Patent Publication Number: US-2019177204-A1

Title: Method for manufacturing optical fiber preform and method for manufacturing optical fiber

Description:
TECHNICAL FIELD 
     The present invention relates to a method of manufacturing an optical fiber preform and a method of manufacturing an optical fiber using the same. 
     BACKGROUND ART 
     Glass rods formed of silica glass are primarily used in manufacturing of optical fibers in the field of optical communication and optics. In the related art, such glass rods (optical fiber preforms) are manufactured, for example, by forming a porous material of silica glass using an outside vapor deposition (OVD) method or a powder molding method so as to surround a glass rod for forming a core portion that is produced by a vapor-phase axial deposition (VAD) method, producing a porous preform, and further heating and sintering the porous preform. 
     In the case of manufacturing what is known as a single-core type optical fiber that includes a single core portion in the optical fiber, one glass rod is used for forming the core portion, and the porous preform has a structure in which one glass rod for forming the core portion protrudes from the vicinity of the center axis of the porous material. When sintering the porous preform, the porous preform is held in a sintering furnace by gripping this single glass rod for forming the core portion, and sintering is performed. 
     In contrast, in order to cope with the recent increase in transmission capacity in optical fiber communication, multi core fibers having a plurality of core portions in the cross section of the optical fiber have been contemplated. As methods of manufacturing the optical fiber preform for a multi core fiber, a perforation method in which a plurality of holes are made in a glass rod, and a glass rod for forming a core portion is inserted into each of the holes, or alternatively, a stack-and-draw method in which glass rods for forming core portions are bundled and drawn, are generally used. However, in the case of the perforation method, as steps for preparing the glass rod and further forming a plurality of holes in the glass rod are involved, there are problems in that it is difficult to manufacture a large optical fiber preform, and the cost increases. Also, in the case of the stack-and-draw method, in addition to the difficulty of manufacturing large optical fiber preforms, there are constraints on the cost and structure, such as difficulty in increasing the positional accuracy of the core. 
     In contrast to this, a method in which the previously mentioned porous preform is produced and sintered is widely used as a manufacturing method of optical fibers, and since it is possible to manufacture large optical fiber preforms, even in cases of manufacturing multi core fibers, it is advantageous from a cost standpoint. 
     For example, as schematically illustrated in  FIG. 15A , cases are considered for manufacturing a multi core fiber  3  in which a core portion  1  is constituted by a core portion  1   a  located at the center and six core portions  1   b  are arranged surrounding it so as to form a regular hexagon, and cladding portions  2  are formed on the outer periphery of these core portions  1 . It should be noted that the region surrounded by the broken line in the multi core fiber  3  is a region formed by a glass rod for forming a core portion, as described later. 
       FIG. 15B  schematically illustrates a porous preform  4  for manufacturing the multi core fiber  3 . The porous preform  4  has a structure in which a porous material  6  formed from fine glass particles is deposited around seven glass rods  5  for forming the core portion, and the seven glass rods  5  protrude from the porous material  6 . Here, the glass rods  5  have a structure in which a cladding portion forming portion for forming a part (the region surrounded by the outer edges of the core portions  1  and the broken lines in  FIG. 15A ) of the cladding portion  2  is formed around the core portion forming portion for forming the core portion  1 . 
     In the case of sintering this porous preform  4 , the porous preform  4  is held in the sintering furnace by gripping the end of the protruding side of the glass rod  5   a  located at the center of the protruding glass rods  5  with the grasping tool  7 , and performing sintering while rotating the porous preform  4  around its axis (See Patent Document 1). 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: Japanese Patent Publication No. 5740065 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the case illustrated in  FIG. 15B , a large stress is partially applied to the porous material  6  due to the weight of the glass rods  5   b  that are not gripped, cracks are generated in the porous material  6 , and there is a risk of the porous material  6  breaking. As a result, there is a problem that the manufacturing yield of the optical fiber preform may be reduced in some cases. 
     The present invention has been made in view of the above, and has an object of providing a method of manufacturing an optical fiber preform in which the reduction in manufacturing yield is suppressed, and a method of manufacturing optical fibers thereof. 
     Solution to Problem 
     In order to solve the above mentioned problems and achieve the object, a manufacturing method for an optical fiber preform, according to an aspect of the present invention includes a step of forming a porous material made of fine glass particles surrounding a plurality of glass rods; and a step of sintering the porous material, wherein: the step of forming the porous material includes forming the porous material such that two or more of the plurality of glass rods protrude from the porous material, and the step of sintering includes supporting end portions of protruding sides of the two or more protruding glass rods collectively with a support jig, and performing the sintering. 
     In the manufacturing method for the optical fiber preform, according to an aspect of the present invention, the support jig is configured to allow the supported glass rods to move in a direction that approaches the center axis of the porous material. 
     In the manufacturing method for the optical fiber preform, according to an aspect of the present invention, the support jig is configured to allow the supported glass rods to be tilted in a direction that approaches the center axis of the porous material. 
     According to another aspect of the present invention, a manufacturing method for optical fiber includes drawing an optical fiber from an optical fiber preform manufactured by the manufacturing method according to an aspect of the present invention. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to realize a method of manufacturing an optical fiber preform in which the reduction in manufacturing yield is suppressed and a method of manufacturing optical fibers. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a schematic diagram for explaining a first configuration example of a support jig used in a forming step of a porous material. 
