Frequency locking of an extended-cavity quantum cascade laser to a frequency comb for precision mid infrared spectroscopy

Extended-cavity quantum cascade lasers (EC-QCLs) enable mode-hope-free frequency sweeps in the mid-infrared region over ranges in excess of 100 cm
^-1
, at speeds up to 1 THz/s and with a 100-mW optical power level. This makes them ideally suited for broadband absorption spectroscopy and for the simultaneous detection of multiple gases. On the other hand, their use for precision spectroscopy has been hampered so far by a large amount of frequency noise, resulting in an optical linewidth of about 30 MHz over 50 ms [1]. This is one of the reasons why neither their frequency nor their phase have been so far locked to a frequency comb. Their use in combination with frequency combs has been performed in an open loop regime only [2], which has the merit of preserving the inherently fast modulation speed of these lasers, yet not to afford high spectral resolution and accuracy.