Polarization-controlled harmonic mode-locking in a soliton fiber laser: isospectral states, continuous tuning, and repetition-rate scaling up to 18 GHz

Harmonic mode-locking (HML) in soliton fiber lasers provides a practical approach to generating multi-gigahertz pulse trains; however, simultaneous control of the pulse repetition rate (PRR) and optical spectrum width (OSW) remains a challenge. Here, we experimentally demonstrate wide and continuous tunability of both PRR and OSW in an HML soliton fiber laser mode-locked through nonlinear polarization evolution (NPE). To the best of our knowledge, this is the first report presenting a two-dimensional map of HML laser states forming a connected domain in the coordinates [pump power; PRR], enabling smooth transitions between different regimes without loss of mode-locking. Experimentally, limited by the available pump power, we achieved maximum repetition rates of approximately 4.2 GHz and 18 GHz for pulses with spectral widths of about 4 nm and 1.7 nm, respectively.