Abstract:
An optical module for connecting a photoelectric conversion device on a substrate to a ferrule connected to an optical fiber includes a body configured to be mounted on the substrate, a first lens disposed on the body at a side thereof connectable to the ferrule, a second lens disposed on the body at a side thereof facing the substrate, and a core disposed in the body between the first lens and the second lens, wherein a refractive index of the core is higher than a refractive index of the body.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The disclosures herein relate to an optical module. 
         [0003]    2. Description of the Related Art 
         [0004]    Advancement in the technology of high-speed, high-volume communication networks and communication control equipment has prompted the wide-spread use of optical fibers for communication and transmission purposes. Generally, an optical transceiver for conversion between an electrical signal and an optical signal is used at the connection point between an optical fiber and a device. Such an optical transceiver has an optical module providing an optical waveguide between an optical fiber and a photoelectric conversion device. 
         [0005]    Conventional optical modules are comprised of a large number of components, which requires a large number of production steps at the time of assembly. The technology disclosed in Patent Document 1 or 2, for example, requires steps of filling a groove for forming an optical waveguide with core material for forming an optical fiber core, applying an over-clad film on the groove which is filled with the core material, and curing with respect to the over-clad film. 
         [0006]    Further, an optical module having a plurality of lenses may be mounted on a printed circuit board (“board”) on which photoelectric conversion devices are disposed. In such a case, misalignment of the optical module with respect to the set of photoelectric conversion devices ends up causing undesirable light loss. Similarly, displacement of lenses in the optical module from their intended positions also ends up causing the loss of light signals.
   [Patent Document 1] Japanese Laid-open Patent Publication No. 2009-20426   [Patent Document 2] Japanese Laid-open Patent Publication No. 2006-309113   
 
       SUMMARY OF THE INVENTION 
       [0009]    According to an embodiment, an optical module for connecting a photoelectric conversion device on a substrate to a ferrule connected to an optical fiber includes a body configured to be mounted on the substrate, a first lens disposed on the body at a side thereof connectable to the ferrule, a second lens disposed on the body at a side thereof facing the substrate, and a core disposed in the body between the first lens and the second lens, wherein a refractive index of the core is higher than a refractive index of the body. 
         [0010]    According to an embodiment, an optical module for connecting a photoelectric conversion device on a substrate to a ferrule connected to an optical fiber includes a body configured to be mounted on the substrate, a first lens disposed on the body at a side thereof connectable to the ferrule, a second lens disposed on the body at a side thereof facing the substrate, and a core disposed in the body between the first lens and the second lens, wherein the core has faces thereof on which a coating film is formed. 
         [0011]    According to an embodiment, an optical module for connecting a photoelectric conversion device on a substrate to a ferrule connected to an optical fiber includes a body configured to be mounted on the substrate, a first lens disposed on the body at a side thereof connectable to the ferrule, a second lens disposed on the body at a side thereof facing the substrate, and a space formed in the body between the first lens and the second lens. 
         [0012]    According to at least one embodiment, the disclosed optical module enables the reduction of light loss even when the positions of disposed lenses or the like are displaced relative to photoelectric conversion devices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
           [0014]      FIGS. 1A and 1B  are illustrative drawings of an optical module of a first embodiment; 
           [0015]      FIGS. 2A through 2C  are drawings illustrating the optical module of the first embodiment; 
           [0016]      FIGS. 3A and 3B  are drawings illustrating the optical module of the first embodiment; 
           [0017]      FIG. 4  is an illustrative drawing of the optical module of the first embodiment; 
           [0018]      FIGS. 5A through 5C  are illustrative drawings of the optical module of the first embodiment; 
           [0019]      FIGS. 6A through 6C  are illustrative drawings of light loss in the optical module; 
           [0020]      FIGS. 7A and 7B  are illustrative drawings of light loss in the optical module; 
           [0021]      FIGS. 8A and 8B  are illustrative drawings of light loss in the optical module; 
           [0022]      FIGS. 9A through 9C  are illustrative drawings of light loss in the optical module; 
           [0023]      FIGS. 10A through 10C  are illustrative drawings of the optical module of the first embodiment; 
           [0024]      FIGS. 11A and 11B  are illustrative drawings of the optical module of the first embodiment; 
           [0025]      FIGS. 12A and 12B  are illustrative drawings of the optical module of a second embodiment; 
           [0026]      FIGS. 13A through 13C  are illustrative drawings of the optical module of a third embodiment; 
           [0027]      FIGS. 14A through 14C  are illustrative drawings of the optical module of a fourth embodiment; 
           [0028]      FIG. 15  is an illustrative drawing of the optical module of a fifth embodiment; 
           [0029]      FIGS. 16A through 16C  are illustrative drawings of the optical module of a sixth embodiment; and 
           [0030]      FIG. 17  is an illustrative drawing of the optical module of a seventh embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    Embodiments will be described by referring to the accompanying drawings. The same or similar elements are referred to by the same or similar numerals. 
