Periodically-poled lithium niobate (PPLN) is a well- established nonlinear material for down converting the output from a solid state laser, the pump wavelength, to generate an emission close to 4 micrometers . A simple OPO configuration was used with a resonated signal beam passing through a heated PPLN crystal, to avoid photo-refraction. A Q-switched Nd:YAG pump laser was used for generating short- duration, high-intensity one micron pulses. Direct modulation of the diode sources enable the emission to be controlled. The pump beam had good beam quality, with an M2 of about 1.8 in each axis, and modulated average power in excess of 6 W. Damage to PPLN crystals used in the solid state laser has been observed. Irreversible damage of the nonlinear medium has been recorded: initially to the output face of PPLN crystals, but latterly to the bulk structure. These observations were made over a range of pumping conditions and OPO configurations. Estimates of the damage threshold have been surprisingly low, of the order of 1.5 J/cm2.