Coherence Deterioration in Ultrafast Fiber Lasers Mode-Locked by Nonlinear Polarization Rotation

Ultrafast lasers based on nonlinear polarization rotation has attracted significant attentions, where artificial saturable absorption is induced by Kerr effect. Due to polarization-dependent device within, nonlinear polarization rotation laser cavity generally operates in scalar regime, while vector dynamics have been observed occasionally. However, coherence deterioration induced by nonlinear polarization evolution are rarely reported. Here, we investigate the coherence deterioration for the ultrafast laser systems based on nonlinear polarization rotation. Three types of vector pulses, group velocity locked vector solitons, polarization rotation vector solitons, and coherence deteriorated states can be generated by finely tuning the birefringence of cavity. Combining dispersive Fourier transform and division-of-amplitude facilitates single-shot measurements of wavelength-resolved states of polarization. We find that birefringence-induced phase shift promotes the wavelength-dependent polarization rotation and various polarization evolution speed. In the coherence deteriorated states, optical polarization rogue waves at certain wavelengths are identified, while optical intensities show different irregular probability distributions.