An LPFG couples light that is propagating along the fiber core of a Single Mode fiber into cladding modes. LPFGs operate in transmission. There is no reflected signal. LPFGs are widely used as optical filters. It is essential for systems sensitive to reflection. LPFG written in a birefringent fiber may also be used as a polarization-depended absorbing spectral filter. Additionally, pair of 50% LPFGs can be used to form in fiber Mach-Zehnder interferometer.