JP5242896B2 - Image fiber and optical fiber preform - Google Patents

Description

  The present invention relates to a silica-based image fiber used as an image transmission medium for an industrial or medical fiberscope and an optical fiber preform.

  A silica-based image fiber is manufactured by bundling and melting a large number of silica-based optical fibers composed of a core and a clad around the core, and is shared by the core and the many cores constituting the pixel. It consists of a common cladding.

  In such an image fiber, it is known that bubbles are generated in the image fiber when a large number of optical fibers are bundled and drawn. Bubbles in the image fiber have a length in the cross-sectional direction (hereinafter referred to as a diameter) of the image fiber of several microns (μm) to several tens of microns, and a length in the longitudinal direction of 1 meter to several meters. . When such bubbles are generated, black dots are generated on the image formed by the image fiber, which causes a deterioration in image quality. Therefore, the image fiber is usually commercialized by cutting out a long image fiber after drawing at a predetermined length avoiding bubbles. Since the portion where the bubbles are present is wasted, the bubbles are a major cause of lowering the production yield.

As a method for reducing bubbles, those described in Patent Document 1 and Patent Document 2 are known, but the effect is not sufficient, and further reduction is required.
JP-A-2-267132 JP-A-8-29630

  The present invention has been made in view of the above circumstances, and provides an image fiber having a structure capable of suppressing the generation of bubbles and an optical fiber preform capable of manufacturing such an image fiber. The purpose is to do.

In order to achieve the above object, a first aspect of the present invention is composed of quartz to which germanium oxide as a refractive index increasing agent is added, and a relative refractive index difference with respect to pure quartz is 2.6% or more and 3.0. % Of a plurality of cores and the outer periphery of each of the plurality of cores, having an outer diameter of 1.10 times to 1.30 times the diameter of the plurality of cores, A plurality of cores including a first clad having a relative refractive index difference of not less than 0.3% and not more than 0.0%, and quartz that surrounds the first clad and is doped with fluorine that is a refractive index lowering agent. The shortest distance between the outer diameters of the first claddings adjacent to each other is 0.2 times or more and 0.4 times or less, and the relative refractive index difference with respect to pure quartz is −1.3% or more− A melt-integrated image with a common cladding that is 1.1% or less To provide a Aiba.

A second aspect of the present invention includes a plurality of cores made of quartz to which germanium oxide as a refractive index increasing agent is added, and having a relative refractive index difference of 2.6% or more and 3.0% or less with respect to pure quartz. , Covering the outer periphery of each of the plurality of cores, having an outer diameter of 1.10 times or more and 1.30 times or less of the diameter of the plurality of cores, and -0.1% or more and 0.0. A first clad having a relative refractive index difference of 0% or less, and a quartz that surrounds the first clad and is doped with fluorine as a refractive index depressant, and has a diameter of 0. Commonly having a shortest distance between the outer diameters of the first claddings adjacent to each other of 2 times to 0.4 times and having a relative refractive index difference of −1.3% to −1.1% with respect to pure quartz And a melt-integrated image fiber.

  It is preferable that the common clad has an inner peripheral portion having a refractive index lower than that of the first clad and an outer peripheral portion surrounding the inner peripheral portion and having a refractive index higher than that of the inner peripheral portion. .

Further, in the second aspect, the relative refractive index difference with respect to pure silica of the outer peripheral portion is preferably is not more than 0.2% or more -0.2%.

According to a third aspect of the present invention, there is provided an image fiber which is made of quartz to which germanium oxide which is a refractive index increasing agent is added and has a relative refractive index difference of 2.6% to 3.0% with respect to pure quartz. A plurality of core parts to be cores, and the outer circumferences of the core parts, an outer diameter of 1.10 times to 1.30 times the diameter of the core parts, and pure quartz A relative refractive index difference of −0.3% or more and 0.0% or less, and a first cladding portion serving as a first cladding of an image fiber, and fluorine serving as a refractive index lowering agent surround the first cladding portion. It is composed of added quartz, and has a shortest distance between the outer diameters of the first claddings adjacent to each other and not less than 0.2 times and not more than 0.4 times the diameter of the plurality of cores. The relative refractive index difference is not less than −1.3% and not more than −1.1%. And a common cladding portion as a common cladding of the image fiber, to provide an optical fiber preform for an image fiber is formed by melt integrally.

