# Laser Absorption Sensing Systems: Challenges, Modeling, and Design Optimization
## 2 Sensor Design Targets and Measurement Challenges
For the purpose of making a direct comparison between different studies, the reported absolute minimal detection limits for several commonly probed species in combustion, including CO, CO\({}_{2}\), H\({}_{2}\)O, and C\({}_{2}\)H\({}_{2}\), are plotted on the same diagram against detection bandwidths. It can be clearly observed that results from different applications naturally separate into different regimes: the well-controlled laboratory environments allow lower absorbance to be detected, where chemical kinetics studies in shock tubes typically demand for a much higher detection bandwidth than stabilized laboratory flames or gas cell; larger scale, more \"realistic\" environments such as industrial burners and engines present harsher conditions that lead to higher achievable minimal detection limits, but industrial process monitoring typically involves stringent emission control targets and trace species detection over continuous, long-term monitoring periods, whereas engine diagnostics are usually associated with high-velocity or turbulent flow and reaction conditions. Figure 1: Schematic of different noise processes on an Allan deviation plot. Figure 2: Representative detection limits and measurement bandwidths of laser absorption sensors in various combustion-related systems. Despite its versatility in measuring temperature, pressure, and velocity, LAS is ultimately a species diagnostic method. It utilizes the \"fingerprint\" molecular spectra and infer the parameters to be measured on the basis of a correct interpretation of the recorded spectral feature. Figure 3 plots the absorption linestrengths over the infrared wavelength range of 0.5-6 um of several of the representative C-O, C-H, N-O, N-H, and H-O compound gaseous species commonly studied in combustion. Today, with semiconductor laser technology, we are able to access almost any wavelength within this region, so that there is no longer much constraint in making an optimized selection of absorption transition, which can be solely based on appropriate strength level and minimal cross interference. For fairly harmonic molecules such as CO, the advantage of using a lower-order vibrational band at longer wavelength is more prominent in achieving lower detection limits as compared with a less harmonic molecule such as NH\({}_{3}\). One can also make the observation that high temperatures, often associated with combustion-related sensing, would by themselves present greater challenges for LAS sensors. The reasoning is twofold from the point of view of molecular spectroscopy: (1) the overall band strength is approximately inversely proportional to temperature, so that absorption is generally weaker at high temperatures, and (2) higher energy levels get more populated at higher temperatures, so that the spectra may get more crowded with hot bands and high J-number lines, especially when a multicomponent gas mixture is being studied. As will be reviewed in the following, many efforts have been devoted to reducing a correct absorption-free background and discriminate the molecular absorption by target species from a medley of various cross interference and noise processes.