Abstract:
A ferrule connecting structure for optically connecting a pair of fibers which are provided in a pair of ferrules, respectively, the ferrule connecting structure comprises: a first holding portion which holds the pair of ferrules; an peripheral portion which is provided on the pair of ferrules and which includes a plurality of groove portions; a second holding portion which is fit in the groove portion so as to hold the ferrule in a particular rotating position about an axis of the optical fiber; a third holding portion which is detachably attached to the first holding portion so as to press the first holding portion in a direction intersecting with the axis; and a fourth holding portion which presses the pair of ferrules in a direction in which the end faces of the pair of ferrules are opposed to each other.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority from Japanese Patent Application No. 2009-073438, which was filed on Mar. 25, 2009, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    The present invention relates to a ferrule connecting structure for use in connecting a pair of optical fibers. In the ferrule connecting structure, ferrules in which the optical fibers are provided individually are opposed to each other and connected with each other. 
         [0003]    An example of a conventional ferrule is shown in  FIG. 10 . Conventionally, a ferrule  101  includes, as shown in  FIG. 10 , a centering portion  102 , a cylindrical body  103  and a flange portion  104  which is formed on the body. The body  103  includes a press fit opening  105  into which the centering portion  102  is press fitted. An optical fiber is exposed only by a predetermined length with a jacket at a leading end thereof stripped off. The exposed optical fiber stock is inserted into an optical fiber insertion hole  106 . A jacketed optical fiber cable is inserted into an optical fiber cable insertion hole  107 . 
         [0004]    Normally, an optical connector used in the field of communication includes a split sleeve. An optical fiber is provided in each of a pair of ferrules  101 . Centering portions  102  of the ferrules  101  are aligned in an axial direction within the split sleeve. End faces of the centering portions  102  are opposed to each other and connected with each other. Then, a working person fits a rotating tool in rotational tool receiving cutouts provided on a flange portion  104  with the pair of ferrules opposed to each other and connected with each other in the split sleeve. Then, by rotating the rotating tool, the eccentricity of one with respect to the other of the pair of optical fibers is minutely adjusted. The pair of optical fibers are connected together concentrically by connecting and aligning one with the other of the pair of optical fibers. The split sleeve is forcibly expanded by the centering portion  102  of each ferrule  101 . Then, the split sleeve exhibits an elastic restoring force and supports the ferrules  101  under the elastic restoring force. 
       SUMMARY 
       [0005]    However, only with the pair of ferrules  101  simply supported by the slit sleeve, in the event that the connecting portion between the pair of ferrules  101  is loosened even a little after assemblage, there is caused a problem that it becomes difficult to obtain high light coupling efficiency. 
         [0006]    In this case, bonding the connecting portion between the pair of ferrules  101  is considered as a solution. However, in the event that the connecting portion between the pair of ferrules  101  is bonded, the following problems are caused. In the event that part of an apparatus which utilizes the ferrule connection mechanism fails, a portion or device of the apparatus which is coupled to the ferrules  101  or ferrules  101  need to be replaced. As this occurs, firstly, the working person needs to cut apart once the ferrules  101 . Next, the working person needs to insert new optical fibers through the ferrules  101 . Then, the working person needs to bond a connecting portion of a pair of ferrules  101  again. In this way, some labor hours are required to replace devices coupled to ferrules or ferrules themselves. 
         [0007]    The invention has been made with a view to solving the problem. An object of the invention is to provide a ferrule connecting structure which requires as few labor hours as possible in replacing devices coupled to ferrules or ferrules themselves. In addition, another object of the invention is to provide a ferrule connecting structure which can maintain a holding condition of ferrules after assemblage of the ferrule connecting structure in an ensured fashion and can maintain high light coupling efficiency. 
         [0008]    In order to achieve the object, one aspect of the invention provides a ferrule connecting structure for optically connecting a pair of fibers which are provided in a pair of ferrules, respectively, the ferrule connecting structure comprising a first holding portion, an peripheral portion, a second holding portion, a third holding portion and a fourth holding portion. The first holding portion holds the pair of ferrules in a state that end faces of the pair of ferrules are opposed to each other and connected with each other. The peripheral portion is provided on at least one of the pair of ferrules and which includes a peripheral surface on which a plurality of groove portions are formed. The second holding portion is configured to fit in the groove portion so as to hole a rotating position of the ferrule about an axis of the optical fiber. The third holding portion is configured to be detachably attached to the first holding portion so as to press the first holding portion by elastic force in a direction intersecting with the axis. The fourth holding portion presses the pair of ferrules in a direction in which the end faces of the pair of ferrules approach each other by elastic force. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1A  is a perspective view showing an external appearance of a ferrule connecting structure  1  according to an embodiment of the invention. 
