Research On Phase Retrieval Algorithmand Resolution In CW Terahertz Inline Digital Holography

Terahertz in-line digital holography has not only simple optical path, low requirements for light source coherence, easy operation etc., but also lower photon energy and high penetration to non-metallic and non-polar substances. And the systemic resolution is only limited by the numerical aperture of the detecting device, so it gains more and more scholars’ attention. However, the zero-order diffraction light and twin image would cause the reconstructed images misty and reduce the image quality. It can be seen that the research and discussion on the reconstruction algorithms are necessary. Besides, due to the instability and low energy of the terahertz source, low sensitivity of detection device etc., the research reports about terahertz in-line digital holography are all stay on the implementation level, so the systemic resolution needs to be improved through more research. Based on the terahertz in-line digital holography imaging system comprised of a CO2 pumped continuous-wave terahertz laser system and a pyroelectric camera, the in-deep and detailed study on the single-exposure, double-exposure phase retrieval algorithms and their key problems are conducted. Under the same experimental conditions, the influences of the recording distance and detector parameters on the single point resolution are discussed by the simulation research.In order to make the phase retrieval algorithms apply to non-isolated object terahertz in-line digital holographic reconstruction, considering the existing phase retrieval algorithms and terahertz imaging characteristics, based on the hologram expansion and apodization operation, a modified, single-exposure, amplitude constrained phase retrieval algorithm(T-APRA) is proposed; The simulation and experiment holograms are used to prove that the single-exposure phase retrieval algorithm(APRA) has good feasibility and applicability in the terahertz in-line digital holography; In addition, when the imaging target is a non-isolated object, the impacts of the extended hologram and hologram superposition frames on T-APRA and its reconstructed results are studied; Meanwhile based on threshold segmentation and morphological filtering, a image processing method is proposed to calculate the object-plane support domain, and combined with the modified single-exposure support-domain constrained phase retrieval algorithm to conduct reconstruction, and then its effectiveness and practicality are verified by simulation and experiment.On the basis of the single-exposure phase retrieval algorithm(T-APRA), exposure number is increased to two times and the basic calculation flow of the two-exposure amplitude constrained phase retrieval algorithm(DT-APRA) is presented; Respectively based on the same simulation holograms and experiment data, the effects of the single-exposure and double-exposure phase retrieval algorithms are compared and analyzed; Finally, the influence of the superimposed frame number of the holograms on double-exposure amplitude constrained phase retrieval algorithm(DT-APRA) is discussed in order to reduce the recording time by reasonably decreasing the frame number of the holograms.Finally, based on the calculating principle of the single point resolution proposed by Fournier et al. under the inverse problem frame, the distribution maps of single point resolution on the object plane, which is as large as the detector size, are calculated in the 2.52 terahertz in-line digital holographic imaging system; The impacts of the different recording distance and detector parameters on single point resolution are emphatically analyzed.