Encapsulated black phosphorus saturable absorber for ultrafast mode-locked lasers in the eye-safe region

Black phosphorus (BP) has been extensively investigated in ultrafast photonics due to its unique direct bandgap energy, which can be tuned by altering the number of layers. However, the reduction of BP layers induces optical limiting effects, such as two-photon absorption and reverse saturable absorption, which hinder stable mode-locked operation at high pump powers. Herein, we demonstrate stable soliton pulse generation by encapsulating bulk BP within polydimethylsiloxane (PDMS). The fabricated BP/PDMS-saturable absorber with a 6% modulation depth was integrated into “eye-safe” thulium-doped fiber laser (TDFL) and thulium-holmium-doped fiber laser (THDFL) systems. We achieved picosecond pulses with high output powers, pulse energies, and peak powers of 63.13 mW (21.90 mW), 5.28 nJ (3.33 nJ), and 4.06 kW (1.35 kW) for the TDFL (THDFL), respectively. Owing to the absence of optical limiting effects from the BP/PDMS-saturable absorber, fundamental soliton regimes were maintained over a wide range of pump powers with no sign of pulse breakup. Finally, 4-hour stability assessments of the TDFL and THDFL under 75% relative humidity validated the reliability of the BP/PDMS-saturable absorber, opening new possibilities for robust ultrafast photonic applications in the near future.