A method for sum-frequency conversion of coherent radiation includes generating two linearly polarized waves at different first f1 and second f2 frequencies (f2>f1), respectively, which coaxially propagate and are characterized in common case by arbitrarily located polarization planes. The waves are further guided through an optical active crystal which rotates their polarization planes at different angles ψ1 and ψ2 determined as ω1=ρ(f1)·L and ψ2=ρ(f2)·L, where L is a length of the optical active crystal, and ρ(f1) and ρ(f2) specific rotations at respective frequencies f1 and f2. Finally the waves with the rotated polarization planes are incident on a non-linear crystal configured to generate a third frequency.