Abstract:
In order to be usable in combination with various receptacles without drawbacks, a plug for an optical connector has a plug tip section defining an optical reference plane that is to coincide with an optical reference plane in a receptacle, and at distances behind the optical reference plane with respect to an insertion direction, the plug has a first mechanical reference plane that is able to be brought into contact with an element within the receptacle, and a second mechanical reference plane that is able to be brought into contact with a front of the receptacle.

Description:
This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 2003-272788 filed in Japan on Jul. 10, 2003, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1 Field of the Invention 
   The present invention relates to a plug and a receptacle for an optical connector that is used for optical transmission. 
   The invention also relates to an optical connector including such a plug and receptacle. 
   The invention also relates to electronic equipment having such an optical connector. 
   2 Description of Related Art 
   The term “electronic equipment” herein broadly encompasses information communication equipment such as digital TV (television) sets, digital BS (broadcasting satellite) tuners, CS (communication satellite) tuners, DVD (digital versatile disc) players, super audio CD (compact disc) players, AV (audio video) amplifiers, audio, personal computers, personal computer peripheral, portable telephones, and PDA (personal digital assistants); amusement equipment such as pachinko machines and other game equipment; and any other equipment using optical transmission. 
   A conventional example 1 of a plug and a receptacle of an optical connector is shown in  FIGS. 1A through 1C  and  FIGS. 2A and 2B , which connector (referred to as a rectangular connector) is conforming to RC-5720B, a standard of JEITA (Japan Electronics and Information Technology Industries Association, formerly EIAJ: Electronic Industries Association of Japan).  FIG. 1A  is a side view of the plug,  FIG. 1B  is a sectional view of the same, and  FIG. 1C  is a front view of the same.  FIG. 2A  is a front view of the receptacle, and  FIG. 2B  is a sectional side elevation of the same. As shown in  FIGS. 1A through 1C , the plug  1001  has a cylindrical plug tip section  1002 , a rectangular plug peripheral section  1003 , and a plug body  1009  to be held by a person at the time of insertion into the receptacle. On the plug peripheral section  1003  are provided guides  1004  for guiding the plug at the plug insertion and protrusions  1005  for engaging with the receptacle. As shown in  FIG. 1B , an optical fiber cable is inserted into a hole  1006  provided at center in the sectional view, and a tip thereof is formed into an optical reference plane  1007  by hot plate processing or polish processing. The plug  1001  is provided with one mechanical reference plane (reference plane for a fit with the receptacle)  1008 , and a distance between the optical reference plane  1007  and the mechanical reference plane  1008  is set at 5.4 mm. In  FIG. 1C , cross-hatching is provided on the mechanical reference plane  1008  for easy understanding. The receptacle  2001  has an insertion hole  2005  to be associated with the plug peripheral section, an insertion hole  2004  to be associated with the plug tip section, grooves  2006  to be associated with the guides of the plug, a plug tip holding section  2010  for holding the plug tip section, an optical element insertion hole  2008 , and holes  2009  to be associated with the protrusions for engaging of the plug, in a rectangular housing  2000 . As shown in  FIG. 2B , an optical reference plane  2007  is provided at a distance of 9.1 mm from an end surface  2002  of the receptacle  2001 , and a mechanical reference plane  2003  is provided at a distance of 5.4 mm from the optical reference plane  2007 . The mechanical reference plane  2003  is an end surface of the plug tip holding section  2010 . 
   By the insertion of the plug  1001  into the receptacle  2001 , in a structure of this conventional example 1, the mechanical reference plane  1008  provided in the plug  1001  is brought into contact with the mechanical reference plane  2003  provided in the receptacle  2001 , so that further insertion is restricted. In a state in which the mechanical reference planes  1008  and  2003  are in contact with each other, the optical reference planes  1007  and  2007  coincide with each other. Thus optical transmission is performed satisfactorily. 
   A conventional example 2 of a receptacle is shown in  FIGS. 3A and 3B  (see JP 2002-48952 A, for example).  FIG. 3A  is a front view in a state with a shutter closed, and  FIG. 3B  is a front view in a state with a shutter opened. As shown in  FIGS. 3A and 3B , the receptacle  3010   a  has a shutter  3011 , a plug insertion hole  3012 , and grooves  3017  associated with guides provided on a plug, in a housing  3000 . Reference character  3010   b  denotes a front of the housing of the receptacle. In the receptacle  3010   a  of the conventional example 2, the plug  1001  of the conventional example 1 can be inserted and fitted into the plug insertion hole  3012 . The shutter  3011  opens and closes inside the plug insertion hole  3012  with assistance of a hinge  3016 , so that foreign matter can reliably be prevented from entering the plug insertion hole  3012 . On a front of the shutter  3011  is provided a groove  3011   a  for guiding the plug tip section  1007  during the insertion of the plug  1001  of the conventional example 1. As shown in  FIG. 3B , blocks  3012   a  according to a shape of the plug are provided on side surfaces inside the plug insertion hole  3012 . 
