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Optical Interconnects

Published on Apr 02, 2024


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


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.

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

Advantages of Optical interconnects

The narrow band nature of optical signals makes it relatively simple to construct high quality, uniform transmission lines that operate at high data rates. An optical signal is modulated onto a very high frequency carrier of an order of 10 Hz and thus the signal serves the purpose of matching the impedance of the transmission line, free space in this context, to the load.

There is no power dissipation associate with this termination as with the resistive termination in the case of electrical signaling. The narrow band nature also eliminate frequency dependent losses for optical interconnects, there is no equivalent to the aspect ratio limit. Although, dissipation limits the distance over which signals can propagate in optical fibres, but this distance limit is not important over the distances encountered in self-contained digital systems.

Various forms of optical crosstalk exist in parallel optical data links, as with electrical signals, these can be controlled by separating the channels by sufficient distance. Optical signals to not in themselves suffer from the crosstalk problem, but the electronic systems at both the ends are still susceptible to the problem. Optical signals do not produce and are immune from electromagnetic interference.

Parallel optical data links also have lower skew than parallel electrical data links. In free-space optical data links, the intrinsic skew is zero-whilst in fibre data links, the skew resulting from manufacturing variations are much smaller than achievable in co-
axial cable eliminates the need to resynchronize each channel in medium-distance optical interconnect and lead to and overall reduction in power consumption.

Optical Interconnects

The technology for implementing single-channel optical links is very mature. The use of single channel optical links in long-haul telecommunication is well established. This technology is not suitable for constructing highly-integrated interfaces to VLSI electronics as it uses a large number of individual optoelectronic devices and fibre components to combine many independent data sources that are generated using special purpose electronics operating at a data rate greater than 20 G bit/s.

It is more likely that interfaces to individual VLSI circuits will be implemented using more channels operating at lower speeds that are compatible with mainstream electronics. There is a requirement for large arrays at optoelectronic devices, a transmission medium that can carry multiple channels and new packaging techniques to interface the optoelectronic devices and associated packaging technologies will be required to provide sufficient channels.

Integration of the optoelectronic devices with mainstream VLSI electronic e.g. CMOS is required to make these multiple optical links economically feasible. Monolithic optoelectronic integrated circuits comprising of a detector and receiver, produced using a special purpose fabrication process have been used to provide high-performance for single-channel links. However, multiple-channel links require a VLSI process to implement arrays of interface circuits i.e. receivers to convert photocurrents into digital logic levels, and transmitters to convert digital logic levels to the signals required to drive modulators or emitters. A capability to include complex digital functionality such as buffering and routing is also desirable.

A huge investment would be required to develop-from scratch, a high yield VLSI process with a full optoelectronic capability. A more realistic approach is to leverage off existing investment in mainstream VLSI by implementing the electronic functionality in a conventional technique for integrating for integrating separately fabricated optoelectronic devices with the VLSI electronics. Silicon CMOS is a strong

candidate for the base VLSI technology because of its low cost and widespread use, but other technologies with VLSI capability that are available at moderate cost as Gas mesfets, could also be considered.

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