# Laser Absorption Sensing Systems: Challenges, Modeling, and Design Optimization
## 1 Introduction
### Scope and Organization of the Paper  
There has been a number of excellent reviews and books on the development and application of LAS sensor technology. Even as early as in the late 1990s, several papers reviewed the fundamental theories and early successes of laser-based spectroscopic methods in gas dynamics and combustion flow measurements [6; 7; 8]. [PERSON] et al. illustrated the evolution of and confinement methods used in semiconductor diode lasers and discussed different techniques used to convert free-running diode lasers into true narrow linewidth tunable diode laser sources [9]. As lasers were much less mature at that time, these papers also reviewed the available laser light sources as an important part of the diagnostics development. More recently, several reviews reported the advances of the LAS technique and its demonstration in various disciplines. [PERSON] et al. presented an overview of combustion kinetics, propulsion, and combustion in practical systems and documented the impact of quantitative laser diagnostics methods [10]. [PERSON] et al. highlighted the temperature, concentrations, and flow velocities in different combustion zones and emphasized the strategies and data processing algorithms for LAS measurements [11]. [PERSON] et al. provided a thorough review of the underlying fundamentals, design, and use of infrared LAS sensors for combustion gases and highlighted recent findings and some of the remaining measurement opportunities, challenges, and needs [12]. [PERSON] et al. summarized the key principles and recent advances of line-of-sight (LOS) LAS techniques and then focused on spatially resolved gas sensing with LAS tomography [13].  
More specifically, a few papers have focused on reviewing signal-to-noise-ratio (SNR) enhancement techniques to improve TDLAS performance. [PERSON] et al. summarized several commonly employed noise reduction schemes including signal averaging, modulation techniques, balanced detection, zero-background subtraction, and adaptive filtering [14]. [PERSON] et al. focused on the various mathematical algorithms applied in TDLAS signal processing to enhance the accuracy and resolution of the sensor [15].  
As discussed above, sensing challenges vary largely with measurement targets and conditions. The maturity of semiconductor lasers and photodetectors allows many researchers in relevant fields to simply use LAS sensor as an established tool, whereas diagnosticians still strive to push the limit of this technique. While previous papers have provided comprehensive reviews on all major elements of LAS sensor systems including fundamental molecular spectroscopy, the development of opto-electronic devices, and various measurement schemes and applications, we refer the readers to other works for the fundamentals and will address the design perspectives of a LAS sensing system. Section 2 will start by summarizing and identifying the measurement challenges and design targets under different measurement conditions; Section 3 will follow by outlining the fundamentals of different diagnostic strategies and their underlying models and control factors; Section 4 will then provide a systematic analysis of approaches for noise reduction and signal optimization, so to extend the use of the LAS sensing technique to much more difficult measurement scenarios. In view of the urging demands in both the use and development of this sensor technology, a goal of this manuscript is to provide understanding and an overview of the key factors and considerations in the measurement targets, as well as a reference and guidelines to a strategic design procedure appropriate for obtaining optimized sensing results.