Two-wavelength, dual regime Q-switched erbium-doped fiber laser based on a microfiber loop coated with gold nanostars as the saturable absorber

• A microfiber-loop saturable absorber coated with gold nanostars was fabricated. • Gold nanostars show three plasmonic resonances with a broadband absorption tail extending to the C band. • The microfiber-loop saturable absorbed has a high modulation depth of 12.2 % • The ring cavity Erbium fiber laser with the microfiber loop saturable absorber exhibited dual Q-switched regimes separated by a CW window. We report a two-wavelength, dual-regime Q-switched erbium-doped fiber laser employing a microfiber loop coated with gold nanostars as the saturable absorber. The gold nanostars are designed to have three resonances, with the broad spectral tail of the longest resonance overlapping the erbium emission band, enabling strong saturable absorption in the coated microfiber loop, resulting in a modulation depth of 12.19% and a nonsaturable absorption of 39.89 %. When inserted into an erbium-doped fiber laser ring, this device produced dual Q-switched regimes separated by a continuous-wave window, along with stable dual-wavelength emission at 1529 nm and 1531 nm. In the pump range of 55.7–188 mW, stable Q-switched pulses with repetition rates ranging from 15.8 to 53.36 kHz and pulse widths as short as 5.2 µs were observed. At pump powers of ∼ 190–210 mW, the laser switched to continuous-wave operation; at higher pump levels (211–257 mW), it returned to a Q-switched regime with shorter pulses. This behavior is attributed to the interplay between saturable bleaching and reverse saturable absorption or thermally induced losses in the gold nanostars. These results highlight the potential of multi-resonant plasmonic nanostructures for broadband, intensity-dependent control of fiber lasers.