Broadband GHz Fundamental Mode-Locking with Low-Quenching Highly Er–Ba Nanoparticle Suspension-Doped Silica Fibers

To address the limitations of conventional Er-doped silica fibers in achieving high gain within short lengths due to concentration quenching, a series of highly Er–Ba nanoparticle suspension-doped silica fibers have been fabricated, achieving Er3+ ion densities ranging from 2.69 × 1025 to 16.8 × 1025 ions/m3 while effectively suppressing concentration quenching. In this work, we systematically evaluate the gain performance of these fibers using short lengths and demonstrate their utility for short-cavity high-repetition-rate femtosecond lasers. Among the investigated variants, the hereafter named Hi8 fiber exhibits the highest single-pass small-signal gain coefficient of 0.85 dB/cm at 1532 nm, while the Lo8 fiber offers an optimal balance between gain coefficient and quantum efficiency, with an Er3+ density of 4.34 × 1025 ions/m3 and a quantum efficiency approximately 25% higher than that of a commercial counterpart fiber with a similar absorption of 110 dB/m at 1532 nm. Based on a 10 cm sample of Lo8 fiber, a robust fundamentally mode-locked fiber laser is demonstrated operating with GHz repetition rate at 1600 nm, producing a 9.4 nm 3 dB bandwidth and 533 fs pulse width. The laser wavelength can be switched in a broadband range through intracavity gain–loss engineering. These results establish these Er–Ba suspension-doped silica fibers as promising gain media for compact silica fiber lasers and low-nonlinearity high-gain Er-doped fiber amplifiers.