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Ph.D. Thesis - Sren Lykke Danielsen - June 1998
Traffic analysis and signal processing in high-capacity optical networks.
Ph.D. thesis, Department of Electromagnetic Systems, Technical University of Denmark, Kgs. Lyngby, Denmark.
19 December 1997.
Supervisors: Kristian Stubkjr.

Sren Lykke Danielsen

The performance of optical WDM networks using photonic switch nodes has been investigated. The analysis has dealt with the traffic throughput of the various switch nodes as well as with the capabilities of the optical functionalities realised with semiconductor optical amplifiers and gain clamped semiconductor optical amplifiers. New models for both random and bursty traffic have been developed to study the traffic performance of packet switched WDM switches and networks. A model that describes the influence of crosstalk in space switches has also been developed, For use in theoretical studies, a model for the semiconductor optical amplifier has been implemented and based on this model, a model for the gain clamped semiconductor optical amplifier has been derived. An optical packet switched network layer has been studied extensively. A model describing the power penalty in photonic packet switches has been implemented showing that the cascadability of the switch is limited by the semiconductor optical amplifier gates due to spontaneous emission noise and degradation of the extinction ratio for signals coupled through. This limits the cascadability to five 4×4 switch blocks at 2.5 Gbit/s. On the other hand, if interferometric wavelength converters are used in the nodes, their capability of increasing the extinction ratio of the incoming signal, increases the cascadability to 28 switch blocks. A traffic analysis of packet switches without add and drop functions shows, that the combined use of WDM and wavelength converters dramatically reduces the size of the fiber delay-lines buffers and the number of gates in the nodes. For random traffic, 37 delay-lines are required without converters while only 9 are required with converters and WDM for a 4×4 packet switch. In a network scenario where add and drop is required for the switch nodes, it is shown, that to overcome packet loss probability floors, the use of WDM is a solution. The possibilities of having wavelength converters at all switch inlets, only at the add inlet of the switch or none at all have been compared. For a 32 node Shufflenetwork build with 2×2 switches without optical buffers but with converters at all inlets, the required number of wavelength channels per fiber is 12 for a packet loss probability of 10⁻¹0. For the other two cases more than 20 wavelength channels are needed for the same packet loss performance. Crosstalk has been investigated with a new detailed and accurate statistical method based on moment generating functions. The model has been verified through experiments and compared with Gaussian models. The detailed model shows that for up to 16 interferometric crosstalk channels, the suppression ratios must be larger than 35 dB to ensure a penalty below 1 dB. Measurements on a semiconductor optical amplifier gate show a high fiber-to-fiber gain of ∼28 dB and input power dynamic ranges of 22, 16 and 13 dB at bit rates of 2.5, 10 and 20 Gbit/s, respectively. A gain of ∼17 dB is measured for a gain clamped SOA together with an input power dynamic range of ∼18 dB at 10 Gbit/s. The cascadability of the GC-SOA is shown to be superior to the cascadability of the SOA: For a gain of 20 dB for both gates, ∼30 GC-SOAs and only ∼7 conventional SOAs can be cascaded at 2.5 Gbit/s if a penalty below 1 dB must be ensured. A bit rate capability analysis of wavelength converters that use cross-gain modulation in SOAs shows that for high bit rate operation the following should be used: co-directional coupling, long SOAs that allow a high current injection, high input power levels and high optical confinement factors realised with a small optical area. Up to ∼50 GHz conversion modulation bandwidth (extrapolated) have been measured for optimised SOAs and a 40 Gbit/s wavelength conversion experiment showed preamplified penalties of only ∼1.5 dB. Converters relying on cross-phase modulation in SOAs placed in Mach-Zehnder or Michelson interferometers have experimentally been shown to operate independent of the ??wavelength and with low and transmission supporting chirp for the converted signals. Optimisations have enabled the fabrication of a high speed MI converter. For this MI, penalty free operation is demonstrated at 20 Gbit/s and at 40 Gbit/s, ∼10 dB extinction ratios as well as signal-to-ASE ratios in excess of 25 dB (1 nm bandwidth) are found independent of the wavelength. The regeneration capability of the XPM converter has been demonstrated at bit rates up to 20 Gbit/s showing an improvement of ∼10 dB of the input power dynamic range for a combined SOA-gate and MI-converter compared to a SOA-gate alone. Schemes for increasing the input power dynamic range of standalone XPM converters showed up to ∼28 dB input power dynamic range at 10 Gbit/s with control compared to ∼3.5 dB without control. Furthermore, the cascadability of IWCs has been shown to be superior to the cascadability of opto-electronic converters. The feasibility of a 320 Gbit/s throughput WDM optical packet switch has been demonstrated showing negligible penalties for the 44 switch with four 20 Gbit/s wavelength channels per in- and out-let. A multiwavelength fiber-loop buffer that uses wavelength converters has been proposed and its operation has been experimentally assessed at 10 Gbit/s. 40 Gbit/s OTDM to 220 Gbit/s WDM translation has been demonstrated using XGM in a high speed optimised SOA. Extinction ratios of 9-10 dB were obtained for the two translated 20 Gbit/s WDM signals using moderate input power levels of ∼5 dBm.

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Record created 2009-02-19, last modified 2009-02-19

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