Abstract

Recently developed Yb-doped fiber lasers with ps and ns pulse duration approach MW peak power level [1]. However, such lasers use photonic crystal fiber (PCF) fiber design that has a number of drawbacks: the complexity of fabrication process, the high bend-sensitivity, the impossibility to realize all-fiber systems. A promising alternative large-mode-area active fiber design based on tapered step-index fiber was proposed recently [2, 3]. The main idea of this approach is to use active step-index fiber with gradually changing outer diameter. The thin end of tapered fiber is single-mode and thick end has few times increased core and cladding diameters. Pump is coupled through a thick end (that allow usage of a low-brightness cheap pump sources) and signal is propagated from thin single-mode end to the thick large-mode area end. A diffraction limited output signal quality together with an increased threshold of nonlinear effects was achieved in this case [3]. However, the demonstrated peak power of 25 kW inside the amplifier [3] is two orders of magnitude lower as compared to PCF fiber [1]. The reasons are a relatively long tapered fiber (~10 m), relatively high core NA (>0.11) and a small ratio of core-to-clad diameters that reduce pump absorption from the cladding.

© 2015 IEEE