Measurements are reported on the open and closed-loop phase stability of a large-mode-area ytterbium-doped fiber amplifier. Phase fluctuations are characterized by a high-frequency low-amplitude jitter superimposed on a slow power-dependent drift. The amplifier may be phase locked to a precision of lambda/20 by using a low-bandwidth feedback loop.

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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 µm 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 2 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.

We report on the phase locking of a fibre bundle laser based on a single frequency oscillator coupled into four fibre amplifiers to provide a coherent beam of over 600 W. The oscillator was phase modulated to a width of up to 2 GHz to increase the threshold for stimulated Brillouin scattering and then a fraction split off and frequency shifted to form a reference beam. The oscillator output was amplified by end-pumped fibre amplifiers based on 20 μm core Yb doped fibre to provide a power of up to 260 W per channel. The beams combined to form a coherent output with phase errors of a twentieth of a wave, unaffected by the spectral broadening.

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