High-Efficiency Frequency Doubling of Nd:YAG

Abstract

The second harmonic of the Nd:YAG output wavelength (1,064 nm) can be produced with both CD*A and KD*P with efficiencies approaching 50% when the crystal indices of refraction are equal at the fundamental and second-harmonic frequencies - the so-called "phase-matching" condition. For Type I doubling processes, this condition can be satisfied by allowing the ordinary index of refraction at the fundamental wavelength to be equal to the extraordinary index at the doubled frequency. For Type II doubling, the necessary condition is that the extraordinary index at the second harmonic be the average of the extraordinary and ordinary indices at the fundamental. For CD*A, phase matching can be accomplished via the Type I process with 0m=90° by elevating the crystal temperature to ~100 deg C, depending on the deuteration level of a specific crystal. For a 21-mm-long CD*A crystal, the width of the temperature versus efficiency curve is 3.25 C FWHM. Ninety-degree phase matching allows efficient doubling with incident beam divergences an order of magnitude greater than diffraction limited and does not introduce walkoff resulting from double refraction. Data are presented showing doubling efficiency as a function of crystal length and incident average power density, including considerations of efficiency and average power saturation. Type II KD*P phase matches at room temperature with 0m=53°36'. To obtain efficient doubling in this case, the incident fundamental beam divergence must be close to diffraction-limited. The angular halfwidth of a 25-mm crystal is 1 mr FWHM for rotation about the ordinary axis. Compared to CD*A, the use of KD*P is advantageous because of its ready availability, high damage thresh-old, high saturation levels and usefulness at room temperature. Using a 3.0-J pulsed Nd:YAG laser with a repetition rate of 10 pps, 10.5 W of 532-nm power has been achieved.