   When the plug  1001  of the conventional example 1 is inserted into the plug insertion hole  3012  of the receptacle  3010   a  of this conventional example 2, an extremity  1011  of the plug peripheral section of the plug  1001  is brought into contact with the blocks  3012   a  provided in the receptacle  3010   a,  so that further insertion is restricted. In a state in which the extremity  1011  of the plug peripheral section of the plug  1001  is in contact with the blocks  3012   a,  optical reference planes of the plug and receptacle coincide with each other. 
   A conventional example 3 of a receptacle is shown in  FIGS. 4A through 4C  (see JP 2000-131564 A, for example).  FIG. 4A  is a front view of the receptacle,  FIG. 4B  is a side view of the same, and  FIG. 4C  is a sectional side view of the same. As shown in  FIGS. 4A through 4C , the receptacle  4200  has a housing  4205  having an insertion hole  4220  into which a plug is inserted, a shutter  4210  that is mounted on an entrance of the insertion hole  4220  with a hinge structure (having a shaft  4240 ) and that is intended for generally closing the insertion hole  4220 , an elastic body  4230  for biasing the shutter  4210  toward the entrance, and grooves  4221  to be associated with guides provided on the plug. An optical element  4290  is provided behind the insertion hole  4220 . When the plug is inserted into the insertion hole  4220 , the shutter  4210  is housed in space  4225  in the insertion hole  4220  and functions so as to hold a side surface of the plug. 
   In the receptacle  4200  of the conventional example 3, a mechanical reference plane is not provided, and it is therefore uncertain which part abuts on the plug when the plug is inserted into the insertion hole  4220 . 
   A conventional example 4 of a plug is a round optical plug  5001  as shown in  FIGS. 5A through 5C  (e.g., Horizon Tec Co., Ltd., “DIY products, OPB-41, circular plug (for Φ 4.0 mm cord),” online, searched on Jul. 1, 2004, see URL: http://www.horizontec.biz/opb41.htm).  FIG. 5A  is a side view of the plug  5001 ,  FIG. 5B  is a sectional side view of the same, and  FIG. 5C  is a front view of the same. The plug  5001  has a cylindrical plug tip section  5002 , a cylindrical plug peripheral section  5003 , and a plug body  5009  to be held by a person on the occasion of insertion into a receptacle. Around the plug peripheral section  5003  is provided an annular protrusion  5004  for engaging with the receptacle. Numeral  5007  denotes a hole in which an optical fiber cable is positioned. The plug  5001 , of which the plug peripheral section  5003  is cylindrical, can be inserted into the rectangular receptacle  2001  defined in JEITA RC-5720B at any angle about a center C thereof. Moreover, the plug can be turned after the insertion, and thus torsion of the cable can be canceled. 
   A positional relation between an optical reference plane  5006  and a mechanical reference plane  5005  in the plug  5001  of the conventional example 4 is the same as the positional relation between the optical reference plane  1007  and the mechanical reference plane  1008  in the rectangular plug  1001  of the conventional example 1. 
   As apparent from the above, there exist various types of plugs and receptacles, and those plugs and receptacles may be used in combination in various manners. 
   For example, insertion of the plug  5001  of the conventional example 4 into the receptacle  2001  of the conventional example 1 causes no problem because the optical reference plane  2007  of the receptacle  2001  and the optical reference plane  5006  of the plug  5001  coincide with each other. 
   When the plug  5001  of the conventional example 4 is inserted into the receptacle  3010   a  of the conventional example 2, however, the plug  5001  does not rest against the blocks  3012   a  provided in the receptacle  3010   a  because a part associated with the extremity  1011  of the plug peripheral section of the plug  1001  of the conventional example 1 is not provided in the plug  5001 , but the end surface  3010   b  of the housing of the receptacle  3010   a  and an end surface  5008  of the plug body come into contact with each other, so that further insertion of the plug is restricted. In this situation, the optical reference plane  5006  of the plug  5001  is in a position deeper than the optical reference plane  2007  of the receptacle, and contact is therefore made between the plug tip  5006  and an optical element provided inside the receptacle. As a result, the optical element may fail, and flaws on the plug tip  5006  may cause transmission loss and deterioration in transmission quality. 
   When the plug  5001  of the conventional example 4 is inserted into the receptacle  4200  of the conventional example 3, as is the case with the conventional example 2, the optical element provided inside the receptacle may fail or damage to the plug tip  5006  may cause transmission loss and deterioration in transmission quality because a mechanical reference plane is not provided in the receptacle  4200  of the conventional example 3. 
   SUMMARY OF THE INVENTION 
   A feature of the present invention is to provide a plug of an optical connector that can be used in combination with a variety of receptacles without causing problems with insertion thereof into a receptacle. 
   Another feature of the invention is to provide a receptacle suitable for such a plug. 
   A further feature of the invention is to provide an optical connector having such a plug, and electronic equipment having such an optical connector. 
   In order to accomplish the above feature, there is provided, according to the present invention, a plug for an optical connector, which plug is to be combined with a receptacle having a plug insertion hole, the plug comprising: 
   a plug tip section defining an optical reference plane that is to coincide with an optical reference plane in the receptacle; 
   a first mechanical reference plane that is positioned a specified distance behind the optical reference plane of the plug tip section with respect to an insertion direction so as to be able to be brought into contact with an element within the receptacle; and 
   a second mechanical reference plane that is positioned a specified distance behind the optical reference plane of the plug tip section with respect to the insertion direction so as to be able to be brought into contact with a front of the receptacle. 
