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Dual-laser technique lowers Brillouin sensing frequency to 200 MHz

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.

The research was in the Journal of Â鶹ÒùÔºics: Photonics on April 25, 2025.

"The dual-laser approach makes BOCDR equipment simpler, more cost-effective, and easier to deploy, giving engineers a practical tool for long-term structural health monitoring, factory process control, and many other sensing tasks," said senior author Associate Professor Yosuke Mizuno of Yokohama National University.

Distributed optical fiber sensors based on Brillouin scattering are widely used to monitor strain and temperature, but conventional BOCDR systems face three hurdles. First, they usually need a physical delay line in one arm to position the measurement point. Second, the Brillouin signal appears near 11 GHz, which forces users to rely on costly wide-band electrical spectrum analyzers or heterodyne receivers. Third, sweeping the laser-modulation frequency during a scan changes the spatial resolution from point to point.

The new dual-laser BOCDR removes these obstacles in one step. Independent modulation of two lasers shifts the correlation peak electronically, so the delay line disappears. Optical heterodyne mixing brings the Brillouin signal down to the 200-MHz range, where standard radio-frequency equipment is sufficient, cutting costs. As the modulation frequency stays fixed, the spatial resolution remains constant at about 0.36 m along the whole fiber.

The team's next steps include raising the scan rate, extending the sensing range beyond the current few-tens of meters, and refining laser-stabilization schemes for long-term accuracy. They also plan field tests on structures such as bridges, tunnels, and industrial pipelines, where only single-ended fiber access is possible.

"We believe that the unique operation and advantages of the dual-laser system offer valuable potential for various applications," Associate Professor Mizuno added. "By providing a practical route to low-frequency, single-ended Brillouin reflectometry, this work not only introduces a fresh configuration but also lays a solid foundation for future research and real-world deployment."