The use of short lengths of large core phosphate glass fibre, doped with highconcentrations of Er or Er:Yb represents an attractive route to achieving highpower erbium doped fibre amplifiers (EDFAs) and lasers (EDFLs). With the aim ofinvestigating the potential of achieving diffraction limited output from suchlarge core fibres, we present experimental results of fundamental modepropagation through a 20 cm length of passive 300 micrometer core multimodefibre when the input is a well-aligned Gaussian beam. Through careful controlof fibre geometry, input beam parameters and alignment, we measured an output Msquared of 1.1 + - 0.05. The fibre had a numerical aperture of 0.389, implyinga V number of 236.8. To our knowledge, this is the largest core fibre throughwhich diffraction limited fundamental mode propagation has been demonstrated.Although the results presented here relate to undoped fibre, they do providethe practical basis for a new generation of EDFAs and EDFLs.
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