Factors reducing the efficiency of ytterbium fibre lasers and amplifiers operating near 0.98 μm

We have studied the factors that reduce the efficiency of cladding-pumped ytterbium fibre lasers and amplifiers operating in the spectral range around a wavelength of 0.98 μm. It has been shown that the core of the active fibre being not single-mode leads to accelerated development of amplified spontaneous luminescence around λ = 1.03 μm, which reduces the highest possible amplifier efficiency. We have examined the effect of spontaneous luminescence propagating in the first reflective cladding. It has been shown theoretically that the pump-to-signal conversion efficiency is highest when the ratio of the core and cladding diameters in a multimode active fibre lies in the range 0.7 – 1. We have fabricated and characterised an optical fibre with a core-to-cladding diameter ratio of 0.76 (core and cladding diameters of 95 and 125 μm, respectively), which allowed a pump conversion efficiency of 66 % to be reached, a record level for cladding-pumped ytterbium fibre laser systems emitting near 0.98 μm. (paper)