A very high capacity optical fibre network for largescale antenna constellations: the RETINA project

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1 1 isbn Proceedings of NOC 2001, pp Ipswich, UK, June 2001 A very high capacity optical fibre network for largescale constellations: the RETINA project Ton Koonen, Huug de Waardt COBRA Research Institute, Eindhoven University of Technology, P.O. Box 513, NL 5600 MB Eindhoven, The Netherlands ( a.m.j.koonen@tue.nl) Jean Jennen, Jack Verhoosel Bell Labs, Lucent Technologies Nederland B.V., P.O. Box 18, NL 1270 AA Huizen, The Netherlands Dion Kant, Marco de Vos, Arnold van Ardenne ASTRON, P.O. Box 2, NL 7990 AA Dwingeloo, The Netherlands Elbert Jan van Veldhuizen KPN Research, P.O. Box 421, NL 2260 AK Leidschendam, The Netherlands Abstract A combination of flexible optical time division multiplexing and dense wavelength division multiplexing techniques is deployed for realising a reconfigurable multi-tbit/s data transport network connecting over telescope s. Both short-range and long-range transport in the 1.3 µm and in the 1.5 µm wavelength range, respectively, are explored. Introduction Very high capacity optical data transport networks usually deploy dense wavelength division multiplexing (DWDM) of many wavelength channels, each carrying data speeds up to 40 Gbit/s which is presently the limit achievable by electronic multiplexing. In this way, capacities up to Tbit/s in a single fibre have been obtained in laboratory experiments, composed of 273 wavelength channels at Gbit/s each spread over the S-, C- and L-bands in the 1.5 µm wavelength region [1]. Increasing the data speed in a single wavelength channel beyond 40 Gbit/s can be achieved by optical time division multiplexing (OTDM). With polarisation multiplexing and OTDM, a single wavelength channel capacity of 1.28 Tbit/s has been demonstrated [2]. Evaluating the performance of these multi-tbit/s systems when loading them up to their full capacity is usually only done in laboratory setups of either DWDM or OTDM systems by deploying data pattern generators. In this paper, a very high capacity data transport network is described using a combination of DWDM and OTDM techniques, which will be fully loaded with reallife signals originating from telescope s. The routing flexibility in this

2 2 RETINA network provides options for dynamically reconfiguring the constellation, in order to meet the observation demands of the users and to improve the system s availability. The LOFAR system For observing events far away in the universe, extremely sensitive radio telescopes with very high resolution are needed. Such a high-performance telescope can be realised by deploying large numbers of elements, spread out over a wide area. The innovative LOFAR (Low Frequency Array) system [3] uses separate broadband dipole s, each of the order of a meter in size, and positioned in an area with a diameter of approximately 350 km, as sketched in Figure 1. The radio signals captured by the individual s are digitized and transported to a central signal processor. Each is omni-directional. By adjusting the delays of the digital signals from several s, and correlating them with the appropriate weight factors in the processor, a well-defined beam for observing the universe in a certain direction in real time can be formed. Since this combination is done digitally, it is well possible to copy the signals from each and combine them also with a different set of weights. This way several independent station beams can be formed. By combining stations in the central processor, the directivity within each beam is further increased. The principle of multiple -beaming is illustrated by Figure 2. signal processing center 350 km Figure 11 The LOFAR system Via the Internet, the LOFAR system can be operated by multiple usersmultiple users can operate the LOFAR system at the same time. Choosing different sets of delay

