Abstract:
Disclosed are reconfigurable optical interconnections for opto-electronic processors in general, and for scalable computer architectures and scalable network servers in particular. The optical-signal interconnects are adaptable, or reconfigurable, during the normal operation of the processor. A large number of optical-signal interconnects may be provided among the components of the processor while using a small number of light transmitters and/or light receivers.

Description:
FIELD OF THE INVENTION 
   The present invention relates to opto-electronic processors with chip-to-chip optical interconnections suitable for use in scalable computer architectures and scalable network servers. More particularly, the present invention relates, to opto-electronic processors with adaptable chip-to-chip optical interconnections. 
   BACKGROUND OF THE INVENTION 
   Mainstay computer systems and network-server systems use electrical interconnects among the integrated-circuit (IC) chips on the component boards, and electrical interconnects on bus-based interconnect planes to interconnect the component boards. The speed of communication in these systems is limited both by the well-known “skin effect,” where resistance increases as signal frequency increases, and by capacitance effects. To overcome the resistance and capacitance of the electrical interconnects, more powerful bus drivers have been used. However, these drivers increase the power consumption of the system, and require additional cooling for the system. 
   Recently, there has been an explosive growth in the amount of information conveyed through the Internet, local networks, and high-speed data exchanges between servers. Current information processing systems and network servers are having difficulty keeping up with the growth, and are now running into the physical limits of the electronic components, electric interconnections, and assembly technologies. 
   The present invention is directed to addressing the ability of information processing systems and network servers to keep up with the growth of information communication, and to addressing the physical limits placed on systems by current electronic components, electric interconnections, and assembly technologies. 
   SUMMARY OF THE INVENTION 
   As part of making their invention, the inventors have recognized that the board-to-board electrical interconnects between component boards could be replaced by optical-signal interconnects. The propagation of optical signals is not limited by resistance, capacitance, or the skin effect, and a light beam only generates a small amount of heat dissipation due to light absorption in waveguides and the light receiver. However, the inventors have recognized that directly replacing each board-to-board electrical interconnect with an optical-signal interconnect would require a dedicated light transmitter device on one board, a dedicated light receiver device on another board, and a dedicated configuration of one or more optical waveguides between the light transmitter device and the light receiver device. Scalable-architecture computer systems and blade-type network servers have large numbers of board-to-board electrical interconnects, and thus the replacement of electrical interconnects with optical-signal interconnects would require large numbers of light transmitters and light receivers incorporated onto each component board. This would greatly increase the size and expense of each component board, calling into question the viability and cost-effectiveness of using optical-signal interconnects in scalable-architecture computer systems and blade-type network servers. 
   As a further part of making their invention, the inventors have discovered that many scalable-architecture computer systems and blade-type network servers have a relatively low utilization of their board-to-board electrical interconnects. That is, while each component board of such a system has several electrical interconnects to each of the other component boards, a much smaller number of the electrical interconnects are used at any one time. Moreover, when an electrical interconnect is used, it tends to be used intensely for a substantial period of time, and then goes idle for a substantial period of time before being used again. 
   As part of making their invention, the inventors have recognized that the implementation of board-to-board optical-signal interconnects can be made viable and cost-effective by using a smaller number of optical-signal interconnects (compared to the number of electrical interconnects being replaced), and by making the optical-signal interconnects adaptable, or reconfigurable, during the normal operation of the computer system or network server. As an example, instead of using three light transmitters on a component board to replace three board-to-board electrical interconnects to three other component boards, a single light transmitter is used, and its output is guided to one of three waveguides (or possibly two of three waveguides, or three of three waveguides) by an optical deflecting device that is under the control of the component board. Similarly, instead of using three light receivers on a component board to replace three board-to-board electrical interconnects to three other component boards, a single light receiver is used and its input is optically coupled to one of three waveguides by another optical deflecting device that is under the control of the component board. 
   Accordingly, a first exemplary embodiment of the present invention encompasses a processor that is suitable for use as a computer system, or a network server, or the like. The processor comprises at least a first IC chip, a second IC chip, and a third IC chip, each IC chip being mounted on a substrate, such as a component board. Usually, the IC chips are mounted on separate substrates, but two or more of the IC chips may be mounted on a common substrate. The IC chips perform tasks in support of the operation of the processor, the first IC chip generating a signal that is to be conveyed to either of the second and third IC chips. The processor further comprises a first light transmitter that receives a first electrical signal from the first IC chip, and that generates an optical signal at an optical output in relation to the first electrical signal. The first light transmitter may be integrated onto the first IC chip, or it may be integrated onto another chip. The processor further comprises a first light receiver having an optical input and generating a second electrical signal in relation to the amount of light received at its optical input. The second electrical signal is electrically coupled to the second IC chip, and the first light receiver may be integrated onto the second IC chip or another chip. The processor further comprises a second light receiver having an optical input and generating a third electrical signal in relation to the amount of light received at its optical input. The third electrical signal is electrically coupled to the third IC chip, and the second light receiver may be integrated onto the third IC chip or another chip. The processor further comprises an optical deflector having an optical input optically coupled to the optical output of the first light transmitter to receive an optical signal therefrom, a first optical output optically coupled to a first output waveguide that enables light to be conveyed to the first light receiver, a second optical output optically coupled to a second output waveguide that enables light to be conveyed to the second light receiver, and an electrical input that receives a first control signal in electrical form. The optical deflector couples the received optical signal more to the first output waveguide than the second output waveguide when the electrical control signal has a first state, and more to the second output waveguide than the first output waveguide when the electrical control signal has a second state. The optical deflector may be disposed on the same substrate as the first IC chip, or may be disposed on an optical interconnect board. The first control signal may be generated by any chip or component within the processor, and may be generated by the first IC chip or another chip located on the same substrate as the first IC chip. 
