The effects of the nanosized high refractive index overlay on tunable long period gratings with normal and reduced cladding diameters

In this paper, we have theoretically demonstrated that the resonant wavelength of long period gratings (LPG) can be shift by a large magnitude by coating with only a thin overlay that has a refractive index higher than that of the glass cladding. The resonant wavelength shift results from the variation of the high refractive index (HRI) of the overlay. The effects of cladding modes re-organization on the sensitivity to the overlay HRI have been outlined. An additional parameter, the cladding diameter, is also considered for improving the sensitivity of LPG to HRIs in this work. The results suggest that the layered structure offers an efficient platform for achieving high-performance index-modulation fiber devices. Moreover, the reduction of the cladding diameter leads to a further improvement in sensitivity.