Topography of hidden objects using THz digital holography with multi-beam interferences

Computational adaptive optics for high-resolution non-line-of-sight imaging

Non-line-of-sight (NLOS) imaging has aroused great interest during the past few years, by providing a unique solution for the observation of hidden objects behind obstructions or scattering media. As such, NLOS imaging may facilitate broad applications in autonomous driving, remote sensing, and medical diagnosis. However, existing NLOS frameworks suffer from severe degradation of resolution and signal-to-noise ratio (SNR) due to aberrations induced by scattering media and system misalignment, restricting its practical applications. This paper proposes a computational adaptive optics (CAO) method for NLOS imaging to correct optical aberrations in post-processing without the requirement of any hardware modifications. We demonstrate the effectiveness of CAO with a confocal NLOS imaging system in Terahertz (THz) band by imaging different samples behind occlusions for both low- and high-order aberrations. With appropriate metrics used for iterative CAO in post-processing, both the resolution and SNR can be increased by several times without reducing the data acquisition speed.

Shape memory of a polymer grating surface fabricated by two-beam interference lithography

Switchable and reversible optical elements have potential applications in self-adaptive optics. Shape-memory polymer devices with adaptive properties could be easily switched under environment or field stimuli. Here, the laser beam interference technique was used to realize the periodic grating structures of the shape-memory polymer, and memory and recovery of the grating structures were performed. A one-dimensional grating structure was fabricated from dual-beam interference lithography of a nanosecond laser and underwent pressure in a condition of 195°C. The vertical height of the grating was reduced, and the diffraction light was weakened. When the sample was cooled down to room temperature, the morphology of the grating could be kept. After raising the ambient temperature of the sample to 120°C, the morphology of the grating was recovered to the original state, which realized the shape-memory function.

Continuous-wave terahertz reflective ptychography by oblique illumination

Massive usage scenarios prompt the prosperity of terahertz (THz) reflective imaging methods. In this Letter, we apply ptychography to continuous-wave THz reflective imaging. Our scheme has a compact lensless layout and uses a full-field oblique-illumination recording mode. Diffraction patterns are corrected through tilted plane correction. This method can be used to retrieve the complex-valued object function and to suppress the negative effect of non-uniform illumination. The feasibility is investigated using two metal samples.

Confocal terahertz SAR imaging of hidden objects through rough-surface scattering

Terahertz (THz) radar imaging has gained great interests in various applications due to its capability of deep penetration in some specific contents such as plastic and non-conductive materials without water. However, the image quality would be highly degraded by the scattering of the rough surfaces, which remains a challenge in the area. Here, we propose a confocal terahertz synthetic aperture radar (SAR) to alleviate the scattering issues with both improved signal-to-noise ratio (SNR) and resolution. Inspired by the confocal non-line-of-sight imaging in visible wavelength, a convex lens is added into the conventional SAR system to localize the effect of scattering within each spatial sampling point by the confocal configuration. Then, the random phase aberration caused by scattering could be corrected through a simple shift in temporal domain. The performance of the proposed method under different roughness occlusions is evaluated through experiments. All the results demonstrate great enhancements in both resolution and SNR over the conventional methods.

General method for complex-wave fields registration with high fidelity

In the field of optical imaging, the image registration method could be applied to realize a large field of view along with high resolution. The traditional image registration methods are mostly conceived for intensity images and might fail for complex-valued images. Especially, those methods do not account for the random phase offset associated with phase. In this paper, we proposed a general method for complex-wave field registration. A similar procedure has been proposed for the reconstruction of the ptychographic dataset, but here is modified for the registration of general wave fields. The method can efficiently separate the illumination and object function, refine the positions of each wavefront, and thus provide a stitched wide-field object wave with high fidelity. Simulation and experimental results applied to register the wave fields obtained from digital holographic microscopy are given to verify the feasibility of the method. This method would have potential applications in large-field high-resolution microscopy, adaptive imaging, remote sensing and the measurement of structured optical fields.

