Selective multipoint interrogation of identical-wavelength fiber Bragg gratings by sweep frequency control in sigma-configured Fourier-domain mode-locked laser

Fiber Bragg gratings (FBGs) have become indispensable elements in optical fiber sensors. We demonstrate a selective multipoint interrogation method for identical-wavelength FBGs based on a sigma-configured Fourier-domain mode-locked (FDML) laser. Unlike conventional wavelength-division multiplexing (WDM) systems with constrained spectral bandwidth, the proposed method leverages cavity-length-dependent sweep periods to achieve spatial discrimination of identical-wavelength FBGs without requiring additional optical hardware. The sigma configuration incorporates multiple optical paths within the FDML resonator, with each path having a distinct round-trip time, which enables selective FBG detection by simply tuning the sweep period of the integrated tunable optical filter. Experimental validation on six FBGs demonstrates distance switching below 10 ms, wavelength resolution of approximately 6 pm, and temporal resolution of approximately 20 μs, potentially doubling the sensing capacity of WDM under the same spectral range. Overall, the proposed method addresses the intrinsic multipoint limitation of WDM and provides a compact and flexible solution for high-speed, high-resolution, and dense distributed sensing, with potential applications in structural health monitoring, aerospace instrumentation, and dynamic strain/vibration analysis.