Very-High-Speed Optical Communications

During 2010 and 2011 coherent transmission technologies have seen a tremendous push towards reaching ever increasing performance in terms of system reach, total capacity per fiber and (capacity x distance) product, or CDP. The latter number has been widely used over the years to provide a powerful overall performance indicator. System experiments extending such CDP indicator beyond existing records are typically awarded great emphasis and visibility in the community and are believed to prove technological superiority in a given system segment.
The current CDP record for (polarization-multiplexed) PM-QPSK is about 142[Pb/s‧km], held by Tyco, followed closely by 117[Pb/s‧km], obtained by Alcatel-Lucent, both published in March 2011. As for PM-16QAM, the record is held by Alcatel-Lucent, with 37[Pb/s‧km], published in September 2011. Finally, regarding PM-64QAM, the record for >100Gb/s tributaries is held by NTT, with 1.8[Pb/s‧km], achieved in September 2010. Using 68Gb/s tributaries then the record is about 10[Pb/s‧km], obtained by NTT in September 2011.
The record CDP value for PM-QPSK is very large and difficult to challenge. On the other hand, it may be argued that there is little interest at present in trying to improve it. In fact, the focus of industrial interest is currently shifting towards higher spectral-efficiency systems than PM-QPSK. Such higher spectral-efficiency systems do not reach the trans-oceanic distances achieved by PM-QPSK, but can deliver much higher capacities over the 500 km to 3000 km terrestrial segment which holds a market significance far greater than the transoceanic segment.
PM-16QAM and PM-64QAM are the obvious candidates for the 500 km to 3000 km terrestrial distance range and potentially deliver a doubled and tripled total capacity, respectively, over PM-QPSK. It is therefore apparent that achieving performance records with these formats is the new strategic goal in the industry.

Concentrating on PM-16QAM, theoretical calculations indicate that a CDP of about 60 [Pb/s‧km], obtained as (25 Tb/s x 2500 km), should be reachable by addressing just the C band, with span lengths and fiber types typical of terrestrial systems and Raman amplification. This would already greatly improve over the current 37 [Pb/s‧km] record. A necessary condition to achieve such challenging goal is to be able to pack channels together to almost minimum theoretical spacing, so as to obtain the highest possible spectral efficiency.
CISCO, together with Politecnico di Torino, has recently obtained lab results at the Photonlab facility (c/ISMB), in the context of the currently ongoing TERACOMP SRA project, which strongly indicate the practical possibility of transmitting 100Gb/s PM-16QAM channels with a gross spectral efficiency exceeding 7 b/s/Hz. This very large value, close to the theoretical maximum of 8b/s/Hz, would enable reaching the mentioned record CDP of 60[Pb/s‧km] provided that enough channels are set-up to fully populate the C band and a suitable transmission line is available.

As for PM-64QAM, there appears to be even more room at present to obtain record results. Theoretical calculations show a potential for reaching a CDP of about 24[Pb/s‧km] (40 Tb/s x 600 km), again provided that the full C band is populated with close-to-theoretical spectral efficiency and a suitable transmission line is available. This value is more than an order of magnitude larger than the present record for >100Gb/s per tributary [4] and more than double the present record with slower tributaries [5].
This project aims at involving the substantial expertise in the set up and running of complex laboratory experiments of optical transmission, available at the Institute, so that with the collaboration of CISCO and of Politecnico di Torino researchers, the above challenging goals can be pursued. This is clearly high-risk research and many obstacles can prevent reaching the mentioned CDP performance values. However, at present, the combined expertise and infrastructures of CISCO, Politecnico di Torino and the Institute appear to constitute critical mass to attempt reaching such goals.

Phase I of ULTRACAP has an intermediate goal and a final goal.
The intermediate goal will be pursued at the Institute laboratory facility PhotonLab and consists of setting up a laboratory experiment using 16 channels at 100Gb/s with PM-16QAM modulation, demonstrating that the target distance of at least 2,500km is reachable with a gross spectral efficiency of at least 7b/s/Hz. This experiment will be carried out using 16 channels, over a fiber re-circulating loop. It will prove that, if the whole C band were fully populated, it would be possible to achieve the target record necessary (capacity x distance) value of 60[Pb/s‧km].
The final goal of Phase I of ULTRACAP is that of setting up another experiment, now fully populating the C band (about 200 channels), so that 60[Pb/s‧km] are indeed achieved. To do so, it will be necessary to transfer the experimental set-up from the PhotonLab to the CISCO premises in Monza. CISCO will make the C-band multi-wavelength source available, as well as the transmission line. Institute researchers will assemble transmitter and receiver and carry out the actual experiment.
Phase II of ULTRACAP has a main goal and a possible extension goal.

The main goal will be pursued at the Institute laboratory facility PhotonLab and consists of setting up a laboratory experiment using 16 channels at 10Gb/s with PM-64QAM modulation, demonstrating that the target distance of at least 600 km is reachable with a gross spectral efficiency of at least 10b/s/Hz. This experiment will be carried out using 16 channels over a fiber re-circulating loop. It will prove that, if the whole C band were fully populated, it would be possible to achieve the target record necessary (capacity x distance) value of 24[Pb/s‧km].
A possible extension goal of Phase II of ULTRACAP is that of setting up another experiment, now fully populating the C band (about 200 channels), so that 30[Pb/s‧km] are achieved. To do so, it will be necessary to either transfer CISCO’s C-band source to PhotonLab or to transfer the experimental set-up from the PhotonLab to the CISCO premises in Monza. If run in Monza, the experiment will take advantage of CISCO’s transmission line. If run in Torino, it will be over a re-circulating loop. CISCO will decide whether this extension experiment is carried out or not. The extension may require some amendment to the financials of the present contract, to be negotiated after Phase I.

Tests will be conducted over the metropolitan fiber test-bed provided by Fastweb, possibly in a recirculating loop configuration in order to perform very-long-distance experiments.

Applied Photonics