Abstract:
A sleeve for optical communication and method for manufacture for the sleeve for optical communication, for which withdrawal force for ferrules, which are generally used in the technical field of optical communication, or connection loss between ferrules is a predetermined value, and for which manufacturing steps are reduced so as to allow reduction of manufacturing cost. The invention is made by press-molding ceramic feedstock, which after cold isostatic pressing and sintering are performed thereto, and is made so that an inner circumferential surface, of a through-hole for inserting a ferrule formed on the length-direction of the axis, has a sintered state.

Description:
TECHNICAL FIELD 
     The present invention relates to a sleeve for optical communication and a method of manufacturing the sleeve for optical communication. 
     BACKGROUND ART 
     An optical connector for connecting optical fibers is constituted of connecting parts which include a sleeve for an apparatus of optical communication made of ceramics such as zirconia or the like. The sleeve for optical communication is a cylindrical member having an open through-hole is configured to insert ferrules from both ends of the sleeve for optical communication so as to abut against each other inside the sleeve for optical communication. The optical fibers are respectively inserted and held in the ferrules. As the sleeve for optical communication, two types of sleeves are used, that is, one is a precision sleeve having a cylindrical shape, and the other is a split sleeve having a slit formed in a cylindrical longitudinal wall (as shown in, for example, Patent 1 to 3). 
     The precision sleeve and the split sleeve are respectively configured so that the inserted ferrules can be firmly held. The precision sleeve is formed so that the inner diameter is slightly larger than the outer diameter of the ferrule. Since the inner diameter of the precision sleeve is not changed by insertion of the ferrule, a highly accurate inner diameter dimension is required. On the other hand, the split sleeve is formed so that the inner diameter is slightly smaller than the outer diameter of the ferrule, and the slit is formed in an axial length direction of the split sleeve. Since the split sleeve is elastically deformed by insertion of the ferrule, the accuracy of the inner diameter dimension is not required compared with the precision sleeve. 
     In general, in the manufacturing of a sleeve for optical communication formed of a raw material mainly composed of zirconia, first, the raw materials containing zirconia, yttria, alumina, titania, and the like are subjected to press forming, extrusion forming, injection forming, or the like to form a cylindrical preform. Then, the cylindrical preform is fired to be hardened and, thus, to obtain a formed body. Next, treatment such as a length adjusting processing or treatment for making a length dimension in a longitudinal direction of the formed body is performed to have the length thereof equal to a predetermined length. An inner peripheral surface of the formed body is subjected to polishing processing such as honing by a diamond wheel, and such as pin grinding by diamond abrasive grains, and the like Namely, the inner diameter adjusting processing is performed so that the formed body becomes a predetermined inner diameter dimension. Next, an outer diameter adjusting processing is performed so that an outer peripheral surface of the formed body is ground with a diamond wheel using a cylindrical grinding machine or the like, so that the formed body has a predetermined outer diameter dimension. When the sleeve for optical communication is subjected to the split sleeve, a slit is formed after the length adjusting processing, the inner diameter processing, or the outer diameter processing, and the like. 
     In the technical field of the optical communication, regarding an optical connector, a plurality of standards are prescribed up to now. In those standards, a dimension or the like is made common for each standard to provide convenience to a user thereby. Presently, as typical optical connectors, there are the so-called SC type, FC type, MU type, and LC type optical connectors. In the outer diameter dimension and the like of the SC type optical connector, the outer diameter dimension and the like of the SC type optical connector are specified in the standard of IEC 61754-4, TIA/EIA-604-3A, and JIS C 5973. The outer diameter dimension and the like of the FC type optical connector are specified in IEC 61754-13, TIA/EIA-604-4A, and JIS C 5970. Next, the outer diameter dimension or the like of the MU type optical connector is specified in IEC 61754-6, TIA/EIA-604-17, and JIS C 5983. Further, the outer diameter dimension or the like of the LC type optical connector is specified in IEC 61754-20 and TIA/EIA-604-10A. 
     The inner diameters of the SC type optical connector and the FC type optical connector, and the outer diameter of a ferrule which holds the connectors thereof are respectively about 2.5 mm And the inner diameters of the MU type optical connector and of the LC type optical connector are respectively about 1.25 mm. Specifically, it is provided that the outer diameter dimensions of the SC type ferrule and the FC type ferrule which are classified into grade 1 in IEC 61754-4, IEC 61754-13, TIA/EIA-604-3A and TIA/EIA-604-4A and grade B in JIS C 5973 and 5970 are respectively specified as 2.499±0.0005 mm. Moreover, the outer diameter dimensions of the MU type ferrule and the LC type ferrule classified into grade 1 in IEC 61754-6, IEC 61754-20, TIA/EIA-604-17 and TIA/EIA-604-10A and grades B and C in JIS C 5983 are respectively specified as 1.249±0.0005 mm. In each optical connector, the outer diameter dimension of a housing is also specified by the IEC standard, the JIS standard, or the TIA/EIA standard. 
     CITATION LIST 
     Patent Literatures 
     Patent document 1: JP 06-15013 Y 
     Patent document 2: JP 2001-91783 A 
     Patent document 3: JP 2011-123221 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In each of the above-stated standards, a plug and an adapter constituting an optical connector are specified. In the adapter, pull-out force for a ferrule and connection loss occurring when the ferrules are butted against each other are specified. In particular, the JIS standard in which the SC type optical connector and the FC type optical connector are classified into the grade B in JIS C 5973 and JIS C 5970 specifies, for example, that the pull-out force for a ferrule is 2.0 to 5.9 N, and the connection loss between the ferrules butted against each other inside the adapter is not more than 0.3 dB in a physical contact connection (PC connection) using a single mode fiber. Accordingly, with regard to the sleeve for optical communication which is one of components constituting the adapter, the pull-out force for ferrules and the connection loss between the ferrules are determined in consideration of the specification of the adapter. In the sleeve for optical communication, it is required to satisfy such the conditions that the pull-out force for the ferrule is 1.5 to 6 N, and the connection loss between the ferrules is 0.3 dB in the PC connection using the single mode fiber, and the conditions are provided as standard conditions generally used in the technical field of the optical communication. However, in a conventional sleeve for the optical communication, when firing is performed, distortion or the like easily occurs, and when only firing is performed, it has been difficult to obtain a sleeve for optical communication in which the pull-out force for a ferrule and the connection loss between the ferrules satisfy the conditions and requirements in the above-stated standards. Thus, in the prior art, in order to satisfy the standard conditions, the inner peripheral surface of the sleeve for optical communication is required to be treated by adjusting processing thereof, and there is problems that the number of manufacturing processes and the manufacturing cost are increased. 
     In order to solve the above problem, an object of the present invention is to provide a sleeve for a device of optical communication and to provide a method of manufacturing the sleeve for optical communication, in which pull-out force for a ferrule and connection loss between ferrules meet such standard conditions generally used in the technical field of the optical communication without treatments such as an adjusting processing on an inner peripheral surface of the sleeve, and which can reduce the number of manufacturing processes or steps and manufacturing costs. 
     Solution to Problem 
     A sleeve for optical communication according to the present invention has the following summaries: 
     (1) a sleeve for optical communication characterizing by integrally forming a ceramics raw material by press forming, performing cold isostatic pressing, and thereafter firing, the sleeve for optical communication having: a through-hole for inserting a ferrule being formed in a cylindrical shape opened in an axial length direction, and an inner peripheral surface of the through-hole being made a fired surface appearing crystal grains of the ceramics raw material formed at the time of firing thereof;
 
