WO2014034774A1 - Optical fiber, method for manufacturing optical fiber, and method for manufacturing optical fiber preform - Google Patents

Abstract

This optical fiber is provided with a core (31), a cladding part (32) surrounding the periphery of the core and having a dopant-added region (32a) to which a dopant is added, and a plurality of vacancies (33) provided in the cladding part parallel to the core and concentrically with the core. When the diameter of the vacancies is defined as d₁, the relative refractive index differences of the dopant-added region with respect to pure quartz glass in an inside portion close to the core and an outside portion distant from the core at a boundary between the vacancies and the dopant-added region are defined as Δr_in and Δr_out, respectively, and (Δr_out - Δr_in)/d₁ is defined as the percent of slope k of a refractive index profile in the dopant-added region, the vacancies are provided in a region of the dopant-added region where the percent of slope k is -0.05%/µm to 0.05%/µm. This optical fiber makes it possible to suppress positional misalignment of the vacancies.

Description

Optical fiber, optical fiber manufacturing method, and optical fiber preform manufacturing method

The present invention relates to an optical fiber, an optical fiber manufacturing method, and an optical fiber preform manufacturing method.
This application claims priority based on Japanese Patent Application No. 2012-192596 for which it applied on August 31, 2012, and uses the content here.

An optical fiber with holes is an optical fiber having a plurality of continuous holes in the waveguide direction, and it is known that the holes can provide optical characteristics that cannot be realized with conventional optical fibers. Such a holey optical fiber is also called a holey fiber (HF). Known types of waveguide structures of holey fibers include photonic crystal fibers (Photonic Crystal Fiber, PCF) and hole assist fibers (Hole-Assisted Fiber, HAF).

An optical fiber with holes is obtained by drawing an optical fiber preform (preform with holes) provided with holes.
The preform with holes is obtained, for example, by a drilling method such as a drilling method in which a hole is drilled by drilling an optical fiber preform (see, for example, Patent Documents 1 to 3).
Furthermore, since the optical fiber with holes affects the loss due to the roughness of the inner surface of the holes and the degree of cleaning in the holes, the smoothing process, dehydration process, and etching process in the holes are performed at the stage of the holed preform. Then, in order to prevent water and impurities from being mixed into the pores until the drawing process, the pore ends are sealed. Drawing of the optical fiber with holes is performed while pressurizing the inside of the holes and controlling the pressure from one end of the preform with holes. Thereby, the optical fiber with a hole which has a desired hole diameter is obtained (for example, refer patent document 1).

Japanese Unexamined Patent Publication No. 2002-293562 Japanese Unexamined Patent Publication No. 2003-206149 Japanese Unexamined Patent Publication No. 2010-173917

In a preform with holes, it is known that the position of holes in a cross section perpendicular to the longitudinal direction greatly affects the optical characteristics of an optical fiber formed from the preform. Therefore, in order to manufacture a holey optical fiber having desired optical characteristics, the cross section of the optical fiber preform is at a predetermined appropriate position and perpendicular to the longitudinal direction of the optical fiber preform. In addition, it is necessary to form a through hole.
In the step of forming a through hole in the optical fiber preform, a through hole is formed along the axial direction of the optical fiber preform by rotating a drill tool that is a drilling tool by drilling with a drill.
A drilling device used for drilling is roughly constituted by a spindle of a rotating body, a drill tool used for drilling, and a gripping part that grips an optical fiber preform. The drill tool also includes a hollow pipe and a grindstone provided at the tip of the hollow pipe to which cylindrical diamond abrasive grains are attached. The outer diameter of the grindstone is determined according to the outer diameter of the optical fiber preform, the designed outer diameter of the hole of the fiber formed from the optical fiber preform, and the outer diameter of the optical fiber. 15 mm.

In order to improve the productivity and cost reduction of holey optical fibers, it is necessary to make longer holey preforms, and the length of the optical fiber material and the length of the drill tool must also be increased. . However, since quartz is a brittle material, it is hard but very brittle, and is a fragile material, so there is a limit to the processing length of quartz. For example, the current drilling apparatus has a limit of a processing length of about 200 mm in the case of drilling with a diameter of 2 mm and a length of about 400 mm in the case of drilling with a diameter of 4 mm (Patent Documents 2 and 3). The reason why the hole length is limited is that the smaller the hole diameter, the more rigid the drill tool used for drilling, for example, a pipe-shaped drill tool with diamond particles embedded at the tip. If the processing length is long, the straightness of the hole is lost, and the design of the hole position in the cross section of the optical fiber is shifted, so that the processing accuracy cannot be maintained.

Further, in Patent Document 3, in order to reduce the cost of an optical fiber with a hole, first, a single short optical fiber preform is drilled by a drill method, and then the number of drilled preforms is several. Next, each glass substrate is stretched and cut, and then quartz glass is deposited on the outside of each stretched and cut hole base material by a jacket method, OVD method, or the like to produce several preforms with holes. Finally, a method is described in which a dummy quartz tube is welded to each of the preforms with holes, and spinning is performed while pressurizing the inside of the holes.
However, in this method, after the hole opening process, a step of stretching and cutting the hole base material and a step of welding the respective preforms with holes and the dummy quartz tube are required. It seems that it is difficult to lower the cost than the manufacturing method of hole opening and spinning.

In order to reduce the processing gap of holes in such an optical fiber preform, it is necessary to optimize the processing conditions. However, only by optimizing the processing conditions, the positional deviation of the through hole does not become zero in the longitudinal direction of the optical fiber preform. For example, when a positional deviation of the through hole occurs in the longitudinal direction of the optical fiber preform, the hole position is far from the desired position on the end face of processing of the optical fiber preform forming the through hole, and the desired optical characteristics are obtained. It cannot be obtained, and the production yield in the drilling process is deteriorated.

The present invention has been made in view of the above circumstances, and an optical fiber, a method of manufacturing an optical fiber, and an optical fiber, in which a positional deviation of the holes is suppressed in a cross section perpendicular to the longitudinal direction of the optical fiber with holes. It aims at providing the manufacturing method of a base material.

In order to solve the above problems, an optical fiber according to a first aspect of the present invention includes a core, a clad portion surrounding a periphery of the core and having a dopant added region to which a dopant is added, and the core in the clad portion. And a plurality of holes provided concentrically around the core. In the optical fiber of the first aspect of the present invention, the diameter of the hole is defined as d _1, and the pure silica glass in the inner part near the core in the boundary part between the hole and the dopant addition region The relative refractive index difference of the dopant-added region with respect to is defined as Δr _in, and the ratio of the dopant-added region to pure quartz glass in the outer portion far from the core in the boundary between the vacancies and the dopant-added region When the refractive index difference is defined as Δr _out and (Δr _out −Δr _in ) / d ₁ is defined as the gradient k of the refractive index profile in the dopant addition region, the vacancies are inclined in the dopant addition region. The rate k is provided in a region within a predetermined range.