  Further, the first cladding part has an inner peripheral part having a refractive index lower than that of the first cladding part, and an outer peripheral part surrounding the inner peripheral part and having a refractive index higher than the refractive index of the inner peripheral part, It is preferable to have

  According to said structure, the image fiber provided with the structure which can suppress generation | occurrence | production of a bubble, and the optical fiber base material which enables manufacture of such an image fiber are provided.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the attached drawings only schematically show the image fiber according to the embodiment. For this reason, it should be noted that the ratio between the diameter of the core and the diameter of the clad is not always displayed as actually designed.

(First embodiment)
FIG. 1 is a cross-sectional view of an image fiber according to a first embodiment of the present invention. As illustrated, the image fiber 10 includes an image circle 12 for image transmission, a quartz glass jacket 14 covering the outer periphery thereof, and a coating layer 16 covering the outer periphery thereof.

  As shown in an enlarged view in FIG. 1, the image circle 12 allows propagation of light and functions as a pixel of the image fiber 10, and a first cladding 2 that surrounds the outer periphery of each of the cores 1, The outer periphery of each of the first clads 2 is formed from a common clad 3 shared by them. The diameter of the image circle 12 is, for example, about 500 μm, and there are about 10,000 cores 1 therein. With such a configuration, an image formed on one end face of the image fiber 10 is spatially divided into pixels and transmitted to the other end face, and an image is formed on this end face.

  FIG. 2 is a diagram showing the refractive index distribution of the core 1, the first cladding 2, and the common cladding 3. The core 1 is made of quartz to which germanium oxide which is a refractive index increasing agent is added, and has a convex refractive index distribution as shown in FIG. This refractive index distribution is a distribution in which the refractive index distribution coefficient α is represented by 2.6, for example. The diameter a of the core 1 is, for example, 2 to 3 μm.

  The refractive index distribution coefficient α is α in the following equation.

n (r) = n ₁ [1-2Δ (r / a) ^α ] ^1/2 (0 ≦ r ≦ a)

Where n (r) is the refractive index at a distance r from the central axis of the glass rod, n ₁ is the refractive index at the central axis of the glass rod, Δ is the relative refractive index difference at the central axis of the glass rod relative to the cladding, and a is the glass Represents the radius of the rod.

  The first cladding 2 is made of pure quartz in the first embodiment, and has a constant refractive index in the radial direction. The first cladding 2 has an outer diameter b that is 1.10 times to 1.30 times the diameter a of the core 1. Note that pure quartz is quartz having a refractive index of about 1.457 with respect to light having a wavelength of 633 nm, and having a metal impurity concentration of about 0.01 ppm or less.

  The common clad 3 is made of quartz to which fluorine as a refractive index lowering agent is added, and has a constant refractive index in the radial direction. The outer diameter c of the common cladding 3 is, for example, about 1.5 times the diameter a of the core 1.

  According to the study by the inventors of the present invention, it is considered that the reason why bubbles are generated in the image fiber is that germanium added to the core diffuses into the common cladding. In the image fiber 10, since the first cladding 2 made of pure quartz is formed on the outer periphery of the core 1, germanium is prevented from diffusing into the common cladding. For this reason, the image fiber of 1st Embodiment has the advantage that generation | occurrence | production of a bubble can be suppressed.

Example 1
Hereinafter, the image fiber 10 described in the first embodiment will be described in detail including its manufacturing method.

<Step 1>
First, a germanium-added quartz glass rod serving as the core 1 of the image fiber 10 was prepared. In this glass rod, the addition concentration of germanium is adjusted so that the relative refractive index difference with respect to pure quartz is + 2.6% to 3.0%, and the refractive index distribution coefficient α is adjusted to 2.5. Has been.

<Process 2>
Subsequently, the 1st glass layer used as the 1st clad of an image fiber was formed in the outer periphery of said glass rod by the plasma external attachment method. That is, SiCl ₄ gas and O ₂ gas as raw materials are decomposed by a plasma flame to deposit glass soot on the outer periphery of the glass rod, and at the same time, the deposited glass soot is transparently vitrified by the plasma flame, and the first glass layer is formed. Formed. Since no refractive index adjusting additive is used, this glass layer is pure quartz, that is, the relative refractive index difference with respect to pure quartz is 0.0%.
The thickness of the first glass layer was adjusted so that the outer diameter b of the first cladding 2 in the image fiber was 1.10 to 1.30 times the diameter a of the core 1.

