Patent Publication Number: US-2002012505-A1

Title: Optical module, optical fiber connecting plug, and optical coupler equipped with the same

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to an optical module, an optical fiber connecting plug, and an optical coupler provided with the foregoing components. More particularly, this invention relates to an optical module, an optical fiber connecting plug, and an optical coupler provided with the foregoing components used for transmission in optical communication or the like.  
       BACKGROUND OF THE INVENTION  
       [0002] To perform optical communication or the like, an optical module equipped with a light emitting device or a light detecting device serving as an optical device is connected to a transmitting optical fiber for transmitting optical signals. The distal end of the transmitting optical fiber must be accurately positioned in relation to the optical device. For this purpose, a plug for connecting the transmitting optical fiber to the optical module is attached to the distal end portion of the transmitting optical fiber, and the plug is coupled to the optical module so as to position the distal end of the transmitting optical fiber in relation to the optical device of the optical module. An example of the configuration of such an optical module and a plug or an optical coupler equipped with the optical module and the plug will be described with reference to FIG. 5.  
       [0003]FIG. 5 shows the configuration of a conventional optical coupler. As illustrated in FIG. 5, an optical coupler  400  is equipped with a light emitting module  200  and a plug  300 . The light emitting module  200  has a package  202 . The package  202  is an approximately hexahedral housing that opens upward. Installed in the package  202  is a silicon substrate  204 . A semiconductor laser device  206  is fixed at a predetermined position of the top surface of the silicon substrate  204 .  
       [0004] The light emitting module  200  is provided with a connecting optical fiber  208  and a ferrule  210 . The connecting optical fiber  208  is an optical fiber that has been cut to a predetermined length, the end surface at one side being fixed between the silicon substrate  204  and a holding plate  212  such that it is positioned in relation to the light emitting portion of the semiconductor laser device  206 .  
       [0005] The end at the other side of the connecting optical fiber  208  is secured in the ferrule  210  with an adhesive agent so that an end surface  208   a , which has been polished to have a convex spherical surface, juts out of the ferrule  210 . The ferrule  210  is secured to the package  202  via a flange  210   a  that has been integrally formed therewith.  
       [0006] Formed on one end of the package  202  is a pair of recessed portions  202   a  and  202   a  that open outwardly. The recessed portions  202   a  and  202   a  are used for coupling the plug  300 . The top open part of the package  202  is covered with a lid which is not shown and which is installed after completing the assembly of the light emitting module  200 .  
       [0007] The plug  300  is provided with a housing  302  and a ferrule  304 . The plug  300  is attached to the distal end portion of the transmitting optical fiber by inserting the distal end portion of a transmitting optical fiber cable C to be connected, a sheath of the distal end portion being removed, into the ferrule  304  and by securing it therein. The distal end portion of the transmitting optical fiber is also polished to have a convex spherical surface and disposed so that it juts out of the ferrule  304 .  
       [0008] The housing  302  has a pair of engaging portions  302   a  and  302   a  that engage the recessed portions  202   a  of the package  202  to couple the housing  302  to the package  202 .  
       [0009] The optical coupler  400  is configured such that, when the engaging portions  302   a  are engaged with the recessed portions  202   a  to couple the light emitting module  200  to the plug  300 , the distal end of the ferrule  210  of the light emitting module  200  is brought near the distal end of the ferrule  304  of the plug  300 . Then, the distal end  208   a , which has the convex spherical surface, of the connecting optical fiber  208  is abutted against the distal end, which has the convex spherical surface, of the transmitting optical fiber. The distal ends of the ferrule  210  and  304  are inserted in a cylindrical split sleeve  306  to align the ferrules  210  and  304  and to also align the distal end  208   a , which has the convex spherical surface, of the connecting optical fiber  208  with the distal end, which has the convex spherical surface, of the transmitting optical fiber.  
       [0010] A light detecting module equipped with a light detecting device as an optical device also has a like configuration.  
       SUMMARY OF THE INVENTION  
       [0011] The present invention provides an optical module, an optical fiber connecting plug, or an optical coupler equipped with the same, which is capable of connecting optical fibers without using a ferrule. The optical module, the optical fiber connecting plug, and the optical coupler equipped with the same comprise a substrate, an optical device mounted on the substrate, and a connecting optical fiber having one end thereof positioned in with respect to the optical device. The distal end of a transmitting optical fiber to be connected is positioned with respect to the other end thereof.  
       [0012] The optical module, the optical fiber connecting plug, and the optical coupler equipped with the same further comprise a guide for guiding the distal end of the transmitting optical fiber to be connected.  
       [0013] The distal end of the transmitting optical fiber is positioned in relation to the other end of the connecting optical fiber by placing the distal end of the transmitting optical fiber in the guide. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014]FIG. 1 is a perspective view illustrating the configuration of an optical coupler in accordance with a first embodiment.  
     [0015]FIG. 2 is an exploded perspective view illustrating a substrate, a connecting optical fiber, and a holding plate used for a light emitting module of the optical coupler in accordance with the first embodiment.  
     [0016]FIG. 3 is a schematic exploded perspective view illustrating a substrate, a connecting optical fiber, and a holding plate used for a light emitting module of the optical coupler in accordance with a second embodiment.  
