Wavelength-modulated planar laser-induced fluorescence for imaging gases

This work presents the development of wavelength-modulated planar laser-induced fluorescence (WM-PLIF) and its initial application to infrared imaging of carbon monoxide in a laminar flame. A continuous-wave quantum-cascade laser producing 50 mW near 4.8 μm was injection-current modulated at 1 kHz and scanned across the P(20) transition of CO at 20 Hz. The corresponding infrared-laser-induced fluorescence from 2065  cm^-1 to 2155  cm^-1 was imaged orthogonal to the laser sheet using a high-speed IR camera, and digital lock-in detection of the WM-PLIF first-harmonic signal (S_F,1f) was performed to provide high-fidelity, background-free imaging of CO. Images of the peak-S_F,1f signal are presented for a laminar CO-H₂ diffusion flame in air at atmospheric pressure. We demonstrate that this technique is sensitive enough to image nascent CO in flames and present a strategy for simulating the WM-PLIF harmonic signals.