 1 in that it includes a trench layer 13 surrounding the inner clad 11a between the inner clad 11a and the outer clad 11b of the optical fiber 1 shown in FIG. . In the present embodiment, the inner clad 11a surrounded by the trench layer 13 is called the inner clad 11a in order to distinguish it from the inner clad 11a of the first embodiment. Such an optical fiber is called a trench optical fiber, and the core 10, inner cladding 11a, and trench layer 13 are sometimes called a core element. In trench optical fibers, light primarily propagates through the core element.

The refractive index of the trench layer 13 is lower than those of the inner clad 11a and the outer clad 11b, and the relative refractive index difference of the trench layer is -0.20% or more and -0.05% or less, for example.

A method for manufacturing an optical fiber preform for manufacturing such a trench optical fiber includes an inner clad glass body forming step P1 and an outer clad glass body forming step P1 in the method for manufacturing an optical fiber preform according to the first embodiment. A step of forming a trench layer glass body that becomes the trench layer 13 is included between step P2 and step P2. However, since the inner clad 11a is read as the inner clad 11a as described above, the inner clad glass body 11Pa is read as the inner clad glass body 11Pa in this embodiment, and the inner clad glass body forming process P1 is replaced with the inner clad glass body forming process. Replace with P1. The inner clad glass body 11Pa becomes the inner clad 11a in the optical fiber. In the trench layer glass body forming step, up to the inner clad glass body forming step P1 is completed, and the glass body having the inner clad glass body 11Pa formed on the outer peripheral surface of the core glass body 10P is formed to have a refractive index similar to that of the trench layer 13. It is inserted into the through-hole of the glass tube and collapsed. Thus, a trench layer glass body is formed on the outer peripheral surface of the inner clad glass body 11Pa. After that, the outer clad glass body forming step P2 is performed. In the outer cladding glass body forming step P2 of the present embodiment, the core glass body 10P serving as the core 10 of the optical fiber and the trench layer 13 having a refractive index lower than that of the cladding 11 of the optical fiber are used as starting base materials. and a trench layer glass body surrounding 10P, soot forming an outer cladding glass body 11Pb is deposited on the outer peripheral surface of the trench layer glass body. Thus, an optical fiber preform used for manufacturing the optical fiber of the present embodiment is obtained. By performing the drawing step P3 using this optical fiber preform, an optical fiber having the refractive index distribution shown in FIG. 7 is obtained.

In the trench layer glass body forming step, the glass body having the inner clad glass body 11Pa formed on the outer peripheral surface of the core glass body 10P is subjected to substantially the same processes as the deposition process, the diffusion process, and the sintering process described above. A layered glass body may be formed. In this case, for example, in the deposition process, the bulk density is made lower than in the deposition process P11 of the first embodiment, and in the diffusion process, the fluorine concentration in the atmosphere is made higher than that in the diffusion process P12 of the first embodiment. Increase the fluorine concentration to be added.

In the method of manufacturing an optical fiber preform according to the present embodiment, the inner clad 11a surrounding the core 10 inside the trench layer 13 is a predetermined layer, the starting substrate is the core glass body 10P, and the inner clad glass body 11Pa is a predetermined layer. can be regarded as a glass body. In the case of such a way of thinking, when a predetermined layer is positioned between the core 10 and the trench layer 13, the relative refractive index difference of the predetermined glass body that becomes the predetermined layer is shallow, and the relative refractive index difference It is possible to suppress a decrease in the manufacturing efficiency of an optical fiber preform having a small variation in . Alternatively, as described above, the starting substrate may be the glass body including the core glass body 10P and the trench layer glass body, the outer clad 11b may be the predetermined layer, and the outer clad glass body 11Pb may be the predetermined glass body. can. In this case, when a predetermined layer is positioned outside the trench layer 13, a predetermined glass body to be the predetermined layer has a shallow relative refractive index difference and a small variation in the relative refractive index difference. A decrease in production efficiency of the base material can be suppressed.

In the method of manufacturing an optical fiber glass body for manufacturing an optical fiber having a trench layer as in the present embodiment, the glass body that becomes the trench layer 13 may be a predetermined glass body. In this case, the starting substrate includes a core glass body 10P and an inner clad glass body 11Pa surrounding the core glass body 10P, and the soot is deposited on the outer peripheral surface of the inner clad glass body 11Pa. In this case, the inner clad glass body 11Pa has a refractive index lower than that of the core glass body 10P and higher than that of the predetermined glass body. In this case, the refractive index of the inner clad glass body 11Pa that forms the inner clad 11a may be constant.

(Third embodiment)
Next, a third embodiment of the invention will be described in detail. Components that are the same as or equivalent to those of the first embodiment are denoted by the same reference numerals, and overlapping descriptions are omitted unless otherwise specified.