       First Embodiment 
       [0032]    An optical module according to an embodiment will be described. In the drawings, the longitudinal direction of an optical module is referred to as an x axis, and the lateral direction of the optical module is referred to as a y axis, with the vertical direction of the optical module being referred to as a z axis. 
         [0033]      FIGS. 1A and 1B  are perspective views of an optical module  100  and an MT (mechanical transfer) ferule  140  according to the present embodiment, respectively. The optical module  100  is made of transparent resin. As illustrated in  FIG. 1A , the optical module  100  has an insertion hole  110  formed therein into which the MT ferule  140  is inserted. By inserting the MT ferule  140  having an optical fiber connected thereto into the insertion hole  110 , the MT ferule  140  and the optical module  100  are connected to each other. Two sloped faces  111  (only one of which is visible in  FIG. 1A ), an end face  112 , and a contact face  114  are formed on the inner surface of the insertion hole  110 . When the MT ferule  140  is inserted into the insertion hole  110 , a front face  142  of the MT ferule  140  comes in contact with the contact face  114 . 
         [0034]    The front face  142  has two pins  143  formed thereon. Each of the two sloped faces  111  has two holes  113  formed therein, respectively, which engage with the pins  143 . The positions, size, and number of holes  113  are in agreement with the positions, size, and number of the pins  143 . In the present embodiment, the pins  143  are cylindrical. Two circular holes, which coincide with the shape of the pins  143 , are provided as the holes  113 . 
         [0035]    The end face  112  is situated between the two sloped faces  111 . The end face  112  has a first lens group  200  formed thereon, which includes reception-purpose lenses and transmission-purpose lenses as will be described later. 
         [0036]    The front face  142  has an opening group  144  formed thereon that includes a plurality of openings through which light signals are received or transmitted. The tip of each optical fiber connected to the MT ferule  140  is situated at the mouth of the corresponding opening. The openings are situated at positions that face the lenses of the first lens group  200 . Optical signals propagating through the optical fibers pass through the openings to enter the lenses of the optical module  100 . Optical signals transmitted from the lenses of the optical module  100  pass through the openings to enter the optical fibers. 
         [0037]    In the following, a description will be given of the structure of the optical module  100  by referring to  FIGS. 2A through 2C .  FIG. 2A  is a drawing illustrating the rear face of the optical module  100  where the insertion hole  110  is situated.  FIG. 2B  is a cross-sectional view of the optical module  100  taken along the line  2 A- 2 A shown in  FIG. 2A .  FIG. 2C  is a view of the bottom face of the optical module  100 . 
         [0038]    As illustrated in  FIGS. 2A through 2C , the first lens group  200  includes four reception-purpose lenses  210  and four transmission-purpose lenses  220 , which are lined up in the y-axis direction. The optical module  100  illustrated in  FIGS. 2A through 2C  enables the transmission and reception of optical signals for four channels. 
         [0039]    The lenses  210 , which are aspherical lenses, convert optical signals received from the optical fibers of the MT ferule  140  into parallel light. This causes the optical signal having propagated through the optical fibers to enter the optical module  100  as parallel light. The lenses  220 , which are aspherical lenses, converge optical signals having propagated inside the optical module  100  for provision into the optical fibers. The use of aspherical lenses provides an advantage in that the loss of optical signal is reduced. 
