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Title: Photonic crystal fibres for coherent supercontinuum generation
Author: Hooper, Lucy
ISNI:       0000 0004 2726 1374
Awarding Body: University of Bath
Current Institution: University of Bath
Date of Award: 2012
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In this research photonic crystal fibres were developed for the purpose of generating coherent supercontinua. Two photonic crystal fibres were fabricated with all-normal group velocity dispersion profiles, with low dispersion at pump wavelengths 800 nm and 1064 nm. Supercontinua generated using these fibres were shown to have superior stability and coherence compared with supercontinua generated in fibres with anomalous dispersion at the pump wavelength. Using a short piece of photonic crystal fibre with all-normal group velocity dispersion, pumped at 1064 nm, a self phase modulation spectrum spanning 200 nm was generated. The supercontinuum was re-compressed using linear chirp compensation to 26 fs, which was within a factor of two of the theoretical transform limit. This demonstrates the high spectral coherence, stability, and almost-linear chirp of the supercontinuum. Simulations showed that pulse compression using a supercontinuum generated in a photonic crystal fibre with anomalous dispersion at the pump wavelength would be limited by shot-to-shot fluctuations in the spectral intensity and phase, and the nonlinear chirp. Using a longer piece of all-normal dispersion photonic crystal fibre, supercontinuum is generated by self phase modulation, and optical wave breaking. A broad flat supercontinuum spanning 700 nm, centred at 1064 nm was generated. This supercontinuum was spectrally filtered, and the pulses obtained analysed in the temporal domain. Clean, stable sub-picosecond pulses were achieved, demonstrating the applicability of such a supercontinuum as part of a compact, tunable laser source. The same experiment was carried out using a photonic crystal fibre with anomalous dispersion at the pump wavelength, resulting in pulses with a large portion of energy contained in broad shoulders, and higher order modes. Interferometric coherence measurements were carried out at 800 nm using a Ti:Sapphire laser. A supercontinuum was generated in all-normal dispersion photonic crystal fibre with low dispersion at 800 nm, spanning 400 nm. Supercontinuum pulses generated by consecutive laser pulses were brought together in time using an interferometer. The interference between consecutive pulses was viewed spectrally, and the interference fringes had high visibility across the whole supercontinuum bandwidth. This demonstrates high spectral coherence. A supercontinuum generated in photonic crystal fibre with anomalous dispersion at 800 nm was tested in the same way, and the interference fringes obtained had lower visibility, indicating low spectral coherence. The research presented demonstrates that photonic crystal fibres with all-normal dispersion profiles can be used to generate supercontinua with high coherence and intensity stability. This type of supercontinuum is applicable to ultra-short pulse compression, and can be spectrally filtered to create a broadband tunable ultra-short laser source.
Supervisor: Knight, Jonathan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: photonic crystal fibres ; supercontinuum ; coherence