High-power superfluorescent fiber light sources have a wide range of applications, including Raman fiber laser pumping, optical coherence imaging, and spectral beam combining. They are favored for their simple structure, low temporal coherence, high temporal stability, absence of relaxation oscillation, and lack of self-locking mode pulses. However, because of the limitation of parasitic lasing, it is challenging to increase the power of a single-stage superfluorescent fiber light source. Currently, its power only reaches a few hundred watts. A master oscillator power amplification (MOPA) structure provides a solution to achieve high power output by amplifying a low-power superfluorescent seed. The highest reported power of 1-μm superfluorescence based on a Yb-doped fiber MOPA structure is 3 kW. Further power scaling is limited by stimulated Raman scattering (SRS) and transverse mode instability (TMI). In this study, we implement backward cascaded pumping to suppress TMI and SRS and boost the superfluorescent output to more than 6 kW.
First, the superfluorescent source is filtered out by a bandpass filter and amplified to 40 W by two pre-amplifiers. In the seed stage, it is important to use isolators to reduce the negative impact of backscattering on the superfluorescent seed source. In the amplification stage, the superfluorescent light is launched into the double-clad ytterbium-doped fiber (YDF) through a mode field adapter, a cladding light stripper, and a combiner. A 1018-nm pump laser is injected into YDF through a backward combiner. Finally, the amplified superfluorescent light is emitted through a cladding light stripper and a beam collimator.
The output power increases almost linearly with the injected pump power. At a pump power of 7554 W, the output power reaches 6200 W with a corresponding optical-to-optical conversion efficiency of 81.5%. As the power increases, the spectral width gradually broadens, and the 3-dB linewidth increases from 2.08 nm at 40 W to 7.72 nm at 6200 W. At an output power of 6200 W, the system experiences severe SRS, and the Raman suppression ratio is only approximately 25 dB. Beam quality factor (M2) first increases and then stabilizes as the power is increased. The seed has an M2 value of 1.71, while M2=1.98 at the maximum power of 6200 W. The temporal and spectral superfluorescence characteristics indicate that the system does not exhibit the TMI phenomenon.
A practical technical approach to designing high-power superfluorescent light sources is proposed. To the best of our knowledge, the 6.2-kW superfluorescent output achieved is the higher power level reported publicly.