           [0010]      FIG. 1B  is a front view showing the external appearance of the ferrule connecting structure  1 . 
           [0011]      FIG. 2  is a sectional view of a ferrule  2   a  according to the embodiment. 
           [0012]      FIG. 3  is a partially cutaway perspective view showing a flange  8   a  alone which is removed from a body  5   a  according to the embodiment. 
           [0013]      FIG. 4A  is a perspective view of a clamp  4  according to the embodiment. 
           [0014]      FIG. 4B  is a side view of the clamp  4 . 
           [0015]      FIG. 4C  is a front view of the clamp  4 . 
           [0016]      FIG. 5  is an explanatory diagram which explains an elastic force exerted on a split sleeve  3  when the split sleeve  3  is accommodated within an accommodating space  19 . 
           [0017]      FIG. 6  is a sectional view of the ferrule connecting structure  1  taken along the line A-A shown in  FIG. 1(   b ). 
           [0018]      FIG. 7A  is a diagram showing a condition in which a pair of ferrules  2   a ,  2   b  according to the embodiment are opposed to and connected with each other within the split sleeve  3 . 
           [0019]      FIG. 7B  is an explanatory diagram which explains a relationship between an angle θ from a core center Mb of an optical fiber  28   b  to a core center Ma of an optical fiber  28   a  of the embodiment and an axis deviation amount between the pair of optical fibers. 
           [0020]      FIG. 7C  is an explanatory diagram which explains a measuring system of a connection loss CL according to the embodiment. 
           [0021]      FIG. 8  is an explanatory diagram which explains a positional relationship between a flange  8   b  according to the embodiment and the optical fibers  28   a ,  28   b.    
           [0022]      FIG. 9  is a graph showing a relationship between light coupling efficiency CE according to the embodiment and grooves  11   a ,  11   b.    
           [0023]      FIG. 10  is a sectional view of a conventional ferrule  101 . 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0024]    Hereinafter, an embodiment of the invention will be described with reference to the drawings. 
       External Appearance of Ferrule Connecting Structure 
       [0025]    Referring to the drawings, an external appearance of a ferrule connecting structure  1  according to an embodiment of the invention will be described. Although  FIG. 3  shows only one flange  8   a , a flange  8   b  has the same construction as that of the flange  8   a.    
         [0026]    As shown in  FIGS. 1A ,  1 B, the ferrule connecting structure  1  includes a pair of ferrules  2   a ,  2   b , a split sleeve  3  and a clamp  4 . Hereinafter, for the sake of simplification, as shown in  FIG. 1A , an axial direction of a pair of ferrules  2   a ,  2   b  is defined as X axis, and directions which intersects with X axis as Y axis and Z axis. The definition of the directions of X axis, Y axis and Z axis is understood to be common with respect to the other drawings. 
         [0027]    The pair of ferrules  2   a ,  2   b  includes bodies  5   a ,  5   b  and centering portions  6   a ,  6   b . The bodies  5   a ,  5   b  are made of a stainless steel, and the centering portions  6   a ,  6   b  are made of zirconia. As shown in  FIGS. 1A ,  1 B, the pair of ferrules  2   a ,  2   b  are connected in such a state that end faces of the centering portions  6   a ,  6   b  are connected with each other within the split sleeve  3 . As shown in  FIG. 2 , the body  5   a  includes a press fit opening  7   a  in which the centering portion  6   a  is press fitted. As shown in  FIGS. 1A ,  1 B and  2 , the body  5   a  includes a flange  8   a . As shown in  FIG. 2 , the centering portion  6   a  includes an optical fiber insertion hole  9   a  through which an optical fiber stock is inserted. The body  5   a  includes an optical fiber cable insertion hole  10   a  through which a jacketed optical fiber cable is inserted. As shown in  FIG. 3 , the flange  8   a  has a gearwheel shape in which grooves  11   a  and projecting portions  12  are aligned alternately along an outer circumference of the flange  8   a . As with the centering portion  6   a , the centering portion  6   b  includes an optical fiber insertion hole through which an optical fiber stock is inserted. As with the body  5   a , the body  5   b  also includes a press fit opening in which the centering portion  6   b  is press fitted and includes a flange  8   b  and an optical fiber cable insertion hole through which an optical fiber cable is inserted. As with the flange  8   a , the flange  8   b  has, as shown in  FIG. 6 , a gearwheel shape in which grooves  11   b  and projecting portions  12   b  are aligned alternately along an outer circumference thereof. 
