Spectroscopy of erbium-doped fibre amplifiers and lasers
This thesis describes work done in investigating Er3+-doped fibre amplifiers and lasers, with emphasis on the spectral properties and diagnosis of ion-ion interaction processes in the glasses and fibres. Ion-pair upconversion and uniform upconversion in Er3+-doped silicate fibres have been directly and separately observed for the first time, by examining upconversion fluorescence. An upper limit to the Er3+ ion-pair lifetime of a few microseconds has been determined. Different effects of the two upconversion mechanisms on the 1.54µm fluorescence have been examined. Ion-pair upconversion in an Al/Ge/Si or an Al/Si host increases far less with increasing Er3+ concentration than in a Ge/Si host, while there is no significant difference in uniform upconversion between them for a given Er3+ concentration. Uniform upconversion constants for various fibres are determined through measuring the maximum decay rate of the 1.54µm fluorescence. The 1.7µm ESA spectrum from the 4H13/2 level has been measured for Er3+-doped silicate fibre and ion-exchanged silicate planar waveguides, and the uniform upconversion constants are estimated. The existence of dual excited-state energy transfer (DESET) in Er3+/Yb3+- doped phospho-silicate fibres has been inferred from measurement of the ESA spectrum around 1.1µm. The absence of ESA at pump wavelength 1.064µm has also been confirmed. The DESET process has been examined for the first time by measurements of 1.064µm pump throughput characteristics of the fibre. Short Er3+/Yb3+ amplifiers pumped at around 970nm have been numerically characterized. The advantage of codoping of Yb3+ is shown to be the potential for higher gain at shorter amplifier length. By examining Er3+-doped fibre preforms with differing co-dopant ions, Ta5+- codoped fibre is shown to be competitive with Al3+-codoped fibre for Er3+-doped fibre devices. A SiGeTaEr fibre laser has been demonstrated for the first time. Spectral properties of Er3+-doped Ga2S3:La2S3 have been studied. Emission and absorption spectra, including for the first time the 2.7µm emission, are measured. Radiative and non-radiative transition rates are calculated and compared with the measured lifetimes. Characteristics of ion-ion energy transfer processes in the glass are discussed. A numerical model has been developed to predict the Performance of Er3+:Ga:La:S fibre devices, including short amplifiers operating at 1.54µm, 980nm upconversion lasers.