Patent Publication Number: US-2011058812-A1

Title: Optically Enabled Broadcast Bus

Description:
TECHNICAL FIELD 
     Embodiments of the present invention are related to optics, and, in particular, to optical broadcast buses. 
     BACKGROUND 
     Typical electronic broadcast buses are comprised of a collection of signal lines that interconnect nodes. A node can be a processor, a memory controller, a server blade of a blade system, a core in a multi-core processing unit, a circuit board, an external network connection. The broadcast bus allows a node to broadcast messages such as instructions, addresses, and data to nodes of a computational system. Any node in electronic communication with the bus can receive messages sent from the other nodes. However, the performance and scalability of electronic broadcast buses is limited by issues of bandwidth, latency, and power consumption. As more nodes are added to the system, there is more potential for activity affecting bandwidth and a need for longer interconnects, which increases latency. Both bandwidth and latency are satisfied with more resources, which results in increases in power. In particular, electronic broadcast buses tend to be relatively large and consume a relatively large amount of power, and scaling in some cases can be detrimental to performance. 
     Accordingly, a scalable broadcast bus that exhibits low-latency and high-bandwidth is desired. 
     SUMMARY 
     Embodiments of the present invention are directed to optical multiprocessing buses. In one embodiment, an optical broadcast bus includes a repeater, a fan-in bus optically coupled to a number of nodes and the repeater, and a fan-out bus optically coupled to the nodes and the repeater. The fan-in bus is configured to receive optical signals from each node and transmit the optical signals to the repeater, which regenerates the optical signals. The fan-out bus receives the regenerated optical signals output from the repeater and distributes the regenerated optical signals to the nodes. The repeater can also serve as an arbiter by granting one node at a time access to the fan-in bus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic representation of an optical multiprocessing bus configured in accordance with embodiments of the present invention. 
         FIG. 2  shows a schematic representation of a beamsplitter configured in accordance with embodiments of the present invention. 
         FIG. 3A  shows how a fan-out bus of the optical multiprocessing bus, shown in  FIG. 1 , distributes optical power to nodes of a computational system in accordance with embodiments of the present invention. 
         FIG. 3B  shows how a fan-in bus of the optical multiprocessing bus, shown in  FIG. 1 , provides an equal amount of optical power output from nodes of a computational system to a repeater in accordance with embodiments of the present invention. 
         FIG. 4  shows a schematic representation of an optical multiprocessing bus configured with delay matching in accordance with embodiments of the present invention. 
         FIG. 5A  show a schematic representation of a first light U-turn system configured in accordance with embodiments of the present invention. 
         FIG. 5B  shows a schematic representation of a second light U-turn system configured in accordance with embodiments of the present invention. 
         FIG. 6  shows a first symmetric optical multiprocessing bus configured in accordance with embodiments of the present invention. 
         FIG. 7  shows a second symmetric optical multiprocessing bus configured in accordance with embodiments of the present invention. 
         FIG. 8  shows a third symmetric optical multiprocessing bus configured in accordance with embodiments of the present invention. 
         FIG. 9A  shows a schematic representation of a first splitter/combiner configured in accordance with embodiments of the present invention. 
         FIG. 9B  shows a schematic representation of a second splitter/combiner configured in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are directed to optical multiprocessing broadcast buses, each of which is composed of a fan-in bus and a fan-out bus. The fan-in and fan-out buses are connected through a repeater. An optical signal generated by a node is sent to the repeater on the fan-in bus where the optical signal is regenerated and broadcast to all of the nodes on the fan-out bus. The repeater can also serve as an arbiter that grants one node at a time access to the fan-in bus. The optical multiprocessing buses can be configured for symmetric multiprocessing where each node on the bus can access or communicate with every other node attached to the bus. The optical multiprocessing buses are enabled by using optical taps that distribute the optical power equally among the nodes over the fan-out bus and ensures that a substantially equal amount of optical power is sent to the repeater from each node on the fan-in bus. 
     For the sake of brevity and simplicity, system embodiments are described below with reference to computer systems having four and eight nodes. However, embodiments of the present invention are not intended to be so limited. Those skilled in the art will immediately recognize that optical multiprocessing bus embodiments can be scaled up to provide optical communications for computer systems composed of any number of nodes. 
