Fibre-taper devices

In this paper I will review recent work at Southampton on tapered single mode optical fibre devices. In particular I will describe our recent work on single mode beam expanders and fused fibre couplers. It is well known that a gap of up to 100 µm in a single mode fibre results in about 1dB loss. By tapering the fibre so that the field expands into an LP cladding mode with increased spot size it is possible to introduce a larger gap of about one mm. This will allow the insertion of small electro-optic devices, such as a liquid crystal cell. By overjacketing the fibre with a silica capillary, or combination of Vycor and silica capillaries it should be possible to achieve a beam diameter of several hundred microns. This would allow a gap of about 1 cm and this would allow the insertion of a very wide range of optical devices, such as acousto optic modulators. Considerable progress has been made recently in the fabrication and analysis of long fused-taper 4-port directional couplers. This has resulted in several important devices, including polarization beam splitters, wavelength division multiplexers, optical fibre filters, and wavelength selective ring resonators