**1. Introduction**

Optical fibre sensors do not only have use in telecommunications but are also extremely useful in a number of sensing applications. Many fields such as medical, oil and gas, civil, automotive as well as aerospace industries (structural health monitoring) have benefitted from optical fibre grating sensors [1–4].

In-fibre gratings are known as intrinsic sensing devices and therefore the propagation of light is guided and controlled within the fibre. Fibre gratings have a perturbation with a certain periodicity which will cause the fibre properties to change. They are also relatively easy to configure, are wavelength encoded enabling stable signals, and offer a high signal-to-noise ratio. One type of in-fibre grating is the long-period grating (LPG), which Vengsarkar et al. [5, 6] were the first to introduce. LPGs typically have periods ranging from around 100 μm to around 1 mm [7]. The principle of operation consists of the forward propagating core mode coupling with one or more of the forward propagating cladding modes [8]. The coupling involves the cladding modes, which means that the evanescent field will extend into the fibre surroundings. This will cause the LPG to be affected by its local environment. Another type of in-fibre grating is the fibre Bragg grating (FBG). The FBG promotes coupling of the propagating core mode with the counter-propagating core mode. FBGs typically have sub-micron periods and will produce a peak (in reflection) at a wavelength that is able to satisfy the Bragg condition. FBGs have also been used for numerous sensing applications [9, 10], but they will not be covered in this chapter.

By appropriately selecting the period of an LPG, it is possible to ensure the core mode will couple to a cladding mode operating at the turn around point (TAP) [11], also known as the phase matching turning point (PMTP), or dispersion turning point (DTP). A feature known as the dual resonance band can also be produced in this region. This type of LPG configuration has become increasingly popular due to its ultra-high sensitivity, a property usually desirable for a sensor. Approaches employed to improve the sensing capability of LPGs have included methods such as tapering [12] and etching [13]; however this can weaken the structure of the fibre and requires more delicate handling or complicated packaging. These sensors have been successfully used for measuring parameters such as temperature [14–16], strain [14–16] and refractive index (RI) [17–20]. The properties of LPGs at PMTP can be tailored further by adding a functional nanoscale coating for chemical and gas sensing [21]. This enables users to adapt the sensor to their own needs and applications. Chemical and bio-chemical based sensors, or those that can be applied to healthcare, are attracting increasing attention as they can have a more direct impact on the wellbeing of people. However, many are still yet to be applied in real situations outside of the laboratory [2].

This chapter aims to provide a more comprehensive coverage of LPGs which operate at and around the phase matching turning point, with respect to what can be found in existing literature [22]. The typical characteristics and fabrication considerations will be discussed. This will be followed by the different applications where PMTP LPGs have been demonstrated.
