Fabrication tolerances in fibre directional couplers designed to maximise coupling between fibres and diode lasers

A fibre directional coupler is described that comprises one fibre with a circular core coupled to one with a suitable rectangular core, which makes it possible in principle to couple 100% of the output of a semiconductor laser or laser amplifier into a single-mode communications fibre, and vice versa. The manufacture of such a coupler depends critically on the control of dimensions, because of the differing dispersion characteristics of the two fibres. This coupler was modelled using a finite element program in a preliminary attempt to study dimensional tolerances, with a view to optimising its design. Calculations are presented of the beatlength as a function of the sizes and the refractive indices of the two cores comprising the coupler, together with estimates of the dimensional tolerances, giving coupling losses under 1 dB