Scientists have developed a dual-laser Brillouin optical correlation-domain reflectometry (BOCDR) system that uses two frequency-modulated lasers. By scanning the relative modulation phase between the pump and reference lasers, the setup measures strain and temperature all along an optical fiber. In a proof-of-concept test on a 13-meter silica fiber, the team recorded Brillouin gain spectra (BGS) at only about 200 MHz—over 50 times lower than the usual 11 GHz band.