One often cited and also actively researched application for Kerr frequency combs is optical data transmission. Because of their large mode spacing, which is compatible with the ITU grid of 25, 50 or 100 GHz, Kerr frequency combs can make the ideal source for coherent data transmission. In principle it would be possible to replace several hundred individual lasers with one Kerr frequency comb. In addition it would be possible to use the coherence of the frequency comb lines to reduce effects of nonlinearity. However, before looking into the advantages of Kerr frequency combs for data transmission applications, I thought it would be a good idea to see, what the current, commercial technology is capable of. And I have to say, I am quite impressed. Here is some information about a recently installed sea fiber link in the Baltic Sea, the C-Lion-1: 15 TBits/s per fiber, 8 fibers, 120 TBits/s aggregated capacity! See below how this is achieved.
The fiber link was constructed by Alcatel-Lucent and is based on the Nu-Wave Optima platform from Xtera. According to the data sheet the 15 Tbit/s are achieved using 150 times 100G technology in 62 nm optical
bandwidth. This bandwidth indicates that over long distances one can not only use erbium doped fiber amplifiers (EDFAs) as their optical bandwidth of around 1530 to 1560 nm is not sufficient. According to Xtera, Raman amplifiers are used possibly in combination with EDFAs. At around 1550 nm a span of 62 nm is equivalent to approximately 7.5 THz. Divided by 150 channels this results in a channel spacing of 50 GHz. This 50 GHz has to carry 100 Gbit/s data such that a spectral efficiency of 2 bit/Hz is required. There are different modulation formats to achieve this and one common one is PM-QPSK which also seems to be employed by Xtera. So, this would be one system to beat!
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