High-sensitivity cavity ring-down spectroscopy for hydroperoxyl radical measurement based on a tunable narrow-linewidth laser using linear cavity optical feedback

The hydroperoxyl radical (HO₂) is an important oxidant, playing a crucial role in atmospheric chemistry processes such as free radical cycling and ozone formation. However, the direct measurement of HO₂ using laser absorption techniques presents significant challenges due to its low atmospheric concentration, high reactivity, and short lifetime. In this paper, a high-sensitivity narrow-linewidth cavity ring-down spectroscopy (CRDS) system for HO₂ detection was developed. By employing dual locking of the feedback phase and laser current to the resonant cavity, the probe laser linewidth was successfully narrowed from 2.3 MHz to 12.3 kHz, and the frequency noise was reduced by 4 to 5 orders of magnitude. Through cavity length adjustments, a continuous laser frequency tuning range of 4.5 GHz was achieved. This narrow linewidth laser significantly heightened the trigger efficiency of the CRDS system. Allan deviation analysis at a fixed laser frequency demonstrated that an improved sensitivity of 3.24 × 10^-11 cm^-1 could be obtained with an averaging time of 0.48 s, which corresponds to an HO₂ detection limit of 1.29 × 10⁸ molecule per cm³. The developed system features a simple design and provides a valuable technical reference for future research on direct HO₂ measurements.