FIG. 8 is a diagram similar to FIG. 2 showing the refractive index profile of the optical fiber of this embodiment. The optical fiber of this embodiment differs from the optical fiber 1 shown in FIG. 1 in that it includes an intermediate clad 14 and a trench layer 13 between the core 10 and the inner clad 11a of the optical fiber 1 shown in FIG. Therefore, the optical fiber of this embodiment is a trench type optical fiber as in the second embodiment. However, the optical fiber of this embodiment differs from the optical fiber of the second embodiment in that the inner clad 11 a is positioned on the outer peripheral surface of the trench layer 13 .

In a method of manufacturing an optical fiber preform for manufacturing such a trench optical fiber, a rod-shaped core element glass body to be a core element is prepared. The core element glass body has an intermediate clad glass body that becomes the intermediate clad 14 on the outer peripheral surface of the core glass body 10P that becomes the core 10, and a trench layer glass body that becomes the trench layer 13 on the outer peripheral surface of the intermediate clad glass body. is a glass body having a circular cross section in which is formed. In the present embodiment, in the inner clad glass body forming step P1 in the method of manufacturing the optical fiber preform of the first embodiment, the trench layer glass body is formed on the outer peripheral surface of the core element glass body instead of the core glass body 10P. Soot 11Sa is deposited. Therefore, in this embodiment, the core element glass body is the starting substrate on which the soot 11Sa that will become the inner clad glass body 11Pa is deposited. After that, other steps are performed in the same manner as in the method of manufacturing the optical fiber preform of the first embodiment. Thus, an optical fiber preform for use in manufacturing an optical fiber having the refractive index profile shown in FIG. 8 is obtained. By performing the drawing step P3 using this optical fiber preform, an optical fiber having the refractive index distribution shown in FIG. 8 is obtained.

According to the method of manufacturing the optical fiber preform of the present embodiment, when the inner clad 11a, which is the layer around the core element in the trench optical fiber, is a predetermined layer, the predetermined glass body that becomes the predetermined layer has a shallow relative refractive index difference, and a decrease in manufacturing efficiency of an optical fiber preform having a small variation in the relative refractive index difference can be suppressed.

In this embodiment, the intermediate clad may be omitted and the core 10 may be in contact with the trench layer 13 . In this case, the glass body prepared in the inner clad glass body forming step P1 is a glass body in which a trench layer glass body, which becomes the trench layer 13, is formed on the outer peripheral surface of the core glass body 10P, which becomes the core 10. FIG.

As described above, the present invention has been described using the above embodiments as examples, but the present invention is not limited to these.

For example, in the above embodiment, the relative refractive index difference of the outer clad glass body 11Pb is −0.20% or more and −0.05% or less, and the maximum value and the minimum value of the relative refractive index difference of the outer clad glass body 11Pb are has been described with an example in which the difference from is 0.03% or more and 0.04% or less. However, unlike the above embodiment, the entire outer clad glass body 11Pb may deviate from this relative refractive index difference. In this case, only the inner clad 11a is the predetermined layer, the inner clad glass body 11Pa is the predetermined glass body, the outer clad 11b is not the predetermined layer, and the outer clad glass body 11Pb is not the predetermined glass body. be able to. Also, some layers of the outer cladding may have such a relative refractive index difference and other parts may not have such a relative refractive index difference. For example, the outer peripheral side of the second region glass body Pb2 may not have such a relative refractive index difference. In this case, the region that satisfies the relative refractive index difference can be regarded as a predetermined glass body that becomes a predetermined layer. Further, for example, only the first region glass body Pb1 may be the predetermined glass body.

Further, in the above-described embodiments, an example has been described in which the optical fiber glass body used for manufacturing the optical fiber is the optical fiber preform. However, the present invention is not limited to this. For example, in the first embodiment, the intermediate preform in which the inner clad glass body 11Pa is provided on the outer peripheral surface of the core glass body 10P is the optical fiber glass body used for manufacturing the optical fiber of the present invention. . Alternatively, in the third embodiment, the intermediate base material in which the inner clad glass body 11Pa is provided on the outer peripheral surface of the trench layer glass body is the optical fiber glass body used for manufacturing the optical fiber of the present invention. .

As described above, according to the present invention, the relative refractive index difference with respect to pure silica glass is -0.20% or more and -0.05% or less, and the difference between the maximum value and the minimum value of the relative refractive index difference is 0.03% or more and 0.05% or more. 04% or less and a predetermined layer disposed outside the core 10 of the optical fiber 1. a deposition step of depositing soot to be a glass body; a diffusion step of heating the porous glass body with the soot deposited on the starting base material in an atmosphere containing fluorine to diffuse fluorine into the soot; and a sintering step of sintering the deposited soot to turn it into transparent glass, wherein the bulk density of the deposited soot decreases from the inner peripheral side to the outer peripheral side, reaching 0.63 g/cm on the inner peripheral side. ³ or more and 0.87 g/cm ³ or less, and 0.54 g/cm ³ or more and 0.79 g/cm ³ or less on the outer peripheral side, and the difference between the maximum value and the minimum value of the bulk density is 0.05 g/cm ³ or more and 0.09 g/cm ³ or less, and in the diffusion step P12, the fluorine concentration in the atmosphere is 1.8% or more and 7.4% or less.

