Patent Publication Number: US-2016238789-A1

Title: Compact optical fiber splitters

Description:
RELATED APPLICATIONS 
     The present application is a continuation in part of U.S. patent application Ser. No. 13/851,178, filed Mar. 27, 2013, and published as US patent publication 2014/0294339, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to optical components, and particularly to compact optical fiber splitters. 
     BACKGROUND 
     US Patent Application Publication 2010/0158446 to Ohta describes an optical path turning device. 
     US Patent Application Publication 2004/0114866 to Hiramatsu describes an optical path changing connector. 
     SUMMARY 
     An embodiment of the present invention described herein provides an optical interconnect to direct optical signals between first and second ferrules of optical fibers, comprising a substrate, a first optical interface, configured to connect to the first ferrule of optical fibers, located on a first face of the substrate, a second optical interface, configured to connect to the second ferrule of optical fibers, located on a second face of the substrate, a plurality of optical waveguides, which are formed in the substrate and are configured to convey respective optical signals between the first optical interface and the second optical interface, one or more first micro-lenses, which are disposed on respective first ends of the optical waveguides and are configured to couple the optical signals between the first ends and the first ferrule and one or more second micro-lenses, which are disposed on respective second ends of the optical waveguides and are configured to couple the optical signals between the second ends and the second ferrule. Optionally, each of the plurality of waveguides includes at least one horizontal bend and wherein the at least one horizontal bends of the plurality of waveguides are included in a single plane. 
     There is further provided an apparatus including one or more optical waveguides, one or more first micro-lenses, and one or more second micro-lenses. The one or more optical waveguides are formed in a substrate and are configured to convey respective optical signals between first ends and second ends of the optical waveguides. The one or more first micro-lenses are disposed on the respective first ends of the optical waveguides and are configured to couple the optical signals between the first ends and respective first optical elements. The one or more second micro-lenses are disposed on the respective second ends of the optical waveguides and are configured to couple the optical signals between the second ends and respective second optical elements. 
     In some embodiments, the first micro-lenses are disposed on a face of the substrate, and the first ends of the optical waveguides terminate at a predefined distance from the face of the substrate, opposite the first micro-lenses. In other embodiments, the first and second optical elements include at least one element type selected from a group of types consisting of optical fibers, optical detectors and optical emitters. In yet other embodiments, the apparatus also includes a mechanical fixture that fixes the first optical elements at a predefined distance from the respective first micro-lenses, so as to form an air gap between the first optical elements and the first micro-lenses. 
     In some embodiments, each optical waveguide includes a respective bending element that bends an optical signal in the optical waveguide between a first axis and a second axis. In other embodiments, the optical waveguides include first and second subsets of the optical waveguides, such that the first ends of the optical waveguides in the first subset are arranged in a first row, and the first ends of the optical waveguides in the second subset are arranged in a second row positioned above the first row. In yet other embodiments, the first and second subsets of the optical waveguides lie in first and second different parallel planes. 
     In some embodiments, the optical waveguides include a first subset of the optical waveguides whose second ends lie on a first face of the substrate, and a second subset of the optical waveguides whose second ends lie on a second face of the substrate, different from the first face. In other embodiments, the first face is parallel with the second face. In yet other embodiments, the first face is perpendicular to the second face. 
     There is additionally provided, in accordance with an embodiment of the present invention, an apparatus, which includes an optical interconnect, which includes a substrate, one or more optical waveguides, one or more first micro-lenses, one or more second micro-lenses, and first and second mechanical fixtures. The one or more optical waveguides are formed in a substrate and are configured to convey respective optical signals between first ends and second ends of the optical waveguides. The one or more first micro-lenses are disposed on the respective first ends of the optical waveguides and are configured to couple the optical signals between the first ends and respective first optical elements. The one or more second micro-lenses are disposed on the respective second ends of the optical waveguides and are configured to couple the optical signals between the second ends and respective second optical elements. The first and second mechanical fixtures are configured to fix the first and second optical elements against the first and second ends of the optical waveguides, respectively. 
