In this paper, coupled-wave equations are solved for large signals by means of numerical integration. A numerical method for calculating SHG and THG efficiencies of beams with Gaussian temporal profiles is given. A high-efficiency frequency tripling seheme for low and moderate power levels is presented. With type-I β-BaB2O4 crystal used in doubling and type-II KD*P used in tripling, energy conversion efficiencies of TEG as high as 41.8% have been achieved at an energy level of 365mJ.