Long period grating has a wide variety of applications, including band-rejection filters, gain flattening filter and sensors. Various gratings with complex structures have been designed: gratings combining several LPFGs, LPFGs with superstructures, chirped gratings, and gratings. In essence, a long period fibre grating (LPFG) is an all-fibre device with wavelength dependent loss. As a band rejection filter, all light in a spectral slice is discarded without affecting the amplitude and phase of neighbouring wavelengths, with the additional advantage of low insertion losses. Microbend gratings, which are antisymmetric with respect to the fiber axis, create a resonance between the core mode and the asymmetric LP1m modes of the core and the cladding. Firstly, the techniques of fabricating HLPGs by CO 2 laser, hydrogen–oxygen flame heating, and arc discharge are summarized. However, loss or gain that can be controlled via optical pumping adds a new degree of freedom and – as will be shown in this chapter – brings many new and interesting properties.
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