Talbot-effect structured illumination: pattern generation and application to long-distance \(\upmu \)-MTV

A new scheme is described and characterized to generate very fine structured illumination patterns that can be used to significantly improve the spatial resolution of molecular tagging velocimetry (MTV) and other techniques that rely on structured illumination. MTV is particularly suited to measure spatial gradients of velocity as well as second-order derivatives, but has traditionally suffered from a limited resolution. MTV has gained a broad adoption in high-speed gas dynamics and has also been applied to a lesser extent in aqueous flows. The present work makes use of the optical Talbot effect, a near-field diffraction phenomenon, to generate an array of quasi-parallel beamlets with very flexible beam spacing and diameter. Both pitch and width have been demonstrated down to the order of 10 \({\upmu \mathrm {m}}\), which is about ten times smaller than that performed in previous MTV studies. Structured illumination with Talbot effect has potential for even smaller sizes. Several parameters are extensively characterized, including beamlet size, spacing, and persistence. Optimization of the optical components is also discussed, including lasers and lenses. Simple analytical models enable to assist in the design of experiments with Talbot effect. The technique is demonstrated with 1D multi-line and 2D grids with simple MTV tests in water. The selected caged dye is used for the first time in MTV and conveniently enables the use of green PIV lasers to read the tagged lines.