Abstract

We report the application of a cw distributed feedback quantum cascade laser to Lamb-dip spectroscopy of $\mathrm{C}{\mathrm{O}}_2$ at $4.3\mu \mathrm{m}$. With the laser operating in the free-running mode, we observed the sub-Doppler profile of the $P\left(28\right)$ line of the $(0,1^1,0)\to (0,1^1,1)$ 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 $3.4\mathrm{MHz}$ (FWHM) for an observation time of $\sim 200\mathrm{ms}$.

© 2006 Optical Society of America

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