This work reports the biosynthesis and application of a colloidal tungsten oxide (WO 3-δ ) solution with reverse saturable absorption behavior for passive short-pulse generation in a Q-switched fiber laser. WO 3 - δ nanoparticles were synthesized through a green chemistry route using Justicia secunda Vahl extract and characterized by EDX, UV-Vis, and TEM analyses. The resulting nanostructures showed a quasi-spherical morphology with an average diameter of 10.33±0.68 nm and a sub-stoichiometric composition identified as WO 1 . 64 . Nonlinear optical characterization using the P-scan technique confirmed a reverse saturable absorption behavior, with a modulation depth of 28%, a nonlinear absorption coefficient of , and a third-order susceptibility of . The reverse saturable absorption was integrated into an erbium-doped fiber ring laser using a capillary cell. Q-switched operation was achieved with a pump threshold of 108.1 mW, repetition rates ranging from 29.41 to 34.48 kHz, pulse widths between 9.2 and 4.1 μs, and a maximum pulse energy of 4.97 nJ. The laser emitted at a central wavelength of 1567.4 nm with a spectral bandwidth (FWHM) of 0.99 nm, confirming single longitudinal mode operation. These results demonstrate that colloidal WO 3 - δ as an eco-friendly, low-cost nonlinear material for passive optical modulation and pulse generation in photonic systems.
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