[en] Optical coupling between a spherical microresonator and a plane dielectric substrate is investigated for a small sphere whose diameter is only a few ten times as large as the optical wavelength. A unique characteristic of small microresonators, relaxation of the phase-matching requirement that is due to the small size, is demonstrated on the basis of experimental results and model numerical calculations. The difference between the frequency and the propagation constant as parameters to describe the resonances of microresonators is clarified. Based on the numerical results, intrinsic restriction on a distance between the resonator and the coupler is also suggested. [copyright] 2001 Optical Society of America