Long Period Fibre Gratings

Abstract

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 and minimum backreflection (Daxhelet, 2003). LPFGs have been used as band rejection filters, sensors, wavelength selective elements in fibre lasers and amplifiers (Su W while parallel LPFGs act as wavelength selective couplers or wavelength multiplexers (Chan & Yasumoto, 2007; Shu et. al.,2002). In a common LPFG, the core mode couples to co-propagating cladding modes supported by the cladding-external medium waveguide structure. The specific cladding through which the core mode is coupled is determined by the following resonance condition:   i ecla eco n n      2 2 (1)