Modal Transition in Nano-Coated Long Period Fiber Gratings: Principle and Applications to Chemical Sensing

In this chapter, the numerical and experimental investigation of the phenomenon of the modal transition in long period fiber gratings (LPGs) coated with a polymeric overlay of nanometric thickness and of high refractive index (HRI) is reported. This layered structure shows a significant modification of the cladding mode distribution with respect to the bare device, which depends on the overlay features (refractive index and thickness) and on the surrounding medium refractive index (SRI). As a consequence, enhanced evanescent wave interaction with the surrounding environment and a powerful guided wave interaction with the overlay itself are obtained. These effects were exploited to build very sensitive refractometers and chemical sensors. Moreover it was shown that the high SRI sensitivity region of the coated LPG can be tuned over the desired SRI range for specific applications by acting on the overlay thickness. The dip-coating technique was adopted to build the thin film coated devices and the syndiotactic polystyrene in its nanoporous form (? form) was used as HRI material, and in particular as molecular sieve for the detection of trace amounts of chloroform in water. The recent applications to chemical sensing of the nano-coated LPGs are reviewed.