JP5203262B2 - Optical fiber manufacturing method - Google Patents

Description

The present invention relates to a method of manufacturing an optical fiber which pores are formed in the cladding portion.

When a normal single mode optical fiber is bent so that the radius of curvature is about 10 mm or less, for example, light leaks from the core portion toward the outside. Therefore, as an optical fiber in which light does not easily leak from the core portion to the outside when bent (bending loss is suppressed), for example, a trench structure as shown in Non-Patent Document 1 below, or Non-Patent Document 2 below. A hole assist fiber as shown in FIG. Among these, the hole assist fiber 100 having a cross-sectional structure as shown in FIG. 17 has a high refractive index between the pores 102 and the glass (cladding portion 103) for light that leaks from the core portion 101 when bent. It is made difficult to leak by being totally reflected due to the difference, and it has excellent bending characteristics compared to an optical fiber having a trench structure. Since this hole assist fiber 100 can withstand small bending, it is resistant to fluctuations in transmission characteristics due to use in indoor wiring, pressure applied from the side (stepping on the fiber, etc.), and fluctuations in the bending state during fiber handling. The use for wiring from the facts, etc., and the use for ensuring safety that requires large power because light does not easily leak are being studied.
By the way, since the hole assist fiber 100 has conventionally been manufactured by drilling a hole in a base material called a preform because the number of the pores 102 is small, the preform is heated and drawn to draw a fiber. It is done by the drill method. The pores 102 are generally circular, and are often arranged in three-fold rotational symmetry, four-fold rotational symmetry, six-fold rotational symmetry, etc. as viewed from the core portion.

Takahiro Hamada, “Technological Trends of Optical Cables and Optical Fibers for Home Wiring”, O plus E, November 2008, vol. 30, no. 11, p. 1192-1197 Kazumasa Otsuki, “Holy Fiber for Indoor Wiring”, O plus E, September 2005, vol. 27, no. 9, p. 1015-1021

However, the above-described drill method has a problem in that the productivity is lower and the cost is higher than that of a normal optical fiber because a machining step of making a hole in the base material is added.
Therefore, the multi-capillary method can be considered as a simple and highly productive manufacturing method. Since this multi-capillary method uses a glass rod and a glass capillary (capillary) having the same outer diameter, the pore size is smaller than that of the drill method. It is difficult to improve the position accuracy, and it is difficult to obtain desired characteristics (low bending loss). Furthermore, for example, when it is necessary to separate the pores from the core portion, it is necessary to place a glass rod around the core portion and further place a glass thin tube outside the core portion. The convenience that is a feature is lost.

This invention is made | formed in view of the situation mentioned above, The objective is to provide the manufacturing method of the optical fiber which can manufacture the optical fiber of a desired characteristic simply.

In order to solve the above problems, the present invention proposes the following means.
In the method of manufacturing an optical fiber according to the present invention, N preforms (where N is an integer of 7 or more) hollow glass capillaries are bundled and arranged in a jacket tube to form a preform, and the preform is heated and stretched. A plurality of pores extending along the direction of the central axis of the core portion are formed on a single concentric circle centered on the central axis in the clad portion covering the periphery of the core portion by a multicapillary method of drawing in a fiber shape. a method for manufacturing an optical fiber to produce a side by side to form the optical fiber, the outer periphery of the central glass rod having the core portion, the outer periphery of the glass capillary, and the impurities that adhere to each of the inner periphery of the glass capillary Impurity removing step to be removed by polishing, and glass for forming the central glass rod and the glass thin tube on the outer periphery or the inner periphery of all the N glass thin tubes An adsorption process for adsorbing a light absorbing medium having a higher extinction coefficient than that of the material, and N glass thin tubes arranged so as to surround the circumference of the central glass rod all around, and the central glass rod and glass A preform forming step of disposing a thin tube in the jacket tube, and the jacket tube is formed in a circular shape in an inner peripheral shape in a cross-sectional view perpendicular to the central axis, and the glass thin tube and the center The glass rod has a circular outer shape in a cross-sectional view orthogonal to each central axis, the diameter of the central glass rod is 2R, the outer diameter of the glass thin tube is 2r, and the inner diameter of the jacket tube is 2 If (R + 2r), then r = (R + r) · sin (π / N).

