Simulations and experiments showing the origin of multiwavelength mode locking in femtosecond, Yb-fiber lasers

A stable and self-starting femtosecond breathing-pulse Yb-fiber oscillator is reported, mode-locked using the non-linear polarization evolution mechanism. A bifurcation between two distinct modes of operation is demonstrated experimentally, producing pulses with a single central wavelength in one state, or following adjustment of the intracavity waveplates, the emission of pulses with three distinct central wavelengths. The maximum bandwidth was 72 nm at the -10 dB level, and the pulses were compressible externally to 70 fs with energies of 0.75 nJ. The multiwavelength pulses reported here are significantly shorter than the picosecond pulses previously observed from similar mode-locked multiwavelength sources. Vector simulations based on the nonlinear Schrodinger equation show that the multiwavelength behavior is produced by overdriving the nonlinear polarization evolution-based saturable absorber at the peak of the pulse, leading to transmission of the two wings of the strongly chirped pulse. This new insight shows clearly that the three pulses output in the multiwavelength state are coherent. The agreement between simulation and experimental data shows nonlinear polarization evolution-based mode-locked fiber lasers are a suitable platform for studying the nonlinear dynamics underlying the bifurcation of the output. (C) 2016 Optical Society of America