         FIG. 1B  is a schematic diagram for explaining the first configuration example of the support jig used in the forming step of the porous material. 
         FIG. 1C  is a schematic diagram for explaining the first configuration example of the support jig used in the forming step of the porous material. 
         FIG. 2  is a schematic diagram for explaining the forming step of the porous material. 
         FIG. 3A  is a schematic diagram for explaining a first configuration example of the support jig used in a sintering step of the porous material. 
         FIG. 3B  is a schematic diagram for explaining the first configuration example of the support jig used in the sintering step of the porous material. 
         FIG. 4  is a schematic diagram for explaining a second configuration example of the support jig used in the forming step of the porous material. 
         FIG. 5A  is a schematic diagram for explaining a structure of an end portion of the glass rod depicted in  FIG. 4 . 
         FIG. 5B  is a schematic diagram for explaining the structure of the end portion of the glass rod depicted in  FIG. 4 . 
         FIG. 5C  is a schematic diagram for explaining the structure of the end portion of the glass rod depicted in  FIG. 4 . 
         FIG. 5D  is a schematic diagram for explaining the structure of the end portion of the glass rod depicted in  FIG. 4 . 
         FIG. 6  is a schematic diagram for explaining a third configuration example of the support jig used in the sintering step of the porous material. 
         FIG. 7A  is a schematic diagram illustrating an example of a configuration of a support member of the support jig depicted in  FIG. 6 . 
         FIG. 7B  is a schematic diagram illustrating an example of a configuration of the support member of the support jig depicted in  FIG. 6 . 
         FIG. 7C  is a schematic diagram illustrating an example of a configuration of the support member of the support jig depicted in  FIG. 6 . 
         FIG. 7D  is a schematic diagram illustrating an example of a configuration of the support member of the support jig depicted in  FIG. 6 . 
         FIG. 8  is a schematic diagram for explaining the movement of the glass rod in the sintering step. 
         FIG. 9A  is a schematic diagram for explaining a fourth configuration example of the support jig used in the sintering step of the porous material. 
         FIG. 9B  is a schematic diagram for explaining a fourth configuration example of the support jig used in the sintering step of the porous material. 
         FIG. 10A  is a schematic diagram for explaining the structure of the end portion of the glass rod. 
         FIG. 10B  is a schematic diagram for explaining the structure of the end portion of the glass rod. 
         FIG. 10C  is a schematic diagram for explaining the structure of the end portion of the glass rod. 
         FIG. 10D  is a schematic diagram for explaining the structure of the end portion of the glass rod. 
         FIG. 11  is a schematic diagram for explaining a fifth configuration example of the support jig used in the sintering step of the porous material. 
         FIG. 12  is a schematic diagram for explaining the tilt and bending of the glass rod in the sintering step. 
         FIG. 13  is a schematic diagram for explaining a sixth configuration example of the support jig used in the sintering step of the porous material. 
         FIG. 14  is a schematic diagram for explaining a forming step of a porous material by a powder molding method. 
         FIG. 15A  is a schematic diagram for explaining a multi core fiber. 
         FIG. 15B  is a schematic diagram for explaining a porous preform for forming a multi core fiber. 
         FIG. 16  is a schematic diagram illustrating a configuration of a multi core fiber. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of a method for manufacturing an optical fiber preform and a method of manufacturing optical fiber according to the present invention will be described in detail with reference to the drawings. It should be noted that the present invention is not limited by these embodiments. In addition, in each drawing, the same or corresponding elements are denoted by the same reference signs, as appropriate. 
     The method for manufacturing an optical fiber preform according to the present invention includes a step of forming a porous material made of fine glass particles surrounding a plurality of glass rods; and a step of sintering the porous material, wherein the step of forming the porous material includes forming the porous material such that two or more of the plurality of glass rods protrude from the porous material, and the step of sintering includes supporting end portions of protruding sides of the two or more protruding glass rods collectively with a support jig, and performing the sintering. In this way, as the weight of the porous preform can be supported by two or more glass rods, generation of cracks or the like in the porous material is suppressed or prevented, whereby the reduction in the manufacturing yield of the optical fiber preform is suppressed. 
     Embodiment 1 
     Hereinafter, a forming step of a porous material and a sintering step of a porous material according to Embodiment 1 will be specifically described. In the forming step of the porous material, a plurality of glass rods are prepared, fine glass particles are deposited around these glass rods, and the porous material are formed. Glass rods manufactured by a VAD method can be utilized. In addition, the forming step of the porous material includes using an OVD method. 
       FIG. 1A ,  FIG. 1B , and  FIG. 1C  are schematic diagrams for explaining a first configuration example of a support jig used in the forming step of a porous material according to Embodiment 1. As illustrated in the overall view of  FIG. 1A , a support jig  10  can support seven glass rods  5 , and is used when manufacturing a multi core fiber  3  illustrated in  FIG. 15A . The support jig  10  includes a rotational axis shaft  11 , seven glass rod support pipes  12 , and twelve connecting rods  13 . These constituent elements are made of, for example, a metal material. 
     The rotational axis shaft  11  is a member that serves as an axis of rotation when the glass rods  5  are revolved in the forming step of the porous material by the OVD method. The glass rod support pipes  12  are members into which the glass rods  5  are inserted, and that support the glass rods  5 . The glass rod support pipes  12  are arranged such that the glass rods  5  are to be arranged as glass rods in the porous preform to be produced. In Embodiment 1, among the seven glass rod support pipes  12 , a glass rod support pipe  12   a  is arranged in the center, and the six glass rod support pipes  12   b  are arranged around this center to form a regular hexagon on the outer periphery. Hereinafter, in cases in which the glass rod support pipe  12   a  and the glass rod support pipes  12   b  arc not distinguished from each other, they will be referred to as glass rod support pipes  12 . 
