Abstract:
An optical fiber preform manufacturing method includes: supporting a drilling jig in a radial direction of a preform that is cylinder-shaped; moving the drilling jig in a longitudinal direction of the preform; and forming a plurality of slits each extending in the longitudinal direction and each directed from an outer side of the preform toward a center the preform, and a plurality of holes each extending in the longitudinal direction and each connecting with an end of one of the plurality of slits in a depth direction of the one of the plurality of slits.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of PCT International Application No. PCT/JP2010/050831 filed on Jan. 22, 2010 which claims the benefit of priority from U.S. Provisional Application No. 61/146,819 filed on Jan. 23, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical fiber preform manufacturing method for manufacturing an optical fiber having holes extending in a longitudinal direction. 
     2. Description of the Related Art 
     Optical fibers having holes extending in a longitudinal direction, so-called microstructure optical fibers, have characteristics that cannot be achieved by optical fibers having a normal structure of confining light with a core and a cladding, and thus are expected to be optical fibers of the next generation. A holey fiber, a hole-assisted fiber, and a photonic bandgap fiber are known as the microstructure optical fibers. 
     The holey fiber has holes formed around a region that has a substantially uniform refractive index and that is near a center axis, and guides light with a core, which is the region near the center axis in which the holes are not formed. The hole-assisted fiber has holes formed around a core of an optical fiber of a normal structure. The photonic bandgap fiber has a hole formed on a center axis and becoming a core, and has holes cyclically formed around this hole, thereby generating a photonic bandgap to guide light. 
     A stack-and-draw method and a drilling method are known as methods of manufacturing a preform for a microstructure optical fiber. The stack-and-draw method is a method of manufacturing a preform for a microstructure optical fiber by bundling plural capillary tubes. The hole drilling method is a method of manufacturing a preform for a microstructure optical fiber by forming through-holes in a longitudinal direction of the preform with a drill (see Japanese Patent Application Laid-open No. 2002-321935 and Japanese Patent Application Laid-open No. 2003-342032). 
     The stack-and-draw method is suitable for arranging many holes in a constant cycle. However, in the stack-and-draw method, capillary tubes need to be prepared as many as the number of holes to be formed, and therefore procurement of parts is difficult. Furthermore, because unintended vacant space is formed between the capillary tubes, characteristics of the optical fiber can become unstable. 
     The drilling method is suitable for arranging a small number of holes at arbitrary positions in a cross-sectional direction. When a microstructure optical fiber is manufactured, desired optical-fiber characteristics need to be achieved by controlling arrangement positions and shapes of its holes precisely. Therefore, in manufacturing a preform for a microstructure optical fiber, the hole drilling method is used in many cases, particularly when the number of holes to be arranged is small. 
     However, when the drilling method is used, a hole having a length greater than that of a drill used in the drilling method cannot be formed in one drilling process. Specifically, the length of a drill is about 500 millimeters at most, and thus a hole having a length larger than 500 millimeters cannot be formed in one hole drilling process. Therefore, when the hole drilling method is used, there is a constraint on the length of manufacturable preforms. Accordingly, when the hole drilling method is used, manufacturing large preforms is difficult, and it is difficult to reduce the manufacturing cost of an optical fiber having holes extending in a longitudinal direction. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According an aspect of the present invention, an optical fiber preform manufacturing method includes: supporting a drilling jig in a radial direction of a preform that is cylinder-shaped; moving the drilling jig in a longitudinal direction of the preform; and forming a plurality of slits each extending in the longitudinal direction and directed from an outer side of the preform toward a center the preform, and a plurality of holes each extending in the longitudinal direction and connecting with an end of one of the plurality of slits in a depth direction of the one of the plurality of slits. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a configuration of an optical fiber preform according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view along a line A-A of the optical fiber preform depicted in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of a modification of the optical fiber preform depicted in  FIG. 1 ; 
         FIG. 4  is a cross-sectional view of another modification of the optical fiber preform depicted in  FIG. 1 ; 
         FIG. 5A  is a conceptual diagram for explaining a manufacturing method of the optical fiber preform depicted in  FIG. 1 ; 
         FIG. 5B  is another conceptual diagram for explaining a manufacturing method of the optical fiber preform depicted in  FIG. 1 ; 
         FIG. 6  is still another conceptual diagram for explaining a manufacturing method of the optical fiber preform depicted in  FIG. 1 ; 
         FIG. 7  depicts a relationship between pressure in a hole at the time of stretching an optical fiber preform and circularity of the hole; 
         FIG. 8A  is a schematic diagram of a change in shapes of holes when a pressure in the holes at the time of stretching an optical fiber preform is less than an optimal pressure; 
         FIG. 8B  is a schematic diagram of a change in shapes of holes when a pressure in the holes at the time of stretching an optical fiber preform is at an optimal pressure; 
         FIG. 8C  is a schematic diagram of a change in shapes of holes when a pressure in the holes at the time of stretching an optical fiber preform is greater than a proper value; and 
         FIG. 9  depicts a relationship between cross-sectional area of a hole after stretching and optimal pressure in the holes at the time of stretching the optical fiber preform. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An optical fiber preform manufacturing method according to an embodiment of the present invention will be explained below with reference to the drawings. 
