Simple Real Time Trace Nitrogen Dioxide Detector Based on Continuous-Wave Cavity Ringdown Spectroscopy Using Passively Locked External Cavity Diode Laser

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We report on a compact trace nitrogen dioxide (NO2) detector based on cavity ringdown spectroscopy (CRDS), using a passively locked external cavity diode laser (PLEC-DL) in the visible wavelength region. Tailoring the spatial mode matching conditions of an antireflection (AR)-coated diode laser with a high finesse external cavity for effective optical feedback into the DL allows for stable resonant enhancement in the external cavity without any electric feedback apparatus. The external cavity having a finesse in excess of $1.4\times 10^{5}$ acts as a ringdown cavity with an effective optical volume (i.e., detection volume) of less than 1 cm3. We successfully demonstrated a noise equivalent absorption loss of $1.4\times 10^{-8}$ cm-1, corresponding to a detection limit of 40 ppb NO2 concentration with a very compact configuration. For differential absorption CRDS, we realized a dual wavelength-locked PLEC-DL in an external cavity simultaneously. Measurements of various NO2 concentrations in N2 based on differential absorption CRDS were performed.
2008-08-25