We employed a 9-mm long periodically poled KTiOPO4 (PPKTP) crystal in an optical parametric oscillator (OPO) to generate sub-nanosecond idler pulses around 2.8 mu m. With a 1-cm long OPO cavity in a singly resonant configuration and double pass pumping by 1-ns pulses at 1,064 nm, the maximum idler energy reached 110 mu J at 1 kHz. Pumping with 500 ps pulses at 1-10 kHz, resulted in an idler energy of similar to 50 mu J and the shortest pulse duration of similar to 250 ps, ever reported for an OPO. The corresponding quantum conversion efficiencies were 32.5 and 34.9 %, respectively.