     As illustrated in an enlarged view of the primary part of  FIG. 1B , six of the twelve connecting rods  13  are provided so as to connect the rotational axis shaft  11  and a corresponding one of the glass rod support pipes  12   b . The remaining six of the twelve connecting rods  13  are provided so as to connect the glass rod support pipe  12   a  and a corresponding one of the glass rod supporting pipes  12   b.    
     In addition, as illustrated in  FIG. 1B , one rod hole position adjustment hole  12   c   1  and three rod fixing screw holes  12   c ,  12   e  are each formed in the glass rod support pipe  12 . The three rod fixing screw holes  12   c ,  12   e  are arranged so as to form an angle of 120° with each other. Both ends of the glass rods  5  are inserted into the glass rod support pipes  12  with which each of the two support jigs  10  is provided, and fixed by screwing fixing screws  14  into the rod fixing screw holes  12   c  and  12   e , whereby the glass rods  5  are supported by the support jigs  10 . It should be noted that although  FIG. 1C  illustrates the rod fixing screw hole  12   e , the rod fixing screw hole  12   c  is similarly arranged. 
     Next, as illustrated in  FIG. 2 , the rotational axis shaft  11  of the support jig  10  is gripped by a chuck  21  of an OVD device  20 , and the glass rods  5  are revolved. Then, while rotating the glass rods  5 , a glass source gas and combustion gases such as H 2  gas and O 2  gas are supplied to a main burner  22 , which is a burner for synthesizing fine glass particles, combustion gases such as H 2  gas and O 2  gas are supplied to an end burner  23 , and fine glass particles are deposited on the glass rods  5 . SiCl 4  or the like can be used as the glass source gas, for example. 
     The main burner  22  synthesizes fine glass particles by flame hydrolysis of the glass source gas in the flame formed by the combustion gas. It should be noted that the main burner  22  moves forward and backward in an extending direction of the glass rods  5 , and deposits fine glass particles uniformly in the extending direction of the glass rods  5  to form a porous material  31  made of silica glass. It should be noted that the end burner  23  is used to make the outer diameters of both ends of the porous material  31  substantially equal to the outer diameter at the center portion in the length direction of the porous material  31 . The fine glass particles that are not deposited are discharged from an exhaust hood  24  via an exhaust pipe  25 . In this way, the porous material  31  is formed, and the porous preform  30  in which the seven glass rods  5  protrude from the porous material  31  is formed. 
     Next, the sintering step of the porous material  31  will be described.  FIG. 3A  and  FIG. 3B  are schematic diagrams for explaining a first configuration example of a support jig used in the sintering step of the porous material according to Embodiment 1. A support jig  40  is capable of supporting seven glass rods  5  protruding from the porous material  31  of the porous preform  30 . As illustrated in  FIG. 3A , the support jig  40  includes a rotational axis shaft  41 , a support member  42 , three connecting rods  43 , and seven fixing pins  44 . These constituent elements are formed of silica glass material. 
     The rotational axis shaft  41  is a member that serves as an axis of rotation when the porous preform  30  is rotated in the sintering step. The support member  42  has a configuration in which cylindrical support portions  42   h  are provided on a disc-shaped base portion  42   a . The support portions  42   b  are disposed at positions corresponding to the arrangement of the glass rods  5 . An end portion of each of the glass rods  5  is inserted into a corresponding one of the support portions  42   b . It should be noted that, although both ends of each of the glass rods  5  protrude from the porous material  31  in the present embodiment, in cases where the porous preform is produced such that only one end portion of the glass rods  5  protrude from the porous material, the end portion on the protruding side of the glass rods  5  are inserted into the support portions. 
     The connecting rods  43  are provided so as to connect the rotational axis shaft  41  and the base portion  42   a.    
     In addition,  FIG. 3B  is a cross-sectional view taken along the line A-A in  FIG. 3A . Through-holes  42   ba  and  5   c  are formed in the support portion  42   b  and the glass rod  5 , respectively. A fixing pin  44  is passed through the through-holes  42   ba  and  5   c . In this way, the glass rods  5  are fixed to the support portion  42   b . As a result, the seven glass rods  5  are collectively supported by the support jig  40 . In this way, by supporting the seven glass rods  5  with the support jig  40 , the porous preform  30  is held in the sintering furnace, and the porous material  31  is heated and sintered while being rotated around the axis. As a result, the porous material  31  is vitrified, and the porous preform  30  becomes an optical fiber preform. 
     It should be noted that in order to pass the fixing pin  44  through the through-holes  42   ba  and  5   c , it is necessary that the glass rod  5  and the support portion  42   b  have a positional relationship such that the through-hole  42   ba  and the through-hole  5   c  communicate with each other. In order to realize this, when attaching the glass rod  5  to the support jig  10  depicted in  FIG. 1A , attachment should be performed such that the through-hole  5   c  of the glass rod  5  and the rod hole position adjustment hole  12   d  of the glass rod support pipe  12  communicate with each other. At this time, by inserting a fixing bolt into the through-hole  5   c  and the rod hole position adjustment hole  12   d  and screwing a nut from a tip of the fixing bolt and fastening it to the glass rod support pipe  12 , it is possible to ensure the positional relationship between the through-hole  5   c  and the rod hole position adjustment hole  12   d . By setting the positional relationship between the rod hole position adjustment hole  12   d  of the glass rod support pipe  12  and the through-hole  42   ba  of the support portion  42   b  so as to correspond to each other, the positional relationship can be made such that the through-hole  42   ba  communicates with the through-hole  5   c.    