   The plug of the present invention has the first mechanical reference plane and the second mechanical reference plane, and is therefore usable in combination with a variety of receptacles without causing problems with insertion thereof into a receptacle. 
   For a receptacle that is to be combined and that has therein an optical reference plane and an element (corresponding to a mechanical reference plane) positioned at a specified distance from the optical reference plane toward a front thereof, such as a rectangular receptacle defined in JEITA (Japan Electronics and Information Technology Industries Association) RC-5720B, insertion of the plug tip section into a plug insertion hole brings the first mechanical reference plane into contact with the element in the receptacle, so that further insertion is restricted. Provided that the distance between the optical reference plane the plug tip section forms and the first mechanical reference plane has been defined in advance in accordance with JEITA RC-5720B, the optical reference plane in the receptacle and the optical reference plane of the plug of the invention are allowed to coincide with each other, with the first mechanical reference plane in contact with the element. Thus optical transmission is performed satisfactorily and problems are prevented from occurring. 
   For a receptacle that is to be combined and that does not have an element which can be brought into contact with the first mechanical reference plane, insertion of the plug tip section into a plug insertion hole brings the second mechanical reference plane into contact with a front surface of the receptacle, so that further insertion of the plug is restricted. Provided that the distance between the optical reference plane the plug tip section forms and the second mechanical reference plane has appropriately been set in advance, the optical reference plane in the receptacle and the optical reference plane of the plug of the invention are allowed to coincide with each other, with the second mechanical reference plane in contact with the front of the receptacle. Thus optical transmission is performed satisfactorily and problems are prevented from occurring. 
   In accordance with the plug of the invention, the optical reference plane in the receptacle and the optical reference plane of the plug of the invention thus coincide with each other even if the plug is combined with a variety of receptacles. As a result, such problems as failure of an optical element provided inside the receptacle and deterioration in transmission quality that might be caused by damage to the plug tip section are prevented. Thus the plug of the invention can satisfactorily be used in combination with a variety of receptacles. 
   In one embodiment, the distance between the optical reference plane defined by the plug tip section and the first mechanical reference plane is set at 5.4 mm, and the distance between the optical reference plane defined by the plug tip section and the second mechanical reference plane is set at 9.1 mm. 
   In accordance with the plug of the embodiment in combination with a receptacle defined in JEITA RC-5720B, insertion of the plug tip section into a plug insertion hole brings the first mechanical reference plane into contact with an element in the receptacle, so that further insertion is restricted. In a state in which the first mechanical reference plane is in contact with the element, the optical reference plane in the receptacle and the optical reference plane of the plug of the invention coincide with each other. Thus optical transmission is performed satisfactorily and problems are prevented from occurring. 
   For a receptacle that is to be combined and that does not have an element which can be brought into contact with the first mechanical reference plane, insertion of the plug tip section into a plug insertion hole brings the second mechanical reference plane into contact with a front of the receptacle, so that further insertion is restricted. In a state in which the second mechanical reference plane is in contact with the front of the receptacle, the optical reference plane in the receptacle and the optical reference plane of the plug of the invention coincide with each other. Thus optical transmission is performed satisfactorily and problems are prevented from occurring. 
   In accordance with the plug of the embodiment, the optical reference plane in the receptacle and the optical reference plane of the plug of the invention thus coincide with each other even if the plug is combined with a variety of receptacles. As a result, such problems as failure of an optical element provided inside the receptacle and deterioration in transmission quality that might be caused by damage to the plug tip section are prevented. Thus the plug of the invention can satisfactorily be used in combination with a variety of receptacles. 
   In this embodiment, a distance between the first mechanical reference plane and the second mechanical reference plane is 3.7 mm. 
   In one embodiment, the plug tip section is cylindrical, and the plug has a cylindrical peripheral section coaxial with the plug tip section and having a diameter that allows the cylindrical peripheral section to be inscribed in a plug insertion hole of a rectangular receptacle defined in JEITA RC-5720B. 
   The plug of the embodiment can be inserted into the rectangular receptacle defined in JEITA RC-5720B, at any angle about a center thereof. Besides, the plug can be turned after the insertion, and thus torsion of a cable can be canceled. 
   In the plug, an end surface of the cylindrical peripheral section may form the first mechanical reference plane. 
   The plug may have a plug body larger than the cylindrical peripheral section in size with respect to a direction perpendicular to the insertion direction, and a front of the plug body with respect to the insertion direction may form the second mechanical reference plane. 
   A receptacle according to the invention is usable in combination with the plug of the invention and has a plug insertion hole. The receptacle has a rear surface that is in depths of the plug insertion hole with respect to an insertion direction tapers off rearwards. 
   With insertion of a plug into the receptacle of the invention for an optical connector, a tip section of the plug is guided by the rear surface of the plug insertion hole that is in the depths of the plug insertion hole with respect to the insertion direction. Thus the plug is smoothly inserted into the receptacle. As a result, the plug tip section is prevented from being damaged, and the problem of deterioration in transmission quality is prevented. 