<Step 3>
Then, the 2nd glass layer used as the common clad of an image fiber was formed in the outer periphery of the 1st glass layer by the plasma external attachment method. Here, in addition to SiCl ₄ gas and O ₂ gas as raw materials, fluorine-containing gas such as SF ₆ , SiF ₄ , C ₂ F ₆ and CF ₄ was used as a fluorine additive. The amount of fluorine added was adjusted so that the relative refractive index difference of the second glass layer (common clad) was −1.3% to −0.7% with respect to pure quartz.
In this embodiment, SiCl ₄ gas is used as a raw material in the plasma external method, but Si (OCH ₃ ) ₄ gas (tetramethoxysilane) can be used instead.
According to the above procedure, five types of optical fiber preforms having different thicknesses of the first glass layer (first clad) were produced. Specifically, the thickness of the first glass layer is 1.10 times, 1.15 times, 1.20 times, 1.25 times, and 1.30 times the diameter of the glass rod.

<Step 4>
Next, the optical fiber preform obtained as described above was drawn to produce an optical fiber strand having an outer diameter of several hundred μm. Then, the produced optical fiber strand was cut | disconnected by predetermined length, and 10,000 optical fiber strands were obtained. By inserting these into a predetermined quartz tube and drawing it, a multi-core optical fiber having an outer diameter of about 500 μm is produced, and an image fiber is obtained by forming a coating layer having a thickness of about 50 μm around it. It was.
Five types of image fibers were prepared using each of the five types of optical fiber preforms.

(Comparative Example 1)
For comparison with the first embodiment, the outer diameter b of the first cladding is 1.05 times and 1.35 times the core diameter a by the same manufacturing method employed in the first embodiment. Various types of image fibers were made.

(Contrast between Example 1 and Comparative Example 1)
Hereinafter, the evaluation results of the characteristics of the five types of image fibers of Example 1 and the two types of image fibers of Comparative Example 1 will be described.
Table 1 summarizes the configuration of each image fiber, the image quality evaluation results, and the number of detected bubbles. In this table, the relative refractive index difference of the core, the first clad, and the common clad is a value for pure quartz, and is equal to the result measured for the optical fiber preform before drawing. The number of bubbles in Table 1 is the number of bubbles per 1000 meters of image fiber obtained by bundling 10,000 optical fiber strands and drawing them. Bubbles to be counted have a diameter of several to several tens of microns in the image fiber strand, and a length of 1 to several meters, and are detected based on fluctuations in the outer diameter of the image fiber strand. It has been done.
Furthermore, the image quality evaluation in Table 1 was based on the contrast and average luminance (or transmitted light amount) of an image obtained by imaging a predetermined imaging target with each image fiber.

  Focusing on the number of bubbles in Table 1, when the ratio b / a between the outer diameter b of the first cladding and the diameter a of the core is 1.05 (sample No. 1), the number of bubbles is seven. On the other hand, in the range where the b / a ratio is 1.10 to 1.35, the number of bubbles is 3 or less. That is, it can be seen that an image fiber having a reduced number of bubbles in this range is obtained. Considering that the length of bubbles in the image fiber reaches several meters, the difference between the number of bubbles of 7 and 3 or less is significant from the viewpoint of manufacturing yield.

  However, in the image fiber having a b / a ratio of 1.35 in this range, the contrast was lowered to an unpreferable level for practical use. This is considered to be due to the fact that the first clad width is increased and the light confinement effect is reduced, so that light is mixed between pixels. FIG. 3 shows the results of investigation by the inventors of the present invention on the b / a ratio dependence of MFD (mode field diameter). This figure shows the results when the spacing between adjacent cores 1 is 3.3 μm and 4.4 μm, and it can be seen that the MFD increases as the b / a ratio increases in both cases. . This means that as the b / a ratio increases, the light confinement effect decreases.

  From the above results, in the five types of image fibers 10 of Example 1 in which the b / a ratio is in the range of 1.10 to 1.30, the image quality is practically good and the number of bubbles generated is reduced. I found out.