     [Configuration of Optical Fiber Preform] 
     A configuration of an optical fiber preform according to an embodiment of the present invention will be explained first with reference to  FIGS. 1 and 2 . 
       FIG. 1  is a perspective view of the configuration of the optical fiber preform according to the embodiment of the present invention.  FIG. 2  is a cross-sectional view along a line A-A of the optical fiber preform depicted in  FIG. 1 . As depicted in  FIGS. 1 and 2 , the optical fiber preform according to the embodiment of the present invention includes a cylindrical preform  1 , plural slits  2   a  to  2   f  extending in a longitudinal direction of the preform  1  and formed on an outer peripheral surface of the preform  1 , and plural holes  3   a  to  3   f  connecting with the slits  2   a  to  2   f  and extending in the longitudinal direction of the preform  1 . 
     The preform  1  is formed of silica based glass. Specifically, the preform  1  is formed of a material having a region near a center axis, which has a substantially uniform refractive index. A material having a core having a higher refractive index than that at an outer peripheral portion on the center axis may be used as the preform  1 . The plural slits  2   a  to  2   f  have a predetermined depth L from the outer peripheral surface of the preform  1  toward a center axis direction of the preform  1 . The plural holes  3   a  to  3   f  connect with the slits  2   a  to  2   f  at ends of the slits in a depth direction. While details are described later, preferably, the predetermined depth L is equal to or larger than 2 millimeters and equal to or smaller than 20 millimeters. A width D of each of the slits is preferably equal to or larger than 0.5 millimeter and equal to or smaller than a quarter of a diameter of the holes  3   a  to  3   f.    
     Although the optical fiber preform has only one layer of the preform  1  according to the present embodiment, the optical fiber preform may be a preform having multiple layers of holes by accommodating a preform  1 A in a tubular preform  1 B having slits and holes formed therein as depicted in  FIG. 3 . Alternatively, as depicted in  FIG. 4 , the preform  1  and a tubular member  5  may be integrated with each other by accommodating the preform  1  in the tubular member  5  such as a glass tube and by fusing the preform  1  with the tubular member  5 . 
     [Method of Manufacturing a Preform for Optical Fiber] 
     A method of manufacturing the optical fiber preform depicted in  FIG. 1  will be explained with reference to  FIGS. 5A and 5B . 
       FIGS. 5A and 5B  are schematic diagrams for explaining the method of manufacturing the optical fiber preform depicted in  FIG. 1 . When the optical fiber preform depicted in  FIG. 1  is manufactured, as depicted in  FIG. 5A , plural slits  2  extending in a longitudinal direction of the preform  1  are first formed on the outer peripheral surface of the preform  1  by using a grindstone or a file, and thereafter a drilling jig  11  having a tubular drill portion  11   a  and an arm portion  11   b  supporting the drill portion  11   a  is set at a longitudinal-direction end of the preform  1 . Next, as depicted in  FIG. 5B , the drilling jig  11  is moved along the longitudinal direction of the preform  1  to pass the arm portion  11   b  through the slits  2 , thereby forming plural holes  3  connecting with the slits  2  and extending in the longitudinal direction of the preform  1 . Consequently, the optical fiber preform depicted in  FIG. 1  is manufactured. 
