Optical-feedback cavity-enhanced absorption spectroscopy for OH radical detection at 2.8 µm using a DFB diode laser

We report the development of an optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS) instrument for OH detection at 2.8 μm using a DFB diode laser. Two different approaches, symmetry analysis and wavelength modulation, were performed to achieve laser frequency locking to the cavity mode. Compared with the symmetry analysis method, the wavelength modulation method continuously locked the laser frequency to the cavity mode and eliminated decoupling the laser from the cavity mode. A detection sensitivity of 1.7×10^-9 cm^-1 was achieved in a 25 s sampling time and was about 3 times better than that of the symmetry analysis method. The corresponding OH detection limit was ∼ 2×10⁸ molecule/cm³. Further improvement can be achieved by using higher reflectivity mirrors and other high-sensitivity approaches, such as frequency modulation spectroscopy and Faraday rotation spectroscopy.