     In some embodiments, the first optical elements include optical fibers, and the first mechanical fixture includes a ferrule that is configured to fix respective facets of the optical fibers to the respective first ends of the optical waveguides. In other embodiments, the first mechanical fixture is configured to fix the first optical elements at a predefined distance from the respective first micro-lenses, so as to form an air gap between the first optical elements and the first micro-lenses. 
     There is additionally provided, in accordance with an embodiment of the present invention, a method including forming one or more optical waveguides in a substrate, for conveying respective optical signals between first ends and second ends of the optical waveguides. One or more first micro-lenses are disposed on the respective first ends of the optical waveguides, for coupling the optical signals between the first ends and respective first optical elements. One or more second micro-lenses are disposed on the respective second ends of the optical waveguides, for coupling the optical signals between the second ends and respective second optical elements. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an optical interconnect with integrated micro-lenses, in accordance with an embodiment of the present invention; 
         FIGS. 2A-2D  are diagrams showing different reflector configurations for a bend in an optical waveguide, in accordance with embodiments of the present invention; 
         FIGS. 3A and 3B  are diagrams showing a T-shaped optical splitter module, in accordance with an embodiment of the present invention; 
         FIGS. 4A and 4B  are diagrams showing an L-shaped optical splitter module, in accordance with an embodiment of the present invention; 
         FIGS. 5A-5C  are diagrams showing three optical splitter module configurations, in accordance with embodiments of the present invention; and 
         FIG. 6  is a diagram showing an optical interconnect with a light monitor, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Many optical systems use optical fibers to couple light between different optical elements. In some systems, individual fibers are held in a bundle, or optical cable, and need to be separated to route the fibers to optical elements at different locations in the system. For example, an optical fiber splitter module may be used to physically separate and bend the individual fibers in the fiber cable into two or more different output fiber cables in a very small module volume, in order to route the fibers to their destination in the system. As the number of fibers in the bundle increase, the size of the splitter increases accordingly to accommodate the density of fibers. This constraint limits reducing the size of the splitter, whereas space constraints are sometimes critical and require modules with small form factors. 
     Embodiments of the present invention described herein provide highly compact optical interconnects, and methods for fabricating such interconnects as building blocks for optical splitter modules. In the disclosed embodiments, optical signals in one or more optical fibers are coupled through an optical interface with integrated micro-lenses to an array of compact waveguides formed in a substrate. The micro-lenses are configured to focus or collimate the light efficiently between the individual fiber and respective waveguides in the substrate. 
     Once the light is coupled into the waveguide array in the substrate, the separate waveguides are bent horizontally or vertically in the substrate. A portion of the waveguides in the array can be split and routed to any desired face of the substrate. The waveguides can be routed in a very small form factor to another micro-lens interface at the other end of the waveguide array. 
     One or more micro-lens based optical interfaces can be located on any face of the substrate to couple light between a first cable to any number of fiber in one or more cables using a multi-level waveguide substrate. Thus in additional embodiments described herein, a variety low-loss optical splitter module configurations and topologies with highly compact form factors, are shown based on the micro-lens based optical interconnect building block. 
     As mentioned above, in some embodiments of the invention, a plurality of the waveguides are bent horizontally, such that the bend areas of the plurality of waveguides are included in a single plane. In some embodiments, the waveguides are entirely planar within the substrate. In these embodiments, interfaces to the waveguides in the substrate on opposite sides of the waveguides are also included in the same plane as the waveguides. In other embodiments, the waveguides are both horizontally and vertically bent within the substrate, allowing the interfaces to the waveguides to be located in different planes. 
     In some embodiments, all the waveguides connecting a pair of interfaces have the same length within the plane of the bend and extend parallel each other. Optionally, each of the waveguides includes at least two bends, which are arranged such that the waveguides are parallel each other when connecting to the interfaces. 
     In other embodiments, the waveguides connecting a pair of interfaces have different lengths according to their relative positions at the bend location. Generally, the waveguide in an inner part of the bend is shorter than the waveguide at the outer part of the bend. 
