Abstract:
An optical circuit board including a top face, a bottom face, an optical layer buried between bottom and top faces, the optical layer being adapted to transmit optical signals, an opto-electronic component adapted to emit or receive light transmitted through the optical layer, a solid heat dissipative element adapted to dissipate heat generated at the opto-electronic component.

Description:
FIELD OF THE INVENTION 
       [0001]    The instant invention relates to optical circuit boards. 
       BACKGROUND OF THE INVENTION 
       [0002]    Today, most communication systems involve a number of system-cards. Such cards are usually manufactured as so-called printed circuit boards (PCBs). Usually, some of the system-cards, which are called daughter boards, are assembled together on a rigid system-card called the backplane, or mother board. 
         [0003]    The daughter boards usually extend parallel with each other and are interconnected together via the backplane, which extends perpendicular to them. There are several practical advantages to such a configuration: Easy insertion, removal, and replacement of the daughter-boards. 
         [0004]    Because of the ever increasing requirements in data rates in communication systems, due for example to the Internet, the limits of using electrical communications between printed circuit boards (PCB) are being reached. It has become difficult to guarantee good signal integrity when transferring information at high frequencies (e.g. 25 Gb/s or higher) through electrical lines between two electrical components such as a printed circuit board. 
         [0005]    To respond to this bandwidth demand, high-speed systems now use optical waveguide light to transfer light-carried information. 
         [0006]    Light enables to improve the transfer of information between two points since light is less sensitive to interference phenomenon. High speed systems are now being built with optical layers (optical fibres or planar waveguides) incorporated in replacement of the electrically-conducting metal. An optical layer is disposed parallel to the average plane of the backplane. An other optical layer is disposed parallel to the average plane of the daughter board. 
         [0007]    The daughter board may comprise a set of opto-electronic components which will either emit or detect light transmitted through its optical layer. U.S. Pat. No. 7,149,376 discloses one such embodiment. 
         [0008]    With the stringent requirements for miniaturization, together with the ever-increasing demands for higher transmission rates, attention is now turning to the efficient thermal management of such systems. 
       SUMMARY OF THE INVENTION 
       [0009]    To this aim, it is provided an optical circuit board comprising a top face and a bottom face. 
         [0010]    The optical circuit board has an optical layer buried between the bottom and top faces. This optical layer can transmit optical signals. 
         [0011]    The board comprises an opto-electronic component to emit or receive light transmitted through the optical layer. 
         [0012]    The board further comprises a solid heat dissipative element adapted to dissipate heat generated at the opto-electronic component. 
         [0013]    With these features, efficient thermal management of the opto-electronic component is provided at the board, using the heat dissipative element, which enables new gains in terms of data rates. 
         [0014]    In some embodiments, one might also use one or more of the features defined in the dependant claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Other characteristics and advantages of the invention will readily appear from the following description of eight of its embodiments, provided as non-limitative examples, and of the accompanying drawings. 
           [0016]    On the drawings: 
           [0017]      FIG. 1  is a top view of a daughter board according to a first embodiment, 
           [0018]      FIG. 2  is a partial sectional view along line B-B of  FIG. 1  for the first embodiment, 
           [0019]      FIGS. 3 to 8  are views similar to  FIG. 2  for second to seventh embodiments of the invention respectively, 
           [0020]      FIG. 9  is a perspective view of heat sink suitable for use in any of the above embodiments, 
           [0021]      FIG. 10  is a view similar to  FIG. 9  for an eighth embodiment. 
       
    
    
       [0022]    On the different figures, the same reference signs designate like or similar elements. 
