What is FBG? An in-fibre Bragg grating is constructed by varying the refractive index of the core lengthwise along the fibre Light of the specified wavelength traveling along the fibre is reflected from the grating back in the direction from which it came Wavelengths which are not selected are passed through with little or no attenuation• An in-fibre Bragg grating is constructed by varying the refractive index of the core lengthwise along the fibre. • Light of the specified wavelength traveling along the fibre is reflected from the grating back in the direction from which it came. • Wavelengths which are not selected are passed through with little or no attenuation. What is FBG?