Microfiber or Tapered FFPIs
FFPIs using optical microfibers or tapered optical fibers with a diameter of several micrometers are specially used for highly sensitive force sensing. Gong et al.  fabricated FFPIs with interference between a low-reflectance FBG and a Fresnel reflection fiber end face. The thin optical microfiber in the FP cavity served as a force-sensitive element. The wavelength-force sensitivity was 0.221 pm/FN, which was much higher than a similar structure with 125 ^m-diameter SMF.
Besides the force-sensitive performance, the microfiber FFPI is also sensitive to refractive index, thanks to its strong evanescent wave penetrated into the medium. Zhang et al.  fabricated such a microfiber FFPI by heating and drawing an FBG in the middle. The two half FBGs acted as the reflective mirrors for the FFPI and the optical microfiber was used for refractive index sensing.
The FBG can be fabricated directly into optical microfibers by the 193-nm UV laser exposure, by either the phase mask or point-bypoint writing technologies. FFPI based on a pair of microfiber FBG was fabricated, and sensing performance was investigated in detail by Guan and coworkers .
In addition to the advantages of force- and index-sensitivity, the FFPI on a fiber taper has a smaller size than the FFPIs based on common SMFs. Kou et al.  developed a miniaturized FFPI on a fiber taper tip. The fiber taper had a total length of 2.1 mm and the tip diameter was less than 1 Дш. After coating a 150-nm-thick aluminum (Al) film, a microcavity was fabricated by FIB milling technology. The cavity was 4.4 Дш long and 5 Дш high. Then the Al film was removed by HF acid etching. High-temperature sensing was carried out. The SEM images of the FP cavity and the tip were shown in Figure 2.22.
Totally speaking, there have been various FFPI microstructures developed for different kinds of applications. Although it has now been developed for decades, the potential of novel FFPI structures is continuing to be explored, for example, the newly developed microbubble and PCF-based FFPI sensor, which pushed the sensitivity of the refractive index to a new level. We believe that new structures
Figure 2.22 FFPI on fiber taper.
and new schemes will continue to enhance the performance of FFPI sensors in future.