Optoelectronic Multi-Chip Modules Based on Imaging Fiber Bundle Structures

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1 Optoelectronic Multi-Chip Modules Based on Imaging Fiber Bundle Structures Donald M. Chiarulli a, Steven P. Levitan b, Matt Robinson c Departments of a Computer Science and b Electrical Engineering, University of Pittsburgh c Schott Fiber Optics, 122 Charleton Street, Cambridge MA ABSTRACT In this paper, we present a new packaging architecture for chip-level optical interconnections based on imaging fiber bundles. Imaging fiber bundles consist of densely packed arrays of small core fibers such that an object imaged at one end of the bundle is correspondingly imaged on the opposite end. In optical communication applications fiber bundles can be directly coupled to an array of optical sources. Each spot is carried by multiple fibers that in turn can directly illuminate each element in a detector array. Neither end requires any additional optical elements. Thus, imaging fiber bundles are capable of supporting the spatial parallelism of free space interconnects with relaxed alignment and geometry constraints. This paper is focused specifically on multi-chip system designs. KEYWORDS: Optical Interconnections, Optical multi-chip-module, Chip-level fiber interconnection 1. Introduction Optoelectronic systems have recently seen significant advances in two enabling technologies, large-scale arrays of vertical cavity surface emitting lasers (VCSEL) and processing technology for bonding these devices to silicon CMOS. The result is a new class of hybrid optoelectronic (OE) chips that include large scale CMOS circuitry with area pads bonded to VCSEL and detector arrays. Using these devices, it is now possible to consider system designs based multiple OE chips connected optically in a monolithic multi-chip module. This paper is focused on the architecture of the optical interconnections within such a module. A number of research groups have considered designs based on free space propagation though both active and passive microoptical systems [1,2]. The success of this approach has been limited by the quality of available diffractive microoptical elements and by the alignment constraints imposed on a system with multiple chips and complex optics. Guided wave designs have also been proposed based on fiber-per-channel guided wave systems using optical ribbon cable or large core fiber arrays [3]. These systems required accurate alignment of between each VCSEL and fiber core. However, manufacturing tolerances make it difficult to maintain this core alignment across the diameter of a large array. The imaging fiber bundles descibed in this paper are fiber image guides (FIG's) produced at Schott Fiber Optics [4]. These bundles have been traditionally used in medical imaging systems and remote inspection devices such as flexible endoscopes. They consist of a dense array of small core fibers arranged in a lattice. Fiber diameters typically range from 5 to 20 microns yielding core densities of two thousand to fifteen thousand cores per square millimeter. The relative spatial position of each fiber within the lattice is maintained throughout the length of the bundle. Imaging fiber bundles can be fabricated as either flexible or rigid packages. We will describe optical interconnection architectures based on both. They typically have very high numerical aperture, which makes them well suited for direct coupling to optical devices.

2 In previous work [5] we have demonstrated that these devices are highly effective for point-to-point links. We demonstrated a 16 channel interconnect between two VCSEL arrays that were directly butt coupled to an imaging fiber bundle with no other optical elements used in the setup. Crosstalk between the channels was low, -22db, and loss mechanisms were largely characterized by insertion loss corresponding to the fill factor of the bundle. This paper looks at the next step, multi-chip interconnection modules. We present designs based on both rigid and flexible imaging fiber bundles. Unlike our previous work in which the chips were mounted in individual carriers with the fiber bundles used as an external interconnect, the chips in these designs would generally be mounted directly to a metalized end surface of the imaging fiber bundle. In these systems, most of the chips in the module will use all optical channels for data I/O. Only power and ground are drawn from the metalization on the surface of the fiber bundle. The key to these designs is that routing is optically passive, but electrically active. Individual regions of the OE chip source and detector arrays are imaged onto different regions of the input surface of a fiber bundle (or a fiber bundle array) and are passively routed to different output surfaces. In the balance of this paper, we present three designs for opto-electronic multi-chip modules. In the first configuration, sets of flexible fiber bundles are bonded side-to-side at the rigid end segments of each bundle. For an n-way interconnect, the array at each bonded end consists of n-1 bundles. If each array contains one bundle for each of the other faces, a full interconnection is achieved. Other simple permutations of the bundles can implement a variety of other networks. In a second approach, sets of flexible bundles are side bonded at one end, as above. Each of the two bonded sets are bonded, in turn, end-to-end with a 90 0 rotation between the ends. The result is a set of spatial regions at the loose end of each bundle that will independently transmit channels to each of the bundles in the other set. The third approach uses rigid fiber bundles that are machined to create internal facets. These facets are designed such that, when the bundles are bonded along these facets, a cube (or other polyhedron) is formed. An optical beam that enters the structure on one face may exit on any other, depending on where it enters the input face and thus which facet is traversed. 2. Side Bonded Structures Imaging fiber bundles can be fabricated with end dimensions from less than a millimeter up to several millimeters. In fact, the final stages of processing a fiber bundle consist of bonding and drawing arrays of smaller bundles into a single large bundle. In the class of structures that we will discuss in this section, the rigid end segments of flexible bundles will be bonded side-to-side in various configurations prior to coupling to an OE chip. This will create a composite structure in which the array of optical channels on the surface of the chip will be partitioned based on the bonding pattern. Each partition will send and receive signals through one of the bundles in the bonded group. Figure 1: Side Bonded, 4-way fan-in/fan-out structure Figure 2: Side bonded, 3-chip full interconnect