     As explained above, in Embodiment 1, because the porous material  31  is heated and sintered while the porous preform  30  is held in the sintering furnace by supporting the end portions of the seven glass rods  5  with the support jig  40  that can collectively support the end portions of the glass rods  5 , the total weight of the porous preform  30  is supported by the seven glass rods  5  and thus the stress applied between the porous material  31  and the glass rods  5  is reduced. As a result, generation of cracks and the like in the porous material  31  is prevented. In this way, reduction in the manufacturing yield is prevented. 
     Embodiment 2 
     A forming step of a porous material and a sintering step of a porous material according to Embodiment 2 will be specifically described.  FIG. 4  is a schematic diagram for explaining a second configuration example of a support jig used in the forming step of the porous material. A support jig  10 A is capable of supporting three glass rods  5 A produced by a VAD method, and is used when producing a multi core fiber  3 A having three core portions  1 A and a cladding portion  2 A as depicted in  FIG. 16 . It should be noted that the glass rods  5  depicted in  FIG. 1A  and the glass rods  5 A are substantially the same, but the differences therebetween will be described in detail later. The support jig  10 A includes a rotational axis shaft  11 A, three glass rod support pipes  12 A, and one connecting plate  13 A. These constituent elements are made of, for example, a metal material. 
     The rotational axis shaft  11 A is a member that serves as an axis of rotation when the glass rods  5 A are revolved in the forming step of the porous material by the OVD method. The glass rod support pipes  12 A are members into which the glass rods  5 A are inserted, and that support the glass rods  5 A. The glass rod support pipes  12 A are arranged such that the glass rods  5 A are to be arranged as glass rods in the porous preform to be produced. In the second embodiment, the three glass rod support pipes  12 A are arranged so as to form an equilateral triangle. 
     The connecting plate  13 A has an equilateral triangular shape, has the glass rod support pipes  12 A provided at corresponding vertexes, and has the rotational axis shaft  11 A erected at the center thereof. 
     In addition, six rod fixing screw holes  12 Ae are formed in each of the glass rod support pipe  12 A. As to the six rod fixing screw holes  12 Ae, three of the rod fixing screw holes  12 Ae, which constitute one set, are arranged so as to form an angle of 120° with each other. Both ends of the glass rods  5 A are inserted into the glass rod support pipes  12 A with which each of the two support jigs  10 A is provided, and fixed by screwing fixing screws into the rod fixing screw holes  12 Ae, whereby the glass rods  5 A are supported by the support jigs  10 A. 
     Here,  FIG. 5A  to  FIG. 5D  are schematic diagrams for explaining the structure of the end portion of the glass rods  5 A. The differences between the glass rods  5  and the glass rods  5 A will be described with reference to  FIG. 5A  to  FIG. 5D .  FIG. 5A ,  FIG. 5B ,  FIG. 5C , and  FIG. 5D  are a side view, an arrow B view, a top view, and a perspective view of the end portion of a glass rod  5 A, respectively. 
     On the side surface of the end portion of the glass rods  5 A, two recessed portions  5 Aa are formed, each of which has a bottom surface  5 Aaa and inner side surfaces  5 Aab that arc parallel to each other. The bottom surfaces  5 Aaa of the two recessed portions  5 Aa are parallel to each other. 
     In the forming step of the porous mother body, a porous material is formed using the support jig  10 A by the OVD method as in the first embodiment, and a porous preform is formed in which both ends of the three glass rods  5 A protrude from the porous material. 
     Next, the sintering step of the porous material will he described.  FIG. 6  is a schematic diagram for explaining a third configuration example of a support jig used in the sintering step of the porous material according to Embodiment 2. A support jig  40 A is capable of supporting the three glass rods  5 A protruding from a porous material  31 A of a porous preform  30 A. The support jig  40 A includes a rotational axis shaft  41 A, a support member  42 A, and three connecting rods  43 A provided so as to connect the rotational axis shaft  41 A and the support member  42 A. These constituent elements are formed of silica glass material. 
     The rotational axis shaft  41 A is a member that serves as an axis of rotation when the porous preform  30 A is rotated in the sintering step. The support member  42 A has a disc shape, and has a configuration in which three long holes  42 Aa are provided. The long holes  42 Aa are arranged at locations corresponding to the arrangement of the glass rods  5 A, and extend radially from the center of the support member  42 A. Each of the glass rods  5 A is supported by the support member  42 A by fitting the recessed portion  5 Aa formed at the end portion of the glass rod  5 A into each of the long holes  42 Aa. As a result, the three glass rods  5 A are collectively supported by the support jig  40 A. 
     Incidentally, in order to fit the recessed portion  5 Aa of each of the glass rods  5 A into a corresponding one of the long holes  42 Aa, the support jig  40 A may have the following structure, for example.  FIG. 7A  to  FIG. 7D  are schematic diagrams illustrating an example of a configuration of the support member  42 A of the support jig  40 A. The support member  42 A is constituted by connecting one member  42 Ab, two members  42 Ac, and one member  42 Ad.  FIG. 7A  is a top view of the support member  42 A,  FIG. 7B  and  FIG. 7C  arc a side view and a perspective view, respectively, of a state in which the member  42 Ab and the member  42 Ad are fitted together, and  FIG. 7D  is a perspective view of the member  42 Ac. 
