Experimental observation of phase mode-locking in multimode graded-index optical fiber

The best-known example of phase locking in multimode optical systems is the laser cavity incorporating an ultrafast saturable absorber [1]. Here, the relative phase between the longitudinal cavity modes turns out to be synchronized in order to form a pulse. This mechanism is based on the presence of dissipation or nonlinear losses. These conditions are far from the case of beam self-cleaning, which occurs in lossless fiber systems [2]. However, in literature there are several example of phase-locking among modes for a purely conservative nonlinear optical system. An example is provided by the interaction among waves with different states of polarization, where phase-locked states emerge as stable attractors [3].