Modelling and dispersion engineering for spectral shaping
The extreme spectral broadening of pulses with an initially narrow spectrum propagating in a nonlinear medium is known as supercontinuum generation (SCG). The SC is spatially coherent and the spectral bandwidth can span several hundreds of nanometres. This has applications in, e.g., component characterization, spectroscopy, optical communications, and optical coherence tomography (OCT). This thesis presents a study of SCG in photonic crystal fibre (PCF) using numerical modelling. The nonlinear physical mechanisms relevant for the thesis are reviewed. It is investigated how the SC spectrum can be shaped by dispersion engineering of the PCF. This is done in 3 different regimes: femtosecond, picosecond, and continuous-wave (CW) pumping. Femtosecond pumping is investigated in five different PCFs with two zero-dispersion wavelengths (ZDWs) and in tapered PCFs. It is found that the spectral broadening is dominated by self-phase modulation in the first millimetres of the fibre, followed by soliton red-shift. The soliton red-shift is limited by the higher ZDW and the generation of dispersive waves. The first observation of an apparent bright-bright soliton pair across the ZDW is also reported. For picosecond pumping it is demonstrated how the spectral width and flatness depends on nanometre scale design of the PCF structure. CW pumping is modelled using a phase noise model to investigate the influence of the pump spectral linewidth on the SC. The results indicate that the broadest and smoothest spectra are obtained using a narrow linewidth pump and a PCF with small anomalous dispersion at the pump wavelength. It is also demonstrated how the time window of the calculations affects the simulation results. Energy transfer during soliton collisions is found to play an important role, and was overlooked in recent work on CW pumped SC generation. Finally, the implications for designing a SC source for OCT are briefly discussed.
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Bang, Ole, Bjarklev, Anders Overgaard, Broeng, Jes, Andersen, Peter E.