arXiv subjects

Hwidon Lee

Publications and source records attributed to Hwidon Lee.

3 recordsLinked to original sources

arXiv2026

Feature-domain Fourier ptychographic tomography with dark-field illumination

Fourier ptychographic tomography (FPT) is an implementation of intensity diffraction tomography that reconstructs three-dimensional (3D) refractive-index (RI) distributions from angle-varied intensity measurements. The distinctive advantage of FPT emerges when incorporating dark-field illumination, which extends the space-bandwidth product toward gigavoxel-scale volumetric imaging, yet dark-field measurements are highly sensitive to system imperfections and often have low signal-to-noise ratios. Here, we propose feature-domain FPT (FD-FPT), which evaluates data fidelity after feature extraction and is optimized using automatic differentiation. In numerical and experimental tests, FD-FPT resolves structures near the synthetic-aperture cutoff far more reliably than the spatial-domain baseline (SD-FPT). Notably, in a whole-mount Oedogonium specimen, the reticulate chloroplast network and transverse septa were resolved only by FD-FPT. We further demonstrate a 1.81-gigavoxel RI reconstruction of a mouse adrenal gland section across a 1.66 by 1.40 square millimeter field of view, establishing FD-FPT as a practical route to label-free volumetric imaging that combines millimeter-scale coverage with cellular-scale structural contrast.

physics.optics↗

arXiv2026

High Spectral Energy Density All-Fiber Nanosecond Pulsed 1.7 $\mu$m Light Source for Photoacoustic Microscopy

We present a high spectral energy density all-fiber nanosecond pulsed 1.7 $\mu$m light source specifically designed for photoacoustic microscopy (PAM). The system targets the first overtone absorption of C-H bonds near 1720 nm within the near-infrared-III (NIR-III) window, where lipids exhibit strong optical absorption and tissues benefit from reduced scattering and high permissible fluence. To achieve narrow linewidth, high pulse energy, and high pulse repetition rate (PRR), we developed a master oscillator fiber amplifier architecture based on stimulated Raman scattering. A 1589.80 nm Raman pump and a custom-built narrow-linewidth Raman seed laser were employed to generate spectrally pure 1719.44 nm pulses with an approximately 0.10 nm linewidth. The proposed light source delivers nanosecond pulses of approximately 5 ns with high pulse energy of at least 2.2 $\mu$J and tunable PRRs up to 300 kHz, resulting in a spectral energy density of approximately 22 $\mu$J/nm, which is significantly higher than that of conventional 1.7 $\mu$m light sources. The performance of the NIR-PAM system was validated through resolution testing with a 1951 USAF target, demonstrating a spatial resolution of approximately 4.14 $\mu$m and an axial resolution of approximately 85.5 $\mu$m. Phantom imaging of CH$_2$-rich polymer films and ex vivo lipid-rich biological tissues confirmed the system's high spatial fidelity and strong contrast for lipid-specific structures. This compact, stable, and spectrally refined light source with high spectral energy density can offer an effective solution for high-resolution, label-free molecular imaging and represents a promising platform for clinical photoacoustic imaging applications involving lipid detection and metabolic disease diagnostics.

physics.optics↗

arXiv2026

Phase-Locked Time-Stretch Optical Coherence Tomography for Contrast-Enhanced Retinal Microangiography

Optical coherence tomography angiography has transformed retinal vascular imaging by providing non-invasive, high-resolution visualization. However, achieving an optimal balance between field of view, resolution, and three-dimensional microvasculature contrast, particularly in deeper retinal layers, remains challenging. A phase-locked time-stretch optical coherence tomography microangiography system is developed to address these limitations with a 5-MHz A-line rate and sub-nm phase sensitivity. Utilizing a dual chirped fiber Bragg grating architecture, the swept-source laser achieves an extended coherence length of approximately 10 mm and a 102-nm bandwidth. A time-stretch analog-to-digital converter overcomes the limitations of conventional multi-MHz optical coherence tomography systems, ensuring a 2-mm imaging depth in the air with high spatial resolution. The proposed system enables high-contrast, depth-encoded mapping of key retinal structures, including the superficial and deep capillary plexuses and the choriocapillaris. Compared to a state-of-the-art system, the proposed approach demonstrates enhanced resolution, improved contrast, and faster imaging speeds, enhancing its potential for diagnosing and monitoring retinal and systemic diseases like age-related macular degeneration, diabetic retinopathy, and Alzheimer's disease.

physics.optics↗