       DETAILED DESCRIPTION 
       [0023]      FIG. 1  schematically shows a system card  1  such as a daughter board to be connected to a mother board  2  of an optical communication system. The system card  1  may comprise a number of electronic components  3 , as well as a number of electrical connectors  4  for electrical connection to the mother board  2 . The system card  1  is an optical circuit board. The foot print of the optical layers is visible as reference  5  on  FIG. 1 . As visible, optical communications may be performed internally in the card  1  between two remote sets of electronic components  3 ,  3 ′ and/or between one set of electronic components  3  and the mother board  2  through an optical connector  6 . As shown on  FIG. 2 , the system card  1  extends between a top face  1   a  and a bottom face  1   b . It is performed as a stack of layers. Electrical layers  7   a ,  7   b  are provided to guide electrical signals of the board  1 . The electrical layer  7   a  is provided close to the top surface  1   a  and the electrical layer  7   b  close to the bottom surface  1   b . Both may be protected by an outer insulating layer  8   a ,  8   b . An optical layer  9  is buried between the top and bottom faces  1   a ,  1   b  of the board, and in the present example, between the electrical layers  7   a ,  7   b , possibly with interposition of further insulating layers  8   c , as shown. The optical layer  9  comprises a plurality of optical waveguides  10   a ,  10   b  embedded in cladding  11 . Light will be propagated in the optical waveguides  10   a ,  10   b  in the system card  1 . Light propagates in a plane parallel to the top and bottom faces of the printed circuit board, for example along a direction X, on  FIG. 2 . In the present example, the waveguides are provided in plurality of rows offset with respect to one another along the direction Z. A top row of waveguides  10   a  extends closer to the top face  1   a  of the circuit board than a bottom row of waveguides  10   b . The present invention could be applied with one or more rows of waveguides. Although only one waveguide  10   a ,  10   b  is shown per row on  FIG. 2 , it will be understood that each row may comprise a plurality of waveguides offset with respect to one another along the direction Y, and separated from each other by cladding  11 . 
         [0024]    A cavity  12  is provided in the system card  1 . For example, the cavity is a through hole extending between the top and bottom faces  1   a ,  1   b  of the system card. The cavity may receive an optical engine  13 . The optical engine  13  comprises a printed circuit board  14  carrying, on its top face  14   a , opto-electronic components  15   a ,  15   b  and electronic control components  16 . For example, it is provided one row of light emitting components  15   a , offset along the direction Y and adapted to emit light to be directed toward the waveguides  10   a  of the top row and photo-diodes  15   b  are provided along one row, offset from one another along the direction Y, to receive light propagated through waveguides  10   b  of the bottom row. However, this lay-out is purely illustrative. 
         [0025]    The printed circuit board  14  mainly comprises a heat-dissipating substrate, for example made of ceramic or of another material enabling effective heat-dissipation as well as being a support for electrical tracks (not shown) electrically joining the electronic control components  16  to the opto-electronic components  15   a ,  15   b . The substrate has its bottom face  14   b  assembled to a heat dissipative element  30 , such as a heat spreader. The heat spreader can for example be a thin foil of heat-dissipating material, such as copper. Thermal conductivity of at least 300 W/m.K are appropriate for such materials. 
         [0026]    The electronic control components  16  are electrically connected to an electrical track of the system card  1 , such as the bottom track  7   b  by wire bonding. 
         [0027]    The electronic control components  16  are electrically connected to the opto-electrical components  15   a ,  15   b  to command and/or assist these components. Such components are for example light-emitting components  15   a  such as suitable VCSELs. The optical engine  13  is provided and assembled to the body  17  of the system card such that the light emitting components  15   a  emit light along the direction Z toward the top face  1   a  of the card. The opto-electrical components  15   b  are for example photo-diodes which are adapted to receive light. These photo-diodes  15   b  are also positioned so that they can detect light propagated along the direction Z toward them, directed toward the bottom face  1   b  of the card. 
         [0028]    An additional heat dissipative element, such as a heat sink  18  is provided in close proximity to the opto-electrical components  15   a ,  15   b . For example, the heat sink  18  has a plane face  18   a  which is assembled, for example glued to the heat spreader  30 , and in particular to a bottom face thereof. The heat sink  18  further comprises a heat transfer region  19  where the ratio of surface to volume is high, and accessible from the outside of the system card so as to receive a suitable cooling fluid such as air ventilated in the device receiving the system card, or the like. For example, the heat transfer region  19  will be accessible from the bottom face  1   b  of the system card  1 . 
         [0029]    The heat sink  18  may further be assembled, for example glued, to the body  17  of the system card  1  for mechanical fixation. Thus, the heat sink  18  is directly supported by the optical circuit board. 
         [0030]    According to a variant, the heat sink  18  (or additional heat dissipative element) is replaced by a heat spreader. Thus the heat spreader  30  and the additional heat dissipative element can be made as a single piece. In such a case, the heat spreader is configured to contact a heat exchanger, e.g. a heat sink. 
         [0031]    The system card  1  further comprises an optical coupling device  20 . 
         [0032]    The optical coupling device  20  is an integral piece made from a translucent material, and adapted to optically couple light between the waveguides  10   a ,  10   b  and the opto-electrical components  15   a ,  15   b.    
