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@phdthesis{SYSDOC_708,
      author       = "Sren Lykke Danielsen",
      title        = "Traffic analysis and signal processing in high-capacity
                      optical networks",
      school       = "Department of Electromagnetic Systems, Technical University
                      of Denmark",
      month        = "Jun",
      year         = "1998",
      abstract     = "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.",
}