Picosecond Kerr-gated Raman spectroscopy for measurements in sooty and PAH rich hydrocarbon flames

The picosecond optical Kerr gate is an effective method for the separation of long-lived laser induced interference from instantaneous Raman scattering. An application of the Kerr-gated Raman spectroscopy in combustion environment was demonstrated. The fast gating provided by a Kerr effect based optical shutter extends Raman spectroscopy measurements to sooty flames and flames rich in polycyclic aromatic hydrocarbons. The optical setup of the Kerr gate is described in detail and the gating time, efficiency and spatial uniformity of the ultrafast shutter are presented. To demonstrate the concept, the Kerr-gated Raman spectroscopy was applied in a laminar CH₄/air diffusion flame to evaluate the capability to extract a Raman spectrum under different interference levels. In the region deemed inaccessible with µs-gated Raman spectroscopy, the laser induced interference was effectively suppressed by the Kerr gate. A test was also performed in a C₂H₄/air premixed flame to explore the limitation of the Kerr-gated Raman technique. The result showed that after combining with polarization separation, Raman spectra could be extracted from locations with 65 ppb soot volume fraction.