Continuously tunable laser sidebands have been generated by mixing radiation from an optically pumped far infrared (FIR) molecular laser, operated at 693, 762, 1627, and 1839 GHz, with that from millimeter-wave klystrons in a Schottky-barrier diode. An enhancement in conversion efficiency over similar systems reported previously is obtained by using a Michelson interferometer to separate the sidebands from the carrier and by placing the Schottky diode in an open structure comer cube mount. With

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... 4 mW oflaser power at 693 and 762 GHz the sideband power was measured to be 3.0 f.L W. This is at least an order of magnitude better than the previously reported results. At higher frequencies, 22m W of 1627 -GHz laser power produced about 2. 5 p W of sideband output, while 3mW of 1839-GHz laser power generated about 100 nW of sideband radiation. The !ower efficiency at the higher frequencies is due primarily to the mismatch between the laser radiation and the fixed-length diode antenna. To demonstrate the tunability of the generated far-infrared radiation, the laser sidebands were swept through absorption lines ofHDO and H 2 CO near 600 and 800 GHz. The absorption signals were easily seen, using either video or lock-in detection techniques.