Near shot noise detection of oxygen in the A-band with vertical-cavity surface-emitting lasers

A dual-beam laser absorption spectrometer with balanced detection for high sensitivity detection of oxygen via the A-band at 760 nm is described. The 2 x 2 vertical-cavity surface-emitting laser arrays used for this set-up are characterized by their wavelength tuning behavior with temperature and current, amplitude modulation, side-mode suppression ratio and polarization contrast. The spectrometer performance is determined over time periods of up to 10 h using the variation in the differential absorption between two beam paths. With the R1IR11 line at STP, 670 muW laser power and 200 mm-long absorption cells, we realized an excellent linearity (R = 0.9999) and over a 5-min interval a record sensitivity for VCSEL-based spectrometers of 35 ppmV, corresponding to an optical density (O.D.) of 7 x 10(-7). For this specific set-up, this sensitivity is only a factor of 2.7 above the shot noise limit, giving us a normalized detection limit of 7.6 ppmV . m . root mW(-1) . s. Over a 10-h interval we achieved a standard deviation of 65 ppmV.