2.01–2.42 $\mu$m All-Fiber Femtosecond Raman Soliton Generation in a Heavily Germanium Doped Fiber

We demonstrated that an all-fiber system generates high-performance mid-infrared Raman solitons in a heavily germanium doped fiber (HGDF). A 10 m-long HGDF with a 12 μm core diameter and 64 mol.% GeO
₂
dopant is designed and pumped by a home-made 1.96 μm femtosecond fiber laser in the strong anomalous dispersion region. Stable and single-color Raman solitons are therefore obtained with a continuous wavelength tunability from 2.01 to 2.42 μm. The pulse duration of the mid-infrared Raman solitons can be as short as ~220 fs. The efficiency of energy transfer to a Raman soliton is about 32.5%, while the maximum average power, peak power, and pulse energy are up to 27 mW, 3.6 kW, and ~1 nJ, respectively. Different from previous multi-color Raman or supercontinnum-like generation from HGDFs pumped in the near-zero or normal dispersion regime, such pure mid-infrared Raman solitons exhibit excellent stability with a radio-frequency signal-to-noise ratio of ~60 dB. This is, to the best of our knowledge, the first demonstration of >2.4 μm stable Raman solitons in an all-fiber system. This work may pave a path towards compact and high-performance mid-infrared femtosecond fiber laser sources.