         [0040]    The holes  113  which extend in the x-axis direction are formed in the sloped faces  111 , respectively, which are situated near the opposite ends of the first lens group  200 . Namely, the holes  113  are formed near the opposite lateral ends of the end face  112 , respectively, to extend perpendicularly to the end face  112  and in parallel to a plane  101 . The plane  101  is an imaginary plane parallel to a bottom face  170  of the optical module  100 . By engaging the pins  143  with the holes  113 , the MT ferule  140  inserted into the insertion hole  110  is aligned with the optical module  100 . With the MT ferule  140  inserted into the optical module  100 , a gap is created between the first lens group  200  and the front face  142 , in other words, the first lens group  200  does not come in direct contact with the front face  142 . 
         [0041]    The bottom face  170  has a second lens group  300  formed thereon. The bottom face  170  has a perimeter  180  on which legs  160  through  162  are formed. The legs  160  through  162  come in contact with a board when the optical module  100  is mounted to the board. 
         [0042]    The second lens group  300  includes four transmission-purpose lenses  310  and four reception-purpose lenses  320 , which are lined up in the y-axis direction. In the present embodiment, the four lenses  310  and the four lenses  320  are aspherical lenses. The lenses  310  converge optical signals having propagated inside the optical module  100  for transmission purposes. The optical signals transmitted from the lenses  310  enter an optical detector (not shown) mounted on the board. 
         [0043]    The lenses  320 , which receive optical signals emitted from light emission devices such as VCSELs (vertical cavity surface emitting lasers) mounted on the board, convert the received signals into parallel light that is input into the optical module  100 . The optical signals received by the lenses  320  propagate in the form of parallel light inside the optical module  100 . 
         [0044]      FIG. 3A  is an enlarged view of the portion enclosed by the chain line  2 C shown in  FIG. 2B .  FIG. 3B  is an enlarged view of the portion enclosed by the chain line  2 D shown in  FIG. 2C . The optical module  100  has a core  250  for providing an optical coupling between the first lens group  200  and the second lens group  300 . As illustrated in  FIG. 3A , a sloped face  251  is formed halfway through the core  250  to reflect light entering the core  250 . The core  250  has a refractive index higher than the refractive index of the transparent resin of the optical module member that surrounds the core  250 . 
         [0045]    In the present embodiment as illustrated in  FIG. 3B , lenses  211 ,  212 ,  213 , and  214  of the first lens group  200  are in one-to-one correspondence with lenses  311 ,  312 ,  313 , and  314  of the second lens group  300 . Further, lenses  221 ,  222 ,  223 , and  224  of the first lens group  200 , are in one-to-one correspondence with lenses  321 ,  322 ,  323 , and  324  of the second lens group  300 . The core  250  connects between the two lenses that correspond to each other. 
         [0046]    As is illustrated in  FIG. 2A , the plane  101  is defined as a flat plane that includes the end faces of the legs  160  through  162 , and is parallel to the surface of the board. In the present embodiment, the angle between the plane  101  and the sloped face  251  of the core  250  is 45 degrees. The plane  101  is perpendicular to the end face  112 . 
         [0047]    In the present embodiment, light emitted in the normal direction of the plane  101  from light emitting devices (not shown in  FIGS. 2A through 2C ), such as VCSELs, situated directly below the lenses  320  enter the core  250  through the lenses  320 . Light propagates through the core  250  perpendicularly to the bottom face  170 , i.e., perpendicularly to the plane  101 , and is then reflected by the sloped face  251 . With the sloped face  251  situated at a 45-degree angle to the plane  101 , the light reflected by the sloped face  251  thereafter propagates parallel to the plane  101  inside the core  250 , and is then transmitted from the lenses  220 . 
         [0048]    Light which is transmitted from the MT ferule  140  (not shown in  FIGS. 2A through 2C ) perpendicularly to the end face  112  enters the core  250  through the lenses  210 , and light is reflected by the sloped face  251 . Light reflected by the sloped face  251  propagates inside the core  250  perpendicularly to the bottom face  170 , and is then transmitted from the lenses  310  to enter an optical detector. 
         [0049]    In the following, a description will be given of an optical transceiver by referring to  FIG. 4  and  FIGS. 5A through 5C . 
         [0050]    As illustrated in  FIG. 4 , a board  400  has optical detectors  411  and light emitting devices  412  mounted thereon. The light receiving faces of the optical detectors  411 , which face the direction perpendicular to a plane  401  of the board  400 , detect incoming optical signals. The light emitting devices  412  such as VCSELs transmit optical signals in the direction perpendicular to the plane  401 . 