(2) the sleeve for optical communication described in (1), wherein a plurality of crystal grains are formed on the fired surface, and a crystal grain boundary is formed between the crystal grains;
 
(3) a sleeve for optical communication, characterizing by: a cylindrical preform made of a ceramic raw material by press forming to form thereof; the preform being treated by cold isostatic pressing to form a formed body in uniformed forming density of a ceramics raw material; and the formed body being fired at a predetermined temperature for predetermined time, an inner peripheral surface of a through-hole thereof become altered fired surface, a crystal grain group comprised of a large number of crystal grains of the ceramics raw material being formed on the fired surface, and a crystal grain boundary being formed between the crystal grains adjacent to each other;
 
(4) the sleeve for optical communication described in (1) or (3), wherein surface roughness R a  of the inner peripheral surface is 0.08 to 0.20 μm; and
 
(5) the sleeve for optical communication described in (1) or (3), wherein a tolerance of an inner diameter of the through-hole is ±2 to 10 μm.
 
     Further, a method of manufacturing a sleeve for optical communication according to the present invention has the following summaries: 
     (6) a method of manufacturing a sleeve for optical communication comprising the steps of: press-forming a powder of a ceramics raw material to form a cylindrical preform having an inner peripheral surface; cold isostatic pressing the preform to form a formed body; and firing the formed body and producing the sleeve for optical communication to form crystal grains of the ceramics raw material formed at the time of firing on the inner peripheral surface;
 
(7) the method of manufacturing a sleeve for optical communication described in (6), wherein the preform is formed by press-forming a ceramics raw material put into a die tool, the formed body is formed by cold isostatic pressing the preform so that the forming density of the ceramics raw material become uniform, the formed body is degreased by application of heat at a predetermined temperature for a predetermined time, the degreased formed body is heated and fired at a predetermined temperature higher than the degreasing temperature, for a predetermined time, and a crystal grain group composed of a large number of crystal grains of the ceramics raw material and a crystal grain boundary formed between the crystal grains adjacent to each other are formed on a fired surface come existence on the inner peripheral surface of a through-hole;
 
(8) the method of manufacturing a sleeve for optical communication described in (6) or (7), wherein the fired formed body is processed to have a predetermined length;
 
(9) the method of manufacturing a sleeve for optical communication described in (6) or (7), wherein an outer periphery of the fired formed body is processed;
 
(10) the method of manufacturing a sleeve for optical communication described in (7), wherein the degreasing is performed while heating at a temperature of 400 to 500° C. for a predetermined time;
 
(11) the method of manufacturing a sleeve for optical communication described in (6) or (7), wherein the firing is performed while heating at a temperature of 1300 to 1500° C. for a predetermined time;
 
(12) the method of manufacturing a sleeve for optical communication described in (6) or (7), wherein finishing processing is performed after cooling the fired formed body.
 
