Optical fibre cavity ring down measurement of refractive index with a microchannel drilled by femtosecond laser

A 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 tim e discloses a nonlinear increase with respect to the RI of the gel and sensitivity up to 300 P s/RI unit (RIU) and resolution of 5 u 10 -4 were obtained. Keywords : cavity ring down, femtosecond laser, microchannel, optical fibre sensor 1. Introduction Many efforts have recently been made towards detection of chemicals or biological agents of small quantities for the interest of life sciences and for security reasons. In this regard, cavity ring down spectroscopy (CRDS) has been recognised as an efficient t ool, especially in analysing gas-phase materi als. Recently, optical-fibre-based CRDS attracted more and more interest because of its obvious advantages in terms of compactness compared to the bulk optical versions along with the absence of alignment difficulties. It was initially implemented by placing mirrors formed by dielectric films at the two ends of the fibre but the desirability of connecting the cavity to other fibre for transmitting of optical signals can bring unwanted effects to these films, such as pressure-induced refractive index (RI) change of the film.