Sensitive shock tube measurements using multipass laser absorption spectroscopy

We report a sensitive laser absorption diagnostic method for shock tube experiments using a multipass configuration with a single-line spot pattern. The multipass setup consists of two silver-coated concave mirrors, which are placed outside the optical windows of the shock tube. The multipass configuration increases the effective path length by a factor of 31. All the reflected beams are aligned within a single plane that is perpendicular to the propagation direction of the shock wave, mitigating the Schlieren and beam steering effects. We validated the approach by measuring shock-heated gases behind reflected shock waves using tunable laser absorption spectroscopy of water vapor at 7447.48 cm^{− 1}. Our approach enables sensitive species detection for high-temperature shock tube/laser absorption experiments.