In a nonlinear optical medium such as the gallium arsenide, two collinear incident monochromatic waves are injected to generate a monochromatic wave. The medium induces between the three waves a propagation phase-shift that has a value .pi. if the three waves travel a coherence length in the medium and that is compensated at the value 0 modulo 2.pi. each time that the three waves have travelled a periodic distance in the medium. In order to significantly increase the conversion efficiency, the periodic distance between two successive bounces of total internal reflection of the waves in zig-zag is strictly less than the coherence length. The high conversion efficiency on a very short material length leads to product high-power coherent optical sources particularly with wavelengths of approximately 10 .mu.m.