Abstract

The sensitivity of mid-IR quantum cascade laser (QCL) off-axis cavity-enhanced absorption spectroscopy (CEAS), often limited by cavity mode structure and diffraction losses, was enhanced by applying a broadband RF noise to the laser current. A pump–probe measurement demonstrated that the addition of bandwidth-limited white noise effectively increased the laser linewidth, thereby reducing mode structure associated with CEAS. The broadband noise source offers a more sensitive, more robust alternative to applying single-frequency noise to the laser. Analysis of CEAS measurements of a ${\mathrm{CO}}_2$ absorption feature at $1890{\mathrm{cm}}^{-1}$ averaged over 100 ms yielded a minimum detectable absorption of $5.5\times {10}^{-3}{\mathrm{Hz}}^{-1/2}$ in the presence of broadband RF perturbation, nearly a tenfold improvement over the unperturbed regime. The short acquisition time makes this technique suitable for breath applications requiring breath-by-breath gas concentration information.

© 2014 Optical Society of America

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