Periodically-poled lithium niobate (PPLN) has been used to convert the 1.064 micrometer emission from a diode-pumped Nd:YAG laser to give an emission close to 4 micrometer. An acousto-optical Q-switch was used for generating short- duration, high-intensity pulses. Modulated waveforms are possible via direct modulation of the diode sources. A simple OPO configuration has been used with a resonated signal beam passing through a PPLN crystal in an oven. The pump beam has good beam quality, having an M2 of about 1.8 in each axis, and modulated average power in excess of 6 W. Damage has been observed to the output face of PPLN crystals over a range of pumping conditions and OPO configurations. The estimated damage threshold has been surprisingly low, of the order of 1.5 J/cm2. First observations indicated the damage occurring consistently at the output face of the crystal, but further analysis has indicated the presence of bulk damage, typically starting at domain boundaries.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.