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Re: Projection displays

> From:  <>

>We have a project looking to do this with Bragg cells (Rich/Brian, you
>listening?).
>A Bragg cell is a solid state crystal that changes its refractive
>index when you put electicity across it.  Used in optical computing.
>Quartz is a low-end example.

TeO2 (paratellurium) is what's commonly used as an acousto-optical spatial
light modulator in holovideo.  The way it usually works is that you launch
an acoustic wave down the medium; zones of compression (longitudinal wave)
or shear (transverse wave) have a higher refractive index, and zones of
rarefaction or no shear have a lower index.  Higher index means phase delay
for light propagating perpendicular to the acoustic axis.

> So, the idea is shine a laser through the first Bragg cell for X
> dimension scanning and through the second Bragg cell for Y dimension.
> Voila', a solid state, no moving parts scanner.  About 15 degree FOV
> we hope.

15 degrees is optimistic.  I'd settle for 1.5 from the Bragg cell itself and
use a 10X telescope to increase the deflection.

> Problems:

> 1) Getting an efficient laser

Most of your power will go into running the AO SLMs (about 1W apiece) and
most of your losses will be in undiffracted light.

> 2) If the scanning fails, not blinding someone with it

Diffractive deflection works in one of two ways: either you suppress the
zeroth-order (input) beam by running it into a stop and work with one of the
first-order beams, or in the case of Bragg diffraction, the input axis is
complementary to the output.  So you only have to worry about the deflection
signal locking to a fixed angle, and since you have two separate deflectors,
you're interested in the likelihood of both systems failing simultaneously.

Some folks at Zenith built a projection TV based on this principle but they
used a liquid-state Bragg cell (water) with ultrasonic drivers.  See "A
Television Display Using Acoustic Deflection and Modulation of Coherent
Light", A. Korpel et al, Applied Optics v.5 #10, Oct. 1966.  A considerably
more recent reference describes the first computational holovideo system;
try http://www.media.mit.edu/people/lucente/holo/spie90.ps for starters.

    -Rehmi

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