Fiber optic implementation of evanescent wave cavity ring-down spectroscopy for chemical detection

Abstract

A fibre optic platform has been fabricated for the field deployment of evanescent wave cavity ring-down spectroscopy with an absorbance sensitivity of 5 ppm. An optical cavity is fabricated by depositing high-reflectivity mirrors onto each end of the fibre and the evanescent field is exposed to the sample in a tapered region of the cavity. The decay time, τ, is dominated by the propagation loss of the radiation in the fibre optic and the loss of the tapered region. The multi-pass configuration can detect molecules adsorbing to the surface of the tapered region if they absorb radiation at the wavelength of the laser. An indicator molecule has been tethered to the glass surface to produce a colour change in response to the bulk pH producing an optical pH sensor with a sensitivity of 0.01 pH units. The fibre cavities have potential to form an optical sensor network to detect target molecules with presumptive detection on functionalised fibre surfaces.