     The use of horizontal bends allows more flexibility in the arrangement of the waveguides and can achieve a more compact fiber splitter. 
     System Description 
       FIG. 1  is a diagram showing an optical interconnect  10  with integrated micro-lenses  13 , in accordance with an embodiment of the present invention. Optical interconnect  10  comprises a lateral array of eight optical waveguides  20  formed in a substrate  15  which terminate with eight respective integrated micro-lenses  13  on either side. 
     Waveguides  20  have an orientation vector that is perpendicular to the cross-section of waveguides  20 . An optical interface  30  is formed on a face  35  of substrate  15  as shown in the dashed region in  FIG. 1 . A similar optical interface is also formed on a face  40  of substrate  15  (not shown in  FIG. 1 ). Light is coupled in waveguides  20  between the two respective optical interfaces on face  35  and face  40  through bends  45  in optical waveguides  20 . The orientation vector of waveguides  20  on face  35  is directed parallel to the Y-axis in the XY plane in  FIG. 1 . The orientation vector rotates to become parallel to the X-axis in the XY plane in bend  45  as waveguides  20  connect into the optical interface on face  40 . 
     As shown in the first inset for optical interface  30 , waveguides  20  terminate on a waveguide end  50  positioned at a distance D from a trench face  47  formed in face  35  of the substrate  15 . The trench causes the light to traverse a distance of D through the substrate material, between ends  50  of waveguides  20  and trench face  47 . An optical material is disposed on trench face  47  at faces  35  and  40  at a distance D from end  50  of each waveguide  20  to form an array of integrated micro-lenses  13 . Each micro-lens  13  in the array is configured to focus a light ray  52  diverging from waveguide end  50  onto an optical element, such an optical fiber facet  55  of an optical fiber  60 , as shown both in the first and second insets of  FIG. 1 . Facet  55  is positioned at a distance G from trench face  47 . 
     In some embodiments, substrate  15  may comprise one or more layers of optical materials, such as polycarbonate (PC), polystyrene (PS), silica, and poly-methyl methacrylate (PMMA). Waveguides are formed by etching grooves in the one or more layers, filling the etched grooves with a second optical material with a higher index of refraction than that of the one or more layers, and bonding the one or more layers together to form substrate  15 . In the exemplary configuration of eight parallel waveguides  20  shown in  FIG. 1 , to form an array of waveguides  20  with two layers of optical material, the filled groove layers on the top layer are aligned and bonded to mirroring filled grooves on the bottom layer to form substrate  15 . 
     Any suitable cross-sectional shape of waveguide  20  can be created in this manner, which depends on the etching process that determines the shape of the etched grooves. Moreover, one or more layers of stacked waveguides can be formed in this manner as will be shown further below. In other embodiments, the waveguides can be directly formed with conventional lithography processes used, for example, in silicon complementary metal oxide semiconductor (CMOS) processes or processes to form Si Micromechanical Systems (MEMS) devices. 
     Micro-lenses  13  can be formed opposite waveguide ends  50  on trench face  47  using fabrication techniques, such as injection molding of polyetherimide (PEI), or other techniques that are known in the art for disposing suitable material on trench face  47  to form micro-lenses  13 . In some embodiments, micro-lenses  13  are designed to filter light in addition to focusing the light. For example, micro-lenses  13  may be produced by a material which filters specific wavelengths and/or micro-lenses  13  are tinted to attenuate the light intensity. Alternatively or additionally, an additional glass window is positioned in front of the micro-lenses  13  to perform the filtering. 
     Optical fibers  60  are typically held in micro-channels formed in a ferrule  70 , which is shown in a dotted outline in the first inset of  FIG. 1 . Optical fibers  60  are typically multi-mode fibers with diameters as follows: a core diameter ranging from 50-62.5 μm, a surrounding cladding layer diameter of 125 μm, and an outer mechanical coil diameter ranging from 250-900 μm. These sizes, however, are given purely by way of example, and any other suitable sizes can be used. 
