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Microdisk structures have been used to achieve low-threshold lasing. For these microcavity lasers, the spontaneous emission coupling factor p is an important parameter since it determines the threshold current of the laser. Theoretical calculation of p based on the exact solution of the modes in a microdisk is complicated. A simple, approximate method for solving the waveguide modes and the density of states is developed here, using conformal transformation and the Wentzel-Kramers-Brillouin approximation and taking into account the effect of the disk thickness. We find that the p value for a microdisk laser is smaller than that of an ideal laser that has a cylindrical waveguide structure with a strong index guiding. A considerably high value of 0 can still be achieved, however, in a microdisk laser.

The modification of the spontaneous emission from various dipoles in a cylindrical dielectric waveguide is studied as a function of the refractive index and the radius of the waveguide. It is found that the emission rates of the axial dipoles and the radial dipoles can be modified to enhance greatly the fraction of radial dipole emission that goes into the guided lasing modes. The total emission from the axial dipoles can be suppressed. This combination gives a high spontaneous-emission factor /3 for a microcavity ring laser. Other microlaser structures that have high /3 values are also discussed. We conclude that microlasers based on strongly guided singlemode dielectric waveguides are promising devices for achieving high /3 values and low lasing thresholds.

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