Fiber-optic biochemical sensors have been extensively investigated for more than 30 years . Wolfbeis has written several review articles on this topic reflecting the progress in this field [63-69]. Actually, the variety of the biochemical parameters is much more diverse than that of the physical parameters, since numerous kinds of analytes need to be measured by biochemical sensors. The specificity for biochemical sensing requires specific sensitive materials being used for certain kinds of analytes. Considering the combination of different fiber microstructures and sensitive materials, the types of fiber-optic biochemical sensors are rather huge. Here, we focus on the fiber-optic FP biochemical sensors for detecting parameters including refractive index, humidity, and gas concentrations.
Refractive Index Sensors
Refractive index is one of the most important parameters for optical waveguiding and photonic devices and is also significant for biochemical sensing as it is directly related to concentration of gas or other biochemical species. Refractive index can be measured precisely with FFPI sensors. One of the most commonly used FFPI schemes is by filling the gas or liquid into the FP cavity and measuring the phase shift by detecting the interference wavelength shift. There are two ways of filling the sample into the cavity. One is through the holes at the end of the FP cavity, that is, end face sampling, while the other is through the side wall of the cavity, that is, side sampling.
By using the solid-core PCF or other microstructured fibers [70,71], the solid fiber core can provide sufficient reflection for the FFPI interference and the hollow holes around the core can serve as the sampling channels. Tian et al.  developed an FFPI by sandwiching silica tube with two microstructured fibers, which had two microfluidic sampling holes in the cladding. One end of the microstructured fiber was cleaved with an angle to reduce the influence of reflection from the third surface. A vacuum pump was used for drawing the liquid into the FP cavity. Good interference fringes were obtained, and a high sensitivity of 1051 nm/RIU was achieved. A similar structure was used for measuring the refractive index of gas at different pressures.
Sampling through the side wall of the FP cavity may be simpler than end face sampling. Duan et al.  developed an open-cavity FFPI refractive index sensor by large lateral offset splicing a short section of SMF between two SMFs. The fabrication process is simple and a sensitivity of 1540 nm/RIU was obtained. Wu et al.  developed an open-cavity FFPI sensor by splicing a section of C-shaped fiber in between two SMFs and got a refractive index sensitivity of 1368 nm/ RIU. Wieduwilt et al.  fabricated a micro FFPI cavity on the fiber taper by focused ion beam milling method and then coating the cavity end faces with improved reflectivity. An ultrahigh sensitivity of 11,500 nm/RIU was achieved.
Different from the previous FFPIs with two-beam interference, Ran et al.  and Gong et al.  developed FFPIs with three- beam interference. The theoretical models were developed for better understanding of the influence of the FFPI structural parameters on the performance of the FFPI sensing based on three-beam interference. One unique characteristic of the three-beam interferometric FFPI sensor is the high fringe contrast. As is well-known, the fringe contrast of the two-beam interference reaches its maximum value when the reflectance of the two surfaces equals, which is a strict constraint. In order to get a high fringe contrast, the requirement on the reflectance of the three surfaces for the three-beam interference can be expressed as 
Th, Thj, and Thn are the reflectances of the three surfaces. Unlike the two-beam interference, the constraint condition for the three- beam interference to obtain the optimal fringe contrast is an inequality. This makes it easier for the three-surface-based hybrid GI-FFP sensor to obtain high performance than the conventional two-surface FFP sensors.
Another advantage of the three-beam interferometric FFPI sensors is its capability for dual-parameter sensing or temperature-insensitive refractive index sensing. It can measure the refractive index by detecting the fringe contrast changes, while measuring temperature via the wavelength shift of the FFPI sensor [77-81].
Quan et al.  developed such an FFPI sensor for detecting the refractive index of gas. The structure of the FFPI is shown in Figure 4.10. They fusion spliced a hollow tube and a solid-core PCF to the SMF in sequence. The FP interference occurred between the end faces of the SMF and the cleaved PCF. The use of the hollow tube formed a cavity for holding the gas samples. The refractive index changed with the gas pressure. The experimental results demonstrated a sensitivity of 30,899 nm/RIU.
Figure 4.10 FFPI refractive index sensors based on end face sampling. (a) Schematic structure and (b) microscopic picture of the fabricated FFPI sensor. (c) The cross-section of the photonic crystal fiber and the hollow micro tube.