Deformation of Brillouin scattered light power spectrum shape by linear strain distribution

A number of distributed fiber optic strain sensing systems are proposed that are based on the frequency shift of the Brillouin scattered light power spectrum in proportion to the strain produced in the fiber. Although the spectral shape under a uniform strain distribution is generally given by a Lorentzian function, it is deformed under a non-uniform strain distribution. It is important to investigate the relationship between the non-uniform strain distribution and the spectral deformation, because it affects the strain measurement error. We have focused on a linear strain distribution where the strain changes at a constant rate along the fiber as a typical non-uniform strain distribution. The power spectrum shape is derived theoretically using the Brillouin frequency shift values at the both ends of the observation section. The power spectrum of the Brillouin scattered light is then observed experimentally. The experimentally observed power spectrum shape was in good agreement with that theoretically obtained and the power spectrum was widened according to the slope of the linear strain distribution.