(Example 2)
Next, another example in which the image fiber 10 described in the first embodiment is manufactured will be described. In the image fiber 10 of Example 2, the relative refractive index difference δ of the first cladding with respect to pure quartz is changed to −0.3%, and the refractive index distribution coefficient α of the glass rod serving as the core 1 is 2. 6 is different from the image fiber 10 of the first embodiment in that the other points are the same. That is, in <Step 2> described in Example 1, in addition to the SiCl ₄ gas and the O ₂ gas, a fluorine-containing gas such as SF ₆ , SiF ₄ , C ₂ F ₆ , CF ₄ is used, and the flow rate thereof is set. By adjusting, a first glass layer having a relative refractive index difference δ of −0.3% was formed.
Also in Example 2, five types of image fibers 20 having different b / a ratios in the range from 1.10 to 1.30 were produced.

(Comparative Example 2)
For comparison with Example 2, the outer diameter b of the first cladding is 1.05 times and 1.35 times the core diameter a by the same manufacturing method as that used in Example 2. Various types of image fibers were made.

(Contrast between Example 2 and Comparative Example 2)
Hereinafter, the evaluation results of the characteristics of the five types of image fibers of Example 2 and the two types of image fibers of Comparative Example 2 will be described.

  Table 2 summarizes the configuration of each image fiber, the image quality evaluation results, and the number of detected bubbles, as in Table 1. Even when the relative refractive index difference δ of the first cladding is −0.3%, when the b / a ratio is 1.05, the number of bubbles is nine, whereas the b / a ratio is In the range from 1.10 to 1.35, the number is 4 or less, and it can be seen that the generation of bubbles is suppressed in this range. For the same reason as described above, when the b / a ratio is 1.35, the result is not preferable from the viewpoint of image quality. From the above, even when the relative refractive index difference δ of the first cladding is −0.3%, the generation of bubbles is suppressed while maintaining the image quality in the range of the b / a ratio from 1.10 to 1.30. I found out that I can do it.

  Further, the results of Example 1 and Example 2 are summarized: the relative refractive index difference δ of the first cladding is in the range from −0.3% to 0.0%, and the b / a ratio is 1.10. In the range from 1 to 1.30, it has been clarified that an image fiber can be obtained in which the image quality is practically good and the number of bubbles generated can be reduced. From the above, the effects of the first embodiment according to the present invention are understood.

For further comparison, an image fiber in which the relative refractive index difference δ of the first cladding is −0.5% was manufactured by the same manufacturing method as that used in Example 2 and Comparative Example 2 (however, The refractive index distribution order α in the glass rod was 2.4). The results are as shown in Table 3. The number of bubbles was 5 or more in any image fiber. That is, it was found that when the relative refractive index difference δ of the first cladding is lowered to −0.5%, the generation of bubbles can be sufficiently suppressed. From this, it was concluded that the relative refractive index difference δ of the first cladding is preferably in the range of −0.3% to 0.0%.

  Moreover, in order to prevent the germanium added to the core 1 from diffusing and generating bubbles, it is desirable that the viscosity of the first cladding 2 is high. For this reason, it is more preferable if the relative refractive index difference δ of the first cladding 2 is −0.1% or more and 0.0% or less.

(Second Embodiment)
The image fiber according to the second embodiment is different from the image fiber 10 according to the first embodiment in that the common cladding 3 is composed of three portions having different refractive indexes, and is the same except for this. Hereinafter, the image fiber according to the second embodiment will be described focusing on the differences.

  FIG. 4 is a diagram showing a refractive index distribution of the image fiber according to the second embodiment. Referring to FIG. 4, in the image fiber 20, the common cladding 3 includes a first portion 31, a second portion 32, and a third portion 33. The first portion 31 is made of fluorine-added quartz on the outer periphery of the first cladding 2, and its refractive index is lower than the refractive index of the first cladding 2 and is constant in the radial direction. Specifically, the relative refractive index difference of the first portion 31 with respect to pure quartz is −1.3% or more and −0.7% or less.

  The second portion 32 is formed on the outer periphery of the first portion 31 and is made of quartz with a fluorine addition amount reduced outward in the radial direction. As the amount of fluorine added decreases, the refractive index increases radially outward as shown in FIG.

