Optical analog computing?

[John S. Denker]

"R. A. Hettinga" wrote:
...
> "the first computer to crack enigma was optical"
> "the first synthetic-aperture-radar processor was optical"
> "but all these early successes were classified -- 100 to 200 projects,
> and I probably know of less than half."
> 
> --> Do these claims compute?! is this really a secret history, or does
> this mean holography, of am I just completely out of the loop?1

Gimme a break.  This is remarkable for its lack of 
newsworthiness.

1) Bletchley Park used optical sensors, which were (and
still are) the best way to read paper tape at high speed.
You can read about it in the standard accounts, e.g.
  http://www.picotech.com/applications/colossus.html

2) For decades before that, codebreakers were using optical
computing in the form of superposed masks to find patterns.
You can read about it in Kahn.

3) People have been doing opto-electronic computing for 
decades.  There's a lot more to it than just holography.  
I get 14,000 hits from
  http://www.google.com/search?q=optical-computing

> Optical info is a complex-valued wave (spatial frequency, amplitude and
> phase)

It isn't right to make it sound like three numbers (frequency, 
amplitude, and phase);  actually there are innumerable 
frequencies, each of which has its own amplitude and phase.

> lenses, refractions, and interference are the computational operators.
> (add, copy, multiply, fft, correlation, convolution) of 1D and 2D arrays
>
> and, of course, massively parallel by default.
> 
> and, of course, allows free-space interconnects.

Some things that are hard with wires are easy with
light-waves.  But most things that are easy with wires
are hard with light-waves.

> Here's a commercialized effort from israel: a "space integrating
> vector-matric multiplier"  [ A ] B = [ C ]
> laser-> 512-gate modulator -> spread over 2D
> "256 Teraflop equivalent" for one multiply per nanosecond.

People were doing smaller versions of that in
the 1980s.

> Unclassified example: acousto-optic spectrometer, 500 Gflops equivalent
> (for 12 watts!) doing continuous FFTs.  Launched in 1998 on a 2-year
> mission. Submillimeter wave observatory.

Not "FFTs".  FTs.  Fourier Transforms.  All you need for
taking a D=2 Fourier Transform is a lens.  It's undergrad
physics-lab stuff.  I get 6,000 hits from:
  http://www.google.com/search?q=fourier-optics

> Of course, the rest of the talk is about the promise of moving from
> optoelectronic to all-optical processors (on all-optical nets & with
> optical encryption, & so on).

All optical???  No optoelectronics anywhere???
That's medicinal-grade pure snake oil, USP.

Photons are well known for not interacting with
each other.  It's hard to do computing without
interactions.

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