Abstract:
An electro-optical interconnection platform is provided. The platform includes an interface medium; a plurality of optical pads; a plurality of electrical pads; and at least one beam coupler adapted to optically couple at least one pair of optical pads of the plurality of optical pads, wherein the at least one pair of optical pads are placed on opposite sides of the interface medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application No. 62/405,476 filed Oct. 7, 2016. This application is also a continuation-in-part of U.S. patent application Ser. No. 14/878,591 filed on Oct. 8, 2015, now pending, the contents of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to coupling an optical fiber to a substrate, and more particularly to coupling the optical components to an optoelectronic integrated circuit (IC). 
       BACKGROUND 
       [0003]    Communications systems and datacenters are required to handle massive amount of data at ever increasing speeds and ever decreasing costs. To meet these demands, optical fibers and optical ICs (such as, a photonic integrated circuit (PIC) or an integrated optical circuit) are used together with high speed electronic ICs. A PIC is a device that integrates multiple photonic functions (similar to an electronic IC or radio frequency (RF) IC). PICs are typically fabricated using indium phosphide or silicon oxide (SiO 2 ), which allows for the integration of various optically active and passive functions on the same circuit. 
         [0004]    The coupling of PICs to optical fibers is not as well advanced as the integration and/or coupling of electronic ICs. Specifically, the challenges facing optical connections are different and much more complex than connecting electronic ICs to, for example, a printed circuit board (PCB). Some difficulties are inherent in wavelength, signal losses, assembly tolerance, and polarization characteristics of optical packaging. 
         [0005]    Existing solutions utilize various techniques for connecting optical fibers to PICs. One technique suggests using various types of butt connections to the edge and surface fiber connections a PIC. The butt of a fiber can be connected to a planar waveguide at the edge of a PIC. This technique is efficient only if the cross section of the propagating mode of the fiber and the waveguide areas of the fiber core and the waveguide are of similar size. In most cases, this technique suffers from poor assembly tolerance. 
         [0006]    Another technique suggests laying a section of fiber on top of the surface of the PIC where the end of the fiber has been cut at an angle to form an angled tip. The angled tip has a flat surface which reflects a light beam down to a waveguide grating coupler disposed on the integrated circuit. The light beam is reflected off the reflective surface of the angled tip by total internal reflection. The waveguide grating coupler is designed to accept the slightly diverging light beam from the reflective surface of the angled tip of the fiber. The light beam can also propagate through the fiber to a chip coupler in the opposite direction, up from the substrate, through the waveguide grating and into an optical fiber after bouncing off the reflective surface of the angled tip. This technique further requires coating the exterior of the reflective surface with an Epoxy. 
         [0007]    Among others, all of the above-noted techniques require precise alignment, and thus active positioning of the optical fiber to the PIC. As such, current techniques suffer from poor and very tight alignment tolerance to gain an efficient connectivity. For example, a misalignment between an optical fiber and a PIC of 1-2 microns (μm) would result in a signal loss of about −3 db. Furthermore, the alignment is now performed with expensive equipment or labor-intensive assembly solutions. As a result, mass production of PICs and/or optical couplers is not feasible. 
         [0008]    The lack of a reliable and efficient solution for optical coupling of PICs also limits the ability of mass production for an electrical-optical interconnection platform that allows multiple-chip connections via electrical and optical connections. 
         [0009]    As an example, one typical arrangement of electrical-optical interconnection platform is based on a silicon photonic interposer. Such an interposer may be utilized in a hybrid integration of photonics and electronics by face-to-face bonding (flip-chip assembly). In such arrangement the I/O fibers are connected to waveguides pre-designed in the silicon interposer. The silicon photonic interposer includes vias created through the silicon to provide the electrical path to/from the PCB substrate and the optoelectronic devices inside the silicon photonics die. The optical transmission is inside the silicon photonics die. 
         [0010]    Another arrangement discussed in the related art, is a multi-substrate electro-optical structure. The structure includes a substrate mounted over a supporting PCB. The substrate interconnections between optical chips, mounted on the silicon photonic interposer, and IC chips. 
         [0011]    The typical arrangements discussed above are based on a silicon photonic interposer. Such an interposer typically includes active and passive optical devices (e.g., photo-detectors, modulators, splitters, etc.). An “active” interposer is an expensive component manufactured using a complex process. Further, the optical source (e.g., laser light) is coupled to the silicon photonic interposer via an optical fiber and grating coupler, or via an edge-coupling or any other equivalent optical coupling. As noted above, such coupling techniques require precise alignment and active positioning of the optical source to the PIC or the interposer. 
