Combined long-period grating and micro-cavity in-line Mach-Zehnder interferometer for refractive index sensing

This work presents combined long-period grating (LPG) and a micro-cavity in-line Mach-Zehnder interferometer (μIMZI), both induced in the same single-mode optical fiber. In order to increase refractive index (RI) sensitivity of the LPG, it has been nanocoated with aluminum oxide (Al2O3). Next, the μIMZI has been fabricated as a cylindrical cavity (d = 60 μm) in the center of the LPG. In transmission measurements for various RI in the cavity and around the LPG we have observed two effects coming from two independently working sensors. There was no significant impact on one or the other in terms of their functional properties. Such a combination of sensorial effects can be applied for cross-reference measurements of the two different parameters at the same time or for discrimination of influence of other parameters on the RI measurements.