Abstract: We Proved theoretically that the week guiding and weak coupling approximatlon must be untilized when our approximate parallel waveguide model takes the place of the original model of the fused tapered wide band coupler.According to the experiment on the weakly fused tapered coupler developed by us,we obtained that the fused tapered wide band coupler with a shape of dumb-bell satisfied the weak guiding and weak coupling conditions.Furthermore,according to the curve of the translation power versus the wavelenth,we obtained that not only the theoretical curve reflecting the wide band characteristic matched well with the experiment curve,but the coupling characteristic was very sensitive to the ratio of two fiber cores and fused length,a fact identical with the practical result.

Key words: fused tapered fiber coupler, coupling, weak guiding weak, simple model