High average power sources operating in the 3 to 5 μm mid-infrared waveband are of interest for a wide variety of applications. We present design and performance results for a high-power engineered breadboard mid-IR source based on near-infrared pumped periodically-poled lithium niobate (PPLN) optical parametric oscillator (OPO) technology. The source design utilises a pair of singly-resonant PPLN OPOs pumped by a commercial 40 Watt, Q-switched, diode-pumped Nd:YLF laser. The mid-IR outputs from each OPO are polarisation recombined into a single output beam. A twin OPO design was chosen to minimise the effect of optical absorption, reduce thermal loading within each PPLN crystal and provide additional flexibility by offering the option for dual-wavelength mid-IR operation. An average output power approaching 4 Watts has been obtained with a corresponding slope efficiency of 15%. The mid-infrared beam is 6 times diffraction limited. Laser operation is controlled by a remote PC link and power, spectral and temporal pulse diagnostics are included within the system.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.