Interplay between Raman and polarization effects in next-generation passive optical networks.

In this paper, we study in details some Raman-induced impairments that arise in Next-Generation Passive Optical Networks (NG-PON2) in a full coexistence scenario between GPON and TWDM-PON. The main new contribution of this paper is to take into account the polarization launches of the different signals at the transmitter, in order to find the best polarization arrangement. We found that launching the TWDM-PON wavelengths on alternately orthogonal polarization minimizes the Raman depletion effect on GPON over all possible PMD values, thus resulting in the optimal polarization launching condition, while any other polarization launch has a higher out of service probability for realistic PMD values.

Cytotoxicity of chalcone derivatives towards glioblastoma.

BACKGROUND: Among seventeen compounds derived from chalcones investigated as potential anticancer drugs towards LN229 glioblastoma cell line, only two were effective. MATERIALS AND METHODS: Anticancer activity was investigated by evaluating the cell growth, cell cycle, mitotic index and the cell death. RESULTS: Two compounds, namely C2 and C12, inhibited cell proliferation associated with a blockade in the G(2)/M phase of the cell cycle and arrested the growth of tumour spheroid mimicking in vivo tumour. C2 blocked cells in the G(2) phase whereas C12 blocked cells in the M phase of the cell cycle. C12 and C2 killed 40% and 95% of the cells respectively using complex mechanisms. The two compounds increased the fluorescence of rhodamine-123 and N-acetylcysteine inhibited their activity, suggesting a role for reactive oxygen species in cell death mediated by these two compounds. CONCLUSION: C2 and C12 are markedly cytostatic and cytolytic to glioblastoma cells and act through different pathways.

Analytical results on channel capacity in uncompensated optical links with coherent detection.

Based on a recently introduced model of non-linear propagation, we propose analytical formulas for the capacity limit of polarization-multiplexed ultra-dense WDM uncompensated coherent optical systems at the Nyquist limit, assuming both lumped and ideally distributed amplification. According to these formulas, capacity fundamentally depends on the transmitted power spectral density and on the total optical WDM bandwidth, whereas it does not depend on symbol-rate. Also, capacity approximately decreases by 2 [bit/s/Hz] for every doubling of link length. We show examples of capacity calculations for specific ultra-long-haul links with different polarization-multiplexed (PM) constellations, i.e. ideal PM-Gaussian, PM-QPSK (quadrature-phase shift keying) and PM-QAM (quadrature amplitude modulation). We show that the launch power maximizing capacity is independent of link length and modulation format. We also discuss the usable range of PM-QAM systems and validate analysis with simulations.

Experimental validation of an analytical model for nonlinear propagation in uncompensated optical links.

Link design for optical communication systems requires accurate modeling of nonlinear propagation in fibers. This topic has been widely analyzed in last decades with partial successes in special conditions, but without a comprehensive solution. Since the introduction of coherent detection with electronic signal processing the scenario completely changed because this category of systems shows better performances in links without in-line dispersion management. This change to uncompensated transmission allowed to modify the approach in the study of nonlinear fiber propagation and in recent years a series of promising analytical models have been proposed. In this paper, we present an experimental validation over different fiber types of an analytical model for nonlinear propagation over uncompensated optical transmission links. Considering an ultra-dense WDM system, we transmitted ten 120-Gb/s PM-QPSK signals over a multi-span system probing different fiber types: SSMF, PSCF and NZDSF. A good matching was found in all cases showing the potential of the analytical model for accurate performance estimation that could lead to powerful tools for link design.

Performance evaluation of coherent WDM PS-QPSK (HEXA) accounting for non-linear fiber propagation effects.

Coherent-detection (CoD) permits to fully exploit the four-dimensional (4D) signal space consisting of the in-phase and quadrature components of the two fiber polarizations. A well-known and successful format exploiting such 4D space is Polarization-multiplexed QPSK (PM-QPSK). Recently, new signal constellations specifically designed and optimized in 4D space have been proposed, among which polarization-switched QPSK (PS-QPSK), consisting of a 8-point constellation at the vertices of a 4D polychoron called hexadecachoron. We call it HEXA because of its geometrical features and to avoid acronym mix-up with PM-QPSK, as well as with other similar acronyms. In this paper we investigate the performance of HEXA in direct comparison with PM-QPSK, addressing non-linear propagation over realistic links made up of 20 spans of either standard single mode fiber (SSMF) or non-zero dispersion-shifted fiber (NZDSF). We show that HEXA not only confirms its theoretical sensitivity advantage over PM-QPSK in back-to-back, but also shows a greater resilience to non-linear effects, allowing for substantially increased span loss margins. As a consequence, HEXA appears as an interesting option for dual-format transceivers capable to switch on-the-fly between PM-QPSK and HEXA when channel propagation degrades. It also appears as a possible direct competitor of PM-QPSK, especially over NZDSF fiber and uncompensated links.

Evaluation of the computational effort for chromatic dispersion compensation in coherent optical PM-OFDM and PM-QAM systems.

Recently, coherent-detection (CoD) polarization multiplexed (PM) transmission has attracted considerable interest, specifically as a possible solution for next-generation systems transmitting 100 Gb/s per channel and beyond. In this context, enabled by progress in ultra-fast digital signal processing (DSP) electronics, both multilevel phase/amplitude modulated formats (such as QAM) and orthogonal-frequency-division multiplexed (OFDM) formats have been proposed. One specific feature of DSP-supported CoD is the possibility of dealing with fiber chromatic dispersion (CD) electronically, either by post-filtering (PM-QAM) or by appropriately introducing symbol-duration redundancy (PM-OFDM). In both cases, ultra-long-haul fully uncompensated links seem to be possible. In this paper we estimate the computational effort required by CD compensation, when using the PM-QAM or PM-OFDM formats. Such effort, when expressed as number of operations per received bit, was found to be logarithmic with respect to link length, bit rate and fiber dispersion, for both classes of systems. We also found that PM-OFDM may have some advantage over PM-QAM, depending mostly on the over-sampling needed by the two systems. Asymptotically, for large channel memory and small over-sampling, the two systems tend to require the same CD-compensation computational effort. We also showed that the effort required by the mitigation of polarization-related effects can in principle be made small as compared to that of CD over long uncompensated links.

Fundamental performance limits of optical duobinary.

We present a rigorous analysis defining the fundamental performance limits of duobinary line-coding for optical communications. First, we compare the back-to-back performance of duobinary and intensity modulation systems in an AWGN channel satisfying the Nyquist criterion, with both linear and quadratic receiver. We show that, also for duobinary and quadratic receiver, matched filtering is the best achievable condition. Then, we derive a detailed performance analysis of duobinary in an ASE-noise-limited direct-detection optical system considering noise on the entire space of polarizations. We show that for duobinary line-coding the expression of the bit error rate depends both on the shape of the transmitted pulse and on the receiver optical filter. Comparing duobinary coded and uncoded intensity modulation systems, we show the intrinsic advantages of using the duobinary line-coding in a system based on quadratic detection. Finally, some results for realistic setups are obtained through simulation and compared to the fundamental limits in order to show how close to those limits state-of-the-art systems can operate.