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Re: Optical repeaters
>
> > > 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
[email protected] Richardson, TX