  We demonstrate experimentally that stable single longitudinal mode operation
can be readily achieved in PT-symmetric arrangements of coupled microring
resonators. Whereas any active resonator is in principle capable of displaying
single-wavelength operation, selective breaking of PT-symmetry can be utilized
to systematically enhance the maximum achievable gain of this mode, even if a
large number of competing longitudinal or transverse resonator modes fall
within the amplification bandwidth of the inhomogeneously broadened active
medium. This concept is robust with respect to fabrication tolerances, and its
mode selectivity is established without the need for additional components or
specifically designed filters. Our results may pave the way for a new
generation of versatile cavities lasing at a desired longitudinal resonance.
Along these lines, traditionally highly multi-moded microring resonator
configurations can be fashioned to suppress all but one longitudinal mode.
