Nunes, Joel
(2024)
Towards mid-infrared fibre lasing.
PhD thesis, University of Nottingham.
Abstract
This thesis focused on the development of trivalent lanthanide doped chalcogenide glasses intended for the fabrication of MIR (mid-infrared) fibre lasers operating at wavelengths beyond 4 μm. The fabrication and spectroscopic evaluation of separate trivalent ‘Ce’ (cerium), ‘Pr’ (praseodymium), ‘Sm’ (samarium), ‘Tb’ (terbium), and ‘Dy’ (dysprosium) cation doped chalcogenide glass samples was undertaken as the first step towards achieving this goal. These RE3+ (rare-earth) doped chalcogenide glass samples were produced using the ‘melt/quench’ technique, then drawn into single material fibres (fibre without a glass cladding) and/or fabricated into a SIF (step-index fibre) using the ‘rod-in-tube’ technique. Emerging from the production and fabrication of these RE3+ doped samples was a novel non-destructive evaluation technique using either NIR or MIR imaging to detect internal defects and inhomogeneities in glass samples. As a result of this work the capability to produce RE3+ doped bulk chalcogenide glass, single material fibre SIF and extruded glass rods and tubes was demonstrated with some RE3+ doped glass compositions exhibiting optical losses as low as 1.81 ± 0.29 dB/m. Additionally, the ability to cause permanent photoinduced refractive-index changes in RE3+ doped glass was also demonstrated. There was also evidence of defects within several chalcogenide glass samples during their production and fabrication, alluding to room for improvement in the employed methodology.
Absorption spectra, PL (photoluminescence) spectra, and PL lifetime measurements were then performed on the RE3+ doped chalcogenide glass samples in the NIR (near-infrared) to the MIR spectral region to assess their spectroscopic behaviour during photoexcitation. These spectroscopic measurements inspired the development of a novel variation on the ‘pump and probe’ technique using an FTIR (Fourier transform infrared) spectrometer to evaluate latent ESA (excited state absorption) in RE3+ doped fibre samples. Importantly, knowledge of these ESA bands offers an opportunity to either avoid or utilise them. As a result of these spectroscopic measurements several absorption and PL emission bands in the NIR and MIR spectral regions were detected and evaluated from Ce3+, Pr3+, Sm3+, Tb3+, and Dy3+ cation doped samples. Notably, some of the RE3+ dopants, exhibited PL emission bands comprised of emissions from disparate transitions each with overlapping wavelengths. Additionally, up-conversion, the transfer of longer wavelength pump energy (lower photon energy) to shorter wavelength PL emissions (higher photon energy), was demonstrated in both the ‘Pr3+’ and ‘Tb3+’ cations while employing ‘in-band’ pumping of their lowest excited state manifolds, the 3H5, and 7F5 manifold, respectively. Furthermore, latent ESA measurements of Pr3+, Sm3+, Tb3+, and Dy3+ cation doped chalcogenide glass fibre samples revealed several prominent transitions from excited RE energy levels for each dopant for the first time. The corresponding absorption measurements and PL emission measurements were used to propose RE energy level transitions for the observed ESA. These results showed the influence of latent ESA on the behaviour of the upper energy levels of RE3+ dopants under excitation.
Finally, and importantly, this thesis presents the results for the first demonstration of room temperature MIR lasing in a chalcogenide SIF with a ‘Ce’ cation doped core at wavelengths greater than 5 µm.
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