         [0028]    The split sleeve  3  has a configuration in which a longitudinal slit is provided in a cylindrical tube. The split sleeve is made of zirconia and has elasticity. The split sleeve wraps a portion where the centering portions  6   a ,  6   b  of the pair of ferrules  2   a ,  2   b  are opposed to each other and connected with each other. Then, when the split sleeve  3  wraps to hold the portion where the centering portions  6   a ,  6   b  are so connected, an inside diameter portion of the split sleeve  3  is elastically deformed so as to forcibly be expanded and maintains the condition in which the pair of ferrules  2   a ,  2   b  are opposed to each other and connected with each other by virtue of elastic force thereof. 
         [0029]    The clamp  4  includes, as shown in  FIG. 4A , a holding portion  13 , a pressing portion  14  and protrusions  15   a ,  15   b . The clamp  4  is made of iron and has elasticity. The clamp  4  is formed through pressing. 
         [0030]    The holding portion  13  includes a first plate portion  16 , a second plate portion  17  and a third plate portion  18 . The first plate portion  16 , the second plate portion  17  and the third plate portion  18  form an accommodating space  19  which can accommodate therein the split sleeve  3 . The split sleeve  3  is accommodated in the accommodating space  19  through open end portions  20 . 
         [0031]    The first plate portion  16  includes a first flat surface  21  which extends parallel to an X axis. The first flat surface  21  contacts with a peripheral surface of the split sleeve  3  when the split sleeve  3  is accommodated in the accommodating space  19 . The second plate portion  17  includes a second flat surface  22  which is formed to stand vertically on one side end  16   a  of the first plate portion  16 . The second flat surface  22  is configured to contact with the circumferential surface of the split sleeve  3  when the split sleeve  3  is accommodated in the accommodating space  19 . The third plate portion  18  stands on the other side end  16   b  of the first plate portion  16  to project towards the second plate portion  17 . The third plate portion  18  includes a curved surface  23  which extends parallel to the X axis. The curved surface  23  projects towards the second plate portion  17  and contacts with the peripheral surface of the split sleeve  3  when the split sleeve  3  is accommodated within the accommodating space  19 . 
         [0032]    The operation and function of the holding portion  13  when the split sleeve  3  is accommodated in the accommodating space  19  through the open end portions  20  will be described. When the split sleeve  3  is accommodated within the accommodating space  19 , the first flat surface  21  and the second flat surface  22  function as a reference plane and the position of the split sleeve  3  within the accommodating space  19  is determined. The third plate portion  18  is elastically deformed to expand in a direction at right angles to the X axis when the split sleeve  2  is accommodated in the accommodating space  19 . As this occurs, the curved surface  23  contacts with the peripheral surface of the split sleeve  3 . Then, with the split sleeve  3  accommodated completely in the accommodating space  19 , elastic force works, and the third plate portion  18  moves to be returned to its initial condition. 
         [0033]    The pair of ferrules  2   a ,  2   b  are opposed to each other and connected with each other within the sleeve  3 , and as shown in  FIG. 7A , an axis AXa of the ferrule  2   a  and an axis AXb of the ferrule  2   b  coincide with each other to constitute a single axis AX. When the split sleeve  3  is accommodated completely in the accommodating space  19 , the split sleeve  3  is pressed towards the X axis, that is, in a direction VT perpendicular to the AX of the pair of ferrules  2   a ,  2   b  by the elastic force of the third plate portion  18  while in abutment with the curved surface  23  and is held in that position. 