     In some embodiments, guide pins  80  are formed on face  35  and face  40  to hold ferrule  70  at each face by inserting guide pins  80  into guide pin channels  85  formed in the body of ferrule  70 . An array of fiber facets  55  can then be placed precisely at a gap distance G from an array of respective micro-lens  13 , i.e., so as to form an air gap of width G between the fiber ends and the corresponding micro-lenses. 
     Typically an optical fiber cable is connected and supported at a first end of the ferrule. The individual fibers from the cable are thread through and held in separate micro-channels formed in the body of the ferrule. At a second (opposite) end of the ferrule, the ends of the fibers coupling light into optical interface  30  are typically cleaved or polished fiber facets  55  that are aligned with the edge of ferrule  70  as shown in the first inset of  FIG. 1 . 
     Examples of ferrules are MT Ferrules produced by Connected Fibers, Inc. (Roswell, Georgia). A datasheet of such MT ferrules, entitled “MT ferrules,” January, 2009, is incorporated herein by reference. International Electrotechnical Commission (IEC) document number IEC61754-5, entitled “Fiber Optic Connector Interfaces—Part 5: Type MT Connector Ferrules,” January, 1996, which is incorporated herein for reference, specifies such ferrule designs. Fiber facets  55  align with the second end of ferrule  70  which connects to the module. The arrangement of the fiber facets at the edge of the ferrules can have different footprints. For example, a ferrule holding twenty-four fibers can be arranged in a footprint of two rows of twelve fibers separated by a predefined distance. 
     In the embodiment of  FIG. 1 , eight waveguides connect the interface trench faces  47  on faces  35  and  40  of substrate  15 . The most inner waveguide is shortest and each following waveguide is longer than the previous waveguide. It is noted that the rightmost waveguide on face  35  corresponds to the leftmost waveguide on face  40  and vice versa. While eight waveguides are shown in substrate  15  connecting the interface trench faces  47  on faces  35  and  40 , substrates in accordance with embodiments of the present invention may include other numbers of waveguides, including more than 10 or even more than 20 waveguides. 
     The coupling efficiency of light between waveguide  20  and fiber  60  at both faces  35  and  40  of interconnect  10  may be optimized empirically, or by simulation. This optimization is done by varying parameters, such as gap distance G, distance D, the shape of micro-lens  13 , the shape of waveguide bends  45 , and any other suitable geometrical or material parameter in optical interconnect  10 . Stated differently, light exiting the waveguide should be matched into the fiber core with a given numerical aperture by varying the above parameters so as to focus the light diverging from waveguide end  50  onto fiber facet  55 . The precision positioning of fiber facets  55  relative to trench face  47  using ferrule guide pins  85  is also an important parameter affecting the overall optical loss in interconnect  10 . 
       FIGS. 2A-2D  are diagrams showing different reflector configurations for optical waveguide bend  45 , in accordance with embodiments of the present invention. The configuration of waveguide bend  45  is also important for minimizing optical losses of light ray  52  in bend  45  propagating in waveguide  20 , which in turn is an important parameter in the design of optical interconnect  10 . 
       FIG. 2A  shows a 45-degree straight optical reflector  100  in waveguide  20 .  FIG. 2B  shows a 90-degree curved concave optical reflector  110  in waveguide  20 .  FIG. 2C  shows a 90-degree straight optical reflector  120  in waveguide  20 .  FIG. 2D  shows a 90-degree parabolic optical reflector  130  in waveguide  20 . The bending reflector configurations shown in  FIGS. 2A-2D  can be formed, for example, by etching, plating a reflecting layer, by placing a reflecting device in the etched groove of the waveguide, or by any suitable fabrication method for forming the reflector. 
     The optical reflector configurations shown in  FIGS. 2A-2D  are by way of example and not by way of limitation of the embodiments of the present invention. Any such configuration can be used for implementing bend  45  in  FIG. 1 , or any of the other waveguide bends shown in the figures below. For example, any suitable bend angle or reflector configuration can be used to rotate the orientation vector of the waveguide from a first axis to a second axis. 
