JP2017043512A - Optical fiber preform manufacturing method, optical fiber manufacturing method, and lens manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber preform manufacturing method capable of suppressing the abnormality of a refractive index distribution due to the increase or decrease of a refractive index at a central portion, in the case where an optical fiber having a refractive index distribution of a gray dead index (GI) type is manufactured, and an optical fiber manufacturing method and a lens manufacturing method utilizing said optical fiber preform.SOLUTION: An optical fiber preform manufacturing method comprises: a soot deposition step of depositing SiOfine particles to acquire a soot deposit 1 having a columnar side face 3; an F diffusion step of diffusing an F element 5 from the side face 3 of the soot deposit 1; and a sintering step of sintering the soot deposit 1 having passed through the F diffusion step. A method manufactures a lens by cutting said optical fibers to a length of the lens.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for manufacturing an optical fiber preform having a graded index (GI) type refractive index profile, and a method for manufacturing an optical fiber and a lens using the optical fiber preform.

As a method for producing an optical fiber preform made of glass (quartz glass) containing SiO ₂ as a main component, a CVD method, an OVD method, and a VAD method are well known.

  The CVD method (Chemical vapor deposition method, chemical vapor deposition method) uses a glass tube as a starting member on which a glass layer is laminated. After a glass layer is laminated on the inner surface of the glass tube, the glass tube is heated by heating. This is a method of reducing the diameter and crushing the cavity inside the glass layer to make it solid (collapse). According to the optical fiber preform manufacturing method using the CVD method, the refractive index distribution of the glass layer along the radial direction can be easily controlled.

  The OVD method (Outside Vapor Deposition method, external method) uses a glass rod as a starting member on which a soot layer is deposited, deposits glass particles on the outer surface of the glass rod, and then forms a soot layer. Is a method of sintering by heating to obtain transparent glass. By removing the glass rod and then reducing the diameter of the cavity by heating, the cavity inside the glass layer can be crushed and solidified. If a glass rod can be used as part of the optical fiber preform, the soot layer can be sintered without removing the glass rod.

  In the VAD method (Vapor phase axial deposition method, vapor phase shaft attachment method), deposition of glass fine particles is started from the tip of a starting member to form a cylindrical soot deposit, and then the soot deposit is fired by heating. It is the method of obtaining transparent glass by making it tie.

  The OVD method and the VAD method are advantageous methods for increasing the size of the optical fiber preform because soot is deposited toward the outside of the starting member. However, when an optical fiber preform having a graded index (GI) type refractive index profile is manufactured, the control of the refractive index profile of the core is important, so the CVD method is generally used. Is.

  An optical fiber having a GI type refractive index profile can be used as a GRIN (Graded Index) lens by omitting the cladding. For example, Patent Document 1 describes that a GRIN lens is formed using quartz glass to which a dopant such as germanium (Ge) that increases the refractive index is added at a higher concentration as it is closer to the central axis of a cylindrical body. Has been.

  Patent Document 2 describes a heat-resistant optical fiber in which a refractive index profile of a glass fiber has the highest refractive index in at least a portion of pure quartz of the core, and a hermetic thin film is formed on the outer periphery of the glass fiber. . This document describes that since the core does not contain germanium, it is possible to prevent an increase in transmission loss over time due to hydrogen, and that the cladding is added with fluorine so that the refractive index is low. Has been.

  Moreover, in patent document 2, as a manufacturing method of an optical fiber preform in which the refractive index changes in three stages, a process of heating and integrating in a state where a glass rod is inserted into a glass pipe (rod in collapse) is repeated twice. A method is described. This document also includes an example of a GI type optical fiber, but a specific manufacturing method thereof is not disclosed. In the GI-type refractive index distribution, the refractive index of the core continuously changes in the radial direction, so that it is considered difficult to realize the GI-type refractive index distribution even if the rod-in collapse method is repeated many times.

JP 2013-190714 A JP 2011-107415 A

When the optical fiber preform is produced by the CVD method, there is a problem that the dopant volatilizes from the glass layer deposited on the inner surface of the glass tube in the solidification step. The volatilization of the dopant is significant at the surface of the glass layer in contact with the cavity to the end, that is, at the center in the radial direction of the solidified glass when the diameter of the cavity is reduced. When dopants (including elements such as Ge, Al, and P) that increase the refractive index of SiO ₂ are volatilized, a refractive index drop (center dip) occurs at the center of the refractive index distribution. When the refractive index of SiO ₂ is small and the dopant (including elements such as F and B) volatilizes, the refractive index increases at the center of the refractive index distribution.

  If an abnormality due to an increase or decrease in the refractive index occurs in the center of the GI type refractive index distribution, the propagation of light inside the optical fiber or the GRIN lens is adversely affected. In particular, in the case of a GRIN lens, the effects of the lens such as divergence, convergence, and parallelization (collimation) are reduced by the action of light divergence or convergence within the lens.

