When Lightning Strikes: Why Fast SOP Tracking is Important

By Alan Gibbemeyer | Posted on March 1, 2018

Did you know that lighting can impact fiber optic communications?

Lightning strikes can cause very fast rotation of an optical signal’s state of polarization (SOP).

State of Polarization Tracking

Today’s coherent transmission systems are all based on encoding of data in two orthogonal polarizations.  Coherent digital signal processors (DSPs) can recover and track the changes in signal’s SOP change as part of the signal decoding process. SOP changes happen naturally in the fiber with mechanical vibration, but the rate of change is orders of magnitude slower than lightning induced SOP transient on aerial optical ground wire (OPGW).  OPGW has twisted metal wires protecting the optical fiber. When lightning strikes, strong electrical current propagates along the twisted metal wires, creating a fast magnetic field transient that rotates optical signal’s SOP.

Lightning Strikes and Acoustic Vibrations

So what does that mean?  Events, such as lightning strikes or acoustic vibrations caused by high speed trains, can cause polarization rotations at speeds an order of magnitude higher than typical, which may be faster than the DSP’s ability to maintain tracking.  When transient events occur, this rotation can accelerate to more than 1 Mrad/sec, which has caused issues for some implementations. Under normal conditions, SOP rotation is generally well below 100krad/sec and are easily managed by DSP solutions.

This is particularly true with aerial OPGW fibers which are sensitive to these events due to their metallic exterior.  Since aerial OPGW fiber can be more prevalent in access networks, it is important to better understand the potential susceptibility in these emerging coherent applications.

The industry is gathering data on the frequency and magnitude of these transient events, while DSP designers work to improve the ability to track these fast rotations and ensure that they recover quickly when errors do occur.

Acacia’s Fast SOP Tracking

At Acacia, we have been working on these issues for several years to engineer our coherent solutions to be robust in all types of environments. Acacia products are designed for fast SOP tracking. The company’s Meru DSP, used in Acacia’s CFP2-DCO module, can track rotations at over 3Mrad/sec in 100G QPSK mode without passing any uncorrected errors.

It is safe to say, that on the off chance lightning does strike – not once, but twice, Acacia’s technology is engineered to ensure that it has the polarization alignment covered.

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