Research Article Fibre Tip Sensor with Embedded FBG-LPG for Temperature and Refractive Index Determination by means of the Simple Measurement of the FBG Characteristics C. Berrettoni, 1,2 C. Trono, 1 V. Vignoli, 2 and F. Baldini 1 1 IFAC-CNR, “Nello Carrara” Institute of Applied Physics, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy 2 Information Engineering and Mathematics Department, Siena University, Via Roma 56, 53100 Siena, Italy Correspondence should be addressed to C. Trono; c.trono@ifac.cnr.it Received 15 December 2014; Accepted 10 March 2015 Academic Editor: Tao Zhu Copyright © 2015 C. Berrettoni et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A novel optical fbre sensing system based on a hybrid long period grating (LPG) and Bragg grating (FBG) confguration is proposed and demonstrated experimentally. Te hybrid confguration, which uses the diference in temperature and refractive index (RI) diferent response of a Bragg grating and a long period grating, makes it possible to discriminate simultaneously the temperature and the refractive index of diferent aqueous solutions. RI (1.33 RIU–1.40 RIU) and temperature (21 ∘ C–28 ∘ C) working ranges have been experimentally determined. Experimental results show that the maximum accuracy in the refractive index measurement (0.004 RIU) with temperature compensation has been achieved within the working ranges. 1. Introduction Owing to their many advantages, such as compactness, electromagnetic interference immunity, high sensitivity, and multiplexing capabilities, optical fbre sensors (OFSs) based on gratings have attracted considerable interest in the context of chemical and biochemical sensing [1]. According to the grating period, fbre gratings can be classifed into short period gratings (or fbre Bragg gratings, FBGs, period in a range of hundreds of nanometres) and long period gratings (LPGs, period in a range of hundreds of micrometres). FBGs induce the coupling of the fundamental core prop- agating mode to its respective counter propagating mode, while LPGs are characterized by mode coupling between the propagating core mode and copropagating cladding modes which gives rise to a series of attenuation bands in the transmission spectrum centred at discrete wavelengths that verify the matching condition of each coupled cladding mode [2, 3]. In the last case, the resonance wavelengths of the LPG coupled cladding modes depend on the external refractive index [4] and any modulation of the core and cladding guiding properties will modify the spectral response of the LPG. Grating based sensors have been proposed for a wide range of applications including measurements of strain, temperature, pressure, and refractive index (RI) and many examples are reported in literature. However, in the context of sensing, one of the most signifcant limitations is the sensitivity of both grating structures to diferent parameters: for FBGs strain and temperature and for LPGs strain, tem- perature, and external refractive index. In order to overcome the strain-temperature cross sensitivity efect and to make possible multiparameter measurements, several techniques and confgurations were thoroughly investigated and proposed in literature. Tese solutions include confgurations based on dual-wavelength superimposed gratings [5, 6], tilted FBG [7], FBGs written in fbres with diferent diameters [8], and hybrid LPG/FBG [9]. To determine separately the temperature and the refrac- tive index of diferent aqueous solutions, many confgurations have been proposed, including Mach-Zehnder interferom- eter realized on tapered optical fbre [10], hybrid cascaded LPG/FBG confguration integrated with a thermostabilized Hindawi Publishing Corporation Journal of Sensors Volume 2015, Article ID 491391, 8 pages http://dx.doi.org/10.1155/2015/491391