Doppler-free saturated-absorption spectroscopy of CO2 at 4.3 mu m by means of a distributed feedback quantum cascade laser

We report the application of a cw distributed feedback quantum cascade laser to Lamb-dip spectroscopy of CO2 at 4.3 mu m. With the laser operating in the free-running mode, we observed the sub-Doppler profile of the P(28) line of the (0, 1(1), 0) -> (0, 1(1), 1) hot band by implementing a pump-probe scheme and using wavelength modulation spectroscopy for highly sensitive detection of saturated absorption signals. We investigated the main limitations to the observation of a narrow resonance, with particular attention to the effect of the laser current noise. We determined the intrinsic laser emission width, which was found to be approximately 3.4 MHz (FWHM) for an observation time of similar to 200 ms. (c) 2006 Optical Society of America.