Re: A New TransAtlantic Cable System

[Chris Tracy <ctracy () es net>]

Heath,

> I just had a thought about EFDA - please forgive my lack of
> terminology though, i'll try to explain:
> Say you have signal coming in to EFDA, the signal is just amplified
> (as you said, also noise - the whole source signal).
> Would it be possible to extract via PLL or similar the source clock
> and use that to modulate the amplifier power?
> Does it work with QPSK / whatever keying is used?
> Would that even help with the noise issue at all, or am I waaaaay off?

Although you can amplify just a single wavelength with an EDFA (has to be in the 1550nm range, not 1310nm), most 
deployments are using EDFAs in a DWDM environment.  The C-band alone consists of ~5THz (5000GHz) of spectrum between 
191.00-195.95 Thz.  Some systems pack 40 wavelengths into this space at 100GHz spacing, some 80 channels @ 50GHz 
spacing, others 160 @ 25GHz.  Each of these signals is independent, they can each be using different 
modulation/bitrate/etc.  The amplifiers are completely ignorant to what is going on with each channel, only the devices 
performing conversion back to the electrical domain need to care about these details (after the incoming light has been 
demultiplexed into individual signals, of course).

Re: amplifier power...  Amplifier gain should really stay constant unless new wavelengths are added/removed from the 
fiber.  There are fixed-gain and variable-gain amps.  VGAs have the advantage that engineers do not need to manually 
re-balance power levels whenever a large number of wavelengths are added or removed from a span, they adjust 
automatically.  Newer DWDM systems should all have VGAs whereas a lot of earlier generation DWDM systems still use 
fixed-gain amps.  With the older fixed-gain amps, you had to have the input power just right -- hence the need to 
re-balance if your aggregate signal changes a lot -- too low and the EDFA would not kick on at all, too high and you'd 
saturate the amp.

-Chris

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