In the optical fiber according to the first aspect of the present invention, it is preferable that the gradient k is −0.05% / μm or more and 0.05% / μm or less.

The optical fiber manufacturing method according to the second aspect of the present invention includes a core, a clad portion surrounding the core and having a dopant-added region to which a dopant is added, and parallel to the core in the clad portion and An optical fiber including a plurality of holes provided on a concentric circle with a core as a center is manufactured. In this optical fiber, the diameter of the hole is defined as d _1, and the ratio of the dopant-added region to the pure silica glass in the inner portion close to the core in the boundary portion between the hole and the dopant-added region. A refractive index difference is defined as Δr _in, and a relative refractive index difference of the dopant added region with respect to pure quartz glass in an outer portion far from the core in a boundary portion between the hole and the dopant added region is Δr _out . When (Δr _out −Δr _in ) / d ₁ is defined as the gradient k of the refractive index profile in the dopant addition region, the vacancies in the dopant addition region have the gradient k within a predetermined range. It is provided in the area. An optical fiber manufacturing method according to the second aspect of the present invention for manufacturing such an optical fiber includes an optical fiber mother including a core and a cladding portion surrounding the core and having a dopant added region to which a dopant is added. A material is manufactured (optical fiber preform manufacturing process), and a plurality of holes are formed on the concentric circle centered on the core in the dopant addition region (hole forming process), and an inner surface of the hole is formed. Smoothing (hole smoothing step), producing an optical fiber preform with holes, and drawing the optical fiber preform with holes while drawing the inside of the holes (drawing step). In the formation of the plurality of holes (hole forming step), the diameter of the hole formed in the dopant added region of the cladding part is defined as d _1, and among the boundary part between the hole and the dopant added region , in the inner portion closer to the core, the relative refractive index difference of the dopant doped region for pure silica glass is defined as [Delta] r _in, among the boundary portion between the air hole and the dopant doped region, farther outer portion from said core Is defined as Δr _out, and (Δr _out −Δr _in ) / d ₁ is defined as the gradient K of the refractive index profile in the dopant added region, The holes are formed in a region where the gradient K is within a predetermined range in the dopant addition region.

In the optical fiber manufacturing method according to the second aspect of the present invention, the gradient k is −0.05% / μm or more and 0.05% / μm or less, and the gradient K is −0.07% / mm or more and 0. 0.07% / mm or less is preferable.

The method for manufacturing an optical fiber preform according to the third aspect of the present invention includes a core, a clad portion that surrounds the core and includes a dopant-added region to which a dopant is added, and is parallel to the core in the clad portion. In addition, an optical fiber preform including a plurality of holes provided on the concentric circle centered on the core and provided in the dopant addition region is manufactured. According to a third aspect of the present invention, there is provided a method of manufacturing an optical fiber preform including an optical fiber preform including a core and a cladding portion surrounding the core and having a dopant added region to which a dopant is added (optical fiber). Manufacturing process of base material), forming a plurality of holes on a concentric circle centered on the core in the dopant addition region (hole forming process), and smoothing the inner surface of the holes (hole smoothing) Process). In the formation of the plurality of holes (hole forming step), the diameter of the hole formed in the dopant added region of the cladding part is defined as d _1, and among the boundary part between the hole and the dopant added region , in the inner portion closer to the core, the relative refractive index difference of the dopant doped region for pure silica glass is defined as [Delta] r _in, among the boundary portion between the air hole and the dopant doped region, farther outer portion from said core Is defined as Δr _out, and (Δr _out −Δr _in ) / d ₁ is defined as the gradient K of the refractive index profile in the dopant added region, The holes are formed in a region where the gradient K is within a predetermined range in the dopant addition region.

In the method for manufacturing an optical fiber preform according to the third aspect of the present invention, the inclination rate K is preferably −0.07% / mm or more and 0.07% / mm or less.

According to the first aspect, the second aspect, and the third aspect of the present invention, it is possible to provide an optical fiber in which the positional deviation of the holes is suppressed.

It is a schematic diagram showing an example of a case where drilling is performed with a drill in a region where the refractive index profile of the optical fiber preform is changed and the refractive index profile of the optical fiber preform is changed. FIG. 7 is a schematic diagram showing another example of drilling with a drill in a region where the refractive index profile of the optical fiber preform is changed and the refractive index profile of the optical fiber preform is changed. FIG. 7 is a schematic diagram showing another example of drilling with a drill in a region where the refractive index profile of the optical fiber preform is changed and the refractive index profile of the optical fiber preform is changed. It is a schematic sectional drawing which shows the optical fiber preform | base_material in one Embodiment of this invention. FIG. 5 is a schematic diagram showing an example of a case where drilling is performed with a drill in a region where the refractive index profile of the optical fiber preform changes in an area where the refractive index profile of the optical fiber preform changes in an embodiment of the present invention. . The schematic diagram which shows the refractive index profile of the optical fiber preform in one Embodiment of this invention, and shows the other example at the time of drilling by a drill in the area | region where the refractive index profile of this optical fiber preform is changing It is. It is a schematic sectional drawing which shows the optical fiber preform with a hole in one Embodiment of this invention. It is a schematic perspective view which shows an example of a drill tool. It is a schematic perspective view which shows a mode when a drill tool is chucked with a drill chuck. It is a schematic perspective view which shows the drilling process by a drill tool. It is a schematic sectional drawing which shows the optical fiber with a hole in one Embodiment of this invention. It is a schematic diagram which shows the measuring method of the hole position of an optical fiber preform with a hole, Comprising: It is a perspective view of an optical fiber preform with a hole. It is a schematic diagram which shows the measuring method of the hole position of an optical fiber preform with a hole, Comprising: It is a side view which shows the process start end surface of an optical fiber preform with a hole. It is a schematic diagram which shows the measuring method of the hole position of an optical fiber preform with a hole, Comprising: It is a side view which shows the process completion | finish end surface of an optical fiber preform with a hole. It is a graph which shows the relationship between the positional offset amount (DELTA) R of the hole of the optical fiber preform with a hole, and the gradient K of the refractive index profile of an optical fiber preform with a hole. It is a schematic diagram which shows the measuring method of the hole position of an optical fiber with a hole, Comprising: It is a perspective view of an optical fiber with a hole. It is a schematic diagram which shows the measuring method of the hole position of an optical fiber with a hole, Comprising: It is a side view which shows the process start end surface of an optical fiber with a hole. It is a schematic diagram which shows the measuring method of the hole position of an optical fiber with a hole, Comprising: It is a side view which shows the process end end surface of an optical fiber with a hole. It is a graph which shows the relationship between the positional offset amount (DELTA) r of the hole of an optical fiber with a hole, and the gradient k of the refractive index profile of an optical fiber with a hole. It is a graph which shows the relationship between the inclination rate K of the refractive index profile in an optical fiber preform with a hole, and the inclination rate k of the refractive index profile of an optical fiber with a hole.

