Passive harmonic hybrid mode-locked fiber laser with extremely broad spectrum

We demonstrate a harmonic mode-locking Erbium-doped fiber laser which is cooperatively mode-locked by nonlinear polarization evolution (NPE) and semiconductor saturable absorber mirror (SESAM). Via effective dispersion and nonlinearity optimization, 8th harmonic at a repetition rate of 666.7 MHz is obtained. The output pulses has a full spectrum width at half maximum (FWHM) of 181 nm and duration of 218 fs. The pulses are compressed to 91 fs by external chirp compensation. The average power of the direct output pulse at an available pump power of 1.5 W is 136 mW, which exhibits a single-pulse energy of 0.2 nJ. The cavity super-mode suppression is up to 60 dB and the signal-to-noise ratio of the 8th harmonic is over 75 dB.