Page 360 - Introduction to Information Optics
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6.7. Polymer Waveguide-Based Optical Bus Structure  345

       from transmission to receiver latching. In addition, there may be skew in the
       transmission lines themselves, due to unequal capacitive loading, unequal
       distances to ac grounds, or for some other reason. (FutureBus transmission
       lines are purposely skewed to ensure that data arrive before strobe.) These
       skews limit the attainable transfer rate to 40 mega transfers/sec or 160 MB/sec
       for a 32-bit bus. Electronic bus lines are not typically terminated in matched
       impedance, since this would require the drive currents to be too high.
       Therefore, the bus line will not settle until all end reflections have subsided
       (several round-trip times). By contrast, polymer bus lines may be terminated
       in antireflection coatings, suppressing end reflections and reducing settling time
       to zero.



       6.7.1. OPTICAL EQUIVALENT FOR ELECTRONIC BUS
            LOGIC DESIGN

         Before discussing an optical backplane design in its entirety, we must
       present optical equivalents of necessary bus components, such as bidirectional
       transmission lines, stubs, transmitters, and receivers. Further, optical equival-
       ents of line voltage, logic levels, and open-collector and tristate line driving and
       receiving must be derived. Once these issues are resolved, the way will be clear
       for defining an optical bus that is fully compatible with existing IEEE
       standardized bus protocol.
         The optical equivalent of a PC board trace is a polymer-based optical
       waveguide. A very important consequence is the ability to provide modulation
       using VCSELs and demodulation using photoreceivers for the same board
       where the polymer waveguide is located. An unloaded (no boards attached) PC
       board trace has a typical signal propagation speed on the order of 0.6 c. The
       speed drops to below 0.2 c for a fully loaded bus line. The polymer, which forms
       the optical waveguide, has an index of refraction n = 1.5. The optical signal
       propagation speed is c/n = 0.67 c, similar to that of the unloaded electronic bus
       line.
         It is important to note, however, that there is no optical analogue to driver
       capacitance from attached boards, which causes loading of electronic bus lines.
       Therefore, the optical signal speed retains the same high value regardless of the
       presence or absence of line drivers in the system. This means that the optical
       bus round-trip delay time will be lower by a factor of 3 than that of the
       electronic bus. A connection to an electronic bus line takes the form of a stub
       or tee junction in the PC board trace; usually, such a stub connects to a line
       transceiver. The optical equivalent of a stub is high-efficiency waveguide
       couplers, such as tilted gratings or 45° TIR waveguide mirrors, which allow
       light from a second waveguide to be coupled into the optical bus line, and
       low-efficiency coupling, used to couple light out of the bus for detection. The
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