What are high-reflection and low-reflection fiber Bragg gratings

A fiber Bragg grating (FBG) is a type of constructed in a short segment of that reflects particular of light and transmits all others. This is achieved by creating a periodic variation in the of the fiber core, which generates a wavelength-specific. Hence a fiber Bragg grating can be used as an inline to block certain wavelengths, can be use. [pdf]