Chakraborty, Arup LalArup LalChakrabortyRoy, AnirbanAnirbanRoy2025-08-312025-08-312021-01-0110.1007/978-981-33-6084-6_132-s2.0-85158841536https://d8.irins.org/handle/IITG2025/27153This chapter presents a detailed discussion of wavelength modulation spectroscopy (WMS), which has emerged as a leading technology in the development of high-sensitivity gas sensing systems that are widely used to detect multiple gases under challenging conditions. The first part of the chapter touches upon a few prominent application of WMS to set the discussion in context. A few basic aspects of molecular spectroscopy are then presented to lay the foundation. The theoretical framework of tunable diode laser spectroscopy is then introduced as a precursor to the more involved wavelength modulation spectroscopy techniques. This is followed by detailed mathematical discussions of 1f and 2f calibration-free WMS techniques that have played a key role in transforming tunable diode laser spectroscopy (TDLS) systems into ultra-high sensitivity systems. The chapter strives to emphasize the practical aspects of these systems by discussing the laser characteristics that have a significant bearing on the performance of systems. The chapter ends with a short discussion on quantifying the detection sensitivity.falseBook Chapter2363510X321-36220215chBook Series3