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We report an original noise-like pulse dynamics observed in a figure-eight fiber laser, in which fragments are continually released from a main waveform that circulates in the cavity. Particularly, we report two representative cases of the dynamics: in the first case the released fragments drift away from the main bunch and decay over a fraction of the round-trip time, and then vanish suddenly; in the second case, the sub-packets drift without decaying over the complete cavity round-trip time, until they eventually merge again with the main waveform. The most intriguing result is that these fragments, as well as the main waveform, are formed of units with sub-ns duration and roughly the same energy.

In this paper, we study noise-like pulse generation in a km-long fibre ring laser including a doubleclad erbium-ytterbium fibre and passively mode-locked through nonlinear polarization evolution. Although single noise-like pulsing is only observed at moderate pump power, pulse energies as high as 120 nJ are reached in this regime. For higher pump power, the pulse splits into several noise-like pulses, which then rearrange into a stable and periodic pulse train. Harmonic mode locking from the 2nd to the 48th orders is readily obtained. At pump powers close to the damage threshold of the setup, much denser noise-like pulse trains are demonstrated, reaching harmonic orders beyond 1200 and repetition frequencies in excess of a quarter of a GHz. The mechanisms leading to noise-like pulse breaking and stable high-order harmonic mode locking are discussed.