A novel fiber laser oscillator employing saddle-shaped core ytterbium-doped fiber

A novel high-power fiber laser oscillator employing a saddle-shaped core ytterbium-doped fiber (SSCYDF) is proposed and demonstrated experimentally. The SSCYDF is designed and fabricated with a long-tapering core (diameter of ~ 30 µm at both ends and ~ 23 µm in the middle) and a constant inner cladding (diameter of ~ 400 µm) in longitudinal dimension. On the one hand, the small core section of the fiber can only support less than two modes, which is helpful for the mitigation of the transverse mode instability. On the other hand, the large core section provides a large mode area for suppression of stimulated Raman scattering. Therefore, this type of laser oscillator holds the potential advantages for both mitigation of transverse mode instability and suppression of stimulated Raman scattering. Based on the homemade SSCYDF, an all-fiber laser oscillator is constructed and investigated by pumping with laser diodes at wavelength of 976 nm and 915 nm, respectively. The maximum output power of 1312 W is achieved in the case of co-pumping at 915 nm. This is the first work, to the best of our knowledge, to validate the feasibility of using saddle-shaped core ytterbium-doped fiber with constant cladding for high-power fiber lasers.