Scattered light modulation cancellation method for sub-ppb-level NO₂ detection in a LD-excited QEPAS system.

A sub-ppb-level nitrogen dioxide (NO₂) QEPAS sensor is developed by use of a cost-effective wide stripe laser diode (LD) emitting at 450 nm and a novel background noise suppression method called scattered light modulation cancellation method (SL-MOCAM). The SL-MOCAM is a variant of modulation spectroscopy using two light sources: excitation and balance light sources. The background noise caused by the stray light of the excitation light sources can be eliminated by exposing the QEPAS spectrophone to the modulated balance light. The noise in the LD-excited QEPAS system is investigated in detail and the results shows that > ~90% background noise can be effectively eliminated by the SL-MOCAM. For NO₂ detection, a 1σ detection limit of ~60 ppb is achieved for 1 s integration time and the detection limit can be improved to 0.6 ppb with an integration time of 360 s. Moreover, the SLMOCAM shows a remote working ability in the preliminary investigation.