Wysocki, G.; Wang, Y. BROADBAND HIGH-RESOLUTION SPECTROSCOPY WITH FABRY-PEROT QUANTUM CASCADE LASERS. Proceedings of the International Symposium on Molecular Spectroscopy, Urbana, IL, June 16-21, 2014.

Simultaneous spectroscopic detection of large molecules with broad ro-vibrational spectra, and small molecules with well-resolved narrow spectral lines requires both broadband optical frequency coverage ($>$50 \wn) and high resolution ($<$0.01 \wn) to perform accurate spectral measurements. With the advent of room temperature, high power, continuous wave quantum cascade lasers (QCLs), high resolution mid-IR spectrometers for field applications became feasible. So far to address the broadband spectral coverage, external cavity (EC) QCLs with $>$100 \wn tuning ranges have been spectroscopic sources of choice in the mid-IR; however EC-QCLs are rather complex opto-mechanical systems, which are vibration-sensitive, and construction of robust transportable systems is difficult.

In this work we present a new method of performing broadband mid-IR spectroscopy using two free-running Fabry-Perot (FP) QCLs to perform multi-heterodyne down-conversion of optical signals to RF domain. The sample transmission spectrum probed by one multi-mode FP-QCL is down-converted to the RF domain through an optical multi-heterodyne process using a second FP-QCL as the local oscillator\footnote{Y. Wang, M. G. Soskind, W. Wang, and G. Wysocki, "High-resolution multi-heterodyne spectroscopy based on Fabry-Perot quantum cascade lasers," Appl Phys Lett 104, 0311141-0311145 (2014)}. Both a broadband multi-mode spectral measurement as well as high-resolution ($~$15 MHz or $~$0.0005 \wn) absorption spectroscopy of NH$_{3}$ and N$_{2}$O are demonstrated and show potential for all-solid-state FP-laser-based spectrometers for chemical sensing.