Page 357 - Introduction to Information Optics
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342 6. Interconnection with Optics
photodetectors cannot be easily achieved. Polymer optical waveguide technol-
ogy, on the other hand, is particularly suitable for intraboard interconnects
applications for its large area waveguide formation, and each lithography layer
is precisely aligned. This can be viewed as an optical equivalent of electrical
printed wiring board technology in which the fabrication cost is independent
of the interconnect functionality and complexity. In order to take advantage of
the polymer waveguides, we must be capable of implementing optoelectronic
devices such as laser diodes and photodetectors in the same PC board package
The polymer waveguides can be fabricated, integrated, and packaged into the
board with the fully embedded architecture shown in Fig. 6.1; the insertion of
optical interconnects becomes an acceptable approach to upgrade microelec-
tronics-based high-performance computers.
An optical H-tree fanout network based on polyimide channel waveguides
was fabricated for optical clock signal distribution in a Cray multiprocessor
supercomputer board. The original design of the Cray T-90 supercomputer was
based on an external laser diode modulating at 500 MHz which went through
a l-to-32 waveguide star coupler to provide a system clock to 32 different
boards of the supercomputer. At the edge of each board, the optical clock was
converted to an electrical clock signal. The l-to-48 fanouts of all 32 board were
realized through delay-equalized electrical transmission lines. The communica-
tion distance of these clock lines within each board is as long as 32.6 cm, which
makes the further upgrade of the clock speed to GHz level unrealistic. To
release such a bottleneck, we demonstrate an optical clock signal distribution
using the building blocks mentioned in the previous sections. Polyimide
waveguides are employed as the physical layer to bridge the system clock
optically to the chip level. As a result, the distance of electrical interconnection
to realize the system clock signal distribution at the GHz level will be
minimized at the chip level instead of board level. The H-tree structure is
selected to equalize the propagation delays of all 48 fanouts. Due to the
relatively short interconnection distance, the waveguide is multimode with a
cross section of 50 /mi wide and 10/mi deep. The horizontal dimension of the
waveguide is to match the 50-/«n multimode glass fiber, and the vertical
dimension can be matched with a three-dimensional tapered waveguide [13].
Both tilted grating couplers and 45° TIR mirror couplers are fabricated to
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efficiently couple light in and out of the H-tree waveguide structure. A 45 TIR
mirror provides better coupling efficiency and a shorter interaction length to
fulfill such a coupling. Figure 6.40(a) shows the broadcasting of the optical
signal through the H-tree structure at 632.8 nm. Forty-eight well defined light
spots are coming out surface-normally. Figure 6.40(b) uses a fabricated l-to-48
H-tree waveguide structure operating at 850 nm. All 48 surface-normal fanouts
are provided through 45° TIR waveguide mirrors. Based on the loss measure-
ment, a VCSEL with enough modulated power is needed to compensate —2
dB coupling loss, — 6 dB waveguide propagation loss, — 17 dB fanout loss, —3
dB bending loss, and —3 dB power margin. The H-tree waveguide structure is