Stable self-pulse behavour of two-core Yb/Sm fiber laser

Q-switched fiber lasers operating intense linear polarized pulses with a narrow linewidth at ∼1.06 µm are of great interest for frequency doubling and suit various applications in medicine, range finding, remote sensing, and optical parametric oscillators. Recently we reported an all-fiber and simultaneously low-cost solution, where a double-clad Yb-doped fiber laser employing Sm-doped fiber as a saturable absorber (SSA) exhibited passive Q-switch operation [1]. There were two drawbacks associated with that laser: sophisticated polarization dynamics explained by a specific Q-switching mechanism that involves the polarization switching [1] and rather broad laser spectrum (∼1nm) that is however typical for standard double-clad fiber lasers with Watt-scale pumping. The latter feature is attributed to the presence of FWM between the laser longitude modes that lead to the broadening of the laser line within the whole FBG reflectivity spectrum. One of the ways to overcome these limitations is to use a multi-core Yb-doped fiber that provides simultaneously higher birefringence for maintaining the linear polarization and a lower concentration of the light energy in each fiber core preventing inter-mode FWM in the laser cavity. It was recently demonstrated [2] that the use of multi-core laser in combination with a multimode fiber Bragg grating (MFBG) written in a gradient fiber is able to provide phase locking between lasing in different fiber cores. Here we report a stable periodic behaviour observed with a two-core Yb-doped fiber laser supplied simultaneously by the MFBG and SSA. Interesting specific features observed in our experiment are an unusually narrow laser spectrum, perfect linear polarization and complicated spatialtemporal behavior of the laser radiation in the far field.