   There is provided, according to another aspect of the invention, a receptacle that is usable in combination with the plug of the invention and that has a plug insertion hole, wherein: 
   the receptacle comprises, at an entrance of the plug insertion hole, a hinged shutter that is openable in contact with the plug tip section as the plug is inserted into the plug insertion hole; 
   the shutter has a groove in a front thereof, the groove extending in a direction perpendicular to a hinge pin of the shutter for guiding the plug tip section; and 
   the groove has a tapered surface at an extremity thereof. 
   The receptacle of the invention has the shutter at the entrance of the plug insertion hole, and therefore foreign matter can reliably be prevented from entering the plug insertion hole in which the plug is not inserted. 
   When the plug tip section is inserted into the plug insertion hole, the plug tip section is guided by the groove and thus the hinged shutter is smoothly opened. Besides, the tapered surface provided at the extremity of the groove prevents the plug tip section from being caught in by the extremity of the groove. Thus the shutter is opened smoothly, and the plug tip section is prevented from being damaged. As a result, such a problem as deterioration in transmission quality that might be caused by damage to the plug tip section is prevented. 
   In the receptacle of one embodiment, an edge of the tapered surface has an arc-shaped pattern bulging out, so that the plug tip section is reliably prevented from being caught in the extremity of the groove. Thus the shutter is opened smoothly, and the plug tip section is prevented from being damaged. As a result, such a problem as deterioration in transmission quality that might be caused by damage to the plug tip section is prevented. 
   The present invention also provides an optical connector including a receptacle having a plug insertion hole, and a plug, wherein 
   the plug comprises:
         a plug tip section defining an optical reference plane that is to coincide with an optical reference plane in the receptacle;   a first mechanical reference plane that is positioned a specified distance behind the optical reference plane of the plug tip section with respect to an insertion direction so as to be able to be brought into contact with an element within the receptacle; and   a second mechanical reference plane that is positioned a specified distance behind the optical reference plane of the plug tip section with respect to the insertion direction so as to be able to be brought into contact with a front of the receptacle.       

   Due to the effects of the plug of the invention, which have been described above, the optical connector of the invention is prevented from suffering from such problems as failure of an optical element provided inside the receptacle and deterioration in transmission quality that might be caused by damage to the plug tip section. Thus reliability of optical transmission can be increased. 
   In the optical connector of the invention, the receptacle may have the structure as described above. 
   According to the present invention, there is also provided electronic equipment that includes the optical connector of the invention. 
   The electronic equipment is therefore prevented from suffering from such problems as failure of an optical element provided inside the receptacle of the optical connector and deterioration in transmission quality that might be caused by damage to the plug tip section. Thus reliability is increased. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not intended to limit the present invention, and wherein: 
       FIGS. 1A ,  1 B, and  1 C are a side view, a sectional side view, and a front view, respectively, of a plug as a conventional example 1 that is defined in JEITA RC-5720B; 
       FIGS. 2A and 2B  are a front view and a sectional side view, respectively, of a receptacle as the conventional example 1 that is defined in JEITA RC-5720B; 
       FIGS. 3A and 3B  are views of a receptacle as a conventional example 2 that is provided with a shutter, wherein  FIG. 3A  is a front view of the receptacle with the shutter closed, and  FIG. 3B  is a front view of the receptacle with the shutter opened; 
       FIGS. 4A ,  4 B, and  4 C are a front view, a side view, and a sectional side view, respectively, of a receptacle as a conventional example 3 that is provided with a shutter; 
       FIGS. 5A ,  5 B, and  5 C are a side view, a sectional side view, and a front view, respectively, of a plug as a conventional example 4; 
       FIGS. 6A ,  6 B, and  6 C are a side view, a sectional side view, and a front view, respectively, of a plug in accordance with one embodiment of the invention; 
       FIGS. 7A and 7B  are a front view and a sectional side view, respectively, of the receptacle as the conventional example 1,  FIG. 7C  is a sectional side view in a case that the plug of the conventional example 4 has been inserted into the receptacle, and  FIG. 7D  is a sectional side view in a case that the plug shown in  FIGS. 6A through 6C  has been inserted into the receptacle; 
       FIGS. 8A and 8B  are a front view and a sectional side view, respectively, of a receptacle in accordance with one embodiment of the invention,  FIG. 8C  is a sectional side view in a case that the plug of the conventional example 4 has been inserted into the receptacle, and  FIG. 8D  is a sectional side view in a case that the plug shown in  FIGS. 6A through 6C  has been inserted into the receptacle; 
       FIG. 9A  is a sectional top view of the receptacle of the conventional example 2, and  FIG. 9B  is a sectional top view of a receptacle in accordance with one embodiment of the invention; 
       FIG. 10A  is a front view of a shutter that is the same as the shutter in the conventional example 2,  FIG. 10B  is a section of the shutter, and  FIG. 10C  is a diagram showing a situation in which an optical plug is being inserted into the receptacle having the shutter; 
       FIG. 11A  is a front view of a shutter of the receptacle in accordance with the embodiment of the invention,  FIG. 11B  is a section of the shutter, and  FIG. 11C  is a diagram showing a situation in which an optical plug is being inserted into the receptacle having the shutter; 
       FIG. 12  is a diagram showing an example in which an optical connector with a combination of the plug and the receptacle of the invention is applied to optical transmission between AV equipment; 
       FIG. 13  is a diagram showing an example in which the optical connector with a combination of the plug and the receptacle of the invention is applied to optical transmission in game equipment; and 
       FIG. 14  is a diagram showing an example in which the optical connector with a combination of the plug and the receptacle of the invention is applied to optical transmission inside a pachinko machine. 