     [0017]FIG. 4 is a perspective view illustrating the configuration of an optical coupler in accordance with a third embodiment.  
     [0018]FIG. 5 is a perspective view illustrating the configuration of a conventional optical coupler.  
     [0019]FIG. 6 is a perspective view illustrating the configuration of an optical coupler in accordance with a fourth embodiment.  
     [0020]FIG. 7 is a perspective view illustrating a state in which a connecting optical fiber and a transmitting optical fiber used for the optical coupler in accordance with the fourth embodiment are connected. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0021] With reference to the accompanying drawings, preferred embodiments of an optical module, an optical fiber connecting plug, and an optical coupler equipped with the optical module and the optical fiber connecting plug in accordance with the present invention will now be described in detail.  
     [0022] The accompanying drawings merely schematically illustrate the sizes and configurations of constituents, and positional relationships there between to aid the understanding of the present invention. Therefore, it should be understood that the present invention is not limited to the embodiments shown in the accompanying drawings.  
     [0023]FIG. 1 is a perspective view showing the configuration of an optical coupler of a first embodiment in accordance with the present invention. FIG. 2 is an exploded perspective view showing mainly a substrate and a holding plate used for a light emitting module of the optical coupler of the first embodiment.  
     [0024] As shown in FIG. 1, an optical coupler  50  is provided with a light emitting module  10  and a plug  40 .  
     [0025] The light emitting module  10  is a so-called receptacle type module and it is equipped with a package  12 . The package  12  is an approximately hexahedral housing that opens upwardly, and it is integrally molded using an epoxy resin or the like. A silicon substrate  14  is installed in the package  12 . A semiconductor laser device  16  for converting external electrical signals into optical signals is secured by soldering or the like at a predetermined position of the top surface of the silicon substrate  14 . Electrical signals are supplied to the semiconductor laser device  16  by a wiring pattern, which is not shown.  
     [0026] The light emitting module  10  is equipped with a connecting optical fiber  18 . The connecting optical fiber  18  is an optical fiber which has been cut to a predetermined length and which has a diameter (approximately 126 to 128 μm) larger than the diameter (approximately 125 μm) of the transmitting optical fiber connected to the light emitting module  10  so as to send optical signals to outside the light emitting module.  
     [0027] As shown in FIG. 2, a V groove (a first groove portion)  20  having a V-shaped section is formed in the top surface of the silicon substrate  14 . The V groove  20  extends from a transverse groove  14   a , which is formed at a position adjoining the semiconductor laser device  16 , to one lengthwise end of the silicon substrate  14 . The V groove  20  is formed by alkali etching that employs, for example, KOH. In this light emitting module  10 , the width of the open end of V groove  20  is approximately 140 μm.  
     [0028] A hexahedral holding plate  22  made of glass is secured to the top surface of the silicon substrate  14  by, for example, a UV-curing adhesive agent. The bottom surface of the holding plate  22  is provided with a V groove (a second groove portion)  24  that extends lengthwise from one end to the other of the bottom surface.  
     [0029] The V grooves  20  and  24  have nearly the same sectional shape. The V grooves  20  and  24  are configured such that they form a bore which is able to house the connecting optical fiber  18  while it is brought in contact with the outer peripheral surface of the connecting optical fiber and which has a foursquare (rectangular) section when the open ends of the V grooves are aligned with each other. In the section of the bore formed by the V grooves  20  and  24 , the opposed sides are placed away from each other by a distance nearly equal to the outside diameter of the connecting optical fiber  18 .  
     [0030] In the light emitting module  10 , the holding plate  22  is installed on the top surface of the silicon substrate  14  such that the open end of the V groove  24  is aligned with the open end of the V groove  20  of the silicon substrate  14 . Hence, the V grooves  20  and  24  form the bore which houses the connecting optical fiber  18  while in contact with the outer peripheral surface thereof, and which has a rectangular section. The bore has a sectional shape that has a larger dimension than the diameter of the transmitting optical fiber. The bore extends from the position adjoining the semiconductor laser device  16  to one end of the silicon substrate  14  and it opens outwardly. The connecting optical fiber  18  is fixed in a portion of the bore (a first bore portion) which is closest to the semiconductor laser device  16 . The remainder of the bore constitutes a second bore portion (a guide or leading portion)  26  that opens outwardly. Thus, the distal end of the transmitting optical fiber can be positioned in relation to a first end surface of the connecting optical fiber  18  by disposing the distal end of the transmitting optical fiber in the second bore  26 .  
     [0031] The V grooves  20  and  24  are configured so that a second end surface of the connecting optical fiber  18  is positioned with high accuracy in relation to the light emitting portion of the semiconductor laser device  16 , which has been installed at the predetermined position of the silicon substrate  14 , when the connecting optical fiber  18  is housed. Hence, the connecting optical fiber  18  is disposed in the bore formed by the V grooves  20  and  24 . When the second end of the connecting optical fiber  18  is jutted out of the bore by a predetermined length toward the semiconductor laser device  16 , the surface on that second end is positioned with high accuracy in relation to the light emitting portion of the semiconductor laser device  16 .  
