Wave Lensing Effects on a Remote Raman Sensor: Enhanced Mean and the Intensity Distribution of Raman Flashes

The water OH Raman band has been used as an internal standard for remote airborne measurements of fluorescence in water, but the Raman and fluorescence signals of airborne sensors can be perturbed by surface waves that induce a focusing or diverging effect. This lensing effect of waves changes the strength and effective sampling depth of optical signals detected by a remote sensor relative to the flat-water condition. In this work, a small remote optical sensor was used to observe the impact of the lensing effect on Raman scattering of water in a diving pool. This manuscript reports the impact of avalanche multiplication noise on the Raman scattering of flat water; the appearance of bright wave-induced Raman "flashes" analogous to, but stronger than, those observed for sunlight focusing; an approximately logarithmic distribution of signal strengths; and an increase in the average signal strength observed in the presence of waves. A mathematical model of the lensing effect in weak lensing conditions was developed for sinusoidal waves and employed to interpret the observed variability of the Raman signal.