     Advantageous Effects of Invention 
     In a sleeve for optical communication or optical communication device according to the present invention, since an inner peripheral surface of a through-hole formed in an axial length direction is a fired surface, the sleeve for optical communication which of pull-out force for a ferrule and connection loss between the ferrules satisfy respectively the conditions and requirements in standards generally used in the technical field of optical communication, can be provided at low cost. 
     In a method of manufacturing a sleeve for optical communication according to the present invention, a powder of a ceramics raw material is treated to form a preform by press-forming, and the preform is conducted through compression forming by a cold isostatic pressing to form a formed body; therefore, while a manufacturing process is simplified, it is possible to obtain the sleeve for optical communication which the pull-out force for the ferrule and connection loss between the ferrules satisfy the conditions and requirements in the standards generally used in the technical field of optical communication. Accordingly, since an inner peripheral surface of the sleeve for optical communication of the present invention is not treated for adjustment thereof, the number of the manufacturing processes or steps is reduced, and consequently, the manufacturing costs are significantly reduced, and the operating efficiency can be significantly enhanced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1( a )  is a perspective view of a precision sleeve for a sleeve for optical communication and a partially enlarged view showing a surface state of an inner peripheral surface of the precision sleeve of the present invention, and  FIG. 1( b )  is a perspective view of a split sleeve for the sleeve for optical communication and a partially enlarged view showing a surface state of an inner peripheral surface of the split sleeve of the present invention. 
         FIG. 2  is a flowchart showing processes for manufacturing the sleeve for optical communication thereof. 
         FIG. 3( a )  is a schematic view for explaining an example of a form used in a press forming, and  FIG. 3( b )  is a schematic view for explaining a state in which a ceramics raw material is formed by the press-forming to form a preform. 
         FIG. 4( a )  is a schematic view for explaining a state in which the preform is put into a soft container, and FIG.  4 ( b ) is a schematic view for explaining a state in which a cold isostatic pressing is conducted to the preform put in the soft container. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of a sleeve for optical communication according to the present invention will be specifically described using the drawings. In this specification, the words of “front”, “rear”, “upper”, “lower”, “left”, and “right” represent directions shown in  FIG. 1 . 
     Although the sleeve for optical communication according to the embodiment will be described while exemplifying two types of sleeves, that is, a precision sleeve  1   a  shown in  FIG. 1( a )  and a split sleeve  1   b  shown in  FIG. 1( b ) , the embodiment is not limited thereto, and sleeves having other configurations may be used. In order to obtain the sleeve for optical communication according to the present invention, a powder of a ceramics material is processed by a press-forming to form a preform, and a cold isostatic press is conducted to the preform to form a formed body, the formed body is degreased and fired, and, if necessary, a length dimension, an outer diameter dimension, and the like of the fired formed body are processed or adjusted. 
     The sleeve for optical communication is formed of various composite ceramics such as ceramics i.e., zirconia, alumina, mullite, silicon nitride, silicon carbide, and aluminum nitride, glass ceramics such as SiO 2 —Al 2 O 3 -based or SiO 2 —B 2 O 3 -based crystallized glass, dispersed zirconia-containing alumina ceramics mainly composed of Al 2 O 3  and mixed with ZrO 2 , dispersed zirconia-containing alumina ceramics in which very fine zirconia grains having a nano-level grain diameter are dispersed in an alumina crystal grain boundary, and grain boundary strength is drastically enhanced, and ceramics mainly composed of Al 2 O 3  and mixed with Al 2 B 2 O 9 . As the composite ceramics or the like, partially stabilized zirconia more excellent particularly in weatherability, bending strength, and the like are more preferable. 
     The partially stabilized zirconia is mainly composed of ZrO 2 , and contains Y 2 O 3 , MgO, CeO 2 , Dy 2 O 3 , CaO, and the like as stabilizers. Moreover, the partially stabilized zirconia is excellent in weatherability, has high toughness or strength, and is easily polished. Thus, by virtue of the use of the partially stabilized zirconia, a high accuracy sleeve for optical communication used for a long period of time can be obtained. 
     The precision sleeve  1   a  shown in  FIG. 1( a )  is formed into a hollow cylindrical shape having a through-hole  2   a  into which a ferrule is inserted in an axial length direction. An inner diameter of the through-hole  2   a  is slightly larger than an outer diameter of the ferrule inserted through the through-hole  2   a , and the through-hole  2   a  is thus formed so that the ferrule is easily inserted therein, and, in addition, the inserted ferrule is prevented from rattling. 
     The split sleeve  1   b  shown in  FIG. 1( b )  is formed into a hollow cylindrical shape having a through-hole  2   b  into which the ferrule is inserted in an axial length direction, and a slit  3  is formed along the axial length direction in a portion of the through-hole  2   b  so as to be connected from an inner peripheral surface of the through-hole  2   b  to an outer peripheral surface  6   b  of the split sleeve  1   b . The inner diameter of the through-hole  2   b  is slightly smaller than the outer diameter of the ferrule inserted through the through-hole  2   b , and the through-hole  2   b  is formed so that the split sleeve  1   b  is elastically deformed in a direction in which the split sleeve  1   b  is slightly expanded when the ferrule is inserted, whereby the ferrule inserted into the through-hole can be firmly held. 
     The precision sleeve  1   a  and the split sleeve  1   b  for the sleeve for optical communication of the present embodiment are formed so that the inner peripheral surfaces  4   a  and  4   b  of the through-holes  2   a  and  2   b  are fired surfaces. Here, the “fired surface” is referred to as a surface formed when a formed body to be described later is fired. Namely, as shown in  FIGS. 1( a ) and 1( b ) , in the precision sleeve  1   a  and the split sleeve  1   b  according to the present embodiment, when portions of the inner peripheral surfaces  4   a  and  4   b  are shown with enlargement, a plurality of crystal grains  5   a  and  5   b  having different grain sizes appear on the inner peripheral surfaces  4   a  and  4   b . Further, a crystal grain group consisting of a large number of crystal grains including the crystal grains  5   a  and  5   b  appears. Furthermore, a crystal grain boundary Sc is formed between the crystal grains  5   a  and  5   b  adjacent to each other. In the crystal grains  5   a  and  5   b , the degree of growth, that is, the grain diameter of the crystal grains  5   a  and  5   b  is different depending on, for example, firing temperature and time in a firing process in the formation of the sleeve for optical communication. And, for example when the firing temperature is low, or when the firing time is short, the grain sizes of the crystal grains  5   a  and  5   b  often become relatively small in large number thereof; otherwise, for example, when the firing temperature is high, or when the firing time is long, the grain sizes of the crystal grains  5   a  and  5   b  become relatively large. 
