Simple Baseband Method for the Distributed Analysis of Brillouin Phase-Shift Spectra

Abstract

The phase shift feature of stimulated Brillouin scattering is rarely used for the development of sensors based on the Brillouin optical time-domain analysis (BOTDA). However, this feature shows significant interest for sensing thanks to its linear dependence around the Brillouin frequency shift. Until now, distributed Brillouin phase shift (BPS) measurements required complex modulations, high-bandwidth detection, and sharp filtering. Recently, we introduced the possibility of developing simple distributed measurements of BPS in BOTDA using the baseband technology. The basic idea was the use of a Sagnac interferometer coupled to a conventional BOTDA without filtering and incorporating balanced detection. Although the concept was demonstrated, the setup proposed (using a 2 $\times $ 2 optical coupler) had significant shortcomings, the main one being its lack of stability due to the need of setting the interferometer bias through the polarization of the interacting waves. This restricted severely the practicality of the method. In this letter, we propose a setup with enhanced stability thanks to the use of a $3\times 3$ optical coupler. With this configuration, the impact of the drifts along the measurement is strongly reduced, and the quality of the measurements improves substantially. The method is theoretically studied and demonstrated experimentally.