Long-Period Grating Inscribed on Concatenated Double-Clad and Single-Clad Fiber for Simultaneous Measurement of Temperature and Refractive Index

In this letter, a kind of long-period fiber grating (LPFG) that is capable of simultaneous measurement of temperature and surrounding refractive index (RI) is fabricated and demonstrated. The compact LPFG is written on a piece of spliced standard single-mode fiber (SMF) and double-clad fiber (DCF) with the CO₂ laser point-by-point irradiation technique. The LPFG section in the DCF is solely sensitive to temperature, while the section of the LPFG in the SMF is sensitive to both temperature and surrounding RI. After temperature and RI calibration, the LPFG has been used to measure the RI change of ethylene glycol with the change of temperature to demonstrate its capability for dual parameter simultaneous measurement.