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

We report on a compact trace nitrogen dioxide (NO) 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 x 10(5) acts as a ringdown cavity with an effective optical volume (i.e., detection volume) of less than 1 cm(3). We successfully demonstrated a noise equivalent absorption loss of 1.4 x 10(-8)cm(-1), corresponding to a detection limit of 40 ppb NO(2) 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 NO(2) concentrations in N(2) based on differential absorption CRDS were performed.