  In the manufacturing method (CVD method and OVD method) for carrying out solidification, in order to eliminate the abnormality in the refractive index of the central portion, the diameter of the hollow portion is to some extent before the hollow portion of the glass tube is closed by the solidification step. A method of removing the surface layer in which the dopant concentration in the vicinity of the cavity portion is reduced by using a method such as gas etching at the stage when the thickness of the substrate has been reduced can be considered. However, since the volatilization of the dopant continues until the solidification process is completed and the cavity is closed, it is difficult to eliminate the refractive index abnormality at the center.

  Further, in the CVD method, the size of the optical fiber preform is difficult to increase because the dimensions of the obtained optical fiber preform are restricted by the dimensions of the first glass tube.

  The present invention has been made in view of the above circumstances, and when manufacturing a GI type refractive index distribution, an optical fiber capable of suppressing an abnormality in the refractive index distribution due to an increase or decrease in the refractive index in the central portion. It is an object of the present invention to provide a method for manufacturing a base material, and a method for manufacturing an optical fiber and a method for manufacturing a lens using the optical fiber base material.

In order to solve the above-mentioned problems, the present invention provides a soot deposition process for obtaining a soot deposit having a cylindrical side surface by depositing SiO ₂ fine particles, and F diffusion for diffusing F element from the side surface of the soot deposit. A glass body obtained through the soot deposition step, the F diffusion step, and the sintering step, and a sintering step for sintering the soot deposit body that has undergone the F diffusion step. An optical fiber preform manufacturing method characterized by having a refractive index profile.

A configuration in which the SiO ₂ fine particles are made of pure SiO ₂ can be adopted.
It is possible to adopt a configuration in which the soot deposit has a portion containing the SiO ₂ fine particles over the entire radial cross section from the side surface to the center.
A configuration having a dehydration step of heating the soot deposit in an atmosphere containing dehydration gas between the soot deposition step and the F diffusion step can be adopted.

It is possible to adopt a configuration in which F is not added to a region having the highest refractive index in the GI type refractive index distribution, and the F concentration in the glass is increased from the center to the outer periphery of the glass body. it can.
A configuration in which the F concentration is increased from the central portion of the soot deposit through the F diffusion step toward the side surface can be employed.

In the F diffusion step, it is possible to adopt a configuration in which the F element is diffused in the soot deposit and the sintering step is started when the F element reaches the center from the side surface.
The structure which has the process of adjusting the refractive index distribution of the said glass body can be employ | adopted by grinding the outer peripheral part of the said glass body after the said sintering process.

After the sintering step, the outside of the glass body, it is possible to adopt a configuration having a step of applying a cladding made of F-doped SiO _2.
After the sintering step, the outside of the glass body, it is possible to adopt a configuration having a step of applying a trench structure consisting of F added SiO _2.

Moreover, this invention provides the manufacturing method of an optical fiber which has the process of manufacturing an optical fiber preform with the manufacturing method of the said optical fiber preform, and the process of drawing the said optical fiber preform.
The present invention also includes a step of manufacturing an optical fiber preform by the method of manufacturing an optical fiber preform, a step of drawing the optical fiber preform, and cutting an optical fiber obtained by drawing into a lens length. And a method for manufacturing a lens.

  According to the present invention, an optical fiber preform having a GI type refractive index distribution can be manufactured by diffusing the F element from the side surface of the soot deposit and sintering the soot deposit having an F concentration distribution. . When the soot deposit is sintered, by not having a solidification step, it is possible to suppress an abnormality in the refractive index distribution due to an increase or decrease in the refractive index in the central portion. By expanding the cross-sectional area in the radial direction of the soot deposit, the optical fiber preform can be increased in size and productivity can be improved.

It is a schematic diagram which shows an example of F spreading | diffusion process. It is a graph which shows an example of F addition amount distribution and refractive index distribution in the radial direction of a glass body. It is a perspective view showing an example of (a) a graph which shows an example of refractive index distribution, and (b) an appearance about a GRIN lens fiber. It is a perspective view showing an example of (a) a graph which shows an example of refractive index distribution, and (b) an appearance about GI type optical fiber. It is a graph which shows an example of the refractive index distribution of the glass body obtained in the Example. It is a graph which shows an example in which abnormality of refractive index distribution arose by the raise of the refractive index of a center part.

  Hereinafter, the present invention will be described based on preferred embodiments.

The manufacturing method of the present embodiment includes a soot deposition step of obtaining a soot deposit having a cylindrical side surface by depositing SiO ₂ fine particles, an F diffusion step of diffusing F element from the side surface of the soot deposit, This is a method for producing a glass body having a GI type refractive index distribution by a sintering process in which the soot deposit body that has undergone the diffusion process is sintered. The obtained glass body can be used for an optical fiber preform.

