Laser‐Induced Plasmas for Broadband Cavity‐Enhanced Absorption Spectroscopy

Abstract

The combination of cavity‐enhanced absorption spectroscopy (CEAS) with the phenomenon of laser‐induced breakdown has yielded a variation of CEAS in which the emission from a laser‐induced plasma (LIP) is utilized as a pulsed incoherent broadband light source in the center of a high‐finesse optical cavity. In comparison to standard CEAS implementations, this approach places light directly inside the resonator without injection through the cavity mirrors. The broadband optical emission from the pulsed LIP facilitates the measurement of spectra through (i) time‐dependent cavity ring‐down spectroscopy (CRDS) and (ii) intensity‐dependent incoherent broadband cavity‐enhanced absorption spectroscopy (IBBCEAS) methodologies. Additionally, from the broadband ring‐down time of light within the cavity, cavity mirror reflectivities may be calibrated in situ, thus enabling absolute extinction coefficient measurements with IBBCEAS. The IBBCEAS approach possesses numerous advantages over the CRDS approach in terms of measurement speed and achievable signal‐to‐noise ratios (SNRs). However, it is limited by the presence of an offset in the measured absorption that arises due to the absorption of light that occurs between LIP formation and commencement of data collection. This offset motivated the development of a numerical correction procedure which allows spectra measured in this way to be corrected to improve their overall accuracy.