         [0051]    The optical module  100  is mounted on the board  400  such that the lenses  310  are situated directly above the optical detectors  411 , and such that the lenses  320  are situated directly above the light emitting devices  412 .  FIGS. 5A through 5C  illustrate the optical module  100  aligned with and mounted on the board  400 .  FIG. 5A  is a drawing illustrating the optical module  100  as viewed from the insertion-hole side.  FIG. 5B  is a lateral cross-sectional view of the optical module  100 .  FIG. 5C  is a view of the bottom face of the optical module  100 . The optical module  100  as illustrated in  FIGS. 5A through 5C  is connected to the MT ferule  140  to form an optical transceiver. 
         [0052]    As illustrated in  FIG. 5C , the positions of the light emitting devices  412  coincide with the positions of the lenses  320 , and, also, the positions of the optical detectors  411  coincide with the positions of the lenses  310  as viewed from the bottom-face side. In such a manner, the optical module  100  is aligned with the board  400  in the x-axis direction and in the y-axis direction. 
         [0053]    The legs  160  through  162  have a certain height such that the light emitting devices  412  and the optical detectors  411  are accommodated in the gap between the bottom face  170  of the optical module  100  and the board  400 . 
       [Light Loss] 
       [0054]    In the following, light loss in the optical module of the present embodiment will be described. In the following, comparison is made between the optical module having the core  250  according to the present embodiment and an optical module  900  having only a sloped face without a core. 
         [0055]      FIGS. 6A through 6C  illustrate the optical module  900  lacking a core. In the drawings, dashed lines represent light paths. Further, two-dot chain lines represent an axis of the light emitted from a light emitting device  412  as well as the axis of the first lens group  200  and the axis of the second lens group  300 . 
         [0056]      FIG. 6A  illustrates the optical module  900  that is positioned with respect to the light emitting device  412  such that the axis of the light emitted from the light emitting device  412  coincides with the center of the lens of the second lens group  300 . In the arrangement illustrated in  FIG. 6A , the light emitted from the light emitting device  412  enters the optical module  900  through the lens of the second lens group  300 , and is then reflected by a sloped face  951 , followed by exiting through the lens of the first lens group  200 . In  FIG. 6A , the axis of the light emitted from the light emitting device  412  coincides with the center of the lens of the second lens group  300 , and also coincides with the center of the lens of the first lens group  200 . With this arrangement, there is substantially no light loss with respect to the light exiting through the lens of the first lens group  200 . 
         [0057]      FIG. 6B  illustrates the optical module  900  that is displaced to the left in the drawing relative to the light emitting device  412 . In  FIG. 6B , the axis of the light emitted from the light emitting device  412  is off the center of the lens of the second lens group  300 . In this case, part of the light reflected by the sloped face  951  reaches a point below the first lens group  200  as illustrated in  FIG. 6B , thereby failing to enter the first lens group  200 . This deviating light causes light loss. 
         [0058]    In  FIG. 6C , the optical module  900  is displaced to the right in the drawing relative to the light emitting device  412 . In this case also, the axis of the light emitted from the light emitting device  412  is off the center of the lens of the second lens group  300 . Part of the light emitted from the light emitting device  412  and reflected by the sloped face  951  reaches a point above the first lens group  200  as illustrated in  FIG. 6C , thereby failing to enter the first lens group  200 . This deviating light causes light loss. 
         [0059]    On the other hand, the optical module  100  of the present embodiment causes substantially no light loss even when the optical module  100  is displaced relative to the light emitting device  412  as illustrated in  FIGS. 7A and 7B . It may be noted that neither explanation nor illustration is given for the case in which the lens center of the lens group coincides with the axis of the light emitted from the light emitting device. In such a case, light loss is substantially low. 
