Multimode-interference-induced femtosecond pulse generation in a large-core erbium-doped fiber laser

In this study, we propose and experimentally demonstrate a new mode-locked fiber laser configuration in which a large-core erbium-doped fiber (EDF) simultaneously functions as the gain medium and an artificial saturable absorber (SA). A 70 cm-long, highly doped large-core EDF was incorporated into a simple all-fiber ring resonator, eliminating the need for any additional conventional SA device. Stable soliton mode-locking was achieved through nonlinear multimode interference occurring within the large-core EDF. The excitation and interaction of multiple transverse modes under high optical intensity produced intensity-dependent transmission, effectively enabling self-starting pulse formation. The laser generated stable soliton pulses centered at 1564.1 nm with a pulse duration of 330 fs. Mode-locking was maintained over a pump power range from 291.6 to 319.2 mW, demonstrating good operational stability. Owing to the short cavity length—since the same EDF segment serves dual roles as both gain medium and artificial SA—the laser achieved a relatively high fundamental repetition rate of 24.1 MHz. The maximum pulse energy reached 143.5 pJ. This compact and simplified architecture highlights the potential of large-core EDF as a multifunctional nonlinear element for efficient ultrafast pulse generation without relying on discrete SA components.