Quantum-cascade-laser-based heterodyne phase-sensitive dispersion spectroscopy in the mid-IR range: capabilities and limitations

Heterodyne Phase Sensitive Dispersion Spectroscopy (HPSDS) is a new method for molecular dispersion spectroscopy that provides an output linearly dependent on the concentration of gas, inherent baseline and normalization-free operation and an extended dynamic range in comparison with absorption-based spectroscopic methods. Besides this, HPSDS provides capabilities for the implementation and deployment of gas analyzers without any need for calibration and all data processing and concentration retrieval procedures are straight forward. HPSDS is based on the measurement of the change in the refractive index of the gas under study in the vicinity of the molecular resonances of interest, and most of the characteristics of the method come from the fact that this change in the refractive index is directly proportional to the concentration of gas. Experimental demonstrations of HPSDS have already been performed in the 1.5 mu m optical range, where it is possible to take advantage of high-speed optical intensity modulators and optoelectronics. Here, we present a HPSDS system operating in the Mid-Infrared based on a directly modulated Quantum Cascade Laser (QCL). This instrument has been experimentally validated through the measurement of the concentration of atmospheric carbon monoxide. Taking also advantage of the study of the performance of the HPSDS system that was preformed, the main capabilities and also current limitations of the method are discussed.