The soot deposition step is a step of obtaining a soot deposition body having a cylindrical side surface by depositing SiO ₂ fine particles. The soot deposit body is a porous glass body formed by deposition of SiO ₂ fine particles. The SiO ₂ fine particles are glass fine particles containing silica (SiO ₂ ) as a main component. Examples of dopant elements that can be added to the SiO ₂ fine particles include various elements such as Ge, Al, P, B, F, Cl, Na, K, Ti, and rare earths (for example, Y, Nd, Er, and Yb). In order to suppress the change in the refractive index due to the volatilization of the dopant, the SiO ₂ fine particles are preferably made of pure SiO ₂ .

  The glass fine particles can be obtained as soot (soot) generated by burning a source gas containing silicon (Si) in a flame. Specifically, various gases including raw material gas and fuel gas are supplied to a burner having a cylindrical nozzle, and glass fine particles generated in a flame generated at the tip of the burner are discharged toward the starting member. Glass particulates are deposited on top. The mixing ratio of various gases can be controlled by adjusting the flow rate supplied to the burner. The number of burners is one or more, and can be selected as appropriate.

Examples of the source gas include a compound gas containing Si, such as SiCl ₄ and SiHCl ₃ . When a dopant element is added to the glass fine particles, a gas containing the dopant element can be supplied to the burner. Examples of the fuel gas include a combination of a combustible gas such as H ₂ or hydrocarbon and an auxiliary combustible gas such as O ₂ . For the purpose of obtaining a SiO ₂ of high purity, a combination of SiCl ₄ and _{H 2} and _{O 2} is preferred. An inert gas such as nitrogen (N ₂ ), argon (Ar), helium (He), neon (Ne), carbon dioxide (CO ₂ ) or the like may be added to the source gas or the like for the purpose of adjusting the gas concentration. it can. In order to stabilize the shape of the flame, an inert gas flow can be formed around the source gas.

  The starting member is a target that is the foundation of the soot deposit. The starting member can be formed from a heat resistant material such as quartz glass or platinum (Pt). In the VAD method, deposition of glass fine particles is started from the tip of a rod-shaped starting member, and a soot deposit is grown in a radial direction with respect to the central axis of the starting member and on an extension line of the central axis. The arrangement of the central axis of the starting member may be a vertical direction along the gravity (up and down direction), a horizontal direction perpendicular to the gravity, or an oblique direction. In the case of the VAD method, the vertical direction is generally used.

  It is preferable to rotate the starting member around the central axis in order to make the thickness at which the glass particles accumulate in the circumferential direction of the starting member uniform. In order to grow the soot deposit on the extension line of the central axis, it is preferable to move one or both of the burner and the starting member in the length direction of the starting member so that the distance between the two increases. Also, when the soot deposit grows in the radial direction of the starting member, one or both of the burner and the starting member can be moved in the radial direction of the starting member so that the distance between the two increases. Examples of the shape of the starting member include a solid columnar shape, a hollow cylindrical shape, and other rod shapes, but are not particularly limited thereto.

Between the soot deposition step and the F diffusion step, a dehydration step of heating the soot deposit in an atmosphere containing a dehydration gas can be provided. Examples of the dehydrating gas include a halogen gas such as Cl _{2 and} a halogen compound gas such as SOCl ₂ , POCl ₃ , and CCl ₄ . The atmosphere containing the dehydrating gas is not particularly limited, but an inert gas atmosphere such as Ar, He, or Ne is preferable. By performing the dehydration step, OH groups (Si—OH) contained in the SiO ₂ glass can be removed. The dehydration step before the F diffusion step can be omitted.

  The F diffusion step is a step of diffusing the F element from the side surface of the soot deposit. FIG. 1 illustrates a schematic diagram of the F diffusion step. The soot deposit 1 is suspended downward from the support member 2. The shape of the soot deposit 1 is substantially cylindrical. It is preferable that the side surface 3 has a cylindrical shape and the distance from the central axis is substantially constant. The shape of the upper part 4a and the lower part 4d of the soot deposit 1 is not particularly limited, and may be a planar shape, a convex shape, a concave shape, or the like. It is also possible to appropriately adjust the shape of the soot deposit 1 produced in the soot deposition process. Note that FIG. 1 is a schematic diagram, and a dimensional ratio such as a diameter and a length and a shape may be different from actual ones.

The soot deposit 1 is supported by a support member 2. In the illustrated example, the tip of the support member 2 is slightly buried in the upper portion 4a of the soot deposit 1, but below the support member 2, the SiO ₂ fine particles extend over the entire radial cross section from the side surface 3 to the center. including. Here, the radial cross section means a cross section perpendicular to the cylindrical central axis of the side surface 3, and if the central axis is the vertical direction, the radial cross section is the horizontal direction. As the support member 2, a starting member (target) used at the time of producing the soot deposit 1 may be used.

In the F diffusion step, an atmosphere containing an F-containing source gas is provided around the soot deposit 1. The F-containing source gas is a gas containing an F element, and examples thereof include fluorine (F) compounds such as SiF ₄ , SF ₆ , CF ₄ , CF ₂ Cl ₂ , and C ₂ F ₆ . Atmospheric gas _can also include N 2, Ar, He, Ne , an inert gas such as CO _2. Although the density | concentration of F containing source gas in atmospheric gas can be set suitably, about several percent is mentioned, for example.

