Sensitization of hybrid LPFG–FBG refractometer using double-pass configuration

We demonstrated a method to enhance the sensitivity of a hybrid arc-induced Long Period Fiber Grating (LPFG)–Fiber Bragg Grating (FBG) system using a double-pass configuration for refractive index (RI) sensing. The sensing principle is based on the interrogation of the reflected signal variation of an FBG caused by the shifting of the LPFG's resonant wavelength by changes in the surrounding medium. Standard oil with RI values ranging from 1.39 to 1.49 was used to monitor the RI response of the sensor in single-pass and double-pass configurations. Comparison of both configurations showed that the double-pass configuration utilizing an optimize-coupled LPFG has a sensitivity of 870.6 dB/RIU as compared to the single-pass configuration which has a sensitivity of 457.3 dB/RIU in the index range of 1.43–1.46.