       FIG. 1  shows a schematic representation of an optical multiprocessing bus  100  configured in accordance with embodiments of the present invention. The optical bus  100  includes a fan-in bus  102 , a fan-out bus  104 , and a repeater  106 . The fan-in bus  102  includes mirrors  108  and  110  and three optical taps  111 - 113 . The fan-out bus  104  includes mirrors  114  and  116  and three optical taps  118 - 120 . Four nodes labeled  0  through  3  are positioned between the fan-in and fan-out buses  102  and  104 . The nodes can be any combination of processors, memory controllers, server blades of a blade system, clusters of multi-core processing units, circuit boards, external network connections, or any other data processing, storing, or transmitting device. Nodes  0 - 3  include electrical-to-optical converters (not shown) that convert electronic data signals generated within each node into optical signals that are sent over the fan-in bus  102  to the repeater  106 . Nodes  0 - 3  also include optical-to-electrical converters (not shown) that convert optical signals sent by the repeater  106  over the fan-out bus  104  into electronic data signals that can be processed by nodes  0 - 3 . 
     As shown in the Example of  FIG. 1 , directional arrows represent the direction optical signals propagate along optical communication paths of the fan-in and fan-out buses  102  and  104 . The term “optical communication path” refers to optical interconnects and to light transmitted through free space. The optical interconnects can be hollow waveguides composed of a tube with an air core. The structural tube forming the hollow waveguide can have inner core materials with refractive indices greater than or less than one. The tubing can be composed of a suitable metal, glass, or plastic and metallic and dielectric films can be deposited on the inner surface of the tubing. The hollow waveguides can be hollow metal waveguides with high reflective metal coatings lining the interior surface of the core. The air core can have a cross-sectional shape that is circular, elliptical, square, rectangular, or any other shape that is suitable for guiding light. Because the waveguide is hollow, optical signals can travel along the core of a hollow waveguide with an effective index of about 1. In other words, light propagates along the core of a hollow waveguide at the speed of light in air or vacuum. 
     The repeater  106  is an optical-to-electrical-to-optical converter that receives optical signals reflected off of mirror  108 , regenerates the optical signals, and then retransmits the regenerated optical signals to the mirror  114 . The repeater  106  can be used to overcome attenuation caused by free-space or optical interconnect loss. In addition to strengthening the optical signals, the repeater  106  can also be used to remove noise or other unwanted aspects of the optical signals. The amount of optical power produced by the repeater  106  is determined by the number of nodes attached to the fan-out bus, the system loss and the receiver sensitivity. In other words, the repeater  106  can be used to generate optical signal with enough optical power to reach all of the nodes. 
     The repeater  106  can also include an arbiter that resolves conflicts by employing an arbitration scheme that prevents two or more nodes from simultaneously using the fan-in bus  102 . In many cases, the arbitration carried out by the repeater  106  lies on the critical path of computer system performance. Without arbitration, the repeater  106  could receive optical signals from more that one node on the same optical communication path, where the optical signals combine and arrive indecipherable at the repeater  106 . The arbiter ensures that before the fan-in bus  102  can be used, a node must be granted permission to use the fan-in bus  102 , in order to prevent simultaneous optical signal transmissions to the repeater  106 . It is also critical that arbitration be precise and fast and must scale as the number of nodes are added to the bus  100 . Arbitration can be carried out by the arbiter using well-known optical or electronic, token-based arbitration methods. For example, the arbiter can distribute a token representing exclusive access to the fan-in bus  102 . A node in possession of the token has exclusive access to the fan-in bus  102  for a specific period of time. When the node is finished using the fan-in bus  102 , the node can be responsible for replacing the token so that other nodes can have access to the fan-in bus  102 . 
     The optical signals broadcast by nodes  0 - 3  over the fan-in and fan-out buses  102  and  104  can be in the form of packets that include headers. Each header identifies a particular node as the destination for data carried by the optical signals. All of the nodes receive the optical signals over the fan-out bus  104 . However, because the header of each packet identifies a particular node as the destination of the data, only the node identified by the header actually receives and operates on the optical signals. The other nodes also receive the optical signals, but because they are not identified by the header they discard the optical signals. 
     The optical taps of the fan-out bus  104  are configured to distribute the optical power approximately equally among the nodes. In general, the optical taps are configured to divert about 1/nth of the total optical power of an optical signal output from a repeater to each of the nodes, where n is the number of nodes. The optical taps of the fan-in bus are configured so that an equal amount of optical power is received by the repeater from each node on the fan-in bus. In other words, the optical taps are configured in the fan-in bus so that the repeater receives about 1/nth of the total optical power output from each node. 
     Beamsplitters are a kind of optical tap that can be used in the fan-in and fan-out buses.  FIG. 2  shows a schematic representation of a beamsplitter  202  configured in accordance with embodiments of the present invention. The beamsplitter  202  identified by BS m  is configured to reflect a fraction of the optical signal power P  204  input to the beamsplitter  202  in accordance with: 
     
