PMD and PDL monitoring of traffic-carrying transatlantic fibre-optic system

Introduction: Polarisation mode dispersion (PMD) is an important factor in the performance of long-distance high speed optical transmission systems. There is clearly a need for a practical approach that supports the network provider’s planning and route design process for possible capacity upgrading in existing fibre plants, such as to retrofit 10 Gbit/s dense wave division multiplexing (DWDM) systems with 40 Gbit/s per wavelength. A comprehensive characterisation of fibre system properties has to be performed before upgrading, but this often has to be done without disrupting customer traffic. Traditional PMD and polarisation-dependent loss (PDL) measurement techniques, such as the Jones matrix and Mueller matrix methods [1], require access to both ends of the fibre, which is not feasible when the transmitter and the receiver are far apart, except by using loop-back techniques. These techniques are also intrusive because of the need for a probe optical signal to make the measurement. We have recently demonstrated a non-intrusive method to directly measure the first-order differential group delay (DGD) and PDL in traffic-carrying terrestrial optical systems using coherent detection and RF signal processing [2, 3]. In this Letter, we report the measurement of PMD and PDL on wavelengths of the transatlantic fibre-optic system known as TAT-14. One segment of this submarine fibre-optic system runs from Manasquan, NJ, USA, to Blaabjerg, Denmark, a path that is approximately 7500 km long, employing 147 erbium doped fibre amplifiers (EDFA). A chirped return-to-zero (RZ) modulation format is used in the DWDM transmitters, and intra-bit polarisation scrambling is used to suppress the effects of polarisation-hole burning [4] and other nonlinear waveform distortions such as four-wave mixing and cross-phase modulation [5]. Since our measurement utilises live traffic as the probe signal, the high-speed polarisation scrambling in the transmitter complicates the measurement of PMD and PDL compared to the previously reported methods, and a modified algorithm is used to analyse the results.