Narrow Lamb Dip of 3.4 µm Band Transition of Methane with Difference Frequency Generation and Enhancement Cavity

We have developed a sub-Doppler-resolution spectrometer tunable in the 3.4 µm region on the basis of difference frequency generation and an enhancement cavity cell. The spectrometer has allowed us to record the Lamb dips of the ν 3 fundamental band of methane even with an IR power level of 3.9 µW, and the linewidth of the Lamb dips can be as narrow as 0.33 MHz. The spectrometer is also so sensitive that a Lamb dip of a less abundant isotopomer, 13 CH 4 , has been successfully observed.