Analysis of the response of long period fiber gratings to external index of refraction

This paper demonstrates that the change in wavelength of a long period fiber grating attenuation band with changes in external index of refraction can be enhanced by proper selection of the grating period. We calculate and experimentally verify that the wavelength shift caused by changing the external index from n = 1 to n = 1.44 of the attenuation band which appears in the 1400-1600 nm region in a 200-mu m period grating is four times that in a 350-mu m period grating, Changes in the spectrum over a wavelength range from 1100 to 1600 nm and 1 < n < 1.72 index range are also presented, and implications for grating design when enhanced or reduced index sensitivity is desired are discussed. Finally, we demonstrate the use of a highly index-sensitive long period fiber grating as a chemical concentration sensor.