We have designed and tested a broadband fan-out PPLN-based optical parametric oscillator (OPO) pumped by a Q-switched Nd:YAG laser (1.064 mm) and operating in the spectral range 2.6 – 4.2 mm. The optical scheme of the OPO utilises a cylindrical pump beam expander for increasing the effective aperture of the PPLN structure and obtaining broadband generation.