Lead-free bismuth-based perovskite saturable absorber for mode-locked fiber laser

Perovskite single crystals, with their low defect density and absence of grain boundaries akin to top-quality silicon, offer exciting possibilities. Lead-based perovskite has shown promise in fiber lasers, but environmental and health concerns drive the search for lead-free alternatives with similar properties. This study introduces methylammonium bismuth bromide (MBB) as a lead-free alternative for saturable absorbers (SAs). MBB single crystals were synthesized with optimal results achieved at specific precursor ratios 2:3, and 3:3 of MaBr:BiBr. MBB/PDMS composites were formed using different solvents, with MBB/PDMS: HEX and MBB/PDMS: TOL maintaining MBB perovskite's properties. These composites were deposited on optical fibers to fabricate SAs, exhibiting nonlinear properties. Characterization revealed transmission loss of 4.20 dB, saturation intensity (Isat) of 200.03 MW/cm2, and modulation depth (MD) of 9.75 % for MBB/PDMS: HEX-SA. Meanwhile, MBB/PDMS: TOL-SA featured transmission loss of 3.58 dB, Isat of 900.00 MW/cm2, and MD of 19.30 % The SAs were incorporated into a ring cavity erbium-doped fiber laser (EDFL) and successfully generated stable ultrashort pulses pulse width (τpulse) of 942.8 fs and 942.1 fs, respectively. Long term stability test of the SA was then performed for MBB/PDMS: HEX-SA with higher SNR value (65.4 dB) compared to its counterpart. Stability tests under different lighting conditions were conducted for 16 weeks, comparing packaged (SA-WP) and unpackaged (SA-WOP) SAs. SA-WOP exhibited greater transmission loss deviation (1.00 dB vs. 0.79 dB) and greater mode-locked threshold deviation (50% at week 7 vs. 9% at week 14). An increase in MD by 6.00 % after 16 weeks attributed to light-induced perovskite bandgap decrease was observed for SA-WOP, while SA-WP displayed consistent decrease in MD by 4.65% due to regular perovskite degradation, which aligned with the observed pulse width deviation of 6.48% and 2.79% in the autocorrelation traces respectively. This investigation offers insights for optimizing fabrication and packaging of lead-free bismuth-based perovskite SAs for fiber lasers and other applications.

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