High-resolution HO2 radical detection by optical feedback linear cavity-enhanced absorption spectroscopy

We report the development of an optical feedback linear cavity-enhanced absorption spectroscopy instrument for HO₂ detection using a distributed feedback (DFB) diode laser operating at 1506 nm. A direct and accurate method of reflectivity measurement based on the analysis of cavity mode signals was proposed. A differential circuit was used to judge the zero crossing point of the optical feedback cavity mode in the center of the frequency locking region, and shift the laser operating current to the non-resonant region. In this way, a ring-down signal was obtained with a time of 17.9 μs, corresponding to an effective absorption pathlength of 5.37 km. Combining the standing wave condition, the relationship between cavity length and drive voltage of the PZT mounted on the cavity rear mirror is translated into a correlation between the transmitted light wavenumber and the PZT voltage. The spectral resolution was improved from 290 MHz to 97 MHz by precisely tuning the PZT voltage. The achieved detection sensitivity of the system was 7 × 10^− 10 cm^− 1 with a data acquisition time of 10.6 s. The absorption spectrum of HO₂ at 6638.205 cm^− 1 was measured at a cell pressure of 50 mbar with a detection limit of 3.24 × 10⁹ molecule/cm³.