Fibre couplers composed of unequal cores

To obtain the simplest results possible for couplers composed of nearly equal cores, terms of second order have been deliberately neglected in the previous conventional perturbation analysis. Merely by retaining these terms, we obtain the full perturbation results¿results which satisfy power conservation exactly and which cannot be improved on within weak coupling. Our derivation shows that the recent `new thoery¿ of Hardy and Streifer, which necessitates consideration of radiation, is convoluted, unnecessary and contains fundamental errors.