FIBER OPTIC SENSOR SYSTEM AND METHOD

A fiber Bragg grating (FBG) is used to measure strain. An optical fiber is used with a grating that comprises periodic spatial variations with mutually different first and second spatial frequencies. The wavelength of a first and second light source is swept over a first and second wavelength range that contain first and second Bragg reflection wavelengths corresponding to first and second spatial frequencies respectively. Light from the light sources is supplied to the optical fiber and to a reference branch. The reference branch contains an interferometer and a first and second filter having a transmission or reflection peak in the first and second wavelength range respectively. Time points of first and second peaks in the reflection of the light from the first and second light source by the optical fiber or detected during said sweeping. A count is made of periods of output of the interferometer, between the time points of detection of the first and second peaks and time points of detection of peaks from the first and second filter. The strain is solved from an equation that relates the variations of the temperature and the strain as shown by the counts.