On limitation of quality factor of single mode resonators with finite   size
  Using realistic numerical models we analyze radiative loss of bound and unbound modes of specially designed high-Q whispering gallery and Fabry-Perot cavities of similar size and shape, and find a set of parameters when they can be treated as single mode structures. We show that these cavities have similar properties in spite of their different loss mechanisms. The cavity morphology engineering does not lead to reduction of the resonator quality factor. 
Fahmida Ferdous, Alena A. Demchenko, Sergey P. Vyatchanin, Andrey B.   Matsko, and Lute Maleki
physics.optics