According to the optical fiber manufacturing method of the present invention, the diameter 2R of the central glass rod, the outer diameter 2r of the thin glass wire, and the inner diameter 2 (R + 2r) of the jacket tube are:
r = (R + r) · sin (π / N)
Therefore, in the preform forming process, N glass thin wires are arranged so as to surround the central glass rod in a line all around the circumference, and the central glass rod and the glass thin tube are jacketed. It is possible to fill the inside of the jacket tube with the central glass rod and the thin glass wire simply by disposing it in the tube, and the central glass rod can be disposed at the center of the jacket tube. In addition, at this time, it becomes possible to arrange the glass thin wire without any gap between adjacent ones over the entire circumference of the central glass rod, that is, without play, and the position of the hole of the glass thin tube relative to the entire preform can be determined. It can be determined with high accuracy. Accordingly, it is possible to easily manufacture an optical fiber having desired characteristics.
Further, the light absorption medium adsorbed in the adsorption process becomes a light absorption layer in the process of drawing the preform into a fiber shape by heating and stretching. Therefore, the light leaking from the core part can be absorbed by the light absorption layer, and the light leaking to the outside and the light returning to the core part can be reduced. As a result, it is possible to suppress the occurrence of waveform distortion in the optical signal due to total reflection of light leaked from the core portion and re-coupling with the core portion, and the propagation characteristics of the optical signal can be reduced. Deterioration can be reduced.

In addition, the optical fiber manufacturing method according to the reference example of the present invention includes a multicapillary in which a plurality of thin glass wires are bundled and arranged in a jacket tube to form a preform, and the preform is heated and drawn into a fiber shape. An optical fiber in which a plurality of pores extending along the central axis direction of the core portion are arranged in a single concentric circle centered on the central axis in a clad portion covering the periphery of the core portion A manufacturing method of an optical fiber to be manufactured, wherein N pieces (where N is an integer of 7 or more) of the thin glass wires are arranged so as to surround the periphery of the central glass rod having the core portion over the entire circumference. And a preform forming step of disposing the central glass rod and the glass thin tube in the jacket tube, and at least two of the N glass thin wires are hollow glass. The jacket tube has a circular inner peripheral shape in a cross-sectional view orthogonal to the central axis thereof, and the thin glass wire and the central glass rod are cross-sectional views orthogonal to the respective central axes. Are formed in a circular shape, and when the diameter of the central glass rod is 2R, the outer diameter of the thin glass wire is 2r, and the inner diameter of the jacket tube is 2 (R + 2r), r = (R + r) · sin ( π / N), and at least one of the outer periphery of the central glass rod, the outer periphery of the thin glass wire, and the inner periphery of the thin glass tube is a glass material that forms the central glass rod and the thin glass wire Impurities having a higher extinction coefficient than that may be attached.

According to the method of manufacturing an optical fiber according to the reference example of the present invention, the diameter 2R of the central glass rod, the outer diameter 2r of the thin glass wire, and the inner diameter 2 (R + 2r) of the jacket tube are:
r = (R + r) · sin (π / N)
Therefore, in the preform forming process, N glass thin wires are arranged so as to surround the central glass rod in a line all around the circumference, and the central glass rod and the glass thin tube are jacketed. It is possible to fill the inside of the jacket tube with the central glass rod and the thin glass wire simply by disposing it in the tube, and the central glass rod can be disposed at the center of the jacket tube. In addition, at this time, it becomes possible to arrange the glass thin wire without any gap between adjacent ones over the entire circumference of the central glass rod, that is, without play, and the position of the hole of the glass thin tube relative to the entire preform can be determined. It can be determined with high accuracy. Accordingly, it is possible to easily manufacture an optical fiber having desired characteristics.
In addition, impurities attached to at least one of the outer periphery of the central glass rod, the outer periphery of the glass thin wire, and the inner periphery of the glass thin tube are heated in the process of drawing the preform into a fiber shape. It becomes an absorption layer. Therefore, the light leaking from the core part can be absorbed by the light absorption layer, and the light leaking to the outside and the light returning to the core part can be reduced. As a result, it is possible to suppress the occurrence of waveform distortion in the optical signal due to total reflection of light leaked from the core portion and re-coupling with the core portion, and the propagation characteristics of the optical signal can be reduced. Deterioration can be reduced.

