Curvature effects on leaky modes in weakly guiding optical fibres

Abstract

The effect of curvature on leaky modes with low losses in weakly guiding optical fibres is investigated, within a theoretical model where the cladding (outer cladding in case of multiclad fibres) is extended to infinity. Such modes already radiate rays in the cladding when the fibre is straight and two cases can be considered according to whether this ray pattern is modified near the core area in the bent fibre (strong curvature) or is not modified (weak curvature). A general loss formula is given for the case of weak curvature. The rays which are emitted in the curvature plane, towards the inner side of the bend are captured later on by the core and induce an oscillating modulation of the leakage loss, with an amplitude varying asR−1/2 but limited to a few per cent in practical cases (for ‘weak curvatures’). The case of strong curvatures is analysed in the special case of the LP 01 mode in fibres with a large depressed inner cladding, through a 2-D ray method. It is found that the loss oscillations are rapidly damped while the average loss is considerably increased.