Characterisation of novel refractometric sensing systems

Simulations and experimental results for novel refractometric-sensing platforms are presented here. The first platform is based on a multi-mode interference coupler (MMIC) in which the top and sidewalls of the coupler are exposed to a humidity-sensing enrichment layer. Sensor operation is based on the creation of self-images of the input field into the coupler, at regular intervals along the coupler. This phenomenon is due to interference between the optical modes in MMICs. Changes in the refractive index of the sensing layer cause predictable shifts in the position of the output image, which in turn affects the amount of light coupled into the output waveguide. Sensitivity enhancement has been demonstrated by fabricating longer MMICs capturing higher-ranking self-images, which are shifted more than the first self-image. Consequently, more significant changes in the amount of light coupled to the output are observed for a given refractive index change. The second platform demonstrated is a Multi-Channel Directional Coupler sensor (MCDC). It differs from the MMIC in that the sensing region now consists of multiple single-mode waveguides, which are in close enough proximity to allow light to transfer between the waveguides. Sensitivity dependance on platform length has been investigated and compared with that of the MMIC. The devices have been fabricated by the direct laser writing process on UV curable hybrid sol-gel materials. Such materials allow implementation of planar technology enabling integration on a silicon substrate. Future applications of these platforms include chemical and bio-chemical sensing is the areas of process, environmental and bio-diagnostic monitoring.