Non-blocking PMD monitoring in live optical systems

Introduction: In high-speed fibre-optic communication systems, polarisation-mode dispersion (PMD) is one of the most important factors of performance degradation. In-situ monitoring of PMD in live, multichannel WDM systems will be a key requirement for future dynamic optical networks to ensure quality of operation. Especially, for optical links with exceptionally high PMD values, an in-situ monitoring of PMD during operation will help network engineers to determine the impact of PMD and to select wavelength bands where PMD appears less damaging [1]. Many PMD measurement techniques have been proposed and demonstrated during the last two decades; the most popular include the fixed analyser method, the Jones matrix method, the Poincare arc method and the pulse delay method [2]. All these methods were developed for PMD characterisation instrumentation, which usually require access to both ends of an optical fibre cable to be measured. In live optical networks with installed optical fibres, the source and receiver are at distance and not accessible at the same time. We have recently introduced a novel and simpler method to evaluate differential group delay (DGD) using coherent detection and RF signal processing. This method utilises the spectral characteristics of the digital signal carried by each wavelength channel and measures the PMD-induced polarisation walk-off between the carrier and the clock components [3]. By tuning the wavelength of the local oscillator, the measurement can be performed across various channels in a WDM optical network. In [3], the PMD monitoring technique was briefly introduced and tested with a fixed PMD emulator. In this Letter, we provide a more detailed signal processing algorithm and report the results of our field measurement applying this technique to a 200 km OC768 SONET optical link. Most importantly, we show that this PMD monitoring technique does not rely on the existence of clock components of the signal and therefore it is independent of the optical modulation format. The results demonstrate clear correlation between the instantaneous DGD and the Q-margin at the receiver. We also demonstrate that PMDinduced outage can be eliminated by an adaptive PMD compensator.