Generation of conventional soliton and 20^th harmonic mode-locking in an erbium-doped fiber laser modulated by carbon-quantum-dots saturable absorbers

Taking advantage of both the saturable absorption and high third-order nonlinear properties of the carbon quantum dots (CQDs) saturable absorber (SA), conventional soliton (CS) mode-locking, fundamental mode-locking, and 20 th harmonic mode-locking of an erbium-doped fiber (EDF) laser are achieved. First, CQDs are prepared by solvothermal method. Then, taper-fiber-structured CQDs-SA and sandwich-structured CQDs-SA are fabricated. Investigated by a balanced twin-detector measurement scheme, the SA devices exhibit excellent nonlinear saturable absorption properties (NSAP) at the wavelength of 1.55 µm. Second, mode-locked by the as-prepared taper-fiber-structured CQDs-SA, CS mode-locking with central wavelength and pulse width of 1561.5 nm and 675 fs is achieved. When sandwich-structured CQDs-SA is integrated, a fundamental mode-locking as well as 20 th harmonic mode-locking (HML) are realized. The fundamental repetition rate is 4.93 MHz with a pulse duration of 1.18 ps. In the 20 th HML operation, the repetition rate is 98.6 MHz and the pulse duration is 1.33 ps. The results provide the first demonstration of the simultaneous applications of both high nonlinear and saturable absorption effects of the CQDs, as well as broaden new avenues and opportunities for extending the methods of realizing ultrafast photonic devices.