Patent Publication Number: US-2013230274-A1

Title: Photonic flexible interconnect

Description:
FIELD 
     Embodiments of the invention generally pertain to optical devices and more specifically to photonic flexible interconnects utilized by electronic and optical devices. 
     BACKGROUND 
     Current state of the art solutions for optical connections to photonic chips are made through either free space coupling to optical fibers/polymers or direct butt coupling to fibers; these prior art coupling solutions may be in or out of the plane of the photonic chip. When these solutions are used for creating optical connections to high speed electronic chips, they require packaging solutions that deviate far from those used by the electronic packaging industry. 
     The problem with current state of the art free space coupling solutions is that they require hermetic (i.e., airtight) enclosures to avoid condensation onto lenses. Such enclosures are difficult and costly to manufacture, and require an additional optical waveguide solution to exist in a printed circuit board (PCB) used to optically interconnect electronic chips and circuit boards. 
     The problem with current state of the art direct butt coupling solutions is that the coupling fibers used are too stiff and brittle to be bent sharply without the risk of breaking; this significantly increases both the necessary package height in the case of vertical coupling to silicon (Si) ships, and the necessary die area when making horizontal coupling due to mechanical features required to align optical fibers. 
     What is needed is a way to make optical interfaces to Photonic integrated circuits (ICs) that is more compatible with electronic packaging. These interfaces should be compatible with electronic packaging (on one end) and with optical fiber connections (on the other end), be very flexible, reduce the coupling area on the photonic IC, and incur a low manufacturing cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following description includes discussion of figures having illustrations given by way of example of implementations of embodiments of the invention. The drawings should be understood by way of example, and not by way of limitation. As used herein, references to one or more “embodiments” are to be understood as describing a particular feature, structure, or characteristic included in at least one implementation of the invention. Thus, phrases such as “in one embodiment” or “in an alternate embodiment” appearing herein describe various embodiments and implementations of the invention, and do not necessarily all refer to the same embodiment. However, they are also not necessarily mutually exclusive. 
         FIG. 1  is a top-view illustration of a photonic interconnect according to an embodiment of the invention. 
         FIG. 2  is a side-view illustration of a photonic interconnect according to an embodiment of the invention. 
         FIG. 3A ,  FIG. 3B  and  FIG. 3C  are side-view and top-view illustrations of a silicon photonic interconnect according to an embodiment of the invention. 
         FIG. 4A ,  FIG. 4B  and  FIG. 4C  are side-view and top-view illustrations of a Si photonic interconnect according to an embodiment of the invention. 
         FIG. 5A  and  FIG. 5B  are side-view illustrations of a silicon photonic interconnect according to an embodiment of the invention. 
         FIG. 6A  and  FIG. 6B  are side-view illustrations of a silicon photonic interconnect according to an embodiment of the invention. 
         FIGS. 7A ,  7 B and  7 C are illustrations of a surface coupler to edge coupler with pitch conversion means according to an embodiment of the invention. 
         FIGS. 8A ,  8 B and  8 C are illustrations of an active photonic flexible interconnect according to an embodiment of the invention. 
     
    
    
     Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein. An overview of embodiments of the invention is provided below, followed by a more detailed description with reference to the drawings. 
     DESCRIPTION 
     Embodiments of the invention describe photonic flexible interconnects utilized by electronic and optical devices. Throughout this specification, several terms of art are used. These terms are to take on their ordinary meaning in the art from which they come, unless specifically defined herein or the context of their use would clearly suggest otherwise. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects. 
       FIG. 1  is a top-view illustration of a photonic interconnect according to an embodiment of the invention. In this embodiment, printed circuit board (PCB)  100  is shown to include integrated circuit (IC) package  102 , which may comprise, for example, a Silicon (Si) Based System in Package, a System on a Chip (SoC) with photonics input/output (I/O), etc. 
     Optical interconnect  104  is shown to provide an optical interface for IC package  102 . In this embodiment, optical interconnect  104  includes Si photonic interconnect component  110 , fiber or waveguide interconnect component  112 , and substrate  114  coupling both of said components together. As described below, in this embodiment optical interconnect  104  is a flexible interface solution that significantly reduces the coupling area necessary compared to prior art optical interconnect solutions. 
       FIG. 2  is a side-view illustration of a photonic interconnect according to an embodiment of the invention. In this embodiment, PCB  200  is shown to be electronically coupled to IC package  202 . Said IC package includes central processing unit (CPU) package  210  electronically coupled to transmission/receiving component  212 , which is further coupled to photonic layer  214 . Said photonic layer receives and converts optical data to electric data, and vice versa. 
     In this embodiment, optical interconnect  204  includes a Si photonic interconnect component, shown to include flexible polymer layer  220  and Si photonic waveguide component  222  (described in further detail below). Other embodiments may not utilize said flexible polymer layer. Optical interconnect  204  further includes fiber interconnect component  224  to send/receive optical data to/from optical components or devices (not shown). 
