Harmonic mode-locking in a fiber laser through continuous external optical injection

The effect of an external continuous wave (cw) on the operating regime of a passively mode-locked double-clad fiber laser, operating in the anomalous dispersion regime, is experimentally investigated. Starting from different soliton distributions, we demonstrate that, under specific conditions, the cw signal forces the principal laser to operate in harmonic mode-locking regime. This effect is fully reversible and does not exhibit any hysteresis phenomena.