     [0032] The ends of the V grooves  20  and  24  located opposite the semiconductor laser device  16  have slope portions  20   a  and  24   a , the widths and depths of which gradually increase outwardly. Thus, the open end portion of the second bore  26  is constituted by the slope portions  20   a  and  24   a  providing a diverging portion  28  that diverges outwardly.  
     [0033] A photodiode device  30  is installed near a side of the semiconductor laser device  16  opposite from the connecting optical fiber  18 . The photodiode device  30  has a function of monitoring laser beams emitted from the rear of the semiconductor laser device  16 .  
     [0034] A coupling portion used for coupling the light emitting module  10  to the plug  40  is formed at one lengthwise end of the package  12 . The coupling portion is composed of a pair of protuberances  32  and  32 , each of which is equipped with a first portion  32   a  that extends outwardly in the lengthwise direction from one longitudinal end surface of the package  12  and a second portion  32   b  that bends outwardly in the direction of the width of the package  12  at the distal end of the first portion  32   a . The opposed surfaces of the first portions  32   a  have a shape that is complementary with respect to the outer periphery of a cylindrical holder of the plug  40 , and they form a recessed surface (a receiving portion)  32   c  shaped to receive (abut against) a part of the outer peripheral surface of the cylindrical holder. A recessed portion  34  that opens outwardly in the direction of the width of the package is formed between each second portion  32   b  and the package  12 .  
     [0035] The top open portion of the package  12  is covered by a lid, which is not shown.  
     [0036] The plug  40  is equipped with a housing  42  that is integrally molded using an epoxy resin or the like. The housing  42  opens upwardly, and it is provided with side walls and has an approximately U-shaped cross section. The plug  40  is provided with a cylindrical holder  44 . The holder  44  is a cylindrical member formed of ceramic such as zirconia or glass. Formed inside the holder  44  is a bore in which the transmitting optical fiber can be inserted. The holder  44  is mounted on the housing  42  by being inserted in an opening  42   b  formed in a U-shaped bottom surface  42   a  of the housing  42 . The plug  40  is installed on a distal end of a transmitting optical fiber cable C by a distal end portion  48  which is inserted in the holder  44  and fixed therein, the distal end portion  48  being a portion of the transmitting optical fiber cable C to be connected, the sheath of which has been removed. The transmitting optical fiber is disposed relative to the holder  44  so that the distal end portion  48  juts out of the holder  44  a predetermined length. This end portion  48  is fixed to the holder  44  with an adhesive agent. A spring  46  is installed between a flange  44   a  formed on the holder  44  and a bottom surface  42   a  of the housing  42 .  
     [0037] The housing  42  engages with the recessed portions  34  of the package  12 , and it is provided with a pair of engaging portions  42   c  for coupling the housing  42  to the package  12 .  
     [0038] The projecting length of the distal end portion  48  of the transmitting optical fiber and the dimensions and shapes of the respective portions of the plug  40  are set such that the distal end portion  48  of the transmitting optical fiber is disposed in the second bore  26 , and so that the distal end of the distal end portion  48  is positioned in relation to the first end of the connecting optical fiber  18  (i.e. so that optical signals sent through the connecting optical fiber  18  are efficiently supplied to the distal end of the transmitting optical fiber) when the outer peripheral surface of the holder  44  is received by the recessed surface  32   c . The engaging portions  42   c  are shaped so as to engage the recessed portions  34  so as to couple the plug  40  to the light emitting module  10 .  
     [0039] In this embodiment, the distal end of the transmitting optical fiber and the first end of the connecting optical fiber  18  have been machined to have planes. The optical coupler  50  is configured so that the distal end of the transmitting optical fiber is abutted against the first end of the connecting optical fiber  18  when the plug  40  is coupled to the light emitting module  10 . The optical coupler  50  in this embodiment is configured such that the flange  44   a  of the holder  44  is urged by the spring  46  toward the light emitting module  10  and the distal end of the transmitting optical fiber housed in the holder  44  is pressed by a given force with respect to the first end of the connecting optical fiber  18  when the plug  40  is coupled to the light emitting module  10 .  
     [0040] The top open part of the housing  42  is covered by a lid, which is not shown.  
     [0041] In the optical coupler  50  in this embodiment, the transmitting optical fiber is connected to the light emitting module  10  by bringing the plug  40  close to the module  10  while holding the outer peripheral surface of the holder  44  against the recessed surface  32   c  of the light emitting module  10  and by engaging the engaging portions  42   c  with the recessed portions  34 . This operation causes the distal end portion  48  of the transmitting optical fiber to be disposed in the second bore portion  26  and the distal end of the distal end portion  48  to be positioned with respect to the first end of the connecting optical fiber  18 . The outer peripheral surface of the holder  44  has been received in the recessed surface  32   c  of the light emitting module  10 . Thus, the movement of the plug  40  relative to the light emitting module  10  in a direction orthogonal to the axial direction of the transmitting optical fiber is suppressed. As a result, the changes in the distance between the distal end of the transmitting optical fiber and the first end of the connecting optical fiber attributable to such movement will be suppressed.  
