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