Intracavity frequency doubling based on BBO crystals in a mode-locked erbium-doped fiber laser

We propose a mode-locked erbium-doped fiber laser that simultaneously exports a fundamental frequency laser and second harmonic (SH) laser. The fiber-based frequency doubling device is prepared by attaching a barium borate crystal on the rear surface of a self-focusing lens, and the input and output terminals are standard single-mode fibers. Microwatt SHs are directly obtained at the rectangular-pulse and the multi-pulses operation, confirming that frequency doubling can be achieved inside fiber resonator without affecting the mode-locked state. Our results also indicate that the rectangular-pulse is a special noise-like pulse constituted of a mass of high-intensity sub-pulses.