Detection of oxygen using short external cavity GaAs semiconductor diode lasers

Abstract

High sensitivity detection of O₂ in the near infrared with short external cavity (SXC) AlGaAs semiconductor diode lasers is reported. The mode control provided by the SXC enhances the performance of these diode lasers by allowing a number of single laser modes (up to ten) to be individually selected and continuously scanned over extended frequency intervals. At a constant laser heat sink temperature, this provides nearly complete spectral coverage of up to ≈40 cm⁻¹ for overlapping modes. When using second harmonic detection techniques, these SXC controlled lasers were found to provide high sensitivity detection of O₂ at atmospheric pressure. A minimum detectable absorbance of ≈4 × 10⁻⁶ has been achieved with a SNR of 1. The stability of these laser systems has also allowed the continuous monitoring of an absorption signal over extended time intervals. Absorbances of 1 × 10⁻² have been continuously monitored for durations of up to 15 h with an rms uncertainty of ±6 × 10⁻⁵. These results are reproducible for measurements made on all the laser modes that could be selected with the SXC.