From: droelke@rdxsunhost.aud.alcatel.com (Daniel R. Oelke)
Date: Tue, 20 Feb 1996 11:46:40 +0800
To: acg@mandrake.cen.ufl.edu
Subject: Re: Optical repeaters
Message-ID: <9602200124.AA09857@spirit.aud.alcatel.com>
MIME-Version: 1.0
Content-Type: text/plain



> 
> > > Optical
> > > repeaters have to pass your signal through an intermediate electronic
> > > stage anyway, since we have no purely optical valve/transistor
> > > equivalents (bosons don't interact with each other at all).
> 
> Dave Emery responds:
> > 	This is not true.   There is now a whole technology of optical
> > amplifiers for fiber communications systems that used Ettrium doped
> > fibers pumped with strong light from a laser at a slightly shorter
> > wavelength. These fiber optical amplifiers have gains in the order of
> > 10-12 db in a section of special doped fiber only about 10 feet long.
> 
> You're right, I do remember reading about these somewhere... didn't
> realize they were already in use.

Optical amplifiers -  they are in use all right.  Want to buy one?
My employer sells them.  (get out a big checkbook though -
gotta cover my salary ;-)

> Even so, I still don't think such a repeater would pass quantum-crypto
> signals, excepting any photons that happened to just "leak" directly
> through.  Your useful quantum state information resides in the
> individual photons originally sent, and any even the optical repeaters
> you describe achieve gain by by gating in *more* photons under the
> incoming signal's control.  In so doing it will collapse the
> wavefunctions of these incoming photons.
> 
> Not to say repeaters on the line aren't possible, but they'll have to
> decode your data using a copy of the "secret" key, then re-encode for
> transmission... so this will be a potential break-in point and need
> good physical security.

This is essentially true for the purpose of quantum-crypto. 
The cascade of phontons triggered from the incoming photons would
mask most of your original phontons - necesitating a secure "repeater".


> > 	The current generation of undersea cables from the US to Europe
> > use these amplifiers instead of the more traditional regenerating
> > repeaters that convert the light to electronic signals, reclock the data
> > stream and convert it back to light with another laser diode.   There is
> > no conversion from light to digital electronic signals all the way from
> > Rhode Island to England - the same light pulses that go into the fiber
> > on one side of the Atlantic come out on the other end without ever
> > having been converted to electronic form in between.
> 
> You said power for the amps comes from a high-intensity,
> shorter-wavelength beam... can this be superimposed on the original
> signal at the point of origin, as with in-line coax-cable amplifiers?

Power for the amps must be electrical.  So, a seperate power cable must
be run seperate with the optical-fiber.  This normally isn't a problem
since the casing/etc of the fibers has lots of metal for protection anyways.

> > 	 These amplfiers have enourmous bandwidth, and can be used to
> > amplify several slightly different wavelengths of light allowing
> > wavelength division multiplexing of multiple streams of light flashes of
> > slightly different "colors" (all the current technology works at around
> > 1500 nm which is well into the infrared).   This can expand the capacity
> > of a single fiber to four to six times the 5 Gb/sec that is the current
> > state of the art.
> 
> Nice... :)
> 
Very nice! ;-)  Flouride based amplifiers should be able to handle up
to 16 channels.  Using state of the art time-multiplex stuff of 10 Gb/sec
gives a total throughput of 160 Gb/Sec.... smoking!

Of course the parts for all of this will set you pack a sizeable 
chunk of change.

If you want to see some further reading about optical amplifiers
as applied in a telephone network - find a copy of "Telephony" 
a trade rag.  There is an article by John Moss and ??? here at Alcatel
that explains stuff in high level terms.  (little to no physics stuff)

Dan

------------------------------------------------------------------
Dan Oelke                                  Alcatel Network Systems
droelke@aud.alcatel.com                             Richardson, TX