Large-field quantitative phase imaging of optical fiber devices via intensity image stitching and the TIE

High-resolution, large-field-of-view, nondestructive characterization of millimeter-scale fiber devices is essential for elucidating their optical performance and guiding fabrication optimization. However, imaging sub-micron microstructures remains challenging due to the fundamental trade-off between field coverage and spatial resolution. For example, conventional quantitative phase imaging (QPI) systems typically achieve resolution better than 0.5 µm only when characterizing fibers with cladding diameters below 400 µm. To overcome this limitation, we propose a large-field QPI method that synergistically combines intensity-image stitching with the Transport of Intensity Equation (TIE). Our approach follows a "stitch-then-retrieval" paradigm: multiple intensity images are first aligned with sub-pixel precision and seamlessly fused in the intensity domain, after which the TIE is solved globally to reconstruct the full-field phase distribution. This strategy effectively avoids the error propagation and boundary artifacts inherent in conventional phase-stitching techniques. Experimentally, we demonstrate high-fidelity phase imaging of both a fiber splice with a 440 µm cladding diameter and a tapered fiber segment spanning over 2 mm. The accuracy of the reconstructed phase is validated through quantitative agreement with the manufacturer-specified numerical aperture. Furthermore, by integrating our method with tomographic reconstruction, we achieve three-dimensional refractive index mapping of the splice region in a thulium-ytterbium co-doped fiber. This work presents a simple, robust, and readily deployable solution for nondestructive, quantitative characterization of large-scale fiber microstructures.