  It is preferable to heat the soot deposit 1 during the F diffusion step. The heating temperature in the F diffusion step is preferably lower than the temperature at which the glass fine particles in the soot deposit 1 are melted or sintered. Depending on the temperature conditions, it is possible not only to circulate the F-containing source gas in the atmospheric gas between the glass fine particles but also to penetrate into the glass fine particles. Although the heating temperature in F diffusion process is not specifically limited, For example, it is 1000-1400 degreeC.

  During the F diffusion step, a flow can be provided in the atmospheric gas around the soot deposit 1. Examples of the flow direction include upward or downward in the vertical direction, inward or outward in the radial direction, clockwise or counterclockwise in the circumferential direction, or a direction formed by combining two or more of these. The speed of the flow can be set as appropriate including the windless state. The F element diffuses when the F-containing source gas passes through the porous space in the soot deposit 1 or permeates into the glass fine particles constituting the soot deposit 1.

  When the F element 5 is diffused into the soot deposit 1 in the F diffusion step, it is preferable that the F element 5 gradually diffuses from the side surface 3 of the soot deposit 1 toward the center as shown in FIG. . Thereby, it is possible to configure a concentration distribution in which the F concentration increases from the central portion of the soot deposit 1 toward the side surface 3 and closer to the side surface 3. When the F element 5 reaches the center of the soot deposit 1, it is preferable to end the F diffusion process or start the sintering process. Thereby, addition of F element can be suppressed in the central part of the soot deposit 1.

  The sintering process is a process of sintering the soot deposit through the F diffusion process. In the sintering process, the soot deposit is heated so that the glass fine particles are integrated while reducing the gap between them. Finally, a transparent glass body is obtained. Although the heating temperature in a sintering process is not specifically limited, For example, it is 1200-1600 degreeC.

  The heating device for the soot deposit is not particularly limited, and examples thereof include a heating furnace such as a soaking furnace and a gradient furnace.

  The soaking furnace is a furnace that heats the entire soot deposit. According to the soaking furnace, the soot deposit can be kept still and heated uniformly to allow the soot sintering to proceed as a whole. The soaking furnace is preferably used when a relatively small soot deposit is sintered because the configuration of the apparatus can be further simplified.

  The inclined furnace is a furnace that heats a part of the length of the soot deposit in a strip shape. While the soot deposit is gradually moved (traversed) along the length direction, the soot is sequentially heated from one end to the other end, and as a result, the soot is sintered sequentially from one end to the other end. According to the inclined furnace, even if unnecessary gas stays in the soot deposit, the gas can be smoothly removed by moving to an unsintered part during sintering. When sintering a relatively large soot deposit, it is preferable to use an inclined furnace in order to suppress defects such as bubbles.

  Whichever heating apparatus is used, it is preferable that there is little unevenness in heating in the circumferential direction of the cylindrical soot deposit. When the heating elements of the heating device are arranged at intervals in the circumferential direction, it is preferable to provide a cylindrical core tube around the soot deposit and to install the heating element outside the core tube. As a result, the heat radiated from the heating element heats the core tube from the outside, and the radiant heat emitted from the core tube heats the soot deposit body accommodated inside the core tube. The amount of heat distribution in can be made more uniform. Examples of the material for the core tube include materials having high heat resistance such as carbon.

  The atmosphere surrounding the soot deposit in the sintering step is not particularly limited, but an inert gas atmosphere such as Ar, He, Ne or the like is preferable. In the sintering process, in order to suppress the desorption of fluorine (F) from the soot deposit, an F-containing source gas can be included in the atmospheric gas. In order to suppress an increase in the amount of F added to the soot deposit during the sintering step, an atmospheric gas containing no F-containing source gas can be used.

  By passing through the above-mentioned soot deposition process, F diffusion process, and sintering process, a transparent glass body having a refractive index distribution based on the radial F concentration distribution is obtained. The radial F concentration distribution in the glass body can have the same or the same tendency as the radial F concentration distribution imparted to the soot deposit by the F diffusion process. By adopting a configuration in which the F concentration in the glass is increased from the center of the glass body toward the outer peripheral portion, a cylindrical glass body having a GI type refractive index distribution is obtained. In addition, a glass body in which F is not added to the central portion of the glass body, which is the region having the highest refractive index, can be manufactured.

  FIG. 2 is a graph showing an example of F addition amount distribution and refractive index distribution in the radial direction of the glass body obtained by the sintering process. “Δ before F diffusion” represents the relative refractive index difference (Δ) of the glass before the F diffusion step, and “Δ after F diffusion” represents the relative refractive index difference (Δ) of the glass after the F diffusion step. “F addition amount” relatively represents the amount of F added to the glass by the F diffusion step.