       
         
           
             
               R 
               m 
             
             = 
             
               1 
               
                 ( 
                 
                   n 
                   - 
                   m 
                   + 
                   1 
                 
                 ) 
               
             
           
         
       
     
     and transmit a fraction of the optical signal power P  204  in accordance with: 
     
       
         
           
             
               T 
               m 
             
             = 
             
               
                 ( 
                 
                   n 
                   - 
                   m 
                 
                 ) 
               
               
                 ( 
                 
                   n 
                   - 
                   m 
                   + 
                   1 
                 
                 ) 
               
             
           
         
       
     
     where ideally R m +T m =1, and m is an integer representing a beamsplitter located along the optical communication paths of the fan-in and fan-out buses such that 1≦m≦n−1, 1 represents the beamsplitter located closest to the repeater and n−1 represents the beamsplitter located farthest from the repeater. Thus, the beamsplitter BS m    202  receiver an optical signal with optical power P  204 , outputs a reflected portion with optical power PR m    206 , and outputs a transmitted portion with optical power PT m    208 , where P=PR m +PT m . 
     As shown in the example of  FIG. 1 , the beamsplitters BS 1 , BS 2 , and BS 3  used in the fan-in bus  102  are identical to the beamsplitters used in the fan-out bus  104 , however, the beamsplitters  111 - 113  of the fan-in bus  102  are oriented so that an equal amount of optical power is received by the repeater  106  from each node on the fan-in bus  102 , and the beamsplitters  118 - 120  are oriented to distribute the optical power of the optical signal output from the repeater  106  approximately equally among nodes  0 - 3 . In particular, according to the reflectance R m  and the transmittance T m  above, the beamsplitter BS 1  has an R 1  of ¼ and a T 1  of ¾, BS 2  has an R 2  of ⅓ and a T 2  of ⅔, and BS 3  has an R 3  of ½ and a T 3  of ½.  FIG. 3A  reveals how the beamsplitters BS 1    118 , BS 2    119 , and BS 3    120  of the fan-out bus  104  are configured and oriented so that the optical power of the optical signal received by each node is P 0 /4, where P 0  is the power of the optical signal output from the repeater  106 .  FIG. 3B  reveals how the beamsplitters BS 1    111 , BS 2    112 , and BS 3    113  of the fan-in bus  102  are configured and oriented so that the optical power of the optical signal received by the repeater  106  is approximately P′/4, where P′ is the power of the optical signal output from each of nodes  0 - 3 . 
       FIG. 4  shows a schematic representation of an optical multiprocessing bus  400  with delay matching configured in accordance with embodiments of the present invention. The optical bus  400  is nearly identical to the bus  100 , shown in  FIG. 1 , except the fan-in bus  102  has been replaced by a fan-in bus  402  comprising a mirror  404 , three beamsplitters  406 - 408 , a light U-turn system  410 , and a mirror  412  that directs optical signals output form each node  0 - 3  to the repeater  106 . The fan-in bus  402  ensures that the round trip path length or distance an optical signal travels back to the node it originated from is approximately the same for all nodes. For example, examination of the bus  400  reveals that the round trip path length of an optical signal generated by node  3  back to itself is substantially the same as the round trip path length of an optical signal generated by node  1  back to itself. By contrast, examination of the bus  100  reveals that the path length of an optical signal generated by node  3  back to itself is longer than the path length of an optical signal generated by node  1  back to itself. Because the length of time for optical signals to be transmitted around the bus  400  is substantially the same, the input and output of optical signals of every node can be timed in accordance with a system clock. 
