All-fiber sensor based on few-mode fiber offset splicing structure cascaded with long-period fiber grating for curvature and acoustic measurement

An all-fiber sensor based on a single-mode–few-mode–single-mode fiber offset splicing structure cascaded with long-period fiber grating is proposed and demonstrated for curvature and acoustic signal measurement. High-order mode is inspired in the few-mode fiber by the offset splicing structure and coupled into the LPFG, resulting in a splitting into two dips of LPFG attenuation band, which are more sensitive to the external environment change. The results of the curvature sensing experiment show that the intensity of the two resonant dips has a linear response to curvature in the range of 0.124–0.304 \({\hbox {m}}^{-1}\), and the sensitivity is about 93.01 \({\hbox {dB/m}}^{-1}\), which is one order of magnitude higher than congeneric curvature sensors demonstrated by other researchers before. Based on the curvature sensing mechanism, the sensor is also demonstrated for acoustic measurement in the range 110–230 Hz. The sensor shows a sensitivity of about 15 mV/Pa at 110 Hz and 4.5 mV/Pa at other frequencies. High sensitivity and easy fabrication make it a preferable candidate for curvature and acoustic sensing in the field of structural health monitoring.