Open-Path Quantum Cascade Measurement System

In the field of environmental monitoring there is a need for highly sensitive open-path systems to detect in real time parts per billion concentration of gases in ambient air that are adverse to human health such as ozone and ammonia. As room temperature mid-infrared quantum cascade laser (QCL) sources have improved, field systems with QCL's are predicted to become a viable alternative to traditional open-path Fourier transform infrared spectrometers (FTIR). This is because the high spectral brightness of the laser source coupled with it's high spectral resolution makes it a promising candidate in such deployments. To evaluate this claim we present a model of the performance of a realistic system based on available mid-infrared technologies as well as preliminary Herriot multi-pass cell measurements of ambient water vapor and ammonia. We modeled a fieldable open-path quantum cascade laser (QCL) system that uses a retro-reflector to measure ambient ozone (20 ppb) and ammonia (2 ppb) concentrations to accuracies of up to 1 % at ranges from 5 m to 5 km. The transmitter was based on the “intra-pulse” tuning of a single distributed-feedback (DFB) laser with an optimal spectral window of 1045 – 1047 cm-1. We also present our multi-pass cell data of water and ammonia absorption in this region. Finally, we discuss the design and construction of our QCL based open-path system based on a Newtonian telescope platform.