         [0060]    In  FIG. 7A , the optical module  100  is displaced to the left in the drawing relative to the light emitting device  412 . In this case, the axis of the light emitted from the light emitting device  412  is off the center of the lens of the second lens group  300 . Despite this arrangement, the light emitted from the light emitting device  412  propagates inside the core  250 , which suppresses deviated light that would travel toward a point below the first lens group  200  as illustrated in  FIG. 6B . There is thus substantially no occurrence of light loss. Namely, the light emitted from the light emitting device  412  enters the core  250 through the lens of the second lens group  300 , and is then reflected by the sloped face  251  to propagate further inside the core  250 , followed by exiting through the lens of the first lens group  200 . In so doing, light propagates inside the core  250  by undergoing total reflection at the outer interface of the core  250 , so that almost all of the light that enters the core  250  enters the lens of the first lens group  200 , thereby resulting in almost no light loss. 
         [0061]    In  FIG. 7B , the optical module  100  is displaced to the right in the drawing relative to the light emitting device  412 . In this case also, the light emitted from the light emitting device  412  propagates inside the core  250 , thereby resulting in substantially no light loss as in the case of  FIG. 7A . Namely, the light entering the core  250  through the second lens group  300  is reflected by the sloped face  251  to propagate further inside the core  250 , followed by exiting through the first lens group  200 . In so doing, light propagates inside the core  250  by undergoing total reflection at the outer interface of the core  250 , so that almost all of the light having entered the core  250  enters the lens of the first lens group  200 . 
         [0062]    What was described above will be elaborated by showing simulation results. In this simulation, a lens of the first lens group  200  had a diameter of 250 micrometers.  FIGS. 8A and 8B  are drawings illustrating the results of simulation performed with respect to the optical module  900  having the sloped face  951  of 45 degrees with no core as illustrated in  FIGS. 6A through 6C . 
         [0063]    When the optical module  900  is mounted at its intended position relative to the light emitting device  412  as illustrated in  FIG. 8A , the axis of the light emitted from the light emitting device  412  coincides with the center of the lens of the second lens group  300 . With this arrangement, a substantial portion of the light having propagated inside the optical module  900  and having been reflected by the sloped face  951  enters the lens of the first lens group  200 . It is difficult for the second lens group  300  to convert the light emitted from the light emitting device  412  into perfect parallel light rays, resulting in the incident light spreading in the optical module  900  while propagating therein. The finite size of the lens of the first lens group  200  means that some of the spreading light propagating in the optical module  900  does not enter the lens of the first lens group  200 . Such a light component accounts for light loss. According to the results of simulation, light loss in this case was 64 dB. It may be noted that this value of light loss represents loss in the ideal arrangement in which the axis of the light emitted from the light emitting device  412  coincides with the center of the lens of the second lens group  300  and the center of the lens of the first lens group  200 . In other words, this value represents the lowest possible light loss observed in the optical module  900  illustrated in  FIGS. 8A and 8B . 
         [0064]    When the optical module  900  is displaced by 10 micrometers relative to the light emitting device  412  as illustrated in  FIG. 8B , the axis of the light emitted from the light emitting device  412  is at 10 micrometers off the center of the lens of the second lens group  300 . With this arrangement, a substantial portion of the light having propagated inside the optical module  900  and having been reflected by the sloped face  951  does not enter the lens of the first lens group  200 . This portion accounts for light loss. According to the results of simulation, light loss in this case was 15.08 dB. 
         [0065]      FIGS. 9A through 9C  illustrate the results of simulation performed with respect to the optical module  100  having the core  250  as illustrated in  FIGS. 7A and 7B .  FIGS. 9A through 9C  illustrate the case in which the optical module  100  is displaced by 10 micrometers relative to the light emitting device  412 . 
         [0066]      FIG. 9A  illustrate the case in which the width W of the core  250  is 100 micrometers. With the 100-micrometer core width W as illustrated in  FIG. 9A , a significant portion of the light emitted from the light emitting device  412  and reflected by the sloped face  251  enters the lens of the first lens group  200  because the light emitted from the light emitting device  412  propagates inside the core  250  by undergoing total reflection, despite the fact that the axis of the light emitted from the light emitting device  412  is at 10 micrometers off the center of the lens of the second lens group  300 . According to the results of simulation, light loss in this case was 0.27 dB. 
         [0067]    The width W of the core  250  is 200 micrometers in  FIG. 9B . Light loss according to the results of simulation was 0.385 in this case. 
         [0068]    The width W of the core  250  is  300  micrometers in  FIG. 9C . Light loss according to the results of simulation was 2.6865 in this case. 