         [0034]    As shown in  FIG. 4C , the pressing portion  14  includes extending portions  24  and pressing pieces  25   a ,  25   b . The extending portions  24  extend in the X-axis direction from opposite sides in the X-axis direction of the holding portion  13  while being bent into a step-like configuration. The pressing pieces  25   a ,  25   b  are formed to stand on the extending portions  24 . As shown in  FIGS. 4A ,  4 B, the pressing pieces  25   a ,  25   b  include projecting portions  26   a ,  26   b  and cutout portions  27   a ,  27   b , respectively. As shown in  FIG. 4A , the projecting portions  26   a ,  26   b  each have a shape in which each projecting portion is bifurcated at its free end side. As shown in  FIG. 4A , the cutout portions  27   a ,  27   b  are formed between bifurcated free end portions of the projecting portions  26   a ,  26   b , respectively. 
         [0035]    The operation and function of the pressing portion  14  will be described when the split sleeve  3  is accommodated in the accommodating space  19  with the pair of ferrules connected with each other within the split sleeve  3 . The projecting portions  26   a ,  26   b  contact with side surfaces of the flanges  8   a ,  8   b , respectively, when the split sleeve  3  is accommodated in the accommodating space  19  as shown in  FIG. 1B . Then, by the flanges  8   a ,  8   b  being pressed by elastic force in the X-axis direction, the pair of ferrules  2   a ,  2   b  are held. In addition, the cutout portions  27   a ,  27   b  permit insertion of the centering portions  6   a ,  6   b  of the pair of ferrules  2   a ,  2   b  therethrough. 
         [0036]    As shown in  FIG. 6 , the protrusions  15   a ,  15   b  are formed on confronting surface of the extending portions  24  which confront grooves  11   a ,  11   b  when the split sleeve  3  is accommodated in the accommodating space  19 . The protrusions  15   a ,  15   b  are fitted in the grooves  11   a ,  11   b  of the flanges  8   a ,  8   b , respectively. By the protrusions  15   a ,  15   b  being fitted in the grooves  11   a ,  11   b , respectively, the rotating positions of the flanges  8   a ,  8   b  are determined. The fitting of the protrusions  15   a ,  15   b  in the grooves  11   a ,  11   b , respectively, will be described in detail later. 
       Setting of Number of Flanges and Grooves 
       [0037]    For the sake of simplification of the drawing,  FIGS. 7A to 7C  show simplified constructions of the ferrules  2   a ,  2   b  and the split sleeve  3  with bodies  5   a ,  5   b  omitted.  FIG. 8  shows only the flange  8   b  for the sake of simplification of the drawing. For the sake of simplification of the drawing,  FIG. 8  shows the flange portion  8   b  which includes eight grooves  11   b  and eight projecting portion  12   b . However, in the embodiment shown in  FIG. 3 , the numbers of grooves  11   a  are 16. So are the numbers of grooves  11   b  and projecting portions  12   a ,  12   b.    
         [0038]    As shown in  FIG. 7A , the pair of ferrules  2   a ,  2   b  are opposed to each other within the split sleeve  3 , and the axis AXa of the ferrule  2   a  and the axis AXb of the ferrule  2   a  coincide with each other so as to constitute the single axis AX. 
         [0039]      FIG. 7B  shows rotating positions of the optical fibers  28   a ,  28   b  about respective core centers Ma, Mb thereof with respect to the axis AX when the ferrule  2   a  and the ferrule  2   b  are seen about the axis AX of respective. The optical fibers  28   a ,  28   b  have respective core eccentricity amounts Ca, Cb (μm) in accordance with the eccentricity of the respective core centers Ma, Mb from the axis AX. Assuming that an angle from the core center Mb to the core center Ma is θ, an axis deviation amount AD (μm) of the pair of optical fibers  28   a ,  28   b  is calculated from the follow equation. 
         [0000]      AD=( Ca   2   +Cb   2− 2 ·Ca·Cb ·cos θ) 1/2   Equation 1 
         [0040]    When the pair of ferrules  2   a ,  2   b  are connected with each other and the pair of optical fibers  28   a ,  28   b  are optically connected together, the light coupling efficiency CE is decreased by the eccentricities of the core centers Ma, Mb, whereby a connection loss CL is produced. In general, a connection loss CL (dB) in connection of optical fibers having the same mode field diameter is calculated by the following equation. Hereinafter, the mode field diameter is denoted by MFD. However, herein, BR denotes a beam radius (μm) which is defined as a half amount of MFD. 
