Intracavity Faraday modulation spectroscopy (INFAMOS): A tool for radical detection.

We present the intra-cavity Faraday modulation spectroscopy technique, whereby optical feedback cavity-enhanced spectroscopy is coupled with Faraday modulation spectroscopy to greatly enhance the interaction path length of a laser beam with a paramagnetic sample in a magnetic field. We describe a first prototype based upon a cw quantum cascade laser targeting a selection of fundamental rovibrational R-branch transitions of nitric oxide (1890 cm-1 ), consisting of a linear cavity (finesse F=6300) and a water-cooled solenoid. We demonstrate a minimum detectable Verdet constant of Vmin =4.7×10-14  rad cm-1  G-1  Hz-1/2 (at SNR = 1), corresponding to a single-pass rotation angle of 1.6×10-10  rad Hz-1/2 and a limit of detection of 0.21 ppbv Hz-1/2 NO.