  The third portion 33 is made of pure quartz on the outer periphery of the second portion 32 and has a constant refractive index in the radial direction. Specifically, the relative refractive index difference of the third portion 33 with respect to pure quartz is −0.2% or more and 0.2% or less.

  As described above, in the second embodiment, the common cladding 3 includes the inner peripheral portion (first portion 32) having a lower refractive index than the refractive index of the first cladding portion 2, and the refraction of the inner peripheral portion. And an outer peripheral portion (second portion 32 and third portion 33) having a refractive index higher than the refractive index.

According to the study by the inventors of the present invention, as another cause of the generation of bubbles, it is conceivable that fluorine added to the common cladding 3 is combined at the time of fusion integration to generate SiF _4. . However, in the image fiber 20, since the second portion 32 and the third portion 33 in which the fluorine addition concentration is reduced are provided in the common clad 3, the generation of SiF ₄ is hindered, and thus generation of bubbles is caused. Can be further suppressed. In other words, the image fiber 20 is added to the common clad 3 with the same advantage as the image fiber 10 according to the first embodiment that bubbles caused by germanium of the core 1 can be reduced by the effect of the first clad 1. This also has the advantage of reducing bubbles caused by fluorine.

(Example 3)
Next, an example in which the image fiber 20 described in the second embodiment is manufactured will be described.
The image fiber 20 of Example 3 was manufactured by basically the same manufacturing method as that of Example 1, although <Process 3> was changed as described below. Specifically, after <Step 1> and <Step 2>, in <Step 3>, after depositing the fluorine-added glass layer that becomes the first portion 31, the first step is performed while gradually decreasing the supply amount of the fluorine-containing gas. A glass layer to be the second portion 32 was formed, and then the supply of the fluorine-containing gas was stopped to form a glass layer to be the third portion 33. The glass rod prepared in <Step 1> had a refractive index distribution coefficient α of 2.5.
In this manner, five types of optical fiber preforms were produced in the same manner as in Example 1. Then, according to the procedure of <Process 4>, five types of image fibers 20 corresponding to each optical fiber preform were produced.

(Comparative Example 3)
For comparison with Example 3, two types of outer diameters of the first cladding are 1.05 times and 1.35 times as large as the core diameter by the same manufacturing method as that used in Example 3. An image fiber was produced.

(Contrast between Example 2 and Comparative Example 2)
Next, evaluation results for the characteristics of the five types of image fibers of Example 3 and the two types of image fibers of Comparative Example 3 will be described.

Table 4 summarizes the configuration of each image fiber, the image quality evaluation results, and the number of detected bubbles, as in Table 1. In Table 4, the c / a ratio is the ratio of the outer diameter c of the first portion 31 to the diameter a of the core 1, and the d / a ratio is the outer diameter of the third portion 33 to the diameter a of the core 1. The ratio of d.

  Referring to Table 4, the b / a ratio is less than 2 when the b / a ratio is 1.10 to 1.35, compared to 6 when the b / a ratio is 1.05. It turns out that generation | occurrence | production of is suppressed. Further, when the b / a ratio was 1.35, as in the case of Example 1 and Example 2, it was not a preferable result in terms of image quality (contrast). From the above, it was found that the generation of bubbles can be suppressed while maintaining the image quality in the range of the b / a ratio from 1.10 to 1.30.

  Further, when Example 3 is compared with Example 1, in Example 3, the number of bubbles is further reduced. This is due to the effects of the second portion 32 and the third portion 33 provided in the common clad 3 and having a reduced amount of fluorine added, whereby the effect of the second embodiment according to the present invention is understood. Is done.

The present invention has been described above with reference to some embodiments and examples. However, the present invention is not limited to these, and various modifications can be made.
In the second embodiment, the case where the common cladding 3 includes the first portion 31, the second portion 32, and the third portion 33 has been described. However, the common cladding 3 includes the third portion 33. It may be omitted and only the first portion 31 and the second portion 32 may be included. Further, regardless of the presence or absence of the third portion 33, the refractive index distribution of the second portion 32 is not limited to linear change as shown in FIG. As long as the value decreases, for example, it may change in a quadratic curve or may change in an exponential function.
Further, in the second embodiment, the common cladding 3 may be configured only by the first portion 31 and the third portion 33, omitting the second portion 32.

