Progressive transitions from periodic waves to soliton rain in an erbium-doped fiber laser

Periodic waves and soliton rain are well-known dissipative structures that are conventionally investigated in saturable absorber (SA)-free and mode-locked fiber lasers, respectively. Yet accessing the transition connecting these two states remains challenging in conventional cavities due to the intrinsic steepness of the SA response curve. Here, we report what we believe to be the first experimental observation of the progressive transition from periodic waves to soliton rain in an erbium-doped fiber laser by introducing tailored polarization-dependent loss (PDL) to flatten the nonlinear transmission curve. The slope reduction in the transmission curve enables experimental access to the evolutionary dynamics connecting periodic waves and soliton rain through standard polarization tuning. Our investigation reveals the critical roles of the self-amplitude modulation and intracavity instability during the transition, offering a unified perspective on periodic waves and soliton rain formation in fiber lasers, which complements the understanding of the extensively studied continuous wave (CW) to soliton rain evolution.