EXAMPLES The content of the present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

(Example 1)
A φ20 core rod glass body made of silica glass with a chlorine concentration of 1.5 wt % produced by the VAD method was prepared. A deposition step P11 was performed on the outer peripheral surface of this core rod glass body to deposit soot to obtain a porous glass body. In this process, the initial deposition temperature was set to 1100°C, and the temperature was gradually lowered to a final deposition temperature of 1040°C. As a result, the bulk density of the soot on the inner peripheral side was 0.68 g/cm ³ and the bulk density of the soot on the outer peripheral side was 0.61 g/cm ³ . The bulk density is calculated by measuring the thickness of each soot layer using a laser and calculating the increase in soot weight for each soot layer. Next, this porous glass body was placed in a heating furnace, and dehydration was performed by heating at 1050° C. for 5 hours in a 150 sccm Cl ₂ and 5 SLM He gas atmosphere in the heating furnace. Next, the inside of the heating furnace was filled with a He gas atmosphere of 200 sccm of SiF4 and 5 SLM, and heated at 1050° C. for 8 hours to perform the diffusion step P12 to add fluorine to the soot. The fluorine concentration in this atmosphere is 2.8%. After 10 hours, the SiF4 was increased to 12.5 sccm, and after the atmosphere in the heating furnace was sufficiently replaced, the soot was heated at 1450° C. to carry out the sintering step P13 to convert the soot into transparent glass. The fluorine concentration in the atmosphere in the sintering step P13 is 0.25%. It took 20 hours from the start of the dehydration treatment to the end of the sintering step P13. Thus, a glass body was obtained in which the inner clad glass body 11Pa was formed on the outer peripheral surface of the core glass body 10P.

Next, after this glass body was stretched to φ25, a deposition step P21 was performed to deposit soot to obtain a porous glass body. In this process, the initial deposition temperature was set to 1165°C, and the temperature was gradually lowered to a final deposition temperature of 1120°C. Next, the dehydration treatment, the diffusion step P22, and the sintering step P23 were performed under the same conditions as the dehydration treatment, the diffusion step P12, and the sintering step P13. Thus, an optical fiber preform was obtained. Table 1 and FIG. 9 show the relative refractive index difference from the inner peripheral side to the outer peripheral side of the inner clad glass body 11Pa of this optical fiber preform. The normalized radius is the radius normalized by the thickness of the inner clad glass body 11Pa of each optical fiber preform.

(Example 2-7)
The deposition step P11 was performed in the same manner as in Example 1 with the initial deposition temperature and the final deposition temperature set to the temperatures shown in Table 1. A porous glass body having the bulk density shown in Table 1 was obtained. Next, the diffusion step and the sintering step were carried out in the same manner as in Example 1 with the concentration of fluorine in the atmosphere as shown in Table 1, and the subsequent steps were carried out in the same manner as in Example 1 to obtain an optical fiber preform. Ta. Table 1 and FIG. 9 show the relative refractive index difference from the inner peripheral side to the outer peripheral side of the inner clad glass body 11Pa of this optical fiber preform.

(Comparative Example 1-6)
The deposition step P11 was performed in the same manner as in Example 1 with the initial deposition temperature and the final deposition temperature set to the temperatures shown in Table 1. A porous glass body having the bulk density shown in Table 1 was obtained. Next, the diffusion step and the sintering step were carried out in the same manner as in Example 1 with the concentration of fluorine in the atmosphere as shown in Table 1, and the subsequent steps were carried out in the same manner as in Example 1 to obtain an optical fiber preform. Ta. The diffusion process in Comparative Example 6 took twice as long as in Example 1. Table 1 and FIG. 10 show the relative refractive index difference from the inner peripheral side to the outer peripheral side of the inner clad glass body 11Pa of this optical fiber preform.

In Example 1-7, by producing the optical fiber preform under the conditions shown in Table 1, the bulk density of the soot that becomes the inner clad glass body 11Pa decreases from the inner peripheral side to the outer peripheral side. It is 0.63 g/cm ³ or more and 0.87 g/cm ³ or less on the peripheral side, and 0.54 g/cm ³ or more and 0.79 g/cm ³ or less on the outer peripheral side. difference was 0.05 g/cm ³ or more and 0.09 g/cm ³ or less. The relative refractive index difference of the inner clad glass body 11Pa is -0.20% or more and -0.05% or less, and the difference between the maximum value and the minimum value of the relative refractive index difference is 0.03% or more and 0.05% or more. 04% or less. In other words, it was found that the inner clad glass body 11Pa can have a shallow relative refractive index, and the fluctuation of the refractive index of the inner clad glass body 11Pa can be suppressed.

