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IBM makes light work of links
We were so busy at PCW Towers last week putting the next print edition to bed (as we say) that we had little time to look into the details of IBM's announcement of a 'breakthrough' photonic modulator that could lead to supercomputers the size of a laptop.
The term photonic modulator sounds alarming, but functionally it is simply an ultra-fast shutter capable of blocking or unblocking light billions of times a second. It can also be seen as an interface between digital electronics and digital photonics, converting electronic information into light information.
The other important point is that it can be made using the same techniques used to manufacture silicon based processors. Intel has also produced silicon modulators; IBM says its design is 100 to 1000 times smaller than similar designs, though it is unclear whether this includes the Intel ones.
Silicon is transparent to the light frequencies used (it is, after all, the main constituent of glass). Silicon waveguides 200 times thinner than a human hair can be etched into processors, acting like short-range glass fibre links using a laser as the light source. They are the photonic equivalent of wires.
The waveguide in the modulator is doped so that its transparency is controlled by a digital electronic signal. The result is a very fast data path that can be used to link processor cores. Additional advantages are that the system generates little heat, and there is no 'crosstalk' between neighbouring data paths.
IBM envisages processors with thousands of cores, like an extended version of the nine-core Cell processor that powers the Sony Playstation 3.
The big problem with optical interconnects is how to switch them without returning the data to an electronic stream and back again, which would be inefficient and create a bottleneck. The switching can be controlled electronically, however.
Above is an IBM diagram of how this would work on a chip. An arbitrary number 'N' of 10Gbit/sec signals enters using the glass fibre input (marked blue, top left). Each signal uses a different colour, or wavelength. The Wave Division Multiplexer (WDM) separates each colour into a dedicated waveguide that leads to an NxN optical cross-connect.
IBM gives few details of how this might work, except that packets would get new headers. The outputs go through another WDM, which pumps them down the output fibre.
The delay line shown in the diagram is used for buffering optical pockets where necessary. Also shown is a Germanium photodetector module for monitoring channels, and CMOS logical circuits (marked VLSI) for monitoring the performance.
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