Demonstration of calibration-free WMS measurement of gas parameters with in-situ real-time characterization of laser parameters using cw-DFB-QCL, VCSEL and DFB lasers

This paper presents a detailed experimental and theoretical study of a new calibration-free nth harmonic wavelength modulation spectroscopy approach and demonstrates that it is applicable to various types of semiconductor lasers that are widely used in near-infrared and mid-infrared tunable diode laser spectroscopy. A 5250 nm continuous wave distributed feedback quantum cascade laser is used to extract the concentration and pressure values of nitric oxide using 2f WMS technique under controlled conditions. The applicability of the technique to different types of current tuned semiconductor lasers is demonstrated by extending it to a 2004 nm vertical cavity surface emitting laser and a 1650 nm distributed feedback edge-emitting laser to extract gas parameters of carbon dioxide and methane respectively. The generality of the technique is demonstrated by extending it to third harmonic detection for the three different kinds of lasers used in this study. The methodology required to provide in-situ realtime measurements of both gas parameters and operating characteristics of the laser is described in detail. Finally, the advantages and limitations of the technique are discussed in view of the fact that the characteristic behaviour of the laser sources is somewhat different.We specifically discuss the issue of targeting non-absorbing wavelength regions and the choice of modulation frequency and modulation amplitude of the laser as well as the choice of the detection harmonic.