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The resonant nonlinearity observed in rare-earth doped fibre is of interest for all-optical switching due to the very low pump powers needed to achieve complete switching. Large nonlinear effects have been observed previously in both erbium and neodymium doped fibres, however, these large effects cannot be explained by taking into account the change in absorption of local transitions alone.Two mechanisms have been suggested to account for these anomalously large index changes, namely thermally induced effects and far from resonance population dependent effects. In this work, an ytterbium doped twin-core fibre is used to investigate the two possible mechanisms, and by allowing the doped fibre to lase and observing the clamping of the induced phase change, it is demonstrated that the effect is population dependent in nature. The observed wavelength response of the effect shows an increase in induced phase towards shorter wavelengths, which suggests that the effect is due principally to changes in strong absorptions in the UV. This far-from-resonance effect has been used to demonstrate large phase changes at the preferred telecommunications windows of l300nm and 1550nm. Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/26/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx 190/SPIE VoL 2841