   In the above example, the reception of the light signals may be handled in a number of ways. In one configuration, each of the first and second light receivers may receive an optical signal from a single dedicated waveguide. In another case, each of the first and second light receivers may receive its optical signal from one of a plurality of waveguides that is adaptively selected during operation by an optical deflector similar to that described above. The latter configuration enables a large number of waveguides to be replaced by a smaller number of bus-type waveguides that are shared in a multiplexed manner. 
   A second exemplary embodiment of the present invention encompasses a processor that is suitable for use as a computer system, or a network server, or the like. The processor comprises at least a first IC chip, a second IC chip, and a third IC chip, each IC chip being mounted on a substrate, such as a component board. Usually, the IC chips are mounted on separate substrates, but two or more of the IC chips may be mounted on a common substrate. The IC chips perform tasks in support of the operation of the processor, the first IC chip receiving a signal in optical form that is conveyed from one of the second and third IC chips. The processor further comprises a first light receiver having an optical input and generating a first electrical signal in relation to the amount of light received at its optical input. The first electrical signal is electrically coupled to the first IC chip, and the first light receiver may be integrated onto either the first IC chip or another chip. The processor further comprises a first light transmitter that receives a second electrical signal from the second IC chip and generates an optical signal at an optical output in relation to the second electrical signal. The first light transmitter may be integrated onto the second IC chip, or it may be integrated onto another chip. The processor further comprises a second light transmitter that receives a third electrical signal from the third IC chip and generates an optical signal at an optical output in relation to the third electrical signal. The second light transmitter may be integrated onto the third IC chip, or it may be integrated onto another chip. The processor further comprises an optical deflector having a first optical input optically coupled to a waveguide that enables light to be conveyed from the first light transmitter, a second optical input optically coupled to a waveguide that enables light to be conveyed from the second light transmitter, an optical output optically coupled to the optical input of the first light receiver, and an electrical input that receives a first control signal in electrical form. The optical deflector further has a first optical coupling efficiency between its first optical input and its optical output, and a second optical coupling efficiency between its second optical input and its optical output. The first optical deflector makes the first optical coupling efficiency greater than the second optical coupling efficiency when the first control signal has a first state, and makes the second optical coupling efficiency greater than the first optical coupling efficiency when the first control signal has a second state. The optical deflector may be disposed on the same substrate as the first IC chip, or may be disposed on an optical interconnect board. The first control signal may be generated by any chip or component within the processor, and may be generated by the first IC chip or another chip located on the same substrate as the first IC chip. 
   In the above example, the transmission of the light signals may be handled in a number of ways. In one configuration, each of the first and second light transmitters couples its optical output to a single dedicated waveguide. In another case, like that of the first exemplary embodiment described above, each of the first and second light transmitters may couple its optical signal to one (or more) of a plurality of waveguides that is adaptively selected during operation by an optical deflector, as described above. The latter configuration enables a large number of waveguides to be replaced by a smaller number of bus-type waveguides that are shared in a multiplex manner. 
   Accordingly, it is an object of the present invention to enable scalable-architecture computers and network servers to process more information. 
   It is a further object of the present invention to provide a large number of optical-signal interconnects among component boards using a small number of light transmitters and/or light receivers. 
   It is a further object of the present invention to reduce the cost of implementing optical-signal interconnects in large processors. 
   These objects and others will become apparent to one of ordinary skill in the art from the present specification, claims, and attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a perspective view of an exemplary processor embodiment according to the present invention. 
       FIG. 2  shows a partial perspective view of a component board and a first exemplary optical deflector that may be used in the processor shown in  FIG. 1  according to the present invention. 
       FIG. 3  shows a top plan view of the first exemplary optical deflector shown in  FIG. 2  according to the present invention. 
       FIG. 4  shows a cross-sectional view of a second exemplary optical deflector that may be used in the processor shown in  FIG. 1  according to the present invention. 
       FIG. 5  shows a partial perspective view of the second exemplary optical deflector shown in  FIG. 4  according to the present invention. 
