Due to the steadily increasing bandwidth demand on transoceanic communication systems, currently installed fiber-optic undersea links are approaching their capacity limits. Upgrading such systems beyond their original design capacity is an attractive alternative to developing new cables. In particular, the use of 20 Gbps RZ-DPSK channels with 50 GHz spacing has been demonstrated as a promising candidate for upgrades of legacy submarine systems. By order of Xtera Communication Ltd., the performance of this modulation format in the gain bandwidth of a typical transatlanic fiber link was experimentally verified with the recirculation loop-testbed of the Fraunhofer HHI.
System upgrades can be carried out either as dark fiber upgrades, i.e. lighting previously unused fiber pairs in a submarine cable, or as overlay upgrades by replacing or adding optical channels on a fiber already carrying traffic. As an example for dark fiber upgrade, results are presented for a 3rd generation submarine system over transatlantic distances operating with N x 21.4 Gbps RZ-DPSK channels. The system performance was investigated by measuring the BER after 6,615km for seven probe channels in different wavelength regions. For experiments a recirculating twin-loop testbed with non-slope compensated submarine fibers was set up, replicating the typical dispersion and OSNR maps of trans-oceanic fiber links (s. Figure 1).
In the WDM transmitter, 17x100 GHz spaced WDM channels with wavelengths between 1546.12 nm and 1558.98 nm were generated. During the measurement of each of the probe channels, the channel spacing around the measured channel was reduced from 100 GHz to 50 GHz by inserting two additional neighboring channels.
After a variable dispersion pre-compensation, all channels were launched into the fiber-loop with the worst-case co-polarized state. The twin-loop setup consisted of three consecutive EDFA-amplified NZDSF sections, each composed of a mix of submarine NZDSF typed. Random polarization scrambling was applied within the loop to mitigate polarization effects. Mach-Zehnder filters (MZ-EQ) and Gain Equalizers (GEQ) were used for flattening the power spectrum. In every third roundtrip, the signal was fed through the outer loop with standard fibers (SSMF) for dispersion compensation. After a total transmission distance of 6,615 km, the WDM comb was coupled out from the loop, dispersion post-compensation was applied and the bit error ratio (BER) of the investigated probe channel was measured within the receiver (RX). Accurate optimization of the pre-compensation was essential for each probe channel.
The measured optimum Q-factor varies between 14.2 dB and 14.7 dB. Therefore apart from these variations, which are in the order of measurement ambiguity and system parameter fluctuations, the optimized Q-factor remains largely independent of the channel’s wavelength. These experimental results have also been cross-checked by computer simulations. The optimum pre-compensation values predicted by the simulations showed excellent agreement with the measurements.
For non-slope-compensated submarine systems, 50 GHz spaced 20 Gbps-RZ-DPSK shows consistent performance within the system bandwidth. A Q-factor > 14 dB across the band means that pure RZ-DPSK upgrades are feasible at 12 Gbps line-rate (s. Figure 2).
Today’s challenge is the upgrade of legacy submarine links for a transmission capacity of 100 Gbps per channel and beyond.