We introduced two continuous-wave terahertz iterative phase-contrast imaging methods. In-line digital holography has the capability to reconstruct the amplitude and phase distributions simultaneously. It is a non-destructive, high-resolution, full-field dynamic phase-contrast imaging technique. Ptychography can reconstruct the complex amplitude distribution of the transmission object from the overlapped diffraction patterns. Both methods can achieve phase-contrast imaging, and are suitable for terahertz region. In this paper, both Gabor in-line holographic and ptychographical configurations are investigated from algorithms to experiments. For in-line holography, the use of extrapolation, synthetic aperture, sub-pixel shifting and multi-plane imaging are introduced to improve the resolution and reconstruction accuracy. For ptychography, we obtained the ptychographical reconstruction results of a polypropylene alphabet sample, which provides a new imaging method for terahertz phase-contrast imaging.
Terahertz (THz) radiation is able to penetrate many different types of nonpolar and nonmetallic materials without the damaging effects of x-rays. THz technology can be combined with computed tomography (CT) to form THz CT, which is an effective imaging method that is used to visualize the internal structure of a three-dimensional sample as cross-sectional images. Here, we reported an application of THz as the radiation source in CT imaging by replacing the x-rays. In this method, the sample cross section is scanned in all translation and rotation directions. Then, the projection data are reconstructed using a tomographic reconstruction algorithm. Two-dimensional (2-D) cross-sectional images of the chicken ulna were obtained through the continuous-wave (CW) THz CT system. Given by the difference of the THz absorption of different substances, the compact bone and spongy bone inside the chicken ulna are structurally distinguishable in the 2-D cross-sectional images. Using the filtered back projection algorithm, we reconstructed the projection data of the chicken ulna at different projection angle intervals and found that the artifacts and noise in the images are strikingly increased when the projection angle intervals become larger, reflected by the blurred boundary of the compact bone. The quality and fidelity of the 2-D cross-sectional images could be substantially improved by reducing the projection angle intervals. Our experimental data demonstrated a feasible application of the CW THz CT system in biological imaging.
Fourier ptychography is a new type of synthetic aperture imaging technique based on phase retrieval method which can improve microscopeic imaging performance beyond the diffraction limit of the employed optical components by illuminating the object with oblique waves of different incident angles where the field of view remains unchanged. illumination angle and the overlap rate of spectrum will have a certain impact on the quality of reconstruction. In this paper, we study the effects of illumination angle and spectral overlap rate on the image quality of Fourier ptychography. The simulation results show that increasing the illumination angle and spectral overlap can improve the resolution, but there is a threshold for the key parameters of spectral overlap rate. The convergence rate decreases when the overlap rate exceeds 70%, and the reconstruction process is more time-consuming due to the high overlap rate. However the results of proposed study shows that an overlap of 60% is the optimal choice to acquire a high-quality recovery with high speed.
Ptychography can reconstruct the complex amplitude distribution of the transmission object from the overlapped diffraction patterns. The extended ptychographical iterative engine (ePIE) algorithm is considered as one of the most popular phase retrieval algorithms of present time which has strong robustness and high image quality. However, no constraint is added in the iteration process, which results in slow convergence and makes the real-time imaging difficult. To overcome this problem, an efficient phase retrieval algorithm is proposed based on the constraint of positive absorption in the object plane where the absorption coefficient and phase distribution of the object are constrained by the positive absorption of the object. The simulation demonstrated that the proposed algorithm has greatly improve the convergence rate and imaging quality of traditional ePIE algorithm and is expected to be applied in real-time imaging.