         [0069]    As is described above, the provision of the core  250  realizes the reduction of light loss in the optical module  100 . It may be noted that increasing the core width W of the core  250  in excess of the lens diameter of the first lens group  200  will increase the amount of light failing to enter the first lens group  200 . In consideration of this, the core width W is preferably smaller than the lens diameter of the first lens group  200 . 
       [Production Method] 
       [0070]    A description will be given of the method of making the optical module  100 .  FIG. 10A  is a cross-sectional view illustrating the optical module.  FIG. 10B  is an enlarged view of the portion enclosed by the chain line  10 A shown in  FIG. 10A .  FIG. 10C  is a cross-sectional view taken along the chain line  10 B- 10 C shown in  FIG. 10A . In  FIGS. 10B and 10C , dashed lines represent light paths, and two-dot chain lines represent an axis. 
         [0071]    Firstly, an optical module member having an inner space in which the core  250  is to be formed is formed. An optical module member having an inner space, the legs  160  through  162 , the second lens group  300 , and the first lens group  200  is formed by injection molding as a unitary, seamless structure of transparent resin. This inner space has a sloped face. 
         [0072]    Subsequently, the inner space is filled with liquid resin, which is then cured to form the core  250 . The resin for the core  250  may be thermosetting resin or photo-curable resin. In the case of thermosetting resin, the inner space of the optical module is filled with thermosetting resin, which is then heated and thermally cured to form the core  250 . In the case of a photo-curable resin, the inner space is filled with photo-curable resin, which is then illuminated by light, such as ultraviolet light, and cured to form the core  250 . In the present embodiment, the refractive index of the resin of the core  250  is higher than the refractive index of the material of the optical module member that surrounds the core  250 . 
         [0073]    The optical module  100  allows the positions of the lenses of the first lens group  200  and the positions of the lenses of the second lens group  300  to be accurately measured. Specifically, the positions of upper and lower edges  252   a  and  252   b  of the core  250  are first identified. Then, the positions in the z-axis direction of the centers of the lenses  210 , the lenses  220 , and dummy lenses  230  are identified with respect to a reference line that is the midline between the core edges  252   a  and  252   b  extending in the y-axis direction. This arrangement allows a check to be made as to whether the first lens group  200  is formed at its intended position in the z-axis direction. The z coordinate, relative to the reference point, of the center of a given lens of the first lens group  200  is a distance in the z-axis direction between the center of the core  250  and the center of the given lens. If z is equal to zero, the center of the core  250  coincides with the center of the lens without any displacement.  FIG. 11A  illustrates an enlarged view of the portion where the core  250  is formed, and  FIG. 11B  illustrates an enlarged view of the optical module  100  as viewed from the insertion-hole side. In  FIG. 11A , two-dot chain lines represent an axis and imaginary extension of the faces of the core  250 .  FIG. 11B  illustrates the structure that has the dummy lenses  230  situated between the lenses  210  and the lenses  220 . The dummy lenses  230  are used to identify the center of the first lens group  200  in such an area that is situated between the lenses  210  and the lenses  220 . 
         [0074]    The above description is directed to an example in which the positions of the lenses of the first lens group  200  are checked after the core  250  is formed in the optical module  100 . The positions of the lenses may similarly be checked before forming the core  250  into the optical module member. 
         [0075]    In this case, the positions in the z-axis direction of the center of the lenses of the first lens group  200  are identified relative to a reference line that is the midline between the edges of the space that is to be filled with the core  250 . 
         [0076]    In the embodiment described above, the lenses  210  and  320  and the lenses  220  and  310  are aspherical lenses. Alternatively, other types of lenses such as spherical lenses which are easy to manufacture may be used to allow the optical module to be produced at lower cost. 
       Second Embodiment 
       [0077]    A second embodiment will be described. In the second embodiment, the core  250  having a shape coinciding with a space  150  of an optical module  100   a  is produced in advance by use of transparent resin as illustrated in  FIG. 12A , and then inserted into the space  150  of the optical module  100   a  as illustrated in  FIG. 12B . In the second embodiment, the refractive index of the core  250  is higher than the refractive index of the optical module  100   a.    
       Third Embodiment 
       [0078]    A third embodiment will be described. 