On the other hand, Comparative Examples 1 and 2 resulted in a large change in the relative refractive index difference of the inner clad glass body 11Pa. This is probably due to the high fluorine concentration in the sintering step P13 in Comparative Example 1, and the low fluorine concentration in the diffusion step P12 in Comparative Example 2, although the bulk density is approximately the same as that of the example. Conceivable. Comparative Examples 3 and 4 resulted in an excessively deep relative refractive index difference of the inner clad glass body 11Pa. This is believed to be due to the bulk density of the deposited soot being too low. Comparative Example 5 resulted in a large change in the relative refractive index difference of the inner clad glass body 11Pa. The reason for this is considered to be that although the bulk density is low, the fluorine concentration in the diffusion step P12 is too low, and the fluorine does not adequately penetrate to the inner peripheral side of the soot. Moreover, as described above, in Comparative Example 6 in which appropriate characteristics were obtained, the diffusion step P12 required a long time.

As described above, according to the embodiments within the scope of the present invention, an optical fiber preform, which is an optical fiber glass body used for manufacturing an optical fiber having a shallow relative refractive index difference in the region on the inner peripheral side of the clad, is manufactured. It has been found that there is provided a method of manufacturing an optical fiber preform capable of suppressing a decrease in efficiency.

According to the invention, there is provided an optical fiber glass body having a shallow relative refractive index difference with respect to pure silica glass in at least a part of the layer of the clad glass body serving as the clad of the optical fiber, and a small variation in the relative refractive index difference in the layer. Also, a method for manufacturing an optical fiber glass body that can be manufactured while suppressing a decrease in manufacturing efficiency is provided, and is expected to be used in fields such as the manufacture of optical fibers.

Reference Signs List 1 Optical fiber 1P Optical fiber preform 10 Core 10P Core glass body 11 Clad 11a Inner clad (inner clad)
11b... Outer clad 11P... Clad glass body 11Pa... Inner clad glass body (inner clad glass body)
11Pb... Outer clad glass body 20... Porous glass body P1... Inner clad glass body forming process (inner clad glass body forming process)
P2: Outer clad glass body forming step P3: Drawing step P11, P21: Deposition step P12, P22: Diffusion step P13, P23: Sintering step

Claims (5)

The relative refractive index difference with respect to pure silica glass is -0.20% or more and -0.05% or less, the difference between the maximum value and the minimum value of the relative refractive index difference is 0.03% or more and 0.04% or less, and the light A method for manufacturing an optical fiber glass body including a predetermined glass body that becomes a predetermined layer arranged outside the core of the fiber,
a depositing step of depositing soot to be the predetermined glass body on the outer peripheral surface of a starting substrate having a circular cross-sectional shape;
a diffusion step of heating the porous glass body in which the soot is deposited on the starting base material in a fluorine atmosphere to diffuse fluorine into the soot;
a sintering step of sintering the soot in which fluorine has been diffused into transparent glass;
with
The bulk density of the soot decreases from the inner peripheral side to the outer peripheral side, and is 0.63 g/cm ³ or more and 0.87 g/cm ³ or less on the inner peripheral side and 0.54 g/cm 3 on the outer peripheral side. ³ or more and 0.79 g/cm ³ or less, and the difference between the maximum value and the minimum value of the bulk density is 0.05 g/cm ³ or more and 0.09 g/cm ³ or less,
A method for producing an optical fiber glass body, wherein in the diffusion step, the concentration of fluorine in the atmosphere is 1.8% or more and 7.4% or less. 2. The method of manufacturing an optical fiber glass body according to claim 1, wherein in said sintering step, the concentration of fluorine in the atmosphere in which said porous glass body is placed is 0.41% or less. 3. The method of manufacturing an optical fiber glass body according to claim 1, wherein the starting base material is a core glass body that becomes the core of the optical fiber. The starting base material includes a core glass body serving as the core of the optical fiber, and a trench layer glass body serving as a trench layer having a lower refractive index than the cladding of the optical fiber and surrounding the core glass body,
3. The method of manufacturing an optical fiber glass body according to claim 1, wherein said soot is deposited on the outer peripheral surface of said trench layer glass body. The starting base material includes a core glass body serving as the core of the optical fiber, and an inner clad glass body surrounding the core and having a higher refractive index than the predetermined glass body,
3. The method of manufacturing an optical fiber glass body according to claim 1, wherein said soot is deposited on the outer peripheral surface of said inner clad glass body.

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