     At this time, each surface roughness R a  of the inner peripheral surfaces  4   a  and  4   b  is 0.08 to 0.20 μm, preferably 0.08 to 0.15 μm, more preferably 0.08 to 0.12 μm. When the surface roughness is reduced, it becomes easy to obtain a sleeve which the pull-out force for a ferrule and the connection loss between the ferrules satisfy the conditions and requirements of the standards generally used in the technical field of optical communication. A tolerance of inner diameters of the inner peripheral surfaces  4   a  and  4   b  is ±2 to 10 μm, preferably ±2 to 8 μm, more preferably ±2 to 5 μm. When the tolerance of the inner diameters of the inner peripheral surfaces  4   a  and  4   b  is ±2 to 5 μm, it becomes easier to obtain the sleeve which the pull-out force for a ferrule and the connection loss between the ferrules satisfy the conditions and requirements of the standards generally used in the technical field of optical communication. 
     It is preferable that the inner diameters of the precision sleeve  1   a  and the split sleeve  1   b  are designed so that force applied to the ferrule when the ferrule inserted into each sleeve is press-fitted or pulled out (the force is hereinafter referred to as “pull-out force”) is in the range of from 1.5 to 6 N, preferably in the range of from 2 to 6 N, more preferably in the range of from 2 to 4 N. When the pull-out force in that range is applied to the ferrule, the ferrule can be easily inserted. Further, when the inner diameter of each sleeve is designed so that the pull-out force is in that range, the inserted ferrule can be reliably held. 
     In outer peripheral surfaces  6   a  and  6   b  of the precision sleeve  1   a  and the split sleeve  1   b , the surface roughness R a  is not more than 0.3 μm, preferably not more than 0.2 μm, more preferably not more than 0.1 μm. Although the outer peripheral surfaces  6   a  and  6   b  may be subjected to finishing processing such as polishing processing, they may be fired surfaces as long as the above conditions of the surface roughness R a  can be met. When the outer peripheral surfaces  6   a  and  6   b  are fired surfaces, finishing processing to be described later is not required to be performed, and therefore, it is possible to significantly reduce the manufacturing cost and significantly enhance manufacturing efficiency. 
     Hereinabove, although an example of the configuration of the sleeve for optical communication according to the embodiment of the present invention has been described, the sleeve for optical communication according to the invention is not limited to the example and may be optionally changed without departing from the scope of the invention. 
     Next, a method of manufacturing a sleeve for optical communication according to the present embodiment will be described. As an example, a case in which ceramics made of zirconia is used as a material of the sleeve for optical communication will be described. 
     As shown in  FIG. 2 , in the manufacturing of the sleeve for optical communication, a material for forming the sleeve for optical communication is provided. In the present embodiment, as a material, partially stabilized zirconia mainly composed of ZrO 2 , containing Y 2 O 3 , MgO, CeO 2 , Dy 2 O 3 , CaO, and the like as stabilizers, and added with a binder is provided. Since the partially stabilized zirconia is excellent in processability and, for example, is easily polished, the sleeve for optical communication can be easily formed. 
     Next, a ceramics raw material M is treated by the press-forming to form a preform M 1 . Press forming step is performed using a die tool  11  shown in  FIG. 3 . The die tool  11  used in the press forming is constituted of a middle die  12 , an upper die  13 , a lower die  14 , and a core  15 . The middle die  12  is formed into a substantially columnar shape in the present embodiment, and a through-hole  16  is opened and formed at a central portion of the middle die  12 . The through-hole  16  is formed to have a circular shape in plan view of the middle die  12 . The through-hole  16  is formed so that the inner diameter of the through-hole  16  is substantially the same as outer diameters of the upper die  13  and the lower die  14 , or is slightly larger than the outer diameter of the upper die  13  and the lower die  14 , and the upper die  13  and the lower die  14  can be easily inserted into the through-hole  16 . 
     The upper die  13  is inserted from above in the through-hole  16  of the middle die  12 . The upper die  13  has an upper insertion portion  17 , formed to have an outer diameter substantially the same as or slightly smaller than the inner diameter of the through-hole  16 , and an upper large diameter portion  18  formed to have an outer diameter larger than the outer diameter of the upper insertion portion  17 . The upper insertion portion  17  is provided with an upper insertion hole  19  into which a front end portion  15   a  of the core  15  is inserted. The lower die  14  is inserted from beneath in the through-hole  16  of the middle die  12 . The lower die  14  has a lower insertion portion  20 , formed to have an outer diameter substantially the same as or slightly smaller than the inner diameter of the through-hole  16 , and a lower large diameter portion  21  formed to have an outer diameter larger than the outer diameter of the lower insertion portion  20 . The lower die  14  has an insertion hole  22  which penetrates through in a longitudinal direction and through which the core  15  is inserted. The insertion hole  22  is formed to have an inner diameter substantially the same as or slightly larger than the outer diameter of the core  15  and is formed so that the core  15  can be inserted through the insertion hole  22 . The core  15  is a bar-like member formed to have a diameter substantially the same as or slightly smaller than the inner diameter of the insertion hole  19  of the upper die  13  and the inner diameter of the insertion hole  22  of the lower die  14  and is formed to have a length that allows the front end portion  15   a  to be inserted into the upper insertion hole  19 . 
     When a preform is formed by press forming, the lower die  14  is inserted from beneath in the through-hole  16  of the middle die  12 , and the core  15  is inserted through the insertion hole  22  of the lower die  14 . At this time, in the through-hole  16 , a space  23  is formed between the inner peripheral surface  16   a  of the through-hole  16  and the outer peripheral surface  15   b  of the core  15 . The ceramics raw material M is entered, i.e. filled into the space  23 . Then, the upper die  13  is inserted into the through-hole  16 , the ceramics raw material M entered or filled into the space  23  is pressed and hardened by application of pressure, whereby a preform M 1  is obtained. 
