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...