Non-blocking PMD monitoring in live optical systems

Abstract

A simple and non-blocking polarisation-mode dispersion (PMD) monitoring technique using coherent detection is demonstrated and applied to a 40 Gbit=s live fibre-optic system. The results demonstrated a clear correlation between the instantaneous differential group delay and the receiver Q-margin. It was also demonstrated that PMDinduced outage can be eliminated by an adaptive PMD compensator.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.

Demonstration of PMD compensation by using a DSP-Based OPMDC prototype in a 43-Gb/s RZ-DQPSK, 1200 km DWDM transmission

“…In the real optical transmission systems, DGD doesn't change very fast, usually several pico-seconds per hour [16]. With FEC, the OPMDC prototype is able to perform BER free in that kind of situation.…”

In-service characterization of optical links and signals with respect to PMD

“…For lower bit rates, coherent approaches such as proposed in [2][3][4][5] are promising to deliver the desired spectral resolution and the necessary sensitivity.…”

“…The easiest way to obtain the SOP spectrum is to use an optical scanning filter combined with a polarimeter [1,9]. [2], [4] and [5] propose different concepts using a coherent detection scheme to analyze the SOP of different spectral components. Another interesting approach is to analyze the signal in the time-domain by observing the eye-diagram while introducing a controlled amount of DGD into the measuring path [10].…”

43Gb/s Adaptive Polarization Mode Dispersion Compensator Field Trial

“…Both transponders used in the trial incorporate Phase Shaped Binary Transmission (PSBT) modulation scheme [8]. One transponder transmitter was connected through the fiber and at the receive end optically split 3 ways, one into the coherent PMD monitor [9] another straight into a 43Gb/s receiver and the third optical split into the PMDC and another 43Gb/s receiver. The field trial was left running for 3 weeks.…”