Discrimination between the refractive index and temperature by using a sandwiched structure of long-period gratings

We present a novel scheme for simultaneous measurement of the external RI and temperature by using the sandwiched long-period gratings (SLPGs). An LPG pair coupling between the guided mode and the low-order cladding mode acts a temperature reference element, and another LPG in-between coupling with the higher-order cladding mode performances an RI sensor. The period and length of the paired LPG (with a center to center distance 47.5mm) are 643 pm and 22.5mm,respectively, and those of LPG in-between are chosen to be 428 mu m and 23mm, respectively. The experimental results show that the RI sensitivities of a paired LPG coupling with the third order cladding mode is -6.71 nm/R.I.U (refractive index unit), and the sensitivity of the LPG in between coupling with the ninth order cladding mode is -27.77 nm/R.I.U. The temperature sensitivities of these LPGs are -0.315nm/degrees C and -0.337nm/degrees C, respectively.