ABSTRACT
In recent years the technology on the fabrication of fiber Bragg gratings (FBGs) in polymer optical fibers (POFs) has been intensively investigated, allowing the fabrication of polymer optical fiber Bragg gratings (POFBGs) with different properties. Recent progress on this fiber technology allowed the creation of POFBGs in different fiber structures, such as step-index (SI) [1], no-core [2], microstructured [3], graded-index (GI) [4,5], highly-birefringent (HiBi) [6], etc. The wide availability of polymers makes them attractive to explore new fiber sensing technologies. FBGs in POFs based on undoped materials, such as polymethylmethacrylate (PMMA) [1,3], TOPAS® [7], Polycarbonate (PC) [8], ZEONEX® [2,9], and CYTOP® [4,5], have already been demonstrated, allowing the development of fiber sensors with special characteristics. Examples of that are the ability to be humidity sensitive [10] or insensitive [2,11], the ability to sustain high temperatures [2,8,12], biological affinity [13], better mechanical properties [14], etc. Furthermore, as we have seen in the previous chapter, the possibility of doping POFs with special photosensitive materials such as fluorescein [1], trans-4-stilbenemethanol (TSB) [15], benzildimethylketal (BDK) [16], 9-vinylanthracene (9-VA) [17], diphenyl disulphide (DPDS) [18], etc., led to the inscription of FBG with high reflectivity [19,20] and with short inscription times [18,19,21]. POFBGs are nowadays not only fabricated at the 1,550 nm region but also at shorter wavelengths namely 960 nm [22], 850 nm [23], and 600 nm [24,25] where most of the polymer materials have lower attenuation. Multiplexing capabilities of these structures were shown to be possible through different techniques such as the inscription with different phase masks [26], the use of a single-phase mask and different POF elongations during inscription [27,28], the use of a single-phase mask with several diffraction orders [24] and the annealing with temperature [29], strain and temperature [30], and mediated by solution [31]. Intense research during the last five years revealed the possibility of fabricating not only uniform FBGs but also tilted [32], chirped [33], phase-shifted [34], and Fabry-Pérot (FP) cavities [35], opening the possibility of using these fiber optic devices for new emerging sensing applications. The possibility to inscribe long-period gratings (LPGs) in POFs also opens an easy route for low-cost POF sensors and it will also be included in this chapter.