  In each of the embodiments, the case where an optical fiber preform is manufactured by a plasma external method has been described. However, the present invention is not limited to this, and an internal CVD (chemical vapor deposition) method, an external CVD method not using a plasma flame, and a VAD. It goes without saying that various methods such as the (Vapor-phase Axial Deposition) method can be used. Furthermore, the c / a ratio in Example 1 and Example 2, and the c / a ratio and d / a ratio in Example 3 are not limited to the illustrated values, and the numerical aperture that the image fiber to be manufactured should have Of course, it may be changed as appropriate.

It is sectional drawing of the image fiber by the 1st Embodiment of this invention. It is a figure which shows the refractive index distribution of the core of the image fiber of FIG. 1, a 1st clad, and a common clad. It is a graph which shows b / a ratio dependence of MFD (mode field diameter). It is a figure which shows the refractive index distribution of the image fiber by the 2nd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Core, 2 ... 1st clad, 3 ... Common clad, 31 ... 1st part of a common clad, 32 ... 2nd part, 33 ... 3rd part .

Claims (6)

A plurality of cores made of quartz to which germanium oxide, which is a refractive index increasing agent, is added, and a relative refractive index difference with respect to pure quartz is 2.6% or more and 3.0% or less;
The outer periphery of each of the plurality of cores is covered, the outer diameter is 1.10 times or more and 1.30 times or less with respect to the diameter of the plurality of cores, and is −0.3% or more and 0.0% with respect to pure quartz. A first clad having a relative refractive index difference of not more than%,
The first clad is surrounded by the first clad, is made of quartz to which fluorine as a refractive index lowering agent is added, and is adjacent to each other by 0.2 to 0.4 times the diameter of the plurality of cores . A melt-integrated image fiber comprising: a common cladding having a shortest distance between the outer diameters of the cladding and having a relative refractive index difference of -1.3% to -1.1% with respect to pure quartz. A plurality of cores made of quartz to which germanium oxide, which is a refractive index increasing agent, is added, and a relative refractive index difference with respect to pure quartz is 2.6% or more and 3.0% or less;
The outer periphery of each of the plurality of cores is covered, the outer diameter is 1.10 times or more and 1.30 times or less with respect to the diameter of the plurality of cores, and is −0.1% or more and 0.0 with respect to pure quartz. A first clad having a relative refractive index difference of not more than%,
The first clad is surrounded by the first clad, is made of quartz to which fluorine as a refractive index lowering agent is added, and is adjacent to each other by 0.2 to 0.4 times the diameter of the plurality of cores . A melt-integrated image fiber comprising: a common cladding having a shortest distance between the outer diameters of the cladding and having a relative refractive index difference of -1.3% to -1.1% with respect to pure quartz.   The common cladding includes an inner peripheral portion having a refractive index lower than that of the first cladding, and an outer peripheral portion surrounding the inner peripheral portion and having a refractive index higher than that of the inner peripheral portion. The image fiber according to 1 or 2.   The image fiber according to claim 3, wherein a relative refractive index difference with respect to pure quartz at the outer peripheral portion is −0.2% or more and 0.2% or less. A plurality of core portions that are cores of an image fiber, which is made of quartz to which germanium oxide as a refractive index increasing agent is added, and whose relative refractive index difference with respect to pure quartz is 2.6% to 3.0%;
The outer periphery of each of the plurality of core portions is covered, the outer diameter is 1.10 times to 1.30 times the diameter of the plurality of core portions, and −0.3% to 0.0% with respect to pure quartz. A first clad portion having a relative refractive index difference of not more than% and serving as a first clad of an image fiber;
The first cladding portion is surrounded by quartz to which fluorine, which is a refractive index lowering agent, is added, and is adjacent to each other by 0.2 to 0.4 times the diameter of the plurality of cores . A common clad portion having a shortest distance between the outer diameters of one clad and having a relative refractive index difference with respect to pure quartz of −1.3% or more and −1.1% or less and serving as a common clad of the image fiber. An optical fiber preform for an image fiber formed by melting and integration.   An inner peripheral part in which the first cladding part has a refractive index lower than the refractive index of the first cladding part; and an outer peripheral part surrounding the inner peripheral part and having a refractive index higher than the refractive index of the inner peripheral part; The optical fiber preform according to claim 5, comprising:

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