4 research outputs found
Photonic Crystal Fibres for Coherent Supercontinuum Generation
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.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Studies of third-order nonlinearities in materials and devices for ultrafast lasers
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Pages 161-162 blank.Includes bibliographical references (p. 133-143).Recent developments in telecommunications, frequency metrology, and medical imaging have motivated research in ultrafast optics. Demand exists for broadband components and sources as well as highly nonlinear fibers and materials. For this thesis, several different devices have been developed for such applications. Broadband saturable absorbers based on III/V and Si materials systems were developed for femtosecond lasers and have high reflectivity over 200 to 300 nm bandwidths. These absorbers were designed for modulation depths ranging from 0.3% to 18%. Self-starting modelocked operation with the absorbers was achieved in a variety of lasers including Ti:Sapphire, Cr:Forsterite, Er:glass, Cr⁴⁺:YAG and erbium-doped bismuth-oxide fiber. In tapered microstructure fiber, highly nondegenerate four-wave mixing was achieved, with a frequency shift of 6000 cm⁻¹ in an interaction length of only 1.4 cm. Amplification in erbium-doped bismuth-oxide fiber was demonstrated, with gains of 12 dB achieved between 1520 - 1600 nm in a 22.7-cm length. With a 55.6 cm length of bismuthoxide erbium-doped fiber, an L-band modelocked laser was constructed, tunable between 1570 - 1600 nm. It produced 288-fs pulses at 1600 nm. Undoped highly nonlinear bismuthoxide fiber was used to generate smooth, controlled supercontinuum between 1200 to 1800 nm.(cont.) Pulse compression of 150-fs pulses to 25 fs was also demonstrated. Finally, the nonlinear refractive index coefficient and two-photon absorption coefficient of Ge-As-Se glasses were measured. Ge₃₅As₁₅Se₅₀ is found to have a nonlinearity 900 times that of silica.by Juliet Tara Gopinath.Ph.D