         [0000]      CL=10·Log {exp(−AD/BR 2 )}  Equation 2 
         [0041]    A connection loss CL of the optical fibers  28   a ,  28   b  is calculated by a measuring system shown in  FIG. 7C . In  FIG. 7C , light from a light source  29  is supplied into the ferrule connection mechanism  1  and is then supplied to an optical power meter  30  via the optical fibers  28   a ,  28   b . With an output of the light source  29  kept constant, an optical power detected by the optical power meter  30  varies in accordance with the connection loss CL of the optical fibers  28   a ,  28   b . Consequently, the connection loss CL of the optical fibers  28   a ,  28   b  can be calculated from the optical power detected by the optical power meter  30 . The working person can identify a position having a least connection loss CL in a positional relationship between the ferrule  2   a  and the ferrule  2   b  by measuring a connection loss CL by the measuring system shown in  FIG. 7C  when carrying out an alignment of the ferrules  2   a ,  2   b . In addition, the working person causes the split sleeve  3  to be accommodated in the accommodating space  19  in the position having the least connection loss CL, whereby by the protrusions  15   a ,  15   b  fitting in the grooves  11   a ,  11   b , respectively, the rotating positions of the flanges  8   a ,  8   b  are determined. By the rotating positions being determined in this way, the ferrule  2   a  and the ferrule  2   b  are held in the position having the least connection loss CL, that is, a position having a largest connection efficiency CE. 
         [0042]    Referring to  FIG. 8 , a relationship between the numbers GN of grooves  11   a ,  11   b  and the connection loss CL will be described when the pair of ferrules  2   a ,  2   b  are opposed to each other and connected with each other within the split sleeve  3  and the split sleeve  3  is accommodated within the accommodating space  19 . As shown in  FIG. 8 , a relationship between the number GN of grooves  11   b  and an interval between the grooves  11   b  of the flange  8   b , that is, a rotational angle t from a crest portion GM of the projecting portion  12   b  to an adjacent crest portion GM is expressed by the following equation. 
         [0000]      Φ=360 /GN   Equation 3 
         [0043]    The connection loss CL becomes largest either when the core center Ma is positioned at a crest portion GM of the projecting portion  12   a  and the core center Mb is positioned at a root portion GV of the groove portion  11   b  or when the core center Mb is positioned at the crest portion GM of the projecting portion  12   b  and the core center Ma is positioned at a root portion GV of the groove  11   a . In either case, as shown in  FIG. 8 , a relationship between the angle θ from the core center Mb to the core center Ma and the rotational angle Φ of the pair of ferrules  2   a ,  2   b  is expressed as follows. 
         [0000]      θ=Φ/2  Equation 4 
         [0044]    When substituting Equation 4 by Equation 3, 
         [0000]      θ=180 /GN    
         [0045]    When substituting Equation 1 by Equation 2 and Equation 5, Equation 6 is obtained which expresses a connection loss CL (dB) and the numbers GN of grooves  11   a ,  11   b  when the connection loss CL becomes largest. 
         [0000]      CL=10·Log {exp(−( Ca   2   +Cb   2− 2 ·Ca·Cb ·cos θ) 1/2 BR 2 )}  Equation 6 
         [0046]    The result of a calculation of an light coupling efficiency CE using Equation 6 is shown in  FIG. 9 . As MFD of the optical fibers  28   a ,  28   b , the types of set values are used which include a visible light single mode fiber whose MFD is 3.5 μm, communication single mode fiber whose MFD is 10.0 μm and a multiple mode fiber whose MFD is 100.0 μm. In  FIG. 9 , an axis of abscissa represents the numbers GN of grooves  11   a ,  11   b  and an axis of ordinates represents the light coupling efficiency CE (%). The core eccentricity amounts Ca, Cb of the optical fibers  28   a ,  28   b  are both set to Ca=Cb=2 (μm). 
         [0047]    As shown in  FIG. 9 , in any case of the visible light single mode fiber, the communication single mode fiber and the multiple mode fiber, it is seen that a larger light coupling efficiency CE can be obtained as the numbers GN of grooves  11   a ,  11   b  increase. In the visible single mode fiber, when the numbers GN of grooves  11   a ,  11   b  are four, the connection efficiency takes a small value of the order of 50%. This is because the connection loss CL becomes large due to MFD of the visible light single mode fiber taking the very small value compared with those of the other two optical fibers. However, even in the case of the visible light single mode fiber, as shown in  FIG. 9 , in the event that the numbers GN of grooves  11   a ,  11   b  are 12 or larger, the connection efficiency CE takes a larger value of 90% or lager. The working person performs a calculation using Equation 6 in advance and provides the numbers GN of grooves  11   a ,  11   b  which can obtain an light coupling efficiency of 90% or lager on the flanges  8   a ,  8   b , thereby making it possible to obtain a high connection efficiency CE. In this embodiment, 16 grooves  11   a ,  11   b  are provided individually on the flanges  8   a ,  8   b , so as to obtain an light coupling efficiency of almost 100%. 
