Experimental Evidences of Non-Hermitian Mode-Locking in Fibre Laser

Mode-locking (ML) is an established technique used to generate high power, ultrashort (ranging from ps to few fs duration) coherent light pulses in lasers. Mode-locking techniques could be classified into two broad categories. First one is passive mode-locking techniques in which modes are locked through dynamical intracavity self-organization processes not requiring extra energy sources. Another one is active mode-locking where locking between cavity modes is induced by external energy source. Active mode-locking could be either amplitude mode-locking (AML), for example achieved by a periodic (in time) modulation of the loss coefficient, or phase mode-locking (PML), for example via periodical modulation of the length/detuning of the cavity. In amplitude mode-locking periodic forcing induces synchronization of the cavity modes symmetrically coupled to the closest neighbors due to the action of the modulator: the spectrum broadens symmetrically, resulting in coherent frequency comb centered at the middle of the gain line, see Fig.1a.