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Optical networking beyond 40 Gbit/s

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Optical networking beyond 40 Gbit/s
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  OWK7 Optical Networking beyond 40 Gbit/s H. de Waardt E. Tangdiongga J.P. Turkiewicz and G.D. Khoe COBRA Research Institute, Eindhoven University of Technology, P.O.Box 513, 5600 MB Eindhoven, The Netherlands Tel: 31 40 2474007, Fax: 31 40 2455197 h.d.waardt@tue.nl Abstract: 160 Gbit/s OTDM networks will need nodes with add-drop multiplexers to extract and insert channels at lower bitrates. We will review the current status of add-drop multiplexing for bit rates up to 160 Gbit/s and beyond. ©2005 Optical Society of America OCIS codes: (060 4510) Optical communications; (230.2090) Electro-optical devices 1. Introduction Although the last years have shown problems in the telecommunications sector, resulting in a delay in the introduction of 40 Gbit/s transmission systems, clear signs appear in a returning interest in the development of very high speed transmission systems operating beyond 40 Gbit/s. One way to achieve higher capacity is to increase the bit rate of a single wavelength channel by the employment of Optical Time Domain Multiplexing, enabling bit rates beyond the electronical bottlenecks up to 160 Gbit/s resp. 640 Gbit/s. Recent progress is illustrated by successful field trials at single carrier rate of 160 Gbit/s [ 1 and 8 x 170 Gbit/s [2] carried out in Europe within the frame work of the EU-funded 6 h IST program. This interest towards high speed high capacity transmission for application in the trunk network will be fuelled further in future with the large scale deployment of broadband connections in the access area world wide, notably in Japan [3]. To take advantage of this large aggregated bit rates, networking functions as channel add- and drop functionalities of lower channel tributaries are required to fully exploit the networking capability. In this paper will we review and comment on the current directions in OTDM networking, focusing in particular on the different approaches how to achieve channel add- and drop multiplexing. 2. OTDM add-drop multiplexers A 160 Gbit/s OTDM channel is composed of time-interleaved Return-to-Zero (RZ) coded lower bit rate channels (tributaries), generally at 10 Gbit/s resp. 40 Gbit/s. In an OTDM network a key functionality is provided by add-drop multiplexers (ADM), enabling the extraction and insertion of lower bit rate data channels at the various network nodes in the network. To enable this extraction/insertion of channels, two essential functional blocks are required: a clock recovery circuit (CR) operating at the base rate of the lowest tributary, and an ultra fast gating element, capable to create a time window to isolate the desired channel from the OTDM signal, thereby suppressing the remaining channels sufficiently. For this purpose a switching window of 3-4 ps is required for demultiplexing of a 160 Gbit/s OTDM signal with a bit interval time of 6 25 ps. For a 640 Gbit/s OTDM signal this requirement scales down proportionally. To avoid coherent cross-talk when a new channel is added, a contrast ratio > 20 dB for the empty bit slots is mandatory. The synchronization of the ultra fast optical gate with the OTDM signal is provided by the retrieved clock signal from the CR. Ultra fast gating/switching technologies rely on various operating principles as phase shifting (interferometer type gate/switch), frequency shifting (parametric frequency conversion e.g. four wave mixing) or gain/absorption variation by electrical or optical excitation. The gating function can be provided in various ways: by Electro-Absorption Modulators (EAM s) [4], semiconductor optical amplifier (SOA) based Ultra fast Non-linear Interferometers (GT-UNI) [5,6], integrated differentially operated SOA-based interferometers (SOA-MZI) [7,8], Sagnac interferometers exploiting Highly-Non-Linear Fibres (HNLF) [9,10], and by exploiting Four-Wave Mixing in either SOA s [ 11 ] or fibres [8] Depending on the gating principle, an electrical or an optical clock signal is required. 3. Clock Recovery The main task of the CR circuit is to provide a stable reference clock signal either in the electrical or the optical domain. For a 160 Gbit/s OTDM signal, a sub-harmonic clock signal needs to be recovered on 10 GHz, resp 40 GHz base rate. Among many possible options, phase-locked loop (PLL) based systems have attracted much attention, due to its stability and robustness against frequency drift. It is c~nmonly achieved by phase comparison of the incoming signal with a pre-scaled clock (10 GHz, resp 40 GHz), provided by a voltage controlled oscillator (VCO) driving an EAM in an opto-electronic feedback configuration [ 12,13]. For activating an all-optical switching gate an  OWK7 optical clock is required. The electrical retrieved clock can then be used to drive an optical pulse source. Alternatively, the optical signal can also be injected directly in a pre-scaled mode-locked laser diode (MLLD) with an integrated EAM-section [14] or in a self-pulsating DFB laser diode [15]. 