       Description of Operation 
       [0048]    The operation of the ferrule connecting structure  1  according to the embodiment will be described by use of the drawings. The pair of ferrules  2   a ,  2   b  include the flanges  8   a ,  8   b  which are provided with the numbers GN of grooves  11   a ,  11   b  set according to the method described above. The pair of ferrules  2   a ,  2   b  are opposed to each other and connected with each other in the split sleeve  3 . The optical fibers  28   a ,  28   b  which are provided in the ferrules  2   a ,  2   b , respectively are optically connected together. 
         [0049]    Aligning the pair of ferrules  2   a ,  2   b  will be described. A calculation of a connection loss CL is carried out based on the method described above using the measuring system shown in  FIG. 7C . Specifically, a position having a least connection loss CL, that is, a position having a largest light coupling efficiency is identified while rotating the pair of ferrules  2   a ,  2   b  within the split sleeve  3 , and the pair of ferrules  2   a ,  2   b  are held in that position within the split sleeve  3 , whereby the alignment of the pair of ferrules  2   a ,  2   b  is completed. 
         [0050]    The split sleeve  3  is accommodated within the accommodating space  19  in the clamp  4  with the pair of ferrules  2   a ,  2   b  kept opposed to each other and connected with each other within the split sleeve  3  after the alignment of the ferrules has been completed. When the split sleeve  3  is accommodated within the accommodating space  19  in the clamp  4 , there may occur such a situation that the protrusions  15   a ,  15   b  on the clamp  4  do not fit in the corresponding grooves  11   a ,  11   b  on the flanges  8   a ,  8   b  but are brought into contact with the projecting portions  12   a ,  12   b  of the flanges  8   a ,  8   b . As this occurs, after the contact of the projecting portions  12   a ,  12   b  with the protrusions  15   a ,  15   b , the protrusions  15   a ,  15   b  are caused to deviate slightly from the projecting portions  12   a ,  12   b  while kept in contact therewith, whereby the protrusions  15   a ,  15   b  are caused to fit in the corresponding grooves  11   a ,  11   b . Since the numbers GN of grooves  11   a ,  11   b  on the flanges  8   a ,  8   b  are set in advance so as to obtain a large light coupling efficiency CE, the slight deviation of the projecting portions  12   a ,  12   b  from the protrusions  15   a ,  15   b  after contact affects little the light coupling efficiency CE. The protrusions  15   a ,  15   b  are fitted in the corresponding grooves  11   a ,  11   b  of the clamp  4 , and the flanges  8   a ,  8   b  are pressed in the direction in which the pair of ferrules  2   a ,  2   b  are opposed to each other and connected with each other by the pressing portion  14 , whereby assembling work of assembling the pair of ferrules  2   a ,  2   b  in the split sleeve  3  and on to the clamp  4  is completed. 
         [0051]    Disassembling work of the ferrule connecting structure  1  will be carried out as follows. The working person grips on the split sleeve  3  and the clamp  4  with both of his or her hands to remove the split sleeve  3  from the accommodating space  19  of the clamp  4 . Next, the working person grips on the split sleeve  3  and the body  5   a  of the ferrule  2   a  to pull the ferrule  2   a  out of the split sleeve  3 . Next, similarly, the working person pulls the ferrule  2   b  out of the split sleeve  3 . In this way, the disassembling work of the ferrule connecting structure can be implemented extremely easily without carrying out labor hour consuming work of cutting apart the optical fibers  28   a ,  28   b.    
       Application Example of Embodiment 
       [0052]    An application of the ferrule connecting structure  1  according to the invention to a retina scanning display will be described. A retina scanning display is one form of a head-mounted display device. The retina scanning display is mounted on a head portion of a person who wears it or in the vicinity of the head portion so as to guide an image light to the eyes of the person who wears the same display. The retina scanning display is configured so that by scanning the retina in two-dimensional directions thereover, an image corresponding to contents information is visualized by the person who wears the retina scanning display. 
