A 1.8 kW fiber laser oscillator employing a section of spindle-shaped core ytterbium-doped fiber

We propose and demonstrate a new type of Yb-doped fiber called spindle-shaped core ytterbium-doped fiber (SSCYDF) and conduct a laser oscillator based on this fiber. The SSCYDF is a kind of long tapered fiber which consists of a small-core section at both ends and a large-core section in the middle. The ratio of core/clad diameter remains a constant along the total length of the SSCYDF (∼31 m). The core/clad diameter of the small-core sections at both ends is 20/400 μm with a length of about 3.5 m, and the corresponding value of the large-core section in the middle is 30/600 μm and 6 m. The laser oscillator performance is investigated with pump wavelength of 976 nm. When the pump source is employed by a counter-pumping scheme, a maximum output power of 1836 W is achieved, with an optical-to-optical efficiency of 83.5%. The beam quality (M ² factor) measured at the operation of 1836 W is 1.65. The laser oscillator based on SSCYDF holds the advantage of suppressing both mode instability and stimulated Raman scattering at the same time because of the small-core sections which can support only one mode and the large-core section which expands the mode-field area in the SSCYDF. (letter)