Abstract
Terahertz digital off-axis holography is demonstrated using a Mach-Zehnder interferometer with a highly coherent, frequency tunable, continuous wave terahertz source emitting around 0.7 THz and a single, spatially-scanned Schottky diode detector. The reconstruction of amplitude and phase objects is performed digitally using the angular spectrum method in conjunction with Fourier space filtering to reduce noise from the twin image and DC term. Phase unwrapping is achieved using the dual wavelength method, which offers an automated approach to overcome the 2π phase ambiguity. Potential applications for nondestructive test and evaluation of visually opaque dielectric and composite objects are discussed.
Highlights
Digital holography at optical wavelengths has been heavily developed over recent years because of newly-available high-resolution CCD cameras and advances in digital and automated image processing techniques [1]
THz holography is an exception to this rule, and early simulations and experiments have explored these principles by producing low-resolution millimeter wave and THz amplitude holograms and pseudo-holograms using continuous wave (CW) techniques [22,23,24] and THz-time domain (THz-TD) techniques [25,26], respectively
A resolution limit based on diffraction and Rayleigh’s criterion can be formulated that states that two neighboring object points can just be resolved if their angular separation ω = 1.22 λ/D where D is the diameter of the objective aperture
Summary
Digital holography at optical wavelengths has been heavily developed over recent years because of newly-available high-resolution CCD cameras and advances in digital and automated image processing techniques [1]. We adapt dual-wavelength digital holographic methods - which have been shown to eliminate reconstruction ambiguities, improve image resolution, reduce coherent noise, and provide quantitative measurements of physical thickness, surface, and refractive index variations - in the THz region [16,27] to reconstruct amplitude and phase objects using highly coherent, monochromatic THz radiation. Such high fidelity images illustrate the potential of THz holography for non-destructive testing and evaluation of visually opaque materials and structures
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