     In the method of manufacturing a sleeve for optical communication according to the present embodiment, since the preform M 1  is obtained by press forming, the binder contained in the ceramics raw material M can be significantly reduced. Thus, forming density of the ceramics raw material M in the preform M 1  can be relatively uniformed. Further, since the preform M 1  can be obtained by press forming, large pressure is easily applied when the ceramics raw material M is pressed and hardened, so that the preform M 1  can be firmly pressed and hardened. 
     Next, a cold isostatic pressing is given for the preform M 1  to obtain the formed body. The cold isostatic press is a method of putting the preform M 1  into a flexible container such as a rubber bag and applying a hydrostatic pressure to the preform M 1  in a liquid medium such as water or oil to form the formed body and is also called a wet process. In the present embodiment, as shown in  FIG. 4( a ) , first, the preform M 1  formed by press forming is put into a flexible container  25  formed into a bag shape. It is preferable that the flexible container  25  is formed of a flexible material easily deformed by force applied from outside of the flexible container  25 , and specifically, it is preferable to use a flexible container formed of rubber, vinyl, nylon, or the like. 
     Next, the flexible container  25  sealed while the preform M 1  is put therein is put in a pressurized container  26 . As shown in  FIG. 4( b ) , the pressurized container  26  is provided with a side wall  27 , an upper lid  28 , and a lower lid  29 . In the pressurized container  26 , a pressurized space  30  surrounded by the side wall  27 , the upper lid  28 , and the lower lid  29  is formed. In the upper lid  28 , a sealing member  31  such as an O-ring is provided at a portion mated to an inner peripheral surface of the side wall  27  and is configured so that it can seal the upper lid  28  and the side wall  27 . In the lower lid  29 , a sealing member  32  such as an O-ring is provided at a portion mated to the inner peripheral surface of the side wall  27  and is configured so that it can seal the lower lid  29  and the side wall  27 . Further, a piping  34  which is connected to a pressurizing pump  33  at an end is connected to the lower lid  29  at the other end thereof, and the lower lid  29  is provided with a flow passage through which liquid flows between the pressuring pump and the pressurized space  30  via the piping  34 . Note that, reference numeral  35  indicates a pressure gauge showing pressure in the pressurized space  30 . And the pressurized space  30  is formed to have a size that allows the flexible container  25  and a liquid such as water to be put therein. 
     In the pressurized space  30 , the flexible container  25  is put therein, and a liquid such as water is injected therein; thereafter, the upper lid  28  is attached to the side wall  27 , and the pressurized space  30  is hermetically sealed. Next, the pressurizing pump  33  is driven to pressurize the inside of the pressurized space  30  at a pressure of 130 to 210 MPa. The pressure at this time is preferably 150 to 200 MPa, more preferably 170 to 190 MPa. In the cold isostatic press, the pressurized space  30  is pressurized at this pressure for a predetermined time, the pressurization by the pressurizing pump  33  is thereafter stopped, and the upper lid  28  is detached to take out the flexible container  25  from the inside of the pressurized space  30 . Next, the formed body obtained by giving the preform M 1  through a cold isostatic press is taken out from the flexible container  25 . Although forming density of the ceramics raw material M in the preform M 1  is relatively uniform because press forming is performed using the ceramics raw material M with little binder, forming density distribution of the ceramics raw material M in the preform M 1  slightly varies. This is because in the press forming, force is applied to the ceramics raw material M only from a certain specific direction. 
     In the method of manufacturing a sleeve for optical communication according to the present embodiment, cold isostatic press is performed after press forming, so that force can be applied to the preform M 1 , formed in the press forming, from all directions, whereby the variation of the forming density distribution of the ceramics raw material M in the preform M 1  can be eliminated. Namely, while a portion in which the forming density of the ceramics raw material M in the preform M 1  is sparse is easily made dense by the force applied in the cold isostatic pressing, a portion in which the forming density of the ceramics raw material M in the preform M 1  is dense is hardly affected by the force applied in the cold isostatic pressing, and the original forming density is maintained. Thus, a difference of the forming density between the portion in which the forming density tends to be sparse and the portion in which the forming density is dense is significantly reduced, and the forming density distribution of the ceramics material as the entire formed body can be substantially uniformed, so that the variation in the forming density can be eliminated. Namely, when the treatment of the cold isostatic press is performed after press forming, each portion of the preform M 1  can be pressed and hardened by uniform strong force, so that the forming density of the ceramics raw material M in each portion of the preform can be further uniformed. In the cold isostatic press, such a formed body can be obtained from the preform M 1 . 
     Next, the formed body is degreased. The degreasing is performed for the purpose of removing a binder contained in the formed body, and after the formed body is put in a degreasing furnace, the formed body is heated at a temperature of 400 to 500° C. for a predetermined time, whereby the degreasing is performed. When the formed body is degreased, the binder in the formed body is eliminated, and a portion where the binder exists becomes a void. However, according to the method of manufacturing a sleeve for optical communication according to the present embodiment, since the preform M 1  is formed by press forming, the amount of the binder contained in the ceramics raw material M can be reduced. Thus, by virtue of the degreasing, the amount of voids existing in the formed body can be significantly reduced, so that distortion can be prevented from occurring when the sleeve for optical communication is formed from the formed body. 
