An in-line in-fibre ring cavity multi-parameter sensor with a tunable refractive index response

Abstract

An in-fibre ring cavity is fabricated by writing two blazed gratings next to each other to form a Fabry Perot cavity. A visibility of fringes as good as 0.032 in the reflection spectrum and 0.76 for transmission is obtianed for interference between the foward propagating guided mode and the reverse propagating ghost mode of the blazed grating. The ability to measure the external refractive index and the variability of this response with cavity length is demonstrated.