Coaxial optical fibres

Abstract

A novel form of directional coupler configuration is presented in which one of the dielectric guides is a conventional single mode rod, and the other a tube guide completely surrounding the rod coaxially. The coaxial combination offers the possibility of efficient optical power transfer between the guides when phase-matching conditions are achieved. The coaxial directional coupler could be used.both as a light modulator and as a sensor. In order to be able to analyse the properties of such a device, the exact eigenvalue problem is solved to obtain the normal modes of the coupler. Power transfer from one guide to the other is described in terms of the interference between the normal modes. The large number of cumbersome computations involved, using the exact approach, is avoided with the aid of the simpler waveguide coupling theory. Numerical calculations for the design of phase matched guides, beatlengths, fabrication tolerances and switching sensitivities are presented. In a further attempt to check the accuracy of our results, the transverse resonance technique was developed. This method is useful for examining the propagation characteristics of cylindrical or slab waveguides, of arbitrary index profiles. Excellent numerical agreement amongst all these alternative approaches was found. The application of the coaxial coupler as a modulator, when an electrooptic material such as KDP is used as outer cladding is also examined. Particular emphasis is given to various modulator configurations and to the coplanar electrode structure.· A study of the capacitance and electric field strength under theelectrodes is also presented. After the successful fabrication of the coaxial fibre, a series of coupling experiments, using for simplicity silicone oil as outer cladding, were performed. Coupling and switching was demonstrated, under conditions which were consistentwith the numerical predictions.