Ring microlasers have proved useful in many applications, including optical communications, spectroscopy and photochemistry. A new approach to ring lasers has been proposed and analyzed by Amnon Yariv and Jacob Scheuer of California Institute of Technology in Pasadena. Advantages of the design are a very high free spectral range for the laser's size and a very low loss per cycle (i.e., a very high Q).The ring resonator is based on a circumferentially guiding defect within a periodic medium, such as a photonic bandgap crystal or radial Bragg reflectors. The researchers started with the Helmholtz equation, describing the electric field in cylindrical coordinates, and they have shown that the free spectral range of such a resonator can be larger than that of a conventional resonator with similar dimensions. Moreover, adding layers to the outside of the cylindrical structure may make the resonator loss arbitrarily small.They described the design in the Sept. 1 issue of Optics Letters.