Continuous-wave off-axis and in-line terahertz digital holography with phase unwrapping and phase autofocusing

Continuous-wave terahertz digital holography is a practical tool to obtain the complete wave-front information of a sample in the terahertz region that has offered unique advantages in various fields. The propagation distance plays a decisive role in the numerical reconstruction of both in-line and off-axis digital holograms. Only when a terahertz hologram is propagated back to a focal object plane can the authentic amplitude and phase distribution be reconstructed, which is actually a troublesome task. In this manuscript, the combination of hologram enhancement, spectrum filtering, phase retrieval and phase unwrapping algorithms is illustrated and organized to benefit the decision-making process of the autofocusing method not only for the amplitude distribution but also for the phase image. To guarantee the accuracy of the autofocusing results, the noise of the hologram, the intrinsic twin image in the amplitude and phase results and the wrapped ambiguity in the phase distribution are suppressed or lessened by using the four techniques stated above. The focused amplitude and unwrapped phase distributions of the sample are reconstructed, and experimental verification confirms that the present hybrid autofocusing method is a feasible, effective and promising technology with broad application prospects.