ECOC 2010, 19-23 September, 2010, Torino, Italy
Mo.1.D.3
Higher capacity with smaller global energy consumption in core optical networks due to elastic channel spacing T. Zami, A. Morea, J. Renaudier, O. Rival, H. Mardoyan, O. Bertran-Pardo, J.-C. Antona, S. Bigo Alcatel-Lucent, Bell Labs, Route de Villejust, 91620 Nozay, France,
[email protected] Abstract We propose a novel model for core optical networks combining benefits of transparent and opaque networks through variable channel spacing. We demonstrate increased capacity at limited energy cost through detailed transmission experiments and network planning. Introduction New modulation, transmission and detection techniques have been extensively investigated to improve the capacity and the flexibility of optical Wavelength Division Multiplexed (WDM) networks. 40Gb/s transmissions systems are now commercially available and the next step, at 100Gb/s, is forthcoming. This paper proposes to mix 40Gb/s, 50GHz-spaced channels in a transparent waveband with 100Gb/s channels in an opaque waveband with an impairment-aware elastic channel spacing. We first show how variable channel spacing remains compliant with current transparent network technologies thanks to coherent detection. We then experimentally assess the impact of physical impairments in elastic-spacing transmissions. Finally, we use these experimental results as basis for detailed network planning studies highlighting the benefits of our networking concept, in particular for the upgrade of deployed networks. The network model In the European Backbone Network (EBN), the connections between next nodes account for 1 more than 30% of the whole exchanged traffic . These connections correspond to opaque transmissions that generally span over shorter distances than transparent ones in optical networks. So it is easier to modulate them at a higher rate. This paper proposes an advanced EBN network combining 40Gb/s transparent connections with 100Gb/s opaque ones (43 and 112Gb/s respectively, accounting for Forward Error Correction –FEC– and protocol overhead). Our novelty is to keep a pragmatic and simple optical routing process despite the elastic channel spacing. Hence, we choose to have two complementary WDM bands per link: an opaque 2 and a transparent one. As in , each connection can use both wavebands along its path to solve wavelength contention while taking into account physical impairments and minimizing the number of required transponders (TRx). The concept of elastic spectral efficiency We consider 40Gb/s transmission based on Differential Phase Shift Keying (DPSK) with differential detection, yielding a maximum reach
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around 1800km over Standard single Mode Fiber (SMF). We use Polarization Division Multiplexing-Quadrature Phase Shift Keying 4 (PDM-QPSK) with coherent detection for 100 Gb/s transmission. Coherent detection tolerates a high level of out-of-band crosstalk because of the tight filtering induced by the beating of modulated signal with the light of the local 5 oscillator . This feature relaxes the constraints on optical filters in front of optical receivers and thus allows reception of channels not aligned on the standard 50GHz ITU grid, e.g. WDM combs with elastic channel spacing (∆f), as proposed 6,7 in . Furthermore, thanks to agile wavelength 8 selective switches such as , it is possible to drop channels regardless of their spectral position in the transmission window. We take advantage of these properties by adapting ∆f in the opaque sub-band for each link, depending on the link length. Therefore the shorter an inter-node link is, the more channels there will be in its opaque band. However, as the harmfulness of nonlinear effects and crosstalk between adjacent channels tends to rise when ∆f decreases, we need to estimate the transmission reach as a function of ∆f to set the optimal ∆f for each transmission link. Experimental assessment of the reach of wavebands with elastic spacing Fig. 1 illustrates two different configurations of wavebands with elastic ∆f. We used an optical re-circulating loop made up of four 100km-long 9 spans of SMF to measure the reach as a function of ∆f. We tested three different ∆f (33, 40 and 50GHz) over a band of 0,5THz width, wide enough to be representative of all the cases considered in this paper. This band was fully filled with 112Gb/s PDM-QPSK channels, meaning that the number of channels inside the band varied with 1/∆f. The remainder of the Cband was filled with 43Gb/s DPSK channels aligned on the 50GHz ITU grid. After each span, we used erbium-doped fiber amplifiers with the same output power (19dBm) for all ∆f. The chromatic dispersion was compensated after each span to achieve about 100ps/nm per loop lap.
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Figure 1: Measured optical power spectra for 2 different channel spacings (Left). Measured Q-factor vs. distance and channel spacing (Right). It is important to mention that the channels were multiplexed with an optical coupler as would be the case in a real optical network handling channels no longer aligned on optical frequencies known in advance. This low-cost flexibility comes at the price of higher crosstalk degradation than what would be obtained through optical multiplexers. We define the reach as the maximum distance with a Q-factor -4 ≥ 11dB (bit error rate=2.10 before FEC correction). From the results of Fig. 1 (right), we find that the optimal ∆f for a given link length L is well-described by the following formula for 33GHz
