Toward a Minimalist Visible Ultrafast Fiber Oscillator via Linear-Cavity Mode-Locking Engineering

Visible ultrafast fiber lasers hold transformative potential for material processing, life sciences, and biomedical research, yet their development is hindered by complex ring- or figure-eight-based cavities, whose reliance on cascading components (e.g., isolator, polarization controller) forces compromises in compactness, efficiency, and cost. To address these limitations, we demonstrate a novel linear-cavity ultrafast fiber oscillator operating robustly in the deep-red region. The design integrates a double-clad Pr3+-doped fiber gain with simplified linear-cavity nonlinear polarization evolution for passive mode-locking, which achieved self-starting operation using only two waveplates and a polarization beam splitter. This configuration delivers mode-locked pulses at 717.19 nm with a 5.94 nm bandwidth, 327 fs duration, and 54.756-MHz repetition rate. The simulations quantitatively verified the experimental performance, demonstrating remarkable agreement. These advances establish a new pathway toward compact, robust, cost-effective, and high-performance visible ultrafast fiber laser sources for materials processing, bioimaging, and medical technology applications.