     Compact Optical Splitter and Routing Modules 
     A variety of optical splitter module configurations can be fabricated using optical interface  30  as a basic building block as shown in  FIG. 1 . Optical interconnect  10  shown in  FIG. 1  comprises one row of waveguides connecting optical interface  30  on face  35  of substrate  15  oriented along the Y-axis that is bent to the same row of waveguides oriented along the X-axis which connects into the optical interface on face  40 . 
     To realize an optical splitter module, two or more arrays of waveguides  20  shown in optical interconnect  30  may be stacked vertically. Optionally, each array of waveguides is located within a respective single plane. The input optical interface may comprise one or more rows of waveguides in the XY plane but stacked vertically at different heights along the Z-axis as shown in  FIG. 1 . Similarly, respective rows of micro-lens arrays may be disposed on trench face  47  at the input optical interface in the same registration as the vertically stacked waveguides so as to couple light from fibers through the micro-lenses into the stacked waveguides at the substrate edge. Each array of waveguides formed in the stacked level at a respective height along the Z-axis can then be routed away from the input optical interface to a different face of substrate  15  so as to form the core of the optical splitter module. 
     The exemplary embodiments of the optical splitters shown in the following figures below have a first array of micro-lenses (e.g., micro-lenses arranged in two rows) on a first edge of substrate  15  that are configured to couple light into a first and a second level of vertically stacked waveguides in the substrate. The first level of the vertically stacked waveguides is routed to a second array of respective micro-lenses formed on a second edge of the substrate, and the second level is routed to a third array of respective micro-lenses on the third edge of substrate  15 . 
     The exemplary embodiments shown herein with eight or sixteen fibers  60  carrying light which are coupled between respective waveguides in substrate  15  through an arrays of micro-lenses  13  are shown merely for conceptual clarity and not by way of limitation whatsoever of the embodiments of the present invention. Typically, any number of fibers M*N arranged with N fibers in M rows, where M and N are integers, may be used so long as the footprint of the ferrule is configured to support the M*N fibers. 
     Moreover, light carried in the M*N fibers arranged in N rows and held in the ferrule should be coupled precisely to a corresponding waveguide array comprising N levels of M waveguides using a suitable multilayered substrate as described previously. However, in accordance with the embodiments of the present invention described herein, any number of fibers may be held in any suitable housing and is not limited to ferrules, which are coupled to any number of waveguides in any arrangement through respective micro-lenses. 
       FIGS. 3A and 3B  are diagrams showing a T-shaped optical splitter module  168 , in accordance with an embodiment of the present invention.  FIG. 3A  shows substrate  15  where a cavity  140  is formed on a front face  155  of substrate  15 . Similarly, two cavities  150  and  151  are formed in a right face  152  and a left face  158 , respectively, of substrate  15 . 
       FIG. 3B  shows T-shaped optical splitter module  168  into which substrate  15  is placed. Cavity  140  is configured to support a multi-fiber ferrule  165  holding sixteen optical fibers that are arranged with a footprint of two rows of eight fibers within the ferrule housing supported by two ferrule guide pins  80 . T-shape optical splitter module  168  shown in  FIG. 3A  and  FIG. 3B  splits sixteen fibers held in ferrule  165  on face  155  perpendicular to the X axis into two bundles of eight fibers each, held in two respective ferrules  70 . The ferrules are mounted on opposite faces  152  and  158  of substrate  15 , both perpendicular to the Y-axis. 
     Individual fibers  60  are held in optical cables. A first optical cable  160  is configured to connect to the footprint of ferrule  165 . Cavities  150  and  151  are configured to support multi-fiber ferrule  70  holding eight optical fibers with a footprint of one row of eight fibers within the ferrule housing. A second optical cable  170  is configured to connect to the footprint of ferrule  70  in cavity  150  at face  152  and in cavity  151  at face  158  of substrate  15 . 