     Further, the preform  1  having the holes  3  formed therein is stretched in a longitudinal direction to make an outer diameter of the preform  1   a  predetermined size by using a well-known heating method such as a method of heating with a flame, a high-frequency-induction plasma torch, or an electric furnace. A long optical fiber preform may be manufactured by stretching the preform  1  after forming the holes  3 . Because not only the outer diameter of the preform  1  but also diameters of the holes are decreased by stretching the preform  1 , the diameters of the holes drilled by the drilling jig  11  may be large. As a result, formation of the holes  3  by the drilling jig  11  and also cleaning the inside of the holes  3  are facilitated. 
     As it is clear from the above explanation, according to the optical fiber preform manufacturing method according to the embodiment of the present invention, the plural slits  2  extending in the longitudinal direction of the preform  1  are formed on the outer peripheral surface of the cylindrical preform  1 , the drilling jig  11  is introduced into a predetermined position in the radial direction of the preform  1  via the slits  2 , and the drilling jig  11  is moved in the longitudinal direction of the preform  1 , thereby forming the plural holes  3  connecting with the slits  2  and extending in the longitudinal direction of the preform  1 . That is, in the optical fiber preform manufacturing method according to the embodiment of the present invention, the drilling jig  11  is introduced via the slits  2  which are open faces, and the drilling by the drilling jig  11  is performed. Therefore, according to the optical fiber preform manufacturing method of the embodiment of the present invention, drilling up to an unlimited length is possible in principle, unlike in a conventional drilling method of forming through-holes using a drill, and thus, an optical fiber having holes extending in a longitudinal direction is economically manufacturable. 
     If, before the stretching, the slits  2  are eliminated to leave only the holes  3  by heating the outer periphery portion of the preform  1  by a well-known heating method such as a method of heating using a flame, a high-frequency-induction plasma torch, and an electric furnace, the preform  1  similar to that formed with through-holes is able to be formed. However, in this case, heating temperature of the preform  1  is preferably within a ±1% range of a softening point temperature of the silica based glass forming the preform  1 . When the heating temperature is low, the shape of the preform  1  does not change, and the slits  2  are not eliminated. On the contrary, when the heating temperature is high, the shape of the holes  3  is changed. 
     The softening point temperature of the silica based glass is 1800° C. When the actually manufactured preforms  1  were heated at 1773° C., 1782° C., 1800° C., 1818° C., and 1827° C., it was confirmed that the slits  2  were not eliminated at 1773° C., and the slits  2  were eliminated at or above 1782° C. However, when the preform  1  was heated at 1827° C., circularity (=(minimum diameter/maximum diameter)×100) of the holes  3  became 85%. From the above, if aiming only to eliminate the slits  2 , the preform  1  may be heated to or above the softening point temperature of the silica based glass forming the preform  1 , but if aiming to maintain shapes of the holes  3  while eliminating the slits  2 , the preform  1  may be heated to a temperature near the softening point temperature of the silica based glass forming the preform  1 . 
     When the depth L of the slits  2  is small, the distance between the hole  3  and the outer periphery of the preform  1  becomes short, and this becomes a cause of cracks being generated upon machining. On the contrary, when the depth L of the slits  2  is large, the length of the arm portion  11   b  becomes long and precision of positions at which the holes  3  are formed becomes low. When the length of the arm portion  11   b  becomes large, load on the arm portion  11   b  becomes large, and this becomes a cause of damaging the drilling jig  11 . Therefore, the depth L of the slit  2  is preferably equal to or larger than 2 millimeters and equal to or smaller than 20 millimeters. 
     When the width D of the slits  2  is small, the arm portion  11   b  may contact a sidewall of the slits  2 , and this may become an obstacle upon movement of the arm portion  11   b . On the contrary, when the width D of the slit  2  is large, the holes  3  may be deformed greatly as the slits  2  are eliminated upon heating the outer periphery portion of the preform  1  to eliminate the slits  2 . Therefore, the width D of the slits  2  is preferably equal to or larger than 0.5 millimeter and equal to or smaller than a quarter of a diameter of the holes  3   a  to  3   f.    