DESCRIPTION OF EMBODIMENTS Embodiments of an optical fiber, an optical fiber manufacturing method, and an optical fiber preform manufacturing method of the present invention will be described.
Note that this embodiment is specifically described in order to better understand the gist of the invention, and the present invention is not limited unless otherwise specified.

As a result of intensive studies, the inventors have different refractive index profiles in the cross section perpendicular to the longitudinal direction of the optical fiber preform, so that the hole positions drilled are in the longitudinal direction of the optical fiber preform. It has been found that the refractive index profile tends to shift in an inclined region. That is, it has been found that there is a limit to improving the positional deviation of the holes only by optimizing the processing conditions.

Here, the relationship between the refractive index profile of the optical fiber preform and the processing gap of the holes in the optical fiber preform will be described.
For example, in an optical fiber preform having a refractive index profile as shown in FIG. 1, a region 2a in which the refractive index profile changes (region where the refractive index profile is not flat) 2a is formed in the cladding portion 2 surrounding the core 1. Exists. In this region 2a, as shown in FIG. 1, the relative refractive index difference Δ of the cladding portion 2 with respect to pure silica glass increases from the center side of the optical fiber preform toward the outside (in the radial direction). . When holes are formed in this region 2a by drilling with a drill, the positions of the holes in the longitudinal direction of the optical fiber preform are changed from a low refractive index side to a high refractive index side, that is, light. In the radial direction of the fiber preform, the optical fiber preform tends to be shifted outward from the center side of the optical fiber preform.
Here, when the refractive index of the additive-free pure quartz glass is defined as n ₂ and the refractive index of the portion of the cladding portion 2 where the dopant is added (dopant added region) is defined as n ₁ The relative refractive index difference Δ is defined by the following equation.

Further, in the optical fiber preform having the refractive index profile as shown in FIG. 2, a region (region where the refractive index profile is not flat) 12 a where the refractive index profile is changed is formed in the cladding portion 12 surrounding the core 11. Exists. In this region 12a, as shown in FIG. 2, the relative refractive index difference Δ of the cladding portion 12 with respect to pure silica glass decreases from the center side of the optical fiber preform toward the outside (in the radial direction). . When holes are formed in the region 12a by drilling with a drill, the positions of the holes are changed from the low refractive index side to the high refractive index side in the longitudinal direction of the optical fiber preform, that is, the light. In the radial direction of the fiber preform, it tends to shift to the center side from the outside of the optical fiber preform.
In the optical fiber preform having the refractive index profile as shown in FIG. 1 or FIG. 2, the reason that the region 2a in which the refractive index profile changes is present in the cladding portion 2 is to manufacture the optical fiber preform. At this time, the addition of fluorine or chlorine to the clad part 2 slightly lowered the refractive index of the clad part 2.

In contrast to the optical fiber preform having the refractive index profile as shown in FIG. 1 or FIG. 2, in the optical fiber preform having the refractive index profile as shown in FIG. The refractive index profile is flat. When the present inventors form holes in the clad portion 22 having a flat refractive index profile by drilling with a drill, the positions of the holes are perpendicular to the longitudinal direction of the optical fiber preform. I found out that

Hereinafter, a manufacturing method of an optical fiber preform having six holes using an embodiment of the present invention, a manufacturing method of an optical fiber, and an optical fiber manufactured by this manufacturing method will be specifically described. Here, an optical fiber preform and optical fiber having six holes are illustrated, but the present invention is not limited to this, and the number of holes and the arrangement of holes may be different.

[Method of manufacturing optical fiber preform]
An optical fiber preform 30 having a core 31 at the center as shown in FIG. 4 and a clad portion 32 around the core 31 is produced.
The core 31 is made of quartz glass to which a dopant such as germanium (Ge) is added. The clad portion 32 is made of quartz glass having a dopant added region to which a dopant such as fluorine (F) or chlorine (Cl) is added.
In the optical fiber preform 30, _{n 1} the refractive index of the core 31, and the refractive index of the cladding portion 32 and _{n 2,} satisfy the relationship of _n 2 <n _1.
Further, the optical fiber preform 30 has a refractive index profile as shown in FIG. 5 or 6, for example, and the refractive index profile is flat or slightly changed in the dopant addition region of the cladding portion 32 surrounding the core 31. The region 32a is present (the refractive index profile is not flat).

When the optical fiber preform 30 has a refractive index profile as shown in FIG. 5, the relative refractive index difference Δ of the dopant added region of the cladding portion 32 with respect to pure silica glass in the region 32 a is the optical fiber preform 30. Slightly increases from the center side toward the outside (in the radial direction).
Further, when the optical fiber preform 30 has a refractive index profile as shown in FIG. 6, in the region 32a, the relative refractive index difference Δ of the dopant added region of the cladding portion 32 with respect to pure silica glass is the optical fiber preform. It slightly decreases from the center side of the material 30 toward the outside (in the radial direction).

For the production of the optical fiber preform 30 having such a structure, a general optical fiber preform production method such as a VAD (Vapor Phase Axial Deposition) method or a CVD (Chemical Vapor Deposition) method can be applied. .

Next, in a region 32a where the refractive index profile in the dopant addition region of the cladding portion 32 is flat or slightly changed, six holes are formed at equal intervals on a concentric circle centered on the core 31 by drilling with a drill. Holes 33 are formed to obtain holey optical fiber preform 30A as shown in FIG.
A method for forming the holes 33 in the optical fiber preform 30 by drilling with a drill will be specifically described with reference to FIGS.
First, a drill tool for forming the hole 33 in the region 32a in which the refractive index profile is flat or slightly changed in the dopant addition region of the cladding part 32 is prepared, and used as a drill chuck of a drilling device (not shown). Chuck.

FIG. 8 is a schematic perspective view showing an example of a drill tool.
The drill tool 40 is schematically configured from a pipe 41 and a cutting portion 42 provided at the tip of the pipe 41 and having a through hole.
The pipe 41 has a cylindrical shape, and the outer diameter of the pipe 41 is slightly smaller than the diameter of the hole formed in the optical fiber preform 30.
Further, the through hole of the pipe 41 and the through hole of the cutting part 42 are connected to each other. Therefore, the drill tool 40 is formed with a through hole 43 including the through hole of the pipe 41 and the through hole of the cutting part 42.

