SiO 2 -C composite derived from rice husk ash as saturable absorber for high modulation depth mode-locked pulses in thulium-holmium doped fiber lasers

Abstract This study presents the synthesis and application of a silicon dioxide-carbon composite (SiO 2 -C) derived from rice husk ash (RHA) as a novel saturable absorber (SA) for passive mode-locking in a thulium-holmium doped fiber laser (THDFL) operating near 2 μm. The SiO 2 -C was extracted through a controlled chemical and thermal process and deposited onto arc-shaped optical fibers to form the SA device. The nonlinear absorption characterization revealed a high modulation depth of 84.8%. When integrated into a dual-pump THDFL, the RHA SA enabled stable operation in both conventional fundamental and soliton rain mode-locking regimes. With a frequency of 14.3 MHz and a pulse width of 1.48 ps, the conventional fundamental mode-locked pulse showed long-term stability for 2 days, as observed over a continuous 5-hour period each day. The transition to soliton rain mode-locking occurred at a higher pump power, with the center wavelength consistently maintained at 1909 nm. During this shift from fundamental mode-locking, the noise level increased by only 3.4 dB, indicating minimal impact on overall signal quality. Overall, RHA outperforms typical SAs by delivering high modulation depth, robust spectral stability, and excellent noise suppression, making it a cost-efficient option for next-generation ultrafast fiber lasers.