Monitoring local temperature and longitudinal strain along a nonuniform AsSe-PMMA tapered fiber by Brillouin gain-profile tracing.

The local temperature and longitudinal strain at spatial resolution of 0.5% of the pulse-width equivalent length along a nonuniform AsSe-PMMA tapered fiber is investigated by a Brillouin gain-profile tracing method. This scheme uses a 20 ns pump pulse with the pulse-width equivalent length longer than the fiber under test (FUT) of 50 cm nonuniform AsSe-PMMA tapered fiber. The whole interaction process of long pump pulse is investigated including pump pulse entering the FUT, overlapping completely with FUT and leaving the FUT. The evolution of Brillouin gain spectrum (BGS) along the nonuniform fiber is formed by the subtraction of frequency-domain BGS of two adjacent sensing points in the trailing edge (where the pump pulse leaves the FUT) of the BOTDA spectrum. The trailing part is preferred due to the pre-amplified acoustic field by the long pumping pulse. Then the local responses of temperature and wide-range longitudinal strain with high spatial resolution of 1.1 cm along the nonuniform AsSe-PMMA tapered fibers are investigated. The change of the local temperature and strain shifts the BGS that is different along the nonuniform fibers, which forms the distributed measurement. The spatial resolution, the fiber length of the detected section in the proposed method, is 1.1 cm for the local temperature and longitudinal strain measurement, which is 0.5% of the pulse-width equivalent length and is limited by the sampling rate of data acquisition and the fall-time of the pump pulse.