This inscription method is considered a major step forward in the recording of Bragg gratings in optical fiber due to its simplicity and ease of recording [36].The usage of sensors based on FBGs to monitor RH requires the presence of a transducer layer that induces a thermal or mechanical actuation to which selleck chemicals Crizotinib the FBG is directly sensitive [37]. In the proposed sensor the transducer layer is an organo-silica-based material with strong adhesion to the optical fiber that swells in the presence of distinct RH levels. The expansion of the material will be reflected in the FBG, resulting in a Bragg wavelength change. This deformation is also dependent on the Silica Young modulus and on the thickness of the material deposited on the fiber [38].
Furthermore, Inhibitors,Modulators,Libraries due to the thermal sensitivity of the FBG, it is required to include a thermal compensation scheme, to correct the environment effects. This can be easily solved by using an uncoated Bragg grating, thus insensitive to Inhibitors,Modulators,Libraries RH variations.2.1. Related WorksThe usage of a FBG in conjunction with a transducer layer to produce and optical sensor was already reported, using polymer-based materials as the swelling media. In particular, the influence of the RH on a FBG coated with a polymer was first described by Limberger et al. [39]. That sensor uses a polyimide coating as the transducer layer with a linear response over a wide range of values of RH (10�C90%). Yeo et al. proposed a polyimide based RH sensor demonstrating that the sensor sensitivity is enhanced by increasing the polymer thickness from 10 to 42 ��m [37].
However, in the latter case, the sensor response is slower and increases the sensitivity to temperature variations. A small degree of hysteresis was observed in all the tested situations. To overcome this problem, Venugopalan et al. inserted the FBG-polyimide optical sensor into a stainless steel tube containing multiple holes, Inhibitors,Modulators,Libraries enabling the contact with the surrounding environment [40]. To compensate the thermal effect on the RH sensor measurements, it was also included one uncoated FBG in the tube. This configuration provides Inhibitors,Modulators,Libraries mechanical Dacomitinib resistance to the sensor, essential to withstand harsh conditions typically found in SHM and contributes to reduce the effect of cross-sensitivity to thermal changes [40]. Nigel et al. proposed a RH sensor based on a Pyralin coated optical FBG with a response time and resolution comparable to a capacitive RH sensor. A thin coating of ~2 ��m was used for a short response time [41]. A fast RH sensor was recently present by Mathew et al., which can operate over a wide RH range, based on a bent single mode optical fiber Sorafenib [42]. The sensor makes use of an agarose hygroscopic coating with a linear response in the RH range between 25 and 90%.