Flyscan terahertz multi-plane lensless imaging with suppressed coherent noise.

Abstract

Coherent lensless imaging usually suffers from coherent noise and twin-image artifacts. In the terahertz (THz) range, where wavelengths are 2 to 4 orders of magnitude longer than those in the visible spectrum, the coherent noise manifests primarily as parasitic interference fringes and edge diffraction, rather than speckle noise. In this work, to suppress the Fabry-Pérot (F-P) interference fringes, we propose a novel method, which involves the averaging over multiple diffraction patterns that are acquired at equal intervals within a sample's half-wavelength axial shift. To address edge diffraction, as well as non-uniform illumination, a normalization operation is applied. As the twin-image disturbances when dealing with a single diffraction pattern, multi-plane configuration is employed. With all these strategies combined, we propose a flyscan THz multi-plane lensless imaging technique that enables subwavelength resolution, and high-quality, full-field, and rapid complex-valued THz imaging. Furthermore, we refine two algorithms for image reconstruction: one based on the regular multi-plane alternating projection and the other based on an optimization model with total variation regularization. We experimentally verify the proposed methods, achieving a lateral resolution of 88 µm (0.74λ) at 2.52 THz, and showcase its potential for biomedical applications by imaging a section of mouse brain tissue.