Generation of non-smooth spike spectrum in Yb-doped mode-locked fiber laser

The output spectral characteristics of passively mode-locked fiber lasers are influenced by various nonlinear effects. Studying these characteristics is of great significance for a deeper understanding of the laser's operating mechanism and for optimizing its output performance. In this paper, we utilize Yb-doped fiber as the gain medium and construct a passively mode-locked Yb-doped fiber laser based on nonlinear polarization rotation (NPR) technology, which can produce a non-smooth spectrum with symmetrically distributed spectral spikes. The experiment reveals that as the pump power increases from 145 mW to 395 mW, the number of spectral spikes gradually increases, forming symmetrically distributed spike clusters on both sides of the central spectrum, eventually evolving into spike peaks. Additionally, under a constant pump power of 335 mW, adjusting the polarization controllers enables different stable pulses with two spectral states: spiked and smooth. What is remarkable is that the spiked spectrum yields more stable pulses with higher energy. Nonlinear phase modulation plays an important role in the dynamic process, especially the influence on the output spectrum. The experimental results provide valuable insights into the dynamic processes and spectral control of mode-locked fiber lasers.