Tunable High-Order Harmonic Mode-Locking in Yb-Doped Fiber Laser With All-Normal Dispersion

A tunable high-order harmonic and passively mode-locked Yb-doped fiber laser with all-normal dispersion is demonstrated. Using the mode-locking mechanism of the nonlinear polarization evolution (NPE) effect and a cascade long period fiber grating (C-LPFG) as an all-fiber format spectral filter for the strong pulse shaping, a maximum of 14th-order harmonic mode-locked (HML) Yb-doped fiber laser can be tunably achieved with a repetition rate of 35.497 MHz compared with the fundamental repetition rate of 2.538 MHz, which is the highest harmonic mode-locked laser with all-normal dispersion so far. At the 14th HML output, the cavity supermodes are suppressed to 23 dB, and the pulses duration is 1.23 ns with a maximum pulse energy of 0.97 nJ.