Further, in the optical fiber according to the reference example of the present invention, a plurality of pores extending along the central axis direction of the core portion are formed in a single concentric circle centered on the central axis in the cladding portion covering the periphery of the core portion. An optical fiber formed side by side, wherein a light absorption layer having a light absorption coefficient higher than that of the clad portion is formed in the vicinity of the concentric circle.

According to the optical fiber according to the reference example of the present invention , it is possible to absorb the light leaking from the core portion by the light absorption layer formed in the vicinity of the concentric circles, that is, in the vicinity of the pores, and the light leaking to the outside and Light returning to the core can be reduced. As a result, it is possible to suppress the occurrence of waveform distortion in the optical signal due to total reflection of light leaked from the core portion and re-coupling with the core portion, and the propagation characteristics of the optical signal can be reduced. Deterioration can be reduced.

  The light absorption layer may be formed along an edge of the pore.

  In this case, since the light absorption layer is formed along the edge of the pore, light returning to the core portion can be surely reduced.

  According to the present invention, an optical fiber having desired characteristics can be easily manufactured.

It is a typical cross section of the core part vicinity in the optical fiber which concerns on 1st Embodiment of this invention. It is a typical cross-sectional view of the preform for manufacturing the optical fiber shown in FIG. It is a typical cross-sectional view of the preform for manufacturing the optical fiber shown in FIG. It is a typical cross section of the core part vicinity in the optical fiber which concerns on 2nd Embodiment of this invention. It is a typical cross section of the core part vicinity in the modification of the optical fiber which concerns on 2nd Embodiment of this invention. It is a figure for examining the example of arrangement | positioning of a center glass rod and a capillary in the preform of the manufacturing method of the optical fiber which concerns on this invention. It is a figure for examining the example of arrangement | positioning of a center glass rod and a capillary in the preform of the manufacturing method of the optical fiber which concerns on this invention. It is a figure for examining the example of arrangement | positioning of a center glass rod and a capillary in the preform of the manufacturing method of the optical fiber which concerns on this invention. It is a figure for examining the example of arrangement | positioning of a center glass rod and a capillary in the preform of the manufacturing method of the optical fiber which concerns on this invention. It is a figure for examining the example of arrangement | positioning of a center glass rod and a capillary in the preform of the manufacturing method of the optical fiber which concerns on this invention. It is a figure for examining the example of arrangement | positioning of a center glass rod and a capillary in the preform of the manufacturing method of the optical fiber which concerns on this invention. It is a figure for examining the example of arrangement | positioning of a center glass rod and a capillary in the preform of the manufacturing method of the optical fiber which concerns on this invention. It is a figure which shows the example of arrangement | positioning of a center glass rod, a capillary, and a glass rod in the preform of the manufacturing method of the optical fiber which concerns on this invention. It is a figure which shows the example of arrangement | positioning of a center glass rod, a capillary, and a glass rod in the preform of the manufacturing method of the optical fiber which concerns on this invention. It is a typical cross section of the core part vicinity in the modification of the optical fiber which concerns on 2nd Embodiment of this invention. It is a typical cross section of the core part vicinity in the modification of the optical fiber which concerns on 2nd Embodiment of this invention. It is a typical cross section of the core part vicinity in the conventional optical fiber.

(First embodiment)
Hereinafter, an optical fiber according to a first embodiment of the present invention will be described with reference to FIG.
The optical fiber 1 includes a core part 2 and a clad part 3 that covers the periphery of the core part 2. In the clad portion 3, a plurality of pores 4 extending along the direction of the central axis O of the core portion 2 are formed side by side on a single concentric circle centered on the central axis O. In the present embodiment, N (10 in the illustrated example) pores 4 are arranged at equal intervals around the central axis O. The optical fiber 1 has, for example, a clad portion 3 made of quartz glass and a core portion 2 made of high-flexibility glass (for example, a mixture of germanium and the like in quartz glass), and a refractive index distribution. Is set to be suitable as a single mode optical fiber.

  Next, the manufacturing method of the optical fiber 1 shown above is demonstrated with reference to FIG.2 and FIG.3. In the present embodiment, as shown in FIG. 2, a plurality of capillaries (glass thin tubes) 12 are bundled and arranged in a jacket tube 11 to form a preform 10, and the preform 10 is heated and stretched to form a fiber. The optical fiber 1 is manufactured by a multicapillary method in which the diameter is reduced by drawing. Details will be described below.

