Abstract:
An optical module is configured such that it is not susceptible to fine particulate matter when an optical fiber is inserted into a fiber insertion hole thereof. The optical module is also configured to prevent any reduction in the optical coupling efficiency and degradation of anti-noise properties with the optical fiber. The optical module includes: a ferrule, having the fiber insertion hole formed on an end surface on which an electrical circuit is formed; a photoelectric conversion element, connected to the electrical circuit and facing the fiber insertion hole; and an optical fiber, optically coupled directly with the photoelectric conversion element. The optical fiber has a glass fiber and a protective coating, and is aligned with the insertion hole with the protective coating interposed therebetween. The optical fiber is retained in the insertion hole with the glass fiber not in contact with the insertion hole.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. National stage application claims priority under 35 U.S.C. §119(a) to Japanese Patent Application No. 2007-303996, filed in Japan on Nov. 26, 2007, the entire contents of which are hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an optical module, which comprises a device array in which photoelectric conversion elements are arranged and a ferrule into which optical fibers are inserted, wherein the photoelectric conversion elements are optically coupled directly with the optical fibers. 
     BACKGROUND ART 
     An optical semiconductor module (optical module) wherein an optical fiber and a photoelectric conversion element are directly optically coupled (butt joint) without an optical element such as a lens interposed therebetween is described in JP-A 2005-43622. Region (a) of  FIG. 5  is a cross-sectional view showing a conventional optical module prior to assembling. The optical module has a device array  1  and an optical ferrule  3 . 
     The device array  1  has a coupling surface  5 , and a plurality of photoelectric conversion elements  9  are arranged in one row (in a direction perpendicular to the plane of the drawing) at the center section of the coupling surface  5 . A plurality of bumps  11 , arranged parallel to the row of photoelectric conversion elements  9 , function as connecting terminals for the elements. The ferrule  3  has a coupling surface  7 , and a plurality of optical fiber insertion holes  15  for holding an optical fiber  13  in position are arranged in an open state in one row on the coupling surface  7 . A plurality of electrical circuits (not shown) connected to the bumps  11  are arranged on the coupling surface  7  in parallel to the row direction of the insertion holes  15 , and are continuously formed up to an orthogonal surface adjacent to the coupling surface  7 . The ferrule  3  is made of a material containing a polyester resin, a PPS resin, or an epoxy resin. 
     The device array  1  and the ferrule  3  are arranged so that the coupling surface  5  and the coupling surface  7  face each other. The insertion hole  15  and the photoelectric conversion element are positionally aligned, and the bump  11  is secured to the electrical wiring, whereby the ferrule  3  and the device array  1  are integrally coupled. An appropriate length of a protective coating  19  is removed at a distal end of the optical fiber  13  to expose a glass fiber  21 , and the optical fiber  13  is inserted into the insertion hole  15 , so that the optical fiber  13  is optically coupled with the photoelectric conversion element  9 . The optical fiber  13  is positionally aligned with the optical fiber ferrule  3  by a position-restricting hole section  15   a  in the insertion hole  15 , the diameter of the hole section  15   a  being smaller on the coupling surface  7  side. Specifically, the optical fiber  13  is positionally aligned by the glass fiber  21  without the coating  19  being interposed therebetween. 
     Region (b) of  FIG. 5  is an enlarged cross-sectional view of a section at the periphery of the distal end of the glass fiber  21  of a conventional optical module under assembling. When the glass fiber  21  is inserted into the insertion hole  15 , the distal end of the glass fiber  21  may scrape against a plastic material that is softer than the glass fiber  21  such as an inner peripheral surface of the position-restricting hole section  15   a . Fine particulate matter can be produced when the inner peripheral surface is so abraded. This particulate may also be present within the insertion hole  15 . In some instances, the particulate  25  adheres to a surface  21   a  on the distal end of the glass fiber  21 , reducing the efficiency of optical coupling with the device array  1 , and degrading the anti-noise property of the optical module. 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     An object of the present invention is to provide an optical module that does not produce fine particulate matter when an optical fiber is inserted into the fiber insertion hole, and thus to prevent a reduction of the optical coupling efficiency of the optical module and a degradation of the anti-noise property of the optical module. 
     Technical Solution 
     In order to achieve the object, there is provided an optical module, comprising: a ferrule, having a fiber insertion hole formed on an end surface on which an electrical circuit is formed; a photoelectric conversion element, connected to the electrical circuit and facing the fiber insertion hole; and an optical fiber, optically coupled directly with the photoelectric conversion element. The optical fiber comprises a glass fiber and a protective coating and is held in the fiber insertion hole in a state where it is positionally aligned in the fiber insertion hole with the protective coating interposed therebetween, and the glass fiber is not in contact with the optical fiber insertion hole. 
