Dual-comb Spectroscopy for Laminar Premixed Flames with a Free-running Fiber Laser

Abstract

ABSTRACT Dual-comb spectroscopy (DCS) has emerged as an important new technique for high-resolution molecular spectroscopy. However, this powerful tool has not been widely utilized in combustion diagnostics due to the difficulty of maintaining the mutual coherence between the two comb sources. In this work, we demonstrated a free-running fiber-laser-based DCS for in situ and non-intrusive H2O and C2H2 measurements in laminar premixed flames. This picometer-resolution dual-comb spectroscopic system was developed by using a single erbium-doped fiber laser at 1.5 μm along with a multipass optical configuration for the enhanced absorption measurement. Because of the mutual coherence of the two comb pulse trains emitted from the same laser cavity, DCS could be performed without using any phase-locking systems. Absorption spectra of CH4/O2/air flames over the spectral range 6503 cm−1 – 6535 cm−1 were measured at different reactant flow rates and heights above the burner, showing a good agreement with the spectral simulation of high-temperature H2O using the HITEMP database. A noise equivalent (1σ) absorption coefficient of 9.5 × 10−5 cm−1 was achieved using the current DCS system. Besides, the dual-comb measurement of a C2H4/air sooting flame was also performed at the equivalence ratio Ф = 2.94 and compared with the spectral simulation of high-temperature H2O and C2H2. All the spectral data were retrieved by 1500 averages of interferograms to increase the spectral signal-to-noise ratio (SNR), but a single interferogram could be acquired at 0.83 ms using the current free-running dual-wavelength fiber laser.