Optical frequency mixing through cascaded second‐order processes in β‐barium borate

We show that the large effective optical nonlinearity, which is produced by cascaded second‐order processes, allows us to accomplish the frequency conversion of a signal pulse with a pump pulse of moderate intensity. In a 1 cm long β‐barium borate crystal, the cascaded effect enhances by 5 orders of magnitude the efficiency of the frequency mixing process. The enhancement is larger than 104 even in non‐phase‐matched condition for the intermediate second‐harmonic field, when the imaginary part of the induced nonlinear susceptibility is negligible.