In this paper we describe work aimed at identifying and measuring properties of rare earth doped fibres which may affect their use and application in oscillators and amplifiers. The main dopants of interest are erbium(1.55μm), and neodymium (0.9, 1.06, 1.3μm). We shall discuss the measurement, and effects, of parasitic absorption from the upper laser level (known as excited state absorption, or ESA), at both pump and signal wavelengths. We shall also consider how detailed measurements of fluorescence decay from the excited level can yield information about quenching, upconversion, and energy transfer processes.
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