  First, a member production process for producing each member constituting the preform 10 is performed. At this time, the central glass rod 13 having the core portion 2 may be produced by a known method such as a VAD method or an OVD method. Further, adding a rare earth element to the core portion 2 to provide functionality can be performed using a known technique. The jacket tube 11 and the capillary 12 can be made of fused silica glass in addition to synthetic quartz glass by the above method or other methods.

At this time, as shown in FIG. 2, the jacket tube 11 is manufactured so that the inner peripheral shape in a cross-sectional view orthogonal to the central axis thereof is circular, and the capillary 12 and the central glass rod 13 are respectively formed. The outer shape in a cross-sectional view orthogonal to the central axis of each is formed in a circular shape. If the diameter (outer diameter) of the central glass rod 13 is 2R, the outer diameter of the capillary 12 is 2r, and the inner diameter of the jacket tube 11 is 2 (R + 2r),
r = (R + r) · sin θ (1)
Each member is produced so that In the mathematical formula (1), θ = π / N. Further, it may be standardized as R + r = 1 at the time of design.

Next, an impurity removal step is performed in which impurities formed by adsorbing moisture in the air on the outer periphery of the central glass rod 13, the outer periphery of the capillary 12, and the inner periphery of the capillary 12 are removed by physical polishing or chemical polishing. . Since impurities are adsorbed by moisture in the air, the extinction coefficient is higher than that of the glass material forming the central glass rod 13 and the capillary 12.
Next, as shown in FIG. 2, N capillaries 12 are arranged so as to surround the central glass rod 13 in a row around the central axis O over the entire circumference, and these central glass rods 13 and 13 A preform forming step for arranging the capillary 12 in the jacket tube 11 is performed. At this time, the capillary 12 and the central glass rod 13 are bundled and arranged in the jacket tube 11 so that the central axes of the jacket tube 11, the capillary 12 and the central glass rod 13 are parallel to each other.

  Here, as shown in FIG. 2, in the present embodiment, each member is manufactured so as to satisfy the formula (1). Therefore, the center glass rod 13 and the capillary can be simply obtained by performing this preform forming step. 12, the inside of the jacket tube 11 can be filled, and the central glass rod 13 can be disposed at the center of the jacket tube 11. In addition, at this time, the capillary 12 can be disposed without any gap between adjacent ones over the entire circumference of the central glass rod 13, that is, without play. The position can be determined with high accuracy. That is, the center-symmetric preform 10 can be manufactured with high accuracy. FIG. 2 is an enlarged view of the vicinity of the center of the cross section of the preform 10, and the actual jacket tube 11 is thicker than that shown in FIG.

Next, a drawing process is performed in which the preform 10 is drawn by heat drawing of the prior art to reduce the diameter. At this time, a plurality of substantially triangular first gaps 14 between the inner periphery of the jacket tube 11 and the outer periphery of the capillary 12, and a plurality of substantially triangular shapes between the outer periphery of the central glass rod 13 and the outer periphery of the capillary 12. The optical fiber 1 shown in FIG. 1 can be manufactured by drawing so as to crush the second gap 15.
According to the manufacturing method of the optical fiber 1 shown above, the optical fiber 1 having desired characteristics (low bending loss) can be easily manufactured.

  In addition, as shown in FIG. 3, you may arrange | position the glass rod 16 for hole filling in the said 1st space | gap 14 in a preform formation process. In this case, the first gap 14 can be easily crushed in the drawing step. Further, the hole filling glass rod 16 may be arranged in the second gap 15 or in both the first gap 14 and the second gap 15. Furthermore, the gaps 14 and 15 may not be completely crushed, that is, they may remain as finer gaps in the optical fiber 1 than the pores 4 of the cladding portion 3. FIG. 3 is an enlarged view of the vicinity of the center of the cross section of the preform 10, and the actual jacket tube 11 is thicker than that shown in FIG.

(Second Embodiment)
Next, an optical fiber 20 according to a second embodiment of the present invention will be described with reference to FIG. In the second embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, description thereof is omitted, and only different points will be described.
Hereinafter, a direction orthogonal to the central axis O is simply referred to as a radial direction.

