Ultraviolet generation in step index optical fibres and microfibres

Abstract

Coherent ultraviolet (UV) generation has various applications in a large number of fields, such as stand-off explosive detection, enhanced Raman scattering and photolithography, to name a few. Typically, the generation of coherent UV light relied on frequency doubling with nonlinear crystals and relatively complicated free space optics, on toxic gases (excimers) and low power UV diodes. In this thesis, the use of solid core step-index optical fibre for the generation of coherent UV radiation is investigated. Here, wavelength conversion in optical microfibres (OMFs) via nonlinear processes is considered. Firstly, efficient generation of UV radiation via intermodal third harmonic generation (THG) in fibres is first studied. While THG is potentially efficient in OMFs, it is not feasible in optical fibres with large numerical apertures. Detuning is found to be a critical parameter which determines the overall efficiency, thereby making the OMF diameter control crucial. Initial experiments indicate that the quasi-continuous wave (CW) sources with long pulses (a few ns) and high peak powers (∼ kW) are the most suitable for intermodal THG in OMFs, as detrimental nonlinear effects become dominant with ultrashort pulses. However, further theoretical investigations show that intermodal THG is fundamentally limited by the intrinsic surface roughness of the silica OMFs, restricting the maximum efficiency to ∼ 10^-3. This leads to the investigation of four wave mixing (FWM), where both parametric amplification and wavelength generation are theoretically investigated. A quasi-CW source working in conjunction with a periodically poled silica fibre (PPSF) is employed to investigate this experimentally, and it is shown that a scheme employing two OMFs allowed the generation of coherent UV radiation down to 0:31 µm, with fundamental limitations being imposed from losses due to the oxygen-deficiency centre inherent to the fibre.

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Identifiers

ORCID for Gilberto Brambilla: orcid.org/0000-0002-5730-0499

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