Abstract:
An optical coupling device includes: a first face facing a support of the optical coupling device, this support having a reception face facing upwards and; a cavity mouthing to the first face, and receiving glue to fix the optical coupling device to the support. The cavity is surrounded by a wall including a second face facing at least partly upwards.

Description:
FIELD OF THE INVENTION 
       [0001]    The instant invention relates to optical coupling devices, optical communication systems comprising such optical coupling devices, and their method of manufacture. 
       BACKGROUND OF THE INVENTION 
       [0002]    Most communication systems involve a number of system-cards. Such cards are usually manufactured as so-called printed circuit boards (PCBs). Because of the ever increasing requirements in data rates, due for example to the Internet, the limits of using electrical communications are being reached. It has become difficult to guarantee good signal integrity over the electrical lines. 
         [0003]    To respond to this bandwidth demand, high speed systems are now being built with optical layers (optical fibre or planar waveguide) incorporated in replacement of the electrically-conducting metal. Indeed, light does not suffer from the same limitations as electricity. 
         [0004]    Optical coupling devices are usually used to interconnect an optical layer of a PCB, or so-called optical circuit board (OCB), with an external optical device. In order to ensure efficient transfer of light through the optical coupling device, a very precise positioning of it along a vertical direction with respect to the circuit board is necessary. Then, a fixation part of the optical coupling device is glued to a fixation surface of the optical circuit board. 
         [0005]    It is required to improve the fixation of the optical coupling devices to the optical circuit boards. 
       SUMMARY OF THE INVENTION 
       [0006]    It is provided an optical coupling device for an optical communication system. The optical coupling device comprises a first face, which is to face a support of the optical coupling device. This support has a reception face facing upwards and adapted to receive the optical coupling device. 
         [0007]    The optical coupling device further comprises a cavity mouthing to the first face, and adapted to receive glue to fix the optical coupling device to the support. 
         [0008]    The cavity is surrounded by a wall comprising a second face facing at least partly upwards. 
         [0009]    With these features, cured glue will act as an anchor, further contributing to the prevention of the tearing away of the coupling device. 
         [0010]    In some embodiments, one might also use one or more of the features defined in the dependant claims. 
         [0011]    According to another aspect, it is provided an optical coupling device for an optical communication system. The optical coupling device comprises: 
         [0012]    a bottom face adapted to face a support of the optical coupling device, and 
         [0013]    a top face opposed to the bottom face. 
         [0014]    A through hole extends between said top face and said bottom face. The through hole can receive glue to fix the optical coupling device to the support. 
         [0015]    This allows glue to be dispensed from over the optical coupling device rather than from the side thereof, which is much easier to perform, and allows to glue in other places than only the periphery of the optical coupling device. 
         [0016]    In some embodiments, one might also use one or more of the features defined in the dependant claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Other characteristics and advantages of the invention will readily appear from the following description of four of its embodiments, provided as non-limitative examples, and of the accompanying drawings. 
           [0018]    On the drawings: 
           [0019]      FIG. 1  is a partial perspective top view of an optical system, 
           [0020]      FIG. 2  is perspective view of the bottom face of an optical coupling device, 
           [0021]      FIG. 3  is a partial sectional view along line III-III of  FIG. 1  for a first circuit board, 
           [0022]      FIG. 4  is a view partially similar to  FIG. 3  for a second embodiment, 
           [0023]      FIG. 5  is a view similar to  FIG. 4  for a third embodiment, 
           [0024]      FIG. 6  is a partial top view of the third embodiment, and 
           [0025]      FIG. 7  is a view similar to  FIG. 4  for a fourth embodiment. 
       
    
    
       [0026]    On the different Figures, the same reference signs designate like or similar elements. 
       DETAILED DESCRIPTION 
       [0027]      FIG. 1  partially shows a hybrid or full optical PCB  1  for example a backplane, which is a layer stack comprising a plurality of layers. In particular, this layer stack  1  comprises, from top to bottom, a copper layer  101 , a pre-preg layer  102 , an optical layer  103 , and further copper  104  and pre-preg  105  layers. The optical layer  103  itself comprises a first top cladding layer  106 , a second transmission optical layer  107  below the first top cladding layer  106 , and a third bottom cladding layer  108  below the second transmission optical layer  107  (see  FIG. 3 ). 
         [0028]    The terms “top”, “bottom”, “up”, “down” or the like are given in reference to the direction Z, normal to the top surface  1   a  of the PCB, and pointing toward a mating optical device  4  to be optically coupled to the PCB. The top surface of the PCB extends parallel to an X-Y plane, with X and Y being artificially defined. For example, X corresponds to the direction of propagation of light in the layer  107  and Y to the direction transverse thereto. 
