Design of long-period gratings in cladding-pumped microstructured optical fiber

An optical coupler constituted by a long-period grating (LPG) written into the single-mode core of a doublecladding microstructured optical fiber (DCMOF) is designed. The LPG allows the power transfer from the inner cladding modes toward the fundamental mode guided in the core. The eigenanalysis of the guided modes is performed via a finite element method routine, and the coupler spectral characteristics are modeled by using the multimode coupled-mode theory. The optical coupler feasibility is numerically demonstrated, and its refinement is obtained via a large number of simulations performed by varying the geometrical parameters of the fiber cross section, the grating period, and length. Moreover, the coupler performance has been further improved via a chirpedlike LPG made of a cascade of gratings having a suitable period and length. The simulations emphasize that the core-mode power at the output end of an DCMOF with an LPG coupler 23.17 cm long is increased by about 575% with respect to its value at the input section. In fact, about 25% of the total input power is transferred from the inner cladding modes toward the fundamental mode guided into the core