Difference frequency mixing of strongly focused Gaussian beams in periodically poled LiNbO 3

Difference frequency mixing of strongly focused Gaussian beams in periodically poled LiNbO 3 was investigated. The beams were focused inside the crystal without damage. Mixing of cw and pulsed laser systems was employed to minimize the thermal lensing effects inside this crystal. A low-power cw 1.064 - μ m Nd:YAG laser beam was used as the pump, whereas the signal beam came from a picosecond optical parametric oscillator. The two independently focused beams were made to overlap inside the crystal. The focusing parameters of the two beams were about the same. The focusing function developed by Boyd and Kleinman for tightly focused beams was extended to periodically poled crystals in order to fit the temperature-tuning curve. A 0.66 % ∕ W efficiency was obtained for the difference frequency mixing process.