         [0079]    For an optical module of the third embodiment, a core  550  illustrated in  FIG. 13A  has the faces thereof coated with coating films  553  as illustrated in  FIG. 13B , which are made by applying resin to the faces except for the reception face  552   a  and the transmission face  552   b.  The core part  560  having the coating films  553  formed thereon is inserted and secured into the space  150  of the optical module  100   a  as illustrated in  FIG. 13C . The coating films  553  are made of resin having a lower refractive index than the resin constituting the core  550 . Further, the resin of the core  550  may have a refractive index higher than, or lower than, the resin constituting the optical module  100   a.    
       Fourth Embodiment 
       [0080]    A fourth embodiment will be described. In the fourth embodiment, a core  550  is produced as illustrated in  FIG. 14A , followed by forming a metal film  571  of light-reflective metal material on the faces of the core  550  as illustrated in  FIG. 14B  through vapor deposition or the like except for the reception face  552   a  and the transmission face  552   b.  The core part  570  having the metal film  571  formed thereon is inserted into the space  150  of the optical module  100   a  as illustrated in  FIG. 14C . 
         [0081]    In the fourth embodiment, light entering the core  550  propagates inside the core  550  by undergoing reflection on the inner faces of the metal film  571  formed on the core  550 . Further, the resin material of the core  550  may have a refractive index higher than, or lower than, the resin constituting the optical module  100   a.    
       Fifth Embodiment 
       [0082]    A fifth embodiment will be described.  FIG. 15  is an enlarged view of the cross-section of an optical module according to the fifth embodiment. Dashed lines represent light paths. 
         [0083]    The optical module of the fifth embodiment has metal films  650  formed on the inner walls of the space  150  as illustrated in  FIG. 15 . The space  150  has a sloped face  151  for reflecting light. The optical module according to the present embodiment has no resin core disposed in the space  150 . The metal films  650  of light-reflective material are formed by vapor deposition or sputtering on the inner walls of the space  150  except for the light reception and transmission faces. During vapor deposition or sputtering, vapor particles or the like easily enter the space  150  and reach every corner of the space  150 , and the metal films  650  are formed on all the walls of the space  150 . 
         [0084]    In the fifth embodiment, light entering the space  150  propagates inside the space  150  by undergoing reflection on the metal films  650 . Despite the absence of a core in the space  150 , thus, the present embodiment enables the reduction of light loss similarly to the configuration having such a core. 
         [0085]    Films coating the inner walls of the space may be made of non-metal material, as long as such films can efficiently reflect light. 
       Sixth Embodiment 
       [0086]    A sixth embodiment will be described.  FIG. 16A  is a cross-sectional view of an optical module member  700   a.    FIG. 16B  is an enlarged view of the portion enclosed by the chain line  16 A shown in FIG.  16 A.  FIG. 16C  is an enlarged view of the cross-section of the optical module. 
         [0087]    The optical module of the sixth embodiment has gaps  751  formed between the optical module member  700   a  and the core  250  as illustrated in  FIG. 16C . This arrangement reduces a critical angle necessary to provide total reflection at the interface of the core  250 , thereby further reducing light loss. The optical module member  700   a  has an inner space  750  that is slightly wider in the vertical direction than the width of the portion of the core  250  situated toward the first lens group  200 , and that is also slightly wider in the horizontal direction than the width of the portion of the core  250  situated toward the second lens group  300 . The core  250  similar to the core of the second embodiment is inserted into the space  750 . This arrangement creates the gaps  751  between the optical module member  700   a  and the core  250  as illustrated in  FIG. 16C . 
       Seventh Embodiment 
       [0088]    A seventh embodiment will be described.  FIG. 17  is a cross-sectional view of an optical module  800  according to the present embodiment. The optical module  800  of the seventh embodiment has a core  850  which does not have a sloped face. Specifically, the core  850 , which is made of a material having a higher refractive index than a surrounding optical module member  800   a,  has a gentle curve halfway through. Light entering the core  850  propagates in the core  850  by undergoing total reflection at the, interface of the core  850 , which provides lower light loss than in the case of light being reflected at a sloped face. The optical module of the present embodiment is made by a method similar to the method used in the first embodiment. 
         [0089]    Further, although the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention. 
         [0090]    The present application is based on and claims the benefit of priority of Japanese priority application No. 2015-112611 filed on Jun. 2, 2015, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.