     The glass fiber may protrude by a length L from the protective coating at a distal end section of the optical fiber. The fiber insertion hole may have a holding section, a tapered section, and an expanded-diameter section arranged in order from the end surface side. In such an instance, the optical fiber is held in the fiber insertion hole at the holding section; and the protrusion length L is such that a rim at the distal end surface of the glass fiber is not in contact with the tapered section when a rim of the protective coating is in a state of contact with the tapered section. The protrusion length L is preferably 5 μm or more and 100 μm or less. The distal end of the protective coating is preferably positioned within the fiber insertion hole. The protective coating material and the glass fiber are preferably severed together at the distal end section of the optical fiber by being irradiated with a laser from above the protective coating. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of an optical module that is an embodiment of the present invention. 
         FIG. 2  is an enlarged partial cross-sectional view showing a periphery of a distal end section of an optical fiber under assembling of the optical module in  FIG. 1 . 
         FIG. 3  is an enlarged partial cross-sectional view for explaining a protrusion length in a first modification example of an optical module that is an embodiment of the present invention. 
         FIG. 4  is an enlarged partial cross-sectional view showing a periphery of a distal end section of an optical fiber in a second modification example of an optical module that is an embodiment of the present invention. 
       In  FIG. 5 , region (a) is a cross-sectional view of a conventional optical module prior to assembling, and region (b) is an enlarged partial cross-sectional view of the periphery of a distal end of a glass fiber under assembling. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention will now be described with reference to the drawings. The drawings are used for the purpose of illustration, and are not intended to limit the scope of the invention. In order to avoid repetition in the description, identical labels are used in the drawings to indicate identical sections. The scales used in the drawings are not necessarily accurate. 
       FIG. 1  is a cross-sectional view of an optical module  100  that is an embodiment of the present invention. The optical module  100  comprises a device array  1 , a plurality of optical fibers  13 , and an optical ferrule  31 . 
     The device array  1  has a plurality of photoelectric conversion elements  9  arranged in one row (in a direction perpendicular to the plane of the drawing) on a surface (coupling surface  5 ) of the device array  1 , the surface facing the ferrule  31 . Representative examples of the photoelectric conversion element  9  are a surface-emitting laser (VCSEL) if used as a light source part, or a photo-detector (PD) if used as a light-receiving part. The pitch at which the photoelectric conversion elements are arranged is, e.g., 250 μm. A light-receiving/emitting section  9   a  is provided on the photoelectric conversion element  9 . A plurality of bumps  11  are provided on the coupling surface  5  of the device array  1 , the bumps being arranged parallel to the photoelectric conversion elements. The plurality of bumps  11  function as electrode terminals for supplying electricity to the photoelectric conversion elements  9 , or for transmitting a signal from the photoelectric conversion elements  9 . 
     The ferrule  31  has optical fiber insertion holes  33 , through which the optical fiber  13  is inserted, arranged in one row at a pitch corresponding with the arrangement pitch of the photoelectric conversion element  9 . The insertion holes  33  open onto an end surface (coupling surface  35 ) that faces the coupling surface  5  of the device array  1 . The holding section of the insertion hole  33  for holding the optical fiber  13  anchors the optical fiber  13  with, for example, a thermo-setting adhesive, with which the insertion hole  33  is filled. An electrical circuit  37 , to which the bumps  11  are connected, is provided on the coupling surface  35 . The electrical circuit  37  is formed continuously on a surface (bottom surface)  39  orthogonally adjacent to the coupling surface  35 . 
     The plurality of optical fibers  13  are arranged in a row. On the outside of the ferrule  31 , the optical fibers form a fiber ribbon coated with an integrated protective coating (not shown). A multi-mode optical fiber is preferably used as the optical fiber  13 . An example of an optical fiber that may be used has a core diameter of 50 μm, a cladding diameter of 80 μm, and a coating outer diameter of 125 μm. Using the multi-mode optical fiber enables allowances to be made for a small displacement in an axial direction or in a direction of adjustment during installation. 
     The device array  1  and the ferrule  31  are coupled to form the optical module  100 . The bumps  11  are secured to the electrical circuit  37 , whereby the device array  1  is integrally coupled with the ferrule  31 . The optical fibers  13  are inserted into the insertion holes  33  after the ferrule  31  and the device array  1  are coupled. The position of the optical fiber  13  is set so that the end surface of a glass fiber  21  at the distal end section of the optical fiber  13  is positioned in the vicinity of the light-receiving/emitting section  9   a  of the photoelectric conversion element  9 . The bottom surface  39  of the ferrule  31  is installed on a substrate or the like, thereby allowing the optical module  100  to readily provide electricity to the photoelectric conversion elements  9 , or to receive a signal from the photoelectric conversion elements  9 , through the electrical circuit  37 . 
       FIG. 2  is an enlarged partial cross-sectional view showing the periphery of a distal end section of the optical fiber  13  under assembling of the optical module  100 . A distal end  19   a  of a coating is removed from the distal end section on the optical fiber  13  such that a glass fiber  21  protrudes from a protective coating  19 . As a result, an annular space  43  is formed on the outer periphery of the glass fiber  21  when the optical fiber  13  is inserted into the insertion hole  33 . 
