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.
The results of theoretical and experimental studies of active tapered double-clad fibers, intending the optimization of its imperative parameters--tapering ratio, longitudinal profile, core/cladding diameters ratio, are presented. Using a refined taper geometry we have demonstrated power scaling of a ytterbium fiber laser pumped by low-brightness, cost-effective laser diodes up to 750 W, with 80% efficiency.
We demonstrate passive mode-locking of a Bi-doped fiber laser using a nanotube-based saturable absorber. We achieve stable mode-locking in both the all-normal and net anomalous dispersion regimes. Near transform-limited 4.7 ps soliton pulses are generated in the average-soliton regime, with a chirped fiber Bragg grating used for dispersion compensation to retain an all-fiber format.
In-fiber Bragg and long-period gratings as well as Mach-Zehnder interferometers based on germanium-and nitrogendoped silica fibers have been investigated under y-rays. The majority of the experimental results suggest that both types of gratings in both types of fibers are stable with respect to y-ray doses of up to 1.47 MGy.
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