Detection of trace amount of Nitrogen Dioxide gas in the atmosphere using tunable blue laser diode

What is it about?

Nitrogen dioxide, one of the main air pollutants, has strong light absorption cross section in the blue region of the optical spectrum. Recent availability of blue laser diodes provides possibility of detecting NO2 in open-air paths with very low detection limits. However, in the blue region, the sharp features of the NO2 spectrum is relatively broad with typical width of tenths of nanometer. This poses a serious challenge for implementing traditional direct or wave-modulated tunable diode laser absorption spectroscopy. In this study, we report the usage of ablue laser diode with multi longitudinal modes tuned over about one nanometer in detecting trace amount of NO2gas. Details of the setup and its optimization will be presented along with a comparison of its performance withother NO2 detection optical methods.

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Why is it important?

In this work, we employ a blue laser diode in a laboratory setting to study the detection of trace amount of NO2 using single-pass absorption. Unlike 17,18, where absorption at only a single wavelength is utilized, here we use absorption over a wavelength range of about 2 nm to positively identify NO2 and to eliminate the possibility of misidentifying absorption from NO2 with absorption from other interferers such as dirt or dust. Moreover, we are using a regular multi-longitudinal laser diode, instead of an expensive external cavity diode laser; hence, making our system is much cheaper and more robust.