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We review the progress of multi-wavelength distributed feedback (DFB) quantum cascade lasers (QCLs) as sources for broadband mid-infrared (mid-IR) spectroscopy. While it is possible to tune the emission wavelength of DFB QCLs in a frequency range of about 5 cm by varying the operating temperature and driving current, continuous, deterministic and mode-hop free tuning remains challenging. To use these laser sources for broadband spectroscopy applications and to exploit their narrow linewidth, the equalization of the power across the multicolor spectrum is another requirement. Various approaches are reviewed: conventional DFB QCL arrays, surface emitting DFB QCL arrays, on-chip beam combined DFB arrays, multi-channel Vernier-based switching DFBs, and dual-wavelength DFBs. The results are presented to highlight the applicability of these sources for broadband and high-resolution trace gas spectroscopy. The results for multi-species trace-gas spectroscopy using dual-wavelength QCLs are presented in more detail, where the best precision as determined from Allan-Werle plots was obtained for measurement of NH3 with 0.02 parts per billion (ppb) at 100 s integration time, and typical values for the other trace gases of the order of 0.1 ppb. These results indicate that spectrometers using dual-wavelength QCLs can serve as an all-in-one solution for high precision gas spectroscopy.

Multi-wavelength distributed feedback quantum cascade lasers for broadband trace gas spectroscopy