Determination of the Thermal Lens of a PPKTP Crystal Based on Thermally Induced Mode-Mismatching

We present a method for determining the thermal focal length of periodically poled potassium titanyl phosphate (PPKTP) through measuring the variation in the mode-matching efficiency between the seed beam and the TEM
₀₀
eigenmode of an external optical cavity. The eigenmode size of the external optical cavity changes with the thermal focal length originating from the absorption of PPKTP inside the external optical cavity, whereas the fundamental wave injected into the external optical cavity keeps the geometrical size unchanged, which continuously deteriorates the mode-matching efficiency between the fundamental wave and external optical cavity's eigenmode. Thus, we can obtain the thermal focal length at different power levels of a harmonic wave according to the corresponding mode-matching efficiency. The measurement is more sensitive to the absorbed power in the material, which can be of high precision under intense pumping. Finally, the uncertainty of the method is analyzed.