Numerical and experimental investigation of long-period gratings in photonic crystal fiber for refractive index sensing of gas media.

We have used the finite-difference frequency-domain (FDFD) method to simulate the core mode to cladding mode couplings in long-period gratings (LPGs) in photonic crystal fiber (PCF). Four sets of LPG-PCF have been fabricated with respective periodicities of 590, 540, 515, and 490 μm, resulting in corresponding resonance wavelengths (RWs) of 1241, 1399, 1494, and 1579 nm. We show both theoretically and experimentally that the longer the RW, the more sensitive the LPG-PCF is to the index change in Ar. We demonstrate a robust sensitivity of 517 nm per refractive index unit using the LPG-PCF at 1579 nm RW.