PNNL

Abstract

An innovative spectroscopic system based on an external cavity quantum cascade laser (EC-QCL) coupled with a mid-infrared (mid-IR) fiber and quartz enhanced photoacoustic spectroscopy (QEPAS) is described. SF6 has been selected as a target gas in demonstration of the system for trace gas sensing. Single mode laser delivery through the prongs of the quartz tuning fork has been obtained employing a hollow waveguide fiber with inner silver–silver iodine (Ag–AgI) coatings and internal core diameter of 300 lm. A detailed design and realization of the QCL fiber coupling and output collimator system allowed almost practically all (99.4 %) of the laser beam to be transmitted through the spectrophone module. The achieved sensitivity of the system is 50 parts per trillion in 1 s, corresponding to a record for QEPAS normalized noise-equivalent absorption 2.7 9 10-10 W cm-1 Hz-1/2.