Optical Fiber Cavity Ring Down Measurement of Refractive Index With a Microchannel Drilled by Femtosecond Laser

A microchannel was inscribed in the fiber of a ring cavity which was constructed using two 0.1% : 99.9% couplers and a 10-m fiber loop. Cavity ring down spectroscopy was used to measure the refractive index (RI) of gels infused into the microchannel. The ring down time discloses a nonlinear increase with respect to RI of the gel and sensitivity up to 300 mus/RI unit and an index resolution of 1.4 times10
^-4
was obtained.