Liquid crystal adaptive planar optical devices

Abstract

This thesis presents a series of experimental studies based on using liquid crystals (LC) with planar optical lightwave components. Adaptive optical devices have been fabricated by combining LCs with direct UV written buried channel waveguide and Bragg grating structures. It has been discovered that the hysteresis seen in previous LC tunable Bragg grating devices is due to a process known as the pincement transition. Pincement involves the transition from a splay-bend wall in the nematic LC to a pair of oppositely charged half-integer disclination lines. The voltage-based transient behaviour of the pincement transition correlated with the tuning curve response seen in voltage controlled LC tunable gratings. In order to reduce the hysteresis effect a new grooved substrate geometry was introduced. Rather than accessing the Bragg grating vertically via a wet etch process, the substrates are precision machined using a dicing saw to allow side access. The result is significantly reduced hysteresis, with a maximum tuning range of over 1nm with the application of under 30V(pp). Tunable chirped Bragg gratings based on standard PCB technology using arrays of resistors as heating elements were designed and tested. The group delay slope of the chirped gratings was tuned by 4ps/nm using a combination of heating elements and thermoelectric cooling. Finally, LC compounds were then tested for suitability as waveguiding media. It was found that both hollow fiber and planar substrate systems using an LC as the core guiding medium successfully guided both visible and IR light at telecomms wavelengths

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Identifiers

ORCID for Peter Smith: orcid.org/0000-0003-0319-718X

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