Examination of optical fibre inhomogeneities and their effects on fibre coupling

Coupling effects from source to optical fibre or from fibre to fibre are very important in the design of communication systems using such fibres, since both the amount of guided power and the output pulse shape depend on them. This work is an investigation into such effects. Various fibre imhomogeneities have been studied analytically. Internally, these are the refractive index difference A, the core ellipticity and the difference in numerical apertures; externally, the factors considered are fibre separation, offset and tilt. The problem is treated using geometrical optics and is applied to various profiles of different refractive index exponent (a). Fibres produced currently by the (MCVD) technique normally have a dip in the refractive index profile at the centre of the core. This dip in the refractive profile affects the launching efficiency and causes pulse broadening in multimode graded index fibres and birefringence in single mode graded index elliptical fibres. parameters has been investigated. The effect of the dip on these Finally experiments have been carried out to test the validity of some of the theoretical derivations.