Thulium-Doped 1940- and 2034-nm Fiber Amplifiers: Towards Highly Efficient, High-Power All-Fiber Laser Systems
Maria Michalska, Pavel Honzátko, Paweł Grześ, Michal Kamrádek, Ondřej Podrazký, Ivan Kašı́k, Jacek Świderski
Abstract
We report on high-power thulium-doped fiber amplifiers built using a nanoparticle-doped double-clad, pedestal-style large mode area fiber. The applied nanoparticle doping technique allowed achieving uniform high doping concentrations of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Tm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> across the fiber core. The fiber outer layer was fluorine-doped providing a decrease in pump radiation interaction with the coating, thereby enhancing the reliability of the entire laser system under high-power operation. The laser system operating at a wavelength of 2034 nm delivered an output power of up to 441 W with a slope efficiency of 57.4%, and the electro-optical conversion efficiency of the amplifier was 25%. The 1940-nm-wavelength all-fiber amplifier seeded with a narrowband Tm-doped fiber laser provided an output power of up to 273 W with a slope efficiency as high as 61.8%. In both cases the output spectrum was characterized by a 3-dB spectral bandwidth below 160 pm. Moreover, a special splicing procedure for large mode area fibers transmitting high power radiation was proposed and successfully demonstrated.