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Ph.D. Thesis - Carsten Jrgensen Videcrantz - March 1997
Optical amplification and processing in high-capacity photonic networks.
Ph.D. thesis, Department of Electromagnetic Systems, Technical University of Denmark, Kgs. Lyngby, Denmark.
29 May 1997.
Supervisors: Kristian Stubkjr.

Carsten Jrgensen Videcrantz


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.


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

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