     Next, the degreased formed body is taken out from the degreasing furnace and then fired. In the firing, the degreased formed body is put in a firing furnace, then heated for a predetermined time at a temperature of 1300 to 1500° C. higher than the degreasing temperature, and thereafter cooled by an air cooling method, a water cooling method, or the like to obtain a fired formed body. In the method of manufacturing the sleeve for optical communication according to the present embodiment, since the amount of the binder contained in the ceramics raw material M can be significantly reduced, the amount of voids formed in the formed body in the degreasing can be reduced. Further, in the method of manufacturing a sleeve for optical communication according to the present embodiment, cold isostatic pressing is performed after press forming, whereby the forming density distribution of the ceramics raw material M in the formed body is uniformed, and the formation of voids or the like between the ceramics raw materials M can be significantly reduced. Thus, distortion occurring in the formed body upon cooling after firing and variation in the inner diameter dimension do not occur. In the present embodiment, the formed body is degreased in a degreasing furnace to be taken out from the degreasing furnace and, thus, to be put in a firing furnace, whereby the formed body is fired; however, the temperature of the firing furnace is adjusted, whereby degreasing and firing may be performed in the firing furnace. 
     In the subsequent length processing, the dimension in the longitudinal direction of the fired formed body is adjusted to be a predetermined length in the longitudinal direction of the sleeve for optical communication. The length adjusting may not be performed when the dimension in the longitudinal direction of the fired formed body is a predetermined dimension. 
     In the subsequent outer diameter processing, when an outer diameter of the fired formed body is larger than a predetermined outer diameter of the sleeve for optical communication, or when the outer diameter of the fired formed body varies in the longitudinal direction, the outer diameter of the fired formed body is processed to be substantially the same as the predetermined outer diameter of the sleeve for optical communication. Here, “substantially the same diameter” does not mean that the outer diameter of the fired formed body is completely same as the predetermined outer diameter of the sleeve for optical communication but means that the outer diameter of the fired formed body is within a predetermined allowable error range with respect to the predetermined outer diameter of the sleeve for optical communication. 
     In the finishing processing, an end surface in the longitudinal direction of the fired formed body is processed. In the present embodiment, various processings such as processing for chamfering an end of the end surface, curved surface machining, and deburring are performed. When the sleeve for optical communication to be manufactured is the split sleeve  1   b , the slit  3  is formed by the finishing processing. The finishing processing may not be performed when various processings such as chamfering processing, curved surface machining, and deburring are not required for the fired formed body and when the slit  3  may not be formed because the sleeve for optical communication to be manufactured is the precision sleeve  1   a.    
     The sleeve for optical communication formed through the above processes is formed so that the inner peripheral surfaces of the through-holes  2   a  and  2   b  are fired surfaces. 
     Next, the operational advantages of the sleeve for optical communication according to the present embodiment will be described. Since the sleeve for optical communication according to the present embodiment is formed so that the inner peripheral surfaces of the through-holes  2   a  and  2   b  are fired surfaces, a sleeve for optical communication in which the pull-out force for a ferrule and the connection loss between the ferrules satisfy the conditions and requirements of the standards in the technical field of optical communication can be provided at very low cost. Specifically, it is possible to provide a sleeve for optical communication, which satisfies such conditions of the standards in the technical field of optical communication that the connection loss between optical fibers connected inside the sleeve for optical communication is not more than 0.3 dB and that the pull-out force applied when the optical fiber is pulled out from the sleeve for optical communication is 1.5 to 6 N. According to the sleeve for optical communication of the present embodiment, even when the inner peripheral surfaces of the through-holes  2   a  and  2   b  are fired surfaces, the optical fiber can be connected so that the pull-out force for the ferrule and the connection loss between the ferrules satisfy the conditions of the standards in the technical field of optical communication. 
     In the sleeve for optical communication according to the present embodiment, the surface roughness R a  of the inner peripheral surface is set to a value in the above range, or/and the tolerance of the inner diameter of the through-hole is set to a value in the above range, whereby it is possible to provide an optical connector having such characteristics that the pull-out force for the ferrule and the connection loss between the ferrules are determined as the conditions of the standards in the technical field of optical communication. 
     In the method of manufacturing a sleeve for optical communication according to the present embodiment, the powder of the ceramics raw material M is press-formed to form the preform M 1 , and the preform M 1  is compression-formed by cold isostatic pressing to form the formed body; therefore, the forming density of the ceramics raw material M in the preform M 1  can be uniformed in the press-forming, and occurrence of distortion of the formed body after degreasing and firing can be reliably prevented. Accordingly, in the method of manufacturing a sleeve for optical communication according to the present embodiment, there is no variation in the hole diameters of the through-holes  2   a  and  2   b  and change of the hole shape due to deformation. Thus, the number of the manufacturing processes can be significantly reduced, and the sleeve for optical communication can be manufactured at very low cost. In the method of manufacturing a sleeve for optical communication according to the present embodiment, although the example in which the outer diameter processing is performed after the length processing has been described, the present invention is not limited to the example, and the length adjusting may be performed after the outer diameter processing. 
     EXAMPLES 
     Hereinafter, although the present invention will be described in detail using examples, the invention is not limited to the examples. 
     Example 
     Manufacturing of Sleeve for Optical Communication 
     In this example, a ceramics raw material mainly composed of ZrO 2 , containing Y 2 O 3  and Al 2 O 3  as stabilizers, and added with a binder was provided. A lower die was inserted from beneath a through-hole formed in a middle die, and the previously provided ceramics raw material was put in a space of a form in which a core was inserted through an insertion hole of the lower die. Subsequently, an upper die was inserted from above the through-hole of the middle die, and the ceramics raw material was pressed and hardened by applying downward force from above the upper die and upward force from beneath the lower die, using a press apparatus, whereby a preform was formed. 
     Next, the preform was put in a bag-like flexible container having a size of 550 mm in the vertical direction and 25 mm in the horizontal direction. As the flexible container, an easily deformed flexible container formed by using a nylon packaging bag was used. Subsequently, the flexible bag containing the preform was entered into a pressurized space into which water in a pressurized container of a cold isostatic pressing machine was injected, and after that, an upper lid was closed to hermetically seal the pressurized container. 
