Demonstration of non-absorbing interference rejection using wavelength modulation spectroscopy in high-pressure shock tubes

We experimentally demonstrate the non-absorbing interference rejection capabilities of wavelength modulation spectroscopy (WMS) speciation in shock tube experiments by directly comparing WMS measurements against direct-absorption spectroscopy (DA) measurements. The improved capability is demonstrated by probing the P(20) transition of the CO fundamental band using a quantum cascade laser in shock-heated mixtures of CO and N ₂ across a wide range of pressures between 3.5 and 18 atm. In the WMS measurements, the second harmonic (2f) served as the detection signal, while the first harmonic (1f) provided normalization to counteract intensity drift and fluctuations. These perturbations occur in shock tubes because of significant beam-steering noise and imperfect optical alignment when experiments are conducted at elevated pressures. The WMS detection system was evaluated at reflected shock pressures of 3.5 atm, 8.5 atm, and 18 atm, demonstrating improvement in signal-to-noise ratio over concurrent DA measurements. To the authors’ knowledge, this work represents the first direct experimental quantification of the intensity-fluctuation rejection capabilities of a WMS-based TDLAS sensor at high pressures.