         [0012]    Another solution for electrical-optical interconnection platform includes assembling of discrete electrical and optical device on a silicon photonics substrate. The electrical connection is achieved using electrical pins formed using through glass vias (TGV) while optical connections are formed using optical pins. The optical pins are structured using optical grating couplers. As noted above, such couplers do not demonstrate good tolerance for misalignment. As a result, a mass production of PICs and/or optical couplers of the various electrical-optical interconnection platform solutions discussed in the related art may not be feasible. 
         [0013]    It would therefore be advantageous to provide an efficient electrical-optical interconnection platform that would overcome the deficiencies of the existing solutions. 
       SUMMARY 
       [0014]    A summary of several example embodiments of the disclosure follows. This summary is provided for the convenience of the reader to provide a basic understanding of such embodiments and does not wholly define the breadth of the disclosure. This summary is not an extensive overview of all contemplated embodiments, and is intended to neither identify key or critical elements of all embodiments nor to delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later. For convenience, the term “some embodiments” may be used herein to refer to a single embodiment or multiple embodiments of the disclosure. 
         [0015]    Some embodiments disclosed herein include an electro-optical interconnection platform comprises an interface medium; a plurality of optical pads; a plurality of electrical pads; and at least one beam coupler adapted to optically couple at least one pair of optical pads of the plurality of optical pads, wherein the at least one pair of optical pads are placed on opposite sides of the interface medium. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The subject matter disclosed herein is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the disclosed embodiments will be apparent from the following detailed description taken in conjunction with the accompanying drawings. 
           [0017]      FIG. 1  is a side view of a fiber-to-chip optical coupler constructed according to one embodiment. 
           [0018]      FIG. 2  is a top view of the “fiber side” of the disclosed arrangement according to one embodiment. 
           [0019]      FIG. 3  is an exemplary and non-limiting top view of the “fiber side” of the disclosed arrangement according to another embodiment. 
           [0020]      FIG. 4  is a side view of a chip-to-fiber optical coupler utilized as a beam coupler arranged according to one embodiment. 
           [0021]      FIGS. 5A through 5D  illustrate a process for creating a fiber trench at a side view of the substrate layer according to an embodiment. 
           [0022]      FIG. 6  is a side view of an electro-optical interconnection platform arranged according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    It is important to note that the embodiments disclosed herein are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed embodiments. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in plural and vice versa with no loss of generality. In the drawings, like numerals refer to like parts through several views. 
         [0024]    By way of example to the various disclosed embodiments, a platform for electro-optical interconnection is provided. The arrangement of the disclosed platform allows efficient interconnection of, for example, different electronic integrated circuits (ICs) (e.g., electrical chips), photonic integrated circuits (PICs) (e.g., photonic chips), and other optical elements (e.g., fibers and lasers), while ensuring low signal losses and thermal stability. 
         [0025]    To this end, the platform includes a passive interposer and a set of optical pads. That is, the interposer does not include any active components or devices, but merely serves as an interface media. The optical pads allow optical connectivity through an optical coupler (or a photonic plug). The optical coupler is designed to provide high tolerance alignment through passive positioning or assembling. Therefore, the disclosed electro-optical interconnection platform allows components assembling with standard packaging machinery, and therefore can enable high volume packaging. In an embodiment, a complete system that utilizes the disclosed platform can be fabricated using a wafer fabrication process, for example, a CMOS fabrication process. Thus, the disclosed embodiments can solve a long unmet need for using wafer fabrication processes in the manufacturing of silicon photonics based systems. 
         [0026]      FIG. 1  shows a high-level schematic diagram of an electro-optical interconnection platform  100  constructed according to one embodiment. The platform  100  includes a plurality of optical pads (collectively shown and referred to as “o-pads”  110  and a plurality of electrical pads (collectively shown and referred to as “e-pads”  120  formed on an interface medium (not shown)). The e-pads  120  can be, for example, bumps, micro solder bumps, or any other type of metal pads to enable electrical connections. Electrical connections between e-pads  120  can be achieved through vias  160 , such as, but not limited to, through silicon vias (TSV), through glass vias (TGV), and the like. 
