We present a measurement series of the efficiency of periodically poled KTP used for second-harmonic generation in an external phase-locked cavity. Due to the high absorption (0.01 cm^−1) in the PPKTP crystal at the pump wavelength a strong thermal dephasing of the periodically poled grating is observed for high pump powers. For four different resonator setups, it was experimentally found that a threshold parameter could be defined as the ratio between the focal intensity in the crystal and the single-pass conversion efficiency. The value of this threshold for the onset of strong thermal dephasing was found to be 1.41×10^10 W^2 m^-2 in our 30-mm long PPKTP sample. This threshold parameter marks the onset of thermally induced instability that leads to a degradation of the SHG conversion efficiency. Above the threshold the shape of the resonance peaks of the resonator changed from symmetrical into triangular making phase locking difficult.
Applied Physics B, 2009, Vol 96, Issue 4, p. 827-831