When fiber optics was being developed for communications, efforts were underway to use the new technology for sensing at the same time. Optical fiber sensors are being broadly advanced as they have numerous advantages over conventional sensors such as the ability to function in harsh environments, small sizes, with electromagnetic interference, and many more. Among them, interferometer-based fiber-optic sensors have been implemented for a wide range of applications since 1980 . After the introduction of FFPI, optical sensors based on FFPI have been extensively studied due to their high performance and ability for signal “amplification” (i.e., resonance). Also, FFPIs are attractive because of high sensitivity, large dynamic range, fast response for measurement of various parameters, such as temperature, strain, pressure, displacement, electrical/magnetic field, refractive index, flow rate, etc. and can also be used as embedded sensors in composite materials [2,3]. There are several ways that are being followed, each of which can be the link between the variable being sensed and an optical resonance of the FFPI: (1) linear expansion of a spacer length, (2) refractive index of the medium between reflectors, (3) reflector absorption or reflectivity, and (4) spectral absorption or scattering in the medium between reflectors. Hence, FFPIs have shown considerable potential for sensing in various fields, such as energy, aerospace, biomedicine, et al., which are discussed below.