[Laser] modulated retroreflectors

[Kerry Banke]

If one searches for modulated retroreflector on the web, you'll find plenty of info about ongoing projects of that type. A few years ago I visited  a project at UCSD here in San Diego doing just that.
 - Kerry N6IZW - 

--- On Wed, 3/18/09, Tim Toast <toasty256 at yahoo.com> wrote:

From: Tim Toast <toasty256 at yahoo.com>
Subject: [Laser] modulated retroreflectors
To: "laser mailinglist" <laser at mailman.qth.net>
Date: Wednesday, March 18, 2009, 6:54 AM

This talk about corner cubes got me wondering about passive optical repeaters
using modulated corner cubes. Has anyone in here experimented with modulating a
retro-reflector either homemade or not?

While googling the subject (mostly research papers) i saw that some are using
an AOM technique to get AM or phase shifts or polarization effects on the beam.
While others use a shutter to mechanically do it at lower frequencies. Or
something as simple as the old Bell photophone type flexible mirror to modulate
a beam with your voice.
 
Seeing how touchy the alignment is when making a good retroreflector from
scratch, it's easy to see how a very small misalignment of one of the 3
mirrors can effect the whole reflection in a major way.

If the final mirror adjustment is left "close enough" and instead, a
slight modulation is impressed on it by moving the mirror in and out of
alignment, or on either side of the exact alignment, it could be modulated. This
would only require a very slight movement of one mirror to fully modulate the
beam, breaking it up into the 6? misaligned beams.

In a communication link consisting of two stations, each would have a modulated
corner reflector pointed roughly at the other station. Alignment is each side
directing its laser at the other till they get a reflection either visually or
with a test signal. 

After this initial alignment is done, the transmitted light beams won't
need to be modulated. The entire two way communication is or can be done with
Unmodulated light sources... Kind of a strange concept at first maybe. Each
receiving station is providing the carrier for the others transmitting station.
One of the two stations would not even be required to have a light source at all
if the other station can modulate its own light beam - to make up for the
missing carrier from the other station. 

All this requires the beams to travel twice the distances they would have
without the passive retroreflectors of course. So a path that is only barely
good enough for a direct link would not be able to use this method. Maybe it is
only good for about half the distances achieved in other ways.





 



      
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