Journal Article

Real-Time Amplitude and Phase Imaging of Optically Opaque Objects by Combining Full-Field Off-Axis Terahertz Digital Holography with Angular Spectrum Reconstruction

Ogawa, Takayuki, Yamamoto, Hirotsugu, Okabe, Kyosuke, Minamikawa, Takeo, Tsurumachi, Noriaki, Iwata, Testuo, Abdelsalam, Dahi Ghareab, Yasui, Takeshi, Mizutani, Yasuhiro, Yamagiwa, Masatomo

2018

Overview

Terahertz digital holography (THz-DH) has the potential to be used for non-destructive inspection of visibly opaque soft materials due to its good immunity to optical scattering and absorption. Although previous research on full-field off-axis THz-DH has usually been performed using Fresnel diffraction reconstruction, its minimum reconstruction distance occasionally prevents a sample from being placed near a THz imager to increase the signal-to-noise ratio in the hologram. In this article, we apply the angular spectrum method (ASM) for wavefront reconstruction in full-filed off-axis THz-DH because ASM is more accurate at short reconstruction distances. We demonstrate real-time phase imaging of a visibly opaque plastic sample with a phase resolution power of λ/49 at a frame rate of 3.5 Hz in addition to real-time amplitude imaging. We also perform digital focusing of the amplitude image for the same object with a depth selectivity of 447 μm. Furthermore, 3D imaging of visibly opaque silicon objects was achieved with a depth precision of 1.7 μm. The demonstrated results indicate the high potential of the proposed method for in-line or in-process non-destructive inspection of soft materials.

• Publisher: Springer US,Springer Nature B.V
• Subject: Amplitudes / Classical Electrodynamics / Digital imaging / Electrical Engineering / Electronics and Microelectronics / Engineering / Fresnel diffraction / Holography / Image reconstruction / Immunity / Inspection / Instrumentation / Real time / Reconstruction / Selectivity / Signal to noise ratio / Wave front reconstruction / Wave fronts