High-resolution refractive index sensor by using thinned fiber Bragg gratings

In this work, high sensitivity refractive index measurements have been experimentally demonstrated by using thinned fiber Bragg grating sensors. When cladding layer is reduced, significant changes in the effective refractive index occur due to external refractive index modifications, this means that consequent shifts in the Bragg wavelength are expected. Reduced cladding layer have been obtained by using wet chemical etching in hydrofluoric acid solutions. Experimental tests to prove sensor sensitivity induced by changes in the external refractive index have been carried out by using glycerine solutions with well known refractive indices. Obtained results agree with the numerical analysis carried out by using three layer fiber model. If the cladding layer is full etched resolutions of ≈10-5 and ≈10-4 for outer refractive index around 1.45 and 1.333, respectively, are possible. In addition, higher sensitivity and robustness could be achieved by using fiber Bragg grating sensors written in multimode graded index fibers as shown in a preliminary study here reported.