Generation of stable clean ultrashort pulses in a simple all-fiber, all-normal dispersion ytterbium-doped mode-locked laser

We report an all-fiber, all-normal dispersion ytterbium (Yb)-doped fiber oscillator mode locked by nonlinear polarization evolution. The mode locking is realized in different pump power regimes by adjusting the polarization controllers. The average output power, pulse energy, and compressed pulse duration varies from 60 to 150 mW, 1.4 to 3.5 nJ, and 350 to 146 fs, respectively, when the pump power to the oscillator is varied from 250 to 500 mW. Particularly at 500 mW pump power, the oscillator generates highly chirped 5 ps duration pulses at 42 MHz that are compressed to a 146 fs duration with a clean temporal profile. The signal-to-noise ratio of the train of mode-locked pulses is 85 dB, indicating stable mode-locking operation. The generalized nonlinear Schrodinger equation is solved numerically to model the passive mode locking in a Yb oscillator, supporting the experimental results. In a chirped pulse Yb amplifier, the signal power is scaled to a 10 W level, and the amplified pulse is compressed to 355 fs duration.