Peculiarites of second harmonic generation of radiation from a pulsed ytterbium-doped fibre laser in KTiOPO4 crystals

Second harmonic generation of radiation from a high-power fibre Yb³⁺ laser is studied upon sf–f phase matching near the X axis of the KTP crystal (KTiOPO₄). The temperature dependence of the phase-matching wavelength as well as its spectral, angular and temperature tuning characteristics of the second harmonic generator are experimentally measured. It is found that the increase in the average output power of the second harmonic in a single crystal is limited, first of all, by the thermal self-focusing of radiation caused by the absorption of radiation from the second harmonic, whose threshold decreases with increasing the crystal temperature. When the two-channel scheme is used, the maximum stable ~4-W output power of the second harmonic is obtained in two tandem KTP crystals.