Sensitive CH₄ detection applying quantum cascade laser based optical feedback cavity-enhanced absorption spectroscopy

We report on sensitive detection of atmospheric methane employing quantum cascade laser based optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS). An instrument has been built utilizing a continuous-wave distributed feedback quantum cascade laser (cw-QCL) with a V-shaped cavity, a common arrangement that reduces feedback to the laser from non-resonant reflections. The spectrometer has a noise equivalent absorption coefficient of 3.6 × 10^-9 cm^-1 Hz^-1/² for a spectral scan of CH₄ at 7.39 μm. From an Allan-Werle analysis a detection limit of 39 parts per trillion of CH₄ at atmospheric pressure within 50 s acquisition time was found.