         [0027]    In an embodiment, each o-pad  110  is constructed as an optical element, such as curved (collimated) mirror, a titled flat mirror, or combination thereof. In an embodiment, the positioning and formation of the o-pads (curved mirror) is performed on the interface medium (not shown) using a photolithography process such as, but not limited to (e.g., grayscale lithography), a nanoimprint lithography process, and the like. In certain configurations, an o-pad  110  may also be constructed to include a tilted flat mirror and fiber trench (not shown in  FIG. 1 ). In such configurations, an o-pad  110  allows connection of an optical fiber. 
         [0028]    A beam coupler  170  can be formed between two opposite o-pads  110 . In an embodiment, the beam coupler  170  allows the optical coupling between an optical source and drain connected the respective o-pads  110 . The structure of the o-pads  110  and the optical coupling formed by the platform  100  is disclosed in great detail below. 
         [0029]    In an embodiment, the light beams travel through an interface medium (not shown). Such a medium is made of transparent and non-conductive material, such as glass, polydimethylsiloxane, or any other index matching materials. The dimensions of the interface medium determine, in part, the efficiency of the light beam (optical signal) that propagates through the interface medium. In one embodiment, the interface medium is realized as a passive interposer. 
         [0030]    It should be noted the geometric of the vias  160  and/or beam couplers  170  shown in  FIG. 1  is for illustration purposes only and should not limit the scope of the disclosed embodiments. 
         [0031]    It should be further noted the o-pads  110  and the e-pads  120  can be positioned at any location on the platform  100 . However, to form a beam coupler between two o-pads  110 , such o-pads should be opposite to each other when placed on the platform  100 . 
         [0032]    The e-pads  120  allow electrical connectivity to an electrical component  140 . An electrical component  140  may include, for example, a driver, a field programmable gate array (FPGA), an application-specific integrated circuit (ASIC), an application-specific standard products (ASSP), a system-on-a-chip system (SOC), a general-purpose microprocessor, a microcontroller, a GPU, a digital signal processors (DSP), and the like, or any other analog or digital circuit. Furthermore, one or more of the e-pads  120  may provide connectivity with a PCB. 
         [0033]    To o-pads  110  are connected or otherwise mounted optical components  150 . An optical component  150  may include, for example, a PIC, a light source (e.g., laser), an optical fiber, a photo-detector, an optical splitter, an optical combiner, an optical tap, or any other passive or active optical component. In an embodiment, a PIC is mounted on the platform  100  and connected to an o-pad  110  using a flip-chip mounting technique. This would provide the mounted PIC electrical and optical connectivity required for its operation. 
         [0034]    Thus, the platform  100  is designed to enable standard die stacking processes (e.g., pick-and-place assembly) of the electrical and optical components through connections to e-pads using a flip-chip mounting technique. As a result, an electro-optical system assembled on the disclosed platform can be manufactured, tested, and packaged using wafer level processes. Further, the disclosed platform can be utilized to provide chip-chip connectivity. For example, connections between two PICs or two ICs can be formed through the disclosed interconnection platform  100 . 
         [0035]    The design of the electro-optical interconnection platform  100  may be achieved by distribution of o-pads  110  and e-pads  120  throughout the platform  100 . The characteristics of the o-pads  110  and e-pads  120  may be selected from design libraries included in a CAD design tool. 
         [0036]      FIG. 2  is a side view of a portion of the electro-optical interconnection platform  100  illustrating a beam coupler  200  formed between o-pads  210 - 1  and  210 - 2  constructed according to one embodiment. The o-pads  210 - 1  and  210 - 2  are positioned on opposite sides of the interface medium  220 . The material of the medium  220  may be any transparent and non-conductive material, such as glass, polydimethylsiloxane or any other index matching materials. In an embodiment, the interface medium  220  may be of a combination of materials. As an example, the interface medium  220  may be structured as an upper substrate layer and a lower substrate layer and a glass or air layer in-between. The substrate layers may be made of silicon oxide (SiO 2 ), plastic, and the like. 
         [0037]    In certain configurations, the interface medium  200  may be adhered to a transparent glue layer (e.g., Epoxy). Such a layer may be placed between the interface medium  220  and the mirrors  210 - 1  and  210 - 2 . 
         [0038]    As noted above, the o-pads  210 - 1  and  210 - 2  are formed on the interface medium  220  using a photolithography, grayscale lithography, nanoimprint lithography process, and the like. Specifically, in an embodiment, the o-pads  210 - 1  and  210 - 2  are formed as curved, collimated mirrors. The o-pads (curved mirrors  210 - 1  and  210 - 2  are placed and created during fabrication, which ensures high accuracy positioning and accurate reflective mirrors. As a non-limiting example, the fabrication process utilized to create the mirrors may include a Silicon-On-Insulator (SOI), complementary metal-oxide semiconductor (CMOS), and the like. 
