Tilted fiber Bragg grating (TFBG) sensors were demonstrated to simultaneously measure the material thermomechanical and refractometric state in which they are embedded. In this work, for the first time, TFBGs are investigated for three-parameter monitoring of space-qualified NuSil® CV16-2500 silicone operating during high-vacuum ultraviolet (UV) exposure. The first part of the work is focused on the ultraviolet effect on the TFBG spectrum when the sensor is 1) directly exposed to the radiation, 2) covered by a thin cover glass, and with a Kapton layer on top. Successively, the silicone is used as an adhesive in a sandwich structure in which the TFBGs are embedded and exposed under high vacuum to various UV/vacuum UV intensity radiations and durations. The sensors’ spectra were acquired and demodulated to detect the silicone strain-temperature-refractive index variations and correlate the silicone refractometric changes with the equivalent exposure solar hours. The second part of the paper is on silicone degradation state evaluation using the same sensor but during a direct exposure of the adhesive to the radiation. This allowed the UV effects on the silicone to be enhanced but needed a method to compensate for the damaging effect of UV radiation on the TFBG spectrum.

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 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.

This research demonstrates the promising abilities of a tilted Fibre Bragg Grating (TFBG) sensor for monitoring the status of a silicone adhesive during a simulated space environment exposure. The silicone is used as adhesive between two thin cover glasses and the TFBG is embedded into the polymer such that it is fully enclosed. Then, the sample is exposed to standard space environment conditions in a vacuum chamber simulated by creating a high vacuum (1.3×10^-6 mbar) and thermal cycles between -120 ℃ to 190 ℃. The TFBG spectra recorded during the exposure were demodulated to obtain the wavelength shifts of the Bragg and Ghost peaks and the envelope area of the upper and lower cladding modes resonances peaks. This will allow the thermomechanical and the refractive index (RI) variations of the silicone to be measured during the testing. In particular, the silicone RI depends on the material chemical and physical state and its thermal history, and the TFBG envelope area is sensitive to these RI changes. Hence, the envelope area of the TFBG spectrum can be used to obtain information on the evolution of the silicone adhesive during the test. The resulting trend of the selected peak wavelengths variation and envelope area were used to detect a variation of the degradation state of the material.