     The member  42 Ad is a connecting ring member, and in addition to being provided with connecting rods  43 A, also has a stepped portion  42 Ada formed on its inner peripheral side for fitting the members  42 Ab and  42 Ac. 
     The member  42 Ab includes a substantially fan-shaped plate portion  42 Abb having a stepped portion (not illustrated in the drawings) fitted to the stepped portion  42 Ada and a recessed portion  42 Abc for forming the long hole  42 Aa, a cylindrical portion  42 Abc that houses a part of the member  42 Ac, and a connecting portion  42 Abd for connecting the plate portion  42 Abb and the cylindrical portion  42 Abc. 
     The member  42 Ac includes a substantially fan-shaped plate portion  42 Acb, formed on the outer periphery, having a stepped portion  42 Ace fitted to the stepped portion  42 Ada, and having a recessed portion  42 Aca for forming the long hole  42 Aa, and an extending portion  42 Acd extending from the plate portion  42 Acb. 
     The support member  42 A is assembled by inserting each of the extending portions  42 Acd of the two members  42 Ac into the cylindrical portion  42 Abc of the member  42 Ab, connecting the member  42 Ab and the two members  42 Ac, and fitting the connected member to the member  42 Ad. At this time, the recessed portions  42 Aba of the member  42 Ab and the recessed portions  42 Aca of the member  42 Ac are combined, and the recessed portions  42 Aca of the two members  42 Ac are combined, thereby to form the long holes  42 Aa. 
     When connecting the member  42 Ab and the two members  42 Ac, by connecting them after fitting the recessed portion  5 Aa of each of the glass rods  5 A into the recessed portion  42 Aba or the recessed portion  42 Aca, the recessed portion  5 Aa of each of the glass rods  5 A can be fitted into a corresponding one of the long holes  42 Aa. 
     By supporting the three glass rods  5 A with the support jig  40 A as described above, the porous preform  30 A is held in the sintering furnace, and the porous material  31 A is heated and sintered while being rotated around the axis. As a result, the porous material  31 A is vitrified, and the porous preform  30 A becomes an optical fiber preform. 
     In Embodiment 2, as in the case of Embodiment 1, because the total weight of the porous preform  30 A is supported by the three glass rods  5 A in the sintering step, the stress applied between the porous material  31 A and the glass rods  5 A is reduced. As a result, as in the case of Embodiment 1, reduction in the manufacturing yield is prevented. 
     Furthermore, even in cases where the glass rods  5 A are not present in the center axis of the porous preform  30 A, it is possible to suppress the application of unbalanced stress to the porous material, and to prevent the reduction in the manufacturing yield. Such an effect can also be obtained in Embodiments 3 to 6 in which a glass rod is not present in the center axis of the porous preform, which will be described below. 
     Incidentally, in the process of sintering the porous material  31 A to form a glass body, the volume of the porous material  31 A contracts. With this contraction, the porous material  31 A exerts a stress on the three glass rods  5 A that causes them to become closer to each other. In particular, the porous material  31 A exerts a stress on the three glass rods  5 A that causes them to become closer to the center of the axis of the porous material  31 A. 
     In the present Embodiment 2, each of the glass rods  5 A is supported by the support member  42 A by fitting the recessed portion  5 Aa into a corresponding one of the long holes  42 Aa. Accordingly, as illustrated in  FIG. 8 , in the process in which the porous material  31 A contracts and becomes a glass body  36 A of an optical fiber preform  35 A, when stress is applied to the three glass rods  5 A, each of the glass rods  5 A is guided by a corresponding one of the long holes  42 Aa, and moves closer to the center axis of the porous material  31 . In this way, as the support jig  40 A is configured to be able to support the glass rods  5 A so as to allow the glass rods  5 A to move in a direction that approaches the center axis of the porous material  31 , and thus bending of each glass rod  5 A is prevented. 
     It should be noted that unlike the support jig  40 A, when a support jig that immobilizes the end portions of the respective glass rods is used, since each glass rod becomes closer to the center axis of the porous material  31 A due to the contraction of the porous material, as the parts located in the glass body are closer to each other than the parts fixed by the support jig, each glass rod is bent. 
     Embodiment 3 
     A forming step of a porous material and a sintering step of a porous material according to Embodiment 3 will be described. The forming step of the porous material according to the present Embodiment 3 is substantially the same as that of Embodiment 2, but the glass rod  5  of Embodiment 1 is used as the glass rod. 
     Next, the sintering step of the porous material will be described.  FIG. 9A  and  FIG. 9B  are schematic diagrams for explaining a fourth configuration example of a support jig used in the sintering step of the porous material to Embodiment 3. A support jig  40 B is capable of supporting three glass rods  5  protruding from a porous material  31 B of a porous preform  30 B. The support jig  40 B includes a rotational axis shaft  41 B, a support member  42 B, three connecting rods  43 B provided so as to connect the rotational axis shaft  41 B and the support member  42 B, three fixing rings  44 B, and three fixing pins  45 B. These constituent elements are formed of silica glass material. 
