Acousto-Optical Frequency Shifters in Dual-Core Optical Fibres.

Abstract : The aim of this project is the design and implementation of a practical acousto-optic frequency-shifter compatible with single-mode optical fibre. The frequency shifter was originally to be based on a special fibre with two dissimilar cores. Normally, little light couples between the cores, but in the presence of an appropriate flexural acoustic wave a large transfer of optical power, with a frequency shift, takes place. This interaction has been analysed theoretically, a dual-core fibre frequency shifter designed, and its component parts assembled. Progress towards this initial experiment was hampered by fibre fabrication problems. In the meantime, an alternative type of frequency shifter was pursued. Here the acousto-optic interaction takes place between two modes in the narrow uniform waist of a special "null" fused taper coupler. This has so many compelling advantages that the earlier dual-core work was put to one side. A prototype null coupler device was built and tested, and immediately satisfied some of the project's goals. The design promises to be able to satisfy the remainder also. (MM)