Coherent crosstalk in ultradense WDM systems

We present extensive numerical studies on the determination of coherent wavelength-division multiplexing (WDM) crosstalk penalties for ultradense wavelength-division multiplexed (DWDM) systems, focusing on carrier-suppressed return-to-zero (CSRZ) as well as on 67% duty cycle differential phase-shift keying (67% RZ-DPSK) at a spectral efficiency of 0.8 b/s/Hz. Our analyses reveal large statistical variations in the predicted required optical signal-to-noise ratio (OSNR) when changing the WDM channels' interference conditions, in particular their relative optical phases and their relative time shifts. The strong impact of the exact WDM interference conditions can lead to simulation inaccuracies of many decibels when using standard OSNR simulations techniques. In measurements of DWDM system performance, the long averaging time of bit error ratio (BER) test sets can hide these burst-error generating penalty variations, and may, therefore, lead to wrong interpretations, especially for systems employing forward error correction (FEC). To overcome the DWDM simulation problem, we introduce and thoroughly assess a new simulation technique that allows us to efficiently and accurately capture the average required OSNR penalty for DWDM systems with negligible statistical error.     

Long-distance quantum communication with entangled photons using satellites

The use of satellites to distribute entangled photon pairs (and single photons) provides a unique solution for long-distance quantum communication networks. This overcomes the principle limitations of Earth-bound technology, i.e., the range of the order of 100 km afforded by both optical fiber and by terrestrial free-space links.     

Performance Optimization of Optically Preamplified Receivers for Return-to-zero and Non Return-to-zero Coding

We determine optimum bandwidths for optical and electrical filters in optically preamplified receivers, both for NRZ coding and RZ coding. Our simulations clearly reveal the trade-offs to be made when optimizing bandwidths: NRZ is typically limited by intersymbol interference, while RZ is limited by energy truncation. Thus, RZ allows for tighter filtering, leading to near quantum limited performance. Further, RZ systems are less susceptible to suboptimum filtering. We also show that the use of RZ with duty cycles below 33% only leads to minor additional receiver sensitivity improvements at the expense of impractically higher receiver bandwidths. Employing an RZ duty cycle of 33%, we achieved a receiver sensitivity of 52 ppb at a data rate of 10 Gbit/s, which is only 1.4 dB off the quantum limit.     

Optimum filter bandwidths for optically preamplified NRZ receivers

We present a comprehensive treatment of optically preamplified direct detection receivers for non-return-to-zero (NRZ) and return-to-zero (RZ) on/off keying modulation, taking into account the influence of different (N)RZ optical pulse shapes, specified at the receiver input, and filter transfer functions; optical Fabry-Perot filters (FPFs) and Bragg gratings as well as electrical fifth-order Bessel and first-order RC low-pass filters are considered. We determine optimum optical and electrical filter bandwidths and analyze the impact of bandwidth deviations on receiver sensitivity. Optimum receiver performance relies on a balance between noise and intersymbol interference (ISI) for NRZ transmission, while for RZ reception detection noise has to be traded against filter-induced signal energy rejection. Both for NRZ and 33% duty cycle RZ, optical filter bandwidths of around twice the data rate are found to be optimum. Receivers using RZ coding are shown to closely approach the quantum limit, and thus to outperform NRZ-based systems by several decibels. We further analyze the impact of important degrading effects on receiver sensitivity and optimum receiver bandwidths, including receiver noise, finite extinction ratio, chirp, and optical carrier frequency (or optical filter center frequency) fluctuations     

Choice of MUX/DEMUX filter characteristics for NRZ, RZ, and CSRZ DWDM systems

This paper investigates the influence of filter bandwidth and flank steepness of both multiplexing and demultiplexing filters in dense wavelength division multiplexed systems (spectral efficiency 0.8 b/s/Hz) in the presence of coherent wavelength division multiplexing (WDM) crosstalk. Using a recently introduced technique for the statistically reliable performance prediction of systems impaired by coherent WDM crosstalk, this paper presents numerical results for nonreturn-to-zero (NRZ), 33% duty-cycle return-to-zero (RZ), and 67% duty-cycle carrier-suppressed return-to-zero signals. This paper confirms that steep filter flanks are generally preferable, both in terms of optical signal-to-noise ratio penalty and in terms of filter bandwidth tolerance.