Generation of sub-7-cycle optical pulses from a mode-locked ytterbium-doped single-mode fiber oscillator pumped by polarization-combined 915 nm laser diodes.

We report on a passively mode-locked ytterbium-doped fiber oscillator pumped by polarization-combined diodes emitting at a wavelength of 915 nm instead of 976 nm. Stable mode-locked operation based on nonlinear polarization evolution generated a broad spectrum of 140 nm, spanning from 950 to 1090 nm. The output power was 16.3 mW at a repetition rate of 93.1 MHz. External compression using a pair of transmission gratings resulted in pulse durations as short as 21.6 fs, which is equivalent to 6.6 cycle optical pulses at a wavelength of around 1000 nm.