Effects of Differential Measurement Scheme on Brillouin Optical Correlation-Domain Analysis

Differential measurement (DM) is a modified lock-in detection technique that was introduced to the Brillouin optical correlation domain analysis (BOCDA) for improving the spatial resolution and dynamic range of distributed measurement. In the first report of the DM-based BOCDA the improvement of spatial resolution by 5-fold as well as the extension of dynamic range have been experimentally confirmed, however the maximum achievable improvement and the validity of DM in various measurement conditions have not been theoretically analyzed so far. In this paper we present an in-depth analysis on the effects of DM applied to the BOCDA by numerical simulations and test experiments. Our results show that the DM can bring up to 6-fold improvement in the spatial resolution of the BOCDA system, and the frequency of the phase modulation in the DM should be optimized according to the target spatial resolution, as a key factor determining its performance. In the experimental confirmation a 5 cm strain-applied section is measured by a DM-based BOCDA system with a nominal spatial resolution of 30 cm. The applicability to the long-range BOCDA system using double modulation and the practical limitation coming from the reduction of signal amplitude are also discussed.