Abstract-Signal degradation due to polarization-mode dispersion (PMD) effects may become significant for signaling rates of 10 Gb/s, 40 Gb/s, and beyond. To assess the utility of various PMD mitigation schemes, temporal and spectral measurements of differential group delay (DGD) were made on 95 km of buried standard single-mode fiber over an 86-d period to determine the distribution and rate of change of high-DGD events. As expected, statistical analysis of variations in DGD indicate that excursions from the mean DGD by factors of 3.7 or higher have very low probability. For this link, the DGD varied slowly with time (having a drift time of about 3.4 d) and rapidly with wavelength. The DGD data agree well with results of similar experiments reported in the literature. Statistical analysis of the measured DGD data shows that high-DGD episodes will be exceedingly rare and short-lived. The impact of PMD on network operations is explored and approaches to ensure network reliability are reviewed for network operators given the task of transporting high-bit-rate channels over fiber links with known PMD characteristics.

Abstract-The performance of high-speed digital fiber-optic transmission using subcarrier multiplexing (SCM) is investigated both analytically and numerically. In order to reduce the impact of fiber chromatic dispersion and increase bandwidth efficiency, optical single-sideband (OSSB) modulation was used. Because frequency spacing between adjacent subcarriers can be much narrower than in a conventional DWDM system, nonlinear crosstalk must be considered. Although chromatic dispersion is not a limiting factor in SCM systems because the data rate at each subcarrier is low, polarization mode dispersion (PMD) has a big impact on the system performance if radiofrequency (RF) phase detection is used in the receiver. In order to optimize the system performance, tradeoffs must be made between data rate per subcarrier, levels of modulation, channel spacing between subcarriers, optical power, and modulation indexes. A 10-Gb/s SCM test bed has been set up in which 4 2.5 Gb/s data streams are combined into one wavelength that occupies a 20-GHz optical bandwidth. OSSB modulation is used in the experiment. The measured results agree well with the analytical prediction.

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.

Abstract-A 10-Gb/s subcarrier multiplexing long-haul optical system is reported. 4 2.5 Gb/s data streams are combined into one wavelength, which occupies a 20-GHz optical bandwidth. Optical single sideband is used to increase bandwidth efficiency and reduce dispersion penalty. The receiver sensitivity is calculated using a simplified receiver model with an optical preamplifier. The measured results agree well with the analytical prediction.Index Terms-Modulation, optical communication, optical modulation, optical signal processing.I N ORDER to use the optical bandwidth provided by optical fibers more efficiently, new transmission technologies have been developed rapidly in recent years, such as time division multiplexing (TDM), wavelength division multiplexing (WDM), and their combinations. Apart from noise accumulation, high-speed TDM optical systems suffer from chromatic dispersion, nonlinear crosstalk, and polarization-mode dispersion (PMD). Optical systems with data rates of 10 Gb/s and higher require precise dispersion compensation, and careful link engineering. On the other hand, WDM technology uses lower data rates at each wavelength. However, due to the limitations in the wavelength stability of semiconductor lasers and selectivity of optical filters, the minimum channel spacing is 50 GHz in current commercial WDM systems. Narrower channel spacing requires better wavelength stability of laser sources and better wavelength selectivity of optical filters.Optical subcarrier multiplexing (SCM) is a scheme where multiple signals are multiplexed in the RF domain and transmitted by a single wavelength. A significant advantage of SCM is that microwave devices are more mature than optical devices: the stability of a microwave oscillator and the frequency selectivity of a microwave filter are much better than their optical counterpart. A popular application of SCM technology in fiber optic systems is analog cable television (CATV) distribution [1], [2]. Because of the simple and low-cost implementation, SCM has also been proposed to transmit multichannel digital-optical signals using direct detection [3], [4] for local area optical networks.The basic configuration of an SCM/WDM optical system is shown in Fig. 1. In this example, -independent high-speed digital signals are mixed by -different microwave carrier frequencies . These are combined and optically modulated onto an optical carrier. -wavelengths are then multiplexed together in an optical WDM configuration. At the receiver, an optical demultiplexer separates the wavelengths for individual optical detectors. Then, RF coherent detection is used at the SCM level to separate the digital signal channels. Channel add/drop is also possible in both wavelength level and SCM level. While this SCM/WDM is in fact an ultradense WDM system, sophisticated microwave and RF technology enables the channel spacing to be comparable to the baseband, which is otherwise not feasible by using optical technology. Compared to conventional high-speed TDM systems, SCM is less sensitiv...

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