Multi-long-period gratings for the optimization of pump absorption in microstructured optical fiber lasers

In this paper, a double-stage optical coupler is designed with the aim to enhance the characteristics of a laser constituted by an ytterbium doped, double cladding microstructured optical fiber (MOF). The optical coupler consists of a cascade of multiple long-period gratings (MLPG) inscribed in the fiber core region. More precisely, it is made up of two different stages, the former is designed outside of the laser cavity, before the input mirror, while the latter is designed in the active fiber region, i.e., inside the cavity. Each stage includes a cascade of different gratings. The whole double-stage multiple long-period grating (DS-MLPG), including both the outside and inside parts, increases the pump power transfer from the inner cladding modes towards the fundamental core mode. A homemade numerical code has been developed for the investigation of the DS-MLPG. It models the coupler by taking into account the coupled-mode equations and the rate equations. The dependence of the fiber laser characteristic on the DS-MLPG parameters, such as the grating period and the length, have been investigated. The simulations highlight that an improvement of the pump power absorption and an increasing at least 10% of the output signal power can be obtained by employing a suitable DS-MLPG. In fact, a large amount the total input pump power can be more efficiently transferred from the inner cladding modes towards the fundamental mode guided into the core. As a consequence, a reduction of the total length of the fiber laser and a minimization of the length-dependent nonlinear effects can be obtained.