     [0042] According to the optical coupler  50  having the configuration described above, the transmitting optical fiber can be connected to the light emitting module  10  simply by coupling the plug  40  to the light emitting module  10  while inserting the distal end portion  48  of the transmitting optical fiber in the second bore portion  26  of the light emitting module  10 . Hence, it becomes possible to connect a light emitting module to a transmitting optical fiber without using an expensive part such as a ferrule. Furthermore, the second bore portion  26  is provided with the diverging portion  28  and the second bore portion has a section having a larger diameter than the outside diameter of the transmitting optical fiber. Hence, the transmitting optical fiber can be easily inserted in the second bore portion  26 .  
     [0043] In the optical coupler  50  in this embodiment, the flange  44   a  of the holder  44  is urged by the spring  46  toward the light emitting module  10  when the plug  40  is coupled to the light emitting module  10 . Further, the distal end of the transmitting optical fiber housed in the holder  44  is pressed by a given force against the first end of the connecting optical fiber  18 . Therefore, even if the light emitting module  10  and the plug  40  move relative to each other, the contact between the distal end of the transmitting optical fiber and the first end of the connecting optical fiber  18  will be maintained.  
     [0044] An optical coupler of a second embodiment will now be described. FIG. 3 is an exploded perspective view showing a substrate, a connecting optical fiber, and a holding plate used for the optical coupler of the second embodiment.  
     [0045] The optical coupler of the second embodiment differs from the optical coupler  50  of the first embodiment in that the diameter of the connecting optical fiber is equal to the diameter of the transmitting optical fiber, and the dimensions of the first bore portion are different from those of the second bore portion. The rest of the configuration is identical to that of the optical coupler of the first embodiment. Hence, only the different aspects will be described. In FIG. 3, the same components as those of the first embodiment will be assigned the same reference numerals as those in the first embodiment.  
     [0046] As shown in FIG. 3, a V groove (a first groove)  62  having a V-shaped section is formed in the top surface of a silicon substrate  60  of the optical coupler of the second embodiment. The V groove  62  extends from a transverse groove  60   a  provided at a location adjoining a semiconductor laser device  16  to one lengthwise end of the silicon substrate  60 . The V groove  62  is constructed of a connecting optical fiber housing portion  66  which is positioned at one side of the semiconductor laser device  16  and which accommodates a connecting optical fiber  64 , a slope portion  68  which is positioned at one lengthwise end of the silicon substrate  60  and which is formed so that the width and depth increase outwardly, and a transmitting optical fiber housing portion  70  disposed between the connecting optical fiber housing portion  66  and the slope portion  68 .  
     [0047] A hexahedral holding plate  72  made of glass is secured to the top surface of the silicon substrate  60  by, for example, a UV-curing adhesive agent. The bottom surface of the holding plate  72  is provided with a V groove (a second groove)  74  that extends lengthwise from one end to the other of the bottom surface.  
     [0048] The V groove  74  has the same shape as the V groove  62 . More specifically, the V groove  74  is formed of a connecting optical fiber housing portion  76  which is positioned at the side of the semiconductor laser device  16  and which accommodates the connecting optical fiber  64 , a slope portion  78  which is positioned at one lengthwise end of the holding plate  72  and which diverges outwardly, and a transmitting optical fiber housing portion  80  disposed between the connecting optical fiber housing portion  76  and the slope portion  78 .  
     [0049] The holding plate  72  is installed on the top surface of the silicon substrate  60  such that the open end of the V groove  74  is aligned with the open end of the V groove  62  of the silicon substrate  60 . The holding plate  72  is installed on the silicon substrate  60  such that the open end of the connecting optical fiber housing portion  76  is aligned with the open end of the connecting optical fiber housing portion  66  of the silicon substrate  60 , the open end of the transmitting optical fiber housing portion  80  is aligned with the open end of the transmitting optical fiber housing portion  70  of the silicon substrate  60 , and the open end of the slope  78  is aligned with the open end of the slope portion  68  of the silicon substrate  60 , respectively.  
     [0050] The connecting optical fiber housing portions  66  and  76  are configured so that, when the opening ends thereof are aligned with each other, they form a bore (a first bore portion) which is able to house the connecting optical fiber  64  while it is in contact with the outer peripheral surface thereof and which has a foursquare (rectangular) section.  
     [0051] In the section of the first bore portion formed by the connecting optical fiber housing portions  66  and  76 , the opposed sides are placed away from each other by a distance nearly equal to the outside diameter of the connecting optical fiber  64 .  
     [0052] The depths and widths of the transmitting optical fiber housing portions  70  and  80  are set to be larger than those of the connecting optical fiber housings  66  and  76 . The bore (the second bore portion, the guide, or the leading portion) formed by the transmitting optical fiber housing portions  70  and  80  has therefore a section that is larger than the outside diameter of the transmitting optical fiber to be connected. This permits easier insertion of the transmitting optical fiber.  
     [0053] Having the section that is larger than the outside diameter of the transmitting optical fiber means that the transmitting optical fiber is able to move in the guide in the direction orthogonal to the axis thereof.  
     [0054] The slope portions  68  and  78  constitute a diverging portion that diverges outwardly.  
     [0055] The connecting optical fiber  64  has the same diameter as the transmitting optical fiber. The connecting optical fiber  64  is fixed in the first bore made up of the transmitting optical fiber housing portions  70  and  80  such that the second end of the connecting optical fiber  64  is positioned in relation to the light emitting portion of the semiconductor laser device  16 , and the first end juts out in the second bore portion.  