       FIG. 6  shows a schematic diagram of an exemplary optical wiring architecture that may be used in the processor shown in  FIG. 1  according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a perspective view of a processor embodiment of the present invention at  100 . Processor  100  comprises a plurality of component boards  120  mechanically coupled to a main optical backplane  110 , preferably in a detachable manner. Optical signals are routed among component boards  120  as described below in greater detail. Component boards  120  may be component boards of any electro-optic-based system, such as daughter boards of a computer system or processor blades of a network server. A component board  120  comprises a base substrate  122 , a plurality of integrated circuit chips (IC chips)  130 , a plurality of opto-electric devices  135  formed on one or more chips, a network of electrical traces  125  formed in and/or on base substrate  122 , a plurality of optical deflectors  140  formed on base substrate  122 , and a plurality of optical waveguides  150  formed on base substrate  122 . For visual clarity in  FIG. 1 , some of these elements and their reference numbers are omitted for the three lower boards  120 . Main optical backplane  110  comprises a base substrate  112 , a plurality of optical waveguides  115  formed on or near the top surface of substrate  112  to interconnect optical signals among component boards  120 , and optionally a network of electrical traces to interconnect electrical signals among component boards  120 , as explained below in greater detail. 
   The network of electrical traces  125  on each component board  120  electrically interconnects IC chips  130  to one another, and preferably interconnects selected IC chips to opto-electric devices  135  and optical deflectors  140 . The IC chips  130  are interconnected by traces  125  according to the desired function performed by the chips and the system. Opto-electric devices  135 , optical deflectors  140 , waveguides  150 , and waveguides  115  provide board-to-board optical-signal interconnections among component boards  120 . Each opto-electric device  135  may comprise a light transmitter or a light receiver, and each chip of opto-electric devices  135  may comprise one or more light transmitters, one or more light receivers, or a combination of light transmitter(s) and light receiver(s). A signal generated in a first IC chip  130  on the top component board  120  may be optically communicated to a second IC chip  130  on a middle component board  120  in the following manner. The first IC chip  130  on the top component board  120  generates a first electrical information signal, which is electrically coupled to a first light transmitter device  135  located on the top component board  120 . The first light transmitter device  135  converts the first electrical information signal to a first optical information signal, and couples it to a first deflector  140 . The first deflector  140  then routes the first optical signal to a first waveguide  150  that will convey the optical signal to a first waveguide  115  on main optical backplane  110 , which in turn will convey the optical signal to a first light receiver on the middle component board  120  for the second IC chip  130 . Optical signals between waveguides  150  and  115  may be coupled by conventional mirrors or gratings formed in main optical backplane  110 . The first deflector  140  receives a first control signal that instructs it to select the first waveguide  150  for routing the first optical signal to the first light receiver. The first control signal is generated by an electrical component of processor  100 , such as an IC chip on the top component board  120 , which can be the first IC chip  130 . 
   On the middle component board  120 , a second waveguide  150  receives the first optical signal from the first waveguide  115  of main optical backplane  110 . Second waveguide  150  may convey the first optical signal directly to the first light receiver device  135  (which is located on a second chip of opto-electric devices  135  on the middle component board  120 ), or by way of a second deflector  140  located on the middle component board  120 . In the first case, the first light receiver device  135  converts the optical signal to an electrical signal, which is then electrically coupled to the second IC chip  130 . In the second case, the second deflector  140  selects the second waveguide  150  from a number of other waveguides  150  (which are optically coupled to other respective waveguides  115 ) before coupling it to the first light receiver device  135  for conversion. The second deflector  140  receives a second control signal that instructs it to select the second waveguide  150 . The second control signal is generated by an electrical component of processor  100 , such as an IC chip on the middle component board  120 , which can be the second IC chip  130 . The first and second control signals may be coordinated by sending control signals through one or more optical waveguides  115  that are dedicated to that purpose, or by sending control signals through optional electrical traces  117  of main optical backplane  110 , which in turn are coupled to selected traces  125  on component boards  120 . 
   In a similar manner, the first information signal generated by first IC chip  130  on the top component board  120  may be optically communicated to a third IC chip  130  on the bottom component board  120 . As before, the first electrical information signal is electrically coupled to the first light transmitter device  135  located on the top component board  120 , and the first light transmitter device converts the first electrical information signal to a first optical information signal and couples the latter to the first deflector  140 . The first deflector  140  then routes the first optical signal to a third waveguide  150  that will convey the first optical signal to a second waveguide  115  on main optical backplane  110 , which in turn will convey the first optical signal to a second light receiver device  135  on the bottom component board  120  for the third IC chip  130 . The first control signal to the first deflector  140  instructs the deflector to select the third waveguide  150  instead of the first waveguide  150 . On the bottom component board  120 , a fourth waveguide  150  receives the first optical signal from the second waveguide  115  of main optical backplane  110 . Fourth waveguide  150  may convey the first optical signal directly to the second light receiver device  135  (which is located on a third chip of opto-electric devices  135  on the bottom component board  120 ), or by way of a third deflector  140  located on the bottom component board  120 . In the first case, the second light receiver device converts the optical signal to an electrical signal, which is then electrically coupled to the third IC chip  130 . In the second case, the third deflector  140  selects the fourth waveguide  150  from a number of other waveguides  150  (which are optically coupled to other respective waveguides  115 ) before coupling it to the second light receiver device for conversion. The third deflector  140  receives a third control signal that instructs it to select the fourth waveguide  150 . The third control signal is generated by an electrical component of processor  100 , such as an IC chip on the bottom component board  120 , which can be the third IC chip  130 . The first and third control signals may be coordinated by sending control signals through one or more optical waveguides  115  that are dedicated to that purpose, or by sending control signals through optional electrical traces  117  of main optical backplane  110 , which in turn are coupled to selected traces  125  on component boards  120 . 