FIG. 9 is a schematic perspective view showing a state when the drill tool 40 is chucked by the drill chuck 50.
The drill chuck 50 is generally configured from a through hole 51 for inserting the drill tool 40, a flange 52, and a shank 53 fixed to the flange 52.
The shank 53 is configured such that the size of the through hole 51 can be changed on the distal end side of the shank 53 (drill chuck 50) by a tightening mechanism (not shown). Therefore, by reducing the size of the through hole 51 by the tightening mechanism, the drill tool 40 inserted through the through hole 51 is chucked on the tip side of the shank 53.

Next, in the region 32a in which the refractive index profile is flat or slightly changed in the dopant addition region of the clad portion 32 by the drill tool 40, the core 31 is centered along the longitudinal direction of the optical fiber preform 30. Six holes 33 are formed on the concentric circles so as to be arranged at almost equal intervals.
FIG. 10 is a schematic perspective view showing drilling with a drill tool.
In this drilling process, the drill chuck 50 is advanced toward the clad portion 32 while rotating the drill chuck 50 by a rotating device (rotating means, not shown). Thereby, the drill tool 40 advances along the longitudinal direction of the clad part 32 (optical fiber preform 30) while the drill tool 40 rotates, and the hole 33 from the one end side of the optical fiber preform 30 to the clad part 32 is pushed. The formation of is started. At this time, liquid is introduced from the rear end side of the through hole 43 of the drill tool 40 (the side opposite to the end portion where the cutting portion 42 is provided). As this liquid, for example, a water-soluble cutting fluid is used.

The liquid introduced into the through hole 43 passes through the through hole 43 and flows out from the tip of the cutting part 42. In the state where the cutting part 42 has advanced to the middle of the cladding part 32, the holes 33 of the cladding part 32 do not penetrate, so the liquid flowing out from the tip of the cutting part 42 loses its place, and the cutting part 42 The air flows backward from the inner wall surface of the cladding portion 32 and is discharged from the surface of the hole 33 where the drill tool 40 is inserted (that is, one end side of the optical fiber preform 30). Therefore, until the air holes 33 penetrate the clad portion 32, the temperature of the clad portion 32 is prevented from excessively rising due to the friction between the cutting portion 42 and the clad portion 32. Further, glass waste generated by the perforation is discharged from the portion where the drill tool 40 is inserted into the clad portion 32 together with the liquid.

As described above, the drill tool 40 performs the drilling process on the clad part 32 to form the holes 33 penetrating the clad part 32 (optical fiber preform 30) in the longitudinal direction.
Similarly, by forming another plurality of holes 33 as shown in FIG. 7, the core 31 is formed in a region 32 a where the refractive index profile in the dopant addition region of the cladding portion 32 is flat or slightly changed. Six holes 33 are formed at substantially equal intervals on a concentric circle centered at.

In the present embodiment, when the optical fiber preform 30 has a refractive index profile as shown in FIG. 5 or FIG. 6, for example, the refractive index in the dopant addition region of the cladding portion 32 is as follows. Holes 33 are formed in the region 32a where the profile is flat or slightly changed.
In this case, the diameter of the hole formed in the dopant addition region of the cladding portion 32 is d ₁ , and the dopant addition to the pure quartz glass in the inner portion near the core 31 in the boundary portion between the hole 33 and the dopant addition region. The relative refractive index difference of the region is Δr _in , and the relative refractive index difference of the dopant added region with respect to pure quartz glass in the outer portion far from the core 31 in the boundary portion between the hole 33 and the dopant added region is Δr _out , (Δr _out −Δr _in ) / d ₁ is defined as a gradient K of the refractive index profile in the dopant addition region, a region in which the gradient K is within a predetermined range in the dopant addition region (the refractive index profile is flat or slightly changed). A hole 33 is formed in the region 32a).

The inclination rate K is preferably −0.07% / mm or more and 0.07% / mm or less, and more preferably −0.03% / mm or more and 0.03% / mm or less.
If the holes 33 are formed in a region where the gradient K in the dopant addition region of the cladding portion 32 is within a predetermined range, preferably in a region where the gradient K is within the above range, an optical fiber mother is formed by drilling with a drill. The holes 33 can be formed perpendicular to the longitudinal direction of the material 30 (the amount of positional deviation between the one end surface and the other end surface of the optical fiber preform 30 is small). That is, as shown in FIG. 5, in the region 32a, the relative refractive index difference Δ of the dopant-added region of the cladding portion 32 with respect to the core 31 increases from the center side of the optical fiber preform 30 toward the outside (in the radial direction). 6) or as shown in FIG. 6, in the region 32a, the relative refractive index difference Δ of the dopant-added region of the cladding portion 32 with respect to pure quartz glass is the optical fiber preform 30. Even in the case where it slightly decreases from the center side toward the outside (in the radial direction), the vacancy 33 is formed in a region where the gradient K in the dopant addition region of the cladding portion 32 is within a predetermined range. If formed, the holes 33 can be formed perpendicular to the longitudinal direction of the optical fiber preform 30 by drilling with a drill.

Next, by polishing or etching the inner surface of the hole 33 by a known method, the inner surface of the hole 33 is smoothed, and an optical fiber mother with a hole as shown in FIG. A material 30A is obtained.

[Optical fiber and manufacturing method thereof]
Next, a holey optical fiber 60 as shown in FIG. 11 is obtained by drawing the holey optical fiber preform 30A manufactured by the above method.
The method of drawing the holey optical fiber preform 30A is not particularly limited, and a known method is used.
Moreover, it is preferable to coat | cover the optical fiber 60 with a hole after drawing with various resin by a well-known method.

The holey optical fiber 60 has a core 61 at the center, a clad portion 62 around the core 61, and roughly includes six holes 63 provided in parallel with the core 61 in the clad portion 62. It is configured.
The six holes 63 are arranged at equal intervals on a concentric circle with the core 61 as the center.
The cladding 62 has a dopant-added region to which a dopant is added. In the region where the refractive index profile is flat or slightly changed in the dopant-added region of the cladding 62, the cladding 62 extends in the longitudinal direction of the optical fiber 60. The air holes 63 are provided so as to be arranged at substantially equal intervals on the concentric circles with the core 61 as the center.

In the holey optical fiber 60, the refractive index profile of the holey optical fiber preform 30A is maintained. Therefore, assuming that the refractive index of the core 61 is n ₁₁ and the refractive index of the cladding 62 is n ₁₂ , n ₁₂ < satisfy the relationship of n _11.

