Development of frequency converters with extended functionalities in periodically poled silica fibres

Abstract

The centrosymmetry of amorphous silica can be broken through the technique of thermal poling, whereby the application of an electric field at elevated temperatures introduces an effective second-order nonlinearity. Quasi-phase-matching is realised through periodic ultra-violet erasure of the uniformly induced nonlinearity. A truly all-fibre laser system operating in the visible was envisioned to replace the crystalline frequency doublers currently employed. Despite the lower second-order-nonlinearity in poled silica fibres, the longer interaction length for comparable acceptance bandwidths, the higher damage threshold and straightforward integration makes them a very attractive substitute.

In this thesis, the routes taken to induce the maximum possible nonlinearity and the optimisation of the quasi-phase-matching technique to achieve the highest possible normalised conversion efficiency are reported. Periodic ultra-violet erasure for quasi-phase-matching allows scalability to longer interaction lengths in comparison to the photolithography technique previously employed. Further circular twin-hole fibres can be used instead of D-shaped fibres. A greater than 16 times improvement in the normalised conversion efficiency over previous results was obtained through the enhancement of key parameters of interaction length, stability and reproducibility. An average conversion efficiency of ~15% was demonstrated in a ~32cm long periodically poled fibre using a fibre laser source with just ~200W of peak power. The functionalities of all-fibre frequency converters have been extended to include broadband wavelength tunability of 45nm and chirped period poling for precise control of the acceptance bandwidth.

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