Robust femtosecond laser sources operating in the mid-infrared promise major technological advances due to their unique properties. They are being considered for use as frequency combs in molecular spectroscopy, in nonlinear conversion processes to achieve even longer wavelength coverage, as well as for materials processing in this untapped spectral region, which is rich in vibrational modes of abundant molecules. Unfortunately, direct generation of ultrafast radiation in robust all-fiber designs has so far been limited by the lack of suitable photonic components operating in the mid-infrared.

In this impressive contribution, Luo et al. reported the generation of ultrafast pulses of 115 femtoseconds duration, more than 50 nanojoules of energy per pulse, and an average output power of 3.4 W around 2.9 microns from an all-fiber MOPA system. The proposed approach begins with a passively mode-locked thulium-doped silica all-fiber source producing seed pulses that are amplified and converted by soliton self-frequency shift up to 2.8 microns. These picosecond shifted pulses are then further amplified using an erbium-doped fluoride fiber amplifier that also compresses them efficiently to femtosecond duration by the self-compression process acting in the gain fiber. Such rugged and powerful femtosecond laser sources operating in the mid-infrared should find numerous promising applications.

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