Linear optical-feedback cavity ring-down spectroscopy using an interband cascade laser at 3-µm wavelength

We report on a linear optical-feedback cavity ring-down spectrometer (LOF-CRDS) operating under ultra-high vacuum (UHV) conditions, designed for high-sensitivity detection of light hydrocarbons. The system employs an interband cascade laser (ICL) operating at 3 µm, in coincidence with the strong absorption features of the asymmetric C-H stretching vibrational mode. Taking advantage of the passive optical feedback from the high-finesse linear cavity, the ICL emission linewidth is drastically narrowed, thus facilitating light coupling into the cavity. An empty-cavity ring-down time of about 18 µs was measured, which translates into a cavity finesse of ∼37000 and a cavity-mode width of ∼9 kHz. As a result of an Allan-Werle analysis, performed at a fixed ICL frequency, we determined a minimum detectable absorption coefficient of ∼2.5×10^-10 cm^-1 for an integration time of approximately 100 s. The spectrometer was tested on the R(25) component of the acetylene ν₃ vibrational band at 3352.2874 cm^-1. In this regard, we report on spectroscopic measurements of the C₂H₂ partial pressure in the UHV chamber. For the selected line, the sensitivity leads to a limit of detection of 2.1×10^-8 mbar.