Heath,

>> By the way, my recollection is the undersea regenerators do purely optical 
>> regeneration.
>> There is no O-E conversions undersea, only at the landing stations and 
>> terrestrial components.
> 
> I'm not clever enough to know of some way that you could do optical
> regeneration without converting the signal to electrical and
> retransmitting back as optical.. How is that done?

Erbium Doped Fiber Amplifiers (EDFAs) do not re-shape or re-time the signals 
(the last 2 R's in 3R -- re-amplification, re-shaping, and re-timing).  Raman 
is another popular amplification technology, widely used in long-haul WDM.  
Some systems have the flexibility of using EDFA and Raman amps on the same 
spans.

EDFAs amplify a band of spectrum (C- and/or L-band, depending on the device) -- 
signal *and* noise.  The amplified noise floor is clearly visible if you 
connect an optical spectrum analyzer to the output of an EDFA -- you see a big 
wide bump across the entire amplified band with spikes for each wavelength.   
An optical signal can only go through so many EDFAs before it becomes too 
degraded to be accurately converted back to an electrical signal by the 
receiver -- either due to dispersion (especially if uncompensated) or noise, 
tolerances of which are different for every device...(EDFAs introduce some 
amount of noise, so OSNR before EDFA != OSNR after EDFA :-) )

That being said, one can find examples of all-optical regeneration [1], but I 
do not know of any transport vendors who have integrated this capability into 
currently shipping products.  (Some have developed various tricks like 
electronic dispersion compensation, but IIRC, these work by pre-distorting the 
signal.)

Getting back to the original post from this thread -- when I first read it, I 
immediately wondered whether the vendor might be using coherent optical 
receivers which have much higher dispersion tolerances, allowing the optical 
signal to travel much further without OEO conversions (see [2] and [3] for some 
background).  Unfortunately, I could not find any evidence one way or the other 
about what Hibernia is doing.  

In fact, Per Hansen from Ciena just so happens to be talking about coherent 
receiver technology [DP-QPSK encoding & DSP analysis] as I write this e-mail...

Cheers,
-Chris

[1] 3R optical regeneration: an all-optical solution with BER improvement, 
http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-14-6414
[2] Coherent receivers enable next-generation transport, 
http://www.lightwaveonline.com/about-us/lightwave-issue-archives/issue/coherent-receivers-enable-next-generation-transport-53426202.html
[3] Optical hybrid, http://en.wikipedia.org/wiki/Optical_hybrid

--
Chris Tracy <ctr...@es.net>
Energy Sciences Network (ESnet)
Lawrence Berkeley National Laboratory





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