       FIG. 5A  show schematic representations of a light U-turn system  500  configured in accordance with embodiments of the present invention. The U-turn system  500  includes a reflective structure  502 , a hollow input waveguide  504  and a hollow output waveguide  506  vertically stacked located proximate to the reflective surface  502 . Directional arrows represent the paths light travels through and is turned around within the U-turn system  500 . In particular, light transmitted along the core  508  of the hollow input waveguide  504  in a first direction  510  emerges from the hollow input waveguide  504  and is reflected off of a first reflective surface  512  to a second reflective surface  514  of the reflective structure  502 . The light is then reflected off of the second reflective surface  514  into the core  516  of the hollow output waveguide  508  in a second direction  518  that is opposite the first direction  510 .  FIG. 5B  shows a schematic representation of a light U-turn system  520  having four U-turns configured in accordance with embodiments of the present invention. The U-turn system  520  includes a reflective structure  522  composed a first reflective surface  524  and a second reflective surface  526 , hollow input waveguides  530 - 533  that terminate proximate to the reflective surface  524 , and corresponding hollow output waveguides  534 - 537  that terminate proximate to the reflective surface  526 . The hollow waveguides  530 - 537  lie in the same plane. Directional arrows represent one of four U-turn paths the optical signal travel through the U-turn system  520 . 
     In other optical multiprocessing bus embodiments, rather than placing the repeater at the end of the nodes as is done with the optical multiprocessing bus  100  described above, the repeater can be centrally disposed between the nodes, in order to reduce the amount of optical power needed to send an optical signal to the repeater and reduce the amount optical power needed to broadcast optical signals to all of the nodes.  FIGS. 6-10  show a number of different optical multiprocessing bus configurations. The optical processing bus embodiments described below all include the same fan-in and fan-out buses  102  and  104  described above with reference to the bus  100  as portions of larger fan-in and fan-out buses. Thus, a detailed description of the operation and function of the larger fan-in and fan-out buses is not repeated. 
       FIG. 6  shows a first symmetric optical multiprocessing bus  600  configured in accordance with embodiments of the present invention. The bus  600  is composed of a fan-in bus  602  and a fan-out bus  604 . A repeater  606  is disposed in the middle of nodes  0 - 7 . The repeater  606  may include an arbiter that controls which of nodes  0 - 7  is granted access to the fan-in bus  602 . The fan-in bus  602  is composed of a first fan-in portion  608  that directs optical signals output from each of nodes  0 - 3  to the repeater  606  and a second fan-in portion  610  that directs optical signals output from each of nodes  4 - 7  to the repeater  606 . The repeater  606  can be configured to separately receive optical signals from the first fan-in portion  608  and the second fan-in portion  610 . The fan-out bus  604  is composed of a first fan-out portion  612  that broadcast optical signals output from the repeater  606  to nodes  0 - 3  and a second fan-out portion  614  that broadcast optical signals output from the repeater  606  to nodes  4 - 7 . The repeater  606  receives optical signals output from one of nodes  0 - 7  over either the fan-in portion  608  or the fan-in portion  610  along the optical communication paths  616  and  618 , respectively, and simultaneously generates two regenerated optical signals that are output on the optical communication paths  620  and  622 , respectively. The regenerated optical signals are then simultaneously broadcast to nodes  0 - 7  over the first and second fan-out portions  612  and  614  of the fan-out bus  604 . 
       FIG. 7  shows a second symmetric optical multiprocessing bus  700  configured in accordance with embodiments of the present invention. The bus  700  is composed of a fan-in bus  702  and a fan-out bus  704 . A repeater  706  is disposed in the middle of nodes  0 - 7 . The repeater  706  may include an arbiter that controls which of nodes  0 - 7  is granted access to the fan-in bus  702 . The fan-in bus  702  is composed of a first fan-in portion  708  that directs optical signals output from each of nodes  0 - 3  to the repeater  706  and a second fan-in portion  710  that directs optical signals output from each of nodes  4 - 7  to the repeater  706 . The fan-out bus  704  is composed of a first fan-out portion  712  that broadcast optical signals output from the repeater  706  to nodes  0 - 3  and a second fan-out portion  714  that broadcast optical signals output from the repeater each of nodes  4 - 7  to the repeater  706 . As shown in the example of  FIG. 7 , the fan-in bus  702  and the fan-out bus  704  also include 50/50 beamsplitters  716  and  718 , respectively. An optical signal output from one of nodes  0 - 3  passes through the first fan-in portion  708  and is directed by a mirror  720  to the beamsplitter  716 , where the transmitted portion of the optical signal is received by the repeater  706 . An optical signal output from one of nodes  4 - 7  passes through the second fan-in portion  710  to the beamsplitter  716 , where the reflected portion is received by the repeater  706 . An optical signal output from the repeater  718  is split into a reflected optical signal that is broadcast to nodes  0 - 3  over fan-out portion  712  and a transmitted optical signal that is reflected by a mirror  722  and broadcast to nodes  4 - 7  over fan-out portion  714 . 
