A grating periodicity of typical FBG is around 1 µm. An FBG with a grating periodicity of several tenth ~ several hundredth µm is called long period fiber grating (LPG or LPFG). Different from the typical FBG, LPG does not reflect light, but radiate a clad mode to the outside due to a wide band range. An LPG is employed as an equivalent gain device of optical fiber amplifier. Because of a large periodicity, the interference is not necessarily used for manufacturing.
In some kind of FBG, a period of grating changes so that the low frequency components and high frequency components in a pulse are reflected at different positions. This is called as chirped fiber Bragg grating (CFBG). Figure shows the schematic structure of CFBG. A CFBG is employed for the dispersion compensation and the pulse stretching.