Real-Time Amplitude and Phase Imaging of Optically Opaque Objects by Combining Full-Field Off-Axis Terahertz Digital Holography with Angular Spectrum Reconstruction
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.
KeywordsTerahertz Digital holography Phase image Off-axis Angular spectrum method
This work was supported by grants for Exploratory Research for Advanced Technology (ERATO) MINOSHIMA Intelligent Optical Synthesizer (IOS) Project (JPMJER1304) from the Japanese Science and Technology Agency and Research Grant 2015 from Mitutoyo Association for Science and Technology, Japan. The authors gratefully acknowledge Prof. Qing Hu of Massachusetts Institute of Technology for his help in preparation of the THz-QCL device and the Kagawa University Nanotechnology Supporting Office as part of the “Nanotechnology Project” in the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, for its help in the preparation and evaluation of the semiconductor sample. We also wish to acknowledge Ms. Natsuko Takeichi of Tokushima University for her help in the preparation of the manuscript.
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