A research team led by Prof. Zhang Weijun from the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS) has recently detected hydroxyl radical (OH) at 2.8 μm wavelength with a distributed feedback diode laser, based on optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS) technology. The study was published in Optics Express.
OH free radicals are the most important oxidant in the atmosphere. Rapid circulation reaction determines the production and removal of major pollutants in the atmosphere. Accurate measurements for OH radicals are very difficult due to their high reactivity, short life and low concentration in the atmosphere. It is an important and challenging research topic in the field of atmospheric chemistry today.
Scientists explained that OF-CEAS used resonant light of the cavity to feedback to the laser. It could effectively narrow the laser linewidth. It could realize optical self-locking to improve the coupling efficiency of the laser and the cavity and achieve high-sensitivity detection.
Scientists used the wavelength modulation method to control the optical phase. They used the 1f signal of the cavity mode demodulated by the lock-in amplifier as an error signal and sent it to the Proportional integral differential servo controller to control the distance from the laser to the cavity. The system achieved real-time phase locking. The detection sensitivity was about three times better than that of the symmetry analysis method.
OF-CEAS can provide a new and higher sensitivity approach for direct detection of atmospheric OH radicals when combined with Faraday Rotation Spectroscopy and Frequency Modulation Spectroscopy.