3 3 times and weight factors enables to create multiple beams for simultaneous observations. Of specific interest is the observation of very remote hydrogencontaining objects, which show spectral absorption lines in the 10 to 240 MHz range. Digitizing these signals yields a bit stream of 2 Gbit/s per dual-polarisation dipole. The s are grouped in 168 stations of 78 s each. Each station thus produces a 160 Gbit/s data stream, and the aggregate data flow to the processor is some 26 Tbit/s. As shown in Figure 1, the stations are arranged in a central area of 2 km in diameter, and along three short (about 65 km) and three long (about 250 km) curved arms, which converge in the central area. The central area will contain 25% of the stations and s. 16 Synthesized beams 12 Station patterns 8 Element pattern 4 Figure 22 Principle of multiple beaming The RETINA optical fibre network The RETINA (Remote Elements Telescope Intelligent Network Architecture) project is investigating an optical fibre network to transport the huge data streams to the central processor. Two architectural options are considered: one using a blend of dense wavelength division multiplexing (DWDM) and optical time division multiplexing (OTDM) techniques, and a second one deploying space division multiplexing (i.e., multiple fibres) in combination with OTDM. In the DWDM/OTDM network architecture option, each of the network arms encompasses a number of stations, and a separate wavelength channel in the 1.5 µm window is assigned to each station (see Figure 3). Adding signals along the way towards the central processor, the aggregate upstream traffic amounts to several Tbit/s. Downstream traffic towards the stations is 155 Mbit/s or less, and is meant for station and network management and control. Bi-directional multiwavelength optical amplifiers are compensating the network splitting and fibre link losses. To enable dynamic remote shaping and pointing of the observation beams, the assignment of channels to the stations can be changed by using flexible wavelength add-drop multiplexers as well as flexible OTDM multiplexers. Thus it is also possible to operate the system at a decreased data rate such as 16 Gbit/s per station, at the expense of a decreased system performance.

4 4 Within a station, the signals of the 78 s are firstly multiplexed into 4 streams of 40 Gbit/s each, and then by OTDM combined into a 160 Gbit/s stream per station. consoles / storage / Internet central signal processor station λ 2.. λ N Netw. Mgt. laser pulse source 40 Gbit/s OTDM mux 160 Gbit/s λ mux/demux N x 160 Gbit/s λ 155 Mbit/s bidir. OA station management & control λ OADM λ 2 station station λ OADM bidir. OA λ N Figure 33 A flexible DWDM/OTDM concept for the RETINA network Within a station, the signals of the 78 s are firstly multiplexed into 4 streams of 40 Gbit/s each, and then by OTDM combined into a 160 Gbit/s stream per station. Concepts for the optical time division multiplexing and demultiplexing functions are investigated; in particular the options for high-speed narrow-pulse sources, for fast optical gates (e.g. electro-absorption modulators) for optical multiplexing and demultiplexing, and for stable clock extraction as required for synchronisation of the demultiplexing. Also an analysis is made of the transmission performance of a multiwavelength signal carrying multiple 160 Gbit/s data streams, in view of required fibre characteristics (dispersion, non-linear effects), source properties, optical amplifier characteristics, etc. OTDM add/drop functions will be studied for injecting the (on level aggregated) data signals into the 160 Gbit/s main stream. The investigations will encompass OTDM techniques in the 1.3 µm wavelength region for the core network part and for the short arms, as well as OTDM techniques in the 1.5 µm region for the long arms. An other architectural option is to deploy multiple fibres (i.e. Space Division Multiplexing, SDM), in combination with OTDM. As shown in Figure 4, each station is connected with a separate fibre. At the central signal processor site, an optical crossconnect can flexibly assign transceivers to one or more stations.

5 5 The DWDM/OTDM architecture requires less fibres and optical amplifiers than the SDM/OTDM one, and thus offers better opportunities for merging with other services in the same cable sheet. In the following, implementation options for the DWDM/OTDM architecture are discussed. consoles / storage / Internet central signal processor.. Netw. Mgt. station optical crossconnect laser pulse 40 Gbit/s OTDM mux 160 Gbit/s 155 Mbit/s N 160 Gbit/s cable split N-1 fibres station management & control cable split station N-2 fibres station 1 fibre Figure 44 A flexible SDM/OTDM concept for the RETINA network Long network arms In the DWDM/OTDM architecture, each of the long arms encompasses 28 stations, and a separate wavelength channel in the 1.5 µm window is assigned to each station (see Figure 3). Adding signals along the way towards the central processor, the aggregate upstream traffic amounts to 4.48 Tbit/s. Downstream traffic towards the stations is much less (around 155 Mbit/s), and is carrying network management (at a common wavelength) and station management and control information. Flexible wavelength add-drop multiplexers and OTDM multiplexers allow to reconfigure the constellation and thus to tailor the observation possibilities. Most important is to keep the stringent timing relations between the signals, as required for defining a specific observation beam in the signal processor. Therefore accurate compensation techniques for combatting chromatic and polarisation mode dispersion are envisaged. In view of the wide wavelength range, the long link length and the high data channel speeds, specific measures to combat dispersion effects in the system s long arms are inevitably needed. Dispersion compensation modules need to offset the chromatic dispersion of the fibre, thus providing a very low overall dispersion in combination with a non-zero local dispersion. The latter is needed to

