Wide-Range and Linear Strain Sensing in SMS Fiber Sensors Using Phase Unwrapping

Single-mode-multimode-single-mode (SMS) fiber sensors exhibit high sensitivity but suffer from two major limitations: (i) a restricted dynamic range due to the periodic nature of the transmission spectrum, and (ii) nonlinear spectral peak shifts near the critical wavelength. We present a phase-unwrapping-based interrogation method that overcomes both limitations. By empirically determining the wavelength-dependent intermodal phase difference and performing local fits to the modal interference model, we extract phase changes between arbitrary pairs of wavelengths. We experimentally demonstrate a linear strain response up to 7120 µϵ, far exceeding what is achievable with conventional peak tracking, and a detection limit better than 11 µϵ.