Coupling constant of triple-clad long-period fiber grating

Abstract

Based on the method of vector-field analysis, the relationship between the core-mode/cladding-mode coupling constantk 1v-11 cl-co and mode orderv is studied. It could be seen that the coupling constants of thel=1 low-orderHE cladding modes are larger than that of thel=1 low-orderEH cladding modes whenv≤41. For cladding modes withv>41, the coupling constants of thel=1 low-orderHE andEH modes are comparable to reach other. The plotted local intensity curves of the six lowest-order (l=1) cladding modes show that the low-orderHE modes have a larger proportion of intensity located in the core than the low-orderEH modes. Further, the relationship between the coupling constantk 1v-11 cl-co and the film refractive index as well as the film thickness is studied. The results indicate that the coupling constant changes monotonously with the film refractive index changing. As the film thickness increases, the coupling constant decreases slowly with irregularly changes in a certain region of the film thickness, which matches the experimental results of Nicholas D. R.