Page 356 - Introduction to Information Optics
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6.6. Optical Clock Signal Distribution     341

       Ironic interconnection layer is constructed using the building blocks mentioned
       earlier. Such a guided-wave optoelectronic interconnect network is inserted
       into the Cray supercomputer boards to become an additional optical intercon-
       nection layer among many other electrical interconnection layers. As a result,
       the future supercomputer system may have a clock speed more than ten times
       higher than current systems.
          Our approach is to construct an additional optoelectronic interconnection
       layer (OIL) for the high-speed optical clock signal distribution using polymer-
       based guided-wave devices. The selection of the guided-wave approach is
       mainly based on system alignment and reliability concerns. Si-CMOS process
       compatibility and planarization of the OIL are the two major technical
       concerns. As shown in Fig. 6.39(a), a polymer-based waveguide H-tree system
       is employed to replace the existing electrical fanout interconnect network
       shown in Fig. 6.39(b). The optical clock signal delivered by an optical fiber is
       coupled into the OIL using an input surface-normal waveguide coupler, and
       distributed throughout the board by the polymer-based channel waveguide
       network. The distributed optical clock signal at each fanout end will be
       coupled into a corresponding photodetector by an output surface-normal
       grating coupler.
          The building blocks required to facilitate such an optical H-tree system (Fig.
       6.39(a)) include high performance low-loss polymer-based channel waveguides,
       waveguide couplers, l-to-2 3 dB waveguide splitters and 90° curved waveguide
       bends. These components allow the formation of a waveguide H-tree for the
       required optical clock signal distribution, where all the optical paths have the
       same length to minimize the clock skew problem. The employment of optical
       channel waveguides and surface-normal waveguide couplers provides a com-
       pact, mechanically reliable system. Due to the massive fanouts (48) over a large
       area, waveguide propagation loss must be minimized while waveguide grating
       coupling efficiency has to be maximized. These two factors are very important
       to ensure enough optical power at the end of the photodetectors for high-speed
       operation. While fiber-optics technology has been successfully implemented
       among cabinets as replacements for coaxial cable point-to-point link, its
       application inside a cabinet on the board-level system is severely limited due
       to the bulkiness of fibers and fiber connectors and significant labor and cost
       involved in parallelism of the interconnects. Recent achievements in plastic
       fiber-based optical interconnects have demonstrated an optical interconnect
       from a centralized light source to 1.-to-many direct fanouts within one board.
       Several Gbit/sec optical signal transmission has been demonstrated experimen-
       tally [41,42,43]. While having the advantages of low loss and easy implemen-
       tation of the optical layer, this type of interconnect has an intrinsic drawback
       in intraboard optical interconnections. The alignments of laser to fiber and
       fiber to detector are difficult to achieve. In the context of optical clock signal
       distribution, alignment of 48 plastic fiber to 48 independently addressed
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