   

   DETAILED DESCRIPTION 
   Hereinbelow, the present invention will be described in detail with reference to embodiments shown in the accompanying drawings. 
   First Embodiment 
     FIGS. 6A through 6C  show a configuration of a plug  1  as one embodiment.  FIG. 6A  is a side view,  FIG. 6B  is a sectional side view, and  FIG. 6C  is a front view. As shown in  FIG. 6A , the plug  1  has a cylindrical plug tip section  2 , a cylindrical plug peripheral section  3  having the same axis as and a larger diameter than the plug tip section  2 , and a rectangular plug body  9  to be held by a person on the occasion of insertion thereof into a receptacle. The diameter of the plug peripheral section  3  is set at a size such that the section is inscribed in a plug insertion hole  2005  (see  FIGS. 7A and 7B ) of a rectangular receptacle defined in JEITA RC-5720B. On the plug peripheral section  3 , an annular protrusion  4  for engaging with the receptacle is provided along a circumference of the section  3 . As shown in  FIG. 6C , outside dimensions of the plug body  9  are set so as to be larger than those of the plug peripheral section  3 . 
   An optical fiber cable (not shown) is inserted into a plug center hole  8  shown in  FIG. 6B , and a tip thereof is finished by hot plate processing or polish processing. The tip of the optical fiber, that is, an end surface of the plug tip section  2  forms an optical reference plane  7 . 
   An end surface  5  of the plug peripheral section  3  forms a first mechanical reference plane. A distance between the optical reference plane  7  the plug tip section  2  forms and the first mechanical reference plane  5  is set at 5.4 mm. 
   An end surface  6  of the plug body  9  forms a second mechanical reference plane. A distance between the optical reference plane  7  the plug tip section  2  forms and the second mechanical reference plane  6  is set at 9.1 mm. 
   As a result, a distance between the first mechanical reference plane  5  and the second mechanical reference plane  6  is 3.7 mm. 
   The distance between the optical reference plane  7  and the first mechanical reference plane  5  is equal to the distance between the optical reference plane  1007  and the mechanical reference plane  1008  in the rectangular plug  1001  of the conventional example 1 shown in  FIGS. 1A through 1C . The distance is also equal to the distance between the optical reference plane  5006  and the mechanical reference plane  5005  in the plug  5001  of the conventional example 4 shown in  FIGS. 5A through 5C . 
   In the rectangular plug  1001  of the conventional example 1, a distance A between the mechanical reference plane  1008  and an end surface  1010  of the plug body  1009  is specified at 4.1 mm, and a distance between the optical reference plane  1007  and the plug body end surface  1010  therefore runs into 9.5 mm. 
   In the round plug  5001  of the conventional example 4, a distance B between the mechanical reference plane  5005  and the end surface  5008  of the plug body  5009  is 4.1 mm as is the case with the conventional example 1, and a distance between the optical reference plane  5006  and the plug body end surface  5008  therefore runs into 9.5 mm. 
   By contrast, the plug  1  of the embodiment is significantly different from the plugs of the conventional examples 1 and 4 in that the distance between the optical reference plane  7  and the second mechanical reference plane  6  is set at 9.1 mm. 
   Hereinbelow, a case in which the plug  1  of the embodiment is inserted into the rectangular receptacle of the conventional example 1 defined in JEITA RC-5720B will be described in comparison with a case in which the round plug  5001  of the conventional example 4 is inserted into the same receptacle. 
     FIG. 7A  is a front view of the rectangular receptacle  2001  defined in JEITA RC-5720B, and  FIG. 7B  is a sectional side view of the receptacle  2001 . In the rectangular receptacle  2001 , a distance between the optical reference plane  2007  and the mechanical reference plane  2003  is set at 5.4 mm. 
   In the case that the round optical plug  5001  of the conventional example 4 is inserted into the rectangular receptacle  2001 , as shown in  FIG. 7C , the plug tip section  5002  is inserted into depths (rightward in  FIG. 7C ) along the plug tip insertion hole  2004  in the plug tip holding section  2010  of the receptacle  2001 . In course of time, the mechanical reference plane  5005  of the plug is brought into contact with the mechanical reference plane  2003  in the receptacle  2001  and is then stopped. In this state, the optical reference plane  5006  of the optical plug and the optical reference plane  2007  of the receptacle coincide with each other. Thus, contact is prevented between the optical element (not shown) provided inside the receptacle and the plug tip  5006 . Between the plug body end surface  5008  and the front surface  2002  of the receptacle, there is no contact, but a clearance of 0.4 mm. This positional relationship is the same as that in the conventional example 1 defined in JEITA RC-5720B in which the rectangular optical plug  1001  has been fitted in the rectangular receptacle  2001 . 