  In the optical fiber 20 of the present embodiment, a light absorption layer 21 is formed in the vicinity of the concentric circle in which the pores 4 are arranged. In the illustrated example, the light absorption layer 21 has a transverse cross section perpendicular to the central axis O so as to circulate around the core part 2 around the central axis O inside the cladding part 3 in the radial direction of any pore 4. It is formed in a circular shape. The size along the radial direction of the inner cladding portion 3 </ b> A that is a portion located between the core portion 2 and the light absorption layer 21 in the cladding portion 3 is larger than, for example, the outer diameter of the core portion 2. Further, the light absorption layer 21 has a larger extinction coefficient than, for example, the core portion 2 and the cladding portion 3.

Next, the manufacturing method of the optical fiber 20 shown above is demonstrated.
First, in this embodiment, the central glass rod 13 is produced so that it may have the core part 2 and the inner clad part 3A which covers the circumference | surroundings of the core part 2 in a member preparation process.
Next, in the impurity removal step, impurities adhering to the outer periphery of the capillary 12 and the inner periphery of the capillary 12 are removed by physical polishing or chemical polishing. At this time, the outer periphery of the center glass rod 13 is not polished, and impurities are left (attached) on the outer periphery of the inner cladding portion 3 </ b> A that is the outer periphery of the center glass rod 13.
And a preform formation process and a drawing process are performed. Thereby, the impurities remaining on the outer periphery of the central glass rod 13 become the light absorption layer 21.

According to the optical fiber 20 described above, the light leaking from the core part 2 can be absorbed by the light absorption layer 21, and the light leaking to the outside and the light returning to the core part 2 can be reduced. As a result, it is possible to suppress the waveform distortion in the optical signal caused by the light leaking from the core part 2 being totally reflected by the pores 4 and coupled to the core part 2 again. It is possible to reduce the deterioration of propagation characteristics.
Moreover, according to the manufacturing method of the optical fiber 20 shown above, since the light absorption layer 21 can be formed by slightly changing the process performed by the conventional multicapillary method, It can be manufactured by directly applying the manufacturing technique related to the capillary method.

  Further, since the central glass rod 13 includes the inner cladding portion 3A, the central glass rod 13 can be formed such that the core portion 2 and the outer periphery thereof are sufficiently separated in the radial direction. Therefore, in the preform forming step, the core portion 2 of the central glass rod 13 and the capillary 12 can be sufficiently separated in the radial direction, and the pore 4 is sufficiently separated from the core portion 2 in the optical fiber 20. be able to. Therefore, for example, by using high-purity glass with little light absorption for the inner cladding portion 3A, even if there is weak absorption in the light absorption layer 21, the influence on the transmission loss of the optical fiber 20 itself can be minimized. . When the position of the pore is close to the core, the light intensity distribution in the fiber cross section extends to the vicinity of the pore, and as a result, the formation of the light absorption layer has an effect on the transmission loss of the optical fiber itself. It may occur.

  In the present embodiment, when forming the light absorption layer 21, the impurities on the outer periphery of the central glass rod 13 are left in the impurity removing step without polishing the outer periphery of the central glass rod 13. It is not limited. For example, in the impurity removal step, the outer periphery may be polished so that impurities slightly remain on the outer periphery of the central glass rod 13. Further, impurities on the inner periphery or outer periphery of the capillary 12 may remain. In this case, as in the optical fiber 30 shown in FIG. 5, the light absorption layer 21A is formed in a circular shape in the cross-sectional view along the edge of the pore 4, and around the central axis O on the concentric circle, that is, They are arranged on a circle equidistant from the central axis O. According to the optical fiber 30, the light absorption layer 21 </ b> A is formed along the edge of the pore 4, so that the light returning to the core portion 2 can be reliably reduced.

  Further, after removing all impurities on the outer periphery of the central glass rod 13, the outer periphery of the capillary 12, and the inner periphery of the capillary 12, and before the preform forming step, the outer periphery of the central glass rod 13, the outer periphery of the capillary 12, and the capillary 12 An adsorption process for adsorbing a light-absorbing medium (for example, metal) having a light absorption coefficient higher than that of the glass material forming the central glass rod 13 and the capillary 12 on any of the inner circumferences of A light absorption layer may be formed. Even in this case, the same effects can be achieved.

