In Chapter 2, we have introduced film-based FFPI structures. Different kinds of sensitive films can be used for the measurement of a variety of parameters. For humidity sensing, cellulose acetate butyrate (CAB) and chitosan were often used as sensitive materials . Nanocomposite polyacrylamide can also be used for humidity sensing.
Xu et al.  developed an FFPI humidity sensor by coating the CAB film, with a thickness of about 50 |!m, at the end of SMF, as shown in Figure 4.11. The fabrication process was very simple. A relative humidity of 8.8%-88.1% was measured. Long stability was
Figure 4.11 FFPI refractive index sensor based on CAB film.
Figure 4.12 Extrinsic FFPI humidity sensor based on chitosan film. The microscopic pictures of (a) the side-view and (b) the cross-section of the FFPI sensor before and after coating.
demonstrated with a precision of 0.03% over 100 min. The response time was around 2.15 s/%RH.
Chen et al.  developed an FFPI humidity sensor by using an extrinsic FP cavity, which was formed by fusion splicing a hollow tube with the SMF and then sealing the end of the tube via coating a thin chitosan film, as shown in Figure 4.12. The cavity length was controlled by cleaving the hollow tube under the optical microscope. The thickness of the chitosan film was about 6.2 |lm. The refractive index of chitosan is about 1.45 so that the film can provide a Fresnel reflection similar to that of the SMF end, thus getting a good fringe contrast for FFPI sensing. A sensitivity of 0.13 nm/%RH was obtained for relative humidity ranging from 20%RH to 95%RH. Thanks to the thinness of the film, a fast response time of 380 ms was observed.