   In the case that the round optical plug  1  of the embodiment is inserted into the rectangular receptacle  2001 , as shown in  FIG. 7D , the plug tip section  2  is inserted into depths (rightward in  FIG. 7D ) along the plug tip insertion hole  2004  in the plug tip holding section  2010  of the receptacle  2001 , as is the case with  FIG. 7C . In course of time, the mechanical reference plane  5  of the plug is brought into contact with the mechanical reference plane  2003  in the receptacle  2001  and is then stopped. At this point, the second mechanical reference plane  6  of the optical plug is in contact with the front surface  2002  of the receptacle. In this state, the optical reference plane  7  of the optical plug and the optical reference plane  2007  of the receptacle coincide with each other. Thus, contact is prevented from occurrence between the optical element (not shown) provided inside the receptacle and the plug tip  5006 . In the case that the plug  1  of the embodiment has been inserted into the rectangular receptacle  2001 , therefore, optical signal can be transmitted without deterioration in transmission quality, as is the case with the plug  5001  of the conventional example 4 and the rectangular plug  1001  of the conventional example 1 inserted into the receptacle  2001 . 
   Hereinbelow, a case in which the plug  1  of the embodiment is inserted into a receptacle having a shutter in a plug insertion hole as in the conventional examples 2 and 3 will be described in comparison with a case in which the round plug  5001  of the conventional example 4 is inserted into the same receptacle. 
     FIG. 8A  is a front view of a subject receptacle  11  with a shutter opened, and  FIG. 8B  is a sectional side view of the same. The receptacle  11  has an insertion hole  15  corresponding to the plug peripheral section, an insertion hole  14  corresponding to the plug tip section, grooves  16  corresponding to the guides of the plug, a plug tip holding section  20  for holding the plug tip section, and a hole  19  corresponding to the engaging protrusion of the plug, in a rectangular housing  10 . 
   As apparent from the front view (the state with the shutter opened) of  FIG. 8A , shapes of the plug insertion holes  14  and  15  of the receptacle  11  are identical to those of the plug insertion holes  2004  and  2005  of the receptacle  2001  of the conventional example 1. Accordingly, any of the rectangular optical plug  1001  of the conventional example 1, the round optical plug  5001  of the conventional example 4, and the optical plug  1  of the embodiment can be inserted into the receptacle  11 . 
   The receptacle  11  employs a structure in which the shutter (not shown) is provided in a hinged manner in vicinity of a front surface  12  of the receptacle so that the shutter pivots around a hinge axis and thereby opens toward inside of the plug insertion hole  15 . As shown in  FIG. 8B , therefore, a size of the plug tip holding section  20  in a direction of the insertion is set smaller than that of the plug tip holding section  2010  of the rectangular receptacle  2001  of the conventional example 1 so that the section  20  may not hinder the shutter from pivoting. In  FIG. 8B , for comparison, an outline of the tip holding section  2010  in the receptacle of the conventional example 1 is shown by a dotted line. 
   In the receptacle  11 , a distance between an optical reference plane  17  on which an optical element (not shown) is provided and the front surface  12  of the receptacle is set at 9.1 mm. 
   In the case that the optical plug  5001  of the conventional example 4 is inserted into the receptacle  11 , as shown in  FIG. 8C , the plug tip section  5002  is inserted into depths (rightward in  FIG. 8C ) along the plug tip insertion hole  14  in the plug tip holding section  20  of the receptacle  11 . The mechanical reference plane  5005  provided in the optical plug goes beyond a position corresponding to the mechanical reference plane provided in the receptacle of the conventional example 1 because the size of the plug tip holding section  20  in the direction of the insertion is small. In course of time, the plug body end surface  5008  is brought into contact with the front surface  12  of the receptacle and is then stopped. In this state, the optical reference plane  5006  of the optical plug and the optical reference plane  17  of the receptacle do not coincide with each other. That is, the optical reference plane  5006  of the plug resides in a position deeper than (on the right side in  FIG. 8C ) the optical reference plane  17  of the receptacle, and there is a gap between both the planes. This is a result of the insertion of the plug  5001  deeper by the clearance of 0.4 mm between the plug body end surface  5008  and the front surface  2002  of the receptacle which clearance is shown in  FIG. 7C . This means that the tip  5006  of the optical plug may come into contact with the optical element (not shown) in the receptacle  11 . 
   That is because the size of the plug tip holding section  20  in the direction of the insertion is decreased for purpose of providing a structure which does not hinder the pivoting movement of the shutter, as already described, and because the decrease of the size of the plug tip holding section  20  eliminates a mechanical reference plane corresponding to the mechanical reference plane  5005  provided in the optical plug  5001  of the conventional example 4. 
   In the case that the optical plug  1001  described as the conventional example 1 is inserted into the receptacle  11 , the optical reference planes of both can be made to coincide with each other by provision of blocks to the receptacle  11  in a position in which the extremity  1011  of the plug peripheral section of the optical plug  1001  is to come into contact with the blocks, as in the receptacle of the conventional example 2. 