Further, in the holey optical fiber 60, the clad portion 62 has a dopant addition region to which a dopant is added, a hole 63 is provided in the dopant addition region, the diameter of the hole 63 is d ₁ , and the hole 63. Δr _in is the relative refractive index difference of the dopant addition region with respect to pure quartz glass in the inner portion near the core 61 among the boundary portion between the hole 63 and the dopant addition region. When the relative refractive index difference of the dopant addition region with respect to pure silica glass in the outer portion far from 61 is defined as Δr _out and (Δr _out −Δr _in ) / d ₁ is defined as the gradient k of the refractive index profile in the dopant addition region, The holes 63 are regions where the gradient k is in a predetermined range in the dopant addition region (the refractive index profile is flat or slightly changed). Area).

The inclination rate k is preferably −0.05% / μm or more and 0.05% / μm or less, and more preferably −0.04% / μm or more and 0.04% / μm or less.
If the hole 63 is provided in a region where the gradient k in the dopant addition region of the cladding 62 is within a predetermined range, preferably in a region where the gradient K is in the above range, the amount of positional deviation of the hole 63 The holey optical fiber 60 having a small wavelength shift amount (position displacement of the hole 63 between the one end face and the other end face of the holey optical fiber 60) and excellent wavelength dispersion characteristics can be obtained.

Although preferred embodiments of the present invention have been described and described above, it should be understood that these are exemplary of the present invention and should not be considered as limiting. Additions, omissions, substitutions, and other changes can be made without departing from the scope of the invention. Accordingly, the invention is not to be seen as limited by the foregoing description, but is limited by the scope of the claims.

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

[Example 1]
An optical fiber preform having a core at the center and a cladding around the core was produced.
The optical fiber preform had an outer diameter of 100 mm and a length of 1000 mm.
Moreover, the core was comprised from the quartz glass to which germanium (Ge) was added, and the clad part was comprised from the quartz glass which has the dopant addition area | region to which the fluorine (F) was added.
Next, six holes were formed at substantially equal intervals on a concentric circle centered on the core by drilling with a drill in the dopant addition region. At this time, the outer diameter of the tip of the drill tool is set to 3.0 mm, the drilling length is set to 1000 mm, a through hole having a diameter of 3.0 mm is formed along the longitudinal direction of the optical fiber preform, and a hole is provided. An optical fiber preform was obtained.
The diameter of the hole formed in the dopant-added region of the clad part is d ₁ , and the relative refractive index difference of the dopant-added region with respect to pure quartz glass in the inner part near the core in the boundary part between the hole and the dopant-added region is Δr _in , the relative refractive index difference of the dopant addition region with respect to pure quartz glass in the outer portion far from the core in the boundary between the hole and the dopant addition region is expressed as Δr _out , (Δr _out −Δr _in ) / d ₁ The gradient K was defined as the gradient K of the refractive index profile in the dopant addition region.
Here, the hole diameter d ₁ was set to 3.0 mm, the relative refractive index difference Δr _in was −0.0295%, and the relative refractive index difference Δr _out was −0.0066%. As a result, the gradient K of the refractive index profile of the obtained optical fiber preform with holes was 0.008% / mm.

[Comparative Example 1]
In the same manner as in Example 1, a holey optical fiber preform was obtained.
In the same manner as in Example 1, the gradient K of the refractive index profile of the obtained optical fiber preform with holes was calculated. Here, the hole diameter d ₁ was 3.0 mm, the relative refractive index difference Δr _in was −0.2825%, and the relative refractive index difference Δr _out was 0.0485%. As a result, the gradient K of the refractive index profile of the obtained optical fiber preform with holes was 0.11% / mm.

[Comparative Example 2]
In the same manner as in Example 1, a holey optical fiber preform was obtained.
In the same manner as in Example 1, the gradient K of the refractive index profile of the obtained optical fiber preform with holes was calculated. Here, the hole diameter d ₁ was 3.0 mm, the relative refractive index difference Δr _in was −0.5043%, and the relative refractive index difference Δr _out was 0.0572%. As a result, the gradient K of the refractive index profile of the obtained optical fiber preform with holes was 0.187% / mm.

[Comparative Example 3]
In the same manner as in Example 1, a holey optical fiber preform was obtained.
In the same manner as in Example 1, the gradient K of the refractive index profile of the obtained optical fiber preform with holes was calculated. Here, the hole diameter d ₁ was set to 3.0 mm, the relative refractive index difference Δr _in was −0.2810%, and the relative refractive index difference Δr _out was −0.5229%. As a result, the gradient K of the refractive index profile of the obtained optical fiber preform with holes was -0.081% / mm.

[Comparative Example 4]
In the same manner as in Example 1, a holey optical fiber preform was obtained.
In the same manner as in Example 1, the gradient K of the refractive index profile of the obtained optical fiber preform with holes was calculated. Here, the hole diameter d ₁ was 3.0 mm, the relative refractive index difference Δr _in was 0.0398%, and the relative refractive index difference Δr _out was −0.4182%. As a result, the gradient K of the refractive index profile of the obtained optical fiber preform with holes was −0.153% / mm.

[Evaluation]
With respect to Example 1 and Comparative Examples 1 to 4, the positional displacement amount of the holes was evaluated by the following method.
[Evaluation of hole misalignment by measuring hole position of holey optical fiber preform]
With respect to the holey optical fiber preforms obtained in Example 1 and Comparative Examples 1 to 4, as shown in FIGS. 12A to 12C, the machining start end face 101 and the machining end end face 102 of the holey optical fiber preform 100 are obtained. , The position of the hole 103 was measured.
Here, in the processing start end face 101, the distances from the center of the holey optical fiber preform 100 (center of the core 104) to the center of the six holes 103 are Rs1, Rs2, Rs3, Rs4, Rs5, and Rs6, respectively. And distances Rs1 to Rs6 were measured. The average value of these distances Rs1 to Rs6 was defined as the hole position Rs in the machining start end face 101.
Similarly, the distance from the center of the holey optical fiber preform 100 (the center of the core 104) to the center of the six holes 103 is set to Re1, Re2, Re3, Re4, Re5, Re6 on the processing end face 102, respectively. And distances Re1 to Re6 were measured. The average value of these distances Re1 to Re6 was defined as the hole position Re in the processing end face 102.
Then, the hole misalignment amount ΔR in the holey optical fiber preform 100 was defined as | Rs−Re |. That is, the positional deviation amount ΔR is defined as the absolute value of “Rs−Re”. The positional deviation amount ΔR of the holes in Example 1 and Comparative Examples 1 to 4 was measured. The results are shown in Table 1.

