Anping LIU and Kenichi UEDA

Institute for Laser Science, University of Electro-Communications, 1-5-1, Chofugaoka, Chofu-shi, Tokyo, 182 Japan

(Received March 7, 1996; Accepted June 3, 1996)

The absorption characteristics of circular double-clad fibers are analyzed in straight and bend cases with geometrical optics and mode coupling theory. A concept of absorbable power ratio is defined to describe absorption behavior and establish a theoretic model. The model investigation indicates that the absorption of doped core is determined by the absorbable power ratio, absorption coefficient, and coupling coefficient. Along with the increase of fiber lengths, the total absorption efficiency tends to saturation. A three-dimensional ray tracing simulation has also been achieved to calculate absorption efficiency and the calculated results are found to agree well with the model on the assumption of inconstant coupling coefficient. To improve the absorption, potential methods such as using proper bend as well as good pump light and bending fiber with variant bend radiuses are also discussed.

Key words : double-clad fibers, absorption, rare earth-doped