   In the case that the optical plug  1  of the embodiment is inserted into the receptacle  11 , as shown in  FIG. 8D , the plug tip section  2  is inserted into depths (rightward in  FIG. 8D ) along the plug tip insertion hole  14  in the plug tip holding section  20  of the receptacle  11 . In course of time, the second mechanical reference plane  6  provided in the optical plug is brought into contact with the front surface  12  of the receptacle and is then stopped. Then the first mechanical reference plane  5  provided in the optical plug is not in contact with any elements (parts) in the receptacle  11 . In this state, the optical reference plane  7  of the optical plug and the optical reference plane  17  of the receptacle coincide with each other. Thus no contact is made between the optical element (not shown) provided inside the receptacle and the plug tip  7 . 
   By the combination of the plug  1  of the embodiment and the receptacle  11 , therefore, optical signal can be transmitted without deterioration in transmission quality, as is the case with the combination of the rectangular plug  1001  and the rectangular receptacle  2001  described as the conventional example 1. 
   The plug  1 , of which the plug peripheral section  3  is cylindrical, can be inserted into the rectangular receptacle  2001  conforming to JEITA RC-5720B, at any angle about a center C thereof. Moreover, the plug can be turned after the insertion, and thus torsion of a cable can be canceled. 
   In the embodiment, setting of the first and second mechanical reference planes is done on the basis of the rectangular plug and receptacle (F05 type) defined in JEITA RC-5720B. The setting of the first and second mechanical reference planes, however, may be done for an optical plug corresponding to connectors of two-core F07 type, two-core PN type, SC type, FC type and the like and other special connectors. Furthermore, a larger number of mechanical reference planes may be set. 
   Second Embodiment 
     FIG. 9B  shows a section, as seen from above, of a receptacle  31  in accordance with an embodiment. For comparison,  FIG. 9A  shows a section, as seen from above, of the receptacle  11  shown in  FIGS. 8A and 8B  and described above. 
   The receptacle  31  shown in  FIG. 9B  has an insertion hole  35  corresponding to a plug peripheral section, an insertion hole  34  corresponding to a plug tip section, and a plug tip holding section  36  for holding the plug tip section, in a rectangular housing  30 . The receptacle  31  is different from the receptacle  11  in that the receptacle  31  has a tapered bottom surface  33  (on upper side in  FIG. 9B ) of the plug insertion hole  35 , the bottom surface opening conically toward the front, so that the plug tip section can easily be guided toward the plug tip insertion hole  34 . 
   In the receptacle  11  in  FIG. 9A , a bottom surface  21  of the plug insertion hole  15  is perpendicular to the direction of the insertion, and there is a step (a plug tip holding section)  20  between the surface  21  and the plug tip insertion hole  14 . In the other points, the receptacle  31  of  FIG. 9B  and the receptacle  11  of  FIG. 9A  are configured similarly. 
   The plug  5001  of the conventional example 4 and the optical plug  1  of the embodiment shown in  FIGS. 6A through 6C  have the cylindrical plug peripheral sections  5003 ,  3 , and therefore sizes thereof in a direction perpendicular to the direction of the insertion are comparatively small. When the plug  5001 ,  1  is inserted into the receptacle  11 , it is difficult to insert the plug horizontally or vertically and the plug may diagonally be inserted because a size of the plug tip holding section  20  of the receptacle  11  in the direction of the insertion is small. With use of the receptacle  11  of  FIG. 9A , in this case, the plug tip section  5002 ,  2  may collide with the bottom surface  21  of the plug insertion hole  15  without being properly inserted into the plug tip holding section  20  of the receptacle. In that event, because the step (the plug tip holding section)  20  exists between the surface  21  and the plug tip insertion hole  14 , the plug must be removed temporarily from the receptacle and reinserted therein, which takes a lot of trouble. In addition, the plug tip section  5002 ,  2  may be damaged and thus deterioration in transmission quality may be caused. 
   In the receptacle  31  of  FIG. 9B , by contrast, the bottom surface  33  of the plug insertion hole  35  is tapered so as to be conically widened forward, or to be narrowed backward, and there is no stepped portion. In the event that the plug is inserted diagonally, therefore, the plug tip section  5002 ,  2  is easily guided and smoothly inserted into the plug insertion hole  34 . Besides, a structure preventing damages to the plug tip section can be provided by mirror grinding on the bottom surface of the plug insertion hole  35 , use of a slippery material, or the like. 
   Though the bottom surface  33  of the plug insertion hole  35  in the embodiment has the tapered shape so as to open conically forward, a shape of the surface is not limited thereto. The surface may be tapered with uncurved or flat slopes, an elliptic shape or the like. 
   Third Embodiment 
     FIG. 11A  shows a front view of a shutter  51  that is suitable for the receptacle  31 , and  FIG. 11B  shows a section of the shutter  51  as seen looking from above. For comparison,  FIG. 10A  shows a front view of a shutter  41  that is the same as the shutter  3011  in the conventional example 2 shown in  FIGS. 3A and 3B , and  FIG. 10B  shows a section of the shutter  41  as seen from above. 
