Highly sensitive and thermally stable simultaneous sensing of surrounding refractive index and temperature using cascaded turnaround point LPG

Abstract This study presents the cascaded turnaround long period grating (CTLPG) based highly sensitive simultaneous sensing of temperature and surrounding refractive index (SRI). This sensor is having added features; enhanced thermal stability and compact in size. A cascaded LPG, a pair of LPGs of 300 μm period and each having length of 6 mm were studied by varying the intra-grating separation of cascaded LPG from 2 to 12 mm, respectively. The optimum number of fringes (two) for two parameter sensing can be obtained by keeping intra grating separation of 2–4 mm. Subsequently, cascaded LPGs of period 170 μm, which operated at turn around point, were inscribed with intra-grating separation of 2 mm and then annealed. The annealing process while enhances the thermal stability also transformed the dual resonance spectra from a two-fringe structure to a four-fringe structure. The spectral evolution of fringes of cascaded LPG investigated in temperature range 27.0–70.0 °C and also ambient RI in the range 1.3336–1.3954. The observed sensitivities for simultaneous temperature and SRI measurements are 1507.3 pm/°C, 742 pm/°C and 411.75 nm/RIU, 188.07 nm/RIU, respectively. The highest observed sensitivities by measuring dual resonance peak are 2.967 nm/°C and 1100.8 nm/RIU for refractive index and temperature, respectively. The resolution observed for change in SRI and temperatures are ~ 1.85 × 10 − 4 RIU and 0.067 °C respectively. This study will be useful for development of high sensitivity and thermally stable cascaded LPGs for bio-chemical sensing to measure change in SRI and temperature simultaneously.