The transfer function of a multimode step-index fibre has been calculated by means of geometrical optics taking into account mode coupling and leaky rays. The light source may have axial symmetry (light emitting diode) or be line shaped (semiconductor laser). Radial displacement of the line source relative to the fibre axis is allowed. Calculations performed for a large numerical aperture fibre (N.A.=0.475) with a coupling length of 2 km show that leaky rays and radial displacement of the line source are significant for fibre lengths shorter than 200m. The influence of the width of the entrance beam is significant for lengths up to 5 km.

Transfer function of a multimode step-index fibre: a geometrical optical investigation

The effects of lasing time delay of injection laser diodes in digital fibre optic communication systems are considered. The solution of applying a constant dc bias is considered taking into account the resulting non-zero extinction ratio which increases the shot noise. An optimisation procedure is formulated for the bias of a laser diode and optimum bias values for different cases are calculated.

Optimistation of the Bias of an Injection Laser in a Digital Fibre Optic Communication System

The influence of the source-drive duty factor on the performance of a digital fibre-optic communication system is explored. The system considered utilises an l.e.d. source and either a p?i?n or avalanche photodetector. It is shown that, for a fibre with known dispersion, an optimum duty factor exists which offers significant s.n.r. advantages over a system with impulsive excitation.

Source-drive optimisation for optical-fibre systems using L.E.D.s

Digital systems using multimode fibres with a step-index profile, a lossy cladding and a low-loss core are treated in the letter. The influence of fibre numerical aperture (n.a.) on repeater spacing is examined when the total core-cladding interface loss results from Fresnel reflection. It is shown, by way of example, that in this case there is no advantage in using low n.a. values, even at moderate modulation rates, owing to a diminishing receiver incident optical power with decreasing n.a.

R.J. Morgan

Papers

Source-drive optimisation for optical-fibre systems using L.E.D.s

Design consideration for digital systems using multimode fibre with lossy cladding