Patent ID: 6950578

Claim:
A long period fiber grating configured to be sensitive to relatively small changes in the ambient refractive index, the long period fiber grating comprising a core area including a grating with a predetermined period Λ; a cladding region surrounding the core area, the cladding region exhibiting a refractive index defined as n cl and having a predefined outer diameter D, the combination of the core area and the cladding region capable of supporting two different propagating modes such that when the group velocity of the two different propagating modes match a zero slope point is formed in their phase matching curve, defined as a “turn-around-point” (TAP); and a coating/ambient area surrounding the cladding region and having a refractive index n coat greater than n cl , wherein the relation between the TAP and the grating period is a function of the value of n coat and changes in the separation between the TAP and the grating period are directly proportional to changes in the coating/ambient refractive index.