We demonstrate a wildly tunable,high-efficiency mid-infrared (mid-IR) output-coupled single resonant optical parametric oscillator (OC-SRO) pumped by a Yb-fiber laser.The compact mid-infrared source employs a 50-mm-long,multi-grating MgO-doped periodically poled lithium niobate (MgO-PPLN) crystal,providing as much as 1.73 W idler power at 3.012μm,and 1.27W signal power at 1645nm,corresponding to an overall conversion efficiency of 41.7％ and a slope efficiency of 77.9％.In particular,the mid-infrared output power of 1.03 W and 0.67 W are obtained at 3.7μm and 3.9 μm,respectively,with an optical-to-optical conversion efficiency of 14.3％ and 9.3％.Furthermore,the idler light is tunable from 3 μm to 3.9 μm by changing the periods from 31 to 28.5 μm,with the output power ＞ 1 W over 78％ of the tuning range.Our experimental results are pump power limited and further mid-IR power and conversion efficiency could be obtained with a suitable high-power pump source.The total OC-SRO output power rms at 2.6 W is about 0.6％ during 2 h measurement.