Here, the allowable range of the positional deviation amount ΔR of the holes is defined as in the following formula (1).
ΔR ≦ Δr / d × D (1)
In the above formula (1), D represents the diameter of the holey optical fiber preform, and d represents the diameter of the holey optical fiber obtained by drawing the holey optical fiber preform. Here, the diameter d of the optical fiber with holes is set to 125 μm, like the outer diameter of a general optical fiber.
In the above equation (1), Δr represents an allowable range of the positional deviation amount with respect to the design position of the hole position of the optical fiber formed from the preform. Here, as described later, −0.4 μm or more 0.4 μm or less. If Δr exceeds 0.4 μm or less than −0.4 μm, the optical characteristics such as the wavelength dispersion characteristic and the cutoff wavelength characteristic of the holey optical fiber deviate from the design.
If the hole misalignment amount ΔR in the holey optical fiber preform in Example 1 and Comparative Examples 1 to 4 is within the allowable range described later, it is determined as “OK”, and if it is outside the above allowable range. It was determined as “NG”. The results are shown in Table 1.

In Example 1, the gradient K of the refractive index profile of the holey optical fiber preform was 0.008% / mm. Also, the hole positional deviation amount ΔR in the holey optical fiber preform was 0.13 mm, which was within the allowable range of the hole positional deviation amount ΔR.
In Comparative Example 1, the gradient K of the refractive index profile of the holey optical fiber preform was 0.11% / mm. Also, the hole positional deviation amount ΔR in the holey optical fiber preform was 0.45 mm, which exceeded the allowable range of the hole positional deviation amount ΔR.
In Comparative Example 2, the gradient K of the refractive index profile of the holey optical fiber preform was 0.187% / mm. Further, the hole position deviation amount ΔR in the holey optical fiber preform was 0.99 mm, which exceeded the allowable range of the hole position deviation amount ΔR.
In Comparative Example 3, the gradient K of the refractive index profile of the holey optical fiber preform was −0.081% / mm. Further, the hole positional deviation amount ΔR in the optical fiber preform with holes was −0.38 mm, which exceeded the allowable range of the positional deviation amount ΔR of the holes.
In Comparative Example 4, the gradient K of the refractive index profile of the holey optical fiber preform was −0.153% / mm. Further, the hole positional deviation amount ΔR in the holey optical fiber preform was −0.63 mm, which exceeded the allowable range of the hole positional deviation amount ΔR.
In this way, it was confirmed that the gradient K of the refractive index profile and the positional deviation amount ΔR of the holes in the optical fiber preform with holes are closely related.

Also, the amount of positional deviation ΔR of holes in the optical fiber preform with holes shown in Table 1 and the gradient K of the refractive index profile of the optical fiber preform with holes are plotted, and a graph as shown in FIG. The following approximate expression (2) showing the relationship between the positional deviation amount ΔR of the hole in the holey optical fiber preform and the gradient K of the refractive index profile of the holey optical fiber preform was obtained. .
ΔR = 4.7K (2)
The permissible range of the hole misalignment amount ΔR in the holey optical fiber preform includes the hole misalignment amount Δr in the holey optical fiber, and the hole misalignment optical fiber preform and the holey optical fiber. It is calculated by a proportional expression (the following expression (3)).
ΔR = (Δr / d) × D (3)
However, in said Formula (3), D represents the diameter of an optical fiber base material with a hole, and d represents the diameter (= 125 micrometers) of an optical fiber with a hole.
When the diameter D of the optical fiber preform with holes is 100 mm and the allowable range of Δr is −0.4 μm or more and 0.4 μm or less as will be described later, the allowable range of ΔR is − It becomes 0.32 mm or more and 0.32 mm or less. Further, from the above equation (2), the allowable range of the gradient K of the refractive index profile of the holey optical fiber preform is −0.07% / mm or more and 0.07% / mm or less. That is, by forming vacancies in the region where the gradient K of the refractive index profile is −0.07% / mm or more and 0.07% / mm or less in the dopant addition region of the optical fiber preform, It was confirmed that the positional deviation amount ΔR was within the allowable range.

[Example 2]
By drawing the optical fiber preform with holes obtained in Example 1, a core is provided in the center part, a cladding part is provided around the core part, and the refractive index profile is flat in the dopant addition region of the cladding part. Alternatively, in a slightly changing region, there are six holes provided so as to be arranged along the longitudinal direction of the optical fiber and at almost equal intervals on a concentric circle centered on the core. A holey optical fiber was obtained.
In the obtained optical fiber with holes, the diameter of the holes is d ₁ , and the relative refractive index difference of the dopant-added region with respect to pure silica glass in the inner portion near the core in the boundary between the holes and the dopant-added region the [Delta] r _in, among the boundary portions between the holes and the dopant doped region, the far outer portion from the core, when the relative refractive index difference of the dopant doped region for pure silica glass is defined as Δr _out, (Δr _out -Δr _in ) / D ₁ The gradient k of the refractive index profile in the dopant addition region represented by ₁ was calculated.
Here, the hole diameter d ₁ was 6.0 μm, the relative refractive index difference Δr _in was −0.0017%, and the relative refractive index difference Δr _out was 0.0112%. As a result, the gradient k of the refractive index profile of the obtained optical fiber preform with holes was 0.002% / μm.

[Comparative Example 5]
In the same manner as in Example 2, by drawing the holey optical fiber preform obtained in Comparative Example 1, in the dopant addition region of the cladding part, the core is located in the longitudinal direction of the optical fiber and the core is centered. As a result, an optical fiber with a hole in which six holes are provided so as to be arranged at substantially equal intervals on the concentric circle is obtained.
In the same manner as in Example 2, the gradient k of the refractive index profile was calculated for the obtained optical fiber with holes. Here, the hole diameter d ₁ was 3.5 μm, the relative refractive index difference Δr _in was −0.2176%, and the relative refractive index difference Δr _out was 0.0803%. As a result, the gradient k of the refractive index profile of the obtained optical fiber preform with holes was 0.085% / μm.

[Comparative Example 6]
In the same manner as in Example 2, by drawing the holey optical fiber preform obtained in Comparative Example 2, the dopant-added region of the cladding part is located along the longitudinal direction of the optical fiber and is centered on the core. As a result, an optical fiber with a hole in which six holes are provided so as to be arranged at substantially equal intervals on the concentric circle is obtained.
In the same manner as in Example 2, the gradient k of the refractive index profile was calculated for the obtained optical fiber with holes. Here, the hole diameter d ₁ was 4.1 μm, the relative refractive index difference Δr _in was −0.4776%, and the relative refractive index difference Δr _out was 0.0681%. As a result, the gradient k of the refractive index profile of the obtained optical fiber preform with holes was 0.133% / μm.

