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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.