From: jim bell <jimbell@pacifier.com>
Date: Tue, 20 Feb 1996 14:17:01 +0800
To: Alexandra Griffin <cypherpunks@toad.com
Subject: Re: Optical repeaters
Message-ID: <m0toiH0-00090QC@pacifier.com>
MIME-Version: 1.0
Content-Type: text/plain


At 07:45 PM 2/19/96 -0500, Alexandra Griffin wrote:
>I wrote:
>
>> > 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.

Actually, they're in very common usage today.


[stuff deleted]

>> 	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?

Uh, no.  See, the problem is that the (long)transmission fiber has even more 
attenuation at about 980 nm (the "pump wavelength) than about 1500, the 
wavelength of interest.  It is necessary to generate and apply the power at 
the amplifier site.

>> 	 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... :)

They are!  Truly amazing technology.  It's like a "negative-loss" cable.  
Invented about 10 years ago, and they vastly improved the ability to do LD 
communications through fiber.  Huge bandwidth, compared to current usage.

Jim Bell
jimbell@pacifier.com