Fibre optic pH sensors using thin-film Langmuir-Blodgett overlay waveguides on single-mode optical fibres

A fibre-optic pH sensor is demonstrated using thin film Langmuir-Blodgett (LB) films to form a single-mode planar waveguide, evanescently coupled to a side-polished single-mode optical fibre. The LB technique shows two distinct advantages in the fabrication of such sensors, the ability to control the overlay waveguide thickness to a single molecular layer (similar to 3nm) and also, as the materials used are organic, the ability to engineer specific materials for different sensing purposes. The response of the sensor is also investigated through changes in the overlay refractive index, caused by changes in its absorption spectrum via a simple Kramers-Kronig model. The mode! is shown to be able to predict the response of the sensor.