Impact of lightning and short circuits on polarization and coherent 100 Gb/s links in aerial fiber cables with metallic components (OPGW)

The ever-increasing data rates beyond 50 Gb/s require new modulation techniques beyond the simple On-Off Keying (OOK), and into the realm of Coherent modulation schemes such Dual Polarization – Quadrature Phase Shift Keying (DP-QPSK), and 64-Quadrature Amplitude Modulation (QAM). In these schemes information is encoded in polarization, phase and frequency of the optical signal. The impact of lightning strikes on optical ground wire (OPGW) can have significant consequences for the polarization of light within the embedded fibers, ultimately negatively affecting coherent modulation and leading to transmission errors. Studies have shown that the electromagnetic pulse generated by a lightning strike can induce currents in OPGW and subsequent transient changes in the birefringence of the fibers inside OPGW cables, attributable to Faraday Effect, altering the State Of Polarization (SOP) of the propagating light. This sudden change in SOP, which can be as high as tens of Mrad/sec, may be beyond the compensation capabilities of the coherent receivers, and disturb the delicate balance required for coherent modulation techniques, resulting in increased bit error rates (BER) and degraded system performance in these systems. Furthermore, the lightning-induced perturbations and the subsequent current in OPGW can persist for several milliseconds, potentially impacting multiple data frames before the system can recover. To mitigate these effects, advanced polarization monitoring and control mechanisms have been proposed. By understanding and addressing the impact of lightning on the SOP of light in OPGW fibers, network operators can ensure the reliability and resilience of their coherent optical communication systems.