   In this manner, a first information signal generated from an IC chip  130  on top component board  120  can be optically transmitted to a selected chip on one of the other component boards by the coordination of the control signals. In a similar manner information signals generated by IC chips  130  on the other component boards  120  may be optically transmitted to top component board  120  and to the other component boards  120 . For this, each component board has an array of light transmitters, an array of light receivers, and a plurality of optical deflectors  140  and waveguides  150 . Exemplary arrangements of these components are provided below after exemplary embodiments of optical deflector  140  and waveguides  150  are described. 
     FIG. 2  shows a partial perspective view of component board  120  that shows the features of an exemplary embodiment  140 - 1  of deflector  140  and waveguides  150 . For the purpose of visual clarity, the chip for opto-electric devices  135  is offset upwards from its normal position, as shown by the small vertical dashed lines below it. Component board  120  comprises a component layer  121  at the top of the board. Deflector  140 - 1  and four waveguides  150   a - 150   d  are embedded in layer  121 . Waveguide  150   a  is disposed between the chip of opto-electric devices  135  and deflector  140 - 1 , and has a first end disposed under the attachment area for the chip of opto-electric devices  135  and a second end disposed adjacent to deflector  140 - 1 . A reflector  152  is disposed at the first end of waveguide  150   a , and serves to couple light between waveguide  150   a  and an opto-electric device  135 , which may be a light transmitter or a light receiver. (It is possible for reflector  152  to couple light between waveguide  150   a  on the one side, and a small number of opto-electric devices  135  on the other side closely grouped together on the chip). Deflector  140 - 1  is a prism-type deflector and comprises a first optical surface  141  facing towards the second end of waveguide  150   a  to couple light therewith, a second optical surface  142  located opposite to first optical surface  141  to couple light beams in or out of surface  142  at a plurality of directions, and a body  145  of opto-electric material disposed between first optical surface  141  and second optical surface  142 . Three waveguides  150   b - 150   d  are disposed between second optical surface  142  and the attachment edge of component board  120  that attaches to main optical backplane  110 . Deflector  140 - 1  is configured and operated to bend the light passing through it principally along three possible paths, and each of waveguides  150   b - 150   d  is located to couple light between itself and deflector  140 - 1  along a respective one of these paths. 
   More specifically, light may flow through deflector  140 - 1  in either direction along the three paths. In the case that a light transmitter is optically coupled to waveguide  150   a  (through reflector  152 ), light flows through waveguide  150   a  to deflector  140 - 1 , and is then directed principally along one of the three paths by deflector  140 - 1  to one of waveguides  150   b - 150   d . From there, the light is directed to main optical backplane  110 . In this case, waveguide  150   a  may be called an input waveguide, and each of waveguides  150   b - 150   d  may be called an output waveguide. In the case that a light receiver is optically coupled to waveguide  150   a , deflector  140 - 1  receives light at its second optical surface from at least one of the waveguides  150   b - 150   d , deflects the path of the received light toward waveguide  150   a . From there, the light propagates in waveguide  150   a  toward opto-electric device  135 . In this case, waveguide  150   a  may be called an output waveguide, and each of waveguides  150   b - 150   d  may be called an input waveguide. Deflector  140 - 1  may also receive light at its second optical surface  142  from the other waveguides, but this light is deflected in a manner that prevents it from substantially entering waveguide  150   a . Since light may flow in either direction in waveguides  150   a - 150   d , they may be given the following more general names: inner coupling waveguide  150   a  (since it is located more toward the interior of component board  120 ), and outer coupling waveguides  150   b - 150   d  (since they are located at the attachment edge of component board  120 ). 
   Deflector  140 - 1  also comprises a top electrode  143  disposed on the top surface of opto-electric body  145 , and preferably a bottom electrode  144  disposed on the bottom surface of opto-electric body  145 . (In place of bottom electrode  144 , an electrode may be formed on the top of substrate  122  or therein, below component layer  121 .) Top electrode  143  comprises a polygon-shape having two non-parallel sides, one such side facing first optical surface  141  and inner waveguide  150   a , and the other such side facing second optical surface  142  and the outer waveguides  150   b - 150   d . A triangle is shown in the figure. If bottom electrode  144  is used, it preferably has the same shape as top electrode  143 , and is aligned opposite to it. An electric field is established between electrodes  143  and  144  by applying a voltage between the electrodes through electrical traces  125   a  and  125   b , which are coupled to electrodes  143  and  144 , respectively, through respective vias. The electric field causes the portion of body  145  that is between the electrodes to undergo a change in refractive index, thereby creating a spatial change in refractive index that underlies each of the polygon sides of the electrodes. Because the two polygon sides facing optical surfaces  141  and  142  are not parallel, the spatial change in refractive index will cause bending of the light as it passes under each polygon side of electrode  143 , substantially according to the well-known Snell&#39;s law. The result is a deflection of the light&#39;s path through deflector  140 - 1 . The degree of deflection depends upon the change in refractive index, which in turn depends upon the polarity and the magnitude of the applied voltage. The workings of prism deflectors are known to the optics arts, and a detailed description thereof is not needed for one of ordinary skill in the optics art to make and use the present invention. 
