THE EFFECTIVE ABSORPTION-COEFFICIENT IN DOUBLE-CLAD FIBERS

Pumping of a fibre laser with a powerful multimode diode laser array is most efficient if the receiver area of the fibre is enhanced by the use of a double-clad configuration. In this case the pump light is guided in the inner multimode cladding of the fibre and is only weakly absorbed by the rare-earth doped monomode core. In a first approximation the absorption coefficient of the core material is reduced by the core/inner cladding area ratio. The effective absorption coefficient has been determined for different lengths of a neodymium doped double-clad fibre. It is shown that in our configuration the effective absorption coefficient for a fibre length of 50 cm is up to 3 times larger than for long fibre lengths, where values close to those predicted by the first approximation are reached. The experimental results are described using a simple model.