     [0056] In the optical coupler of the second embodiment having such a configuration, the distal end of the transmitting optical fiber can be positioned in relation to the first end of the connecting optical fiber to connect the transmitting optical fiber to the semiconductor laser device  16  of the light emitting module simply by inserting the distal end portion of the transmitting optical fiber through the diverging portion formed by the slope portions  68  and  78  and disposing it in the bore (the second bore portion, the guide, or the leading portion) formed by the transmitting optical fiber housing portions  70  and  80 .  
     [0057] In the optical coupler of the second embodiment, the optical fiber having the same diameter as the transmitting optical fiber is employed for the connecting optical fiber. The optical coupler of the second embodiment provides an additional advantage in that the optical fiber for the transmitting optical fiber can also be used for the connecting optical fiber.  
     [0058] Referring now to FIG. 4, an optical coupler  130  of a third embodiment will now be described.  
     [0059]FIG. 4 is a perspective view illustrating the configuration of the optical coupler  130  of the third embodiment in accordance with the present invention.  
     [0060] As shown in FIG. 4, the optical coupler  130  is equipped with a light emitting module  90  and a plug  110 .  
     [0061] The light emitting module  90  is a so-called receptacle type module and it is equipped with a package  92 . The package  92  is an approximately hexahedral housing that opens upwardly, and it is integrally molded using an epoxy resin or the like. A silicon substrate  94  is installed in the package  92 . A semiconductor laser device  96  for converting external electrical signals into optical signals is secured by soldering or the like at a predetermined position on the top surface of the silicon substrate  94 . Electrical signals are supplied to the semiconductor laser device  96  by a wiring pattern, which is not shown.  
     [0062] The light emitting module  90  is equipped with a connecting optical fiber  98 . The connecting optical fiber  98  has the same diameter as the diameter (approximately 125 μm) of the transmitting optical fiber connected to the light emitting module  90  so as to send optical signals to outside the light emitting module. The connecting optical fiber  98  is secured to the silicon substrate  94 .  
     [0063] A hexahedral holding plate  100  made of glass is secured to the top surface of the silicon substrate  94  by, for example, a UV-curing adhesive agent. The top surface of the silicon substrate  94  and the bottom surface of the holding plate  100  are provided with V grooves. The two V grooves have the same sectional shapes. These two V grooves are formed such that they form a bore which is able to accommodate the connecting optical fiber  98  while it is in contact with the outer peripheral surface thereof and which has a foursquare (rectangular) section when the open ends thereof are aligned with each other.  
     [0064] In the light emitting module  90 , the holding plate  100  is secured to the top surface of the silicon substrate  94  such that the open end of the V groove is aligned with the open end of the V groove of the silicon substrate  94 . One end of the connecting optical fiber  98  is fixed in the bore by an adhesive agent or the like. Hence, in the light emitting module  90 , the silicon substrate  94  serves as the mounting portion for the connecting optical fiber  98 .  
     [0065] These V grooves are configured so that a second end surface of the connecting optical fiber  98  is positioned with high accuracy in relation to the light emitting portion of the semiconductor laser device  96 , which has been installed at the predetermined position of the silicon substrate  94 , when the second end of the connecting optical fiber  98  is housed therein. When the connecting optical fiber  98  is disposed in the bore formed by the V grooves, and the second end thereof is jutted out of the bore by a predetermined length toward the semiconductor laser device  96 , the end surface on that second end is positioned with high accuracy in relation to the light emitting portion (active layer) of the semiconductor laser device  96 .  
     [0066] A portion  98   a  on the first end of the connecting optical fiber  98  juts out of the silicon substrate  94  which serves as the mounting portion.  
     [0067] A photodiode device  102  is installed at a position opposite from the connecting optical fiber  98  of the semiconductor laser device  96 . The photodiode device  102  has a function of monitoring laser beams emitted from the rear of the semiconductor laser device  96 .  
     [0068] A coupling portion used for coupling the light emitting module  90  to the plug  110  is formed at one lengthwise end of the package  92 . The coupling portion is composed of a pair of protuberances  93  and  93 , each of which is equipped with a first portion  92   a  that extends outwardly in the lengthwise direction from one longitudinal end surface of the package  92  and a second portion  92   b  that bends outwardly in the direction of the width of the package  92  at the distal end of the first portion  92   a . The opposed surfaces of the first portion  92   a  have a shape that is complementary with respect to the outer periphery of a cylindrical holder of the plug  110  and they form a recessed surface (a receiving portion)  92   c  shaped to receive (abut against) a part of the outer peripheral surface thereof. A recessed portion  95  that opens outwardly in the direction of the width of the package is formed between each second portion  92   b  and the package  92 .  
     [0069] In the light emitting module  90 , the portion  98   a  on the first end of the connecting optical fiber  98  that juts out from the silicon substrate  94  on which it is mounted terminates between the recessed surfaces  92   c  and  92   c , so that the connecting optical fiber  98  does not extend beyond the package  92 .  
     [0070] The top open portion of the package  92  is covered by a lid, which is not shown.  
