Electric- and Magnetic-Related Sensors
Electric-related parameters, including partial charges, current, and electric field, are of great importance in the field of energy and power transmission. Partial charge detection in oil-filled transformers can be performed by the aforementioned FFPI acoustic sensors. The acoustic wave bandwidth generated by partial discharges is at the 100 kHz level. Song et al.  developed an FFPI acoustic sensor for partial charge detection, by securing SMF in a silica tube and then sealing the tube with a deformable silica diaphragm.
Magnetic fluid can be used as a sensitive medium for current detection. Xia et al.  demonstrated an FFPI current sensor by measuring the magnetic-induced refractive index changes of the magnetic fluid. A detection resolution of 1 mA was obtained.
FFPI electric field sensor was developed by measuring the deflection of the micro cantilever, which also served as one of the reflective surfaces for the FFPI. Priest et al.  fabricated an FFPI sensor by aligning SMF and a 17-p.m-thickness aluminum cantilever. The displacement of the cantilever was induced by the external electric field and measured by the changes of the FFPI interference fringes.
Except for electric-related parameters, FFPI sensors are also promising for weak magnetic field detection. Anbo Wang and coworkers developed an FFPI sensor by using Metglas (Fe775B15Si75) wire as both the magnetic transducer and one of the reflectors . DC magnetic fields of 100-35,000 nT were measured with low-vibration cross sensitivity and 99% compensation of thermal-induced fluctuations. A resolution of 50 nT was further achieved, and also a related theoretical model was developed for the magnetic field sensing .
Zhao et al.  developed an FFPI magnetic field sensor by filling the FP cavity with magnetic fluid. The refractive index of the magnetic fluid increases as the magnetic field increases. The measurement resolution of 0.5 Gs over the range of 0-400 Gs was achieved (1 Gs = 10-4 T). Further, the temperature compensation was obtained by integrating the FFPI structure with an FBG . Zhang et al.  developed a simple FFPI magnetic field sensor, which was capable of measuring magnetic field strength up to 560 Oe with good linearity. A sensitivity of 854 pm/Oe was obtained.