This work is conducted in collaboration with the Office of Naval Research (ONR) and focuses on developing a LiDAR-based method for detecting backscattered Brillouin scattering in water. Brillouin scattering occurs when light interacts with thermally driven density fluctuations in a fluid, producing frequency-shifted components relative to the incident light. In a backscattered LiDAR geometry, these features can be useful for remote sensing, but the signal is very weak and is typically buried under much stronger Rayleigh and Mie scattering.
To get around this, Slow-Light Imaging Spectroscopy (SLIS) is used as a temporal filtering technique. A narrow-band pulsed laser operating at 774 nm is frequency doubled to 387 nm to access the water transmission window. The backscattered signal is then passed through a heated cesium vapor cell, where strong dispersion near resonance introduces a frequency-dependent group delay. This delay separates the Brillouin signal in time from the dominant elastic scattering. An intensified CCD camera with a 0.5 ns gate is used to capture the delayed return and isolate the signal. Experimental results show a Brillouin frequency shift of approximately 10.1 GHz at 387 nm, and the signal appears as a clearly delayed feature relative to the background. These results show that SLIS can effectively isolate Brillouin scattering and demonstrate that this approach works for detecting backscattered Brillouin signals in a LiDAR configuration for underwater environments.
Figure 3: Experimental setup for SLIS measurements at 387 nm using a cesium vapor cell and
time-gated iCCD detection.