Open path trace gas measurements using a pulse quantum cascade laser

Abstract

We report the application of a pulsed distributed feedback (DFB) quantum cascade laser (QCL) for a 6 km long open path spectroscopic monitoring of ozone, ammonia, water vapor and carbon dioxide. The thermal chirp during a 200 ns long excitation pulse is used for fast wavelength scanning of about 1 cm^-1 in the spectral range 1043-1049 cm^-1. This tuning method has the advantage of not being affected by the atmospheric turbulence, which is essential for long open path measurements. The intrinsic haze immunity of mid IR laser sources is an additional important advantage of mid-IR open path spectroscopy, compared with standard UV-visible DOAS. The third major advantage of the method reported is the possibility to measure many more organic and inorganic atmospheric species compared to the UV-visible DOAS. The obtained sensitivity for ozone and ammonia of the order of 10 ppm.m retrieved from the absorption spectra for averaging times less than 1 min are comparable with teh UV DOAS values. The open path of 6 km is covered using average laser power of less than 0.2 mW, which shows much higher efficiency of spectroscopy using narrowband laser source, compared to broadband light as Xe lamp.