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
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