Gas sensors are useful for the chemical industry, environmental monitoring, and even health diagnostics. Similar to humidity sensing, gas sensors also employ films that are sensitive to certain types of gases.
Fiber-optic gas sensors are special for flammable gas detection as they are inherently safe and unlike electric-based sensors, will never cause sparks during detection. Therefore, FFPI hydrogen sensors were extensively investigated for developing and using clean energy, and also for the semiconductor and other industrial fields. Pd-related films were widely used for hydrogen sensing due to their volume expansion (up to 900 times) after absorbing the hydrogen. Yang et al.  coated an FFPI structure with Pd-Ag film, which absorbed hydrogen and added mechanical stress onto the FFPI. The tiny cavity length changes can be detected, and the hydrogen concentration was determined. The sensitivity is not high as the radial size of the FFPI was large, hundreds of micrometers, compared to the 100-nm-thickness film.
Yang and coworkers [87,88] contributed a lot to the development of FFPI hydrogen sensors. They tried different kinds of hydrogen- sensitive materials and also various fiber microstructures. Pt/WO3 film was coated on the capillary, in which an FFPI was formed based on polymer materials. The exposure of Pt/WO3 film in hydrogen will generate heat, introducing a local temperature rise, which was further measured by the sensitive FFPI sensor via the wavelength shift detection. A high sensitivity of -5.1 nm/% was achieved at the low hydrogen concentration range of 0%-0.5%. Further, the sensitivity was enhanced to -10 nm/% (Figure 4.13), corresponds to 1 pm/ ppm and a temperature sensitivity of -5.5 nm/°C, which is very high compared to the intrinsic temperature sensitivity of ~10 pm/°C for
Figure 4.13 Highly sensitive FFPI hydrogen sensor based on Pt/WO3 film and its temperature responses.
FBGs. Besides the hydrogen detection, there were also FFPI sensors developed for other kinds of gas sensing, including volatile organic compounds (VOCs) and mixed gases [89-92].
Other Types of Biochemical FFPI Sensors
There exists huge demand for the detection of numerous biochemical analytes and related parameters. FFPI sensors have great potential for exploring this field, and a few examples have been demonstrated for surface tension analysis , thermo-optic coefficient measurement , and dissolved organics detection . Optical label-free methods like refractive index detection are only a small group of biochemical sensing examples. There are several other technologies being used for biochemical detection.