LPG (Long Period Grating)

An LPG is a periodic structure created in an optical fiber with a period much longer than a Bragg grating (typically hundreds of micrometers), coupling the core mode to the fiber's cladding modes instead of reflecting light back.

How does a long-period grating work?

Unlike an FBG, an LPG doesn't reflect light — instead it gradually transfers energy from the core-guided mode to the cladding modes, which quickly decay — the result is a characteristic, wide attenuation band in the transmission spectrum. The spectral position of this band is very sensitive to the refractive index of the medium surrounding the fiber cladding.

Applications

LPGs are used as chemical and biological sensors that detect changes in the surrounding refractive index (e.g. in liquids), gain-flattening filters for EDFA amplifiers, and dispersion-compensating elements.

Related terms

FBG

SPR

EDFA