     Next, a pressurizing pump of the cold isostatic press machine was driven to increase water pressure in the pressurized container to 200 MPa, and the water pressure was maintained for two minutes. After that, the water pressure in the pressurized container was decreased, and the upper lid was opened to take out the flexible container in the pressurized space. Then, after the cold isostatic press, the formed body was taken out from the flexible container. 
     Next, after the formed body was put into the degreasing furnace, the temperature of the degreasing furnace was increased to 400° C., and the formed body was degreased. Subsequently, after the temperature of the degreasing furnace was decreased, the degreased formed body was taken out from the degreasing furnace. 
     Next, the degreased formed body was put into the firing furnace, and a temperature of the firing furnace was increased to 1350° C., and the formed body was fired. Subsequently, after the temperature of the firing furnace was decreased, the fired formed body was taken out from the firing furnace. An inner diameter of the fired formed body was 2.49 mm. 
     Next, the fired formed body was processed to have a length dimension of 11.4 mm and an outer diameter of 3.2 mm by using a grinding machine or the like, and chamfering processing for an end of a through-hole and processing for slit formation were performed to obtain a sleeve for optical communication of the example. 
     [Measurement of Surface Roughness R a  of Inner Peripheral Surface of Sleeve for Optical Communication] 
     The surface roughness R a  of an inner peripheral surface of the sleeve for optical communication was measured by using a surface roughness measure. As a result of the measurement, the surface roughness R a  in the example was 0.0989 μm in average. 
     [Measurement of Connection Loss] 
     A first optical fiber to which a ferrule was attached at the front end was inserted from one end side of the sleeve for optical communication according to the example, and a second optical fiber to which a ferrule was attached at the front end was inserted from the other end side of the sleeve for optical communication. Inside the sleeve for optical communication, respective end surfaces of the first optical fiber and the second optical fiber were butted against each other to be in contact with each other. Subsequently, light was made to enter the first optical fiber, whereby an amount of light emitted from the second optical fiber was measured. Then, the connection loss was calculated from the amount of the incident light and the amount of the emitting light. As a result of the measurement, the calculation result of the connection loss obtained when the sleeve for optical communication according to the example was used was 0.19 dB in average. 
     [Measurement of Pull-Out Force] 
     The magnitude of the pull-out force applied when the first optical fiber and the second optical fiber inserted into the sleeve for optical communication according to the example was pulled out from the sleeve for optical communication was measured. As a result of the measurement, the magnitude of the pull-out force in the sleeve for optical communication according to the example was 2.1 N in average. 
     The measurement results of the surface roughness R a , the connection loss, and the magnitude of the pull-out force measured for the sleeve for optical communication according to the example are shown in Table 1. 
     Comparative Example 
     Manufacturing of Sleeve for Optical Communication 
     In the comparative example, a ceramics raw material similar to that in the above example was provided. A lower die was inserted from beneath a through-hole formed in a middle die, and the previously provided ceramics raw material was entered into a space of a form in which a core was inserted through an insertion hole of the lower die. Subsequently, an upper die was inserted from above the through-hole of the middle die, and the ceramics raw material was pressed and hardened by applying downward force from above the upper die and upward force from beneath the lower die, using a press apparatus, whereby a preform was formed. A pressure applied to the ceramics raw material at this time was 2 ton/cm 2 . 
     Next, after the formed body was put into the degreasing furnace, the temperature of the degreasing furnace was increased to 400° C., and the formed body was degreased. Subsequently, after the temperature of the degreasing furnace was decreased, the degreased formed body was taken out from the degreasing furnace. 
     Next, the degreased formed body was put into a firing furnace, and a temperature of the firing furnace was increased to 1350° C., and the formed body was fired. Subsequently, after the temperature of the firing furnace was decreased, the fired formed body was taken out from the firing furnace. 
     Next, the fired formed body was processed to have a length dimension of 11.4 mm by using a grinding machine or the like. Since an inner diameter of the fired formed body varied, the formed body was processed to have an inner diameter of 2.49 mm. Further, the fired formed body was processed to have an outer diameter of 3.2 mm. After that, chamfering processing for an end of a through-hole and processing for slit formation were performed to obtain a sleeve for optical communication of the comparative example. Since the inner peripheral surface of the sleeve for optical communication was subjected to the inner-diameter processing, the inner peripheral surface was a ground surface on which crystal grains and a crystal grain boundary are not exposed. 
     [Measurement of Surface Roughness R a  of Inner Peripheral Surface of Sleeve for Optical Communication] 
     The surface roughness R a  of the inner peripheral surface of the sleeve for optical communication was measured by a method similar to that in the example. As a result of the measurement, the surface roughness R a  in the comparative example was 0.0213 μm in average. 
     [Measurement of Connection Loss] 
     By a method similar to that in the example, the connection loss between a first optical fiber and a second optical fiber occurring when the sleeve for optical communication of the comparative example was used was measured. As a result of the measurement, the calculation result of the connection loss obtained when the sleeve for optical communication according to the comparative example was used was 0.12 dB in average. 
     [Measurement of Pull-Out Force] 
     The pull-out force in the comparative example was measured by a method similar to that in the example. As a result of the measurement, the magnitude of the pull-out force in the sleeve for optical communication according to the comparative example was 2.8 N in average. 
     The measurement results of the surface roughness R a , the connection loss, and the magnitude of the pull-out force measured for the sleeve for optical communication according to the comparative example are shown in Table 1. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Surface roughness 
                 Connection 
                 Pull-out 
               