   The material of body  145 , the shape of electrodes  143  and  144 , and the distances between second optical surface  142  and waveguides  150   b - 150   d  are preferably selected such that:
         1. light is principally coupled between deflector  140 - 1  and waveguide  150   b  when a voltage of +M (or alternatively −M) is applied between electrodes  143  and  144 ,   2. light is principally coupled between deflector  140 - 1  and waveguide  150   c  when zero volts is applied between electrodes  143  and  144 , and   3. light is principally coupled between deflector  140 - 1  and waveguide  150   d  when a voltage of −M (or alternatively +M) is applied between electrodes  143  and  144 .
 
M may have the value of 5 to 10 volts, given that recent polymer-based electro-optic materials have electro-optic coefficients of over 100 picometers per volt (for example, see the chromophoric organic electro-optic materials from Lumera, and U.S. Pat. No. 6,716,995). Instead of using the voltages +M, 0, and −M to set the beam deflection to select waveguides  150   b - 150   d , one may use the voltages 0, +½M, +M, or 0, −½M, −M (i.e., all voltages of one polarity).
       

     FIG. 3  shows a top plan view of deflector  140 - 1  and waveguides  150   a - 150   d  and the three optical paths that can be established between deflector  140 - 1  and waveguides  150   b - 150   d . The deflection angle of the paths to waveguides  150   b  and  150   d  may be as small at 0.5 degrees in either direction. Waveguides  150   b  and  150   d  have small curvatures to aid in steering the deflected light back into a line substantially parallel with the optical axis of waveguide  150   c . The curvature of waveguides  150   b  and  150   d  may be around 5 degrees. Waveguides  150   a - 150   d  and deflector  140 - 1  may be formed by conventional waveguide processing steps without undue experimentation. 
   When deflector  140 - 1  receives light from opto-electric device  135 , first optical surface  141  provides an optical input for the deflector to receive an optical signal, and second optical surface  142  provides three optical outputs at the exit points of the three optical paths to waveguides  150   b - 150   d , as shown at reference numbers P 1 , P 2 , and P 3  in  FIG. 3 . The first control signal may have three states, each state to select a respective optical path and a respective optical output P 1 -P 3 . In the first state, deflector  140 - 1  couples the received optical signal more toward optical output P 1  than to optical outputs P 2  and P 3 , so that most of the optical signal goes to waveguide  150   b . In the second state, deflector  140 - 1  couples the received optical signal more toward optical output P 2  than to optical outputs P 1  and P 3 , so that most of the optical signal goes to waveguide  150   c . And in the third state, deflector  140 - 1  couples the received optical signal more toward optical output P 3  than to optical outputs P 1  and P 2 , so that most of the optical signal goes to waveguide  150   d.    
   When deflector  140 - 1  conveys light to opto-electric device  135 , first optical surface  141  provides an optical output for the deflector to output an optical signal, and second optical surface  142  provides three optical inputs at the entry points of the three optical paths from waveguides  150   b - 150   d , again as shown at reference numbers P 1 , P 2 , and P 3  in  FIG. 3 . The prism deflector provides a respective variable-coupling efficiency between each optical input P 1 -P 3  and the optical output. Each of the coupling efficiencies changes as the voltage to electrode  143  is varied. The first control signal coupled to electrode  143  may have three states, one state to select a respective optical path and a respective optical input P 1 -P 3  by increasing the coupling efficiency to that optical input with respect to the other coupling efficiencies. In the first state, deflector  140 - 1  makes the coupling efficiency to optical input P 1  significantly greater than the coupling efficiencies to optical inputs P 2  and P 3 . In the second state, deflector  140 - 1  makes the coupling efficiency to optical input P 2  significantly greater than the coupling efficiencies to optical inputs P 1  and P 3 . And in the third state, deflector  140 - 1  makes the coupling efficiency to optical input P 3  significantly greater than the coupling efficiencies to optical inputs P 1  and P 2 . 
   As a closing note to the discussion of  FIGS. 2 and 3 , the electo-optic device  135  indicated in  FIG. 2  has its electrical terminals coupled to chip  130   a  by way of the electrical traces shown between chip  130   a  and the chip for device  135 . If device  135  is a light transmitter, these traces provide the electrical signal that will be converted to the optical signal. If device  135  comprises a light receiver, these traces provide the electrical signal that has been converted from the optical signal. 