     The rotational axis shaft  41 B is a member that serves as an axis of rotation when the porous preform  30 B is rotated in the sintering step. The support member  42 B has a disc shape, and has a configuration in which three long holes  42 Ba and guide grooves  42 Bb provided at outer edges of the long holes  42 Ba are provided. The long holes  42 Ba are arranged at locations corresponding to the arrangement of the glass rods  5 , and extend radially from the center of the support member  42 B. Fixing rings  44 B are fitted into corresponding ones of the guide groove  42 Bb, and corresponding ones of the glass rods  5  are inserted thereinto. 
     In addition,  FIG. 9B  is a cross-sectional view taken along the line C-C in  FIG. 9A . Through-holes  44 Ba and  5   c  are formed in the fixing ring  44 B and the glass rod  5 , respectively. A fixing pin  4513  is passed through the through-holes  44 Ba and  5   c . In this way, the glass rods  5  are fixed to the fixing ring  44 B and supported by the support member  42 B. As a result, the three glass rods  5  are collectively supported by the support jig  40 B. In this way, by supporting the three glass rods  5  with the support jig  40 B, the porous preform  30 B is held in the sintering furnace, and the porous material  31 B is heated and sintered while being rotated around the axis. As a result, the porous material  31 B is vitrified, and the porous preform  30 B becomes an optical fiber preform. 
     In the present Embodiment 3, as in the case of Embodiments 1 and 2, because the total weight of the porous preform  30 B is supported by the three glass rods  5  in the sintering step, the stress applied between the porous material  31 B and the glass rods is reduced. As a result, as in the case of Embodiments 1 and 2, reduction in the manufacturing yield is prevented. 
     Further, in the present Embodiment 3, as in Embodiment 2, the support jig  40 B is configured to be able to support the glass rods  5  to allow the glass rods  5  to move in a direction that approaches the center axis of the porous material  31 B. Specifically, in the process in which the porous material  31 B contracts and becomes a glass body, when stress is applied to the three glass rods  5 , each of the glass rods  5  moves closer to the center axis of the porous material  31 B as the fixing ring  44 B fixed to a corresponding one of the glass rods  5  is guided by the guide grooves  42 Bb. As a result, as in the case of Embodiment 2, bending of each of the glass rods  5  is prevented. 
     Embodiment 4 
     A forming step of a porous mother body and a sintering step of a porous material according to Embodiment 4 will be specifically described. The forming step of the porous material according to the present Embodiment 4 is substantially the same as that of Embodiments 2 and 3, but a glass rod described below is used as the glass rod. 
       FIG. 10A  to  FIG. 10D  are schematic diagrams for explaining the structure of the end portion of a glass rod  5 C.  FIG. 10A ,  FIG. 10B ,  FIG. 10C , and  FIG. 10D  are a side view, an arrow D view, a top view, and a perspective view of the end portion of the glass rod  5 C, respectively. 
     On the side surface of the end portion of the glass rod  5 C, two recessed portions  5 Ca are formed, each of which has a bottom surface  5 Caa, as well as a planar inner side surface  5 Cab and an inner side surface  5 Cac being a curved surface of a cylindrical shape, which are opposed with each other. The bottom surfaces  5 Caa of the two recessed portions  5 Ca are parallel to each other. In addition, the inner side surface  5 Cac is formed closer to the end portion of the glass rod  5 C than the inner side surface  5 Cab. 
     Next, the sintering step of the porous material will be described.  FIG. 11  is a schematic diagram for explaining a fifth configuration example of a support jig used in the sintering step of the porous material. A support jig  40 C is capable of supporting the three glass rods  5 C protruding from a porous material  31 C of a porous preform  30 C. The support jig  40 C includes a rotational axis shaft  41 C and a disc-shaped support member  42 C. These constituent elements are formed of silica glass material. 
     The rotational axis shaft  41 C is erected at the center of the support member  42 C, and is a member that serves as an axis of rotation when the porous preform  30 C is rotated in the sintering step. The support member  42 C has a configuration in which three notches  42 Ca are provided on the outer edge. The notches  42 Ca are provided at locations corresponding to the arrangement of the glass rods  5 C. Each of the glass rods  5 C is supported by the support member  42 C by fitting the recessed portion  5 Ca formed at the end portion of the glass rod  5 C into a corresponding one of the notches  42 Ca. As a result, the three glass rods  5 C are collectively supported by the support jig  40 C. 
     In this way, by supporting the three glass rods  5 C with the support jig  40 C, the porous preform  30 C is held in the sintering furnace, and the porous material  31 C is heated and sintered while being rotated around the axis. As a result, the porous material  31 C is vitrified, and the porous preform  30 C becomes an optical fiber preform. 
     In the present Embodiment 4, as in the case of Embodiments 1 to 3, because the total weight of the porous preform  30 C is supported by the three glass rods  5 C, the stress applied between the porous material  31 C and the glass rods  5 C is reduced. As a result, as in the case of Embodiments 1 to 3, reduction in the manufacturing yield is prevented. 
     Incidentally, in the process of sintering the porous material  31 C to form a glass body, along with the contraction thereof, a stress is exerted on the three glass rods  5 C that causes them to become closer to the center axis of the porous material  31 C. In this way, as illustrated in  FIG. 12 , in the process in which the porous material  31 C contracts and becomes a glass body  36 C of an optical fiber preform  35 C, when stress is applied to the three glass rods  5 C, each of the glass rods  5 C is bent such that the distance between each other is closer in a part located within the glass body  36 C than in a part fixed to the support jig  40 C. 
