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@phdthesis{SYSDOC_709,
      author       = "Kim Stockholm Jepsen",
      title        = "High-speed optical signal processing for high capacity
                      optical networks",
      school       = "Department of Electromagnetic Systems, Technical University
                      of Denmark",
      month        = "Feb",
      year         = "1999",
      abstract     = "High-speed photonic switching technologies aimed at
                      developing optical time division multiplexing (OTOM) as a
                      networking technology are investigated. The experimental
                      investigations are mainly based on the use of monolithically
                      integrated interferometers incorporating semiconductor
                      optical amplifiers as phase shifting elements. A statistical
                      model for describing interferometric cross-talk is developed
                      and used for investigating interferometric cross-talk in
                      OTDM networks. The influence of interferometric cross-talk
                      is analysed theoretically using moment generating functions.
                      The extended MGF gives predictions that are confirmed by
                      experimental results both for the probability density
                      function of the received signal and for the calculated
                      system penalties in the presence of one and multiple
                      coherent interferers. Calculations show that when the number
                      of interferers is below 6 or 7, the detailed model must be
                      used rather than the commonly used Gaussian approximation.
                      MI-optical phase detection based on semiconductor optical
                      amplifiers has been investigated for 40 Gb/s OTDM signals,
                      using both cross-gain modulation and a novel scheme based on
                      differential cross-phase modulation in a semiconductor
                      optical amplifier-based Michelson interferometer (SOA-Ml).
                      The differential cross-phase modulation scheme shows
                      superior performance in terms of SNR over the XGM scheme
                      (improvement of 20 dB), and an estimated improvement of 12
                      dB compared to previously published results using FWM. A
                      high sensitivity of optical phase comparator schemes to both
                      average input, power fluctuations and imperfections in the
                      multiplexing is established: even small average power
                      changes in the sub-dB range can give rise to a static phase
                      error of more than 20°. This sensitivity can be overcome by
                      the use of a compensation scheme. By using the scheme, the
                      average-power related phase error can be completely
                      compensated; further, a reduction of the phase error due to
                      imperfect multiplexing from ∼40° to less than 1° is
                      found experimentally. All-optical add-drop multiplexing is
                      investigated extensively. All-optical add-drop multiplexing
                      of OTDM signals can lead to signal degradation because of
                      interferometric cross-talk. Therefore, very stringent
                      requirements to both clearing efficiency of the add timeslot
                      and pulse- quality in terms of absence of pedestals in the
                      pulse tails must be met. As an example, in a bus- configured
                      ring structure with only 5 nodes, where each node
                      communicates with all others, a required pedestal level
                      below 43 dB for the pulses and at the same time a clearing
                      of the add timeslot of 30 dB is required, according to
                      calculations. All-optical add-drop multiplexing is
                      demonstrated experimentally at 40 Gb/s using a
                      monolithically integrated Mach-Zehnder interferometer
                      incorporating SOAs as phase shifting elements. The SOA-MZI
                      can simultaneously perform both the demultiplexing and the
                      required clearing of the add time slot prior to insertion of
                      the add channel. After the add-drop multiplexing a
                      pre-amplified sensitivity of -34.4 dBm for the drop channel
                      is recorded, while the penalty for adding into the cleared
                      timeslot is 1.3 dB. A dynamic range of 7-8 dB is estimated
                      for the OADM. Optimisation for the add-operation alone
                      enables penalty free insertion of the add- channel and
                      penalty free carry-through of the undropped channels.
                      Polarisation independent, RZ-to-RZ wavelength conversion is
                      performed using a SOA-MZI. Conversion over 30 nm - including
                      conversion to the same wavelength - is demonstrated at 40
                      Gb/s with zero conversion penalty. The wavelength conversion
                      is accompanied by a small pulse broadening from 8 ps to 10
                      ps, demonstrating ultra-fast operation. Optical 3R
                      regeneration is demonstrated at 20 Gb/s using a novel scheme
                      for reducing jitter- to-amplitude modulation. The scheme is
                      based on the use of a polarisation independent, SOA-based
                      monolithically integrated Michelson interferometer (SOA-MI).
                      The penalty for passing the regenerator is 1 dB for a high
                      input signal-to-ASE ratio. Jitter levels of 5 p ??s (10
                      percent of the 20 Gb/s timeslot) can be accommodated with an
                      excess penalty of below 0.5 dB. Two types of all-optical
                      interfaces providing all-optical interconnection between an
                      OTDM trunk-line and optical network sub-sections are
                      demonstrated. One interface provides access from the OTDM
                      bus to a WDM based sub-network (NRZ-format assumed), the
                      other interface provides access to the OTDM bus from the
                      sub-network, which can be either NRZ- or RZ-format based.
                      The OTDM-to-WDM interface is demonstrated at 40 Gb/s using a
                      SOA-MZI to perform both wavelength and RZ-to-NRZ format
                      adaptation. The penalty for passing the interface is 1.3 dB.
                      The WDM-to-OTDM interface provides all-optical
                      synchronisation of a 10 Gb/s tributary channel to the
                      add-vacancy in an OTDM bus in addition to the necessary
                      format and wavelength conversion. The synchronisation is
                      obtained using a tuneable, all-optical wavelength converter
                      (AOWC) and dispersive fibre. A second AOWC provides a
                      regenerative wavelength and format conversion for adapting
                      the signal to the OTDM bus. A temporal tuning range for the
                      conversion of up to than 120 ps demonstrated with a negative
                      penalty across the entire range. The dynamic range is 3.5
                      dB, while the wavelength tolerance is around 5 nm. The
                      scheme is also tested with a RZ input signal.",
}