New method for optical fiber sensing that achieves world-record spatial resolution of 6 mm

A research team including Associate Professor Heeyoung Lee (Lightwave Sensing Laboratory) of the Graduate School of Engineering and Science at Shibaura Institute of Technology, and Associate Professor Yosuke Mizuno of the Faculty of Engineering at Yokohama National University, announced on April 8 that they have achieved a world-record spatial resolution of 6 mm for Brillouin optical correlation-domain reflectometry (BOCDR). This optical fiber sensing technology employs a reflective measurement technique to measure strain (expansion and contraction) and temperature distribution along an optical fiber. Furthermore, the team succeeded for the first time in the world in detecting temperature changes occurring within a 7 mm section of less than 1 cm as a distribution. The results were published in the Journal of Lightwave Technology, an international journal in the field of optical communications and optical sensing.

As infrastructure facilities such as bridges and tunnels built during periods of high economic growth continue to age, optical fiber sensing has attracted attention as a method for continuously monitoring the safety of these structures. This is because by laying optical fibers along the structure, changes in strain and temperature can be measured as a distribution over long distances.

As such a distribution measurement, a phenomenon called Brillouin scattering, which occurs in optical fibers, is used. In this scattering process, the frequency of the scattered light shifts slightly, and the amount of strain or temperature change can be estimated from this shift.

Among various methods, BOCDR used in this study has the potential to capture minute changes on a millimeter scale. Mizuno noted that since it operates by injecting light from only one end of the fiber, it offers high flexibility in installation. Measurements can continue even if the fiber is severed midway, and its relatively simple configuration makes it advantageous in terms of cost.

In BOCDR, the measurement position is controlled by applying frequency modulation to the laser beam. Higher modulation speed (modulation frequency) provides finer spatial resolution. However, Brillouin scattering has an inherent frequency width (Brillouin bandwidth). If a modulation frequency close to or exceeding this bandwidth is used, periodic fluctuations occur in the measurement signal, making it difficult to accurately determine temperature or strain, which is a significant barrier to practical application.

Consequently, the mainstream spatial resolution for distributed sensing using Brillouin scattering has generally been tens of centimeters (or a few centimeters at best). While there have been attempts to approach the millimeter scale, many required special optical fibers or complex equipment configurations, leaving challenges for practical use.

Mizuno emphasized the unique feature of this achievement, "This time, we achieved 6 mm while using standard optical fibers and a relatively simple configuration."

The research team conducted a detailed investigation into the signal disturbances occurring near the Brillouin bandwidth, which acts as a barrier, and identified them as periodic components caused by the modulation frequency. Subsequently, they developed a method to selectively suppress these periodic components through signal processing, enabling accurate distributed measurement even under high modulation frequency conditions previously considered difficult. This achievement is expected to be applied as a technology for diagnosing the health of social infrastructure, such as bridges, more minutely and efficiently than ever before.

Furthermore, because millimeter-scale spatial resolution is now possible, applications are anticipated in monitoring the internal state of optical waveguides and "shape sensing" for structures and robots. In the future, the technology could serve a role similar to "nerves," allowing robots to sense their surroundings with high precision.

Mizuno shared his outlook: "For infrastructure monitoring, a spatial resolution of a few centimeters is often considered sufficient, so I believe this achievement has already reached a practical level in terms of resolution. In fact, demonstration experiments have already begun in the construction field. In the future, we would like to further our research with the goal of balancing this high spatial resolution with longer measurement distances, as well as improving the portability and downsizing of the equipment for easier use at actual sites."

Journal Information
Publication: Journal of Lightwave Technology
Title: BOCDR Achieving 6-mm Spatial Resolution at Modulation Frequencies Close to Brillouin Bandwidth
DOI: 10.1109/JLT.2025.3640608