Appl. Phys. Express 9, 092701 (2016)

Mode-locked multiwavelength fibre lasers are useful for numerous applications ranging from wavelength-division multiplexing to bioimaging. In such systems, saturable absorbers are often employed within the laser cavity as a mode-locking element but shortcomings can include limited bandwidth or small optical modulation depth. Now, Ruwei Zhao and colleagues from China have demonstrated a triwavelength synchronously mode-locked erbium-doped fibre laser that uses black phosphorus as a saturable absorber. Stable operation was achieved with lasing wavelengths of 1,557.2, 1,557.7 and 1,558.2 nm, and pulse repetition rate and duration of 1.65 MHz and 9.41 ps, respectively. To make the saturable absorber, eight layers of black phosphorus from solution were deposited onto an erbium-doped fibre core. Pumping the device with 80 mW of power resulted in stable triwavelength lasing, whereas pumping beyond 170 mW disrupted mode-locking. By comparing devices with and without the black phosphorus, the team showed that its inclusion not only provided the mode-locking but also aided generation and stability of the three lasing wavelengths. Without the black phosphorus, homogeneous gain broadening in the erbium-doped fibre resulted in unstable multiwavelength oscillation. By contrast, over 30 min stable operation was demonstrated with the black phosphorus.