A source of femtosecond laser pulses (50) comprising a 980 nm picosecond seed pulse source (12), a Ytterbium (Yb) doped fiber amplifier (14) operating in the three-level regime, a passive air-clad fiber (52) and a pulse compressor (16). The seed pulses are spectrally broadened due to self phase modulation (SPM) in the air-clad Yb doped fiber (18) and further broadened due to SPM in the passive core of the passive air-clad fiber (52), to produce sufficient spectral broadening to allow the pulses to be compressed in the dispersion compensator (16) into femtosecond pulses. The Yb doped fiber may have a phosphosilicate host composition in order to mitigate photodarkening.