For a typical size ELMO Bumpy Torus (EBT) reactor (approx. 1000 MWe), microwave frequencies required lie in the range of 60 to 110 GHz at power levels of 50 to 75 MW. As the frequency rises, the unloaded cavity (i.e., without plasma) quality factor Q decreases. Because of the short wavelengths of microwave heating power and the large cavity dimensions of a reactor, it is possible to apply quasi-optical principles in the efficient coupling of power to the plasma. The use of a confocal Fabry-Perot resonator with spherical mirrors is discussed; these serve to confine the microwave power to the region occupied by the plasma. The potential advantages of these resonators include high efficiency utilization of microwave power, minimal thermal burden on the cryopumping system, and significant benefit in preventing microwave leakage from the device. An estimation of the unloaded cavity quality factor Q and the design considerations of Fabry-Perot resonator are given.