Recently, based on optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS) technology, a research team led by Prof. ZHANG Weijun from Hefei Institutes of Physical Science (HFIPS), Chinese Academy of Sciences (CAS) successfully detected OH radicals at 2.8 μm wavelength with a distributed feedback (DFB) diode laser.
The results have been published in Optics Express.
OH free radicals are the most important oxidant in the atmosphere. The rapid circulation reaction determines the production and removal of the main pollutants in the atmosphere. The accurate measurements for OH radicals are very difficult due to their characteristics of the high reaction activity, short life, and low concentration in the atmosphere. It is important and challenging research in the field of atmospheric chemistry today.
"This research provides a new direct detection method for OH radicals," said YANG Nana, first author of the paper.
She further explained the OF-CEAS technology. OF-CEAS used the resonant light of the cavity to feed back to the laser, which could effectively narrow the laser linewidth. Besides, it could achieve optical self-locking to improve the coupling efficiency of the laser and the cavity and achieve high-sensitivity detection.
In this research, the team 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, therefore, achieved real-time phase locking. The detection sensitivity was 1.7×10-9 cm-1 with an effective pathlength of 800 m corresponding to the detection limit of ~2×108 molecule/cm3 for OH radicals.
Combined with Faraday Rotation Spectroscopy (FRS) and Frequency Modulation Spectroscopy (FMS), OF-CEAS can provide a new and higher sensitivity approach for direct detection of atmospheric OH radicals.
This research was supported by the National Natural Science Foundation of China, the Second Tibetan Plateau Scientific Expedition and Research program, the Youth Innovation Promotion Association CAS, and the HFIPS Director's Fund.
Schematic diagram of optical-feedback cavity-enhanced absorption spectroscopy system. (Image by YANG Nana)
On the left is the cavity transmission signal as a function of time recorded by applying a linear current ramp to the diode laser injection current. On the right is a zoomed-in profile showing the cavity mode. The dotted line indicates the peak position, dividing the cavity mode into left (A) and right (B) parts. (Image by YANG Nana)