The technical scope of the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.
For example, in each of the above embodiments, the case where the number N of capillaries 12 is 10 has been described as an example, but the present invention is not limited to this.
Here, the relationship between the number N of capillaries 12 and the diameter 2R of the central glass rod 13 and the outer diameter 2r of the capillary 12 in a state where the inner diameter 2 (R + 2r) of the jacket tube 11 is constant will be examined. As shown in FIGS. 6 to 8, when the number N of capillaries 12 is 3 to 5, the outer diameter of the central glass rod 13 is smaller than the outer diameter of the capillary 12. As shown in FIG. 9, when N is 6, so-called closest packing, in which the outer diameter of the central glass rod 13 is equal to the outer diameter of the capillary 12. Therefore, as shown in FIGS. 10 to 12, in order to sufficiently secure (separate) the interval (distance in the radial direction) between the core portion 2 and the pores 4 as shown in the second embodiment, The number N of capillaries 12 is preferably 7 or more. 6-12, illustration of the core part 2 in the center glass rod 13 is abbreviate | omitted for the legibility of drawing.

  In each of the above embodiments, the entire periphery of the center glass rod 13 is surrounded by the capillary 12, but the present invention is not limited to this, and instead of the hollow capillary 12, the inside diameter of the capillary 12 is equal. A real glass rod 17 may be used. In any case, it is sufficient that there are at least two capillaries 12 and the total number of capillaries 12 and glass rods 17, that is, the total number of thin glass wires is N. When the glass rod 17 is used, for example, when it is desired to arrange the pores 4 so as to be positioned at triangular corners in the cross-sectional view, as shown in FIG. 13, three capillaries 12 and six glasses are used. The rods 17 (that is, N = 9) are produced, and one capillary 12 may be arranged every two glass rods 17. Further, when it is desired to arrange the pores 4 so as to be positioned at the corners of the square in the cross sectional view, as shown in FIG. 14, four capillaries 12 and glass rods 17 (that is, N = 8) are provided. ) May be arranged alternately. In FIG. 13 and FIG. 14, illustration of the core portion 2 in the central glass rod 13 is omitted for easy viewing of the drawings. Furthermore, even when the glass rod 17 is used, the light absorption layers 41A and 41B may be provided on the capillary 12 and / or the glass rod 17 as in the optical fibers 40A and 40B shown in FIGS. good. In this case, when the light absorption layers 41A and 41B are provided only on either the capillary 12 or the glass rod 17, the light absorption layers 41A and 41B are intermittently arranged around the central axis O.

  In addition, it is possible to appropriately replace the constituent elements in the embodiment with known constituent elements without departing from the spirit of the present invention, and the above-described modified examples may be appropriately combined.

DESCRIPTION OF SYMBOLS 1, 20, 30, 40A, 40B Optical fiber 2 Core part 3 Cladding part 4 Pore O Center axis 10 Preform 11 Jacket tube 12 Capillary (glass thin tube)
13 Central glass rod 17 Glass rod (glass thin wire)
21, 21A, 41A, 41B Light absorption layer

Claims (1)

A multi-capillary method in which N (where N is an integer of 7 or more) hollow glass thin tubes are bundled and arranged in a jacket tube to form a preform, and the preform is heated and drawn into a fiber shape, Light for manufacturing an optical fiber, in which a plurality of pores extending along the central axis direction of the core part are formed in a clad part covering the periphery of the core part and arranged in a single concentric circle centered on the central axis A fiber manufacturing method comprising :
The outer periphery of the central glass rod having the core portion, and the impurity removing step of removing by polishing impurities adhering the outer periphery of the glass capillary, and each of the inner periphery of the thin glass tube,
An adsorption step of adsorbing a light absorption medium having a higher extinction coefficient than the glass material forming the central glass rod and the glass thin tube on the outer periphery or inner periphery of all the N glass thin tubes ,
The N glass tubules are arranged so as to surround the periphery of the central glass rod over the entire circumference, and a preform forming step of arranging the central glass rod and the glass tubules in the jacket tube,
The jacket tube has a circular inner peripheral shape in a cross-sectional view orthogonal to the central axis, and the glass thin tube and the central glass rod have an outer shape in a cross-sectional view orthogonal to the central axis. Are each formed into a circular shape,
When the diameter of the central glass rod is 2R, the outer diameter of the glass thin tube is 2r, and the inner diameter of the jacket tube is 2 (R + 2r),
r = (R + r) · sin (π / N)
A method of manufacturing an optical fiber, wherein

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