Francesco Vacondio

On nonlinear distortions of highly dispersive optical coherent systems

We investigate via experiments and simulations the statistical properties and the accumulation of nonlinear transmission impairments in coherent systems without optical dispersion compensation. We experimentally show that signal distortion due to Kerr nonlinearity can be modeled as additive Gaussian noise, and we demonstrate that its variance has a supra-linear dependence on propagation distance for 100 Gb/s transmissions over both low dispersion and standard single mode fiber. We propose a simple empirical model to account for linear and nonlinear noise accumulation, and to predict system performance for a wide range of distances, signal powers and optical noise levels.

Estimation of Soft FEC Performance in Optical Transmission Experiments

We investigate the feasibility of using hard-decision bit error rates or, alternatively, mutual information, both measured before a soft input forward error correction decoder, as a means to estimate performance after soft-decision decoding. Both methods are compared based on a large set of measurement data. We conclude that mutual information seems to be the more reliable measure to estimate soft-decision FEC performance.

Empirical modeling and simulation of phase noise in long-haul coherent optical transmission systems

An empirical phase noise channel model suitable for performance evaluation of high spectrally efficient modulations in 100G long-haul coherent optical transmission systems using polarization-division multiplexed and wavelength-division multiplexing channels is presented. The derivation of the model is worked out by exploiting the similarity between the power spectral density of the carrier extracted from the analysis of propagation measurements and the Lorentzian spectrum that is usually adopted to describe instabilities of semiconductor lasers. The proposed channel model is characterized by only two parameters: the linewidth of the carrier and the signal-to-noise ratio. We show that in the case of quadrature phase-shift keying transmission a good agreement exists between quantitative measures of performance extracted by processing experimental data and those obtained from simulations based on the use of the empirical model.

Modeling nonlinearity in coherent transmissions with dominant intrachannel-four-wave-mixing

By extending a well-established time-domain perturbation approach to dual-polarization propagation, we provide an analytical framework to predict the nonlinear interference (NLI) variance, i.e., the variance induced by nonlinearity on the sampled field, and the nonlinear threshold (NLT) in coherent transmissions with dominant intrachannel-four-wave-mixing (IFWM). Such a framework applies to non dispersion managed (NDM) very long-haul coherent optical systems at nowadays typical baudrates of tens of Gigabaud, as well as to dispersion-managed (DM) systems at even higher baudrates, whenever IFWM is not removed by nonlinear equalization and is thus the dominant nonlinearity. The NLI variance formula has two fitting parameters which can be calibrated from simulations. From the NLI variance formula, analytical expressions of the NLT for both DM and NDM systems are derived and checked against recent NLT Monte-Carlo simulations.

Pilot-Symbols-Aided Carrier-Phase Recovery for 100-G PM-QPSK Digital Coherent Receivers

A feedforward pilot-symbols-aided carrier-phase recovery scheme is described. The approach relies on pilot symbols that are time-division multiplexed with the transmitted data. In the scheme, an initial coarse estimation of the optical carrier phase, exploiting the knowledge of pilot symbols, is followed by a fine nondata-aided, or blind, estimation. The main advantage of the proposed solution is that of avoiding the phase- ambiguity problem after a cycle slip. The overhead and the impact on the performance of pilot symbols are investigated for long-haul transmission of 100-Gb/s polarization-multiplexed quadrature phase-shift keying wavelength-division-multiplexing signals in different scenarios. For homogeneous transmission, the proposed scheme outperforms blind carrier recovery with differential decoding.

A Silicon Modulator Enabling RF Over Fiber for 802.11 OFDM Signals

We investigate the linearity properties of silicon modulators and show that, contrary to the traditional lithium niobate Mach-Zehnder modulators (MZMs), the third-order intermodulation distortion (IMD3) for silicon modulators is a function of the modulator bias point. The bias point for silicon modulators can be chosen to reduce the IMD3 well below that of standard lithium niobate MZMs. Given the cost and integration advantages of the silicon photonics technology, silicon modulators offer significant advantages for emerging radio over fiber applications. As an example, we examine, for the first time to our knowledge, a silicon modulator for converting analog 802.11 RF signals to the optical domain, achieving an error vector magnitude of -30 dB.

Elastic optical networks: The global evolution to software configurable optical networks

Worldwide operator deployment of high-speed 100G coherent optical networks is currently underway. To ensure a competitive solution offering significant performance improvements to cope with the ever-increasing traffic demand, a novel network concept has been proposed for improved resource utilization based on “elasticity”; specifically, the ability to make a number of previously fixed transmission parameters tunable, for example optical data rate or channel spacing. The benefits are numerous, including increased network capacity, lower cost per bit, and improved energy efficiency and scalability. In this paper, we review the work carried out within the Cooperation for a Sustained European Leadership in Telecommunications (CELTIC) Elastic-Optical NETwork (EO-Net) project towards advancing the state of software-configurable optical networking. We identify the key building blocks for enabling elastic optical networks to provide desired performance improvements over static optical networks. We examine the design of elastic transponders capable of data rate adaptation, interfaces between client packet devices and transponders supporting flexible traffic aggregation, and associated algorithms for traffic grooming and routing. We also perform network cost/energy analyses. Finally, we review the experimental demonstration of such elastic functionalities.

Directly modulated and fully tunable hybrid silicon lasers for future generation of coherent colorless ONU

We propose and demonstrate asymmetric 10 Gbit/s upstream--100 Gbit/s downstream per wavelength colorless WDM/TDM PON using a novel hybrid-silicon chip integrating two tunable lasers. The first laser is directly modulated in burst mode for upstream transmission over up to 25 km of standard single mode fiber and error free transmission over 4 channels across the C-band is demonstrated. The second tunable laser is successfully used as local oscillator in a coherent receiver across the C-band simultaneously operating with the presence of 80 downstream co-channels.

SOA Intensity Noise Suppression in Spectrum Sliced Systems: A Multicanonical Monte Carlo Simulator of Extremely Low BER

We present a thorough numerical study of intensity noise mitigation of spectrum sliced wavelength-division multiplexing (SS-WDM) systems employing a nonlinear semiconductor optical amplifier (SOA) before the modulator. Our simulator of the SS-WDM link, embedded inside a Multicanonical Monte Carlo (MMC) platform, estimates the tails of the probability density functions of the received signals down to probabilities smaller than 10-16. We introduce a new, simple, and efficient technique to handle intersymbol interference (ISI) in MMC simulations. We address the impact of optical postfiltering on SOA noise suppression performance. While previous research experimentally observed the SOA-induced noise cleaning in SS-WDM systems, this is the first complete simulator able to correctly predict the ensuing BER improvement. We measure the BER at different bit-rates and validate predicted BERs with and without post filtering.

Low-Complexity Compensation of SOA Nonlinearity for Single-Channel PSK and OOK

Carrier density fluctuations in semiconductor optical amplifiers (SOAs) impose penalties on phase-shift keying (PSK) signals due to nonlinear phase noise (NLPN), and on-off keying (OOK) signals due to self-gain modulation. In this paper, we propose a simple scheme to equalize the impairments induced by SOA nonlinearities, derived from the small signal analysis of carrier density fluctuations. We demonstrate via simulation almost complete cancelation of the NLPN added by a saturated SOA on a differential PSK signal. We demonstrate via both simulations and experiment the effectiveness of the method for mitigation of nonlinear distortions imposed by SOAs on an OOK signal.