  We demonstrate single transverse-mode operation of broad-area coupled
waveguide lasers enabled by parity-time (PT) symmetry. The PT symmetric laser
operates on coupled waveguide cavities with electrically tuned gain and loss.
Such counterintuitive waveguide design enables PT symmetric breaking, causing
unique mode selection and ultimately enabling single mode operation. By
electrically tuning the loss in the loss region of the coupled waveguide
cavity, several different PT-symmetric regimes are analyzed theoretically, and
the corresponding PT symmetric phase transition is demonstrated experimentally
in the coupled waveguide laser.
