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**117**

**Chapter 7**

**Abstract**

**1. Introduction**

*and Perry Ping Shum*

Optical Fibre Long-Period Grating

Sensors Operating at and around

the Phase Matching Turning Point

Optical fibres have been exploited as sensors for many years and they provide a versatile platform with a small form factor. Long-period gratings (LPGs) operating at and around the phase matching turning point (PMTP) possess some of the highest sensitivities to external perturbations in the family of LPG-based sensor devices. This type of optical fibre grating has been demonstrated as a sensor for use in a wide range of applications. In this review chapter, an overview of PMTP LPGs is presented and the key developments, findings and applications are highlighted. The

fabrication considerations and sensor limitations are also discussed.

monitoring) have benefitted from optical fibre grating sensors [1–4].

matching turning point, turn around point

**Keywords:** optical fibre, fibre optics, fibre sensors, long-period gratings, phase

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

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.

*Rebecca Yen-Ni Wong, Dora Hu Juan Juan, Morten Ibsen* 
