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Optical fibre-based devices (e.g. fibre gratings) play an important role in the optical communications and sensing industry. One type of fibre grating, the long-period grating (LPG),is becoming more and more popular as a simple and versatile component for a number of applications in optical engineering. Long period grating is obtained by introducing a periodic refractive index modulation in the core of an optical fibre. The phase matching condition causes light to couple from core mode to forward propagating cladding mode. These cladding modes attenuate rapidly on propagation and thus resulting in distinct resonance bands in transmission spectrum. The sensitivity of LPGs to various external perturbations and their ability to manipulate selectively light propagating in optical fibres make them well-suited to creating fibre-based devices. LPGs can be used in various applications, for example as gain equalisers for erbium-doped fibre amplifiers, as channel routers in optical add-drop multiplexers and as sensors. LPGs are typically fabricated by exposing photosensitive optical fibre to ultraviolet light.

Long Period Grating: An Axial Strain Sensor