A Review of Theory and Practical Considerations of Tunable Diode Laser Absorption Spectroscopy Diagnostics

TL;DR

This paper addresses the challenge of accurately diagnosing temperature, pressure, gas composition, and velocity in harsh environments using Tunable Diode Laser Absorption Spectroscopy (TDLAS). It combines a rigorous theoretical treatment of Beer–Lambert law, lineshape broadening (Lorentzian, Gaussian, and Voigt), and transition linestrength with a comprehensive development of wavelength-modulation spectroscopy (WMS), including calibration-free WMS and MHz-rate SWMS. A key contribution is the calibration-free WMS model that links laser tuning parameters to WMS harmonics, paired with practical sensor design guidelines, background subtraction, and hardware debugging, enabling high-speed, robust diagnostics. The work also provides concrete methods for temperature and pressure inference via two-line ratios, Doppler-based velocimetry, and uncertainty propagation, along with detailed strategies for line selection, nonuniformity handling, and post-processing algorithms for peak-picking SWMS. Overall, the paper offers a cohesive framework that merges theory, numerical modeling, and practical engineering to advance high-speed TDLAS sensing in combustion and propulsion applications, aided by open-source simulation tools and a focus on MHz-rate implementations enabled by bias-tee circuitry.

Abstract

Tunable Diode Laser Absorption Spectroscopy (TDLAS) has emerged as a versatile and reliable diagnostic tool for measuring temperature, pressure, gas composition, and velocity in power generation and propulsion systems. This paper provides a comprehensive review of TDLAS principles and practical considerations for sensor design and implementation. The discussion begins with a mathematical introduction to the theory of gas absorption including: lineshape modeling and broadening mechanisms, quantitative measurements and challenges, and practical line selection rules. The analysis progresses to wavelength-modulation spectroscopy (WMS), highlighting its advantages in noise rejection and robustness in harsh environments. Furthermore, the calibration-free WMS model and the connection between WMS harmonics and lineshape derivatives is derived. Quantitative measurements through use of multiple harmonics is discussed and challenges surrounding measurement rate are presented. The end of the discussion focuses on practical aspects regarding the implementation of scanned-WMS sensors including laser characterization, background subtraction, and hardware debugging.

Figures (15)

• Figure 1: A comparison of the Gaussian, Lorentzian and Voigt lineshape profiles with $\Delta\nu_{\mathrm{c}} = \Delta\nu_{\mathrm{d}}$. All lineshapes are scaled by 1/$\phi_{\mathrm{D}}(\nu_{\mathrm{o}})$.
• Figure 2: Simulated absorption spectrum of two overlapping transitions. The integrated absorbance of each transition is shaded.
• Figure 3: Voigt profile fitting procedure for inferring gas properties.
• Figure 4: Typically experimental set ups for velocity measurement.
• Figure 5: (a) Temperature sensitivity at various $\Delta E"$ values at combustion relevant temperatures. (b) Normalized linestrength function for H2O at various values of $E"$. The dashed line is at a normalized value of 3.