Phase-wrapping-free terahertz in-line digital holography via projected refractive index

Terahertz (THz) in-line digital holography is a powerful full-field phase imaging technique with a simple optical configuration and a high-space-bandwidth product. However, the phase reconstructed via conventional methods is usually wrapped in the range from -π to π. This introduces phase ambiguity, as some constraints of phase retrieval are ineffective for the wrapped phase. In this work, we propose a projected refractive-index-based phase retrieval algorithm, termed PRI-PR, to reconstruct an unwrapped phase distribution. PRI-PR avoids phase wrapping during the reconstruction by reformulating the phase retrieval problem with the projected refractive index function as the unknown, rather than the transmittance function. Meanwhile, we incorporate physical constraints, Plug-and-Play, and sparsity priors to ensure high-quality imaging for large-phase objects. We experimentally demonstrate high-quality quantitative phase-wrapping-free imaging of large-phase objects using a single hologram on a lensless THz in-line holographic imaging system.