This paper provides an overview of the different types of fiber optic sensors (FOS) that can be used with composite materials and also their compatibility with and suitability for embedding inside a composite material. An overview of the different types of FOS used for strain/temperature sensing in composite materials is presented. Recent trends, and future challenges for FOS technology for condition monitoring in smart composite materials are also discussed. This comprehensive review provides essential information for the smart materials industry in selecting of appropriate types of FOS in accordance with end-user requirements.

A novel fiber optic sensing configuration for simultaneously measuring ammonia vapor (NH₃) concentration and relative humidity (RH) in air is proposed and experimentally demonstrated. The system comprised two silica whispering gallery mode (WGM) microsphere resonators coated with different polymer layers. One of the microspheres was dip-coated with sol gel silica polymer and another with a 0.5 % wt./vol. agarose hydrogel. WGMs in both microspheres were excited simultaneously by evanescent coupling using a single adiabatic fiber taper. The optical properties of both coating layers change due to their exposure to ammonia and water molecules in the surrounding atmosphere, resulting in the spectral shifts of the WGM resonances relevant to each of the microspheres. By measuring the relevant WGMs' spectral shifts, the NH₃ concentration in air and the RH can be determined simultaneously. The experimentally demonstrated sensitivity of the proposed sensor array to ammonia was estimated as 19.07 pm/ppm (NH₃ molecules in air) and its sensitivity to relative humidity as 1.07 pm/% RH. Detailed studies of the coatings' cross-sensitivity and temperature dependence are also presented. The proposed sensor array is compact, highly sensitive and potentially low cost.

The characteristics of polymer fiber Bragg gratings (FBGs) embedded in composite materials are studied in this paper and are compared with characteristics of their silica counterparts. A polymer FBG of 10 mm length which exhibits a peak reflected wavelength circa 1530 nm is fabricated and characterized for this purpose. A silica FBG with a peak reflected wavelength circa 1553 nm is also embedded in the composite material for a comparison study. The fabricated composite material sample with embedded sensors is subjected to temperature and strain changes and the corresponding effects on the embedded polymer and silica FBGs are studied. The measured temperature sensitivity of the embedded polymer FBG was close to that of the same polymer FBG in free space, while the silica FBG shows elevated temperature sensitivity after embedding. With an increase in temperature, spectral broadening was observed for the embedded polymer FBG due to the stress induced by the thermal expansion of the composite material. From the observed wavelength shift and spectral bandwidth change of the polymer FBG, temperature and thermal expansion effects in the composite material can be measured simultaneously.

This paper proposes a hybrid optical fiber sensor system for simultaneously sensing the strain, temperature, and thermal strain of composite materials. The hybrid fiber optic sensor system involves a combination of three sensors: 1) a polarimetric sensor based on an acrylate coated polarization maintaining photonic crystal fiber (PM-PCF); 2) a coating stripped PM-PCF sensor; and 3) a fiber Bragg grating sensor (FBG). Temperature is sensed using the FBG sensor, axial strain is sensed using the acrylate coated PM-PCF sensor, and thermal strain is sensed using the coating stripped PM-PCF. The hybrid sensor system presented operates in the intensity domain by converting the polarization and wavelength information from the polarimetric sensors and the FBG, respectively, into detectable linear intensity variations. Subsequently, by deriving a characteristic matrix for the hybrid sensors, information about temperature, axial strain, and thermal strain can be simultaneously determined. An experimental demonstration of the hybrid sensor system is described based on a glass fiber reinforced composite material sample within which the three different sensors are embedded. The proposed sensor configuration can be employed in composite material structural health monitoring applications.

Low frequency vibration measurements in composite materials using embedded optical fiber sensors are presented in this paper. The two types of fiber sensors embedded are a polarization maintaining photonic crystal fiber (PM-PCF) polarimetric fiber sensor and a fiber Bragg grating (FBG) sensor. A glass fibre reinforced polymer composite material sample is fabricated with both fiber optic sensors embedded in it. A comparison between the performance of both the PM-PCF polarimetric fiber sensor and the FBG sensor embedded in the composite material is also presented.