     [0071] The plug  110  is equipped with a housing  112  that is integrally molded using an epoxy resin or the like. The housing  112  opens upwardly, and it is provided with side walls and has an approximately U-shaped cross section. The plug  110  is provided with a cylindrical holder  114 . The holder  114  is attached to the housing  112  by being inserted in an opening  112   b  formed in a U-shaped bottom surface  112   a  of the housing  112 .  
     [0072] The holder  114  is a cylindrical member formed of ceramic such as zirconia or glass. The holder  114  has a through hole  116  that runs in the axial direction at the center thereof. The through hole  116  has an inside diameter that is nearly equal to the outside diameters of the connecting optical fiber  98  and the transmitting optical fiber. A first end (the end to be connected to the light emitting module  90 ) of the through hole  116  is provided with a diverging portion  116   a , the diameter of which increases outwardly.  
     [0073] The plug  110  is attached to the distal end portion of the transmitting optical fiber cable C by inserting the distal end portion, from which the sheath has been removed, of the transmitting optical fiber cable C to be connected from the other end of the through hole  116  to a midpoint, and fixing it thereto. In this embodiment, a portion (the portion on the other end) that projects from the silicon substrate  94  of the connecting optical fiber  98  is inserted in the remainder (the leading portion) of the through hole  116 .  
     [0074] A spring  118  is installed between a flange  114   a  formed on the holder  114  and the bottom surface  112   a  of the housing  112 .  
     [0075] The housing  112  is provided with a pair of engaging portions  112   c  that engage recessed portions  95  of the package  92  to couple the housing  112  to the package  92 .  
     [0076] The projecting length of the connecting optical fiber  98  from the silicon substrate  94  serving as the mounting portion, and the dimensions and shapes of the respective portions of the plug  110  are set such that the portion (the portion on the first end) of the connecting optical fiber  98  that extends from the silicon substrate  94  is disposed in the through hole  116  of the holder  114  and the distal end of the other distal end portion is positioned in relation to the distal end of the transmitting optical fiber (i.e. optical signals sent through the connecting optical fiber  98  are efficiently supplied to the distal end of the transmitting optical fiber) when the outer peripheral surface of the holder  114  is received by the recessed surface  92   c  and the engaging portions  112   c  are engaged with the recessed portions  95  so as to couple the plug  110  to the light emitting module  90 . In this embodiment, the distal end of the transmitting optical fiber and the first end of the connecting optical fiber  98  have been machined to have planes. The optical coupler  130  is configured so that the distal end of the transmitting optical fiber is abutted against the first end of the connecting optical fiber  98  when the plug  110  is coupled to the light emitting module  90 .  
     [0077] In the optical coupler  130  of this embodiment, the flange  114   a  of the holder  114  is urged by the spring  118  toward the light emitting module  90  when the plug  110  is coupled to the light emitting module  90 . At this time, the holder  114  is pressed by a given force with respect to the first end of the connecting optical fiber  98 .  
     [0078] The top open part of the housing  112  is covered by a lid, which is not shown.  
     [0079] In the optical coupler  130  in this embodiment, the transmitting optical fiber is connected to the light emitting module  90  by bringing the plug  110  close to the light emitting module  90  while holding the outer peripheral surface of the holder  114  against the recessed surface  92   c  of the light emitting module  90  and by engaging the engaging portions  112   c  with the recessed portions  95 . This operation causes the distal end portion of the connecting optical fiber  98  to be inserted in the through hole  116  from one end of the through hole  116  of the holder  114 . Further, the other end portion of the connecting optical fiber  98  is disposed in the through hole  116 , and the distal end of the first end portion is positioned with respect to the distal end of the transmitting optical fiber.  
     [0080] The outer peripheral surface of the holder  114  has been received in the recessed surface  92   c  of the light emitting module  90 . Thus, the movement of the plug  110  relative to the light emitting module  90  in a direction orthogonal to the axial direction of the transmitting optical fiber is suppressed. As a result, the changes in the distance between the distal end of the transmitting optical fiber and the first end of the connecting optical fiber attributable to such movement will be suppressed.  
     [0081] According to the optical coupler  130  having the configuration described above, the transmitting optical fiber can be connected to the light emitting module  90  simply by coupling the plug  110  to the light emitting module  90  while inserting the other end portion of the connecting optical fiber  98  in the through hole  116  provided in the plug  110 . Hence, it becomes possible to connect a light emitting module to a transmitting optical fiber without using an expensive part such as a ferrule.  
     [0082] In the optical coupler  130  in this embodiment, the flange  114   a  of the holder  114  is urged by the spring  118  toward the light emitting module  90  when the plug  110  is coupled to the light emitting module  90 . At this time, the holder  114  is pressed by a given force against the other end of the connecting optical fiber  98 . Therefore, even if the light emitting module  90  and the plug  110  move relative to each other, the contact between the distal end of the transmitting optical fiber and the other end of the connecting optical fiber  98  will be maintained.  