  As shown in FIG. 2, at the center (central axis) in the radial direction of the glass body and the central portion that is the vicinity thereof, Δ after F diffusion tends to be high, and the amount of F added tends to be low. In addition, at the outer periphery (outer peripheral surface) in the radial direction of the glass body and the outer peripheral portion that is the vicinity thereof, Δ after F diffusion tends to be low and the F addition amount tends to be high. In the glass body, the region having the highest refractive index is not necessarily located on the center of the glass body, but is preferably present in the central portion including the vicinity thereof. In the glass body, the region having the lowest refractive index is not necessarily located on the outer periphery of the glass body, but is preferably present on the outer periphery including the vicinity thereof.

  The glass body obtained by the sintering process can be used as it is as a glass preform such as an optical fiber preform. After the sintering step, the outer peripheral portion of the glass body can be processed by grinding or the like and then used as a glass preform such as an optical fiber preform. Here, “the outer peripheral portion of the glass body” means the outer peripheral portion of the glass body itself.

  Moreover, after providing a glass layer further outside the glass body, it can also be used as a glass preform such as an optical fiber preform. Here, the “outside of the glass body” is not limited to the case where it is directly on the outer peripheral portion of the glass body, but also includes the case where another glass layer is provided on the outer peripheral portion of the glass body.

About processing of glass body, glass preform, optical fiber preform, etc., selected from inspection process, storage process, stretching process, cutting process, grinding process, polishing process, processing process, cleaning process, heat treatment process, etc. 1 or Two or more steps can also be provided. The composition of the obtained glass body is preferably in the range of F-added SiO ₂ to pure SiO ₂ , and preferably contains no Ge, P, Al, or Ti as a dopant. The dopant (excluding inevitable impurities) significantly contained in the obtained glass body is preferably F alone, or F and Cl.

The refractive index distribution of the glass body can be adjusted by grinding the outer periphery of the glass body after the sintering step.
As one aspect of grinding the outer peripheral portion of the glass body, when a region deviating from the GI type refractive index distribution exists in the outer peripheral portion of the glass body, the region where the refractive index deviates from the GI type may be removed. Can be mentioned. The regions where the refractive index deviates from the GI type include the case where the refractive index is too low due to the excessive amount of F added at the outer peripheral portion, and the F element is desorbed from the outer peripheral portion of the soot deposit during the sintering process. For example, the amount of F added to the outer peripheral portion may be reduced, and the refractive index may be higher than the target.
As another aspect of grinding the outer peripheral portion of the glass body, by removing a part of the region having a high F concentration in the outer peripheral portion of the glass body, the refractive index in the outer peripheral portion is increased, and the region having the highest refractive index Adjustment of the refractive index difference with the region having the lowest refractive index can be mentioned.

  You may provide the process of providing glass layers, such as a clad, a trench layer, and an outer core, on the outer side of a glass body after a sintering process. In this case, the glass body obtained by the sintering process may constitute a part of the core of the optical fiber, may constitute the whole of the core of the optical fiber, or may comprise the whole of the core of the optical fiber and the cladding. You may comprise a part.

  The method for applying the glass layer to the outside of the glass body obtained by the sintering step is not particularly limited, and a known method can be adopted. The jacket method is a method in which a glass tube is put on the outside of a glass body, that is, after a rod-shaped glass body is inserted into a cavity inside the glass tube, the glass body and the glass tube are integrated by heating.

  As in the OVD method, soot is deposited on the outside of a rod-shaped glass body, and the soot is sintered while leaving the glass body, so that a glass layer derived from the soot is formed on the outer periphery of the glass body. Can be granted. As a method of adding a dopant such as F to a glass layer derived from soot, when depositing soot, a method of depositing glass fine particles containing F from a source gas containing F-containing source gas, or after depositing soot And a method of diffusing the F element in the soot.

The refractive index of the glass layer applied to the outside of the glass body is preferably lower than the refractive index of the glass body obtained by the sintering process. Examples of the glass constituting the clad include low refractive index glass made of F-added SiO ₂ or the like. The clad may be composed of two or more layers having different refractive indexes and compositions.

You may provide the process of providing a trench structure on the outer side of a glass body after a sintering process. The trench structure is a structure in which a layer having a refractive index lower than that of a normal clad is provided concentrically with the clad outside the core of the optical fiber, and the refractive index distribution is grooved. For example, three or more clad layers are formed concentrically, and the refractive index of the second clad layer from the inside, the second clad layer from the outside, or the intermediate clad layer between them is set to be the innermost clad layer and the most clad layer. By making it lower than the refractive index of the outer cladding layer, a trench structure can be formed. Examples of the glass constituting the trench structure include low refractive index glass made of F-added SiO ₂ or the like.

  You may provide the process of providing a depressed structure on the outer side of a glass body after a sintering process. In the depressed structure, a layer having a lower refractive index than the outer cladding is provided on the outermost periphery of the core concentrically with the cladding, and the relative refractive index difference between the core and the depressed portion is determined by the relative refractive index between the normal core and the cladding. The structure is larger than the rate difference. In some cases, a ring core layer having a refractive index higher than that of the cladding is provided outside the depressed portion.

