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@phdthesis{SYSDOC_706,
      author       = "Carsten Jrgensen Videcrantz",
      title        = "Optical amplification and processing in high-capacity
                      photonic networks",
      school       = "Department of Electromagnetic Systems, Technical University
                      of Denmark",
      month        = "Mar",
      year         = "1997",
      abstract     = "Semiconductor optical amplifier based functional elements
                      for amplification and all-optical signal processing
                      applications in optical communication systems operating in
                      the 1.55 μm wavelength window are experimentally and
                      theoretically investigated. Imperfections related to the
                      deployment of SOAs for optical switching purposes have been
                      identified and the power penalty calculated. Modelling
                      revealed that the optical power levels should generally be
                      high throughout the entire switch block to ensure a high
                      signal-to-noise ratio. However, with SOA gates that are used
                      due to their large on/off ratios, extinction ratio
                      degradation limits the power levels and thus the
                      cascadabiity to 2 - 3 switch blocks. On the other hand, if
                      the switch block contains all-optical wavelength converters,
                      capable of improving the extinction ratio, the degradation
                      due to the gates can be counteracted enabling concatenation
                      of up to ∼20 switch blocks. Measurements on SOA gates have
                      demonstrated 28 dB fiber-to-fiber gain with an optical
                      bandwidth of 50 nm for a 450 μm long SOA. The SOA gate
                      exhibits on/off ratios of up to 50 dB. The input power
                      dynamic range is 17 dB at 10 Gbit/s and decreasing with
                      bit-rate. A new gate relying on laser oscillation inside the
                      SOA to stabilise the gain has been examined demonstrating an
                      input power dynamic range of 23 dB at 10 Gbit/s. Successful
                      operation of a DBR-laser wavelength converter at bit-rates
                      of up to 5 Gbit/s has been demonstrated. In addition, simple
                      polarisation independent all-optical wavelength conversion
                      has been carried out using cross gain modulation (XGM) in a
                      SOA at 10 Gbit/s. The practicality of the SOA converter has
                      been demonstrated in a 20 Gbit/s system experiment at the
                      ECOC'95 exhibition showing a bit-error rate below
                      3×10⁻¹5. Wavelength conversion by cross phase modulation
                      in SOAs integrated in a Mach Zehnder (MZI) or Michelson (MI)
                      interferometer has been demonstrated. The conversion
                      principle results in non-inverted converted signals as well
                      as wavelength independent operation within ∼30 nm. 5
                      Gbit/s experiments verified fine spectral properties of the
                      converted signals by transmission on 60 km standard fiber
                      without penalty. Assessment of the input power dependency
                      without adjustment of the operating conditions revealed a
                      dynamic range of 3-4 dB for the MZI converter. A method for
                      adjusting the performance for larger power variations is
                      proposed thereby increasing the dynamic range to 8 dB.
                      Furthermore, 10 dB improvement of the input power dynamic
                      range of a SOA gate is obtained at 10 Gbit/s by exploiting
                      the non-linear characteristic of an integrated MZI
                      wavelength converter. Detailed modelling results verified by
                      experiments have shown that the modulation bandwidth of SOA
                      converters increases with the optical power levels, the
                      total injected bias current, the confinement factor as well
                      as the differential gain. Up to as much as ∼50 GHz
                      modulation bandwidth (extrapolated) is experimentally
                      obtained following these guidelines. In addition, successful
                      polarisation independent 40 Gbit/s XGM conversion in two
                      concatenated SOAs is performed. Recent experiments at 40
                      Gbit/s using the MI wavelength converter demonstrated
                      converted signals of high quality, exhibiting <10 dB
                      extinction ratio and <25 dB signal-to-ASE ratio (in 1 nm).
                      Experiments assessing the detection capability of SOAs have
                      emphasised that the detection function is not a stand-alone
                      function but only attractive in combination with the SOA's
                      main function. Thus, error free detection is successfully
                      combined with amplification at 2.5 Gbit/s and wavelength
                      conversion at 5 Gbit/s. Polarisation independent,
                      all-optical 40 to 2×20 Gbit/s OTDM (Optical Time Division
                      Multiplexing) to WDM (Wavelength Division Multiplexing)
                      translation has been demonstrated by XGM in a SOA. The high
                      quality of the two 20 Gbit/s translated WDM signals at 1549
                      and 1547 nm wavelength is emphasised by the 9-10 dB
                      extinction ratio.",
}