|MadSci Network: Physics|
Hi, Greg - although you referred your question to John, somehow it was sent to me.
Let me address a comment towards the end of your message first: yes, the sort of research you are referring to is going on. It is called Free Space Optics.
In "free space optics", the data is transmitted as light from lasers, but there is no fiber - the light travels through the area, between locations on the board - or even from board to board, or through transparent "waveguides" built into the boards. Some expected advantages of free space optics include:
1) Improved isolation (reduction of high voltage/current feedback in computer controlled high power equipment)
2) Reduction in power dissipation, possibly
3) Reduction in clock and data skew in high data rate systems
4) Elimination of EMI (reduces need to shield noise and noise sensitive lines with large, grounded shields -- EMI is a big issue, for example, in airplanes; hence, they ask you to turn off cellular phones and such while the plane is enroute)
In terms of how many could fit in how small of an area - I have seen research examples with about 2000 interconnections on a 1 x 1 cm chip area using a smart pixel array. You will find other examples in the links I list later.
The beams can be made very small - perhaps 5 to 50 microns diameter coming out of a VCSEL (Vertical Cavity Surface Emitting Laser) but the laser beams do diffuse, and diffuse more the greater the distance the beams must travel - and the photodetectors must be large enough to gather in sufficient light energy for detection of a "1" vs a "0". I've been told that, if the data is to be transmitted a relatively short distance, it is possible that the spacing between photodetectors could be on the order of 100 microns. However, in general, the spacing between "pixels" will need to be on the order of several hundred microns, or even a few millimeters.
Here are some links that explore this interesting approach:
Free Space fiber optic communication:
Try the links in the MadSci Library for more information on Physics.