  A glass body obtained by sintering, or a glass base material including at least a part thereof can be used as an optical fiber base material. An optical fiber can be manufactured by drawing an optical fiber preform.

  In the drawing process of the optical fiber, the axial direction of the optical fiber preform is arranged up and down, and the lower part of the optical fiber preform is pulled downward in a state of being melted by heating. It can be pulled out. The drawn glass fiber is gradually cooled in the air during drawing and then wound on a bobbin or the like. In order to protect the glass fiber, one or two or more coating layers such as a resin can be provided on the outer periphery of the glass fiber before being wound. Examples of the resin include ultraviolet (UV) curable resins such as various acrylates.

  Regarding the GRIN lens fiber, a graph of the refractive index distribution is illustrated in FIG. FIG. 3B illustrates the appearance of the GRIN lens fiber. As shown in FIG. 3A, the GRIN lens fiber 10 shown in FIG. 3B has a core 11 having a GI type refractive index profile. When the diameter of the core 11 is 2r, the GI type refractive index distribution is distributed in the range of −r to + r as shown in FIG.

  The GRIN lens manufacturing method may include a step of cutting an optical fiber manufactured by drawing an optical fiber preform into a predetermined length. The GRIN lens is not limited to being manufactured from an optical fiber having only a core, but can also be manufactured from an optical fiber having a core and a cladding. A cylindrical casing made of metal, resin, or the like can be fitted on the outer periphery of the GRIN lens.

The GI type refractive index profile typically follows an α power distribution. The α power distribution means that the maximum refractive index at the center is n ₁ , the minimum refractive index at the outer periphery is n ₂ , the distance from the center in the radial direction is r, the radius is a, the shape factor of the refractive index distribution is α, and the relative refractive index difference Is a refractive index distribution in which the refractive index n (r) at a distance r (where 0 ≦ r ≦ a) can be normalized by the following formula 1.

n (r) = n ₁ [1-2Δ (r / a) ^α ] ^1/2 (Equation 1)

The relative refractive index difference Δ at the center (refractive index n ₁ ) with respect to the outer periphery (refractive index n ₂ ) is defined by the following formula 2.

Δ = (n ₁ ² −n ₂ ² ) / 2n ₁ ² (Equation 2)

According to Equation 1, n (0) = n ₁ when r = 0, and n (a) = n ₂ when r = a. For reference, the refractive index distribution parameter g described in JIS C 6820 (General rules for optical fibers) is expressed as δ () with respect to the refractive index n (X) at X when X = r / a. X) = refers to ^{1-X g} and parameters g when normalized. Here, the normalized refractive index distribution δ (X) is defined as the following Expression 3.

δ (X) = [n (X) −n (1)] / [n (0) −n (1)] (Equation 3)

  In Equation 3, X represents a radial position. According to the normalization by δ (X), X = 0 and δ (0) = 1 at the center and X = 1 and δ (1) = 0 at the outer periphery. In JIS C 6820, g of the GI type optical fiber is defined as 1 ≦ g <3. In the present embodiment, as the expression of the GI type refractive index distribution, the above-described Expression 1, Expression 3, or an expression similar to this can be used. The parameter value of the GI type refractive index distribution in the present embodiment is preferably α = 2, and examples include 1.5 ≦ α ≦ 2.5.

  In the F diffusion step, the F element is diffused from the side surface of the soot deposit, and a refractive index distribution of about α = 2 can be obtained at the time when the F element reaches the center of the soot deposit. If F diffusion is terminated before the F element diffused from the side surface reaches the central portion of the soot deposit, α> 2 tends to occur. If F diffusion that has diffused from the side surface reaches the central portion of the soot deposit, the F tends to become α <2.

  The length of the GRIN lens can be appropriately adjusted according to the pitch of the GRIN lens. When a light beam propagating through the GRIN lens travels along a sinusoidal optical path with respect to the central axis of the GRIN lens, the period of the sinusoidal optical path is called one pitch. The length when the GRIN lens is used as a collimating lens is preferably a length represented by (2n + 1) / 4 pitch, such as about 0.25 pitch or about 0.75 pitch (where n is an integer of 0 or more) And).

  Regarding the GI type optical fiber, FIG. 4A illustrates a graph of the refractive index distribution. FIG. 4B illustrates the appearance of a GI type optical fiber. A GI type optical fiber 20 shown in FIG. 4B has a core 21 having a GI type refractive index distribution and a clad 22 having a refractive index lower than that of the core 21. When the diameter of the core 21 is 2r, the GI type refractive index distribution is distributed in a range from −r to + r as shown in FIG. The clad 22 corresponds to a range where the radial position is smaller than −r or larger than + r. The clad 22 generally has a lower refractive index than the core 21. The refractive index of the cladding 22 may be substantially constant. Examples of the value include the value of the minimum refractive index of the core 21. The clad 22 may have two or more regions having different refractive indexes.

