Sandwiched long-period gratings for simultaneous measurement of refractive index and temperature

A simultaneous measurement of external refractive-index (RI) and temperature is presented by using a sandwiched structure of long-period gratings (LPGs). An LPG pair with codirectional coupling between the guided mode and a low-order cladding mode acts as a temperature sensor with low RI sensitivity, whereas another in-between LPG which couples the guided mode with a higher order cladding mode performs as an RI sensing element. The experimental results show such an all-LPG structure is not only suitable for use as a compact temperature-compensated RI sensor, but also convenient for fabrication.