         [0053]    The retina display system includes a light source part, a light transmission part and a wave combining part. The light source unit includes three image light generating portions which generate individually respective image lights including red, blue and green lights. The light transmission part is provided one for each of the image light generating part. The light transmission part includes a condensing lens, the pair of ferrules, a light source portion side optical fiber  28   a  and a wave combining part side optical fiber  28   b . The condensing lens concentrates the image light emitted from the image light generating portion on to the optical fiber  28   a . The optical fiber  28   a  is provided within the ferrule  2   a . The optical fiber  28   b  is provided in the ferrule  2   b . The image light concentrated to the optical fiber  28   a  by the condensing lens is supplied to the wave combining part via the optical fibers  28   a ,  28   b . The wave combining part combines the image lights which are supplied via the respective optical fibers  28   a ,  28   b.    
         [0054]    When the light source part fails, it is extremely difficult to replace only the failed light source portion within a housing of the retina scanning display and thereafter align the optical axis of a replacement light source part. Because of this, normally, a light source part for each image light, a condensing lens and an optical fiber  28   a  are assembled individually as one unit in advance. Then, the ferrule connecting structure  1  is used in connecting a centering portion  6   a  of a ferrule  2   a  which is provided at a leading end of the three units with a centering portion  6   b  of a ferrule  2   b  which is connected with a wave combining part. By adopting this configuration, when the light source part fails, requiring the light source part to be replaced, the ferrule connecting structure  1  is disassembled in the way described above for replace of the failed light source part as a unit. In contrast to this, in the event that the ferrules  2   a ,  2   b  are bonded together, the following labor consuming work has to be carried out in which the optical fibers are cut apart, the wave combining part is removed from the housing, the optical fiber  28   b  at the wave combining part is passed through a new ferrule  2   b , and leading ends of the ferrules  2   a ,  2   b  are ground, thereafter, the ferrules  2   a ,  2   b  being bonded together again. In the event that the light source part, the condensing lens and the optical fiber  28  are not assembled as a unit, in addition to the work described above, some labor hours have to be spent in inserting the optical fiber  28   a  on the light source part side through the ferrule  2   a . In the event that the optical fibers are cut every time light source parts are replaced, there is caused a problem that the optical fibers get shorter every replacement. However, by applying the ferrule connecting structure  1  according to the embodiment to the retina scanning display, the labor hours do not have to be spent which would otherwise be required for the pair of ferrules  2   a ,  2   b  which are bonded together, and the working person is allowed to easily replace the failed light source part. 
       Advantageous Effects of the Embodiment 
       [0055]    The rotating positions of the ferrules  2   a ,  2   b  about the axis AX of the optical fibers  28   a ,  28   b  are held by the protrusions  15   a ,  15   b . The split sleeve  3  is pressed in the direction in which the split sleeve  3  approaches vertically the axis AX by the holding portion  13 . The pair of ferrules  2   a ,  2   b  are pressed in the direction in which the connecting end faces of the pair of ferrules  2   a ,  2   b  confront each other by the pressing portion  14 . 
         [0056]    Consequently, the pair of ferrules  2   a ,  2   b  is held while confronting each other from two directions, the direction perpendicular to the axis AX by the holding portion  13  and the direction in which the connecting end faces of the pair ferrules  2   a ,  2   b  are opposed to each other and connected with each other by the pressing portion  14 . Therefore, the holding condition after assembling of the ferrule connecting structure  1  is maintained in an ensured fashion. 
         [0057]    The rotating positions of the ferrules  2   a ,  2   b  are maintained by the protrusions  15   a ,  15   b . Consequently, the post-aligning condition is maintained in an ensured fashion, thereby high light coupling efficiency being maintained. 
         [0058]    As a result, even in the event that the connecting portion between the pair of ferrules  2   a ,  2   b  is not bonded, the high light coupling efficiency is maintained. The working person is freed from labor hour consuming work of cutting apart once the optical fibers  28   a ,  28   b  in replacing the device coupled to the ferrules  2   a ,  2   b  or the ferrules  2   a ,  2   b  themselves. 
         [0059]    The rotating positions of the ferrules  2   a ,  2   b  around the axis AX of the optical fibers  28   a ,  28   b  are set finely in accordance with the numbers GN of groove  11   a ,  11   b  provided on the flanges  8   a ,  8   b.    
