A New TransAtlantic Cable System

Chris Tracy ctracy at es.net
Tue Oct 5 09:09:23 CDT 2010


>> 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...


[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 at es.net>
Energy Sciences Network (ESnet)
Lawrence Berkeley National Laboratory

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