         [0029]    The optical layer  107  of the layer stack  1  is made of a plurality of tubes  2  integrated or embedded in a body  3  having a lower refractive index than the tubes  2 . Thus, the tubes  2  and the body  3  constitute respectively the cores and the cladding of waveguides. Embedded waveguides may be polymer waveguides, glass sheet waveguides or waveguides obtained by embedded fibre technology, or the like. 
         [0030]    It will be understood that a part of the PCB is removed from  FIG. 1  to ease representation, and that what appears as a face  1   c  is in reality not a face but is internal to the PCB  1 . 
         [0031]    As can be seen on  FIG. 1 , a cut-out  27  is formed in the PCB  1 . In particular, the cut-out  27  is shaped with a very simple form of a right parallelepiped. The cut-out is defined by straight walls. The cut-out can also have a plane bottom  27   b,  as shown. 
         [0032]    The wall where the tubes  2  mouth into the cut-out defines an optical interface of the PCB. Namely, all cores  2  mouth into the cut-out  27  to define the optical interface  9  of the PCB ( FIG. 3 ). This optical interface  9  comprises discrete light transmission regions arranged as an array. The spacing of transmission regions along the direction Y might be constant or not, depending on the requirements. For example, in the present drawing, the spacing between neighbour transmission regions is set constant to 250 μm. 
         [0033]    Optical signals, transferred to or from a mating optical device  4 , such as an optical device or opto-electrical device or an other PCB, are provided over a first optical path  6  to/from the cores  2  of the layer stack  1 , which core  2  provides a second optical path  7  for the optical signal parallel to the X-Y plane. In the present example, the optical device  4  can for example comprise a mechanical-transfer ferrule (“MT-ferrule”) comprising a high precision sleeve  21  in which ends of optical fibers  22  extend in precisely defined relative locations. The mating optical device  4  thus has an optical interface  10  defined as the set of optic fibre ends directed toward the PCB. In the present drawing, this interface extends parallel to the X-Y plane. 
         [0034]    The optical interface  10  of the mating connector has the same number of transmission regions as the optical interface  9  of the PCB. Each transmission region of the optical interface  10  of the mating optical device corresponds to a respective transmission region of the optical interface  9  of the PCB. This means that transmission regions are associated two by two and that light normally exited through the transmission region of one of the interfaces is to be transmitted to the corresponding transmission region of the other interface. 
         [0035]    The printed circuit board  1  further comprises a Z-reference. The Z-reference is a part of the printed circuit board the location of which along the Z direction is precisely known with respect to the optical interface  9 . For example, it corresponds to the bottom of the bottom cladding layer (or rather to the coinciding top  23  (see  FIG. 3 ) of the underlying copper layer  104 ). However, other locations are possible, such as the top of the top cladding layer, for example. 
         [0036]    In order to achieve an optimal optical coupling between the first and second optical paths, that are perpendicular to each other for the optical system here, an optical coupling device  8  is provided for alignment purposes. In the present example, the optical coupling device  8  is provided as a single unitary component, although this is not necessarily always the case. Only the central part of the optical coupling device, which is used for optical coupling, is visible on  FIG. 1 . 
         [0037]    The coupling device  8  is, for example, a unitary piece manufactured by moulding a translucent suitable material. The optical coupling device  8  comprises a first face  24  defining a first optical interface  25  which is to be put in optical coupling with the optical interface  9  of the PCB. The first optical interface  25  has transmission regions  13  which are to be placed opposite in free space (sometimes through a translucent coupling medium such as air or a suitable glue) a corresponding transmission region of the interface of the PCB. Hence, the arrangement of the first optical interface  25  directly derives from that  9  of the printed circuit board, and it will not be described in further details here. 
         [0038]    The optical coupling device  8  comprises a second face  11   b  which, in the present case, extends normal to the first face, i.e. extends parallel to the X-Y plane. It defines a second optical interface  26  which is to be put in optical coupling with the optical interface of the mating optical device  4 . The second optical interface  26  has transmission regions  13 ′ which are to be placed opposite (sometimes through a translucent coupling medium such as air or a suitable glue) a corresponding transmission region of the interface of the mating optical device  4 . Hence, the arrangement of the second optical interface  26  directly derives from that of the mating optical device  4 , and it will not be described in further details here. 
         [0039]    An optical path is defined between the first and second interfaces  25 ,  26  of the coupling device  8 . Namely, diverging light entering the coupling device  8  at its first interface  25 , coming from the interface of the printed circuit board  1  will be propagated through the coupling device  8  to the second interface  26  as a substantially collimated light beam, and will be focussed into the interface of the mating optical device  4 . Light propagates in the opposite direction in a similar way. 