     [0083] Further, in the optical coupler  130  of this embodiment, the portion  98   a  at the other end of the connecting optical fiber  98  of the light emitting module  90  terminates between the recessed surfaces  92   c  and  92   c . Hence, the connecting optical fiber  98  does not extend beyond the package  92 . The distal end of the transmitting optical fiber is disposed in the through hole  116  of the plug  110 . Therefore, neither the transmitting optical fiber nor the connecting optical fiber is exposed outside. This minimizes the possibility of damage to the transmitting optical fiber and the connecting optical fiber.  
     [0084] Referring now to FIG. 6, an optical coupler  150  of a fourth embodiment will be described. FIG. 6 is a perspective view showing the configuration of the optical coupler  150  of the fourth embodiment in accordance with the present invention.  
     [0085] The optical coupler of the fourth embodiment differs from the optical coupler  130  of the third embodiment in that a sleeve is provided between the recessed surfaces  92   c  and  92   c  of the light emitting module, and that the distal end of a transmitting optical fiber is abutted against the other end of a connecting optical fiber  168  in a through hole of the sleeve. Therefore, only the different aspects will be described. In FIG. 6, the same components as those of the third embodiment will be assigned the same reference numerals as those in the third embodiment.  
     [0086] As shown in FIG. 6, the optical coupler  150  is equipped with a light emitting module  160  and a plug  170 .  
     [0087] The light emitting module  160  is a so-called receptacle type module and it is equipped with a package  92 . The package  92  is an approximately hexahedral housing that opens upwardly, and it is integrally molded using an epoxy resin or the like.  
     [0088] The light emitting module  160  is provided with the connecting optical fiber  168 . The connecting optical fiber  168  has the same diameter as the diameter (approximately 125 μm) of the transmitting optical fiber connected to the light emitting module  160  so as to send optical signals to outside the light emitting module  162 . The connecting optical fiber  168  is secured to the silicon substrate  94 .  
     [0089] A hexahedral holding plate  100  made of glass is secured to the top surface of the silicon substrate  94  by, for example, a UV-curing adhesive agent. The top surface of the silicon substrate  94  and the bottom surface of the holding plate  100  are provided with V grooves. The two V grooves have the same sectional shapes. These two V grooves are formed such that they form a bore which is able to accommodate the connecting optical fiber  168  while it is in contact with the outer peripheral surface thereof and which has a foursquare (rectangular) section when the open ends thereof are aligned with each other.  
     [0090] In the light emitting module  160 , the holding plate  100  is secured to the top surface of the silicon substrate  94  such that the open end of the V groove is aligned to the open end of the V groove of the silicon substrate  94 . One end of the connecting optical fiber  168  is fixed in the bore by an adhesive agent or the like. Hence, in the light emitting module  160 , the silicon substrate  94  serves as the mounting portion for the connecting optical fiber  168 .  
     [0091] These V grooves are configured so that a second end surface of the connecting optical fiber  168  is positioned with high accuracy in relation to the light emitting portion of the semiconductor laser device  96 , which has been installed at the predetermined position of the silicon substrate  94 , when the second end of the connecting optical fiber  168  is housed therein. The connecting optical fiber  168  is disposed in the bore formed by the V grooves. Further, when a first end of the connecting optical fiber  168  is jutted out of the bore by a predetermined length toward the semiconductor laser device  96 , the end surface on that first end is positioned with high accuracy in relation to the light emitting portion (active layer) of the semiconductor laser device  96 .  
     [0092] An end surface  168   a , which has been polished into a convex spherical surface, at the other end of the connecting optical fiber  168  is inserted as far as a midpoint of a through hole  166  of a sleeve  164 , which will be discussed later, and it is fixed in the sleeve  164  by an adhesive agent.  
     [0093] A photodiode device  102  is installed at the position opposite from the connecting optical fiber  168  of the semiconductor laser device  96 . The photodiode device  102  has a function of monitoring laser beams emitted from the rear of the semiconductor laser device  96 .  
     [0094] A coupling portion used for coupling the light emitting module  160  to the plug  170  is formed at one lengthwise end of the package  92 . The coupling portion has a pair of protuberances  93  and  93 , each of which is equipped with a first portion  92   a  that extends outwardly in the lengthwise direction from one longitudinal end surface of the package  92  and a second portion  92   b  that bends outwardly in the direction of the width of the package  92  at the distal end of the first portion  92   a . The opposed surfaces of the first portion  92   a  have a shape that is complementary with respect to the outer periphery of a cylindrical holder of the plug  170 , and they form a recessed surface (a receiving portion)  92   c  shaped to receive (abut against) a part of the outer peripheral surface thereof. A recessed portion  95  that opens outwardly in the direction of the width of the package is formed between each second portion  92   b  and the package  92 .  
     [0095] The sleeve  164  has a through hole  166  that runs in the axial direction at the center thereof. The through hole  166  has an inside diameter that is nearly equal to the outside diameters of the connecting optical fiber  168  and the transmitting optical fiber. The other end (the end to be connected to the plug  170 ) of the through hole  166  is provided with a diverging portion  166   a , the diameter of which increases outwardly.  
     [0096] The sleeve  164  is fixed by gluing or the like between the recessed surfaces  92   c  and  92   c  of the light emitting module  160 . The sleeve  164  is disposed such that it juts from the package  92 .  
     [0097] The top open portion of the package  92  is covered by a lid, which is not shown.  
