Generation of the fifth harmonic of a neodymium laser and two-photon absorption in KDP and ADP crystals

An investigation was made of cascade generation of the fifth harmonic of radiation from a wide-aperture neodymium laser in KDP and ADP crystals. It was found that the main factor limiting the efficiency of the process was two-photon absorption of the radiation. The coefficients of two-photon absorption of the radiation at the frequency of the fourth harmonic of a neodymium laser were determined for KDP (0.5 cm/GW) and ADP (0.35 cm/GW). The efficiency of conversion of the second harmonic radiation energy of a neodymium laser to the fourth harmonic was 92%, the highest achieved so far. The efficiency of conversion to the fifth harmonic was 19%, close to the maximum attainable for KDP crystals.