Wavelength Modulation Spectroscopy
Arup Lal Chakraborty and Anirban Roy
Abstract This 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.
1 Introduction to Wavelength Modulation Spectroscopy
Tunable diode laser spectroscopy has firmly established itself as a leading technology in the field of high-sensitivity gas sensing. The technique derives its name from
the tunable diode laser, which is at the heart of the technique. Wavelength modulation spectroscopy has been key to transforming the the basic TDLS system into
ultra-high sensitivity portable systems that are widely used to detect multiple gases
under challenging conditions. The extremely low cross-sensitivity of WMS makes it
possible to identify various components in a gas mixture and to extract gas parameters such as mole fraction, pressure, temperature and isotopic ratio. The chapter first
discusses the basic spectroscopic aspects and the direct detection technique to lay
the foundation. The details of WMS are described in detail next with specific focus
A. L. Chakraborty (B) · A. Roy
Indian Institute of Technology, Gandhinagar, Gandhinagar, Gujarat 382355, India
e-mail: arup@iitgn.ac.in
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science
and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_13
321
Arup Lal Chakraborty and Anirban Roy
Abstract This 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.
1 Introduction to Wavelength Modulation Spectroscopy
Tunable diode laser spectroscopy has firmly established itself as a leading technology in the field of high-sensitivity gas sensing. The technique derives its name from
the tunable diode laser, which is at the heart of the technique. Wavelength modulation spectroscopy has been key to transforming the the basic TDLS system into
ultra-high sensitivity portable systems that are widely used to detect multiple gases
under challenging conditions. The extremely low cross-sensitivity of WMS makes it
possible to identify various components in a gas mixture and to extract gas parameters such as mole fraction, pressure, temperature and isotopic ratio. The chapter first
discusses the basic spectroscopic aspects and the direct detection technique to lay
the foundation. The details of WMS are described in detail next with specific focus
A. L. Chakraborty (B) · A. Roy
Indian Institute of Technology, Gandhinagar, Gandhinagar, Gujarat 382355, India
e-mail: arup@iitgn.ac.in
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science
and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_13
321