We demonstrate an efficient CH3F oscillator based on an anti-reflection coated Ge dichroic beam splitter. When pumped by the 10R32 line of a CO2 laser (10.17 mu m), 0.81-mJ far-infrared laser is obtained with the wavelength of 192 mu m. The energy conversion efficiency of 0.16 % is the highest for an optically pumped 192-mu m laser system to our knowledge. The beam quality factor of M-x(2) and M-y(2) is 1.53 and 1.57, respectively. Further, this oscillator can be extended to optically pumped far-infrared lasers with various wavelengths.