μA microchannel was inscribed in the fibre of a ring cavity which was constructed from two 0.1%:99.9% couplers and a
10m fibre loop. Cavity ring down spectroscopy (CRDS) was used to measure the refractive index (RI) of gels infused
into the microchannel with high resolution. The ring down time discloses a nonlinear increase with respect to the RI of
the gel and sensitivity up to 300μs/RI unit (RIU) and resolution of 5×10<sup>-4</sup> were obtained.
Detection of radiological hazards in the solution phase using conventional means is considerably more
difficult than in the gas phase. A new approach is required to provide a reliable, specific and low cost
method of protecting sensitive national assets, such as water supplies, from a terrorist dirty bomb attack.
Fibre optic sensors provide the required speed of response, the optical platforms are mature and of
relatively low cost with proven reliability in the field. This paper describes the combination of a low cost
sensor platform and smart sensor molecule (Isoamethyrin) for the selective determination of uranyl and
other actinide species in water at sub ppm levels. Isoamethyrin is a synthetic porphyrin which has been
demonstrated to show high selectivity for uranyl ions with an associated colour change on complexation.
Fibre optic sensors are created by revealing an evanescent wave in a section of the fibre and covalently
bonding the isoamethyrin to the fibre surface in this region. Colour changes occurring as a result of
interaction between isoamethyrin and uranyl ions are monitored over 3 wavelength ranges covering the
red, green and blue regions of the visible spectrum. Sensors created in this manner were found to be fast
responding (<5s), sensitive (detection threshold <500ppb), specific (response restricted to certain
actinides and lanthanides) and low cost.
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.