[Comparative Example 7]
In the same manner as in Example 2, by drawing the holey optical fiber preform obtained in Comparative Example 3, the dopant-added region of the cladding part is located along the longitudinal direction of the optical fiber and is centered on the core. As a result, an optical fiber with a hole in which six holes are provided so as to be arranged at substantially equal intervals on the concentric circle is obtained.
In the same manner as in Example 2, the gradient k of the refractive index profile was calculated for the obtained optical fiber with holes. Here, the hole diameter d ₁ was 4.3 μm, the relative refractive index difference Δr _in was −0.1770%, and the relative refractive index difference Δr _out was −0.4861%. As a result, the gradient k of the refractive index profile of the obtained optical fiber preform with holes was −0.072% / μm.

[Comparative Example 8]
In the same manner as in Example 2, by drawing the holey optical fiber preform obtained in Comparative Example 4, in the dopant addition region of the cladding part, the core is located in the longitudinal direction of the optical fiber and the core is centered. As a result, an optical fiber with a hole in which six holes are provided so as to be arranged at substantially equal intervals on the concentric circle is obtained.
In the same manner as in Example 2, the gradient k of the refractive index profile was calculated for the obtained optical fiber with holes. Here, the hole diameter d ₁ was 3.9 μm, the relative refractive index difference Δr _in was 0.0592%, and the relative refractive index difference Δr _out was −0.3620%. As a result, the gradient k of the refractive index profile of the obtained optical fiber preform with holes was −0.108% / μm.

[Evaluation]
With respect to Example 2 and Comparative Examples 5 to 8, the positional deviation amount of the holes was evaluated by the following method.
[Evaluation of hole misalignment by measuring hole position of holey optical fiber]
With respect to the optical fibers with holes obtained in Example 2 and Comparative Examples 5 to 8, as shown in FIGS. 14A to 14C, an optical microscope is formed on the processing start end face 111 and the processing end end face 112 of the optical fiber with holes 110. Thus, the position of the hole 113 was measured.
Here, on the processing start end face 111, the distances from the center of the holey optical fiber 110 (center of the core 114) to the center of the six holes 113 are defined as rs1, rs2, rs3, rs4, rs5, and rs6, respectively. The distances rs1 to rs6 were measured. The average value of these distances rs1 to rs6 was defined as the hole position rs in the machining start end face 111.
Similarly, the distance from the center of the holey optical fiber 110 (center of the core 114) to the center of the six holes 113 on the processing end face 112 is defined as re1, re2, re3, re4, re5, and re6, respectively. The distances re1 to re6 were measured. The average value of these distances re1 to re6 was defined as the hole position re in the processing end face 112.
The hole positional deviation amount Δr in the holey optical fiber 110 was defined as | rs−re |. That is, the positional deviation amount Δr is defined as the absolute value of “rs−re”. The hole misregistration amount Δr in Example 2 and Comparative Examples 5 to 8 was measured. The results are shown in Table 2.

Here, as will be described later, Δr (allowable range of positional deviation with respect to the design position of the hole position of the optical fiber (optical fiber with holes) formed from the preform) is −0.4 μm or more and 0.4 μm. It was as follows.
If the hole misalignment amount Δr of the holey optical fiber in Example 2 and Comparative Examples 5 to 8 is within the allowable range described later, it is determined as “OK”, and if it is out of the above allowable range, “NG” is determined. Was determined. The results are shown in Table 2.

In Example 2, the gradient k of the refractive index profile of the optical fiber with holes was 0.002% / μm. In addition, the hole positional deviation amount Δr in the holey optical fiber was 0.16 μm, which was within the allowable range of the hole positional deviation amount Δr.
In Comparative Example 5, the gradient k of the refractive index profile of the optical fiber with holes was 0.085% / μm. Further, the hole positional deviation amount Δr in the optical fiber with holes was 0.56 μm, which exceeded the allowable range of the positional deviation amount Δr of the holes.
In Comparative Example 6, the gradient k of the refractive index profile of the holey optical fiber was 0.133% / μm. In addition, the hole position deviation amount Δr in the holey optical fiber was 1.24 μm, which exceeded the allowable range of the hole position deviation amount Δr.
In Comparative Example 7, the gradient k of the refractive index profile of the holey optical fiber was −0.072% / μm. Further, the hole positional deviation amount Δr in the holey optical fiber was −0.48 μm, which exceeded the allowable range of the hole positional deviation amount Δr.
In Comparative Example 8, the gradient k of the refractive index profile of the holey optical fiber was −0.108% / μm. Further, the hole positional deviation amount Δr in the optical fiber with holes was −0.78 μm, which exceeded the allowable range of the positional deviation amount Δr of the holes.

Further, the amount of positional deviation Δr of holes in the optical fiber with holes shown in Table 2 and the gradient k of the refractive index profile of the optical fiber with holes are plotted, and a graph as shown in FIG. 15 is created. The following approximate expression (4) showing the relationship between the positional deviation amount Δr of the hole in the optical fiber with holes and the gradient k of the refractive index profile of the optical fiber with holes was obtained.
Δr = 7.9k (4)
When the allowable range of Δr is set to −0.4 μm or more and 0.4 μm or less as will be described later, the allowable range of the gradient rate k of the refractive index profile of the optical fiber with holes is −0. It becomes 05% / μm or more and 0.05% / μm or less. That is, if a hole is formed in a region where the gradient k of the refractive index profile is −0.05% / μm or more and 0.05% / μm or less in the dopant-added region of the optical fiber, the position of the hole It was confirmed that the deviation amount Δr was within the allowable range.

Further, as shown in FIG. 16, the relationship between the gradient K in Example 1 and the gradient k in Example 2, the relationship between the gradient K in Comparative Example 2 and the gradient k in Comparative Example 6, and Comparative Example 4 When the relationship between the slope K in the optical fiber preform and the slope k in Comparative Example 8 is plotted, the slope K of the refractive index profile in the holey optical fiber preform and the slope k of the refractive index profile in the holey optical fiber are plotted. It was found that there is a high correlation between the two.

