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

Abstract

An in-line 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 obtained for the interference between the forward 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.

An in-line in-fibre ring cavity sensor for localized multi-parameter sensing

“…As the strain and temperature responses of fibre gratings have already been well established, it was considered sufficient to be able to measure the external refractive index (without using peak wavelength shifts) in order to demonstrate multiparameter sensing (the sensitivity matrix would then have a nonzero determinant allowing inversion to occur). For testing, a preliminary report of which has been made in [30], each grating was in turn immersed in water (refractive index = 1.33) and paraffin oil (refractive index = 1.42) and the transmission spectrum monitored. The results are shown in figures 3 and 4 for BFPG1 and BFPG2 respectively.…”