Research on object-plane constraints and hologram expansion in phase retrieval algorithms for continuous-wave terahertz inline digital holography reconstruction

In terahertz inline digital holography, zero-order diffraction light and conjugate images can cause the reconstructed image to be blurred. In this paper, three phase retrieval algorithms are applied to conduct reconstruction based on the same near-field diffraction propagation conditions and image-plane constraints. The impact of different object-plane constraints on CW terahertz inline digital holographic reconstruction is studied. The results show that in the phase retrieval algorithm it is not suitable to impose restriction on the phase when the object is not isolated in the transmission-type CW terahertz inline digital holography. In addition, the effects of zero-padding expansion, boundary replication expansion, and apodization operation on reconstructed images are studied. The results indicate that the conjugate image can be eliminated, and a better reconstructed image can be obtained by adopting an appropriate phase retrieval algorithm after the normalized hologram extending to the minimum area, which meets the applicable range of the angular spectrum reconstruction algorithm by means of boundary replication.