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From: ga1056@sdcc6.ucsd.edu (George D.T. Lu)
Newsgroups: comp.arch
Subject: Re: Optical Computers
Message-ID: <14020@sdcc6.ucsd.edu>
Date: 9 Nov 90 00:45:50 GMT
References: <5506.9011011201@olympus.cs.hull.ac.uk> <10789@milton.u.washington.edu>
Organization: University of California, San Diego
Lines: 61

In article <10789@milton.u.washington.edu> whit@milton.u.washington.edu (John Whitmore) writes:

>	The patch-panel style of computers (like most old analog computers)
>is a good model for the best of optical computing devices.  Lenses do
>a kind of Fourier transform, gratings are a kind of resonant multipole
>filter, and so forth.  You build the physical elements into the gizmo
>you want, rather than programming it on the fly.

This is ture for the more traditional optical information processing.
I said that optics is more suitable for global interconnection,
therefore it is not economical to build a system with optical gates.
You will still get the conventional programming model with the
electronic processors.  Holograms are also gratings, but rather than
using them as lens subsitutes (holographic optical elements, used in
compact disk readout heads and the barcode scanner in your local
supermarket), they can also be used for interconnection.  For WSI, you
tend to encounter clock-skew at high clock-rate, this can be addressed
by optical clock distribution using holograms.  (We have developed an
expert system here that can generated various holograms using
different encoding schemes and fabricate them with the electron-beam
machine down the hall.)

>	Optical switching elements for digital computation are awkward,
>need multiple light sources (like multicolor laser beams), and will
>(IMHO) NEVER achieve current LSI device densities, because the wavelength
>of light used is too long. 

Not ture.  There are several ways to modulate laser beams, frequency
tuning is not the best way.  Most of the spatial light modulators
works with polarization.  Bell Lab's Self-electro-optic-effect device
(SEED)is pretty promising.  In the februrary 1990 issue of Ooptics and
Photonics News, David Miller showed a 64 x 32 array of symmetric
SEEDs, occupying 1.3 mm square.  It is designed to work in reflection
with 16,384 light beams in arrays. Each SEED consists of two diodes
doing some sort of differential detection.

>	That said, there is real benefit to optical subsystems as
>peripherals to electronic computers.  FDDI and Ethernet bridge fiber optic
>data links are a good example, ROM and WORM optical disks are another,
>and the future will no doubt reveal many more.
>
>	John Whitmore
>	whit@milton.u.washington.edu

There are several groups, including UCSD, working on parallel optical
disks that can read an image off the disk in parallel.
Three-dimensional optical memories are also in the works. I see optics
becoming more than just high-speed data links, but we may need time to
get these things out of the labs.  The May 10, 1990 issue of Applied
Optics is a special issue for optical computing.  It has near two
dozen papers on very recent progress made in this field, highly
recommanded reading if you can find a copy of Applied Optics (it says
on the cover that library use prohibited until 1995, I have no clue
why they want to do that).

George Lu
UCSD Optical Information Processing Group
lu@poet.ucsd.edu
dlu@ucsd.edu
dlu@UCSD.bitnet
uscd!dlu