A study on the possibility of using thermally-poled silica waveguides for optical communication devices

Abstract

In the early 80s a series of experiments showed that it was possible to induce second harmonic generation in silica-glass fibres. The initial excitement was dampened when it was realised that the efficiency was very much limited only to be kindled once again by the first demonstration in 1991 of efficient second harmonic generation in bulk glass by thermal poling. Since then great progress has been made but many efforts are still being made to re-transfer this technology to fibre waveguides.
This thesis describes work done in order to assess the current state of art of thermal poling, the possibility of applying the technique to obtain efficient telecom devices, and if there are any feasible solutions to the problems encountered in poling silica fibres.
The highest nonlinearity obtained in bulk silica glass by thermal poling is of the order of 1 pm/V and may be considered as the achievable limit in silica fibres. On this basis it can be shown that electro-optic modulation or switching in poled waveguides is not competitive with other more established technologies whereas all-fibre frequency conversion remains an interesting application.
It has, however, been noted that the induced nonlinearities in fibres are usually much smaller than in bulk glass. After an initial study of various characterisation methods and a substantial refinement of the Makers fringe technique, we proceeded to model and experimentally determine the ionic migration process underlying thermal poling in silica. Further experiments showed that the presence of a germanium doped region hinders considerably the nonlinearity formation and some solutions were put forward. Most notably, we thermally poled for the first time microstructured holey fibres which are of great interest for nonlinear applications.

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