Photonic Crystal Fibres for Coherent Supercontinuum Generation

Abstract

Inthisresearchphotoniccrystalfibresweredevelopedforthepurposeofgeneratingcoherent supercontinua. Two photonic crystal fibres were fabricated with all-normal group velocity dispersionprofiles,withlow dispersion atpump wavelengths 800 nm and 1064 nm. Supercontinua generatedusing these fibreswereshowntohavesuperiorstability and coherencecompared 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 compressionusing asupercontinuumgeneratedinaphotoniccrystal fibrewith anomalousdispersionat the pump wavelength would be limited by shot-to-shot fluctuations in the spectral intensity and phase, and the nonlinear chirp.Using alongerpieceof all-normaldispersionphotoniccrystal fibre,supercontinuumisgeneratedby 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 crystalfibre 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 supercontinuumbandwidth. Thisdemonstrateshighspectral coherence. Asupercontinuumgenerated 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.Theresearchpresenteddemonstratesthatphotoniccrystal fibreswith all-normaldispersion profiles can be used togenerate supercontinua withhigh 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.

Date of Award	1 Jan 2012
Original language	English
Awarding Institution	University of Bath
Supervisor	Jonathan Knight (Supervisor)