Laser absorption spectroscopy (LAS) diagnostics offer several advantages over traditional mechanical sensors. They provide time-resolved measurements of gas properties in harsh environments at MHz rates, are non-intrusive, and only require optical access. LAS sensors rely on experimental databases and models that correlate spectral absorbance, as defined by Beer-Lambert’s law, with the thermodynamic properties of the absorbing species.
When the injection current of the laser diode is driven by a waveform generator, the wavelength of the diode is tuned across a specific range while following the waveform. This enables the acquisition of spectral information, which is then used to extract gas properties. By analyzing the two-color ratio of integrated absorbances, temperature can be determined. From there, the partial pressure of the gas can be inferred using the definition of integrated absorbance along with the newly determined temperature.
Scanned-Wavelength-Modulation Spectroscopy
Scanned-Wavelength Modulation Spectroscopy (SWMS) is an LAS technique in which the wavelength of the laser diode is tuned across a molecular transition while simultaneously being rapidly modulated. This modulation encodes absorption information into the harmonics of the modulation frequencies and their sidebands. SWMS signals are then extracted using digital lock-in amplifiers, which reject noise outside the filter’s passband. This technique improves the signal-to-noise ratio (SNR) compared to direct absorption methods and allows for frequency-multiplexing the light from two laser diodes on to the same fiber.
However, some disadvantages of this method include the more complex post-processing techniques, which rely on characterizing the laser to simulate signals. Measured signals are then compared with simulated signals to infer gas properties.
In our lab, SWMS has been used to measure gas properties behind incident-shock waves generated in our MHExT facility and at the exit throat of a rotating detonation engine at a measurement rate of 1 MHz.
Publications
Guerrero, Jose, and Mirko Gamba. “Post-Incident Shock Wave Measurements of Gas Properties at 1 MHz Using Scanned-Wavelength-Modulation Spectroscopy.” AIAA SCITECH 2025 Forum. 2025.
Guerrero, Jose, and Mirko Gamba. “Combustion Efficiency Measurements in a Rotating Detonation Engine Using Scanned-Wavelength-Modulation Spectroscopy.” AIAA SCITECH 2025 Forum. 2025.
Guerrero, Jose, and Mirko Gamba. “Synthetic LAS Measurements of Combustion Efficiency in a Rotating Detonation Engine Using 3D-DNS Data.” AIAA SCITECH 2025 Forum. 2025.