     A protrusion length L is set to 5 μm or more and 100 μm or less. As a result, any spillage which may occur from the annular space  43  having inadequate volume, that is, the length L is set to 5 μm or less will be less, of a particulates  25  will be less likely to occur. Furthermore, any scraping, which may be caused by the distal end of the glass fiber having the length of 100 μm or more, of the inner peripheral surface can be effectively prevented. 
     Although a small amount of the distal end  19   a  of the coating is removed from the optical fiber  13 , the distal end of the glass fiber  21  does not scrape against the inner surface of the insertion hole  33  upon insertion of the optical fiber  13 . Even if the coating  19 , which is softer than the glass fiber  21 , scrapes the inner peripheral surface of the insertion hole  33 , there will be little likelihood of the inner peripheral surface being abraded and producing fine particulate matter. 
     In order to facilitate the insertion operation, the insertion hole  33  may have a holding section, a tapering section  47 , and an expanded-diameter section  45  arranged in order from the coupling surface  35  (first modification example). In such an instance, the optical fiber  13  is held in the insertion hole  33  at the holding section. The protrusion length L of the glass fiber  21  is such that the rim  53  of a distal end surface  21   a  of the glass fiber  21  does not contact the tapering section  47  when the rim  51  of the coating  19  is in a state of contact with the tapering section  47  ( FIG. 3 ) 
     The distal end of the optical fiber  13  may be severed using a laser. In such an instance, a step can readily be formed because the protective coating tends to burn (melt) under laser irradiation. Furthermore, the periphery of the optical fiber  13  melts slightly, and an edge section of the distal end becomes rounded; as a result, chipping is unlikely to occur even if the distal end of the optical fiber  13  contacts the inner peripheral surface of the insertion hole  33  upon insertion into the optical fiber insertion hole, and there is little risk of the resin being abraded and producing fine particulate matter. 
       FIG. 4  is an enlarged partial cross-sectional view showing the periphery of the distal end section of the optical fiber  13  in a second modification example of an optical module that is an embodiment of the present invention. In the second modification example, the glass fiber  21  protrudes slightly at the distal end section of the optical fiber  13 , and an outer periphery of the glass fiber  21  is coated in an adhesive  55  made of a transparent resin. The result allows the optical fiber  13  and the adhesive  55  to adhere strongly with each other in comparison to an instance in which the coating  19  extends to the distal end, and reduces the likelihood of a deviation occurring in positional alignment with the light-receiving/emitting section  9   a  even under the effect of external factors such as a temperature change. The adhesive  55  functions as reinforcement means for making it possible to improve the strength of connection between the photoelectric conversion element  9  and the glass fiber  21  exposed at the distal end of optical fiber  13  in the direction of insertion while it also anchors the optical fiber. 
     Preferably, the refractive index of the adhesive substantially matches the refractive index of the glass fiber. Reflection caused by a difference in refractive indices is minimized, thereby allowing the optical properties to be improved. A UV/thermo-setting resin is preferably used as the adhesive. After UV light is used to perform preliminary anchoring, the article may be taken out of a jig and heated in order to harden sections that have not been hardened by the UV light because they were out of its range. Productivity is accordingly improved. 
     In the optical module  100 , when the optical fiber  13  is inserted into the insertion hole  33 , the outer periphery of the coating  19  is in contact with and slides against the inner peripheral surface of the insertion hole  33 , and movement of the optical fiber  13  in the radial direction within the insertion hole  33  is restricted. The glass fiber  21  is kept apart from the inner peripheral surface of the insertion hole  33 , and does not slide in contact with the inner peripheral surface of the insertion hole  33 . Because the coating  19  is made of a soft material, even if the coating  19  is in contact with and slides against the inner peripheral surface of the insertion hole  33 , there is little risk of fine particulate matter being produced by scraping. Even if by chance a particulate is produced or is present, the annular space  43  becomes a space in which the particulate  25  will be accommodated, and the likelihood of the particulate  25  adhering to the distal end surface  21   a  of the fiber is reduced. Because the distal end  19   a  of the coating is positioned within the optical fiber insertion hole  33 , the accommodated particulate  25  does not spill out of the hole and does not adhere to the light-receiving/emitting section  9   a.    
     According to the optical module  100 , therefore, the particulate  25  does not adhere to the distal end surface  21   a  of the fiber or to the photoelectric conversion element, thus preventing a reduction in the efficiency of the optical coupling with the device array and degradation of the anti-noise property of the optical module. 
     The present application is based on JP-A 2007-303996, which was submitted on Nov. 26, 2007, the content of which being included here as reference. 
     Industrial Applicability 
     The optical module according to the present invention is useful for optical transmission within or between electrical devices.