Quasi-distributed temperature sensing using apodized fiber Bragg grating array

Abstract

Drawing-tower fiber Bragg grating (FBG) array can be used in health monitoring of bridges, pipelines and some largescale projects to realize temperature, stress and vibration quasi-distributed sensing. The grating array sensing system can detect the changes of the tested physical parameters by measuring the central wavelength of the grating. However, for the sensing system based on identical grating array consisting of uniform FBGs, it cannot measure accurately the shift of the reflective wavelength of a grating in the array due to the sidelobe superposition of the gratings. Compared with uniform Bragg grating, apodized FBG has smaller reflection peak sidelobes. It means that the reflection power of the apodized FBG can be concentrated to the central wavelength region, and the reflection peak signal will be not interfered with those of other gratings. In this paper, an apodized FBG array was in-line fabricated in a single mode fiber by on a drawing tower and used for dense quasi-distributed temperature sensing. The obtained apodized FBGs have a full width at half maximum (FWHM) of 0.09 nm and their sidelobes were decreased by -21.9 dB. A 1000-meter-long array consisting of 10000 apodized gratings was successfully fabricated and used for quasi-distributed temperature sensing. These gratings have good consistency, low transmission loss and good mechanical properties. Experimental results show that the array can accurately measure quasi-distributed temperature fields and the temperature sensitivity is 10.15 pm/°C. It is expected that the apodized grating array can find more applications in many fields.