A passively mode-locked ytterbium-doped all fiber laser has been demonstrated based on the nonlinear polarization
rotation technique in a all normal dispersion cavity. By optimizing the coupling ratio and position of the output coupler,
stable mode-locked pulses is generated with an average power of 200 mW at a repetition rate of 3.3 MHz, and
corresponding to single pulse energy of 60 nJ. The slope efficiency of power is as high as 68%.
We have demonstrated a passively mode-locked fiber laser with a composite cavity structure for repetition rate control.
An optical delay line is used to control the length so that the lengths of the main cavity and the sub-ring cavity are
accurately co-prime. Using this method we have obtained 46th harmonic pulses with a fundamental repetition of
17.39MHz. The fundamental mode-locking is substantially suppressed. The SNR of rf spectrum is higher than 45 dB.
Stable 800MHz repetition rate mode-locked pulses in duration of 14.27 ps are generated. The detuning phenomenon
appears when two cavity’s lengths are not matched. A larger pump power is required to maintain the oscillation.