         [0060]    The numbers GN of grooves  11   a ,  11   b  are determined in advance based on the light coupling efficiency of the optical fibers  28   a ,  28   b . Thus, desired light coupling efficiency CE is obtained. 
         [0061]    The flanges  8   a ,  8   b  are provided on the pair of ferrules  2   a ,  2   b , respectively. Thus, the rotating positions are set for both of the pair of ferrules  2   a ,  2   b  and the rotating positions so set are then held. Consequently, the high light coupling efficiency CE is obtained in a more ensured fashion. Accordingly, the high light coupling efficiency is maintained in a more ensured fashion. 
         [0062]    The split sleeve  3  is pressed to be held by the third plate portion  18  in such a state that the split sleeve  3  is positioned in the predetermined position by the first surface  21  and the second surface  22  within the accommodating space  19 . Consequently, the holding of the split sleeve  3  is ensured, thereby making it possible to prevent an abrupt movement of the split sleeve  3  after assemblage of the ferrule connecting structure. 
         [0063]    The working person can insert the ferrules  2   a ,  2   b  in the cutout portion while maintaining the condition in which the pair of ferrules  2   a ,  2   b  are left opposed to each other and connected with each other within the split sleeve  3 . Consequently, the pressing portion  14  can be mounted on the flanges  8   a ,  8   b  without any problem. 
         [0064]    The working person can fit the protrusions  15   a ,  15   b  in the grooves  11   a ,  11   b  of the flanges  8   a ,  8   b  which the protrusion  15   a ,  15   b  confront when the split sleeve  3  is accommodated within the accommodating space  19  with the pair of ferrules  2   a ,  2   b  made to confront each other within the split sleeve  3 . Consequently, the working person can easily achieve the maintenance of the condition in which the pair of ferrules  2   a ,  2   b  are made to confront each other and the setting of the rotating positions of the ferrules  2   a ,  2   b.    
         [0065]    The protrusions  15   a ,  15   b , the third holding portion  13  and the pressing portion  14  are combined as an integral unit. Consequently, the fabrication process is simplified and a reduction in costs can be realized. In addition, compared with the ferrule connecting structure in which such three holding members are configured as separate members, the holding members which are combined as an unit is mounted on the ferrules  2   a ,  2   b . Consequently, three different holding actions are completed simultaneously, whereby the holding work by the working person becomes simple and easy. 
       Modified Example 
       [0066]    In the embodiment, while the holding portion  13 , the pressing portion  14  and the protrusions  15   a ,  15   bb  are fabricated integrally, the invention is not limited thereto. For example, any of those members may be formed as a separate member, or all these members may be formed as separate members. For example, only the holding portion  13  is formed as a separate member, which is configured to be mounted on the extending portion  24  of the pressing portion  14 . 
         [0067]    In the embodiment, while the split sleeve  3  is made of zirconia and the clamp  4  of iron, the invention is not limited thereto. Any material may be used as long as it is a member having elasticity. 
         [0068]    In the embodiment, while the flanges  8   a ,  8   b  are formed integrally with the bodies  5   a ,  5   b , respectively, the invention is not limited thereto. The flanges  8   a ,  8   b  may be formed separately from the bodies  5   a ,  5   b . As this occurs, the working person needs to perform work of bonding the flanges  8   a ,  8   b  to the bodies  5   a ,  5   b , respectively. 
         [0069]    In the embodiment, while the flanges  8   a ,  8   b  are provided on both of the pair of ferrules  2   a ,  2   b , the invention is not limited thereto. A flange may be provided only either of the pair of ferrules  2   a ,  2   b.    
         [0070]    In the embodiment, the numbers GN of grooves  11   a ,  11   b  are 16. The invention is not limited thereto. The numbers of grooves  11   a ,  11   b  may be, for example, 10 or 20, provided that relatively high light coupling efficiency is obtained. 
         [0071]    In the embodiment, while the holding portion  13  includes the first plate portion  16  the second plate portion  17  and the third plate portion  18  and the split sleeve  3  is held by these three members, the invention is not limited thereto. For example, the holding portion  13  may include another third plate portion, not shown, in place of the second plate portion  17 . As this occurs, when the split sleeve  3  is accommodated in the accommodating space  19 , the split sleeve  3  is pressed in a direction in which the split sleeve  3  approaches vertically the axis AX of the pair of ferrules  2   a ,  2   b  by the elastic force of the third plate portion  18  and the additional third plate portion and is then held in that position.