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From: lindsay@gandalf.cs.cmu.edu (Donald Lindsay)
Newsgroups: comp.arch
Subject: Optical Interconnect
Message-ID: <12618@pt.cs.cmu.edu>
Date: 8 Apr 91 22:34:12 GMT
Organization: Carnegie Mellon
Lines: 82


I posted recently about using lasers to communicate short distances,
e.g between chips. I was asked by mail about how you route light, so,
he's some answers. Disclaimer: I don't do this stuff: corrections and
donations welcome.

The most obvious idea is to use optic fibers, because you can buy
them, and they have unbelievable optical quality. This clearly solves
the routing problem - it would be a bit like wire-wrap.  But how to
attach a fiber to a chip?

There are several answers. I think one group made a notch in the edge
of a chip, so that a fiber in the notch would be properly aligned
with an edge-emitting diode.  Now that we can make arrays of
face-emitting lasers, there's a simpler answer: just epoxy the fiber
end onto the chip. One of the lasers will happen to be on the fiber's
optical axis, and one is enough. One Bell Labs estimate was that they
could build 1024 lasers in the space of a single conventional bonding
pad.

Fibers are bulkier than we might like, and aren't integral, so other
ideas have been followed. One of the older ideas is optical layers.
Various people have sent light sideways through films [as in the
phrase "thick film", not as in camera film]. It turns out that e.g.
lenses can be formed by manipulating the fabrication of the film.
Since beams of light can cross and interpenetrate without
interference or heavy S/N problems, there were suggestions for e.g.
shuffle-exchange networks sharing the same film.

There has been talk about "free space" communication, meaning that
beam travel isn't guided. There problem here is to keep the
geometries fixed well enough so that multiple beams can be used,
without problems from e.g. vibration.

The "free space" doesn't have to filled with air, however. It could
be filled with the next wafer in a stack of wafers. In this idea, you
send light vertically, right through a wafer. Since a wafer is only a
few mils thick, it doesn't necessarily matter that it isn't all that
transparent.  One can even do interesting S/N tricks, like etch
Fresnel lenses onto the back face of the next wafer up.

A newer "free space" proposal fills the free space with quartz.
Specifically, Bell Lab's "optical LEGO(tm)" would use rectangular
prisms of quartz, with all but one face silvered. The remaining
face is etched in various useful ways. They have fiddled with
making gratings and lenses: the optical path is essentially

            ^
  \        /
  -----------
  | \/\/\/  |
  -----------

with the lenses (etc) in the middle region of the top face.  The
suggestion is to place chips there, face down, and run the optical
interconnection through the quartz. Further, it is suggested that
multiple quartz blocks be connected, LEGO fashion, ie

       +---------+
       |         |
+------+-+-----+-+------+
|        |     |        |
+--------+     +--------+

The idea here is that by etching mechanical alignment guides, it
would be possible to plug the pieces together (while clean!) with
geometrical accuracies fairly near the order of lithographic
accuracies.

To sum up, I don't know that any of the above will become popular.
But these ideas, and the numerous non-optical proposals, have
completely convinced me that little gold wires have got to go.  The
bonding pads are football fields: the power is brutal:  pin
inductance chokes the signal. The physics to to better is there:  a
mere few billion dollars of engineering development costs shouldn't
stop an industry like ours.




-- 
Don		D.C.Lindsay .. temporarily at Carnegie Mellon Robotics