Broadband Multi-State Soliton Molecule Generation and Regulation in a Polarization-Dependent Long-Period Grating-Based Ultrafast Fiber Laser

Polarization-dependent fiber grating-based polarizers have garnered significant attention in ultrafast lasers due to their simple fabrication, excellent polarization properties, and cost-effectiveness, making them a promising alternative to conventional commercial polarizers. Here, we designed and constructed a near-zero dispersion nonlinear polarization-rotating fiber laser utilizing a polarization-dependent long-period fiber grating (PDLPFG). This laser demonstrated the generation of various soliton molecule states, all exhibiting a 3 dB spectral bandwidth of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 20nm and a repetition rate of 30.47MHz. By adjusting the intracavity polarization state and pump power, stable loosely and tightly bound states (2 pulses), multi-pulse soliton molecules (with up to 11 pulses), and structural soliton molecules were generated and characterized. Additionally, the evolution of a 10-pulse soliton molecule as the pulse number decreased from 10 to 3 with decreasing pump power was investigated, revealing that the pulse separation and spectral modulation period remained constant at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 6.6ps and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 1.2nm, respectively. The spectral modulation characteristics and temporal evolution of this process were analyzed in detail. These results confirm the feasibility of using PDLPFGs as intracavity polarizers in broadband multi-pulse soliton molecule fiber lasers. Moreover, this laser system holds promise as a platform for novel encoding schemes, contributing to advancements in all-optical communication and data storage.