Use of first-order diffraction wavelengths corresponding to dual-grating periodicities in a single fiber Bragg grating for simultaneous temperature and strain measurement

A fiber Bragg grating sensor fabricated by a phase mask with 536 nm uniform pitch is presented. Two peaks/dips occur, at 785 and 1552 nm, due to reflection/transmission at the Bragg wavelength and at double the Bragg wavelength, and arising from FBG periodicities associated with half the phase mask periodicity and the phase mask periodicity, respectively. It provides simultaneous measurement of temperature and longitudinal strain, with similar intensities in both wavelengths making it better suited for long-distance operation and multiplexing compared with similar schemes.