Ultraviolet generation in silica fibre and microspheres

Abstract

Ultraviolet (UV) light sources have found numerous applications in medical science and engineering, such as disinfection, water purification, food production, UV curing and lithography, as short wavelengths have relatively high photon energy and provide high resolution. This thesis focuses on the UV light generation in silica glass. The UV spectral range is per se challenging both regarding material transparency and the availability of pump sources and optical devices. Two approaches are theoretically and experimentally demonstrated using submarine telecom silica fibres and Gd-doped silica and phosphosilicate fibres and microspheres.
The first approach relates to the sixth harmonic generation of deep UV from a 1550 nm laser source in a fully fiberised system by cascading second- and third- harmonic generation using a periodically poled silica fibre, and an optical sub-micron diameter optical fibre, respectively. Phase matching is achieved by harnessing intermodal phase matching in optical microfibres and a permanent χ⁽²⁾ induced via thermal poling. As a result, nonlinear processes can be observed despite the low third-order nonlinear susceptibility of silica glass.
The other approach presented in this thesis is based on the spectral properties of the rare earth ion Gd³⁺, which has its first excited state located at a wavenumber of 32 000 cm^-1 above the ground level, corresponding to an UV emission at 312.5 nm. Relevant defects of the host material in the UV wavelength range are detrimental to lasing and are therefore investigated. Luminescence generated by the ⁶P_7/2→⁸S_7/2 transition in the Gd-doped fibre is observed when pumped with various UV sources including a femtosecond laser, a UV LED and a lamp. A theoretical analysis of lasing is then carried out, including the estimation of the emission cross section and the related laser threshold and the selection of a suitable resonator, followed by experiment demonstration.

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Identifiers

ORCID for Yun Wang: orcid.org/0000-0001-9125-8047

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

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