     Said Si photonic interconnect component and fiber interconnect component are coupled together via substrate  226 . In some embodiments, substrate  226  may be formed to act as a heat sinking material for embodiments including active optical components (described below). Fiber interconnect component  224  may comprise one or more tapers to create large, fiber matched mode sizes or v-groove arrangements to enable alignment of optical fibers; other embodiments may include vertical couplers or evanescent couplers that are mode matched to the optical fiber or fiber receptacle in the event an optical connector is desired. 
       FIG. 3A ,  FIG. 3B  and  FIG. 3C  are side-view and top-view illustrations of a Si photonic interconnect according to an embodiment of the invention. In this embodiment, Si photonic interconnect  300  and photonic layer  302  are optically coupled via adiabatic transfer means as described below. 
     Si photonic interconnect  300  includes polymer layer  310  and Si waveguide component  312 , which may be formed by using silicon wafer processing to pattern the respective waveguide (similar to Si photonic processing). Said Si waveguide component is formed to couple optical data from photonic layer  302  via matching structures (i.e., waveguide components  312  and  320 ). 
     In this embodiment, waveguides  312  and  320  are electrically bonded via bonding pads  314  and  322 , respectively. Said bonding pads transfer electrical signals between Si photonic interconnect  300  and photonic layer  302 . As shown in  FIG. 3B , said waveguides are optically bonded—i.e., “direct bonded” via the use of optical-grade adhesive,  330 . In other embodiments, said waveguides may be molecularly bonded. 
     In this embodiment, waveguides  312  and  320  are parallel couplers that couple light between two parallel and different waveguides that are on top of each other, as shown in  FIG. 3B , coupling light with these parallel couplers is based on adiabatic coupling. A waveguide transition is adiabatic if it occurs sufficiently slowly so as not to transfer energy from the occupied mode to other modes. In this embodiment, adiabatic coupling occurs via tapered structures in waveguides  312  and  302 , as shown in  FIG. 3C . Said tapered structures are monotonic (e.g., linear) variations in the width or height of the waveguide, which causes the mode to change its shape. An advantage of adiabatic coupling is that the transfer allows for misalignment between the waveguides  312  and  302 —i.e., misalignment does not affect performance. Adiabaticity also implies fault tolerance to most operational and fabrication errors. In this embodiment, representations of the fundamental mode within waveguides  312  and  320  are shown as elements  332  and  334 , respectively. The taper of the waveguide cores are such that the transmission of the fundamental mode through the coupler is adiabatic. Thus, in this embodiment, high coupling efficiency occurs without lenses due to the matched mode shapes of the two couplers. 
       FIG. 4A ,  FIG. 4B  and  FIG. 4C  are side-view and top-view illustrations of a Si photonic interconnect according to an embodiment of the invention. In this embodiment, Si photonic interconnect  400  and photonic layer  402  are optically coupled via adiabatic transfer means as described above. Similar to  FIG. 3 , Si photonic interconnect  400  includes polymer layer  410  and Si waveguide component  412 , which may be formed by using silicon wafer processing to pattern the respective waveguide (similar to Si photonic processing). Said Si waveguide component is formed to couple optical data from photonic layer  402  via matching structures (i.e., waveguide components  412  and  420 ). 
     Si photonic interconnect  400  includes polymer layer  410  and Si waveguide component  412 . In this embodiment, Si photonic interconnect  400  further includes dielectric waveguide component  440  coupled to SI waveguide component  412 , while photonic layer  402  further includes dielectric waveguide component  450  coupled to waveguide component  420  (in other embodiments, only one of SI photonic interconnect  400  and photonic layer  402  may include a dielectric waveguide component. Said dielectric waveguide component  440  is formed to couple optical data from photonic layer  402  via matching structures (e.g., tapered structures, as shown in  FIG. 4C ). Said dielectric waveguide components may comprise a lower index of refraction that waveguide components  412  and  402  and transmit light having a larger mode, thereby increasing the alignment tolerance between Si photonic interconnect  400  and photonic layer  402 . 
       FIG. 5A  and  FIG. 5B  are side-view illustrations of a Si photonic interconnect according to an embodiment of the invention. In this embodiment, Si photonic interconnect  500  and photonic layer  502  are optically coupled via grating transfer means as described below. 
     Si photonic interconnect  500  includes polymer layer  510  and Si waveguide component  512 , which may be formed by using silicon wafer processing to pattern the respective waveguide (similar to Si photonic processing). Similar to the embodiments discussed above, waveguides  512  and  520  are electrically bonded via bonding pads  516  and  522 , respectively, to vary the optical properties of each waveguide to which it is connected. As shown in  FIG. 5B , said waveguides are optically bonded—i.e., “direct bonded” via the use of optical-grade adhesive,  530 . Representations of the fundamental mode within waveguides  512  and  520  are shown as  532  and  534 , respectively. 
