Processing of ultra-short optical pulses for high bit-rate optical communications
In this thesis, the possibility of integrating linear pulse shapers into various all-optical signal processing devices for applications in high speed optical communication systems, to enhance the overall system performance, is investigated. The linear pulse shaping is performed using superstructured fibre Bragg gratings, which seem very promising passive devices for such application due to their compactness, easy integrability and compatibility with fibre based devices. At the same time, optical switching is facilitated by nonlinear effects in state of the art highly nonlinear fibres. The generation and manipulation of waveforms with specific shapes also requires suitable techniques for their precise characterization. For this reason, optical sampling oscilloscope or linear and nonlinear frequency resolved optical gating techniques are presented in this thesis. As a first example of all optical signal processing using pre-shaped pulses, the incident noisy data pulses are expanded into rectangular pulses at the input port of a nonlinear optical switch (nonlinear optical loop mirror). The flat top of the shaped pulses allows for the mitigation of any mistiming of the original signal across a time window defined by their width, simply by switching them with shorter clean clock pulses. By using a nonlinear switch with full regenerative properties as well, it is demonstrated that amplitude noise reduction as well as timing jitter reduction can be achieved in a single nonlinear switch. In a different switch configuration, where cross-phase modulation is utilized in a single-pass configuration, retiming is obtained by preshaping clean control pulses into pulses with a parabolic shape. XPM induced by such pulses can provide linear frequency-shifting to shorter mistimed data pulses across a temporal window corresponding to the full width of the parabolic pulses. This frequency-shift is proportional to the relative pulse displacement from the control bit-slot centre. Propagation in a suitable length of a dispersive medium can then be used to correct for the mistiming. The parabolic pulse shape is also very interesting for nonlinear propagation in normal-dispersion fibres, since it can propagate at high peak powers without undergoing deleterious pulse distortion (avoiding wave-breaking effects). It is demonstrated that the nonlinearly broadened parabolic pulse spectrum is highly flat and coherent, with a high spectral density, as required for spectral slicing or pulse compression applications. Finally, the use of Bismuth oxide highly nonlinear fibre is investigated in order to enhance the compactness and stability of the switching system. 2R-regeneration at 10- and 40-Gb/s is demonstrated using just 2 m of this fibre.