Long-Period Gratings for the Optimization of Microstructured Optical Fiber Laser

A Long Period Grating (LPG) coupler written into the doped single-mode core of a Double- Cladding Microstructured Optical Fiber (DCMOF) is designed to enhance the output signal power, to reduce the total length of the laser and to minimize the length-dependent nonlinear effects. The coupler is optimized to enhance the power transfer from the inner cladding modes toward the fundamental mode guided in the core at the pump wavelength. The eigenanalysis of the guided modes is performed via a Finite Element Method (FEM) routine. The coupler spectral characteristics and the laser behavior are modeled by an ad-hoc written computer code based on the multimode coupled-mode theory and ion population rate equations. The refined coupler has been designed by identifying a multi-LPG configuration, obtained via a cascade of gratings having suitable period and length.