1 Networks, Department of Photonics Engineering, Technical University of Denmark2 Department of Photonics Engineering, Technical University of Denmark3 Department of Electromagnetic Systems, Technical University of Denmark4 Department of Electrical Engineering, Technical University of Denmark
Optical packet switched networks are investigated with emphasis on the performance of the packet switch blocks. Initially, the network context of the optical packet switched network is described showing that a packet network will provide transparency, flexibility and bridge the granularity gap between the electrical switched layer and the WDM transport layer. Analytical models are implemented to determine the signal quality ghrough the switch blocks in terms of power penalty and to assess the traffic performance of different switch block architectures. Further, a computer simulation model is used to investigate the influence on the traffic performance of asynchronous operation of the switch blocks. The signal quality investigation illustrates some of the component requirements in respect to gain saturation in SOA gates and crosstalk in order to obtain high cascadability of the switch blocks. Cross gain saturation is identified as a potential critical impairment and methods to circumvent this probelm are discussed. Additionally, the compoent requirements to sufficiently suppress crosstalk in the switch blocks are determined. In particular, the influence of regeneration in interferometric wavelength converters is investigated showing that a 10 Gbit/s 19 4x4 swich blocks can be cascaded at a BER of 10-14. An analytical traffic model enables the calculation of the traffice performance of a WDM packet network. Hereby the importance of WDM and wavelegth conversion in the switch blocks is established as a flexible means to reduce the optical buffer, e.g., the number of fibre delay lines for a 16x16 switch block is reduced from 23 to 6 by going from 2 to 8 wavelength channels pr. inlet. Additionally, a component count analysis is carried out to illustrate the trade-offs in the switch block architectures, e.g., between complexity in terms of component count and signal quality in terms of reduction of cross gain saturation in the gates. The requirement for synchronisation in optical packet switch blocks is discussed and asynchronously operated switch blocks are proposed, In these, the randomly arriving packets are managed by the electronic control circuitry so that, e.g., the gates in the space switching section is controlled asynchronously instead of adjusting the asynchronous packets to a synchronous switch block. An assessment of the traffic performance using a simulation model shows that asynchrounous packet switching is feasible. The buffer utilisation is, however, less efficient due to the randomly arriving pakcets, which leads to a higher packet loss probability. This impairment can be counteracted by the use of multiple wavelengths because this in turn renders the buffer more flexible. The basic chracteristics of semiconductor optical amplifier gates for space switching are analysed showing that a gain of -30 dB, an on-off ration of -50dB, a large optical bandwidth of -50 nm, and a fast switching times -1-10 ns are obtainable. For multi-channel gating the gain-clamped SOA (GC-SOA) gate is investigated showing more constant gain characteristics and an increased saturation input power from -17 to -4 dBm. Dynamically, the GC-SOA is demonstrated to perform better at bit rates over 10 Gbit/s for RZ than NRZ signals. Additionally, a 4x4 passive InP mach-Zehnder interferometric space switch has been chracterised demonstrating a switching time of -300 ps and an on-off ration of -30 dB when operated as a 2x2 switch, which is sufficient for use in an optical synchronisation unit. A basic characterisation of interferometric wavelength converters demonstrates a large optical bandwidth including a wavelength range that covers the EDFA window and conversion speeds of 20 Gbit/s and 40 Gbit/s are presented. Furthermore, the regenerative capability due to a nonlinear transfer function is verified at 20 Gbit/s. Following, the transmission characteristics of the IWC is analysed. The chirp measurements indicate that there is a difference in the transmission properties for co- and counter propagation conversion, which is supported by transmission experiments. The combined use of SOA gates and interferometric wavelength converters illustrates the regenerative capability of the IWCs at 2.5, 10 and 20 Gbit/s by increasing the input power dynamic range (IPDR). Moreover, the IPDR of the IWC alone is only -3.5 dB at 10 Gbit/s, however, control schemes are introduced yielding an IPDR of both -28 dB and -40 dB. Finally, the feasibility of an all-optical WDM packet switch block is demonstrated at 20 Gbit/s for a 4x4 switch block with 4 wavelength channels pr. inlet yielding a total throughput of 320 Gbit/s. The results show excellent performance with a negligible penalty.