     In some embodiments, T-shaped optical splitter module  168  shown in  FIG. 3B  is formed directly from substrate  15  with support pins  80 , support cavities  140 ,  150  and  151  for the three ferrules (as shown in the example of  FIG. 3A ). In other embodiments, module  168  can be fabricated by forming cavities  140 ,  150  and  151  in any suitable housing material where substrate  15  is placed inside the housing. The ferrules are then supported both with guide pins  80  in substrate  15  and cavities  140 ,  150  and  151  formed in the housing material. 
     In  FIG. 3A , substrate  15  routes the light from sixteen fibers from ferrule  160  with a footprint of two rows of eight fibers in eight respective waveguides  20  parallel to the X-axis to two ferrules  70  each with a footprint of one row of eight fibers. The two rows of eight fibers are separated in ferrule  165  by a predefined distance. Two levels of waveguide  20  leaving the ferrule with a footprint of two rows of eight waveguides in cavity  140  are physically at different heights separated in the Z-direction by the predefined distance of two rows of the fiber facets in the footprint of ferrule  165 , which is incorporated into the design and fabrication of substrate  15 . As a result, the one row of eight micro-lenses  13  in cavity  150  on face  152  and the one row of eight micro-lenses  13  in cavity  151  on face  158  are offset by the same predefined distance along the Z-axis. 
       FIGS. 4A and 4B  are diagrams showing an L-shaped optical splitter module  190 , in accordance with another embodiment of the present invention.  FIG. 4A  shows substrate  15  where a cavity  182  is formed on a top face  185  of substrate  15  in order to support ferrule  165  with two rows of eight fibers. Similarly, two cavities  183  and  184  are formed in a left face  180  of substrate  15  to support ferrule  70  with one row of eight fibers. 
     For the embodiment shown in  FIG. 4A , two levels of eight waveguides  20  are formed in substrate  15 , which is configured to split the light in the two rows of sixteen fibers in ferrule  165  inserted into cavity  182  to two ferrules  70  with one row of eight fibers inserted in cavities  183  and  184 . In each cavity  182 ,  183  and  184 , guide pins  80  are formed in the substrate and inserted into guide pin channels  85  formed in the body of the ferrule so as to position the end of the fiber facets  55  precisely with a gap G distance from a respective array of micro-lenses  13 . 
     Most of the length of waveguides  20  extends within a single plane. In this single plane, each waveguide has two bend points, one close to cavity  182  and the other close to cavity  184  or cavity  183 . In this embodiment, all the waveguides in a first group, connecting cavities  182  and  183  have the same length and extend in parallel. Optionally, also all the waveguides in a second group, connecting cavities  182  and  183  have the same length. Possibly, although not necessarily, the waveguides of both the first and second groups have the same length. It is noted that close to cavity  182 , waveguides  20  have a vertical bend which leads the waveguides out of the single plane to cavity  182 . 
       FIG. 4B  shows L-shaped optical splitter module  190  where cavity  182  is configured to support multi-fiber ferrule  165  holding sixteen optical fibers that are arranged in a footprint of two rows of eight fibers. Individual fibers  60  are held in the optical cables. First optical cable  160  is configured to connect to the footprint of ferrule  165 . Cavities  183  and  184  are configured to support multi-fiber ferrule  70  holding eight optical fibers that are arranged in a footprint of one row of eight fibers within the ferrule housing supported by two ferrule guide pins  80 . Two optical cables  170  are configured to connect to the footprint of the two respective ferrules  70  at left face  180  supported in cavities  183  and  184  formed in substrate  15 . 
     In some embodiments, L-shaped optical splitter module  190  as shown in  FIG. 4B  is formed directly from substrate  15  with support pins  80 , and support cavities  182 ,  183 , and  184  for the ferrules. In other embodiments, module  190  can be fabricated by forming cavities  182 ,  183 , and  184  in any suitable housing material where substrate  15  is placed inside the housing. The ferrules are then supported both with guide pins  80  in substrate  15  and cavities  182 ,  183 , and  184 , which are formed in the housing material. 
