The use of short lengths of large core phosphate glass fibre, doped with high
concentrations of Er or Er:Yb represents an attractive route to achieving high
power erbium doped fibre amplifiers (EDFAs) and lasers (EDFLs). With the aim of
investigating the potential of achieving diffraction limited output from such
large core fibres, we present experimental results of fundamental mode
propagation through a 20 cm length of passive 300 micrometer core multimode
fibre when the input is a well-aligned Gaussian beam. Through careful control
of fibre geometry, input beam parameters and alignment, we measured an output M
squared of 1.1 + - 0.05. The fibre had a numerical aperture of 0.389, implying
a V number of 236.8. To our knowledge, this is the largest core fibre through
which diffraction limited fundamental mode propagation has been demonstrated.
Although the results presented here relate to undoped fibre, they do provide
the practical basis for a new generation of EDFAs and EDFLs.Comment: 5 figure