Abstract

AlGaAs semiconductor diode lasers with short external cavities (SXC) have been used for high sensitivity detection of O₂ in the near infrared. The mode control provided by the SXC enhances the performance of diode lasers by forcing lasing on any one of a number (up to 10) of the laser modes. The lasing mode is selected by altering the length of the SXC and the selected mode can be continuously scanned over extended frequency intervals. This provides nearly complete spectral coverage in discrete segments of up to ~40 cm⁻¹ at constant laser heat sink temperatures. 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 15-h (overnight) durations with a rms uncertainty of ±6 × 10⁻⁵.

© 1989 Optical Society of America