Double clad tapered fiber for high power applications.

We report a novel type of active fiber - tapered double clad fiber suitable for pumping by low brightness sources with large beam parameter product of 50/300 mm x mrad. Ytterbium double clad all-silica fiber (core/1(st) clad/2(nd) clad diameters 27/834/890 mum, NA(core)=0.11, NA(clad)=0.21), tapered down by a factor 4.8 for a length of 10.5 m was drawn from a preform fabricated by plasma chemical technologies. At a moderate Yb-ion concentration and 1:31 core/cladding ratio, the tapered double clad fiber demonstrates 0.9 dB/m pump absorption at 976 nm and excellent lasing slope efficiency. An ytterbium fiber laser with 84 W of output power and 92% slope efficiency, a 74 W superfluorescent source with 85% slope efficiency and amplifiers operating both in CW and pulsed regimes have been realized. All devices demonstrated robust single mode operation with a beam quality factor of M(2)=1.07.