We have proposed and designed zigzag-folded U-shaped waveguides to realize highly efficient wavelength conversion with an extremely small footprint. Extreme high efficiencies are achieved with a combination of modal phase matching and 4 quasi phase matching in inversion-stacked AlGaAs/Alox waveguides. Numerical simulations reveal that the conversion efficiency of second harmonic generation pumped at 1.55 mu m as high as 12000 % W-1 can be achieved in an 8.0-mm-long AlGaAs/Alox waveguide that is folded up into a small domain of 0.8x0.6 mm(2) area. A phase matching signal bandwidth of difference frequency generation, 29 nm, covers 83 % of the telecommunication C band. (C) 2017 Optical Society of America