In this paper we present a full-wave algorithm for the design and the optimization of quasi-optical frequency multipliers and discuss its implementation in a specialized computer code, able to simulate the non-linear device, the planar antenna and the embedding layered structure as a whole. The analysis of the multiplier is performed under the simplifying approximation of an infinite array excited by an uniform plane wave incident from the broadside direction. The array parameters are deduced from a full-wave analysis, based on the Method of Moments, while the solution of the non-linear circuit is found by the Harmonic Balance method. This analysis algorithm is integrated into an optimization routine, which adjusts the antenna geometry and the layered structure, in order to maximize the overall conversion efficiency of the multiplier. As an example, we discuss the design of quasi- optical frequency tripler, operating at 430.5 GHz, based on Hetero-structure Barrier Varactors (HBVs). We present the characterization of the non-linear devices, the design and the fabrication of the antenna array and the optimization of the measurement setup, using the external filters and dielectric slabs.