     In this embodiment, waveguides  512  and  520  are parallel couplers that couple light between two parallel and different waveguides that are on top of each other, as shown in  FIG. 5B . Coupling of light with these parallel couplers is based on grating-assisted coupling. As described above with reference to  FIGS. 3A and 3B , when two waveguides are made from material with similar or identical indices of refraction, the evanescent coupling efficiency can be quite high and can suffice adiabatically without the assistance of a grating; however, in this embodiment the use of a gratings  514  and  524  make coupling even more efficient, and allows different refractive index materials or different waveguide dimensions to be used for waveguides  512  and  520 . In embodiments where said waveguides are edge coupled (e.g., as shown in  FIG. 6  and discussed below), the alignment tolerance is based on the mode size of each waveguide. In this embodiment, vertically coupling light via gratings  514  and  524  may increase the size of the mode in both directions, increasing the alignment tolerance between Si photonic interconnect  500  and photonic layer  502 . 
     It is to be understood that while this example embodiment illustrates an interconnect from an IC to an optical fiber component, in other embodiments, interconnects may be used to for chip-to-chip connections (e.g., an embodiment similar to that of  FIGS. 5A-5C , but also having another set of gratings to couple to a photonic layer of another IC). 
       FIG. 6A  and  FIG. 6B  are side-view illustrations of a Si photonic interconnect according to an embodiment of the invention. In this embodiment, Si photonic interconnect  600  and photonic layer  602  are optically coupled via edge coupling means as described below. 
     Si photonic interconnect  600  includes polymer layer  610  and Si waveguide component  612 , which may be formed by using silicon wafer processing to pattern the respective waveguide (similar to Si photonic processing) to match waveguide  620  of photonic layer  602 . 
     Si waveguide component  612  includes edge coupler  614 . In some embodiments, said edge coupler comprises an etched facet or cleaved and polished surface facet. In other embodiments, said edge coupler comprises a taper on an end of a waveguide. Mechanical features such as v-grooves to facilitate passive alignment of optical fibers to the edge coupler may be included (as described below). As shown in  FIG. 6B , said waveguides are electrically bonded via bonding layer  630 . 
       FIGS. 7A ,  7 B and  7 C are illustrations of a surface coupler to edge coupler with pitch conversion means according to an embodiment of the invention. In this embodiment, surface coupling means  702  is shown to comprise plurality of gratings  710 ,  711 ,  712  and  713  (similar to the gratings of FIG.  5 A/ 4 B), to exchange optical data with edge coupling means  720 ,  721 ,  722  and  723 , respectively. 
     In this embodiment, the variation in pitch between gratings  710 - 613  (shown as reference element  730 ) and edge coupling means  720 - 623  (shown as reference element  732 ) allows interconnect  700  to have pitch conversion functionality—i.e., said interconnect is capable of connecting two multi-fiber optical connecters with different pitches via a single component by adhering two connection members equipped with plural fiber holes with different pitches. In this embodiment, interconnect  700  includes v-grooves  740  to facilitate passive alignment of optical fibers to the edge coupler. Said v-grooves may be formed from Si substrate material, or any other functionally equivalent means. 
     In this embodiment the Si photonic interconnect may also perform the function of polarization management required for the photonic interconnect its connected to, where in a unknown polarization state is coupled into the edge coupled interface  742  and each polarization component of the incoming optical data is separated, optionally rotated and directed toward the surface coupling elements  720 - 723  (e.g., as illustrated by polarization splitter and/or rotator  750 ). This feature enables the photonic IC to operate in a single polarization, but still receive both polarizations of optical data. 
       FIGS. 8A ,  8 B and  8 C are illustrations of an active photonic flexible interconnect according to an embodiment of the invention. In this embodiment, Si photonic interconnect  800  and photonic layer  802  are electrically coupled via electrodes  814  and  822  as described above. 
     Si photonic interconnect  800  includes polymer layer  810  and Si waveguide component  812 , which may be formed by using silicon wafer processing to pattern the respective waveguide (similar to Si photonic processing). As shown in  FIG. 8B , said waveguides are optically bonded—i.e., “direct bonded” via the use of optical-grade adhesive,  830 . 
     In its active form, active optical components such as lasers, modulators, switches, detectors, etc., (shown as components  840 ) are added to the passive silicon photonics components. Interconnect metallization of the flexible optical interconnect  814  are now added such that it can be directly attached to the IC&#39;s electrical interconnect  822 . In other words, the optical transceiver that was previously integrated with the IC in SoC solutions is now contained in the silicon photonic optical flex interconnect. 
     In this embodiment, electrical connections are made to the IC via jumpers (combinations of both electrical and optical connections are envisioned). Heat sinking material may also be used to dissipate heat from the active components. This heat sinking can be in the form of Si substrate material left underneath active components, heat spreading pieces bonded to the active portions of the silicon photonic optical flex interconnect while it is in wafer form after the substrate has been removed, or any other functionally equivalent means. 
     Reference throughout the foregoing specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. In addition, it is appreciated that the figures provided are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale. It is to be understood that the various regions, layers and structures of figures may vary in size and dimensions. 
     In the foregoing detailed description, the method and apparatus of the present invention have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.