Nonlinear Optical Properties of Quinaldine Red and Its Application in a 2 μm Passively Mode-Locked Fibre Laser

Ultrafast fibre lasers in the 2 μm spectral region are highly desirable for applications such as laser surgery, environmental sensing, and micromachining. Saturable absorbers (SAs) currently used to generate pulses in this region perform admirably in laboratory settings. Still, their high costs, complex fabrication processes, and narrow operating bandwidths prevent their widespread adoption in real-world applications. In this work, we demonstrate, for the first time, the use of Quinaldine Red as a highly effective and viable alternative. The fabrication of the SA only requires dissolving Quinaldine Red powder in isopropyl alcohol to form an aqueous solution at a 10 mg/mL concentration before being drop-casted onto an arc-shaped single-mode fibre (SMF-28) to form the SA. The SA showed good nonlinear optical properties including a high modulation depth of 12.0% and a low saturation intensity of 0.56 MW/cm 2 , and when integrated into a thulium-holmium-doped fibre laser (THDFL) the SA was able to generate stable mode-locked pulses at 1911.8 nm as well as a pulse width of 1.96 ps, repetition rate of 13.4 MHz and pulse energy of 0.16 nJ with a peak power of 83 W. The laser also demonstrated a slope efficiency of 1.78% and exceptional long-term stability over a two-hour observation period and after a two-week dormant state, with a measured signal-to-noise ratio (SNR) of 43 dB. These findings suggest that dye-based SAs like Quinaldine Red offer a robust, giving a pathway for the development of future optical modulators.