Trace gas measurement using IR laser diodes

The properties of semiconductor laser diodes for trace gas measurements are reviewed. First, the modulation characteristics of these lasers are discussed. Then, two sensitive spectroscopic methods using a frequency-modulated near-IR laser are presented and their performance compared. In the λ = 2 μm range, second-harmonic optical detection has shown a 20 times better detection limit than photoacoustic detection. The extension of these methods to longer wavelengths is finally discussed.