         [0033]    The optical coupling device  20  is precisely positioned with respect both to the optical waveguides  10   a ,  10   b  and to the opto-electrical components  15   a ,  15   b . In particular, the position of the optical coupling device  20 , along the direction Z with respect to the optical waveguides  10   a ,  10   b  is precisely defined. For example, a Z-reference  21  of the optical coupling device  20  will cooperate with a Z-reference layer  22  of the system card  1 , the position of which is precisely known with respect to the optical waveguides  10   a ,  10   b . The Z reference layer  22  of the card  1  is for example the electrical layer  7   a , in direct contact with the cladding  11 . 
         [0034]    Further, the position of the optical coupling device  20  along the direction X with respect to the opto-electronic components  15   a ,  15   b , is precisely defined. For example, the optical coupling device  20  is placed, along direction X, taking into account reference marks performed in the system card  1  and exactly showing the location of the opto-electronic components  15   a ,  15   b  along the direction X. 
         [0035]    The position of the optical coupling device  20  with respect to both the optical waveguides  10   a ,  10   b  and the opto-electronic components  15   a ,  15   b  along direction Y is precisely defined, for example by precisely defining the position of the optical coupling device  20  with respect to the opto-electronic components using the same reference marks as for the positioning along the direction X. By construction, the positioning of the opto-electronic components  15   a ,  15   b  along direction Y with respect to the optical waveguides  10   a ,  10   b  needs to be very precise. Therefore, when the optical coupling device  20  will be aligned along direction Y with the opto-electronic components  15   a ,  15   b , it will necessarily be aligned along direction Y with the optical waveguides  10   a ,  10   b.    
         [0036]    When the optical coupling device  20  is located in the precise position, it is fixed in position to the body  17  for example by gluing or the like. 
         [0037]    The cavity  12  is sealed to prevent any ingress of matter. For example, the bottom of the cavity  12  is sealed by the heat sink  18 , and the top of the cavity is sealed by a protective cover  23 . 
         [0038]    The optical coupling device comprises a mirror  24  to reflect light between the opto-electrical components  15   a ,  15   b , which emit or receive light along direction Z, and the optical waveguides  10   a ,  10   b  which propagate light along direction X. For example, the mirror  24  is a planar mirror forming a 45 degrees angle with the X-Y plane. 
         [0039]    The optical coupling device  20  further comprises light-beam forming structures  25  to improve the coupling of the light between the optical waveguides  10   a ,  10   b  and the opto-electrical components  15 . Such light-beam forming structures may for example be suitable lenses. 
         [0040]      FIG. 3  now shows a second embodiment of the invention. Compared to the embodiment of  FIG. 2 , the embodiment of  FIG. 3  differs in that one does not use a heat-dissipating printed circuit board  14 . It does not either use a heat spreader. In the following embodiments, as well as in the above embodiment, the heat spreader may be optional. Thus, the opto-electrical components  15   a ,  15   b  are directly fixed to the heat sink  18 . The opto-electrical components  15   a ,  15   b  are connected electrically to an electrical track of the system card  1 , such as the bottom card  7   b , by wire bonding. The electronic control components  16  are therefore provided directly on the body of the system card and electrically connected to the electrical layer  7   b  by any suitable way. 
         [0041]      FIG. 4  now shows a third embodiment. Compared to the embodiment of  FIG. 3 , the main difference lies in the geometry of the optical coupling device  20 . There will be a longer optical path inside the optical coupling device, for example by providing a plurality of mirrors  24 ,  24   b ,  24   c  between the interfaces of the optical coupling device. The optical coupling device  20  may even project from the top face  1   a  of the board  1 , and light may even propagate inside the optical coupling device in a plane higher along direction Z, than the top face  1   a  of the card  1 . 
         [0042]      FIG. 5  now shows a fourth embodiment. Compared to the embodiment of  FIG. 3 , it differs in that the opto-electrical components  15   a ,  15   b  are electrically connected to the top electrical layer  7   a  of the board  1 . Two cavities  12   a ,  12   b  are provided in the body  17  of the card  1 . The cavity  12   a  receives the heat sink  18  which extends also at the level of the optical layer  9 , up to the top electrical layer  7   a . The second cavity  12   b  receives a portion of the optical coupling device  20 . Light emitted from a laser  15   a  along direction Z upward is reflected by a first mirror  24   a  to propagate inside the coupling device  20  along direction X toward the left until it reaches a second mirror  24   b  where it is reflected along direction Z downwards, until a third mirror  24   c , which extends in the second cavity  12   b , directs light leftward again, along direction X, to the suitable optical waveguide  10   b . In the present embodiment, light will propagate on the left hand side part of the optical layer  9 . Light directed from a waveguide  10   a  toward a photo-diode  15   b  will follow the reverse path. 