6 6 combat non-linear effects such as Four-Wave Mixing (FWM) which can seriously deteriorate the performance of multi-wavelength systems. Dispersion maps together with power evolution maps are set up to come to an optimal scheme for compensation of chromatic dispersion, while at the same time minimising the build-up of non-linear effects. Also polarisation mode dispersion (PMD) will impact the system s performance at the link lengths and data speeds envisaged; measures for compensating PMD over the broad wavelength used are investigated. Various options for implementing dispersion compensation are analysed, deploying fibre gratings, dispersion-compensating fibre, etc. Dynamic adjustment of the compensation in order to handle network reconfiguration are is investigated. The wavelength channels carrying 160 Gbit/s are spaced at 500 GHz, thus requiring a 108 nm wide wavelength operation range of the optical amplifiers. For handling the C-band ( nm), erbium-doped fibre amplifiers with gain flattening are used. In addition, by adequate selection of the pump laser wavelength, Raman amplification covers the L-band, from 1560 up to 1610 nm. Also for the S-band ( nm), Raman amplification is used. It can be realised in the regular transmission fibre by non-linear effects generated by high-power optical pump sources with adequately selected wavelengths. At the optical amplifiers, optical signal monitoring points are added for remote supervision. Also facilities for controlling remotely the amplifier s operation (e.g., gain adjustment) are included. By deploying the S- and the L-band for carrying the LOFAR signals, the C-band is largely still available for carrying regular telecommunication traffic. Thus network operators can support the LOFAR infrastructure as an overlay on their (existing) telecommunication network. Short network arms Each of the short arms of 65 km contains 14 stations, and adding signals along this route amounts to 2.24 Tbit/s upstream traffic. Due to the shorter fibre lengths, operation in the 1.3 µm window with its added advantage of low dispersion is envisaged. However, the low dispersion in standard single-mode fibre may generate sizeable non-linear effects such as FWM, asking for a controlled amount of non-zero local dispersion. Preliminary work in 40 Gbit/s 1.3 µm OTDM systems, performed within the ACTS project UPGRADE (1995-'98), did result in successful transmission over 216 km standard single mode (SMF) fibre. However, in the vicinity of the zerodispersion wavelength spectral broadening due to FWM was observed, which was strongly dependent on the exact wavelength allocation [4]. In contrast to the OTDM studies conducted within the Long network arm configuration, the Short network arm configuration is using a 10 Gbit/s OTDM granularity. This provides a splendid opportunity to evaluate the pros and cons of the 10 Gbit/s granularity with respect to those of the 40 Gbit/s channel granularity. To improve the spectral efficiency, polarisation division multiplexing is investigated, which relaxes the requirements on pulse width and residual chromatic dispersion. To overcome fibre and optical add/drop multiplexer losses in this 1.3 µm window, optical semiconductor amplifiers (SOAs) as well as Raman amplification and Praseodymium-doped fibre amplifiers are being investigated. Considering a 500 GHz

