Wavelength-Modulation Spectroscopy
Molecules can absorb photons at certain wavelengths which causes them to transition from a lower energy level to an upper energy level. The amount of light that is absorbed at a given wavelength is proportional to the fraction of molecules in the absorbing quantum state. Laser-absorption spectroscopy (LAS) exploits this relationship to provide quantitative measurements of gas temperature and composition. Wavelength-modulation spectroscopy (WMS) is a LAS technique that is used to provide improved measurements of gas conditions in hostile environments, such as detonation engines. Professor Goldenstein has invented several WMS techniques which have been adopted by researchers around the world, and we are continually working to improve the measurement capabilities and quality of WMS-based diagnostics. The figure below illustrates the basic operating principles of WMS sensors.
The laser wavelength is modulated at frequency fm to embed discrete frequencies in the detector signal while the nominal wavelength of the laser is scanned across a molecule’s absorption transition. The modulation shifts absorption information to harmonics of fm which can be extracted via digital lock-in filters during post-processing, and the wavelength scanning enables spectra of WMS signals to be measured. The WMS signals are then used to calculate gas conditions using calibration-free WMS models.
WMS exhibits several noise-rejection benefits which we leverage to enable study of hostile combustion environments. For example, the figures below illustrate WMS-based measurements of temperature and H2O that were acquired at rates from 10 to 50 kHz in several detonation engines by Professor Goldenstein and his colleagues at Stanford University. Currently we are using similar techniques to characterize post-detonation fireballs that are used to eradicate biological weapons.
More information regarding these diagnostics and results can be found here for the pulsed-detonation engine and here for the rotating-detonation engine.