ABSTRACT
Interesting possibilities for increasing the amount of optical power coupled in the evanescent field give the using of new types of fibers that appeared a few years ago and have been discussed in the present chapter. Among them, one can mention hollow core fibers. Holey fibers (HFs), also known as microstructured optical fibers (MOFs) and photonic crystal fibers (PCFs) are a new type of fibers in which the air holes are incorporated within the silica-cladding region along its entire length. These cavities enhance the interaction between the evanescent field and the gas to be analyzed. A MOF is used simultaneously as a light guide and as a fluidic channel. Parameters such as the size, shape, and the relative position of the air holes provide an extra degree of freedom in controlling the light propagation and also the way the optical evanescent signal is affected by the surrounding. According to the cross-sectional distribution of the dielectric function, PCFs can be categorized as follows: photonic-bandgap fibers, HFs, hole-assisted fibers, and Bragg fibers. The PCF has a periodic dielectric structure whose periodicity is on the order of a wavelength, giving rise to the photonic bandgap. At present there are many humidity sensors designed on the base of photonic crystals. Some of them are discussed in the present chapter. The analysis carried out in this chapter has shown that the usage of specific fibers for humidity measurements is an interesting trend in the development of optical humidity sensors, as it extends possibilities of the fiber-optic technique for sensor design. However, it must also be recognized that to achieve results comparable to those obtained using conventional approaches, additional studies are needed.