         [0040]    In particular, each transmission region of each interface of the coupling device  8  can be provided with a light beam forming structure  15 ,  15 ′ such as a lens. The lenses  15  optimise the optical coupling of the optical signals of the cores  2  to/from the coupling device  8 . The lenses  15 ′ optimise the optical coupling of the optical signals of the ferrule  4  to/from the coupling device  8 . 
         [0041]    Since lenses  15  and  15 ′ focus the optical signals at the entry of each core  2  and respectively at the entry of each optical fibre  22 , the manufacture tolerance of the coupling device  8 , the ferrule  4  and the layer stack  1  are increased in comparison with an optical coupling system without lenses. 
         [0042]    As shown in the present example, the lenses  15 ,  15 ′ may form an integral part of the coupling device  8 . They are located at the first and second interfaces. They could be of the Fresnel-type or of the aspheric type, for example. It will be appreciated that, for each interface, all lenses of the interface could be performed identical. 
         [0043]      FIG. 2  now shows in more details the bottom face of the coupling device  8 . (It is now shown entirely). The coupling device  8  is provided as a thin plate having a first (bottom) face  11   a  and an opposite parallel second (top) face  11   b  ( FIG. 1 ). A body  16  projects from the bottom face  11   a  downwards, rather centrally. This body carries the optical interface  25 , as well as a mirror  18  used to deflect light from the X direction to the Z direction. 
         [0044]    Further, the optical coupling device  8  is provided with Z-reference parts  12 . Z-reference parts  12  are parts of the optical coupling device  8 , the location of which along the direction Z is precisely known with respect to the first optical interface  25 . These parts are for example surfaces extending parallel to the X-Y surface. For example, three such parts can be provided on three feet  14  which project from the face  11   a.  These feet can be provided unaligned, and of the same length, so that the three Z-reference parts  12  precisely define a plane. 
         [0045]    The optical coupling device  8  further comprises fixation parts. These fixation parts are used to fix the optical coupling device  8  to the printed circuit board  1 . The fixation parts are for example provided at the periphery of the optical coupling device  8 , such as in the present first embodiment. For example, a first fixation part is a peripheral ridge  17  which extends continuously around the whole periphery of the device. Further, a second fixation part is provided as a second peripheral ridge  19 , which extends continuously around the whole periphery of the device. The second peripheral ridge also surrounds the first peripheral ridge  17 . Thus, the second peripheral ridge  19  is an outer fixation part, while the first peripheral ridge  17  is an inner fixation part. Hence, the first peripheral ridge  17  is located between the second peripheral ridge  19  and the body  16 . 
         [0046]    The fixation parts  17 ,  19  project from the face  11   a  of the optical coupling device. 
         [0047]    As can be seen in  FIG. 3 , the optical coupling device will be placed over the cut-out  27  of the printed circuit board  1  so that the Z-reference parts  12  will cooperate with the Z-reference of the printed circuit board, so as to precisely define the position of the optical coupling device  8  with respect to the Z-reference of the printed circuit board along the Z axis. For example, the Z-reference parts  12  are simply laid on the Z-reference  23  of the printed circuit board  1 . However, other ways to precisely define the location of the optical coupling device  8  along the Z direction with respect to a Z-reference of the printed circuit board exist. 
         [0048]    In theory, in this position, the optical coupling device and the printed circuit board are so positioned with respect to one another along the direction Z, that an efficient optical coupling occurs between the interface  9  (out of the plane of  FIG. 3 ) of the printed circuit board and the optical interface  25  of the optical coupling device (not visible on this drawing). This is due to the precisely known relative positioning along the direction Z of: 
         [0049]    the interface  9  of the circuit board with the Z-reference  23  by construction of the circuit board, 
         [0050]    the Z-reference  23  with the Z-reference part  12  of the optical coupling device  8  by co-operation, and 
         [0051]    the Z-reference part  12  with the optical interface  25 , by construction of the coupling device. 
         [0052]    If necessary, X-Y reference means (not shown) are used to carefully place the coupling device with respect to the circuit board in the X-Y plane. 
         [0053]    The Z axis is oriented in a direction out of the main plane of the circuit board, toward the mating optical device  4 . This is the direction of light exiting the circuit board. 
         [0054]    The fixation surface  20  of the printed circuit board is used to cooperate with the fixation parts  17 ,  18  of the optical coupling device  8  to fix the optical coupling device  8  to the circuit board  1 . For example, the fixation surface  20  corresponds to the accessible top face  1   a  of the printed circuit board, either being for example the top face of the copper layer  101  or that of the pre-preg layer  102  if the copper layer  101  has been removed in this area. 
         [0055]    When the optical coupling device  8  is placed on the printed circuit board, the inner and outer fixation parts  17 ,  19  are spaced apart from the surface  20 , to enable the Z-reference part  12  to lay on the Z-reference  23  of the circuit board. 