     In the present Embodiment 4, although each of the glass rods  5 C is supported by the support member  42 C by fitting the recessed portion  5 Ca into a corresponding one of the notches  42 Ca, the inner side surface  5 Cac being the curved surface is substantially in line contact with the upper surface of the support member  42 C. When each of the glass rods  5 C is bent as described above, the inner side surface  5 Cac being the curved surface rolls while maintaining line contact with the upper surface of the support member  42 C. That is, the glass rods  5 C are configured to allow the glass rods  5 C to be inclined with respect to the support jig  40 C in a direction that approaches the center axis of the porous material  31 C. Here, the inclination of the glass rods  5 C means that the glass rods  5 C is inclined with respect to the center axis of the porous material  31 C. As a result, even in a case where each of the glass rods  5 C bends, it is possible to prevent a stress that would damage the glass rods  5 C from being applied between the support member  42 C and the glass rods  5 C. It should be noted that, in order to prevent a stress that would damage the glass rods  5 C from being applied, it is preferable to set the distance between the inner side surface  5 Cab and the inner side surface  5 Cac such that, even in the case where the glass rod  5 C bends, the inner side surface  5 Cab of the recessed portion  5 Ca does not contact the lower surface of the support member  42 C. 
     Embodiment 5 
     A forming step of a porous material and a sintering step of a porous material according to Embodiment 5 will be described. The forming step of the porous material according to the present Embodiment 5 is substantially the same as that of Embodiment 4. In contrast, the sintering step of the porous material includes using a support jig of the sixth configuration example illustrated in  FIG. 13 . A support jig  40 D is capable of supporting the three glass rods  5 C protruding from a porous material of a porous preform. The support jig  40 D includes a rotational axis shaft  41 D, a support member  42 D, and three connecting rods  43 D provided so as to connect the rotational axis shaft  41 D and the support member  42 D. These constituent elements are formed of silica glass material. 
     The rotational axis shaft  41 D is a member that serves as an axis of rotation when the porous preform is rotated in the sintering step. The support member  42 D has a disc-shape, and has a configuration in which three notches  42 Da are provided on the outer edge. The notches  42 Da are arranged at locations corresponding to the arrangement of the glass rods  5 C, and extend toward the center of the support member  42 D. Each of the glass rods  5 C is supported by the support member  42 D by fitting the recessed portion  5 Ca formed at the end portion of the glass rod  5 C into a corresponding one of the notches  42 Da. As a result, the three glass rods  5 C are collectively supported by the support jig  40 D. 
     In this way, by supporting the three glass rods  5 C with the support jig  40 D as described above, the porous preform is held in the sintering furnace, and the porous material is heated and sintered while being rotated around the axis. As a result, the porous material is vitrified and becomes the glass body  36 D as depicted in  FIG. 13 , and the porous preform becomes the optical fiber preform  35 D. 
     In the present Embodiment 5, as in the case of Embodiments 1 to 4, because the total weight of the porous preform is supported by the three glass rods  5 C, the stress applied between the porous material and the glass rods is reduced. As a result, as in the case of Embodiments 1 to 4, reduction in the manufacturing yield is prevented. 
     In addition, in the present Embodiment 5, as in Embodiments 2 and 3, because the notches  42 Da extend toward the center of the support member  42 D, the support jig  40 D is configured to be able to support the glass rods  5 C to allow the glass rods  5 C to move in a direction that approaches the center axis of the porous material. Furthermore, the glass rods  5 C are configured to allow the glass rods  5 C to be inclined with respect to the support jig  40 D in a direction that approaches the center axis of the porous material. As a result, in addition to suppressing bending of each of the glass rods  5 C, it is possible to prevent a stress that would damage the glass rods  5 C from being applied between the support member  42 D and the glass rods  5 C even in the case where each glass rod  5 C bends. 
     Embodiment 6 
     A forming step of a porous material according to Embodiment 6 and a sintering step of a porous material will be described. The forming step of the porous material according to Embodiment 6 includes using a powder molding method. In addition, a glass rod  5 A is used as the glass rod. 
       FIG. 14  is a schematic diagram for explaining a forming step of a porous material by a powder molding method. In the powder molding method, three glass rods  5 A are gripped in a pressurizing mold  50 , and granulated particles  51  of silica glass are inserted into the pressurizing mold  50  and pressure-formed by a pressurizing plunger  52  to form a porous material as a pressure-molded body. In this way, a porous preform in which three glass rods  5 A protrude from the porous material is formed. It should be noted that, in the upper part of the drawing, the reason why the tips of the glass rods  5 A are machined into spherical bodies having a diameter larger than an outer diameter of a part of the glass rods  5 A that have a substantially constant outer diameter is to make the glass rods  5 A less likely to fall out of the porous material in the subsequent sintering step. 
     The subsequent sintering step of the porous material can be performed using the same method as in Embodiment 2. In this way, as in the case of Embodiments 1 to 5, reduction in the manufacturing yield is prevented. 
     It should be noted that optical fiber can be manufactured by drawing an optical fiber from the optical fiber preform manufactured according to the above embodiments by a known method using a known fiber-drawing furnace. 
     EXAMPLE 1 
     As Example 1 of the present invention, three porous preforms were produced according to the method of Embodiment 1 and sintered in accordance with the method of Embodiment 1 to produce three optical fiber preforms. Although the three optical fiber preforms had cracks on the upper part, no abnormalities such as cracks were observed in most of the other parts, and the three optical fiber preforms were favorable. 
