Postfabrication spectral tuning of multimode interference fiber-optic sensors

A practical limitation of multimode interference (MMI) fiber-optic sensors is the inability to tune the interference spectra after the fabrication stage. Here, a postfabrication spectral tuning method is demonstrated that uses a commercial fusion splicer for localized diameter modification of the multimode fiber (MMF) in a single-mode fiber-MMF-single-mode fiber (SMS) configuration. A wavenumber-based analytical model incorporating both the effective radius modification and length variation of the MMF was formulated, and experimental results showed that tapering and bulging the MMF caused the fourth-order interference peak wavelength to shift toward shorter and longer wavelengths, respectively, even for identical MMF lengths. The measured peak wavelengths agreed well with the theoretical predictions, while SMS structures tuned only by MMF length variation followed the conventional model. Although larger diameter modifications reduced the peak intensity, the wavelength shift remained predictable, which demonstrates the practicality of the proposed method.