         [0039]    In an embodiment, the o-pads  210 - 1  and  210 - 2  are fabricated using the same or a substantially similar grayscale lithography process. In another embodiment, the o-pads  210 - 1  and  220 - 2  may be fabricated using a plastic molding technique. Such fabrication techniques ensure high accuracy of the mirrors and their assembly to form a beam coupler. 
         [0040]    Furthermore, by fabricating and placing the first and second curved mirrors  210 - 1  and  210 - 2  on the interface medium  220 , thereby allowing relaxed alignment tolerances in 3-dimenstions. For example, if the o-pad  210 - 2  provides a connection to a light drain (e.g., PIC, not shown) and the o-pad  220 - 1  is connected to a light source (not shown), even if the o-pad  220 - 2  is not perfectly aligned with the o-pad  220 - 1 , the optical signal is not significantly attenuated. It should be noted that in certain embodiments, the o-pads  210 - 1  and/or  210 - 2  can be also fabricated as a part of the interface medium  220 . 
         [0041]    The disclosed arrangement of the beam coupler  200  achieves high signal efficiency with a relaxed alignment between a light beam and/or drain due to the specific locations and shape of the curved mirrors (forming the o-pads) placed against each other. The locations of the o-pads  210 - 1  and  210 - 2  are determined at least with respect to the source/drain light beam. This allows the light beam to be reflected from the o-pads  210 - 1  and  220 - 2 . Specifically, the curved mirrors of the o-pads  210 - 1  and  210 - 2  are shaped in such a way that all light beams from the source are reflected and collimated at a certain angle at a center of one o-pad and focused to a drain after reflecting from the second o-pad. 
         [0042]    For example, as illustrated in  FIG. 2 , the o-pad  210 - 2  reflects a diverging light beam  280 - 1  from a light source (not shown) into parallel or substantially parallel light beams  280 - 2 . The light beams  280 - 2  reach the o-pad  210 - 1  which reflects a focused light beam  280 - 3  to a light drain (not show). The same optical path is true if the location of the source and drain are replaced. It should be noted that all light beams  280  travel through the interface medium  220 . 
         [0043]    It should be noted that a typical application of the electro-optical interconnection platform  100  would include a plurality of beam couplers formed through a plurality of o-pads. 
         [0044]    In certain embodiments, an o-pad can be constructed to further allow connectivity to an optical fiber (including, but not limited to, a single mode fiber). Such an o-pad may be integrated in an o-pad shaped as a curved mirror (e.g., o-pad  210 - 2 ) or serve as a stand-alone connection. 
         [0045]      FIG. 3  illustrates a portion of the electro-optical interconnection platform  100  including a beam coupler  300  formed using an o-pad  310  construed to provide optical coupling to an optical fiber according to an embodiment. The o-pad  310  is shaped as a tilted flat mirror  311  in a fiber trench (not shown in  FIG. 3 ). 
         [0046]    In the beam coupler  300 , the tilted flat mirror of the o-pad  310  is utilized to direct a light beam from the optical fiber  320  to the o-pad  340  (having a curved mirror). 
         [0047]    The tilted flat mirror of the o-pad  310  is fabricated in the interface medium  330  and can be formed by means of anisotropic grayscale etching and tilted at a predefined angle. The angle is determined based on the optical path between the optical fiber  320  and the titled mirror. 
         [0048]    In an embodiment, the o-pad  310 , its tilted flat mirror  311  and the fiber trenches are placed using the same lithography mask alignment accuracy. In another embodiment, the o-pad  310  and its tilted flat mirror  311  are placed using a first lithography mask alignment accuracy, and the fiber trenches are placed using a second lithography mask alignment accuracy. 
         [0049]    It should be noted that in a typical arrangement, a plurality of o-pads  310  can be utilized to connect a plurality of optical fibers using a plurality of fiber trenches. Such an arrangement is further illustrated in  FIG. 3  which shows an exemplary and non-limiting top view of the “fiber side” of the disclosed arrangement. In a further embodiment, an o-pad  310  may be constructed to include a titled flat mirror and a curved mirror. 
