The distributed side-coupled cladding-pumped (DSCCP) fiber comprises an active signal fiber with gain characteristics and several passive multimode pump fibers, collectively coated to form an integral package. The pump laser injected into the pump fiber couples between the pump and signal fibers in the form of an evanescent wave. Upon entering the signal fiber's cladding, it excites rare-earth ions in the signal fiber's core, thus achieving laser gain amplification. In the realm of high-power single-fiber lasers, the primary challenges limiting power enhancement are pump injection and extremely high thermal loads. Therefore, combining cascaded pumping and distributed side-pumping has emerged as a promising and feasible pathway to achieve ultra-high power in the tens of kilowatts range.

The experimental setup is based on a master oscillator power amplifier scheme. A pair of fiber Bragg gratings and 20/400 μm ytterbium-doped fiber form an optical cavity to generate a hundred-watt seed. A homemade 35 m (1+1) DSCCP fiber with high-concentration Yb-doped in active core and a core/cladding size of 60/300 μm is activated by five groups of 1018 nm pump sources in a counter way from pump core with a core size of 310 μm. A Raman suppression array, consisting of a few homemade tilted fiber Bragg gratings, is placed between the oscillator and the amplifier to filter noise within the Raman range.

The experimental results demonstrate the highest output power of 20.13 kW from the signal fiber, with an optical-optical conversion efficiency of 81.0%. The fiber slope efficiency, fitted across the entire power range, reaches 82.3%. Spectral measurements exhibit a 3 dB linewidth of 0.44 nm for the seed laser at the hundred-watt level, expanding to 1.1 nm at the amplified power of 20.13 kW. The experiment also reveals a Raman suppression ratio of approximately 37.65 dB, indicating effective suppression of stimulated Raman scattering components in the spectrum.

This achievement represents the first publicized report of a 20 kW single-fiber laser output using the (1+1) type distributed side-pumping approach. The success not only highlights the efficacy of the distributed side-pumping scheme for realizing high-power outputs but also paves the way for future research on further improving beam quality and achieving high-quality laser outputs approaching diffraction limits in the tens of kilowatts range. In the next stage, we will focus on the improvement of beam quality by controlling core design and enhancing coupling ability.