     For optical fiber splitter module  190  shown in  FIGS. 4A and 4B , two rows of eight micro-lenses  13  focus the light from the two-row footprint of eight fibers in ferrule  165  inserted into cavity  182  to waveguides  20  in substrate  15 . However, the light is directed vertically in a vertical portion  187  of waveguide  20  parallel to the Z-axis in  FIG. 4A . The vertical portion  187  of waveguide  20  has a vertical orientation vector which is then rotated horizontally into the XY plane by a vertical to horizontal bend, or any suitable transition. 
     Vertical waveguide  187  can be formed by filled waveguides in substrate  15  by etching vertical vias in the substrate material layers that are filled with an suitable optical material with a higher index of refraction than the substrate material. Similarly the vertical to horizontal waveguide bend which rotates vertical portion  187  of waveguide  20  into waveguides  20  oriented in the X-Y plane can be formed using any of the reflectors shown in the embodiments of  FIGS. 2A-2D , or any suitable vertical to horizontal transition. 
       FIGS. 5A-5C  are diagrams showing three optical splitter module configurations, in accordance with embodiments of the present invention.  FIG. 5A  shows a planar optical splitter configuration where three ferrules are mounted on two XY planes located at different X-positions.  FIGS. 5B and 5C  show optical splitter configurations in which all three ferrule connectors are on the same top face of the splitter module with the ferrules at different orientations on the top face.  FIGS. 5A-5C  show only substrate  15  and the routing of internal waveguides  20  to illustrate the different module configurations. 
     In the first embodiment shown in  FIG. 5A , a planar optical splitter configuration is shown where the optical ferrule connectors are oriented perpendicular to the X axis. On a front face  200  of substrate  15 , a cavity  202  is formed to support multi-fiber ferrule  165  holding sixteen optical fibers that are arranged in two rows of eight fibers within the ferrule housing supported by two ferrule guide pins  80  also formed in the substrate. Similarly on a back face  210 , two cavities  212  and  214  are formed in substrate  15  perpendicularly to the X-axis to accommodate two ferrules  70 , each with one row of eight fibers. 
     Optionally, in the embodiment of  FIG. 5A , each waveguide  20  is included entirely in a single plane. The waveguides connecting cavity  202  to cavity  214  are located in a first plane, while the waveguides connecting cavity  202  to cavity  212  are in a second plane. 
     In the second embodiment shown in  FIG. 5B , an optical splitter configuration is shown where the optical ferrule connectors are on a same top face  220  oriented perpendicularly to the Z axis. A cavity  222  is formed to support multi-fiber ferrule  165  holding sixteen optical fibers that are arranged in two rows of eight fibers within the ferrule housing supported by two ferrule guide pins  80  also formed in the substrate. Similarly, two cavities  224  and  226  are formed in substrate  15  to accommodate two ferrules  70 , each with one row of eight fibers. However, the two rows on micro-lenses  13  in cavity  222  are oriented parallel to the X-axis, and the one row of micro-lenses  13  in cavities  224  and  226  are oriented parallel to the Y axis as shown in  FIG. 5B . 
     In the embodiment of  FIG. 5B , the waveguides include both vertical bends leading to the cavities  222 ,  224  and  226  and horizontal bends to match the different orientation of cavities  224  and  226  relative to cavity  222 . While the horizontal bends may be in two different planes, e.g., one for the waveguides leading to cavity  224  and one for the waveguides leading to cavity  226 , in some embodiments all the horizontal bends are in a single plane. 
     Finally in the third embodiment shown in  FIG. 5C , an optical splitter configuration is shown where the cavities to support the optical ferrule connectors are on a same top face  230  oriented perpendicularly to the Z axis. A cavity  235  is formed to support multi-fiber ferrule  165  holding sixteen optical fibers that are arranged in two rows of eight fibers within the ferrule housing supported by two ferrule guide pins  80  also formed in the substrate. Similarly, two cavities  237  and  240  are formed in substrate  15  to accommodate two ferrules  70 , each with one row of eight fibers. However, the two rows on micro-lenses  13  in cavity  222  are oriented parallel to the X-axis, and the one row of micro-lenses  13  in cavities  224  and  226  are also oriented parallel to the X axis as shown in  FIG. 5C . 
