Terahertz digital holography: Two- and four-step phase shifting technique in two plane image recording

A two- and four-step phase shifting (PS) technique in terahertz (THz) digital holography is proposed. Relying on the Mach–Zehnder interferometer-based setup, it was demonstrated that the two-step and four-step PS in Fresnel holograms can assist in a five times greater background subtraction. It allows us to improve the quality of the obtained holographic images, in particular when objects introduce phase changes. It was shown that the recording of holograms of an object consisting of two separated planes can enable qualitative reconstruction of 3D images. Here, the planes were separated by 30 mm, thus defining the longitudinal (depth) resolution in this experiment. It is shown that the PS can serve in distinguishing transparent objects and, by a proper selection of phase variation within the 0–2π range, enable us to increase the quality of the reconstructed hologram. Finally, the advantages of the suggested holographic technique are illustrated by comparing the results with the data of weak absorbing objects obtained via point-to-point, plane-to-plane (4f setup), and dark-field THz imaging approaches. Experiments were performed at frequencies of 0.3 and 0.6 THz recording THz images using resonance antenna-coupled titanium microbolometers.