Detection of nitrogen dioxide with tunable multimode blue diode Lasers

A new spectroscopic method using a multimode tunable blue diode laser and direct absorption spectroscopy is developed to detect trace amount of nitrogen dioxide (NO₂) gas. The method produces distinctive signatures for NO₂ absorption that are utilized to accurately measure the gas concentration. The NO₂ signatures are obtained by measuring the transmission through a calibrated NO₂ gas sample as a function of laser injection current at a fixed laser temperature. These transmission curves are also calculated from the NO₂ absorption cross section and laser emission spectra. The measured and calculated transmission curves are found to agree reasonably well within 5%. In this study, two different blue diode lasers emitting around 445 nm are investigated. For both lasers, a detection limit of about 0.1 ppm can be achieved in a 40-cm long gas cell under standard atmospheric conditions.