     In the embodiment of  FIG. 5C , the waveguides only have vertical bends. 
     For the embodiments shown in  FIGS. 5B and 5C , both vertical  187  waveguides (orientation vector parallel to the Z-axis) and horizontal waveguides (orientation vector in the X-Y plane) are formed in substrate  15  by the methods previous described in order to realize the embodiments shown here. 
     In the exemplary embodiments shown in the foregoing figures, optical interface  30  and the ferrules are oriented along Cartesian axes. This orientation is shown merely for the sake of visual clarity and not by way of limitation of the embodiments of the present invention. Optical interface  30  may alternatively be configured at any suitable angle relative to substrate  15 . The optical interface may be configured to couple light between a waveguide in the substrate and a fiber in the ferrule at any desired angle, for example, by cutting substrate  15  at an appropriate bevel angle, forming micro-lenses  13  on the beveled edge of the substrate, and positioning the ferrule on the bevel. Any suitable fabrication technique and materials may be used to form a beveled optical interface so as to support a ferrule mounted at any suitable angle relative to the body of the optical fiber splitter module. 
     Optical interface  30  as described in the embodiments of the present invention is not limited to optical fiber splitter modules. The light in the array of waveguides  20  may be directed by optical interface  30  in interconnect  10  into any suitable optical element in accordance with the embodiments of the present invention shown in  FIG. 1  as previously described. Interface  30  is designed where in place of the multi-fiber ferrule  70  shown in  FIG. 1 , any suitable optical element may be mounted at the same plane of fiber facets  55 . Thus by varying parameters D from the edge  50  of waveguide  20  and gap G from the micro-lens  13  to the optical element focuses the light diverging from edge  50  onto the optical element. 
     An exemplary configuration to illustrate this embodiment may comprise, for example, the light in sixteen fibers held in a ferrule  165  are coupled into two levels of waveguide  20  in substrate  15  and are split into two stacked levels of eight waveguides in substrate  15  as shown in  FIG. 3A . However, each level of eight waveguides arrives to two opposite faces of the substrate as in cavities  150  and  151 . However in place of ferrules in the respective cavities, an optoelectronic transducer array of eight Vertical Cavity Surface Emitting Lasers (VCSEL) devices on a first chip and an array of eight photodiode (PD) devices on a second chip may be mounted at gap distance G from micro-lenses  13 . Alternatively, any other suitable type of optical emitters and optical detectors can be used. 
     Similarly the waveguide may terminate at a distance D from the edge of the substrate faces where the PD and VCSEL array chips are mounted in substrate  15  (in place of ferrule  70  in cavities  150  and  151 ). The optical interface is designed in accordance with the embodiments of the present invention shown in  FIG. 1  as previously described. 
     It is noted that in addition to the waveguides  20 , substrate  15  may include additional optical elements, such as fiber splitters and an optical power monitor. 
       FIG. 6  is a schematic illustration of an optical interconnect  300 , in accordance with an embodiment of the invention. In addition to waveguides arranged in a manner similar to those in optical interconnect  10  of  FIG. 1 , interconnect  300  comprises a light splitter  302  on an outer waveguide. Light splitter  302  leads a portion of the light passing in the outer waveguide into a side waveguide  304 , which leads to a light monitor  306 . Light splitter  302  may be of any type known in the, for example as described in US patent publication 2011/0150390 to Meyer et al., titled “In-Plane Optical Wave Guide with Area Based Splitter”, which is incorporated herein by reference. 
     Although only a single splitter  302  is shown, the optical interconnect may include a plurality of splitters on a plurality of the waveguides, for example on more than 35% of the waveguides or even on all the waveguides. Side waveguide  304  optionally extends in a same plane as includes waveguides  20 . Alternatively, side waveguide  304  connects splitter  302  to a different plane than includes waveguides  20 . 
     Although the embodiments described herein mainly address a low loss optical interface for coupling light in fibers between fiber bundles in input/output ferrules in an optical splitter module, the optical interface described herein can also be used in other applications, for precision coupling light in a fiber to any suitable optical element through a micro-lens array. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.