     [0098] The plug  170  is equipped with a housing  112  that is integrally molded using an epoxy resin or the like. The housing  112  opens upwardly, and it is provided with side walls and has an approximately U-shaped cross section. The plug  170  is provided with a cylindrical holder  174 . The holder  174  is attached to the housing  112  by being inserted in an opening  112   b  formed in a U-shaped bottom surface  112   a  of the housing  112 . The holder  174  is a cylindrical member formed of ceramic such as zirconia or glass. A bore in which a transmitting optical fiber can be inserted is formed in the holder  174 .  
     [0099] The plug  170  is attached to the distal end portion of a transmitting optical fiber cable C by inserting the transmitting optical fiber cable C, which is to be connected, in the holder  174  and fixing it thereto, the sheath having been removed from the transmitting optical fiber cable C. A distal end portion  179  of the transmitting optical fiber is disposed with respect to the holder  174  such that it juts out of the holder  174  by a predetermined length. The transmitting optical fiber is fixed to the holder  174  by an adhesive agent.  
     [0100] In this embodiment, the distal end portion  179  from which the sheath of the transmitting optical fiber cable to be connected has been removed is inserted in a through hole  166   a  provided in the sleeve  166  of the connecting optical fiber  168 .  
     [0101] A spring  118  is installed between a flange  174   a  formed on the holder  174  and a bottom surface  112   a  of the housing  112 .  
     [0102] The housing  112  is provided with a pair of engaging portions  112   c  that engage recessed portions  95  of the package  92  to couple the housing  112  to the package  92 .  
     [0103] As illustrated in FIG. 7, the distal end portion  179  of the transmitting optical fiber is tapered in order to prevent chipping or to reduce the contact area to ease surface accuracy. The portion  168   a  on the other end of the connecting optical fiber  168  is machined to be a plane. The optical coupler  150  is configured so that the distal end portion  179  of the transmitting optical fiber and the portion  168   a  at the first end of the connecting optical fiber  168  are abutted against each other in the through hole  166  of the sleeve  164  when the plug  170  is coupled to the light emitting module  160 .  
     [0104] In the optical coupler  150  in this embodiment, the flange  174   a  of the holder  174  is urged by a spring  118  toward the light emitting module  160  when the plug  170  is coupled to the light emitting module  160 . Further, the holder  174  is pressed by a given force against the first end of the connecting optical fiber  168 .  
     [0105] The top open part of the housing  112  is covered by a lid, which is not shown.  
     [0106] In the optical coupler  150  in this embodiment, the transmitting optical fiber is connected to the light emitting module  160  by bringing the plug  170  close to the light emitting module  160  while holding the outer peripheral surface of the holder  174  against the recessed portion  92   c  of the light emitting module  160  and by engaging the engaging portions  112   c  with the recessed portions  95 . This operation causes the distal end portion  179  of the transmitting optical fiber to be inserted in the through hole  166  from one end of the through hole  166  of the sleeve  164  and to be disposed in the through hole  166 . After that, the distal end portion  179  of the transmitting optical fiber is positioned with respect to the first end of the connecting optical fiber  168 .  
     [0107] According to the optical coupler  150  having the configuration described above, the connecting optical fiber  168  can be connected to the plug  170  simply by coupling the light emitting module  160  to the plug  170  while inserting the distal end portion of the transmitting optical fiber in the through hole  166  provided in the sleeve  164  of the light emitting module  160 . Hence, it becomes possible to connect a transmitting optical fiber of the plug  170  to the connecting optical fiber  168  of the light emitting module  160  without using an expensive part such as a ferrule.  
     [0108] In the optical coupler  150  in this embodiment also, the flange  174   a  of the holder  174  is urged by the spring  118  toward the light emitting module  160  when the plug  170  is coupled to the light emitting module  160 . After that, the holder  174  is pressed by a given force against the other end of the connecting optical fiber  168 . Therefore, even if the light emitting module  160  and the plug  170  move relative to each other, the contact between the distal end of the transmitting optical fiber and the other end of the connecting optical fiber  168  will be maintained.  
     [0109] The holders  44 ,  114 ,  174  used in the foregoing first, second, and third embodiments provide accuracy of about 1 μm for the inner peripheral portion that holds an optical fiber as in the case of ferrules. Regarding the outside diameter, eccentricity, cylindricity, etc., the accuracy is not required to be as strict as in the case of ferrules. Hence, the prices are not as high as those of ferrules.  
     [0110] The present invention is not limited to the embodiments described above, and various modifications can be made within the scope of the present invention as defined in the appended claims.  
     [0111] In the foregoing first, second, and third embodiments, the second bore portion has been provided with the diverging portion. However, the diverging portion may be omitted.  
     [0112] The foregoing first, second, and third embodiments have been the optical modules equipped with the light emitting devices (semiconductor laser devices) as the optical devices. The present invention is also applicable to an optical module equipped with a light detecting device (semiconductor laser device) as the optical device, a plug for connecting a transmitting optical fiber to an optical module, or an optical coupler provided with the same.  
     [0113] In the present specification, “to position” means to place an object in a state that permits efficient transfer of optical signals. Further, “optical fiber” refers to a “bare fiber” provided with no sheath.