       FIG. 8  shows a third symmetric optical multiprocessing bus  800  configured in accordance with embodiments of the present invention. The bus  800  is composed of a fan-in bus  802  and a fan-out bus  804 . A repeater  806  is disposed in the middle of nodes  0 - 7 . The repeater  806  may include an arbiter that controls which of nodes  0 - 7  is granted access to the fan-in bus  802 . The fan-in bus  802  is composed of a first fan-in portion  808  and a second fan-in portion  810  both of which are coupled to a first splitter/combiner  812 . The fan-in portion  808  and the fan-in portion  810  direct optical signals output from each of nodes  0 - 7  to the first splitter/combiner  912  where the optical signals are directed to the repeater  806 . The fan-out bus  804  is composed of a first fan-out portion  814  and a second fan-out portion  816 , both of which are coupled to a second splitter/combiner  818 . The repeater  806  outputs optical signals to splitter/combiner  818  which splits the optical signals that are broadcast to nodes  0 - 3  over the fan-out portion  814  and to nodes  4 - 7  over the second fan-out portion  816 . 
       FIG. 9A  shows a schematic representation of a splitter/combiner  1000  configured in accordance with embodiments of the present invention. The splitter/combiner  900  includes a prism  902  with a first reflective planar surface  904  and a second reflective planar surface  906 . The splitter/combiner  900  also includes a first waveguide portion  908 , a second waveguide portion  910 , and a main waveguide portion  912 . As shown in the example of  FIG. 9A , the first and second waveguide portions  908  and  910  are disposed substantially perpendicular to the main waveguide portion  912 . The waveguide portions  908 ,  910 , and  912  can be optical fibers or hollow waveguides. The splitter/combiner  900  can be operated as a 50/50 beamsplitter for incident light propagating in the main waveguide  912  toward the prism  902 , as indicated by directional arrow  914 . The light is split at the edge  916  into a first beam of light and a second beam of light, each beam carrying substantially one-half of the optical power of the incident beam of light. The angle between reflective surfaces  904  and  906  is selected so that the first beam of light is reflected off of the first reflective surface  904  and propagates along the first waveguide  908  in the direction  918 , and the second beam of light is reflected off of the second reflective surface  906  and propagates along the second waveguide  910  in the direction  920 . 
     The splitter/combiner  900  can also be operated as a light combiner. For example, a first incident beam of light propagating in the first waveguide portion  908  toward the prism  902  in the direction  922  is reflected off of the first reflective surface  904  into the main waveguide  912 , and a second incident beam of light propagating in the second waveguide portion  910  toward the prism  902  in the direction  924  is reflected off of the second reflective surface  906  into the main waveguide  912 . The first and second beams of light combine within the main waveguide and propagate in the direction  926 . The prism angle is chosen to minimize the insertion loss of the splitter/combiner junction. A 90 degree angle prism has a splitter efficiency of better than 93%. 
     In other embodiments, the main waveguide  912  can be configured with a tapered region  928 , as shown in  FIG. 9B . The tapered region  928  can be used to spread light traveling along the main waveguide  912  as it reaches the prism  902 , or the tapered region  928  can be used to improve the loss of the combiner/splitter junction by funneling the light reflected into the waveguide  912  from waveguides  908  and  910 . An efficiency of greater than 78% is predicted for the combiner. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. The foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive of or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations are possible in view of the above teachings. The embodiments are shown and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents: