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In our previous research, a novel demodulation technique based on α-shape Delaunay triangulation (D-T) was developed to obtain the refractive index of the medium surrounding the optical fibre using the envelope of the cladding peaks from the spectrum of the tilted fibre Bragg grating (TFBG) sensor. This technique was demonstrated to be efficient, easy to implement, powerful, faster than the previous ones and applicable for real-time measurements. In this paper, a deep parametric analysis of the resolution, repeatability and accuracy of the D-T demodulation technique for a TFBG refractometer sensor is performed and presented. The spectral properties of the TFBG sensor as a refractometer are explored using the same demodulation technique. Specifically, supposing the use of the TFBG as a two-parameter optical sensor, the influence of the strain on the envelope area is analysed, and the measurement stability regarding the external RI is reported. Then, the cladding resonance peaks in the spectrum are observed experimentally as the TFBG undergoes partial immersion in a defined refractive index liquid. This last experiment allowed a better understanding the evolution of the TFBG transmission spectrum when the Bragg gratings were partially surrounded by a medium with a different RI.

In this research the ageing of a silicone adhesive in a simulated space environment is monitored through an embedded three parameter tilted Fibre Bragg Grating (TFBG) sensor. Here, the silicone is used as an adhesive between two thin cover glasses, and the space environmental ageing is simulated by thermal cycles in high vacuum conditions (better than 10-5 mbar). These operational conditions can induce variations in the silicone adhesive with respect to its original properties such as dimensional stability, chemical composition, generated contaminants, discoloration and, mechanical or optical degradation. Therefore, surrounded by the adhesive, in the centre of the cover glass sandwich, a weakly tilted FBG sensor was placed to obtain information from its spectra on the state of the polymer during the test. Specifically, the temperature, strain and refractive index (RI) of the silicone can be, simultaneously and separately, measured from the spectrum of a single TFBG from selected resonance peaks. These parameters can be used to evaluate the “health” state of the silicone during the vacuum thermal cycles. The simultaneous TFBG thermomechanical measurements gave a solution to the non-localized measuring issues when using classical fibre optic or electrical strain-gauges and a thermocouple (TC) to compensate the temperature and to better understand the material behaviour. The trends of the measured parameters are reported during the entire testing time, and at the end of the test, the optical fibre sensor measured a negative strain of ~100 με and a positive RI variation of ~0.002.