Abstract: The manufacture and theory of a wide band single-mode fibre directional coupler are presented in this paper. Using the different fuse-tapered characteristics of two fibres,we made a two-section quasi-cone-shaped waveguide of unsymmet-rical arms.By means of variational caleulus and the standard theory of eoupledwaves,the coupling behaviors of the waist zone and the trapezoidal zone are ana-lysed;the coupling power of the coupler equals the sum of the coupling powers of these two areas; and the light power of the unsymmetrical arm coupler formed in this way can not be completely transformed between the two fibres.
With proper differences of the characteristics in the fuse-tapered area of the two fibres,the transformation Point of certain power splitting ratio (e,g.,3dB) can be located at the point of extreme value of the ratio against the change of wavelength. In the area near the point,sensitivity of the power change to wave-length is low. Thus a single-mode coupler of wide band is obtained.

Key words: fuse-tapered coupler, wide band fibre coupler, fibre directional coupler