Imaging of transient phenomena with low coherence lasers comprising arrays of independent microbeams: A laser version of Harold Edgerton’s stroboscope

Abstract

A longstanding barrier to laser imaging with high spatial and temporal resolution is speckle, the granular interference pattern arising from the coherent interaction of laser radiation with the topography of an illuminated surface. Over the past five decades, scores of mechanical and optical approaches to mitigating or eliminating the impact of speckle have been proposed, including dynamic diffusers, degenerate optical cavities, and random lasers. We describe a laser resonator architecture that allows the spatial coherence and the associated speckle contrast ratio (C) of the laser output to be varied continuously while providing the power necessary for optical imaging of dynamic objects and phenomena with sub-10 ns resolution. Stabilization of a Fabry–Pérot optical cavity with an internal array of microlenses generates thousands of mutually incoherent, parallel microlaser beams, which merge in the far field to form a single beam having a near-Gaussian transverse intensity distribution. For this laser illuminator, C scales as 1/N, where N is the number of microlasers in the array. When Ti:Al2O3 serves as the gain medium, composite beams comprising N > 1000 microbeams are generated with a divergence angle of ∼5 mrad and C < 0.03 for single pulse energies of 8 mJ (∼1 MW peak power). To illustrate the capability of this tunable spatial-coherence laser, images of Drosophila melanogaster in flight and turbomolecular pump vanes rotating at 56 000 rpm are presented. Owing to the brightness and pulse energies available with this laser, imaging a target at a distance of 5 m through dense fog with ∼250 μm resolution has been demonstrated.