     Comparative Example 1 
     As Comparative Example 1, three porous preforms were produced according to the method of Embodiment 1. Only one in the center among the seven glass rods was supported and sintering was performed. While two of the three porous preforms were able to be sintered, cracks were formed on the upper part of the glass body. In addition, one of the three porous preforms had cracks generated in the porous material during sintering, and one glass rod on the outer peripheral side fell out. 
     EXAMPLE 2 
     As Example 2 of the present invention, three porous preforms were produced according to the method of Embodiment 2 and sintered in accordance with the method of Embodiment 2 to produce three optical fiber preforms. No abnormalities such as cracks were observed in the three optical fiber preforms, and they were favorable. 
     EXAMPLE 3 
     As Example 3 of the present invention, three porous preforms were produced according to the method of Embodiment 4 and sintered in accordance with the method of Embodiment 4 to produce three optical fiber preforms. Although the three optical fiber preforms had cracks on the upper part, no abnormalities such as cracks were observed in most of the other parts, and the three optical fiber preforms were favorable. It should be noted that when the vicinity of the support jig was verified after sintering, the glass rods were inclined between the glass body and the support jig. 
     EXAMPLE 4 
     As Example 4 of the present invention, three porous preforms were produced according to the method of Embodiment 5 and sintered in accordance with the method of Embodiment 5 to produce three optical fiber preforms. Although the three optical fiber preforms had cracks on the upper part, no abnormalities such as cracks were observed in most of the other parts, and the three optical fiber preforms were favorable. It should be noted that when the vicinity of the support jig was verified after sintering, the glass rods were inclined between the glass body and the support jig. 
     EXAMPLE 5 
     As Example 5 of the present invention, three porous preforms were produced according to the method of Embodiment 6 and sintered in accordance with the method of Embodiment 2 to produce three optical fiber preforms. 
     In particular, a silica particle slurry was produced by adding commercially available gas phase synthetic silica particles having an average primary particle size of 10 μm and pure water as a solvent to polyvinyl alcohol (PVA) as a particle bonding agent. Silica granulated particles having a volume value of 50% and a particle diameter of 100 μm were prepared from the produced silica particle slurry using a spray dryer device. 
     Next, the silica granulated particles were inserted into a pressurized mold in which three core rods were gripped, and a porous material to serve as a pressure-molded body was obtained using a pressurizing plunger. The pressurizing mold was a division type, and the porous preform was divided and taken out after pressurizing. The obtained porous material was heat-treated in an oxygen atmosphere to oxidize and remove the PVA, and then sintered using the same support jig as in the case of Example 2 to produce three optical fiber preforms. No abnormalities such as cracks were observed in the three optical fiber preforms, and they were favorable. 
     Further, the present invention is not limited by the embodiments described above. The present invention includes configurations created by appropriately combining each of the above-described constituent elements. Further effects and modifications can be easily derived by those skilled in the art. Accordingly, the broader aspects of the present invention are not limited to the above embodiments, and various modifications are possible. 
     INDUSTRIAL APPLICABILITY 
     As described above, the present invention is suitable for application to the manufacturing of, for example, optical fiber. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  1   a ,  1   b ,  1 A Core portion 
           2 ,  2 A Cladding portion 
           3 ,  3 A Multi core fiber 
           4  Porous preform 
           5 ,  5   a ,  5   b ,  5 A,  5 C Glass rod 
           5 Aa,  5 Ca,  42 Aba,  42 Aca Recessed portion 
           5 Aaa,  5 Caa Bottom surface 
           5 Aab,  5 Cab,  5 Cac Inner side surface 
           5   c ,  42   ba ,  44 Ba Through-hole 
           10 ,  10 A Support jig 
           11 ,  11 A Rotational axis shaft 
           12 ,  12   a ,  12   b ,  12 A Glass rod support pipe 
           12   c ,  12   e ,  12 Ac Rod-fixing screw hole 
           12   d  Rod hole position adjustment hole 
           13  Connecting rod 
           13 A Connecting plate 
           14  Fixing screw 
           20  OVD device 
           21  Chuck 
           22  Main burner 
           23  End burner 
           24  Exhaust hood 
           25  Exhaust pipe 
           30 ,  30 A,  30 B,  30 C Porous preform 
           31 ,  31 A,  31 B,  31 C Porous material 
           35 A,  35 C,  35 D Optical fiber preform 
           36 A,  36 C,  36 D Glass body 
           40 ,  40 A,  40 B,  40 C,  40 D Support jig 
           41 ,  41 A,  41 B,  41 C,  41 D Rotational axis shaft 
           42 ,  42 A,  42 B,  42 C,  42 D Support member 
           42 Aa,  42 Ba Long hole 
           42 Ab,  42 Ac,  42 Ad Member 
           42 Abb,  42 Acb Plate portion 
           42 Abc Cylindrical portion 
           42 Abd Connecting portion 
           42 Acd Extending portion 
           42 Ace,  42 Ada Stepped portion 
           42 Bb Guide groove 
           42 Ca,  42 Da Notch 
           42   a  Base portion 
           42   b  Support portion 
           43 ,  43 A,  43 B,  43 D Connecting rod 
           44 ,  45 B Fixing pin 
           44 B Fixing ring 
           50  Pressurizing mold 
           51  Granulated particles 
           52  Pressurizing plunger