THz coherent lensless imaging

Imaging with THz radiation has proved an important tool for both fundamental science and industrial use. Here we review a class of THz imaging implementations, named coherent lensless imaging, that reconstruct the coherent response of arbitrary samples with a minimized experimental setup based only on a coherent source and a camera. After discussing the appropriate sources and detectors to perform them, we detail the fundamental principles and implementations of THz digital holography and phase retrieval. These techniques owe a lot to imaging with different wavelengths, yet innovative concepts are also being developed in the THz range and are ready to be applied in other spectral ranges. This makes our review useful for both the THz and imaging communities, and we hope it will foster their interaction.

Iterative phase-retrieval-assisted off-axis terahertz digital holography

In terahertz digital holography, the off-axis configuration is the appropriate choice when the investigated object is non-sparse and complex. The limitation of recording distance in the off-axis configuration restricts the imaging quality. Either low-resolution or spectra overlap can potentially occur. We propose an iterative phase-retrieval approach to improve the quality of reconstruction results obtained from an off-axis hologram. One additional capture of object wave intensity is recorded to perform iterative phase retrieval with off-axis reconstruction as the initial guess. Apodization operation can be applied to the object wave intensity capture to suppress undesired border diffraction effects. The image quality using the proposed method has been improved both from simulation and experimental verification.

Visualization of internal structure and internal stress in visibly opaque objects using full-field phase-shifting terahertz digital holography

We construct a full-field phase-shifting terahertz digital holography (PS-THz-DH) system by use of a THz quantum cascade laser and an uncooled, 2D micro-bolometer array. The PS-THz-DH enables us to separate the necessary diffraction-order image from unnecessary diffraction-order images without the need for spatial Fourier filtering, leading to suppress the decrease of spatial resolution. 3D shape of a visibly opaque object is visualized with a sub-millimeter lateral resolution and a sub-µm axial resolution. Also, the digital focusing of amplitude image enables the visualization of internal structure with the millimeter-order axial selectivity. Furthermore, the internal stress distribution of an externally compressed object is visualized from the phase image. The demonstrated results imply a possibility for non-destructive inspection of visibly opaque non-metal materials.

Coherent imaging using laser feedback interferometry with pulsed-mode terahertz quantum cascade lasers

We report a coherent terahertz (THz) imaging system that utilises a quantum cascade laser (QCL) operating in pulsed-mode as both the source and detector. The realisation of a short-pulsed THz QCL feedback interferometer permits both high peak powers and improved thermal efficiency, which enables the cryogen-free operation of the system. In this work, we demonstrated pulsed-mode swept-frequency laser feedback interferometry experimentally. Our interferometric detection scheme not only permits the simultaneous creation of both amplitude and phase images, but inherently suppresses unwanted background radiation. We demonstrate that the proposed system utilising microsecond pulses has the potential to achieve 0.25 mega-pixel per second acquisition rates, paving the pathway to video frame rate THz imaging.

Video-rate terahertz digital holographic imaging system

Terahertz (THz) imaging has been demonstrated in numerous applications from medical to non-destructive evaluation (NDE), but current systems require expensive components, provide slow frame-rates and low resolutions. THz holography offers a potentially low-cost, high-performance alternative. Here we demonstrate the first full video-rate THz digital holography system at 2.52 THz (118.8 µm) using low-cost optical components. 2D digital reconstructions of samples are performed at frame-rates of 50 Hz - an order of magnitude higher than previous systems, whilst imaging of samples concealed in common packaging types demonstrates suitability for NDE applications. A lateral resolution of 250 µm was determined using a 1951 USAF target.

Twenty years of terahertz imaging [Invited]

The birth of terahertz imaging approximately coincides with the birth of the journal Optics Express. The 20^th anniversary of the journal is therefore an opportune moment to consider the state of progress in the field of terahertz imaging. This article discusses some of the compelling reasons that one may wish to form images in the THz range, in order to provide a perspective of how far the field has come since the early demonstrations of the mid-1990's. It then focuses on a few of the more prominent frontiers of current research, highlighting their impacts on both fundamental science and applications.

Terahertz ptychography

We realized a phase retrieval technique using terahertz (THz) radiation as an alternative to THz digital holography, named THz ptychography. Ptychography has been used in x-ray imaging as a groundbreaking improvement of conventional coherent diffraction imaging. Here we show that ptychography can be performed at THz frequencies too. We reconstructed an amplitude and a phase object with both simulated and real data. Lateral resolution accounts to <2λ, while depth variations as low as λ/30 can be assessed.