Scientists have demonstrated a new fiber-optic sensing method that detects strain and displacement by reading interference patterns directly in the electrical spectrum of a photodetected signal. The approach uses a polymer optical fiber-based single-mode–multimode–single-mode (SMS) structure, in which multimode propagation creates relative modal delays that appear as measurable dips in the electrical-frequency domain.
Their research was published online in IEEE Sensors Journal on April 27, 2026.
"The key point of this work is that the interference pattern appears directly in the electrical domain," said corresponding author Associate Professor Yosuke Mizuno of YOKOHAMA National University. "This gives us a new way to read out fiber-optic sensor signals without relying on conventional optical-spectrum interrogation, while still exploiting the rich modal behavior of polymer optical fibers."
Optical fiber sensors are widely used to measure strain, temperature, displacement, and other physical quantities. Among them, multimode-interference sensors based on SMS structures are attractive because of their simple configuration and low cost. However, conventional systems usually monitor changes in the optical transmission spectrum using an optical spectrum analyzer, which can increase system cost and limit measurement speed.
The new method takes a different approach. The researchers transmitted light through a polymer optical fiber-based SMS structure, detected the output light with a photodetector, and analyzed the resulting electrical spectrum. When they used a light source centered around 1070 nm, distinct interference dips appeared in the electrical spectrum. In contrast, when they repeated the experiment using a 1550-nm laser, these dips disappeared, confirming that the effect originates from multimode propagation and modal beating during photodetection.
When axial strain was applied to a 57-cm polymer optical fiber segment, the interference dips shifted clearly and reversibly. The team also extended the principle to displacement sensing by introducing a variable air gap between silica fibers. In this configuration, changes in the gap length shifted the electrical interference dips, with sensitivity reaching approximately 3.7 MHz/µm for larger air gaps.
"We believe that this electrical-domain readout could make multimode-interference fiber sensors more practical for fast and compact measurements," Associate Professor Mizuno added. "Our next step is to clarify the dominant modal contributions, optimize the fiber structure and light source conditions, and evaluate the temperature response."
The research team also includes Ryo Takano from the Faculty of Engineering, YOKOHAMA National University, Japan, and Professor Marcelo A. Soto from the Department of Electronics Engineering, Universidad Técnica Federico Santa María, Chile. The study was partially supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI grants 21H04555 and 26H02136.