3 As an example, Figure 1, shows a simple mechanism for implementing a 4 to 1 fan-in/fan-out structure based on this approach. In this case, one end of four fiber bundles is bonded side-to-side with the other end remaining free. Five OE chips would be coupled to the structure, one on the bonded end and the others on each of free ends. The OE chip mounted on the bonded end, will send and receive data to and from each of the four other chips through channels in of the four quadrants of the array defined by the bonding pattern. Figure 3: Side bonded nearest neighbor array This simple structure provides a basic fan-in/fan-out building block that can be the basis for a variety of complex interconnection networks. For example, Figure 2, shows a three-way fully interconnected system based on three bundles implementing a coupled fan out structure. In this case, an OE chip bonded to one end face of the structure communicates with the other two through channels in the left or right half of the array. This is a 3-way fully connected non-blocking network. Using side-bonding techniques, a general n-way connection of this type can be n 1 built by bonding i bundles in this manner. Figure 3, i= 1 shows another alternative, in which the nearest neighbor network has been implemented the bonding the ends of the 4-way fan-out structure shown above. 3. End-Bonded Structures Side bonded structures have the disadvantage that there is a direct relationship between the number of chips connected to the size of the bonded structure at the OE chip interface. Most current VCEL arrays have device pitch on the order of a hundred microns and array sizes of a few millimeters. Even using small bundles, the size of the side-bonded structure is limited by the size of the VCSEL array. To overcome this problem, we have designed a structure that combines side-to-side bonding and end-to-end bonding in an interconnection network that can support n-way, non-blocking, full interconnection using a fixed sized structure at the OE chip interface that requires only one fiber bundle per chip. An example of this structure is diagrammed in Figure 4. It is based on two stacks of rectangular imaging fiber bundles side bonded into a fan-out structure as in the previous section. The bonded end of each stack is then bonded end-to-end with a 90 o relative rotation. Figure 9 is a 3 x 3 implementation in which three bundles are stacked and bonded vertically on the left side of the diagram and three more are stacked and bonded horizontally on the right side of the diagram. Figure 4: End Bonded 3x3 crossbar interconnection As before, the structure provides for spatial partitioning of the optical channels of the OE chip coupled to the end of each fiber. To understand the network, consider the top fiber bundle in the vertical stack on the left side of Figure 4. Optical channels imaged on to the left third of this bundle, represented by the arrow labeled A, are coupled into a corresponding region of the left fiber bundle in the horizontal

4 stack. Channels imaged on middle third, represented by the arrow labeled B, are transmitted to the middle bundle in the horizontal stack and correspondingly channels imaged on the right region, represented by the arrow labeled C, are coupled to the right bundle in the horizontal stack. Moreover, channels from each of the bundles in the vertical stack exit the corresponding bundles in the horizontal stack within a specific region and are thus spatially identifiable. Since imaging fiber bundles are bi-directional, this structure can implement either a 3 to 3 bi-directional crossbar or, if the loose ends of each of the horizontal and vertical stacks are pair-wise bonded and coupled to single devices, a 3x3 full interconnect. Figure 5: Experimental core element for end bonded 3x3 crossbar To test the basic optical properties of this a structure, a 3 x 3 test element was built by bonding six rigid rectangular fiber bundles in the manner described above. A photograph of this structure is shown in Figure 5. Since there were no loose ends in this test structure, a flexible bundle was butt coupled on one of the input surfaces and the other end coupled to a VCSEL array for testing. This bundle can clearly be seen in Figure 5 as the dark hexagonal structure at the center of the top row. The excess material shown above the top is an overlap caused by the mismatch in the aspect ratio the stacks. The active region is the 3x3 array on the bottom formed by the intersection of the horizontal and vertical stacks. From this photograph it can be seen that individual spots are clearly resolvable through this structure even thought the spots have crossed two coupling boundaries for which no attempt was made to align the fiber cores. Figure 6: Exploded View of segments with machined facets Figure 8: Cube with metallization applied Figure 7: Assembled Cube Figure 9: Fully assembled OE- MCM

