Abstract

Diode-pumped rare-earth-doped (RE-doped) fiber laser is a potential candidate of high-efficient high-power coherence source, which have many applications in optical communications, material process and bio-physics. The limitation of the output power generated by fiber lasers are determined principally by how much pumping light can be launched into the fiber. Since double-clad fiber (DCF) can provide larger cross-section and numerical aperture, DCF have been applied extensively for high-power fiber lasers. In DCF lasers, because only the doped core has significant absorption, the absorption efficiency of the pump light is much lower. To improve the absorption efficiency in double-clad fibers, there are many schemes (e.g. fiber bending, multicore fiber, etc.) have been suggested in the past years. Among these schemes, fibers with non-round inner cladding were widely applied due to its simplicity and high efficiency. In the DCF with a non-round inner cladding, symmetry of ray propagation is destroyed and absorption efficiency is developed by the blending of pumping waves. Although various cross-sectional shapes have been employed in practice, only few researches have been focused on the influence of cross-sectional shape on the absorption characteristics of DCF lasers. Based on 3-D ray tracing model, Liu¹ investigated absorption characteristics of circular and rectangular DCF but ignored the coupling effects between absorbable and un-absorbable field. Bedo et al., ² separated the pumping field into directly and indirectly absorbed parts, which interchange the energy through the coupling effects. However, the separation of directly and indirectly absorbed parts is arbitrary in their theory and should be determined by the experiments. In this paper, the influence of cross-sectional shapes on the absorption characteristics are investigated by directly solving the wave-propagation equations in that DCF. The mode-coupling caused by nonuniform absorption of the doped core are considered. The effective absorption depends on both directly absorption and coupling effects between the eigenmodes. Finally, a characteristic absorption parameter is proposed to compare the effects of the cross-sectional shape of the inner cladding on absorption characteristics of DCFs.

© 2002 Optical Society of America