     FIG. 4  shows a cross-sectional view of a second embodiment  240  of an exemplary optical deflector according to the present invention, and  FIG. 5  shows a partial perspective view. Optical deflector  240  is useful when an optical signal is to be simultaneously transmitted to two IC chips, or to only one of two IC chips. Deflectors  240  and  140 - 1  may be used on the same substrate, and may be coupled in series (that is, an output of deflector  140 - 1  may be coupled to an input of deflector  240 , and an output of deflector  240  may be coupled to an input of deflector  140 - 1 ). In optical deflector  240 , inner waveguide  150   a  is not used, and two outer waveguides  150   b  and  150   c  are used. Waveguide  150   c  is disposed above waveguide  150   b , with each having a first end disposed under or near the chip-holding device  135 , and a second end disposed near the attachment edge of substrate  120  to main optical backplane  110 . In the figures, the core bodies of waveguides  150   b  and  150   c  are shown with clear areas, and the cladding layers are shown with stippled-fill patterns. The waveguides are preferably disposed such that their core bodies are separated by ˜15 microns or more for single mode waveguides, and by ˜50 microns or more for multi-mode waveguides. A sub-layer  121   b  of component layer  121  described below can be used to provide the desired spacing distance (it preferably has a refractive index equal to or less than that of the cladding layers). While  FIG. 5  shows the waveguide  150   c  being disposed above and directly over waveguide  150   d  in a parallel manner, it may be appreciated that waveguide  150   c  may still lie above waveguide  150   d  but be in a non-parallel relationship. As explained below, the construction of optical deflector  240  is based on Snell&#39;s law and the actions of total internal reflection, partial reflection, and partial transmission. 
   Optical deflector  240  comprises an angled reflector  245  and a variable reflector  250 . Angled reflector  245  is disposed at the left end of waveguide  150   b ; it reflects light between the core body of waveguide  150   b  and the bottom surface of variable reflector  250 . Angled reflector  245  preferably comprises a metal layer formed over a beveled side surface of a sub-layer  121   a  of component layer  121 , the side surface being beveled with respect to the top surface of base substrate  122  and the top surface of component layer  121 , and with respect to the optical propagation axis of waveguide  150   b . As used herein, the adjective “beveled” means formed to a bevel angle with respect to a reference surface or reference line (e.g., an optical propagation axis of a waveguide), with the bevel angle being any angle except 90 degrees (right angle). Angled reflector  245  may be formed by the following sequence of processing steps: forming sub-layer  121   a  using a photo-imageable polymer; photo-exposing the sub-layer through a gray-scale mask that defines the beveled side surface; developing the exposed layer to form the beveled side surface; and then forming a metal layer over the beveled side surface. Thereafter, the layers of waveguide  150   b  may be formed, followed by the formation of sub-layer  121   b , which acts as a planarizing layer as well as a spacer layer between waveguides  150   b  and  150   c . In this way, the metal surface of angled reflector  245  makes an inclined angle with respect to the top surface of base substrate  122 . It is also possible to make angled reflector  245  by the steps of: initially forming sub-layer  121   a  as a slab waveguide comprised of photo-imageable core and cladding layers having closely matched photo-chemistries; photo-exposing the slab waveguide through a gray-scale mask that defines both the beveled side surface and waveguide  150   b ; developing the exposed layer to form the beveled side surface and waveguide  150   b ; and then forming a metal layer over the beveled side surface. 
   Variable reflector  250  comprises a top surface  251  facing device  135 , a bottom surface  252  facing towards the angled reflector  245 , a first side surface  254  facing the first end of waveguide  150   c , a body  255  of electro-optic material disposed between top surface  251  and bottom surface  252 , and a body  260  of high-refractive index material disposed between top surface  251  and bottom surface  252  and adjacent to body  255  of electro-optic material. An interface surface  253  is disposed between bodies  255  and  260 , and is beveled with respect to the top surface of base substrate  122 , with respect to the top surface of component layer  121 , and with respect to the optical propagation axis of waveguide  105   c . In other words, interface surface  253  is formed to a bevel angle with respect to the top surfaces of substrate  122  and component layer  121 , and the optical propagation axis of waveguide  105   c , with the bevel angle being any angle except 90 degrees. Preferably, the bevel angle is within one or two degrees of 45 degrees (45°), and more preferably within a half-degree of 45 degrees. Interface surface  253  is preferably planar. The refractive index of body  260  is substantially greater than the intrinsic refractive index of body  250  of electro-optic material, preferably being at least 10% higher. As used herein, the intrinsic refractive index of an electro-optic material is the refractive index of the material when no electric field is present in the material. The refractive index of body  260  can also be higher than the intrinsic refractive index by 20% or more, 25% or more, and 35% or more. 
     FIG. 5  shows a partial perspective view of optical deflector  240  and waveguides  150   b  and  150   c . Sub-layer  121   c  of component layer  121  has been omitted for visual clarity. As can be seen therein, variable reflector  250  further comprises a second side surface  256  and a third side surface  257  that are oriented substantially transverse to first side surface  254 . A first electrode  258  is disposed on second side surface  256  and a second electrode  259  is disposed on third side surface  257 . Electrodes  258  and  259  apply an electric field to body  255  of electro-optic material in relation to the first control signal. The electric field changes the refractive index of body  255 , and both the field and the refractive index vary with the value of the first control signal. Electrodes  258  and  259  may be spaced from one another by a distance on the order of the width of the core body in waveguide  150   c , typically ranging from 5 microns to 10 microns for single mode waveguides, and 25 microns to 50 microns for multi-mode waveguides. The electrodes may be made of tungsten to minimize reflections from their surfaces. Bodies  255  and  260  can be formed by conventional photolithographic methods. To achieve the bevel of interface surface  253 , one may first form a rectangular strip of electro-optic material over layer  121   b , thereafter form a metal mask for laser ablation, and then cut the bevel with laser ablation with the laser light at an angle (e.g., 45-degree angle) to the top surface of base substrate  122 . Also, plasma etching through a tapered mask may be used, with the tapered mask comprising a photoresist that has been exposed through a gray-scale mask. Dicing using a blade with beveled edge is also possible. After interface surface  253  has been formed, body  260  may be formed and patterned by conventional process steps. Thereafter, the electrodes may be formed by conventional sputtering and etching steps. 
