Range-selective digital holography for on-axis geometries and vibrating outdoor objects

Abstract

Optical imaging systems often experience significant performance degradation in turbulent or scattering environments such as fog or dust. To address these challenges, coherent range-selective imaging techniques inspired by frequency-modulated continuous-wave (FMCW) lidar have been developed. These approaches enable holographic imaging of closely spaced objects while suppressing backscatter from objects outside the system's range resolution by reconstructing only those located at specific, tunable ranges. Traditionally, range-selective digital holography has been demonstrated only in off-axis geometries, which limit spatial resolution. This work advances the field through the development of a novel on-axis temporal heterodyne technique that achieves range selectivity without compromising spatial resolution. Experimental results are shown at distances of a few meters, although the method is applicable to longer distances. A corresponding mathematical framework was established to analyze range-selective temporal heterodyne digital holography, which operates by coherently integrating over multiple FMCW chirps to reconstruct a hologram. The developed theoretical formulation is broadly applicable to other holographic systems that employ multi-chirp integration. To enhance outdoor range-selective digital holographic operation, where object motion, vibration, and atmospheric turbulence present major challenges, two additional techniques were developed. First, long-range holographic capability was achieved by combining a scanning lidar system with a telescope-based digital holography platform, enabling imaging at distances exceeding one kilometer. Second, a stabilization subassembly was implemented to mitigate turbulence and motion-induced phase distortions. This system successfully demonstrated coherent holographic imaging of strongly vibrating objects that exhibited displacement greater than 18 optical wavelengths during an integration at 90 meters, thereby confirming the system's ability to maintain coherence and support reliable outdoor imaging under adverse conditions.