7 7 channel spacing, a minimum amplifier bandwidth of 40 nm is required. All three amplifier technologies can meet this bandwidth requirement. For the implementation of SOAs, a recently developed novel orthogonal gain-clamped version (LOA, linear optical amplifier) [5] could be an interesting candidate, provided it becomes available in the 1.3 µm wavelength domain. Aspects of self-phase modulation in the SOAs need to be assessed carefully. Experimental PDFAs and SOAs will be evaluated within a re-circulating loop configuration to study aspects of cascadability in terms of noise accumulation and gain saturation. By deploying the 1.3 µm window for the LOFAR applications, the 1.55 µm window remains available for regular telecommunication services. This offers advantages to network operators, who thus may share (a part of) their network to host the LOFAR infrastructure. Special measures (such as 1.3/1.5 µm coarse WDM-(de)multiplexers) are required to process the two wavelength windows at critical network nodes. Central area In the central area, fibre lengths are short (less than 2 km), and point-to-point links from smaller groups of s to the central processor are foreseen, operating in the 1.3 µm window. Given the very short transmission distance, very simple directly modulated DFB laser diodes developed to support 10 Gbit-Ethernet (representing the 1.3 µm arena par excellence) can be applied. Very recently, direct laser diode modulation up to 20 Gbit/s at 1.3 µm has been reported [6], providing an even more cost-effective approach. Options to link a high-speed source directly to the central processor with a separate fibre or the employment of a WDM-scheme are evaluated in terms of costs and complexity. The first option allows uncooled laser operation as stable wavelength allocation is not a prerequisite, the latter advocates a more simple fibre infrastructure and the implementation of potentially low-cost integrated multiwavelength receivers. Conclusions By providing real-time very high capacity data transport, the RETINA network enables a breakthrough in remote user-adaptable astronomical observation capabilities as aimed at in the LOFAR project. The powerful network techniques developed will, when applied in (public) telecommunication networks, also result in breakthroughs in the capabilities for network operators to deliver broadband services. The highcapacity network techniques developed and the experimental results obtained in RETINA may promote the roll-out of high-bandwidth (public) telecommunication services, in particular in regions where such systems are planned to be installed and where the public telecommunication infrastructure is less extensive. Acknowledgement The RETINA project is part of the research programme of the Dutch academicindustrial joint R&D-alliance BreedBand BraBant (B4). The Dutch Ministry of Economic Affairs is gratefully acknowledged for partly funding this work in the BTS programme. References

8 8 [1] K. Fukuchi et al., "10.92 Tb/s (273 x 40 Gb/s) triple-band/ultra-dense WDM opticalrepeatered transmission experiment", Proc. of OFC 2001, Post-deadline paper PD24, Anaheim, CA, Mar. 22, 2001 [2] M. Nakazawa et al., "1.28 Tbit/s - 70 km OTDM transmission using third- and fourthorder simultaneous dispersion compensation with a phase modulator", Proc. of ECOC 2000, Post-deadlinme paper 2.6, München, Sep. 7, 2000 [3] J.D. Bregman, "Concept design for a low frequency array", SPIE Proc. of Astronomical Telescopes and Instrumentation 2000, Radio Telescopes, Vol. 4015, Mar [4] R.C.J. Smets, All-Optical Time-Domain Demultiplexing and Signal Regeneration using 1300 nm Semiconductor Optical Amplifiers, ISBN , PhD thesis, Eindhoven University of Technology, Sep [5] D.A. Francis et al., A single-chip linear optical amplifier, Proc. of OFC 2001, Postdeadline paper PD13, Anaheim, CA, Mar. 22, 2001 [6] J.K. White et al., 20 Gbit/s Directly Modulated 1310 Ridge Laser, Proc. of OFC 2001, paper WDD71, Anaheim, CA, Mar. 21, 2001

FIBER OPTIC NETWORK TECHNOLOGY FOR DISTRIBUTED LONG BASELINE RADIO TELESCOPES

Experimental Astronomy (2004) 17: 213 220 C Springer 2005 FIBER OPTIC NETWORK TECHNOLOGY FOR DISTRIBUTED LONG BASELINE RADIO TELESCOPES D.H.P. MAAT and G.W. KANT ASTRON, P.O. Box 2, 7990 AA Dwingeloo,