   Variable reflector  250  works as follows. We take the case of opto-electric device  135  comprising a light transmitter that directs a beam of light toward the top surface of component layer  121 , and specifically toward top surface  251  and body  260  of high-refractive-index material. The intrinsic refractive index of body  255 , the refractive index of body  260 , and the bevel angle of interface surface  253  are selected such that interface surface  253  is near or at the initial point of total internal reflection (i.e., critical angle) for light emitted from the light transmitter (i.e., for light directed perpendicular to the top surfaces of layer  121  and base substrate  122 ). This selection can be done by the application of Snell&#39;s law and computer simulation programs available for purchase or on the Internet (e.g., http://www.physics.nwu.edu/ugrad/vpl/optics/snell.html). Then, by changing the refractive index of body  255 , as directed by the first control signal, the majority of the light from device  135  can be reflected off interface surface  253  to the first end of waveguide  150   c  through side surface  254 , or the majority of the light can be transmitted through the interface surface  253  to exit bottom surface  252 , where it strikes angle reflector  245  and enters the first end of waveguide  150   b . Also, the refractive index can be changed to cause the light to split at interface surface  253  such that approximately one-half is reflected to waveguide  150   c  and approximately one-half is transmitted to waveguide  150   b.    
   In this manner, top surface  251  of variable reflector  250  acts as an optical input of deflector  240 , the first side surface  254  acts as a first optical output of deflector  240 , and bottom surface  252  and angled reflector  245  act as a second optical output of deflector  240 . Furthermore, a first coupling efficiency is provided between the optical input and the first optical output, a second coupling efficiency is provided between the optical input and the second optical output, and the variation in the refractive index of electro-optic body  255  enables the relative values of these coupling efficiencies to change in relation to the state of the first control signal. The first coupling efficiency may be greater than the second coupling efficiency in one state of the first control signal, less than the second coupling efficiency in a second state, and substantially equal to the second coupling efficiency in a third state. 
   We give the following example. Body  255  has a refractive index of 1.39 with no electric field. The refractive index can be changed to a value of 1.38 with the application of negative voltage to electrode  258  with respect to electrode  259 , and can also be changed to a value of 1.41 with the application of positive voltage to electrode  258  with respect to electrode  259 . On the other hand, body  260  has a refractive index of 1.95 (40% higher than the intrinsic refractive index of body  255 ). When body  255  is caused to have a refractive index of 1.38, 78% of the light is reflected off interface surface  253  to waveguide  150   c  and 22% is transmitted through the surface to waveguide  150   b . In this state, the first coupling efficiency is 78% and is greater than the second coupling efficiency (22%). When body  255  is caused to have a refractive index of 1.39, 48% of the light is reflected off interface surface  253  to waveguide  150   c  and 52% is transmitted through the surface to waveguide  150   b . In this state, the first coupling efficiency is 48% and is substantially equal to the second coupling efficiency (52%). And when body  255  is caused to have a refractive index of 1.41, 30% of the light is reflected off interface surface  253  to waveguide  150   c  and 70% is transmitted through the surface to waveguide  150   b . In this state, the first coupling efficiency is 30% and is less than the second coupling efficiency (70%). In general, the photo-detectors that receive this light can be designed such that a value of 30% lies below the detection threshold of the photo-detector, and that a value of 50% lies above the detection threshold. The first control signal may have three states to select among the three above divisions of the light beam from device  135 . 
   The high-refractive-index material for body  260  may be provided by the OptiNDEZ A14 material manufactured by Brewer Science Inc. This is a pure polymer-based material. As another option, one may use a conventional polymer that has been loaded with micro particles of a high-refractive crystal, such as sapphire. The electro-optical material in the above example may be a chromophoric organic electro-optic material from Lumera, or one described in U.S. Pat. No. 6,716,995 or other recent patents disclosing new polymers with high electro-optic coefficients. Currently, one can readily obtain a polymer-based electro-optic material with an electro-optic coefficient of about 120 picometers per volt, which can yield a change from 1.39 to 1.405 with the application of about 92 volts per micron between electrodes  258  and  259 . A high voltage driver chip would be needed to drive the electrodes, and such chips are commercially available. The first control signal may be provided as an input to such a chip to generate a voltage amplified version thereof. In the near future, it is expected that the chromophoric organic electro-optic materials will have coefficients of several hundred picometers per volt, which will significantly reduce the voltage that needs to be applied across electrodes  258  and  259 . 