         [0043]      FIG. 6  now shows a fifth embodiment of the invention, which is described below in relation to  FIG. 5 . In this embodiment, there is an optical coupling system which comprises both an optical coupling device  20  and a separate mirror mount  26  fixed to the body of the card. Also, in this embodiment, there is no insulating layer  8   c  between the optical layer  9  and the bottom electrical layer  7   b  so that this layer  7   b  can act as a reference layer, the position of which is precisely known along direction Z with respect to the optical waveguides, to precisely position the mirror mount  26  along direction Z with respect to the waveguide. In this embodiment, the optical coupling device  20  comprises the top part of the optical coupling device  20  of the embodiment of  FIG. 5 , but not the third mirror  24   c . The third mirror  24   c  is now integral with the mirror mount  26 . The light beam forming structures  25  are provided where light enters in or exits from the optical coupling device  20 , and dimensioned accordingly. 
         [0044]      FIG. 7  now shows a sixth embodiment. In this embodiment, the optical coupling device  20  is similar to the one of the embodiment of  FIG. 5 , however with some differences which are detailed below. 
         [0045]    Compared to the above embodiments, where the heat sink  18  is provided from the bottom side  1   b  of the card, the heat sink  18  is provided, in the present embodiment, from the top side  1   a  of the card. The heat transfer region 
         [0046]    thus oriented toward the top face  1   a , i.e. upside down compared to the previous embodiments. Contrarily to the previous embodiments, the opto-electrical components  15   a ,  15   b  are mounted on the same face of the heat sink  18  as the face which comprises the heat transfer region  19 . Hence, the opto-electrical components  15   a ,  15   b  are located around this region  19 . As a consequence, they may be spaced apart from one another more than in the previous embodiments. To cope with this additional spacing, the mirror  24   a  may be split into separate mirror portion  24   a   1 ,  24   a   2 , which are connected together by a flat horizontal portion  24   a   3  extending in the X-Y plane (or a less inclined region). Providing such offset mirrors enables to cope with the difference of spacing between the spacing of the opto-electrical components  15   a ,  15   b  and the spacing of the waveguides  10   a ,  10   b  along direction Z. 
         [0047]      FIG. 8  now shows a seventh embodiment of the invention. This embodiment will be described here in relation to the embodiment of  FIG. 3 . Compared to the embodiment of  FIG. 3 , the optical layer  9  comprises only one row of optical waveguides  10 . The system is provided as being symmetrical with respect to a mid-plane P parallel to the Y-Z axis, whereby, to the right of the plane P, light will be coupled between the waveguide  10  and the opto-electrical component  15  in a way similar to  FIG. 3 . On the left of plane P, light will be coupled between an optical waveguide  10  and an opto-electrical component  15  in a symmetrical way. In such way, both sides (left-hand side and right-hand side) of the optical layer  9  are used, compared to the above embodiments where only one side of the optical layer is used. 
         [0048]    It should be noted that, in the context of the present invention, it is possible to replace the heat sink of the embodiments depicted in  FIGS. 2 to 8  by a heat spreader. 
         [0049]      FIG. 9  shows an example of a heat sink  18  suitable for any of the above embodiments. It is a rigid component which can be assembled to the body of the card. The heat sink is for example provided in a heat-dissipating material, such as aluminium. Thermal conductivity of at least 150 W/m.K may be appropriate for such a device. If the heat sink is provided in an electrically conductive material, care will be taken to insulate the heat sink from the electrical tracks  7  of the card  1 . Preferably, the heat sink will be assembled to insulative material  8  of the card  1 . Assembly can be provided using suitable heat-dissipative glue, for example. In the embodiment of  FIG. 9 , the heat sink  18  is provided with a base plate  27  from which the heat transfer region  19  extends. The heat transfer region  19  is for example provided as a plurality of straight pins which extend along the direction Z. As shown in the variant embodiment of  FIG. 10 , the pins  28  are replace by plates  29  which extend in the Y-Z plane. This embodiment may be useful when the coolant flows preferably along the direction Y in the device. 
         [0050]    Within the frame of the invention, some embodiments may be obtained by combining some of the features of different above-described embodiments, when applicable.