  The ray dynamics of optical cavities exhibits bifurcation points: special
geometries at which ray trajectories switch abruptly between stable and
unstable. A prominent example is the Fabry-Perot cavity with two planar
mirrors, which is widely employed for broad-area semiconductor lasers. Such
cavities support lasing in a relatively small number of transverse modes, and
the laser is highly susceptible to filamentation and irregular pulsations. Here
we demonstrate experimentally that a slight deviation from this bifurcation
point (planar cavity) dramatically changes the laser performance. In a
near-planar cavity with two concave mirrors, the number of transverse lasing
modes increases drastically. While the spatial coherence of the laser emission
is reduced, the divergence angle of the output beam remains relatively narrow.
Moreover, the spatio-temporal lasing dynamics becomes significantly more stable
compared to that in a Fabry-Perot cavity. Our near-planar broad-area
semiconductor laser has higher brightness, better directionality and hence
allows shorter integration times than an incandescent lamp while featuring
sufficiently low speckle contrast at the same time, making it a vastly superior
light source for speckle-free imaging. Furthermore, our method of controlling
spatio-temporal dynamics with extreme sensitivity near a bifurcation point may
be applied to other types of high-power lasers and nonlinear dynamic systems.

Optics