Temperature dependence of fiber optic Bragg gratings at low temperatures

The temperature dependence of the Bragg wavelength of fi- ber optic gratings was measured at temperatures from 4.2 to 350 K. The measured wavelength dependence at room temperature (293 K) corre- sponds to a refractive index dependence of dn/dT59.1310 26 /°C, which agrees with earlier results for fused silica. However, dn/dT de- creases with lower temperatures, declining to 3.5310 26 /°C at liquid ni- trogen temperature (77 K), and to approximately zero at liquid helium temperature (4.2 K). The Bragg wavelength of a fiber grating embedded in a composite material displayed the same temperature dependence as a non-embedded grating.