Page 316 - Introduction to Information Optics

6.1. Introduction 301

optical interconnects will certainly provide handsome payoffs to the computer
industry, how to accommodate this in the existing systems architecture cost
effectively remains a major issue.
Machine-to-machine interconnection has already been significantly replaced
by optical means such as optical fibers, and interconnection distances from 1 m
6
to t() m have been realized. The present challenge is for interprocessor
memory interconnections, where the transmission bus line such as backplane
bus represents the most serious problem for upgrading system performance.
Therefore, major research thrusts in optical interconnection are in the back-
plane and board level where the interconnection distance, the associated
parasitic RLC effects, and the large fanout-induced impedance mismatch
impose the most serious problems in fulfilling bandwidth requirements. Optical
interconnection, in general, has been widely thought to be a better alter-
native for upgrading system performance. Optical interconnect approaches
can be divided into two main categories, free-space and guided-wave optical
interconnects. A number of free-space approaches have been proposed and
demonstrated. However, reliability and packaging compatibility of these dem-
onstrated optical interconnect systems [6, 7, 8] have impeded the integration of
optical interconnects into a real system. For example, the board-level optical
interconnections reported in [6] all use hybrid approach where both electronic
and optoelectronic components are located at the surface of the board. Such
an approach makes packaging difficult and costly. Furthermore, the employ-
ment of free space instead of guided-wave optical interconnections reported in
[6, 7, 8] makes the system vulnerable in harsh environments. On the other
hand, the guided-wave approach offers several inherent advantages over the
free-space approach. For example, in guided-wave interconnects all optical
components, including source and detector, can be fully imbedded in the
multilayer board, making them less sensitive to environmental variables, and
also leaving the top surface of the board available for other necessary electronic
components. This chapter describes the research being carried out at Micro-
electronics Research Center, University of Texas at Austin, and its collabor-
ators on similar guided-wave interconnects based on polymer waveguides
functioning as optical transmission bus lines.
To make optical interconnects acceptable to computer companies, it is
important to make the insertion of optics compatible with the standard PC
board fabrication process so that technology improvement can be achieved in
a cost-effective manner. The polymer-based optical bus can be employed to
upgrade interconnection speed and distance. High parallelism can be realized
by implementing a linear optical waveguide array. A board-level polymeric
channel waveguide array-based optical bus incorporating both transmitting
(electrical-to-optical) and receiving (optical-to-electrical) functions within the
embedded interconnection layers of the three-dimensionally (3-D) integrated
multilayer PC board involving both electrical and optical interconnections is