Generation of harmonic mode-locked pulses and noise-like pulses in an all-PM Tm-doped fiber laser via linear-cavity interferometric NPE

This study demonstrates the generation of both harmonic mode-locking (HML) and noise-like pulses (NLPs) in an all-polarization-maintaining (PM) Tm-doped fiber laser mode-locked by a linear-cavity interferometric nonlinear polarization evolution (NPE) mechanism. Adjusting the pump power and manipulating the intra-cavity pulse distribution along the axes of the PM fiber enable transitions between fundamental mode-locking, various harmonic orders, and NLPs. In the HML regime, a 19th-order harmonic soliton pulse is achieved, marking a record-breaking repetition rate of ∼229 MHz for NPE-based all-PM linear-cavity fiber lasers. Detailed analyses of pulse characteristics across different harmonic orders are presented, with the 3rd-order HML exhibiting a high supermode suppression level. The stability of 3rd-order HML operation is further confirmed through noise evaluations and long-term assessments of power and spectrum. In the NLPs regime, the output is centered at 1962 nm with a 3-dB bandwidth of ∼30 nm, achieving an average power of 284 mW and a pulse energy of ∼24 nJ. The long-term stability and noise profile of the laser are examined to ensure sustained NLPs operation. Furthermore, the influence of pump power variations on the optical properties of NLPs is investigated, revealing an increased spectral width, red-shifted spectrum, and reduced duration of coherent peaks with higher pump intensities.