*1.2. Simultaneous Detection of Multiple Species at Atmospheric Pressure*

A wavelength-multiplexing scheme that combines multiple lasers onto a single path through the test gases and then separates the lasers by wavelength was exploited very early in the development of laser absorption sensing of combustion gases in the harsh environment of a pulse detonation engine [17]. Diffraction gratings were used to combine multiple lasers onto a common measurement line-of-sight and to demultiplex the wavelengths for individual detection. This approach enables a continuous and simultaneous acquisition of multiple wavelengths. However, this early sensor relied on negligible interference absorption in the ambient air in the optical paths between sensor and engine and between engine and detectors. For the sensor designed here, the light from four lasers (wavelengths spanning from 7185.59 cm−<sup>1</sup> (1.3917 μm) in the NIR to 2059.91 cm−<sup>1</sup> (4.8546 μm) in the MIR) is multiplexed onto a common beam path and then coupled into an optical fiber (single mode in the MIR) in a nitrogen-purged enclosure, which allows the light to be delivered to the combustion rig without background absorption of the hot, humid ambient air surrounding the test facility. To our knowledge, wavelength-multiplexing systems using fiber delivery have not previously spanned such a wide range (NIR–MIR) of laser wavelengths (although relevant work is described below using a hollow-core fiber bundle as an alternative solution).

Other lasers are also available in the MIR; for example, using DFB and interband cascade (IC) lasers (not operated simultaneously), Wei et al. [18] were able to measure temperature, CO concentration (via two CO transitions at 2008.53 and 2006.78 cm−1, respectively), and CO2 concentration (via probing a CO2 transition at 2384.19 cm<sup>−</sup>1) in an atmospheric-pressure pilot-stabilized C2H4/air jet flame. Profiles of these scalars were

obtained by translating the sender/receiver units as a whole—and thus the analysis beam horizontally and vertically through the fixed burner flame.

Nau et al. [19] also used simultaneously a DFB (2.3 μm) and an IC laser (3.1 μm) in a fiber-based absorption spectrometer to measure temperature and concentrations of CO, CH4, C2H2, and H2O in an entrained flow gasifier. A wavelength division multiplexing approach using a single ZrF4 fiber was used to measure both wavelength regions simultaneously. Weng et al. [20] recently demonstrated simultaneous detection of multiple species (i.e., hydrogen cyanide (HCN), acetylene, and water) by scanning the water vapor spectra using a single NIR laser (1.5 μm). Highly detailed water spectra modelling based on the measured absorption spectra of H2O at different temperatures allowed quantification of the small overtone and combinations bands of HCN and acetylene in the same spectral region.