         [0050]      FIG. 4  illustrates four (4) fiber trenches  410 - 1  through  410 - 4  (hereinafter referred to individually as a fiber trench  410  and collectively as fiber trenches  410 , merely for simplicity purposes). Each fiber trench  410  adjoins a tilted flat mirror  420  to form an o-pad  310 . The fiber trench  410  is shaped as a groove etched in the interface medium (or a substrate layer attached thereon). Optical fibers  430 - 3  and  430 - 4  are placed in the fiber trenches  410 - 3  and  410 - 4 , respectively. 
         [0051]    In an optional embodiment, illustrated in  FIG. 4 , a curved mirror  450  is also formed. In this embodiment, as an example, a fiber trench  410 - 1 , a tilted flat mirror  420 - 1 , and a curved mirror  450 - 1  form an o-pad  310 - 1 . 
         [0052]    It should be noted that only 2 optical fibers  430 - 3  and  430 - 4  are shown in  FIG. 4  merely for illustrative purposes. Other numbers of optical fibers may be utilized without departing from the scope of the disclosed embodiments. The exemplary arrangement shown in  FIG. 4  can support the formation of 4 different beam couplers. It should be noted that the number of optical fibers that can be supported can be greater than four. It should be further noted that the fiber trenches illustrated in  FIG. 4  are shaped as V-grooves. However, any type of groove shape can be utilized, such as square, cylinder, diamond, and the like. 
         [0053]    The process for creating a fiber trench  410  is further described with reference to  FIGS. 5A through 5D .  FIG. 5A  is a side view of the beam coupler  200 . At first, only the curved mirror of an o-pad  510  is placed on the interface medium  500 . Then, as shown in  FIG. 5B , a groove is etched in the interface medium  500  to create the fiber trench  520  (similar to the trench  410 ,  FIG. 4 ). Finally, an optical fiber  530  is placed in the fiber trench  520  ( FIG. 5C ).  FIG. 5D  shows a side view of the beam coupler  200  with the attached optical fiber  530 . 
         [0054]      FIG. 6  is a side view of an electro-optical interconnection system  600  arranged according to one embodiment. In this arrangement, a PIC  610  is flipped and placed on an electro-optical interconnection platform. 
         [0055]    IC  630 - 1  is connected to the PCB  630 - 2 , both including only electrical elements, are also coupled to the platform  610 . The ICs  630 - 1  and  630 - 2  are coupled to the platform  610  using e-pads  640 - 1 , and  640 - 2 , respectively. The connection between the ICs  630 - 1  and  630 - 2  are through vias or wire bounding  650 . 
         [0056]    The connection of the PIC  610  to the platform  600  is by means of an o-pad  670 - 1 . The connection of the optical fiber  660  to the platform  600  is by means of an o-pad  670 - 2 . The o-pad  670 - 1  is structured using, for example, a curved mirror, while the o-pad  670 - 2  is structured to include a curved mirror, a tilted mirror, and a fiber trench as discussed in detail above. 
         [0057]    The optical connection between the PIC  610  and an optical fiber  660  is through a beam coupler  601  which comprises the o-pads  670 - 1  and  670 - 2 . In this arrangement, the light beam travels through the beam coupler  601  from the PIC  610  to the optical fiber  660  serving as a drain. The o-pad  670 - 2  reflects a diverging light beam from the PIC  610  into parallel light beams which reach the o-pad  670 - 1 , which reflects a focused light beam to a titled mirror in the o-pad  670 - 2  to output the light to the optical fiber  660 . 
         [0058]    The various optical couplers have been discussed herein with a reference to a specific embodiment with curved mirrors utilized for propagating light beams. However, the disclosed embodiments can be realized using other reflective or focusing elements, such as optical lenses, zone plates (e.g., Fresnel zone plates), and the like. 
         [0059]    It should be understood that any reference to an element herein using a designation such as “first,” “second,” and so forth does not generally limit the quantity or order of those elements. Rather, these designations are generally used herein as a convenient method of distinguishing between two or more elements or instances of an element. Thus, a reference to first and second elements does not mean that only two elements may be employed there or that the first element must precede the second element in some manner. Also, unless stated otherwise, a set of elements comprises one or more elements. In addition, terminology of the form “at least one of A, B, or C” or “one or more of A, B, or C” or “at least one of the group consisting of A, B, and C” or “at least one of A, B, and C” used in the description or the claims means “A or B or C or any combination of these elements.” For example, this terminology may include A, or B, or C, or A and B, or A and C, or A and B and C, or 2A, or 2B, or 2C, and so on. 
         [0060]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the disclosed embodiments and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.