Time-resolved continuous-filtering Vernier spectroscopy of H₂O and OH radical in a flame

We use broadband near-infrared continuous-filtering Vernier spectroscopy (CF-VS) for time-resolved detection of H₂O and OH radical in a premixed CH₄/air flat flame. The CF-VS spectrometer is based on a femtosecond Er:fiber laser, an external cavity that contains the flame, and a detection system comprising a rotating diffraction grating and photodetectors. Spectra of H₂O and OH radical around 1570 nm are continuously recorded with 6.6 GHz spectral resolution, 4.0 × 10^-7 cm^-1 absorption sensitivity, and 25 ms time resolution, while the fuel-air equivalence ratio is periodically modulated with a square wave. The concentrations of the two analytes are retrieved with percent level precision by a fit of a Vernier model to each spectrum spanning 13 nm. The temporal profiles of both concentrations in each modulation cycle are repeatable and the steady-state concentration levels are in good agreement with predictions based on one-dimensional simulations of a static flat flame. The robust CF-VS spectrometer opens up for quantitative monitoring of multiple products of time-varying combustion processes with relatively simple data acquisition procedures.