Long Wave Infrared Wavefront Sensing through a Thin Diffuser

Long wave infrared (LWIR) radiation between 8-14 mu m allows imaging without external illumination, and is the fingerprint region for spectroscopic chemical species identification. Infrared (IR) imaging has wide-ranging applications in defense thermography, airborne sensing, structural fault detection, atmospheric measurements and non-invasive medical testing. The need to characterize infrared optics, and probe the thermal emission from controllable metasurfaces of IR nano-antennas,(1) has prompted our development of a high-resolution LWIR imaging methodology. The technique of speckle imaging enables the detection of phase or intensity through complex scattering media - at visible(2) and IR wavelengths.(3) One-shot speckle images of the light scattered through inhomogeneous complex media contain adequate information to enable scene reconstruction and for imaging around obstacles. In this paper, we describe a novel broadband LWIR speckle imaging configuration for wavefront sensing, utilizing a thin diffusive medium and an uncooled microbolometric camera. The thin diffuser has a high angular memory effect, which produces deformations in the speckle image when the locally varying phase of the impinging wavefront is scattered by the diffuser. The spatial shifts of the speckle grains in the images are registered using a rapid diffeomorphic image registration algorithm, generating a high-resolution map of the local phase gradients. The local phase gradients are then integrated in 2-D to reconstruct the wavefront profile. We demonstrate thermal wavefront reconstruction through the LWIR speckle imaging technique in IR optical samples, with promising future applications for imaging through visually non-transparent media like semiconductor wafers, engineered nano-electronic surfaces, and infrared optics.