  Broad-area semiconductor lasers are employed in many high-power applications,
however, their spatio-temporal dynamics is complex and intrinsically unstable
due to the interaction of several transverse lasing modes. A dynamical and
spatio-spectral analysis with ultra-high resolution of commercial broad-area
lasers reveals multiplets of phase locked first- and second-order transverse
modes that are spontaneously created by the nonlinear dynamics for a wide range
of operation parameters. Phase locking between modes of different transverse
order is demonstrated by comparing the linewidths of the lasing modes to that
of their beat note as well as by a direct measurement of their phase
fluctuation correlations. Since the laser and the setup lack any feature
designed to induce locking and the overall dynamics is unstable, the
observation of this spontaneous phase locking effect is very unexpected. Our
findings indicate that chimera-like states featuring groups of synchronized
oscillators coexisting with unsynchronized ones can spontaneously form in
broad-area lasers, and may thus be found in a wider range of optical systems
and beyond than previously assumed. Moreover, some of the phase locked modes do
not even exist on the passive-cavity level, but are created by the nonlinear
dynamics, an effect not previously observed in the context of chimera states.