   The shutter  51  shown in  FIG. 11A  has a shutter body  50  generally shaped like a flat plate, and hinge pins  56 ,  56  protruding upward and downward, as viewed in the figure, along one side (a left side in  FIG. 11A ) of the shutter body  50 . The front of the shutter body  50  is formed with a groove  52  that extends in a direction perpendicular to the hinge pins  56 ,  56  (lateral direction in this example) for guiding the plug tip section. As shown in  FIG. 11B , a tapered surface  53  sloped linearly is formed at a right end (i.e., a deeper end or extremity in a state in which the shutter is opened) of the groove  52 . As shown in  FIG. 11A , an edge  53   r  of the tapered surface  53  has a semicircular pattern bulging out. 
   As is the case with the shutter  51 , the shutter  41  shown in  FIG. 10A  has a shutter body  40  generally shaped like a flat plate, hinge pins  46 ,  46 , and a groove  42 . An extremity  43  of the groove  42 , however, is defined by a surface  43  perpendicular to the shutter body  40 , as shown in  FIG. 10B , and an edge of the surface  43  has a pattern perpendicular to a longitudinal direction of the groove  42 , as shown in  FIG. 10A . 
     FIG. 10C  shows a situation in which the optical plug  5001  of the conventional example 4 or the optical plug  1  of the embodiment shown in  FIGS. 6A–6C  is being inserted into the receptacle  31  having the shutter  41  shown in  FIG. 10A . As shown in  FIG. 10C , the tip  5006 ,  7  of the optical plug  5001 ,  1  being inserted is guided along the groove  42  formed in the front surface of the shutter body  40  while pushing the shutter  41 . The shutter  41  is opened while pivoting on the hinge pins  46 ,  46 . When the optical plug  5001 ,  1  is further inserted, the plug tip  5006 ,  7  may be caught in the end surface  43  and thus may be damaged because the right end surface  43  of the groove  42  is perpendicular to the shutter body  40 . This might cause deterioration in transmission quality. 
     FIG. 11C  shows a situation in which the optical plug  5001  of the conventional example 4 or the optical plug  1  of the embodiment shown in  FIGS. 6A through 6C  is being inserted into the receptacle  31  having the shutter  51  shown in  FIG. 11A . As shown in  FIG. 11C , the tip  5006 ,  7  of the optical plug  5001 ,  1  being inserted is guided along the groove  52  formed in the front surface of the shutter body  50  while pushing the shutter  51 . The shutter  51  is opened while pivoting on the hinge pins  56 ,  56 . Then, because the right end of the groove  52  forms the tapered surface  53  sloped relative to the shutter body  50  and because the border or edge of the tapered surface  53  has the arc-shaped pattern bulging out, the plug tip  5006 ,  7  is prevented from being caught in the extremity of the groove  52 . Accordingly, the shutter  51  is smoothly opened and the plug tip  5006 ,  7  is prevented from being damaged. As a result, such a problem as deterioration in transmission quality that might be caused by damage to the plug tip section is prevented. 
   Though the tapered surface  53  at the extremity of the groove  52  of the shutter  51  is sloped linearly in the described embodiment, the shape of the surface is not limited thereto. The surface may be tapered in shape of a curve, an ellipse or the like. 
   Fourth Embodiment 
     FIGS. 12 ,  13 , and  14  show examples in which an optical connector with a combination of the plug and the receptacle of the invention as described above is applied to optical transmission between various electronic equipment. 
     FIG. 12  shows an example of the application to optical transmission between AV equipment. In this example, a CD (compact disc) player and a PC (personal computer)  61  are connected to an MD (mini disc) player  63  through an optical fiber  62 . On the other hand, a CD and DVD (digital versatile disc) player  64  is connected to an STB (set top box)  66  through an optical fiber  65 . To connections between the optical fibers  62 ,  65  and the equipment are applied the optical connectors (not shown) with the combination of the plug and the receptacle of the invention. 
     FIG. 13  shows an example of the application to optical transmission in a game machine. In this example, a graphic display board  73  is connected to a main board  71  through an optical fiber  72 , a sound source board  75  is connected to the main board  71  through an optical fiber  74 , and a controller board  77  is connected to the main board  71  through an optical fiber  76 . To connections between the optical fibers  72 ,  74 ,  76  and the boards are applied the optical connectors (not shown) with the combination of the plug and the receptacle of the invention. The main board  71  is connected to a power supply board  79  through an electric wire  78 . 
     FIG. 14  shows an example of the application to optical transmission inside a pachinko machine (a kind of a game machine). In this example, a graphic display board  83  is connected to a main board  81  through an optical fiber  82 , and a sound source board  85  is connected to the main board  81  through an optical fiber  84 . To connections between the optical fibers  82 ,  84  and the boards are applied the optical connectors (not shown) with the combination of the plug and the receptacle of the invention. The main board  71  is connected to a delivery unit  87 , a shooting unit  89 , and a power supply board  91  through electric wires  86 ,  88 , and  90 , respectively. 
   In any of the examples, functions of the plug  1  and the receptacle  31  described above prevent such problems as failure of the optical element provided inside the receptacle and deterioration in transmission quality that might be caused by damage to the plug tip section. Thus reliability of optical transmission can be increased. 
   Embodiments of the invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.