Simultaneous strain and temperature measurement by cascading few-mode fiber and single-mode fiber long-period fiber gratings.

A hybrid optical fiber structure of two cascading long-period fiber gratings (LPFGs), respectively, inscribed on a segment of few-mode fiber (FMF) and single-mode fiber (SMF), is proposed and experimentally demonstrated. This structure could be used for simultaneous measurement of strain and temperature. The FMF-LPFG exhibits an opposite temperature response and higher strain sensitivity compared to that of the SMF-LPFG, which can improve the measurement resolutions. Experimentally, the strain and temperature sensitivities of the proposed sensor are -2.9  pm/με and -17.6  pm/°C, respectively, for the FMF-LPFG; for the SMF-LPFG, these are -1.47  pm/με and 46.4  pm/°C, respectively. The maximum errors are ±7.98  με and ±0.54°C for strain and temperature, respectively.