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INTROUCTION
Parallel optical interfaces
can be conceived that consist of arrays of optoelectronic devices of the order
of one thousand optical channels each 'running at speeds around I Gbit/s and hence
offering and overall capacity of 1 Gbit/s to a single integrated circuit. Although
there are still unresolved difficulties in the areas of architectural design,
manufacturing processes, simulation and packaging (as explained later), the technology
has now developed to the point that it is possible to contemplate its use in commercial
systems within a time-frame of 5-10 years. Fig 1st shows the concept of chip-to-chip
communication using optics. The
idea of using optical techniques to address the chip-to-chip interconnection problems
has been around for a long time. However, it is only in the last few years that
technology with a realistic promise of eventual commercial applications has emerged.
Progress can be attributed to a shift away from trying to develop custom VSLI
techniques with in-built optoelectronic capability, towards developing techniques
to allow parallel arrays of separately fabricated optoelectronic devices to be
tightly integrated with standard foundry VLSI electronics, e.g. CMOS 2.
Motivation The
optics can reduce the energy for irreversible communication at logic level signals
inside digital processing machines. This is because quantum detectors, quantum
sources can perform and effective impedance transformation that matches the high
impedance of small devices to the low impedances encountered in the electromagnetic
propagation. This energy argument suggests that all except the shortest intrachip
communication should be optical. We
see that there is a limit to the total number of bits per second, of information
that can flow in simple digital electrical interconnection set by the aspect ratio
at the interconnection. This limit is largely independent of the details of the
design at the electrical lines. As the limit is scale-invariant, neither growing
nor shrinking the system substantially changes the limit. Exceeding the limits
will require additional multi-level modulation. Such a limit will become a problem
for a high band-width machine. Optical interconnect can solve this problem since
they avoid the resistive losses that gives the limit. 3.
Capability and Limitations of Electrical Interconnects Now
the physical origins of the limitation of conventional electrical interconnects
are listed 3.1
Frequency Dependent loss: - The main physical limitation on the use of electrical
signaling over long distances in frequency dependant loss due to the skin effect
and dielectric absorption. Attenuation due to the skin effect increases in proportion
to ?f above a certain critical frequency. This given rise to a so called 'aspect-ratio'
limit on the
The
constant of proportionality Bo is related to resistivity of copper interconnects
and is only dependant on the particular fabrication technology. It ranges from
a 1015 bit per second to 1016 bit per second
The
aspect-ratio limit is scale invariant and applies equally to band to band interconnect
as well as to connection on a Multichip-Module. Also, for a fixed cross-section,
the limit is independent of whether the interconnect is made up of many slow wires
or a few fast wires. The aspect ratio limit is part of the reason why fiber-optics
has replaced co-axial cables in telecommunication networks. Attention
due to dielectric absorption increases in proportion to frequency leadint and
upper limit on operating speed, which is inversely proportional to distance. It
is independent of conductor Cross-section and is not scale-invariant. For a 1
Gbit/s interconnect, it would limit the distance to 1 m in a standard fiber-glass
interconnect and may be to 10m in a good low-loss material like poly tetra fluoroethene
(PTFE).
However, attenuation due to dielectric absorption does not limit the
overall band width of an interconnect over a certain distance in the same way
as the skin effect, because a higher overall bandwidth could be obtained by using
more conductors within the same cross-section
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