Scientists have successfully demonstrated a low-coherence Brillouin optical correlation-domain reflectometry (BOCDR) system that overcomes longstanding challenges related to spatial resolution and measurement range in mapping strain and temperature distributions along optical fibers.
Their research was published in the Journal of Lightwave Technology on September 15, 2024.
"We tackled the persistent issue of balancing spatial resolution and measurement range in our original fiber-optic distributed strain sensing technique called BOCDR," said Associate Professor Yosuke Mizuno of Yokohama National University. "Our purpose was to develop a more efficient system that overcomes this trade-off without relying on complex components like variable delay lines."
The conventional BOCDR technique offers advantages such as operation with light injection from one end of the sensing fiber, relatively high spatial resolution, and random-access capability to sensing points. However, it also faces trade-offs between spatial resolution and measurement range. Previous efforts to mitigate this issue have included special schemes, such as temporal gating, double modulation, and chirp modulation. Yet, these methods have not completely eliminated the fundamental trade-off. Researchers began to look toward using low-coherence BOCDR with a randomly modulated light source, but this approach requires a variable delay line for scanning the measurement position, which limits the measurement range.
To address this limitation, the research team—including collaborators from NTT Corporation, with support from Dr. Kohei Noda of the University of Tokyo—developed a low-coherence BOCDR system based on periodic pseudo-random modulation and demonstrated its proof-of-concept operation. They began by investigating the light source output spectrum based on modulation parameters using a delayed self-homodyne method. This method shows the potential to solve the conventional trade-off between spatial resolution and measurement range, maintaining high spatial resolution while extending the measurement range.
Next, they demonstrated the capability to measure strain distribution along optical fibers under multiple conditions without a variable delay line. Through simulation and experiment, their results showed that this new method performs more accurate distributed strain measurements than the conventional BOCDR technique, with the added advantage of not requiring a variable delay line. Additionally, their method is free from the systematic errors related to the amplitude modulation-frequency modulation (AM-FM) phase delay.
With the team's low-coherence BOCDR, they directly applied the periodic pseudo-random modulation created by an arbitrary waveform generator to the driving current of the light source, leading to the modulation of its output frequency. While conventional low-coherence BOCDR is based on non-periodic random modulation, this new method creates multiple correlation peaks—measurement points—along the fiber under test. This allows for the sweeping of correlation peaks other than the zeroth order along the fiber without the need for a variable delay line.
"The most crucial takeaway from our work is the successful demonstration of a low-coherence BOCDR system based on periodic pseudo-random modulation," Mizuno said. "This approach not only preserves high spatial resolution and extends the measurement range but also simplifies the system design, making it more practical for real-world applications."
The team's proof-of-concept with the low-coherence BOCDR technique sets the stage for future explorations into system optimization and in-depth performance analysis, such as enhanced spatial resolution, extended measurement range, increased measurement speed, and improved accuracy.
Looking ahead, the team aims to refine this low-coherence BOCDR technique to further enhance its performance in terms of resolution, range, and speed. "Ultimately, we aim for this method to be widely adopted for precise strain sensing in critical applications, such as structural health monitoring and industrial diagnostics," Mizuno said.
The research team includes Kenta Otsubo, Guangtao Zhu, Takaki Kiyozumi, and Yosuke Mizuno from the Faculty of Engineering, Yokohama National University; Kohei Noda from the Graduate School of Engineering, the University of Tokyo; and Hiroshi Takahashi and Yusuke Koshikiya from Access Network Service Systems Laboratories, NTT Corporation.
The research was partially funded by the Japan Society for the Promotion of Science (JSPS) KAKENHI grant (21H04555).
