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Ph.D. Thesis - Kim Stockholm Jepsen - February 1999
High-speed optical signal processing for high capacity optical networks.
Ph.D. thesis, Department of Electromagnetic Systems, Technical University of Denmark, Kgs. Lyngby, Denmark.
16 October 1998.
Supervisors: Kristian Stubkjr, Benny Mikkelsen.

Kim Stockholm Jepsen


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.


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

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