Re: A New TransAtlantic Cable System

[Chris Tracy <ctracy () es net>]

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 <ctracy () es net>
Energy Sciences Network (ESnet)
Lawrence Berkeley National Laboratory