Portable cavity ring-down spectrometer for an HO₂ radical measurement: instrument's performance and potential...

We report the development of a portable cavity ring-down spectrometer (CRDS) for direct and absolute measurement of HO₂ radical concentration using a distributed feedback (DFB) diode laser operating at 1506 nm. The spectrometer has a compact design with all optics in a 1000 × 400 × 140 mm³ box. At a pressure of 100 mbar and a ring-down time (τ₀) of 136 µs, the detection limit of the CRDS spectrometer was ∼ 7.3 × 10⁷ molecule/cm³ (1σ, 10s). The corresponding detection sensitivity was 1.5 × 10^-11 cm^-1, which was close to the state-of-the-art performance. By replacing the DFB diode laser with a narrow linewidth erbium-doped fiber (EDF) laser, the amplitude fluctuation caused by the laser phase noise was reduced and the cavity mode injection efficiency was improved. The sensitivity was improved to 3.9 × 10^-12 cm^-1 with a short data-acquisition time of 0.2 s. Compared with the DFB laser, the improvement was nearly an order of magnitude. The use of the narrow linewidth laser is attractive. The instrument can achieve very high sensitivity without the need for a complex locking technique, ensuring simple and ease of use in future field applications.