arXiv subjects

Yunchan Hwang

Publications and source records attributed to Yunchan Hwang.

4 recordsLinked to original sources

arXiv2026

Visible Spectral-Domain Optical Coherence Tomography for Photonic Integrated Circuits Characterization

Visible photonic integrated circuits underpin applications ranging from AR/VR to quantum control, yet lack a high-resolution, nondestructive diagnostic comparable to the optical frequency-domain reflectometry used in infrared silicon photonics. Here we adapt spectral-domain optical coherence tomography to measure guided-mode back-reflections in visible PICs. Broadband visible light injected into a circuit generates back-reflections that interfere with a depth-referencing local oscillator, and the resulting spectral fringes are recorded on a spectrometer. We validate the approach by resolving multiple round-trip echoes in a waveguide-coupled ring resonator using only single-port access. We then extend it to circuits integrated with diamond quantum micro-chiplets, clearly resolving input and output facets as well as PIC--QMC transition regions. The system achieves shot-noise-limited sensitivity, 50 dB dynamic range, 8 um axial resolution in silicon nitride, and a 2 mm imaging depth at 6 dB roll-off. SD-OCT therefore provides a practical, high-resolution diagnostic for visible PICs that uses a broadband probe source and requires only single-port optical access, enabling rapid characterization of propagation loss, backscattering, and dispersion.

physics.optics↗

arXiv2023

Unsupervised detection of small hyperreflective features in ultrahigh resolution optical coherence tomography

Recent advances in optical coherence tomography such as the development of high speed ultrahigh resolution scanners and corresponding signal processing techniques may reveal new potential biomarkers in retinal diseases. Newly visible features are, for example, small hyperreflective specks in age-related macular degeneration. Identifying these new markers is crucial to investigate potential association with disease progression and treatment outcomes. Therefore, it is necessary to reliably detect these features in 3D volumetric scans. Because manual labeling of entire volumes is infeasible a need for automatic detection arises. Labeled datasets are often not publicly available and there are usually large variations in scan protocols and scanner types. Thus, this work focuses on an unsupervised approach that is based on local peak-detection and random walker segmentation to detect small features on each B-scan of the volume.

eess.IV↗

arXiv2023

Retinal blood flow speed quantification at the capillary level using temporal autocorrelation fitting OCTA

Optical coherence tomography angiography (OCTA) can visualize vasculature structures, but provides limited information about the blood flow speeds. Here, we present a second generation variable interscan time analysis (VISTA) OCTA, which evaluates a quantitative surrogate marker for blood flow speed in vasculature. At the capillary level, spatially compiled OCTA and a simple temporal autocorrelation model, ρ(τ) = exp(-ατ), were used to evaluate a temporal autocorrelation decay constant, α, as the blood flow speed marker. A 600 kHz A-scan rate swept-source provides short interscan time OCTA and fine A-scan spacing acquisition, while maintaining multi mm2 field of views for human retinal imaging. We demonstrate the cardiac pulsatility and repeatability of α measured with VISTA. We show different α for different retinal capillary plexuses in healthy eyes and present representative VISTA OCTA of eyes with diabetic retinopathy.

eess.IV↗

arXiv2019

B1+ Homogenization in 7T MRI Using Mode-shaping with High Permittivity Materials

Ultra high field (UHF) brain MRI has proved its value by providing enhanced SNR, contrast, and higher resolution derived from the higher magnetic field (B0). Nonetheless, with the increased B0 of UHF MRI, the transmit RF magnetic field (B1+) inhomogeneity also became one of the critical issues requiring attention. As the effective wavelength of RF becomes comparable or smaller than the dimension of the brain at B0 larger than 7 Tesla, the increased B1+ inhomogeneity of UHF MRI results in poor SNR and uneven contrast. While parallel transmission techniques (PTx) and high permittivity material (HPM) structures for the mitigation of B1+ inhomogeneity have been suggested, the associated complexity in PTx and restricted volume of homogenization with HPM approach still remain as challenges. In this work, we address the B1+ inhomogeneity in the notion of mode-shaping. Treating a brain phantom as a dielectric potential-well resonator, we apply a phantom-conformal HPM potential-well in combination with a low-index potential barrier (air), to achieve the homogeneity of B1+ in the region of interest (ROI). Based on the electromagnetic simulations using a realistic brain model at 7T, we show that the proposed HPM structure reduces both the average deviation of B1+ in axial slices by 54% and peak SAR by 42%, respectively.

physics.med-ph↗