   In the above cases where 52% and 70% of the light is transmitted through to angled reflector  245 , the transmitted light beam makes an angle of between 38 degrees and 33 degrees with a line vertical to the top surface of substrate  122 . This is an average angle of approximately 35 degrees. With this average angle, angled reflector  245  should be disposed to the side of interface surface  253  to receive the angled light, and the beveled surface of angled reflector  245  should be around 27.5 degrees (which is less than 45 degrees by half the value of 35 degrees). The average angle of 35 degrees (and the range from 38 degrees to 33 degrees) can be reduced by disposing high refractive index material between the bottom surface  252  of variable reflector  250  and angled reflector  245 . 
   While optical deflector  240  has been described with opto-electric device  135  being a light transmitter, it may be appreciated that device  135  may be a light receiver, and that the direction of light in deflector  240  may be reversed. In other words, each of waveguides  150   b and  150   c  may couple light beams to the interface surface  253 , and a majority of one of the light beams may be coupled to device  135  based on the refractive index of electro-optic body  255 . In this case, top surface  251  of variable reflector  250  acts as an optical output of deflector  240 , the first side surface  254  acts as a first optical input of deflector  240 , and bottom surface  252  and angled reflector  245  act as a second optical input of deflector  240 . Furthermore, a first coupling efficiency is provided between the first optical input and the optical output, a second coupling efficiency is provided between the second optical input and the optical output, and the variation in the refractive index of electro-optic body  255  enables the relative values of these coupling efficiencies to change in relation to the state of the first control signal in the same manner as the coupling efficiency changed in the above case where the light flowed in the opposite direction. (In other words, optical deflector  240  is a linear system.) The first coupling efficiency may be greater than the second coupling efficiency in one state of the first control signal, less than the second coupling efficiency in a second state, and substantially equal to the second coupling efficiency in a third state. 
   In  FIGS. 4 and 5 , the top surface of high-refractive body  260  has been shown to be substantially parallel to the top surfaces of component layer  121  and base substrate  122 . However, the top surface of body  260  may be inclined to create a more shallow incident angle of light from device  135  onto interface surface  253 . In this case, interface surface  253  would be inclined at an angle of less than 45 degrees with respect to the top surface of base substrate  122 . 
   As another variation, interface surface  253  can be inclined at an angle substantially greater than 45 degrees with respect to the top surface of base substrate  122 , such as 50 degrees and 55 degrees. This would enable one to achieve the initial point of total internal reflection with less of a difference between the refractive indices of bodies  255  and  260 . However, the steeper angle causes the light reflected from interface surface  253  to angle downward into component layer  121  toward base substrate  122 , rather than being substantially parallel to the top surface of base substrate  122 . This can be addressed by positioning waveguide  150   c  at a level in component layer  121  that is lower than that shown in  FIGS. 4 and 5 , so that the left end of the waveguide can capture the downward-angled light. If needed, an angled deflector similar to that of angled deflector  245  can be positioned at the left end of waveguide  150   c , or, more simply, the face of high-refractive body  260  at first side surface  254  may be beveled in order to bend the light from interface surface  253  to a horizontal direction or more a horizontal direction when it exits surface  254 . 
   Claims of the present application encompass the above variations. 
   Having thereby described exemplary embodiments of deflector  140  and waveguides  150 , we return to the global view of the processor shown in  FIG. 1 . As we indicated before, optical connections between each of the boards may be configured by deflectors  140 , with optical routing done through main optical backplane  110 . In  FIG. 6 , we show a schematic diagram of the optical wiring architecture that may be used in processor  100 , which shows one way in which the optical waveguides  115  of backplane  110  may be interconnected with waveguides  150   b - 150   d . The four component boards are indicated at reference numbers  120 - 1  through  120 - 4 . At each end of waveguide  115 , a mirror or grating element is placed to optically couple the end of the waveguide to one of waveguides  150   b - 150   d  in a component board. Twelve optical interconnects are provided, two interconnects between each pair of component boards  120  (one transmission and one reception). Only eight waveguide channels are needed in backplane  110  to implement the twelve interconnects since the six shortest optical interconnects may all be placed in two waveguide channels. Only one pair of a light transmitter and a light receiver is shown for each component board  120  in  FIG. 6  for visual clarity. In practice, each board would have multiple pairs of transmitters/receivers, and the wiring diagram shown in  FIG. 6  would be duplicated for each additional pair. As an alternative to the architecture shown in  FIG. 6 , one can use optical bus waveguides for waveguides  115 , each of which has a bidirectional optical coupler to each component board. This would reduce the number of waveguides  115  to four (one for the receiver of each component board). However, bidirectional optical couplers often degrade the optical signal as it propagates through the waveguide, and so signal strength in the waveguide may become an issue. 
   While we have focused the description on board-to-board interconnects, it may be appreciated that the optical communications described herein may be applied within a single board, such as chip-to-chip communications. Claims of the present application encompass this application as well. In addition, one may place some or all of deflectors  140  in main optical backplane  110 , with control signals for the deflectors being generated within backplane  110  and/or within the component substrates. Also, one may place some or all of the deflectors on intermediate boards that interface between the component boards  120  and main optical backplane  110 . Claims of the present application encompass these variations as well. 
   While the present invention has been particularly described with respect to the illustrated embodiments, it will be appreciated that various alterations, modifications and adaptations may be made based on the present disclosure, and are intended to be within the scope of the present invention. While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention is not limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.