     In the present embodiment, although the holes  3  are formed by using the drilling jig  11  depicted in  FIG. 5A , the slits  2  and the holes  3  may be simultaneously formed, as depicted in  FIG. 6 , by setting, in a longitudinal-direction end of the preform  1 , a drilling jig  21  made of a spherical drill portion  21   a  and an arm portion  21   b , such as a file for forming slits, which supports the drilling unit  21   a , and moving the drilling jig  21  along a longitudinal direction of the preform  1  while rotating the drill portion  21   a  with the arm portion  21   b  as a rotation axis. 
     When the optical fiber preform is stretched after eliminating the slits  2 , the circularity of holes depends on pressure in the holes at a heat-softened part.  FIG. 7  depicts a relationship between pressure in holes and circularity of the holes when a radius r of the holes after stretching is 2 millimeters. When a pressure in the holes is within a range of a region R 1  depicted in  FIG. 7 , the outer diameter of the holes  3  decreases in a direction from a center position O of the preform  1  toward an outer periphery and the circularity decreases, as depicted in  FIG. 8A . When a pressure in the holes is within a range of a region R 2  depicted in  FIG. 7 , the hole  3  keeps its completely round state and the circularity does not decrease greatly, as depicted in  FIG. 8B . On the contrary, when a pressure in the holes is within a range of a region R 3  as depicted in  FIG. 7 , the outer diameter of the holes  3  increases in a direction from the center position O of the preform  1  toward the outer periphery and the circularity decreases, as depicted in  FIG. 8C . 
     Therefore, when the optical fiber preform is stretched after the slits  2  are eliminated, the pressure in the holes at the heat-softened part needs to be controlled to be at an optimal value. Specifically, when the radius r of the holes after the stretching is to be 2 millimeters, a pressure in the holes at the heat-softened part is preferably set at a value corresponding to the region R 2  depicted in  FIG. 7 , that is, at a pressure that is 0.3 kPa to 1 kPa greater than atmospheric pressure. Consequently, the deformation of the holes becomes small, and characteristics of the optical fiber are able to be stabilized. 
     However, an optimal value of a pressure in the holes changes according to a cross-sectional area of a hole after the stretching as depicted in  FIG. 9 . That is, an optimal value of a pressure in the holes becomes smaller when the cross-sectional area of the hole after the stretching becomes larger. Therefore, when the optical fiber preform is stretched, an optimal value of a pressure in the holes needs to be set correspondingly to a targeted value of the cross-sectional area of the hole. A pressure in the holes may be adjusted by a method of sealing both ends of the holes or by a method of supplying an inert gas. 
     As depicted in  FIG. 4 , the preform  1  and a tubular member may be integrated by inserting the preform  1  into the tubular member such as a glass tube, heating the preform  1  to a ±1% range of a softening point temperature of the silica based glass forming the preform  1  to fuse the preform  1  with the tubular member. A heating method of the preform  1  may be, for example, a well-known heating method such as a method of heating using a flame, a high-frequency-induction plasma torch, and an electric furnace. When fusing, by making a clearance between an inner diameter of the tubular member and an outer diameter of the preform  1  about one millimeter, fusing with both ends of the preform  1  open may be performed, without controlling an internal pressure of the holes. The optical fiber may be manufactured by performing fiber-stretching simultaneously with fusing. 
     A cladding may be further formed around the preform  1  having the holes  3  formed therein. A method of forming the cladding may be, for example, a method of inserting into the tubular member the preform  1  having the holes  3  formed therein and crushing the tubular member, or a method of depositing soot around the preform  1  having the holes  3  formed therein and vitrifying. When the cladding is formed around the preform  1  having the holes  3  formed therein, positions to form the holes  3  and the size of the holes  3  are set in advance by taking into consideration a quantity of the cladding to be formed. When the cladding is formed by the method of crushing the tubular member, an optical fiber may be manufactured by simultaneously performing fiber-stretching. 
     While embodiments to which the invention made by the present inventors is applied have been explained above, the present invention is not limited to the descriptions and the drawings, which form a part of the disclosure of the present invention according to these embodiments. That is, all of any other embodiments, examples, operational techniques, and the like, which are realized by those skilled in the art based on the present embodiments, are included in the scope of the present invention. 
     According an embodiment of the present invention, an optical fiber having holes extending in a longitudinal direction is economically manufacturable. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.