Crystal Violet Saturable Absorber for Stable Hybrid Mode-Locking in Ytterbium-Doped Fibre Lasers

While two-dimensional (2D) nanomaterials have become the primary choice for saturable absorbers (SAs) in ultrafast fibre lasers, their use remains limited by complex fabrication processes and high costs, hindering widespread adoption. Here, we propose and demonstrate an innovative SA using crystal violet (CV) dye, an effective yet easily fabricated material, which was drop-cast onto an arc-shaped fibre. The inherent optical properties of the CV dye enable a robust nonlinear absorption response, with a modulation depth of 9.5% and a saturation intensity of 3.0 MW/cm 2 . When placed in a ytterbium-doped fibre laser (YDFL) cavity, it initiates and stabilises mode locking, accompanied by nonlinear polarisation rotation (NPR). The cavity successfully generates stable picosecond pulses at 1035.16 nm, with a pulse width of 14.14 ps, a 3 dB bandwidth of 3.06 nm, a fundamental repetition rate of 3.33 MHz, and a high signal-to-noise ratio (SNR) of 45.28 dB. This CV-based SA matches the mode-locking performance of advanced 2D materials while offering unparalleled advantages by leveraging a commercially available, highly cost-effective dye with a vastly simplified fabrication process. This work highlights the vital role of organic dyes in modern ultrafast photonics applications and demonstrates that CV dyes can provide compact, stable, and economically viable pathways for advanced optical modulators.