5 4. Rigid Structures Both of the previous designs are based on flexible fiber structures. The minimum working length of these bundles along with the mechanical constrains of the bonding surfaces limits these structures to circuit board style interconnections. As with their electronic counterparts, latency for this type of interconnection is a significantly greater that on-chip or multi-chip-module traces. Our third design addresses this problem by implementing a densely packed structure built from segments of rigid fiber bundles. In this case a small segment of a rigid bundle is machined to create internal facets. These facets are designed such that when the bundles are bonded along these facets, a cube (or other polyhedron) is formed. Figure 6 shows a set of segments machined to form a cube structure and Figure 7 shows the cube after assembly. The end surfaces of each bundle, that have now become the faces of the cube, are plated with metal traces and OE chip are mounted to these faces. An example metalization pattern is shown in Figure 8. Finally, OE chips are mounted to all but one of the cube faces. An OE conversion chip with both optical an electrical I/O is mounted the remaining face and this chip is in turn mounted to a ceramic substrate for electrical I/O. Figure 9 shows a drawing of the finished OE multi-chipmodule package. In operation, each chip communicates through optical channels that traverse the structure internally. As with the side and end bonded structures, routing is passive and based on the location of the optical beam entering the structure on a particular face. This, in turn, determines which facet is traversed. Figure 10 is a front view of a wire frame representation of the cube. It clearly shows the four facets and center pass through region defined by the machining pattern. Thus, a bean in the top center region traverses the top facet, is coupled into the vertical fiber bundle segment, and exits. Similarly the left, right, and bottom regions couple into the bundle segments for the left, right, and bottom faces respectively. In this particular cube, we have left a small region in the center of the bundle un-faceted in order to allow for communication between opposite faces. For this structure, this is only possible between four of the six faces. Figure 10: Front View wire frame representation of rigid fiber cube Since the dimensions of the cube are only slightly larger the dimensions of each chip, signal latency is comparable to intra-chip connections. Assuming a small OE/EO conversion latency, it is possible to design a system in which a collection of small chips can operate with performance comparable to a single large die. 5. Summary and Future Work We have demonstrated three multi-chip architectures for chip level optical interconnection using imaging fiber bundles. These include side-bonded, end-bonded and rigid structures. The rigid structures appear to hold a great deal of promise for supporting large scale low-latency interconnects. We are currently in the process of fabricating and characterizing the performance of the rigid cube multi-chip module described here.

6 6. References [1] Xuezhe Zheng, Philippe J. Marchand, Dawei Huang, Osman Kibar, Nur S.E. Ozkan, and Sadik C. Esener, "Optomechanical design and characterization of a printed-circuit-board-based free-space optical interconnect package," Applied Optics, vol. 38, No. 26, pp , 10 September [2] Michael W. Haney, Marc P. Christensen, Predrag Milojkovic, Jeremy Ekman, Premanand Chandramani, Richard Rozier, Fouad Kiamilev, Yue Liu, and Mary Hibbs-Brenner, "Multichip free-space global optical interconnection demonstration with integrated arrays of vertical-cavity surface-emitting lasers and photodetectors," Applied Optics, vol. 38, No. 29, pp , 10 October [3] A.V. Krishnamoorthy and J.E. Ford, "Optoelectronic-VLSI forswitched-data networking," in Optics in Computing, Vol 8, OSA Technical Digest Series, Optical Society of America, Washington DC, pp 269,270. [4] Schott Fiber Optics, Inc. [5] Donald M. Chiarulli, Steven P. Levitan, Paige Derr, Robert Hofmann, Bryan Greiner and Matt Robinson. Demonstration of Multi-channel Optical Interconnection using Imaging Fiber Bundles Butt Coupled to Optoelectronic Circuits, Applied Optics, Vol. 39, #5, pp