4.1 EAM-based Add-Drop Multiplexing By using Traveling-Wave EAM s, interesting opportunities for 160 Gb/s OTDM add-drop multiplexing have been demonstrated [ 16]. By exploiting the absorption properties of EAM s, both channel dropping and insertion can be achieved simultaneously. In a Y-branch configuration, a 160 Gbit/s OTDM signal is injected simultaneously in two TW-EAM s, one TW-EAM driven with a 40 GHz electrical clock and the other driven with n-phase delayed 40 GHz clock. One branch blocks one 40Gbit/s channel (through function) supporting the transmission of the other three 40 Gbit/s channels, whereas the second TW-EAM drops the selfsame 40 Gbit/s channel, blocked in the other branch. A contrast ratio of 21.6 dB and a window of 11.7 ps was measured for the add-function and a window of 5.2 ps for the drop-function. 4.2 GT-UNI based Add-Drop Multiplexing Interferometric structures are of particular interest for ultra-high speed time domain add-drop operations as both the drop channel and the through channels are simultaneously available. Recent examples operating a gain-transparent ultra fast non-linear interferometer (GT-UNI) as time domain add-drop multiplexer are reported for 160 Gbit/s OTDM signals dropping and adding a 10 Gbit/s channel [1,5,6]. As the drop channel is labeled with a different polarization orientation with respect to the through channels, successful add-drop multiplexing was made possible by proper polarization discrimination, despite of the fact that complete x-phase shifting of the drop-channel was not achieved. A switching window of 4.2 ps resp. 6.2 ps with > 20 dB contrast ratio was shown for the drop- resp. the add-function. Here an optical clock signal at 1.3 ~tm was required to operate the GT-UNI. Therefore a 10 GHz electrical clock (jitter < 210 fs) was retrieved from the incoming 160 OTDM signal, which subsequently was used to drive a hybridly mode-locked 1.3 ~tm laser diode providing an optical clock signal with 2-3 ps pulses at 10 GHz repetition rate. Operating the GT-UNI at 40 Gbit/s base rate remains a challenge due to the SOA dynamics involved. 4.3 Add-Drop Multiplexing Based on OTDM-WDM conversion A different approach towards add-drop multiplexing of OTDM signals is given by exploiting the wavelength domain. By simultaneous injection of a linear chirped pulse and an OTDM signal in a HNLF-Non-Linear-Loop- Mirror (HNLF-NOLM), it is possible to image each of the lower tributaries on different wavelengths, thereby creating a WDM replica of the srcinal data. By using Bragg-grating based WDM filters with appropriately designed pass-bands, the channels can be filtered and extracted. Channel insertion is achieved by simple circulators, and subsequent WDM to OTDM conversion is performed by using the WDM channels as control signals to switch a Continuous Wave beam in a second HNLF-NOLM. This concept was proposed and demonstrated for 40 Gbit/s OTDM signal with 10 Gbit/s tributaries [ 17]. In an other experiment OTDM to WDM conversion based on four- wave-mixing was demonstrated from a 160 Gbit/s OTDM signal to four 40 Gbit/s WDM channels [ 18]. A suitable combination of both approaches could lead to highly flexible time domain add-drop multiplexers. The purely passive conversion mechanisms should allow for add-drop operations for OTDM bitrates beyond 160 Gbit/s. 5. Outlook and Discussion From the 160 Gbit/s add-drop multiplex principles considered previously, the version with the TW-EAM s represent the most simple one. It only needs an electrical clock, is suited for a base rate of 40 Gbit/s and has a potential for monolithic integration. To facilitate cascading, the contrast ratio should be improved further. From the three known SOA-based interferometric add-drop multiplexers (GT-UNI, SOA-MZI, TOAD) only the GT- UNI has shown successful add-drop operation of 10 Gbit/s channels from a 160 Gbit/s OTDM signal for so far. Operation for a base rate of 40 Gbit/s is questionable. The main problem is identified as an incomplete phase change in the SOA at this repetion rate due to slow gain recovery dynamics in the SOA. Add-drop multiplexing based on OTDM-WDM conversion looks promising in terms of potentially achievable bit rates beyond 160 Gbit/s, and the flexibility in the number of channels to be added and dropped. 6. References [1] E. Tangdiongga et al., ECOC 04, Tul.l.4, p. 42 [2] M. Schmidt et al., ECOC 04, PD Th4.1.2, p. 4  OWK7 [3] Y. Maeda, ECOC 04, Mo 1.1.2, p. 4 [4] Ellis et al., lEE El. Lett., vol 34, no 18, p. 1788 [5] C. Schubert et al., IEE El. Lett., vol 39, no 19, 1074 [6] J. Turkiewicz et al., ECOC 03, PD Th4.4.5, p. 84 [7] M. Heid et al., ECOC 02, We8.4.3 [8] T. Morioka, ECOC 04, Thl.2, p. 234 [9] T. Yamamoto et al., IEE E1.Lett., vol 34, p. 1013 [10] J. Seoane et al., ECOC 04, Wel.5.4, p. 326 [ 11 ] S. Jansen et al., IEE El. Lett., vol 38, no 17, p. 978 [12] C. Boerner et al., IEE El. Lett., vol 39, no 14, p. 1071 [ 13] J. Turkiewicz et al., IEEE Phot. Techn. Lett., vol 16, no 6, p 1555 [14] T. Ohno et al., ECOC 04, We.P. 158, p. 784 [15] B. Sartorius, ECOC 04, Mo3.5.1, p. 38 [ 16] H.F. Chou et al., IEEE Phot. Techn. Lett., vol 16, no. 6, p. 1564 [17] P.J. Almeida et al., ECOC 04, We3.5.5, p. 438 [18] Y. Awaji et al., ECOC 04, Wel.5.6, p. 330

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