Fiber Bragg grating (FBG) is a structure manufactured by inducing periodic change of light refractive index in the optical fiber core. This structure behaves like a band-stop filter, which means that some wavelengths are reflected from the grating, while others are transmitted.

Bragg grating based filters are characterized by low excess loss in the transmission band, high transmission in the reflection band, and narrow reflect bandwidth. They can be applied for example in reflecting 1650 nm signal for OTDR measurements.