The absorption characteristics of circular, offset, and rectangular double-clad fibers

The absorption characteristics of circular, offset, and rectangular double-clad fibers are investigated with phenomenological, simple, and comprehensible 2-D analysis models. For offset and rectangular fibers, almost all propagation rays are absorbable, so it is possible to achieve very high absorption efficiency. However, for circular fibers, it is not, there are few absorbable rays and absorption is lower. The dependence of absorption on fiber length is calculated with 3-D ray tracing simulations. The calculation indicates that, for circular fibers, the bend radius influences the absorption efficiency markedly. But for rectangular and offset fibers, the bend effect is negligible. The effective absorption coefficients of offset and rectangular fibers are proportional to doped concentration and area ratio of core to first cladding. The influences of offset distance for offset fibers, as well as cross section shape for rectangular ones on absorption is also discussed.