  As the GI type optical fiber, a multimode fiber (MMF) having two or more propagation modes is generally used. Among them, a fusing mode fiber (FMF) having a few propagation modes (Few) such that the number of propagation modes is 2 to 4 or slightly more than that is compared with a single mode fiber (SMF) having only one propagation mode. It is preferable because the amount of information can be increased by mode division multiplexing (MDM). About the diameter (core diameter) of the core 21, the core diameter of MMF is about 50-100 micrometers, for example, The core diameter of FMF is about 10-30 micrometers, for example. An optical fiber 20 shown in FIG. 4B is a single core fiber having one core 21 in one clad 22. Examples of other optical fibers include multi-core fibers having two or more cores in one clad. Although the diameter (clad diameter) in the outer periphery of the clad 22 is not specifically limited, For example, 80-1000 micrometers is mentioned.

  As mentioned above, although this invention has been demonstrated based on suitable embodiment, this invention is not limited to the above-mentioned embodiment, A various change is possible in the range which does not deviate from the summary of this invention.

  In the soot deposition step, it is preferable to produce a soot deposit having a cylindrical side surface by the VAD method, but a soot deposit with a rod-like starting member remaining in the center can also be produced by the OVD method. In this case, it is preferable to perform the F diffusion step and the sintering step while leaving the starting member in order to suppress the desorption of the F element of the dopant from the center in the F diffusion step and the sintering step.

According to the manufacturing method of the optical fiber preform of the present embodiment, since the soot deposit is not sintered, there is no solidification step, so that the refractive index is increased or decreased at the center of the optical fiber preform. Abnormality of the refractive index distribution can be suppressed. Further, even when a SiO ₂ glass having a large amount of dopant such as F and having a relatively low viscosity is sintered, since there is no solidification step, eccentricity and non-circularity can be suppressed. Therefore, it is possible to reduce the deterioration of optical characteristics such as optical fibers and lenses. As a result of the F diffusion step, a refractive index profile near α = 2 can be easily obtained. In addition, by expanding the radial cross-sectional area of the soot deposit, the optical fiber preform can be easily enlarged and the productivity can be improved.

  According to the manufacturing method of this embodiment, it is possible to manufacture a glass preform, an optical fiber preform, an optical fiber, a lens, optical glass, an optical component, and the like. These glass and optical products can be suitably used for optical communication, optical fiber laser, optical measurement, and the like. Applications of the optical fiber laser include various uses such as communication, processing, measurement, illumination, sensor, and heating. The wavelength of light propagating through the optical fiber and lens can be selected from infrared, visible light, and the like.

  Hereinafter, the present invention will be specifically described with reference to examples.

Example 1 (GRIN lens fiber)
Pure SiO ₂ soot was deposited in a cylindrical shape by the VAD method to produce a soot deposit with a uniform particle size. The soot deposit was heated to 1000 ° C. in an SOCl ₂ atmosphere and held for 3 hours to perform a dehydration step of removing OH groups contained in the SiO ₂ soot. The soot deposit subjected to the dehydration step was heated to 1200 ° C. in a SiF ₄ atmosphere and held for 11 h, thereby performing an F diffusion step of diffusing the F element into the SiO ₂ soot. In this F diffusion step, the F element diffuses into the SiO ₂ soot, and a concentration distribution of the F element is generated in the radial direction of the soot deposit. The soot deposit through the F diffusion step was heated at 1400 ° C. to perform a sintering step of sintering SiO ₂ soot containing F element. By grinding the outer peripheral portion of the SiO ₂ glass body containing F element obtained by sintering, an optical fiber preform having a target refractive index of Δ = 0.1% was manufactured. The optical fiber preform was drawn to produce a GRIN lens fiber with α = 2.

Example 2 (GI type FMF having a trench structure)
Pure SiO ₂ soot was deposited in a cylindrical shape by the VAD method to produce a soot deposit with a uniform particle size. The soot deposit was heated to 1000 ° C. in an SOCl ₂ atmosphere and held for 3 hours to perform a dehydration step of removing OH groups contained in the SiO ₂ soot. The soot deposit subjected to the dehydration step was heated to 1200 ° C. in a SiF ₄ atmosphere and held for 11 h, thereby performing an F diffusion step of diffusing the F element into the SiO ₂ soot. In this F diffusion step, the F element diffuses into the SiO ₂ soot, and a concentration distribution of the F element is generated in the radial direction of the soot deposit. The soot deposit through the F diffusion step was heated at 1400 ° C. to perform a sintering step of sintering SiO ₂ soot containing F element. By grinding the outer peripheral portion of the SiO ₂ glass body containing F element obtained by sintering, a core rod having a refractive index adjusted to Δ suitable for the core of the GI type FMF was produced.