               
                   
                 Ra (μm) 
                 loss (dB) 
                 force (N) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Example 
                 0.0989 
                 0.19 
                 2.1 
               
               
                   
                 Comparative 
                 0.0213 
                 0.12 
                 2.8 
               
               
                   
                 example 
               
               
                   
                   
               
             
          
         
       
     
     From the above results, in the sleeve for optical communication according to the example and the sleeve for optical communication according to the comparative example, it was found that the pull-out force applied to the ferrule was 1.5 to 6 N, and the connection loss between the ferrules butted against each other in the sleeve for optical communication was not more than 0.3 dB. In the sleeve for optical communication according to the example, since there is no treatment of the inner-diameter processing step, the number of manufacturing processes or steps in the formation of the sleeve for optical communication concerned is smaller or fewer compared to the comparative example, and the sleeve for optical communication according to the example can be manufactured at lower cost than the sleeve according to the comparative example. 
     REFERENCE SIGNS LIST 
     
         
           1   a  precision sleeve 
           1   b  split sleeve 
           2   a ,  2   b  through-hole 
           3  slit 
           4   a ,  4   b  inner peripheral surface 
           5   a ,  5   b  crystal grain 
           5   c  crystal grain boundary 
           6   a ,  6   b  outer peripheral surface 
           11  die tool 
           12  middle die 
           13  upper die 
           14  lower die 
           15  core 
           16  through-hole 
           23  space 
           25  flexible container 
           26  pressurized container 
           27  side wall 
           28  upper lid 
           29  lower lid 
           30  pressurized space 
           33  pressurizing pump 
         M ceramics raw material 
         M 1  preform