Doppler-free nonlinear absorption in ethylene by use of continuous-wave cavity ringdown spectroscopy

We report what we believe to be the first systematic study of Doppler-free, nonlinear absorption by use of cavity ringdown spectroscopy. We have devel oped a variant of cavity ringdown spectroscopy for the mid-infrared region between 9 and 11 mu m, exploiting the intracavity power buildup that is pos sible with continuous-wave lasers. The infrared source consists of a contin uous-wave CO2 laser with 1-mW tunable infrared sidebands that couple into a high-finesse stable resonator. me tune the sideband frequencies to observe a saturated. Doppler-free Lamb dip in the v(7), 11(1,10) <-- 11(2,10) rovi brational transition of ethylene (C2H4). Power studies of the Lamb dip are presented to examine the intracavity effects of saturation on the Lamb-dip linewidth, the peak depth, and the broadband absorption. (C) 2000 Optical S ociety of America. OCIS codes: 230.5750, 300.6190, 300.6320, 300.6340, 300. 6420, 300.6460.