         [0056]    Further, the heights of these fixation parts differ from one another. In the present embodiment, they may differ by at least 50 micrometers. 
         [0057]    In particular, the first (inner) fixation part  17  is closer to the fixation surface  20  than the second (outer) fixation part  19 . 
         [0058]    Once the optical coupling device is positioned, glue is made to flow from the periphery of the coupling device, for example using a syringe along the arrow  29 . Glue  28  will flow between the bottom surface of the first fixation part  17  and the fixation surface  20  of the circuit board, directly opposed thereto. Fixation will occur between these two surfaces. 
         [0059]    The coupling device is provided with a recess  30  located between the first and second fixation parts. In the present example where the first and second fixation parts are peripheral ridges running all along the periphery of the coupling device, the recess  30  can be provided as a groove also running all along the periphery of the coupling device (see  FIG. 2 ). The recess  30  will absorb glue flown between the coupling device and the circuit board. 
         [0060]    As can be seen on  FIG. 3 , the recess  30  comprises two distinct portions  31 ,  32 . The first portion  31  extends from the bottom of the optical coupling device upwards along direction Z and mouth into the second portion  32 . In particular, the second portion  32  is broader, along direction X, than the first portion  31 , so that the wall which defines and surrounds the recess  30  has a face  33  facing upward. In the present example, the face  33  lies in the X-Y plane and has its normal oriented along the direction Z. 
         [0061]    When the glue  28  flows in the recess  30 , it will flow in the second portion  32 . Once cured, the glue  28  hardens, so that it will mechanically cooperate with the face  33  of the optical coupling device in case removal forces are exerted along the axis Z which would tend to remove the optical coupling device from the circuit board  1 . The glue will act as an anchor means to improve the retention of the optical coupling device to the circuit board  1 . 
         [0062]      FIG. 4  now shows partially a second embodiment of the invention. When compared to the first embodiment of  FIG. 3 , the shape of the recess  30  differs. In particular, the recess does not have the first and second portions  31 ,  32  of the first embodiment. However, instead of having the recess  31  narrowing along direction Z as on  FIG. 3 , the recess now broadens along direction Z so that the recess  30  comprises faces  33  which face only partly upwards. Hence, a face  33  is considered to face at least partly upwards when the projection of its normal on the Z axis is directed upwards. A suitable angle for the faces  33  is considered to be of at least 15 degrees with respect to the Y-Z plane. 
         [0063]      FIG. 5  now shows a third embodiment of the invention. The embodiment of  FIG. 5  is similar to the one of  FIG. 3 , with the difference that the second portion  32  extends up to the top face of the optical coupling device, as shown. Hence, the optical coupling device  8  is provided with a through hole extending from its top face  11   b  to its bottom face. The second portion has a geometry similar to the second portion of the first embodiment, with the top facing faces  33 , as well as further slanted surfaces  33 ′ extending from the face  33  to the top face  11   b.  These slanted faces  33 ′ may also face partly upwards, as shown. 
         [0064]      FIG. 6  partially shows a top view of the optical coupling device  8  according to the embodiment of  FIG. 5 . 
         [0065]    It is only in some local areas  34  that the recess  30  is performed as a through hole. In intermediate regions  35 , intermediate between two local areas  34 , the cross-section of the recess  30  may be as shown on  FIG. 3 , for example or there might not even be any recess in these locations. However, such recesses allow glue to flow uniformly along the periphery of the optical coupling device  8 . The location of the various local areas  34  can be as shown on  FIG. 6 . However, to ease the dispensing of the glue, there may not be any such local area  34  in the corners of the optical coupling device, as shown on  FIG. 6 . 
         [0066]    When the recess  30  is provided as a through hole, such as shown on  FIGS. 5 and 6 , glue may be dispensed through the optical coupling device, from its top face  11   b,  as shown on  FIG. 7 , rather than from the side (arrow  29  of  FIGS. 3 to 5 ). A glue-dispensing nozzle  36  is schematically shown on  FIG. 7 . Dispensing glue from the top rather than from the periphery may be advantageous, because it means that glue needs not necessarily be dispensed only at the periphery of the optical coupling device, but may be dispensed in other locations. In particular, glue may be dispensed closer to the central region comprising the optically relevant components, which would enable to provide the fixation in a more strategic location. Of course, a glue barrier may have to be finely defined to prevent any flow of glue from interfering with the transfer of optical signals at the optical coupling device. 
         [0067]    As shown on  FIG. 7 , according to this embodiment of the invention, the recess  30  needs not necessarily exhibit any upward facing face. For example, the walls defining the recess  30  may extend straight along the Z axis, i.e. normal to the reception face of the PCB 1 .