For ultra-long haul (ULH) DWDM systems, polarization mode dispersion has been considered as a critical factor for 10G/s and beyond. Besides the impact of PMD, the accumulated polarization dependent loss (PDL) from optical components/subsystems and polarization dependent gain (PDG) in EDFAs also introduce significant penalty. Mitigating the polarization induced penalties has become a constant challenge in system designing, so far there is no cost-effective and practical solution available. For deployed DWDM transmission systems, the changes of state of polarization (SOP) of the transmission signals are mainly driven by environmental temperature variations and the physical movements and shaking of the fibers, and it was observed the changes are quite slow and the induced outage event duration can last as long as several hours. In this work, we demonstrated that using a slow polarization scrambler can improve the transmission performances of all DWDM channels for deployment-ready ULH DWDM systems where the polarization induced penalties (like power variation and long-last worst events) can be suppressed via slow polarization scrambling. This is because using polarization scrambling can reduce the duration for the long-last event at the worst SOP.Our deployment-ready ULH system consists of 19 spans of standard single mode fiber (80 km per span) and 80 10G/s channels with 50GHz space over C-band and with forward-error correction (FEC). A commercial available slow polarization scrambler was inserted between the output of transmitter amplifier and the input of the first span of transmission fiber. By monitoring the power at the receiver end, we observed that fluctuation of channel power can be reduced from 4 to about 2 dB via the polarization scrambling as shown in Fig. 1. The preFEC bit error rates (BER) for all the 80 channels were also recorded. We also observed the worst preFEC BER can be reduced by using polarization scrambling for all the channels, the improvement of nearly one order of magnitude have been achieved for some of the channels. It is worthy to mention that our system employed a continuous power pre-emphasis scheme to stable the channel power at the receiver end, and a dynamic decision level adjustment scheme to optimize the PreFEC BER based on the quality of incoming signal. We can expect that the improvement in suppressing the power fluctuation and worst preFEC BER using slow polarization scrambling shall be more significant for the systems which use the fixed decision level or do not use the continuous power pre-emphasis (which are the cases for the vast majority of current deployed systems). Moreover, the improvement using polarization scrambling could be more significant for the system over long period of time (e.g., several months to years) compared to our observation period of less than 24 hrs.In conclusion, using slow polarization scrambling can improve transmission performances simultaneously for multichannel of deployment ready ULH DWDM links, which is very simple, inexpensive and practical.
© 2006 Optical Society of America