An inner cladding layer made of F-added SiO ₂ was formed on the outer periphery of the core rod by the OVD method. Next, a trench layer made of F-added SiO ₂ having a larger amount of F addition than the inner cladding layer was formed on the inner cladding layer by an external method. Next, an outer cladding layer made of F-added SiO ₂ having the same amount of F addition as the inner cladding layer was formed on the trench layer by an external method. A GI type FMF having a trench structure was manufactured by drawing an optical fiber preform composed of a core rod, an inner cladding layer, a trench layer, and an outer cladding layer.

Measurement example 1 (GI-type refractive index distribution)
FIG. 5 shows an example of the result of measuring the refractive index distribution of a glass body having a GI type refractive index distribution through the soot deposition process, the F diffusion process, and the sintering process, as in this example. The horizontal axis of the refractive index distribution is a radius, and the vertical axis is a relative refractive index difference (Δ). In FIG. 5, “upper side” indicates a refractive index distribution in a cross section corresponding to the upper side 3 a of the side surface 3 of the soot deposit 1 in FIG. 1. Similarly, “intermediate” indicates a refractive index distribution in a cross section corresponding to the intermediate 3 b of the side surface 3, and “lower” indicates a refractive index distribution in a cross section corresponding to the lower side 3 c of the side surface 3. In the “upper side”, α was slightly larger than 2, and in the “lower side”, α was slightly smaller than 2. However, in the “middle”, a refractive index distribution of α = 2 was obtained. Further, in any of the “upper side”, “middle” and “lower side”, no refractive index anomaly such as a center dip occurred.

Comparative Example 1 (CVD method)
After adding F-added SiO ₂ on the inner surface of the quartz glass tube using the CVD method, an F-added SiO ₂ glass base material was produced by a method of solidifying (collapse) the cavity at the center. FIG. 6 shows the refractive index distribution. At the center of the F-added SiO ₂ glass layer 101 formed inside the quartz glass tube 102, an abnormal refractive index portion 103 due to center dip was observed. In this refractive index anomalous portion 103, Δ in the central portion increases as compared with Δ in the quartz glass tube 102 to which no F element is added. This is considered to be because the F element of the dopant was desorbed at the central portion close to the cavity during solidification.

DESCRIPTION OF SYMBOLS 1 ... Soot deposit body, 2 ... Support member, 3 ... Side surface, 3a ... Upper side surface, 3b ... Middle of side surface, 3c ... Lower side surface, 4a ... Upper part, 4d ... Lower part, 5 ... F element, 10 ... GRIN Lens fiber, 11 ... core, 20 ... GI type optical fiber, 21 ... core, 22 ... cladding.

Claims (12)

A soot deposition step of obtaining a soot deposit having a cylindrical side surface by depositing SiO ₂ fine particles;
F diffusion step of diffusing F element from the side surface of the soot deposit,
A sintering step of sintering the soot deposit through the F diffusion step;
And a glass body obtained through the soot deposition step, the F diffusion step, and the sintering step has a GI type refractive index distribution. The method for producing an optical fiber preform according to claim 1, wherein the SiO ₂ fine particles are made of pure SiO ₂ . 3. The method of manufacturing an optical fiber preform according to claim 1, wherein the soot deposit has a portion containing the SiO ₂ fine particles over the entire radial cross section from the side surface to the central portion.   The light according to any one of claims 1 to 3, further comprising a dehydration step of heating the soot deposit body in an atmosphere including a dehydration gas between the soot deposition step and the F diffusion step. Manufacturing method of fiber preform.   F is not added to a region having the highest refractive index in the GI-type refractive index distribution, and the F concentration in the glass increases from the center to the outer periphery of the glass body. Item 5. The method for producing an optical fiber preform according to any one of Items 1 to 4.   The optical fiber preform according to any one of claims 1 to 5, wherein the F concentration increases from a central portion of the soot deposit through the F diffusion step toward the side surface. Method.   7. The F diffusion step, wherein the F element is diffused into the soot deposit, and the sintering step is started when the F element reaches the center from the side surface. The manufacturing method of the optical fiber preform of any one of Claims 1.   The method according to any one of claims 1 to 7, further comprising a step of adjusting a refractive index distribution of the glass body by grinding an outer peripheral portion of the glass body after the sintering step. Manufacturing method of optical fiber preform. After the sintering step, the outside of the glass body, according to any one of claims 1-8, characterized in that it comprises a step of applying a cladding made of F-doped SiO ₂ optical fiber preform Production method. After the sintering step, the outside of the glass body, an optical fiber preform according to any one of claims 1-9, characterized in that it comprises a step of applying a trench structure consisting of F added SiO ₂ Manufacturing method. A step of producing an optical fiber preform by the method of producing an optical fiber preform according to any one of claims 1 to 10,
Drawing the optical fiber preform;
The manufacturing method of the optical fiber which has this. A step of producing an optical fiber preform by the method of producing an optical fiber preform according to any one of claims 1 to 10,
Drawing the optical fiber preform;
Cutting the optical fiber obtained by drawing into a lens length;
The manufacturing method of the lens which has this.

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