Detailed analysis of the R1f/1I1 WMS technique and demonstration of significantly higher detection sensitivity compared to 2f WMS for calibration-free trace gas sensing

Recognizing that wavelength modulation spectroscopy (WMS) is particularly important in the development of high-sensitivity gas sensing systems, this paper presents a detailed analysis of the R<inf>1 f</inf>/1I<inf>1</inf> WMS technique that has recently been successfully demonstrated for calibration-free measurements of the parameters that support detecting multiple gases under challenging conditions. In this approach, the magnitude of the 1 f WMS signal (R<inf>1 f</inf> ) was normalized by using the laser's linear intensity modulation (1I<inf>1</inf>) to obtain the quantity R<inf>1 f</inf>/1I<inf>1</inf> that is shown to be unaffected by large variations in R<inf>1 f</inf> itself due to the variations in the intensity of the received light. In this paper, different simulations have been used to explain the approach taken and the advantages that it shows. A 40 mW, 1531.52 nm near-infrared distributed feedback (DFB) semiconductor laser was used to extract the mole fraction of acetylene in a single-pass configuration. The work has shown a detection sensitivity of 0.32 ppm for 28 cm (0.089 ppm-m) with an optimum integration time of 58 s. The detection limit achieved has been shown to be better than the value of 1.53 ppm (0.428 ppm-m) for R<inf>2 f</inf> WMS by a factor of 4.7, which is a significant improvement.