[Allowable range of hole misalignment amount Δr in holey optical fiber]
In the case of a holey optical fiber in which holes are arranged in the cladding part around the high refractive index core, the change in the hole position affects the waveguide dispersion, and as a result, the wavelength dispersion characteristics change. When the chromatic dispersion characteristics change, the holey optical fiber has some chromatic dispersion at the communication wavelength, and as a result, the signal waveform deteriorates. The holey optical fiber as in Example 2 is classified as a low bending loss optical fiber that realizes a reduction in bending loss by providing holes.
The international standard ITU-T G657 is applied to such a low bending loss optical fiber.
Within this standard, the range of chromatic dispersion characteristics is specified. Specifically, there is a standard in which the zero dispersion wavelength is in the range of 1300 to 1324 nm and the zero dispersion Slope is 0.092 ps / nm ² / km. .
In addition, the wavelength dispersion characteristics of the holey optical fiber of Example 2 prepared by drawing the holey optical fiber preform of Example 1 were measured. The results are shown in Table 3.
Further, as Comparative Example 9, a holey optical fiber preform with a hole misalignment amount ΔR of 0.33 mm was prepared, and the holey optical fiber preform was drawn to obtain a hole misalignment amount Δr. Produced a holey optical fiber with a diameter of 0.41 μm. The wavelength dispersion characteristics of the holey optical fiber of Comparative Example 9 were measured. The results are shown in Table 3.
Further, as Comparative Example 10, a holey optical fiber preform with a hole misregistration amount ΔR of −0.33 mm was prepared, and the holey optical fiber preform was drawn to obtain a hole misalignment amount. A holey optical fiber with Δr of −0.41 μm was produced. The wavelength dispersion characteristics of the holey optical fiber of Comparative Example 10 were measured. The results are shown in Table 3.

From the results shown in Table 3, in the examples, the processing bending can be suppressed, and the zero dispersion wavelength and the zero dispersion Slope on the processing end face side are both within the above-mentioned standard.
On the other hand, in the optical fiber with holes of Comparative Example 9, the zero dispersion wavelength on the processing end face side was shifted to the longer wavelength side than the above standard.
In addition, in the optical fiber with holes of Comparative Example 10, the zero dispersion Slope on the processing end face side exceeded the standard value.
Thus, when the hole misalignment amount Δr in the holey optical fiber is not in the range of −0.4 μm or more and 0.4 μm or less, the wavelength dispersion characteristic of the holey optical fiber is the international standard ITU- It was found that TG657 was not satisfied.
That is, if holes are provided in a region where the refractive index profile gradient k is −0.05% / μm or more and 0.05% / μm or less in the dopant-added region of the optical fiber with holes, It was confirmed that the amount of positional deviation of the holes in the optical fiber with a hole is in the range of −0.4 μm to 0.4 μm and the wavelength dispersion characteristic of the optical fiber with a hole satisfies the international standard ITU-T G657. .

21, 31, 61... Core, 22, 32, 62... Cladding portion, 30... Optical fiber preform, 30 A. Optical fiber preform with holes, 33, 63. , 60... Optical fiber with holes.

Claims (6)

1. An optical fiber,
   The core,
   A cladding portion surrounding the core and having a dopant-added region doped with a dopant;
   A plurality of holes provided on the concentric circle centering on the core in the clad part, in parallel with the core;
   The diameter of the hole is defined as d _1, and the relative refractive index difference of the dopant-added region with respect to pure quartz glass in the inner portion near the core in the boundary between the hole and the dopant-added region is Δr _In, the relative refractive index difference of the dopant addition region with respect to pure quartz glass in the outer portion far from the core in the boundary portion between the vacancies and the dopant addition region is defined as Δr _out , (Δr _{When out} −Δr _in ) / d ₁ is defined as the gradient k of the refractive index profile in the dopant addition region, the vacancies are provided in the dopant addition region in the region where the gradient k is within a predetermined range. Optical fiber.
2. 2. The optical fiber according to claim 1, wherein the gradient k is −0.05% / μm or more and 0.05% / μm or less.
3. An optical fiber manufacturing method comprising:
   A core, a clad portion surrounding the core and having a dopant-added region to which a dopant is added, and a plurality of voids provided in a concentric circle parallel to the core and centered on the core in the clad portion A diameter of the hole is defined as d _1, and the dopant-added region with respect to the pure silica glass in an inner portion near the core in a boundary portion between the hole and the dopant-added region is provided. The relative refractive index difference is defined as Δr _in, and the relative refractive index difference of the dopant added region with respect to pure quartz glass in the outer portion far from the core in the boundary between the vacancies and the dopant added region is Δr _out. When (Δr _out −Δr _in ) / d ₁ is defined as the gradient k of the refractive index profile in the dopant addition region, Is a manufacturing method for manufacturing an optical fiber provided in a region in which the gradient k is within a predetermined range in the dopant addition region,
   Producing an optical fiber preform including a core and a clad portion surrounding the core and having a dopant-added region to which a dopant is added,
   In the dopant addition region, a plurality of holes are formed on a concentric circle centered on the core,
   Smoothing the inner surface of the hole, producing a holey optical fiber preform,
   Drawing the optical fiber preform with the hole while drawing the inside of the hole,
   In the formation of the plurality of holes, wherein the diameter of pores formed in the dopant doped region of the cladding portion is defined as d _1, of the boundary between the holes and the dopant doped region, inside closer to the core The relative refractive index difference of the dopant added region with respect to the pure silica glass in the portion is defined as Δr _in , and the boundary portion between the hole and the dopant added region in the outer portion far from the core with respect to the pure silica glass When the relative refractive index difference of the dopant added region is defined as Δr _out and (Δr _out −Δr _in ) / d ₁ is defined as the gradient K of the refractive index profile in the dopant added region, in the dopant added region, An optical fiber manufacturing method in which the holes are formed in a region where the inclination rate K is within a predetermined range.
4. 4. The light according to claim 3, wherein the gradient k is −0.05% / μm or more and 0.05% / μm or less, and the gradient K is −0.07% / mm or more and 0.07% / mm or less. Fiber manufacturing method.
5. An optical fiber preform manufacturing method comprising:
   A core, a clad portion surrounding the core and having a dopant-added region to which a dopant is added; and the dopant addition provided in a concentric circle parallel to the core and centered on the core in the clad portion A manufacturing method of manufacturing an optical fiber preform including a plurality of holes provided in a region,
   Producing an optical fiber preform including a core and a cladding portion surrounding the core and having a dopant-added region to which a dopant is added,
   In the dopant addition region, a plurality of holes are formed on a concentric circle centered on the core,
   Smoothing the inner surface of the pores;
   In the formation of the plurality of holes, wherein the diameter of pores formed in the dopant doped region of the cladding portion is defined as d _1, of the boundary between the holes and the dopant doped region, inside closer to the core The relative refractive index difference of the dopant added region with respect to the pure silica glass in the portion is defined as Δr _in , and the boundary portion between the hole and the dopant added region in the outer portion far from the core with respect to the pure silica glass When the relative refractive index difference of the dopant added region is defined as Δr _out and (Δr _out −Δr _in ) / d ₁ is defined as the gradient K of the refractive index profile in the dopant added region, in the dopant added region, An optical fiber preform manufacturing method in which the holes are formed in a region where the inclination rate K is within a predetermined range.
6. 6. The method of manufacturing an optical fiber preform according to claim 5, wherein the inclination rate K is −0.07% / mm or more and 0.07% / mm or less.

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